| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Vol. 53, Issue 4, 487-525, December 2001
2-Adrenergic Receptor Stimulation Regulate
CD4+ T and B Lymphocyte Function in Vitro and in Vivo
Department of Cell Biology, Neurobiology, and Anatomy (A.P.K., V.M.S.), and Department of Microbiology and Immunology (V.M.S.), Loyola University, Stritch School of Medicine, Maywood, Illinois
Abstract
I. Background
A. Adaptive/Acquired Immunity
B. Bidirectional Communication Between the Nervous and Immune Systems
C. Norepinephrine and the2-Adrenergic Receptor
II. Evidence and Mechanisms for the Release of Norepinephrine in Lymphoid Organs
A. Lipopolysaccharide- and Antigen-Induced Norepinephrine Release
1. Infection/Endotoxin.
2. Particulate Antigens/Sheep Red Blood Cells.
3. Soluble Protein Antigen.
B. Cytokine Receptor Expression on Nerves
C. Afferent Splenic Innervation
D. Cytokine-Induced Norepinephrine Release
III.
A. CD4+ T Lymphocytes
1. Receptor Expression.
2. Mechanisms Regulating Differential Receptor Expression on CD4+ T Cell Subsets.
B. B Lymphocytes
IV. Effects on CD4+ T Lymphocytes
A.
B. Proliferation, Differentiation, and Cell Trafficking
1. In Vitro Proliferation and Differentiation.
2. In Vivo Proliferation and Cell Trafficking.
C. In Vitro and In Vivo Cell Surface Molecule Expression
D. T Cell Cytokine Production
1. In Vitro Th1-Like Cytokines.
2. In Vitro Th2-Like Cytokines.
3. In Vivo Cytokine Production.
4. Differential Effects on Th1 versus Th2 Cytokines.
V. Effects on B Lymphocytes
A.
B. B Cell Proliferation
C. B Cell Surface Molecule Expression and Function
1. In Vitro Surface Molecule Expression and Function.
2. In Vivo Surface Molecule Expression.
D. B Cell Differentiation and Antibody Production
1. In Vitro Direct Alterations Induced by Elevations in Intracellular cAMP.
2. In Vitro
3. In Vivo B Cell Differentiation and Antibody Production.
VI. Disease- and Health-Specific Implications
Acknowledgments
References
 |
Abstract |
|---|
 Top NextReferences
|
|---|
Historically, norepinephrine and the sympathetic nervous system
have been associated with the "fight or flight" response, and they
also contribute to the regulation of autonomic activity within the
body, such as cardiovascular function. In addition, evidence over the
past 30 years suggests that norepinephrine may also regulate the
function of immune cells that protect the body against pathogens. The
presence of sympathetic nerve fibers and the release of norepinephrine
within lymphoid organs represent a mechanism by which signals from the
central nervous system may influence immune cell function. The T
cell-dependent antibody response is essential to successful host
defense against numerous environmental pathogens. It is during this
response that CD4+ T and B lymphocytes are activated to
produce cytokines and antibody, respectively, leading to immune
competence and protection. The goal of this review is to discuss the
evidence supporting the release of norepinephrine within lymphoid
organs and the expression of the 
2-adrenergic receptor by
CD4+ T and B lymphocytes. We also discuss the mechanisms by
which 2-adrenergic receptor stimulation affects the level of
cytokine and antibody produced by these cells both in vitro and in
vivo. In cases where conflicting findings have been reported, we
discuss potential variables that may have contributed to these
conflicting findings. To conclude, we discuss the disease- and
health-specific implications of the basic research being done in the
area of sympathetic nervous system regulation of T and B lymphocyte function.
| |
I. Background |
|---|
|
TopPreviousNextReferences
|
|---|
A. Adaptive/Acquired Immunity
The basic function of the immune system is to clear "nonself" or "foreign" antigens such as bacteria and viruses from the body. The immune system is comprised of two general systems, the innate and the adaptive immune systems. Typically, the innate immune system is considered to be the "first line of defense," and its cells are the first to nonspecifically clear antigen from the body. Unlike the innate immune system, the adaptive immune system is characterized by two distinct features: specificity and memory. The specificity of adaptive immunity originates from the development of a diverse repertoire of T and B lymphocyte receptors that recognize a specific peptide sequence or "antigenic epitope". Therefore, since cells of the adaptive immune system possess the capacity to recognize and respond to minute amounts of antigen, it is essential that immune cell function be carefully regulated to prevent responses to "self" peptide antigens, while at the same time permitting the effective clearance of foreign antigens from the body.
The T cell-dependent antibody response is a critical component of
adaptive immunity and serves as both a sentinel and a defender against
bacterial and viral infections. In addition, the potential exists for
the T cell-dependent antibody response to contribute to the development
of autoimmune diseases, such as multiple sclerosis, rheumatoid
arthritis, and systemic lupus erythematosus (reviewed in Boitard, 1992
;
Goodnow, 1997). In light of this potential for antibody production to
both protect and damage the body, the immune system has developed a
number of autoregulatory mechanisms to augment the antigen-specific
response directed against a foreign antigen and, at the same time, to
prevent responses directed against autoantigens. These regulatory
mechanisms govern both B cell and T cell activation, as well as
effector function during the T cell-dependent antibody response.
The two-signal hypothesis of B cell activation, as first described by
Bretscher and Cohn (1970), represents one of these autoregulatory mechanisms (Fig. 1A). They proposed that
B cell activation requires two signals, with signal 1 originating from
stimulation of the antigen-specific B cell receptor
(BCR) by a foreign antigen. Upon stimulation of the BCR, the B cell begins to prepare itself to produce
antibody. However, without receiving another signal originating from
the CD4+ T-helper (Th) cell, the B cell will not
differentiate into either an antibody-secreting plasma cell or a memory
B cell. This second signal from the Th cell was originally proposed to
be in the form of cytokines. Thus, during a T cell-dependent antibody
response, the B cell will differentiate into either an
antibody-secreting plasma cell or a memory cell following both BCR
stimulation and CD4+ T cell cytokine production.
|
Similar to the process of B cell activation, an antigen-specific Th
cell also requires two distinct signals to become activated to provide
"help" to a B cell (Fig. 1B). As first proposed by Lafferty and
Cunningham (1975), the first of these signals is generated by the
recognition of the peptide antigen by the antigen-specific TCR
expressed on the Th cell surface, which is now known to be presented by
the B cell or another antigen-presenting cell in the context of MHC
class II (the peptide-MHCII complex). In addition, if the Th cell
receives costimulatory signals from the B cell, then the Th cell
becomes fully activated to produce cytokines that provide the second
signal, or "help", required by the B cell to differentiate into
either an antibody-secreting plasma cell or a memory B cell. However,
if the Th cell does not receive the additional costimulatory signals
required for cell activation, such as a B7:CD28 interaction, the cell
is either anergized or induced to undergo apoptosis (Schwartz, 1990).
Thus, the antigen-specific physical interaction between the
CD4+ Th cell and the B cell represents a potent
regulator of the Th cell-dependent antibody response (Sanders et al.,
1986, 1988), which includes antibody secretion from plasma cells,
affinity maturation of the BCR, antibody isotype switching, and memory B cell formation (Liu and Banchereau, 1997).
B. Bidirectional Communication Between the Nervous and Immune Systems
In addition to regulatory mechanisms that are provided by immune
cells, it is now known that complex bidirectional interactions (Fig.
2) between the cells of the immune system
and the nervous system contribute to additional regulatory mechanisms
that influence the function of cellular activities associated with both
systems (reviewed in Sanders and Munson, 1985a; Ader et al., 1990;
Madden and Felten, 1995; Straub et al., 1998; Kohm and Sanders, 2000).
|
One mechanism by which signals from the immune system may regulate
nervous system activity is via the stimulation of cytokine receptors
expressed both on cells within the CNS (reviewed in Rothwell et al.,
1996) and on peripheral sympathetic nerves and ganglia (Hart et al.,
1993; Gadient and Otten, 1996; Marz et al., 1996; Cunningham et al.,
1997). Cytokine receptor stimulation on peripheral nerves and ganglia
alters the level of CNS activity possibly by afferent signal
transduction. But more importantly, alterations in the level of CNS
activity may alter the level of efferent nerve activity and
neurotransmitter release in the periphery. Thus, peripheral cytokine
production may influence efferent nerve activity and neurotransmitter
release by binding to cytokine receptors expressed on peripheral
nerves. In addition, cytokines produced in the periphery may cross the
blood-brain barrier (BBB) via a number of specific cytokine transporter
systems within the BBB to directly affect targets within the CNS (Banks
and Kastin, 1987, 1991; Banks et al., 1991; Gutierrez et al., 1993;
Plotkin et al., 1996). Additionally, activated immune cells may pass
through the BBB to release cytokines directly into the CNS (reviewed in
Weller et al., 1996), thus bypassing the need for afferent signaling pathways from the periphery. The integrity of the BBB can be disrupted under certain pathological conditions, such as viral infection of the
CNS, allowing immune cells and other blood-borne mediators to enter the
CNS (reviewed in Persidsky, 1999). Therefore, several mechanisms exist
for the immune system to communicate with the CNS. However, in order
for the CNS to influence an immune response, reciprocal pathways of
communication from the CNS to the immune system must also exist.
The sympathetic nervous system is typically associated with the
physiological "fight or flight" response, such that it is involved
in the regulation of cardiovascular and respiratory function, especially during times of critical need. In addition, the sympathetic nervous system regulates gastrointestinal tract smooth muscle contraction/relaxation, gastric secretions, and other autonomic functions. Sympathetic neurotransmission from the CNS to the periphery occurs via projections extending from the paraventricular nucleus of
the hypothalamus, rostral ventrolateral medulla, ventromedial medulla,
and caudal raphe nucleus to preganglionic neurons of the spinal cord
(Sawchenko and Swanson, 1982). The preganglionic cell bodies of
sympathetic nerves reside in the intermediolateral cell column of the
lateral horn of the spinal cord at T1-L2. These cell bodies send
myelinated projections that exit the spinal cord via the ventral roots
to synapse primarily on the superior mesenteric ganglia. From these
ganglia, a second projection follows the vasculature to innervate
target organs. Within the target organ, sympathetic nerves form
terminals from which the sympathetic neurotransmitter norepinephrine
(NE) is released to bind to adrenergic receptors expressed by various
cell populations.
Most studies of sympathetic innervation of lymphoid organs incorporated
immunohistological techniques in which the rate-limiting enzyme of
norepinephrine synthesis, tyrosine hydroxylase, was detected. These
studies demonstrated a rich sympathetic innervation of all primary
(thymus and bone marrow) and secondary (spleen and lymph nodes)
lymphoid organs (Calvo, 1968; Reilly et al., 1979; Williams and Felten,
1981; van Oosterhout and Nijkamp, 1984; Felten et al., 1988a,b).
Additionally, these studies reported the presence of sympathetic
innervation in both the splenic capsule and trabeculae, but more
importantly, in the immune cell compartment of the spleen (the white
pulp), especially the T cell-rich periarteriolar lymphoid sheath, the B
cell-rich marginal zone, and marginal sinus areas (Felten et al., 1985,
1987a,b; Livnat et al., 1985; Ackerman et al., 1987; Felten and
Olschowka, 1987). Whereas innervation is prominent in the white pulp,
little innervation is present in the red pulp and represents less than
1% of the total splenic innervation. Electron microscopic studies of
the white pulp reveal that sympathetic nerve terminals are in direct
apposition to T cells and adjacent to both interdigitating dendritic
cells and B cells (Felten et al., 1987a,b), with the neuro-immune
junction being approximately 6 nm wide (Felten and Olschowka, 1987), in contrast to a typical CNS synapse that is approximately 20 nm wide. The
close proximity of sympathetic nerve terminals to immune cells provides
a mechanism not only for specific targeting of norepinephrine release
to immune cells, but also for the containment of neurotransmitter
release, possibly to permit differential modulation of only resident
immune cells, depending on the specific immune response being evoked.
Finally, GAP-43 (a marker for an activated neuron)-positive sympathetic
fibers enter the outer periarteriolar lymphoid sheath, marginal zone,
and marginal sinus within the spleen following immunization, suggesting
that not only can the immune response influence sympathetic outflow,
but also that immune cell-derived neurotrophic factors may direct
innervating fibers to the site of the response (Yang et al., 1998;
Besser and Wank, 1999) to release norepinephrine to bind to
-adrenergic receptors (ARs) expressed on immune cell populations.
Thus, a complete "circuit" appears to exist between the immune
system and the CNS, such that the initiation of an immune response in
the periphery signals the CNS, resulting in subsequent regulation of
the immune response via activation of the sympathetic nervous system.
In summary, whereas behavioral conditioning studies provided the
initial suggestion that an interaction between the CNS and immune
system existed (Ader and Cohen, 1975; Rogers et al., 1976; Wayner et
al., 1978; Cohen et al., 1979; Exton et al., 1998), research findings
over the past 20 to 30 years have documented a number of complex
bidirectional interactions between the nervous system and the immune
system that appear to be necessary for the maintenance of homeostasis
in both systems, as well as for the regulation of immune responses
during the development and progression of immune-related disease states.
C. Norepinephrine and the 2-Adrenergic Receptor
The catecholamine norepinephrine is released from both
postganglionic sympathetic nerve terminals found innervating all
internal organs and from chromaffin cells residing in the adrenal
medulla. Norepinephrine is the principal neurotransmitter of the
sympathetic nervous system and is released into the periphery upon
activation of the sympathetic nervous system. Norepinephrine is
produced via multiple enzymatic alterations of tyrosine, of which the
hydroxylation of tyrosine by tyrosine hydroxylase is the rate-limiting
step (Zigmond et al., 1989). This enzymatic cascade is initiated upon the activation of sympathetic postganglionic nerve fibers. The final
step in norepinephrine synthesis occurs within the nerve terminal
storage vesicles and is mediated by the membrane-bound dopamine
-hydroxylase. Various fates await norepinephrine upon release from
the nerve terminal, such as metabolization into normetanephrine by
catechol-O-methyltransferase, reuptake back into the nerve terminal, diffusion, or receptor binding to influence target cell function.
The -adrenergic family of receptors (ARs) binds norepinephrine
and contains three subtypes: the 1AR, the 2AR, and the 3AR
(reviewed in Bylund et al., 1994). The AR is a seven-transmembrane receptor that classically leads to heterotrimeric guanine
nucleotide-binding protein (G-protein) activation upon stimulation.
Historically, the signaling capacity of the AR has been attributed
to the association of the cytoplasmic tail of the receptor with
stimulatory G-proteins, in which stimulation of the receptor results in
adenylyl cyclase activation, increased intracellular accumulation of
adenosine 3',5'-cyclic monophosphate (cAMP), and increased protein
kinase A (PKA) activity (reviewed in Kobilka, 1992; Meinkoth et al., 1993). Upon activation, PKA regulates the activity of multiple targets
via phosphorylation, including various transcription factors, such as
NF-
B. Although stimulation of each of the three AR-subtypes results in adenylyl cyclase activation, the 2AR appears to be more
efficiently coupled to adenylate cyclase than is the 1AR or 3AR
(reviewed in Strosberg, 1997).
However, over the last 5 to 10 years, a number of other signaling
pathways have been reported to be activated following 2AR stimulation. One such pathway that is also relevant to lymphocyte function is the activation of protein kinase C (PKC). 2AR
stimulation induces PKC activity (Kelleher et al., 1984), which may
then mediate a number of intracellular events, including
down-regulation of 2AR surface expression (Kelleher et al., 1984),
positive or negative effects on adenylyl cyclase activity (reviewed in
Houslay, 1991), and activation of Bruton's tyrosine kinase (reviewed
in Mohamed et al., 1999) which ultimately activates the
mitogen-activated protein kinase (MAPK) pathway. These same
2AR-induced signaling pathways are also involved in BCR signaling
and, therefore, represent a mechanism by which 2AR stimulation may
influence intracellular events in B cells following antigen
recognition. Similarly, 2AR and BCR stimulation both result in Src
kinase activation, which may induce down-regulation of 2AR
expression (Daaka et al., 1997; Cornall et al., 1998; Lankar et al.,
1998) and Ras activation (Daaka et al., 1997), as well as a number of
additional intracellular events associated with BCR stimulation. Thus,
as will be discussed later, due to the existence of overlapping
intracellular signaling pathways associated with stimulation of the
2AR and the BCR, it is not surprising that stimulation of the 2AR
by either an agonist or norepinephrine may influence B cell function.
| |
II. Evidence and Mechanisms for the Release of Norepinephrine in Lymphoid Organs |
|---|
|
TopPreviousNextReferences
|
|---|
As previously discussed, lymphoid organs are heavily innervated by
sympathetic nerve fibers. However, in order for norepinephrine to
influence immune cell function, it must be released at the immediate
site of action, since it is either rapidly degraded by
catechol-O-methyltransferase and monoamine oxidase, diffused into the circulation, or taken back up into the nerve terminal following release (reviewed in Glowinski and Baldessarini, 1966). Therefore, if norepinephrine is to influence immune cell function in
response to antigen, it may be critical that mechanisms exist for
enhancing the normally low basal level of norepinephrine released within the microenvironment in which immune cells reside (findings summarized in Table 1).
|
Splenic norepinephrine is derived from local sympathetic nerve
terminals, as opposed to circulating catecholamine (Williams et al.,
1981; Shimizu et al., 1994), and the electrical stimulation of the
postganglionic splenic nerve results in norepinephrine release within
the spleen (Lundberg et al., 1989). The rate of norepinephrine release
from sympathetic nerve terminals is regulated by both positive and
negative feedback mechanisms. For example, the release of
norepinephrine from sympathetic nerve terminals is inhibited via
stimulation of the 
2-adrenergic receptors
(2AR) expressed on the presynaptic nerve terminal itself but is
enhanced by stimulation of the presynaptic 2AR (Elenkov and Vizi,
1991; Hasko et al., 1995; Vizi et al., 1995). In addition, although there are conflicting reports concerning the level of sympathetic nerve
activity and the release of norepinephrine within the spleen during an
immune response, there is increasing evidence that immune-derived factors may also influence the rate of norepinephrine release within
lymphoid organs.
A. Lipopolysaccharide- and Antigen-Induced Norepinephrine Release
1. Infection/Endotoxin.
For many years, it has been known
that systemic infection induces sympathetic nervous system activity via
the endotoxin released from bacterial cell walls. Early studies
reported alterations in the level of sympathetic nerve activity
both in times of infection and shock by measuring circulating levels of
norepinephrine and epinephrine as an indirect indicator of systemic
sympathetic nerve activity (Heiffer et al., 1960; Rosenberg et al.,
1961; Spink et al., 1966; Devereux et al., 1977; Feuerstein et al.,
1981). In all of these studies, endotoxin exposure increased the levels of circulating norepinephrine, suggesting enhanced sympathetic nerve
activity and norepinephrine release. In other studies, the total tissue
content of norepinephrine was determined as a measure of sympathetic
nerve activity following infection with Escherichia coli or
injection of E. coli-derived endotoxin. Immunization of animals with endotoxin resulted in a significant decrease in the total
tissue content of norepinephrine in the spleen (Zetterstrom et al.,
1964; Pohorecky et al., 1972), possibly via altering norepinephrine reuptake mechanisms (Pardini et al., 1982). Such observations were
interpreted in several ways, i.e., the decreased splenic norepinephrine
levels may have been due to decreased norepinephrine production,
increased norepinephrine release, increased norepinephrine diffusion/metabolism, and/or decreased reuptake of norepinephrine back
into the nerve terminal.).
The uptake of [3H]norepinephrine into splenic
nerve terminals of endotoxin-injected animals was significantly lower
than the rate of norepinephrine uptake in saline-injected control
animals. Interestingly, endotoxin administration did not alter the
activity of norepinephrine reuptake mechanisms in the heart, suggesting
a lymphoid organ-specific effect of endotoxin on norepinephrine
regulatory mechanisms. Therefore, while it is difficult to interpret
data from studies measuring endotoxin-induced alterations in the total
norepinephrine tissue content, one mechanism for infectious challenge
to alter the level of splenic norepinephrine may involve a decrease in
the efficiency of norepinephrine reuptake mechanisms, possibly mediated
by stimulation of cytokine receptors on the local nerve terminal.
; Brodie et al., 1978).
To further study the role of endotoxin in regulating the level of
norepinephrine release, norepinephrine turnover analysis was used to
determine the level of sympathetic nerve activity and norepinephrine
release in the spleens of animals injected with lipopolysaccharide
(LPS) (Pardini et al., 1983). LPS-induced activation of immune cell
populations increased the rate of norepinephrine release in both the
spleen and the heart during the first 12 h of exposure, suggesting
that LPS exposure may have enhanced the level of systemic sympathetic
nerve activity. In addition, since a previous study demonstrated a
spleen-specific suppression of norepinephrine reuptake mechanisms
following endotoxin exposure (Pardini et al., 1982), the effect of the
norepinephrine reuptake inhibitor desmethylimipramine on the
LPS-induced enhancement of norepinephrine release was also evaluated.
In this same study, treatment of animals with desmethylimipramine did
not alter the rate of norepinephrine release in either the spleen or
the heart, suggesting that the LPS-induced increase in the rate of
norepinephrine release was not due to alterations in norepinephrine
reuptake mechanisms.
More recently, the effect of Pseudomonas aeruginosa
infection on the rate of norepinephrine turnover in the bone marrow has been investigated (Tang et al., 1999). Using both isotopic and nonisotopic methods to measure the rate of norepinephrine turnover following Pseudomonas aeruginosa infection, it was shown
that infection increased the rate of norepinephrine turnover in both the heart and bone marrow. Unfortunately, the exact mechanism by which
infectious challenge enhanced the rate of norepinephrine release is
currently unknown. However, additional studies have begun to
investigate the mechanisms by which infectious challenge influences the
level of sympathetic nerve activity in lymphoid organs (MacNeil et al.,
1996, 1997). For example, systemic exposure to LPS (10-100 µg) via
intravenous injection increased the level of splenic nerve activity
within 15 to 25 min of injection (MacNeil et al., 1996). Importantly,
indomethacin inhibited the LPS-induced enhancement of splenic nerve
activity, suggesting a potential role for PGE2
synthesis in mediating the effects of LPS exposure on nerve activity
(MacNeil et al., 1997). Similarly, others have reported that infectious
challenge results in a PGE2-dependent increase in
neuronal c-fos expression in brain regions known to control sympathetic
outflow (Wan et al., 1993, 1994). Taken together, these studies support
the hypothesis that infectious challenge induces
PGE2-dependent alterations in the level of both
efferent sympathetic nerve activity and norepinephrine release in
lymphoid organs.
Thus, immune cell activation following either infectious challenge or
administration of bacterial products may increase the rate of
norepinephrine turnover in lymphoid organs. In addition, these same
stimuli may also increase the level of CNS nerve activity in brain
regions known to control the level of efferent sympathetic nerve
activity, suggesting that CNS-mediated regulatory mechanisms may
respond to a peripheral endotoxin/bacterial insult to influence the
rate of norepinephrine turnover in lymphoid organs following infectious challenge.
2. Particulate Antigens/Sheep Red Blood Cells.
In addition to
infectious challenge, other types of immune stimuli may also influence
the rate of norepinephrine release within lymphoid organs. One of the
earliest studies reporting a correlation between the splenic content of
norepinephrine and an ongoing immune response to a particulate antigen
was performed by Besedovsky et al. (1979). Immunization of animals with
the particulate T cell-dependent antigen sheep red blood cells (sRBC)
decreased the total norepinephrine content of the spleen in comparison
with control animals. In later studies, this same group extended their findings to note that 3 days after immunization of rats with sRBC, the
total norepinephrine content was lower in the spleen, lymph nodes, and
thymus of immunized animals in comparison with nonimmunized control
animals (Del Rey et al., 1981). Importantly, the effect of sRBC-induced
immune cell activation on sympathetic nerve activity may be influenced
by central regulatory mechanisms. For example, since central
norepinephrine inhibits hypothalamic neuronal activity and efferent
sympathetic nerve activity, the observation by Besedovsky and
colleagues (1983) that sRBC-induced immune cell activation decreased the rate of norepinephrine release in the hypothalamus suggested less norepinephrine-mediated inhibition of CNS activity and
increased sympathetic nerve activity in the periphery. Taken together,
these findings support the hypothesis that sRBC-induced immune cell
activation decreases the total lymphoid tissue content of
norepinephrine by increasing the level of sympathetic nerve activity
and norepinephrine release.
). Therefore, as
with the early studies reporting effects of infectious challenge on the
level of sympathetic nerve activity, it is difficult to determine the
exact effect that sRBC-induced immune cell activation exerts on
sympathetic nerve activity when measuring the total tissue content of
norepinephrine alone. In addition, the rate of norepinephrine release
was inferred from observations that sRBC administration resulted in
lower tissue norepinephrine concentrations. This observation could be
interpreted as the result of either an enhanced rate of norepinephrine
release and metabolism, a suppressed level of norepinephrine
production, or a suppressed level of norepinephrine reuptake by the
nerve terminal.
Although they do not directly measure the rate of norepinephrine
release, the levels of the dopamine metabolite 3,4-dihyroxyphenylacetic acid (DOPAC) were measured in the spleens of mice immunized with sRBCs
(Fuchs et al., 1988b). Because the level of DOPAC correlates with the
rate of norepinephrine synthesis, and because the rate of
norepinephrine synthesis is equivalent to the rate of norepinephrine release during steady-state conditions, the concentration of DOPAC should correlate with the rate of norepinephrine release as long as
nerve terminal homeostasis is maintained. Whereas the splenic norepinephrine concentration decreased following immunization with sRBC
48 h after immunization, there was no difference in the total
norepinephrine content, suggesting that the decrease in norepinephrine
concentration resulted from an increase in spleen size. Importantly,
sRBC-induced immune cell activation increased the total level of
splenic DOPAC within 48 h of immunization, suggesting an increase
in the rate of norepinephrine synthesis and release. Thus, these
findings suggested that sRBC exposure increased the rate of
norepinephrine release in the spleen without disrupting the homeostatic
mechanisms responsible for maintaining constant levels of norepinephrine.
Taken together, these studies suggest that sRBC-induced immune cell
activation may influence sympathetic nerve activity to varying degrees,
such that the steady-state nerve terminal dynamics may be disrupted in
some model systems while being maintained in others. Regardless,
sRBC-induced immune cell activation appears to increase the rate of
norepinephrine synthesis and release in lymphoid organs. But thus far,
few studies have investigated the effects of immune cell activation by
a soluble protein antigen on sympathetic nerve activity and
norepinephrine release in lymphoid organs.
3. Soluble Protein Antigen.
In light of the previously
discussed studies concerning the role of infectious challenge and
particulate antigens on the level of sympathetic nerve activity, one
study has used an antigen-specific model system to investigate the
effects of a cognate soluble protein antigen on the rate of
norepinephrine release in lymphoid organs by norepinephrine turnover
analysis (Kohm et al., 2000). Severe combined immunodeficient
(scid) mice were reconstituted with keyhole limpet
hemocyanin (KLH)-specific Th2 cell clones and freshly isolated trinitrophenyl (TNP)-specific B cells prior to immunization with the
cognate antigen TNP-KLH. Activation of Th2 cells and B cells increased
the rate of norepinephrine release in the spleen and bone marrow 18 to
25 h, but not 1 to 8 h, following immunization. Since the
rate of norepinephrine release was not measured between 8 and 18 h
following immunization in these studies, it is possible that immune
cell activation increased the rate of norepinephrine release at a time
earlier than 18 h after immunization. Importantly, it was shown
that immunization of scid mice reconstituted with antigen-specific cell populations with a noncognate antigen
(fluorescein ovalbumin) that would not activate either the Th2
cells or B cells, but would activate resident macrophages, did not
alter the rate of norepinephrine release in the spleen and bone marrow.
Thus, these findings suggested that macrophage activation and
inflammatory cytokine production are not responsible for the soluble
protein antigen-induced increase in sympathetic nerve activity in this model system and that a cognate interaction between Th2 cells and B
cells is necessary for soluble protein antigen-induced enhancement in
norepinephrine release by a currently undetermined mechanism.
). These findings suggest a role for
signals originating above or at the preganglionic cell body in
regulating the level of antigen-induced nerve activity in lymphoid
organs in this model system. One possible mechanism mediating the
effects of antigen-induced lymphocyte activation on the rate of
norepinephrine release may involve the production of immune
cell-derived cytokines. The binding of cytokines to their specific
receptors expressed on either the postganglionic nerve terminal or the
postganglionic cell body may initiate afferent signals that must first
be transmitted back to the CNS prior to the alteration in the rate of
norepinephrine release from the local sympathetic nerve terminal. In
this case, the blockade of ganglion signal transmission would block the
ability of immune cell activation to increase the rate of
norepinephrine release. However, because ganglionic blockade failed to
completely block the antigen-induced enhancement of sympathetic nerve
activity in the spleen and bone marrow, it is possible that local
cytokine production may not only serve to initiate an afferent signal
from the site of the immune response to the CNS, but may also modulate local nerve activity by binding to cytokine receptors on local nerve
terminals or the postganglionic cell body. Thus, whereas signals
emanating from the ganglion may exert a significant regulatory influence on the level of sympathetic nerve activity in response to a
specific cognate antigen, which is blocked by chlorisondamine, local
cytokine receptor stimulation may also enhance norepinephrine release,
which cannot be blocked by chlorisondamine. Thus, there may be multiple
levels of cytokine-induced regulation of local sympathetic nerve
activity and norepinephrine release within immune organs.
The following sections will discuss various mechanisms by which
cytokine receptor stimulation may influence the level of sympathetic nerve activity and norepinephrine release in lymphoid organs, including
the expression of cytokine receptors on peripheral nerves, the presence
of afferent innervation in lymphoid organs, and the effects of
cytokines on sympathetic nerve activity.
B. Cytokine Receptor Expression on Nerves
The hallmark experiments of Besedovsky et al. (1983) suggested
that activated immune cells secrete "soluble factors" into the
circulation that ultimately enter the CNS to stimulate neuronal activity in both the hypothalamus and brainstem. These studies were
some of the first to show that soluble factors produced by cells of the
immune system could alter noradrenergic nerve activity in the brain, as
measured by changes in hypothalamic and brainstem norepinephrine
content following -methyl-p-tyrosine inhibition of
norepinephrine synthesis. It is now known that these soluble factors
were cytokines and that their transport into the CNS represents one
possible mechanism of immune-to-brain communication. However, in order
for cytokines to leave the blood and enter the CNS, a major obstacle
must be overcome. The BBB, which is characterized by the
astrocyte-mediated formation of tight junctions between endothelial
cells composing the CNS vasculature, limits the entry of blood-borne
proteins and cells into the CNS. The passage of molecules across the
BBB is regulated on a variety of levels, including size, charge,
lipophilicity, and adhesion molecule expression (Banks and Kastin,
1985a,b, 1987). Thus, the BBB serves as a biological filter for entry
into the CNS. However, although several mechanisms exist for either the
passage of, or signaling by, blood-borne cytokines into the CNS, this
communication pathway is not considered a primary line of communication
from the immune system to the CNS for a number of reasons, including:
1) the low concentration of cytokines present at the BBB, 2) the lack
of specificity of cytokine signaling directly to the CNS, and 3) the
observation that certain cytokine-related illnesses occur in the
absence of detectable serum cytokine elevation (Kluger, 1991). Thus,
although mechanisms exist for the transport of cytokines into the CNS, one alternative mechanism for immune cell-derived cytokines to signal
the CNS is through the stimulation of cytokine receptors expressed on
peripheral sensory nerves. By this mechanism, immune responses
occurring near sites of sensory innervation could easily communicate
signals to the CNS.
The interleukin-1 receptor (IL-1R) was the first cytokine receptor
reported to be expressed on peripheral sensory nerves and, thus, became
the focus of early studies concerning cytokine communication from the
periphery to the CNS. IL-1 is a primary product of activated macrophages (Dinarello, 1998) and was a leading candidate for a
mediator of the LPS-induced increase in sympathetic nerve activity and
norepinephrine release. In addition, early studies suggested the
presence of the IL-1R on peripheral nerves, because the peripheral administration of IL-1 increased CNS activity (Saphier and Ovadia, 1990; Dunn, 1992). However, these studies did not directly measure the
expression of IL-1 receptors on peripheral nerves. A number of other
studies have reported that peripheral administration of IL-1
resulted in increased vagus nerve activity, suggesting not only that
IL-1 receptors are expressed on peripheral nerves, but that stimulation
of these receptors by their specific cytokines may induce afferent
nerve activity to the CNS (reviewed in Maier et al., 1998). In
addition, some of these studies reported a CNS-localized effect of
peripheral IL-1 administration as measured by cytokine-induced hyperalgesia, which can be blocked via administration of an IL-1R antagonist. Thus, not only did peripheral administration of IL-1 stimulate IL-1 receptors expressed in the periphery to induce vagal
nerve activity, but in addition, it altered the CNS response to pain.
In later studies, the role of the vagus nerve in transmitting
IL-1-induced signals to the CNS was further explored. For example, the
injection of either LPS, a bacterial protein product that activates
macrophages to secrete cytokines, including IL-1, or the injection
of IL-1 itself into the peritoneal cavity of mice and rats resulted
in fever, hypothalamic norepinephrine depletion, and increased c-fos
and acetylcholine expression in the brain (Fleshner et al., 1995;
Gaykema et al., 1995; Sehic and Blatteis, 1996). Importantly, the
effect of peripheral IL-1 on hypothalamic levels of norepinephrine
were blocked by subdiaphragmatic vagotomy, suggesting a role for vagal
afferents in mediating the effect of IL-1 on norepinephrine levels
within the CNS (Fleshner et al., 1995). These findings were later
supported by the observation that vagal paraganglia express IL-1
receptors, providing a direct mechanism by which IL-1 can directly
activate vagal nerve afferent fibers (Goehler et al., 1997). Finally,
others have shown that cultured sympathetic neurons express a
functional IL-1R and that stimulation of this receptor results in the
activation of NF-B (Bai and Hart, 1998). Thus, it appears that the
expression of functional IL-1 receptors on sympathetic nerves, such as
the vagus nerve, provides one mechanism by which immune-derived
cytokines can signal the CNS.
In addition to IL-1 receptors, the expression of other cytokine
receptors on sympathetic nerves has been studied to a lessor extent.
For example, sympathetic neurons appear to express too low a level of
IL-6R to allow a functional effect of endogenous IL-6 on the neuron,
but the exposure of sympathetic neurons to soluble IL-6R in vitro
results in IL-6-induced neuron survival (Marz et al., 1998). This may
be explained by the fact that the IL-6R ligand binding subunit does not
possess tyrosine kinase activity, and therefore, IL-6-stimulated
signaling relies on the dimerization of the ligand binding subunit of
the IL-6R with the signaling subunit gp130 (reviewed in Dinarello,
1998). These studies suggest that although sympathetic neurons may
express low levels of the ligand binding subunit of the IL-6R, they do
express adequate levels of the signaling gp130 subunit. Thus, although
sympathetic neurons may not constitutively express adequate levels of
IL-6 binding subunits to respond to endogenous IL-6, either soluble IL-6R production or nerve injury may enhance the level of functional IL-6R expression on sympathetic nerves.
Finally, one study has detected the expression of IL-2 receptors on
sympathetic neurons (Haugen and Letourneau, 1990). Using immunofluorescence staining, sympathetic neurons were shown to express
detectable levels of IL-2R on their surface. In addition, treatment of
cultured sympathetic neurons to IL-2 enhanced neurite outgrowth,
suggesting that the IL-2R expressed by these cells is functional.
Therefore, the presence of cytokine receptors on peripheral nerves
provides a potential mechanism by which local immune cell-derived
cytokines produced in the periphery may transmit signals to the CNS or
to the peripheral nerve directly.
C. Afferent Splenic Innervation
As previously discussed, the effect of antigen-specific Th2 cell
and B cell activation on the rate of norepinephrine release in lymphoid
organs was significantly decreased by ganglionic blockade (Kohm et al.,
2000). These studies suggested that the activation of antigen-specific
cell populations induced the local release of norepinephrine via a
mechanism that relied partially on ganglionic transmission. In light of
these findings, it is reasonable to hypothesize that an immune
cell-derived signal stimulated a neuronal reflex mechanism of
norepinephrine release dependent upon structures at, or above, the
sympathetic ganglia. Because the diffusion of locally produced
immune-derived factors into the circulation would produce extremely low
concentrations of circulating cytokine and, thus, would unlikely be
able to induce CNS-regulated norepinephrine release in the spleen, it
was more plausible to hypothesize that some local mechanism existed
that was capable of responding to immune-derived signals to induce
norepinephrine release from local sympathetic nerve terminals.
An early study noted the presence of afferent unmyelinated type C nerve
fibers in the spleen (Herman et al., 1982), although others have
observed that a small percentage of afferent fibers of the splenic
nerve are myelinated (Utterback, 1944; Calaresu et al., 1984). Later
studies suggested that approximately 5% of the splenic nerve is
composed of afferent nerve fibers as determined by horseradish
peroxidase retrograde tracing (Baron and Janig, 1988). These afferent
splenic nerve fibers arose from the spinal cord at levels ranging from
T4 to L2. However, the most significant origin of sympathetic afferent
fibers (approximately 60%) appeared to be from levels T10 to T13.
Importantly, the stimulation of these splenic afferent nerve fibers
activated a reflex response via the splenic nerve increasing the level
of cardiopulmonary sympathetic efferent nerve activity (Herman et al.,
1982). Because activation of splenic afferents influenced cardiac
efferent sympathetic nerve activity in these studies, such a mechanism
may also play a role in the low level of cardiac norepinephrine release
following activation of antigen-specific cell populations (Kohm et al., 2000). Interestingly, afferent signals from the spleen did not seem to
originate from the capsule innervation but, instead, from vasculature-associated interior innervation, which is the location of
cytokine-producing cells in the spleen (Herman et al., 1982). Finally,
other studies reported that afferent fibers supplying the spleen may be
activated by immune-derived stimuli (Niijima et al., 1991; Fleshner et
al., 1995). Taken together, these findings support the participation of
afferent innervation in transmitting the signals induced by locally
produced immune-derived products to increase the rate of local
norepinephrine release in lymphoid organs.
Interestingly, other studies in rats reported the absence of afferent
innervation of the spleen using techniques similar to those previously
discussed (Nance and Burns, 1989). The origin of these conflicting data
is currently unclear. However, because these conflicting studies were
performed in different animal species, the presence of afferent fibers
in the splenic nerve may be a species-dependent observation. The
existence of splenic afferent innervation is further supported by the
report of afferent nerve fibers in another species, the guinea pig
(Elfvin et al., 1992). Thus, the presence of afferent innervation in
the spleen provides a specific mechanism by which locally produced
cytokines or other immune cell-derived products may stimulate
sympathetic nerve activity and norepinephrine release.
D. Cytokine-Induced Norepinephrine Release
Cytokines, which were once thought to only influence immune cell
function, have now been shown to affect glial cell proliferation, neuron survival, neuronal proliferation and differentiation, and neurotransmitter expression (Giulian and Lachman, 1985; Yamamori et
al., 1989; Jonakait and Schotland, 1990; Barbany et al., 1991; Freidin
and Kessler, 1991; Hart et al., 1991; Schwartz et al., 1991; Brenneman
et al., 1992). In addition, a number of studies have reported that a
variety of cytokines may influence peripheral sympathetic nerve
activity and the rate of norepinephrine release.
As previously discussed, numerous studies have suggested that exposure
of animals to infectious challenge or bacterial products, such as
endotoxin, increases the rate of norepinephrine release in lymphoid
organs. In light of the critical role of macrophage activation and
IL-1 production in clearing infections and the role of
norepinephrine in regulating macrophage activity (Miles et al., 1996),
it is not surprising that a significant number of studies have
investigated the role of IL-1 in regulating the level of
norepinephrine release in vivo.
One indication that IL-1 may influence the level of sympathetic
nerve activity is demonstrated by its ability to influence CNS
activity. Because the hypothalamus is an area within the CNS that
controls efferent sympathetic nerve activity, an IL-1-induced increase in hypothalamic activity may enhance the level of efferent sympathetic nerve activity and the rate of norepinephrine release in
the periphery. For example, peripheral injection of IL-1 enhanced both hypothalamic nerve activity and the level of CRF secretion from
the hypothalamus (Sapolsky et al., 1987; Akiyoshi et al., 1990; Dunn,
1992; Fleshner et al., 1995). Also, peripheral IL-1 administration
induced c-fos expression in CRF-producing cells in the paraventricular
nucleus of the hypothalamus, suggesting that IL-1 increased
hypothalamic neuronal activity (Ericsson et al., 1994). Because a
number of studies have reported that peripheral IL-1 increases
neuronal activity in the hypothalamus, it is reasonable to hypothesize
that these IL-1-induced alterations in hypothalamic activity may
translate into alterations in efferent sympathetic nerve activity.
Using a nonisotopic technique employing either
-methyl-p-tyrosine to measure norepinephrine turnover in
the spleen (Akiyoshi et al., 1990) or direct measurements of
sympathetic nerve electrical activity (Niijima et al., 1991), it was
shown that peripheral administration of IL-1 increased the rate of
norepinephrine turnover in the spleen 1 to 6 h following exposure
in a dose-dependent manner. Other studies measuring the level of
sympathetic nerve electrical activity reported that peripheral IL-1
exposure increased the level of sympathetic nerve activity within 10 to
15 min of exposure in a dose-dependent manner (Takahashi et al., 1992)
and that the rate of norepinephrine release in the spleen peaks within 40 min after peripheral IL-1 exposure (Ichijo et al., 1992; Shimizu et al., 1994). Finally, the effect of IL-1 on sympathetic nerve activity was specific for certain nerves, because it increased the rate
of norepinephrine release in the spleen, but not in the heart (Akiyoshi
et al., 1990). Because these studies administered IL-1 directly, it
is not surprising that the rate of norepinephrine turnover increased
much quicker than that in a study in which immune cells were activated
via antigen exposure (Kohm et al., 2000). In contrast, others have
reported an IL-1-induced inhibition of splenic sympathetic nerve
activity as measured by microdialysis or inhibition of
[3H]norepinephrine release from atria (Bognar
et al., 1994; Abadie et al., 1997). Although the reason for these
conflicting findings is currently unknown, the concentration of IL-1
used in these studies does not seem to be the source of these
conflicting findings, inasmuch as studies reporting an IL-1-mediated
enhancement of splenic norepinephrine release have used varying
concentrations of IL-1.
Although the exact mechanism by which peripheral IL-1 increases the
level of sympathetic nerve activity and the rate of norepinephrine release is currently unknown, prostaglandin synthesis may be a critical
mediator of IL-1's effect on sympathetic nerve activity. For example,
peripheral administration of cyclo-oxygenase inhibitors blocked the
effect of IL-1 on sympathetic nerve activity in the spleen,
suggesting a role for IL-1-induced prostaglandin synthesis in
regulating norepinephrine release (Niijima et al., 1991). In addition,
the production of CRF within the CNS appears to be another critical
mediator of IL-1's effect on norepinephrine release, because
central administration of a neutralizing antibody directed against CRF
blocked the ability of peripherally administered IL-1 to increase
the level of splenic norepinephrine release (Ichijo et al., 1992;
Shimizu et al., 1994).
In summary, although there are conflicting reports concerning the level
of splenic sympathetic nerve activity and norepinephrine release during
an immune response, it appears that IL-1 may play an important role
in mediating the level of sympathetic outflow in the spleen. However,
IL-1-induced regulation of norepinephrine release may only occur
during immune responses involving macrophage activation, because these
cells are the principal source of the cytokine. Therefore, a few
studies have determined the role of other cytokine receptors in
modulating norepinephrine release in lymphoid organs.
For example, whereas IL-6 does not affect the uptake of
[3H]norepinephrine into sympathetic nerve
terminals, IL-6 does exert dose-dependent effects on sympathetic nerve
activity. For example, 1 ng/ml IL-6 stimulated, 10 ng/ml IL-6 had no
effect, and 100 ng/ml IL-6 inhibited
[3H]norepinephrine release from sympathetic
nerve terminals in vitro within 2 h of cytokine exposure (Ruhl et
al., 1994). Importantly, the combination of subthreshold concentrations
of IL-6 (10 ng/ml) and IL-1 (0.1 ng/ml) significantly suppressed the
level of sympathetic nerve activity and was blocked by an antagonist of
either the IL-6 or the IL-1 receptor. Finally, others have shown that
low concentrations of both IL-2 (Bognar et al., 1994) and TNF-
(Foucart and Abadie, 1996; Abadie et al., 1997) inhibited the rate of
norepinephrine release in the spleen. Thus, IL-6, IL-2, and TNF- may
either enhance, inhibit, or have no effect on the rate of
norepinephrine release, depending on both the cytokine concentration
and the presence of other cytokines in the microenvironment of the
nerve terminal.
Taken together, these studies suggest that a physical mechanism for immune cell-derived cytokines to influence local sympathetic nerve activity is in place. Several studies have reported the presence of afferent innervation in the spleen, the presence of cytokine receptors on peripheral nerves, the ability of cytokine receptor stimulation to initiate afferent signals to the CNS resulting in alterations in hypothalamic neuronal activity, and finally, cytokine-induced alterations in sympathetic nerve activity and the rate of norepinephrine release in lymphoid organs. In light of these findings, immune cell-derived cytokine production may represent one mechanism by which an ongoing immune response may influence the rate of local norepinephrine release. Importantly, different types of antigen may lead to the activation of different populations of immune cells and affect which cytokines are produced during an immune response. The specific cytokines produced may, in turn, determine the mechanism that regulates the level of norepinephrine release within immune organs. Finally, it is possible that greater levels of infection involve the CNS-mediated regulatory mechanisms, whereas lower levels of infection may involve only local regulatory mechanisms of sympathetic nerve activity. However, in order for local norepinephrine release to influence immune cell function, lymphocytes must express receptors for the neurotransmitter.
| |
III. -Adrenergic Receptor Expression on CD4+ T and B
Lymphocytes |
|---|
|
TopPreviousNextReferences
|
|---|
A. CD4+ T Lymphocytes
1. Receptor Expression.
Although few studies have reported
the presence of ARs on T cells, early studies suggested the presence
of a functional AR on their surface. An important premise that made
these studies possible was that stimulation of the AR was found to
increase the level of adenylyl cyclase activity and intracellular cAMP accumulation in other nonlymphoid cell types (reviewed in Wolfe et al.,
1977). Therefore, using AR agonists, early reports demonstrated that
the exposure of lymphocytes to AR agonists resulted in adenylyl cyclase activation and increased cAMP production (Bourne and Melmon, 1971; Makman, 1971; Bach, 1975). Thus, although AR expression would
not be measured directly on lymphocytes for another 6 years, early
pharmacological and biochemical data suggested their functional presence. A recent review provides comprehensive discussion concerning the expression of adrenergic receptors on immune cells (Sanders et al.,
2001). Figure 3 summarizes 2AR
expression on both CD4+ T cells and B cells.
View larger version (21K):
[in a new window]
Fig. 3.
2-Adrenergic receptor expression on
CD4+ cells and B cells. The predominant adrenergic receptor
expressed on resting and activated B cells is the 2AR. Similarly,
naive CD4+ T cells also predominantly express the 2AR.
However, whereas 2AR expression is retained on clones and newly
generated Th1 cells, 2AR expression is repressed on clones and newly
generated Th2 cells.
performed the original studies to measure the
level of AR expression on total human lymphocyte membranes directly
via [3H]alprenolol saturation binding assays.
These studies reported approximately 2000 AR binding sites per
lymphocyte. However, a number of subsequent binding studies reported a
lower level of AR expression on purified populations of T cells, as
opposed to total lymphocytes (Pochet et al., 1979; Bishopric et al.,
1980; Loveland et al., 1981; Krawietz et al., 1982; Bidart et al.,
1983; Pochet and Delespesse, 1983; Khan et al., 1986; Westly and
Kelley, 1987; Fuchs et al., 1988a; Van Tits et al., 1990; Radojcic et al., 1991). In general, the reported absolute number of ARs
expressed on T cells varied, and this variance might be explained by
the use of either different T cell isolation techniques, different types of radiolabel, i.e., 3H versus
125I, pharmacological ligands for which the
receptor has differing affinities, and/or radioligand specific
activity. Similarly, the cell population composition used in these
studies may have also contributed to the varying number of AR
binding sites reported to be expressed by T cells, since it is now
known that the different subsets of murine T cells
(CD8+, naive CD4+, Th1, and
Th2 cells) all express different levels of the AR. Nevertheless, on
average, most reports measured approximately 200 to 750 AR binding
sites per T cell.
Until the early 1980s, very little was known about the specific
subtypes of AR expressed on T cells. Subsequently, the primary AR-subtype expressed on lymphocytes was found to be the 2AR, inasmuch as the 1AR-selective antagonist was unable to
compete for the specific binding of
[125I]HYP, whereas
L-propranolol, a nonselective AR antagonist,
competed for the specific binding (Loveland et al., 1981). This
finding was supported by a number of other studies, suggesting that the primary subtype expressed on T cells was the 2AR (Bourne and Melmon,
1971; Williams et al., 1976; Conolly and Greenacre, 1977; Pochet et
al., 1979; Loveland et al., 1981; Meurs et al., 1982; Ramer-Quinn et
al., 1997; Sanders et al., 1997). Finally, functional studies indicated
the lack of ARs on splenic and thymic T cells (Cook-Mills et al.,
1995). Therefore, because no radioligand binding data showed the
presence of a high affinity 1AR or 3AR on T cells, these findings
suggest that previous studies measuring AR expression on T cells
were in fact measuring the level of 2AR expression.
Interestingly, some studies have reported that the number of ARs
expressed on T cells varies during development. For example, immature T
cells in the thymus may express a significantly lower number of ARs
on their surface in comparison with circulating peripheral T cells
(Pochet and Delespesse, 1983; van de Griend et al., 1983). These
findings were supported by others who reported that thymocytes
expressed a lower number of ARs than did peripheral T cells isolated
from lymph nodes (Staehelin et al., 1985) or the spleen (Fuchs et al.,
1988a), suggesting that 2AR expression may increase on the cell
surface during T cell differentiation. The reason for such alterations
in 2AR expression on developing T cells is unclear. However, it is
possible that 2AR stimulation may impede T cell development. In this
case, it would be beneficial for 2AR expression to be lower on
developing T cells. However, future studies are needed to investigate
this and other potential explanations.
It wasn't until the mid-1980s that the level of AR expression was
measured specifically on CD4+ T cells.
Approximately 750 AR binding sites were reported to be expressed on
Th cells (Khan et al., 1986), but these studies employed a nonselective
AR agonist (isoproterenol) and antagonists (propranolol and
[125I]CYP), leaving the subset of AR
expressed on the surface of the CD4+ T cells
unknown. However, later studies suggested that
CD4+ T cells expressed a 2AR with a
"normal" affinity for isoproterenol (Dailey et al., 1988;
Robberecht et al., 1989). Importantly, these studies used mixed
populations of CD4+ T cells, containing naive,
Th1, and Th2 cell populations.
Recently, the expression of AR subtypes has been measured on
CD4+ T cell subsets at both the protein and the
mRNA level. In general, Th1 cells, but not Th2 cells, preferentially
expressed the 2AR, and this was demonstrated by a number of
techniques using both T cell clones and newly generated Th1 and Th2
cell populations. For example, resting Th1 cell clones, but not Th2
cell clones, showed a detectable level of 2AR protein
expression using both radioligand binding with iodopindolol and
immunofluorescence staining with a polyclonal anti-2AR antibody
directed against the cytoplasmic region of the 2AR (Sanders et al.,
1997). This finding was later confirmed at the mRNA level (A. P. Kohm, M. A. Swanson, and V. M. Sanders, manuscript submitted for
publication). Importantly, these studies were also performed
using newly generated populations of Th1 and Th2 cells. Naive
CD4+ T cells receiving either antigen-presenting
cells and antigen or anti-CD3 stimulation in the presence of IL-12 will
preferentially differentiate into Th1 cells (Seder et al., 1993),
whereas the same naive CD4+ T cells stimulated in
the presence of IL-4 will differentiate into Th2 effector cells (Hsieh
et al., 1992; Seder et al., 1992). Thus, newly generated
CD4+ Th1 and Th2 cells provide another mechanism
to study the phenotype and function of these two effector cell
populations in vitro. Freshly isolated naive CD4+
T cells expressed a functional 2AR, but not a 1AR or 3AR, as
determined by mRNA analysis and functional studies (Swanson et al.,
2001). Importantly, whereas 2AR mRNA expression was retained in
newly generated Th1 cells, 2AR mRNA expression was repressed in
newly generated Th2 cells (A. P. Kohm, M. A. Swanson, and V. M. Sanders, manuscript submitted for publication). Taken together, these findings suggested that the 2AR is differentially expressed on
CD4+ T cell subsets, with detectable receptor
expression on naive CD4+ T cells and Th1 cells,
but not on Th2 cells.
The T cell activation status may also influence the level of AR
surface expression. For example, splenocyte activation by the T cell
mitogen concanavalin (Con) A increased the level of AR surface
expression 24 h after cell activation, while exerting no effects
on the affinity (Kd) of the receptor
(Westly and Kelley, 1987). Others have also reported a similar effect
of T cell activation on AR expression both in vitro (Sanders and
Munson, 1985b; Radojcic et al., 1991) and in vivo (Madden et al.,
1989). Similarly, T cell activation may also influence AR expression
on subsets of CD4+ T cells. As discussed
previously, Th1 cell clones, but not Th2 cell clones, expressed
detectable levels of 2AR surface protein (Sanders et al., 1997).
Stimulation of the CD3 complex associated with the T cell receptor
activates Th1 and Th2 cell clones, and more importantly, 2AR
expression is up-regulated on the surface of activated Th1 cell clones,
but 2AR expression remains undetectable on the surface of activated
Th2 cell clones (Ramer-Quinn et al., 1997). However, other studies have
reported contrasting findings. For example, Con A-induced activation of
lymph node T cells decreased the number of
[125I]CYP binding sites from 750 to 850 sites
per cell to approximately 350 sites per cell within 3 days of culture,
a time that also correlated with the peak in cell proliferation but
induced no change in AR number within 24 h of activation
(Cazaux et al., 1995). These findings are supported by others who
reported that mitogen-induced activation of T cells results in a
PKC-dependent increase in the expression of the -adrenergic receptor
kinase-1 (ARK1) and ARK2 mRNA within 48 h of stimulation,
whereas no alterations were seen in G-protein receptor kinase-5 (GRK5)
and GRK6, suggesting a selective regulation of the receptor-associated kinase subtypes (De Blasi et al., 1995). Because ARK is a
serine-threonine kinase that regulates the level of AR expression
(reviewed in Inglese et al., 1993), ARK activation may contribute to
the down-regulation of AR expression at later times following T cell
activation, e.g., at times longer than 24 h following T cell
activation. Thus, whereas most studies report that T cell activation
elevates the level of surface AR expression, cellular activation may
result in decreased levels of AR expression at later times following T cell activation.
2. Mechanisms Regulating Differential Receptor Expression on
CD4+ T Cell Subsets.
Currently, the mechanisms
regulating the differential expression of the 2AR on Th1 and Th2
cells is unknown. However, a few recent studies have begun to
investigate possible mechanisms that may influence the level of 2AR
expression on CD4+ T cells.
2AR mRNA, but not 1AR or 3AR mRNA (Swanson et al.,
2001). More importantly, Th1 cells newly generated from naive
CD4+ T cells retained the expression of the
2AR, whereas newly generated Th2 cells did not (A. P. Kohm, M. A. Swanson, and V. M. Sanders, manuscript submitted for
publication). Because the cytokine microenvironment is the only
difference between Th1- and Th2-promoting conditions in this model
system, it seems reasonable that intracellular signals resulting from
cytokine receptor stimulation during CD4+ T cell
differentiation may influence 2AR expression on subsequent generations of effector cells.
One mechanism by which cytokine receptor stimulation regulates
gene expression is via alterations in both the level of histone acetylation (Ohno et al., 1997; Taplick et al., 1998; Cheung et al.,
2000; Gray et al., 2000; Ito et al., 2000; Vanden Berghe et al., 2000)
and DNA methylation (Hmadcha et al., 1999; Kang et al., 1999). To
investigate whether either of these epigenetic mechanisms contributes
to the regulation of 2AR expression in Th1 and Th2 cells,
2AR-negative Th2 cells were exposed to pharmacological agents that
resulted in either histone hyperacetylation or DNA hypomethylation,
both of which have been shown previously to regulate the level of AR
expression in other types of cells (Kassis et al., 1988; Buscail et
al., 1990). Exposure of Th2 cells to the histone deacetylase inhibitor
butyrate resulted in a dose- and time-dependent induction of 2AR
mRNA expression in these cells (A. P. Kohm, M. A. Swanson, and V. M. Sanders, manuscript submitted for publication). Similarly,
exposure of Th2 cells to the methyltransferase inhibitor 5-azacytidine
also resulted in a dose- and time-dependent induction of 2AR mRNA
expression, but with a longer time of onset. Not surprisingly,
5-azacytidine-induced DNA hypomethylation prior to butyrate-induced
histone hyperacetylation resulted in a synergistic enhancement of
2AR mRNA expression in Th2 cells. Finally, pretreatment of Th2 cells
with either the transcription inhibitor actinomycin D or the protein
synthesis inhibitor cycloheximide revealed that the induction of 2AR
mRNA expression following histone hyperacetylation and/or DNA
hypomethylation was transcription-dependent, but
translation-independent, suggesting that the basal levels of
transcription factor expression may have been sufficient to induce
2AR gene transcription once the gene locus was accessible. Thus,
epigenetic mechanisms such as histone acetylation and DNA methylation
may play a critical role in regulating the differential expression of
the 2AR in Th1 and Th2 cells. However, future studies are necessary
to further investigate whether basal transcription factor expression
plays a critical role on 2AR gene transcription in this model system.
In addition to epigenetic mechanisms, others have investigated
the role of protein kinase activation in the regulation of AR
expression on T cells. One study has reported that Con A-induced T cell
activation decreased the level of AR surface expression in a
PKC-dependent manner (Cazaux et al., 1995). In support of these
findings, exogenous activation of PKC via a phorbol ester (PMA)/ionophore also decreased the level of AR expression. Such a
role for PKC-induced down-regulation of the 2AR has been reported previously in other cell types as part of the endogenous mechanisms inducing receptor desensitization (reviewed in Lefkowitz et al., 1998;
Dzimiri, 1999). Finally, whereas treatment of cells with PMA alone did
not down-regulate the level of AR expression on T cells within 3 days, the addition of recombinant IL-2 (12.5 U/ml) significantly
down-regulated AR expression (Cazaux et al., 1995). It is
interesting to note that IL-2R stimulation induces PKC activation
(reviewed in Gomez et al., 1998), and thus, future studies may reveal a
PKC-dependent component to the ability of IL-2R stimulation to
influence 2AR expression on T cells. Thus, these studies suggest
that PKC activation plays an important role in the regulation of 2AR
expression in Th1 and Th2 cells.
Taken together, these studies suggest that CD4+ T
cells differentially express the 2AR, with detectable expression on
CD4+ naive T cells and Th1 cells, but not on Th2
cells. In addition, activation of CD4+ T cells
may result in an initial up-regulation of the level of 2AR surface
expression, but then a later down-regulation of 2AR expression by
PKC-dependent mechanisms possibly involving ARK activation. Although
both the mechanism and the purpose of these biphasic effects of T cell
activation on 2AR expression are currently unclear, such a mechanism
may be critical to allow for the maintenance of
CD4+ T cell function while the cell participates
in an ongoing immune response.
B. B Lymphocytes
An isoproterenol-induced accumulation of intracellular cAMP was
the first finding to suggest the presence of a functional AR on the
B cell surface (Bach, 1975). In addition, early radioligand binding
studies reported that peripheral lymphocytes, which contain between 30 and 50% B cells, expressed the AR (Williams et al., 1976). However,
it was not until a few years later that an enriched population of B
cells was shown to express approximately 400 to 600 AR binding sites
per cell using a radioligand binding assay (Pochet et al., 1979). In
addition, using three different AR agonists, isoproterenol,
epinephrine, and norepinephrine, an order of potency was observed that
was consistent with the presence of a 2AR. Finally, T cells and B
cells isolated from peripheral blood expressed a similar low level of
AR expression on their surface, and the affinity of the receptor was
similar in both cell populations (Bishopric et al., 1980).
In contrast, other reports suggested that purified B cells
expressed a higher level of the AR on their surface in comparison with peripheral T cells (Pochet et al., 1979; Miles et al., 1981, 1984,
1985; Krawietz et al., 1982; Bidart et al., 1983; Paietta and
Schwarzmeier, 1983; Pochet and Delespesse, 1983; Fuchs et al., 1988a;
Griese et al., 1988; Korholz et al., 1988; Van Tits et al., 1990;
Cremaschi et al., 1991). For example, some reported that B cells
expressed approximately twice the number of surface ARs as T cells,
but that the Kd values of the AR
were similar on both cell populations (Miles et al., 1984, 1985).
Studies employing salbutamol displacement curves determined that the
AR expressed by both cell populations were of the 2AR subtype
(Griese et al., 1988; Korholz et al., 1988), which is in agreement with
the findings of others (Krawietz et al., 1982; Pochet and Delespesse,
1983; Fuchs et al., 1988a). These findings were supported by a recent study that investigated the expression of AR-subtypes on
antigen-specific B cells freshly isolated from the spleens of
unimmunized mice (Kohm and Sanders, 1999). Radioligand binding analysis
suggested that antigen-specific B cells isolated from the spleens of
unimmunized mice expressed approximately 620 AR binding sites per
cell with an affinity of 0.1 nM. The AR expressed on the B cell was
shown to be of the 2AR-subtype, because B cells stained with an
antibody directed against the cytoplasmic tail of the 2AR, but not
with antibodies directed against the cytoplasmic tails of the 1AR. Thus, in agreement with previous studies investigating the expression of the AR on purified B cell populations, these studies suggested that freshly isolated antigen-specific B cells preferentially expressed
the 2AR.
| |
IV. Effects on CD4+ T Lymphocytes |
|---|
|
TopPreviousNextReferences
|
|---|
A. 2-Adrenergic Receptor Signaling Components
Early studies measuring isoproterenol-induced intracellular cAMP
accumulation in T cells supplied some of the initial suggestions that T
cells expressed a functional AR on their surface. For example, one
early study observed that thymocyte exposure to isoproterenol elevated
the level of cAMP accumulation 4 to 5 times that observed in similarly
exposed peripheral T cells (Bach, 1975). This study not only suggested
that both thymocytes and peripheral T cells expressed a functional
AR on their surface but, in addition, suggested that alterations in
either the level or function of AR expression may occur
during T cell development. Other studies also reported that stimulation
of either the AR or 2AR results in increased levels of cAMP
(Bishopric et al., 1980; Pochet and Delespesse, 1983; Staehelin et al.,
1985; Khan et al., 1986; Dailey et al., 1988; Bartik et al., 1994;
Cazaux et al., 1995; Sanders et al., 1997) or increased adenylyl
cyclase activity (Bartik et al., 1994; Bauman et al., 1994) in T cells.
In contrast, other studies reported that isoproterenol did not induce
cAMP accumulation in human thymocytes, but did increase cAMP levels in
both mouse thymocytes and peripheral human T cells (van de Griend et
al., 1983). These findings correlated with binding studies showing that
human thymocytes expressed a very low number of AR binding sites
compared with either mouse thymocytes or human peripheral T cells (van
de Griend et al., 1983). Finally, the 2AR-selective agonist
terbutaline induced an increase in the intracellular concentration of
cAMP in clones of Th1 cells, but not in clones of Th2 cells (Sanders et
al., 1997), a finding that is in agreement with the previously
discussed data concerning the differential expression of the 2AR on
Th1 and Th2 cell clones. Therefore, these studies suggest that
stimulation of the 2AR expressed by mature T cells results in
increased levels of intracellular cAMP accumulation.
Studies by Pochet and Delespesse (1983) reported that although maturing
thymocytes possessed a lower number of ARs than did mature
peripheral T cells, the few receptors that these cells did possess were
more efficiently coupled to adenylate cyclase. They observed that even
though thymocytes had 4.6 times fewer ARs per cell than splenocytes,
as determined by radioligand binding studies, thymocyte stimulation by
the AR agonist isoproterenol resulted in 20 times higher levels of
intracellular cAMP in comparison with stimulated splenocytes.
Importantly, they also showed that the affinity of the AR in both
cell populations for isoproterenol was equivalent, and thus,
differences in the efficiency of receptor stimulation could not be
responsible for the observed differences in cAMP generation. In
addition, immature thymocytes expressed a lower AR density, but a
greater cAMP response, to isoproterenol than more mature thymocytes.
One interpretation of these findings may be that the efficiency of
AR coupling to adenylyl cyclase may be dependent upon the
developmental status of the T cell. This proposal is supported by the
observations that the different subtypes of AR are coupled to
adenylate cyclase with varying efficiencies (Dixon et al., 1986;
Frielle et al., 1987; Emorine et al., 1989). However, future studies
are necessary to determine whether the same AR subtype can be
coupled to adenylyl with varying affinities and whether the
developmental status of the T cell influences this coupling affinity.
Thus, although the level of AR surface expression may increase on T
cells during maturation in the thymus, the level of
isoproterenol-induced cAMP accumulation appears to decrease, suggesting
that differentiation-dependent alterations in the efficiency of
adenylyl cyclase coupling may exist.
In addition to comparing T cell populations at different developmental
stages, other studies have compared the levels of AR-induced cAMP
accumulation in mature T and B cells. For example, even though purified
B cells expressed a 2-fold higher number of ARs on their surface in
comparison to peripheral T cells, both cell types responded equivalently to isoproterenol-induced cAMP accumulation (Pochet and
Delespesse, 1983). Importantly, there were no differences in the
Kd values of the AR in either
population of cells, and the AR subtype expressed by both cell
populations was determined to be of the 2-subtype by using
salbutamol displacement curves. One explanation of these findings is
that both cell types expressed varying 2AR affinities, with T cells
expressing an increased number of the higher affinity 2AR in
comparison with B cells. This difference may explain why the levels of
isoproterenol-induced cAMP accumulation were equivalent in both cell
populations, even though the T cells in these studies expressed a lower
number of 2AR. Interestingly, the T cell cAMP response to
isoproterenol was higher following 4 to 5 days of IL-2 induced
proliferation in the absence of antigen (Dailey et al., 1988). Possible
explanations include an increase in AR expression, an increase in
the catalytic subunit expression, or an increase in receptor coupling
to adenylate cyclase. Thus, although T cells may express a lower level
of the 2AR on their surface in comparison with B cells, the 2AR
expressed on T cells may be more efficiently linked to adenylyl
cyclase, and this efficiency of adenylyl cyclase coupling may be
modulated by cytokine receptor stimulation.
Classically, stimulation of the 2AR initiates an intracellular
signaling cascade leading to adenylyl cyclase activation, cAMP
accumulation, and PKA activation. Exposure of T cells to either
isoproterenol or PGE2 induced PKA activity in a
dose-dependent manner, but the PKA isoform activated in these studies
was stimuli-dependent (Bauman et al., 1994). For example,
PGE2 exposure resulted in equal activation of two
different isoforms of PKA, PKAI and PKAII, whereas isoproterenol
exposure preferentially lead to the activation of PKAI. These data
contradict previous studies suggesting that stimulation of either the
PGE2R or AR induced PKA activation via
identical pathways (Smith et al., 1971a; Goodwin et al., 1977; Baker et
al., 1981; Johnson et al., 1981; Rappaport and Dodge, 1982; Makoul et
al., 1985; Aussel et al., 1987; Hausdorff et al., 1990). One possible
explanation of these findings may involve the cellular distribution of
the different PKA isoforms, but there is still some controversy
concerning the distribution of PKA in T cells (Chaplin et al., 1979;
Hasler et al., 1992; Skalhegg et al., 1994). Regardless, stimulation of
the T cell 2AR leads to PKA activation, and future studies are
necessary to further investigate whether the PKA isoforms activated by
receptor stimulation are cell type-, developmentally, and/or
activation-dependent.
Other nonclassical signaling pathways have been described for AR
signaling in T cells. For example, one study used mutant T cell lines,
which lack various classical components of the 2AR signaling
cascade, to describe a PKA-independent component to 2AR-induced
thymocyte apoptosis (Gu et al., 2000). Stimulation of the T cell 2AR
resulted in the activation of Lck, a Src family tyrosine kinase, via
physical interactions between the Gs subunit of the 2AR and Lck.
However, future studies are required to determine the functional role
of 2AR-induced Lck activation on T cell differentiation, proliferation, and function.
B. Proliferation, Differentiation, and Cell Trafficking
1. In Vitro Proliferation and Differentiation.
Two of the
major cellular activities of CD4+ T cells are
cell proliferation and cytokine production. Upon activation by
recognition of an antigen peptide presented in the context of MHC class
II by their antigen-specific T cell receptor, the small population of
antigen-specific T cells must be expanded to magnify and successfully complete their effector functions, such as providing "help" to the
B cell for antibody production. Therefore, cellular proliferation of
CD4+ T cells is a critical determinant of the
magnitude of the ongoing immune response. Table
2 summarizes past findings concerning the
effects of norepinephrine, 2AR stimulation, and cAMP-elevating agents on T cell proliferation in vitro and in vivo.
TABLE 2
The effects of norepinephrine, 2AR stimulation, and cAMP-elevating
agents on T cell proliferation in vitro and in vivo
). In addition, exogenous addition of db-cAMP
(10
5 M) also inhibited PHA-stimulated
lymphocyte proliferation, but only if present within the first hour of
cell activation. Interestingly, low concentrations of db-cAMP
(108-109 M) had the
opposite effect, to slightly increase the rate of lymphocyte
proliferation. Therefore, these studies suggested that AR
stimulation may either inhibit or enhance the level of T cell proliferation, depending on the concentration of cAMP generated intracellularly.
Later studies supported the hypothesis that AR stimulation decreased
the level of mitogen- or anti-CD3 antibody-induced T cell
proliferation. For example, isoproterenol exposure inhibited PHA-induced T cell proliferation in a dose-dependent manner using concentrations of isoproterenol ranging from
106 M to 104 M (Feldman
et al., 1987; Carlson et al., 1989). Importantly, isoproterenol-induced
intracellular cAMP levels peaked within 10 to 20 min after exposure
(Carlson et al., 1989). In the same manner, AR stimulation by
isoproterenol (109-105
M) also decreased the rate of anti-CD3 antibody-induced T cell proliferation in a dose-dependent manner (Bartik et al., 1993), and
isoproterenol-induced PKA activation correlated with an inhibition of T
cell proliferation (Bauman et al., 1994). However, even though a number
of reports suggested that either stimulation of the AR, increased
intracellular cAMP accumulation, or increased PKA activity inhibits T
cell proliferation (Smith et al., 1971b; Johnson et al., 1981; Glibetic
and Baumann, 1986; Feldman et al., 1987; Griese et al., 1988;
Scordamaglia et al., 1988; Minakuchi et al., 1990; Bartik et al., 1993;
Bauman et al., 1994), the exact mechanism by which these events may
influence T cell proliferation was unknown.
Although early elevations in intracellular cAMP and PKA activation have
been proposed as candidates for mediating the effect of 2AR
stimulation on the rate of T cell proliferation, other mediators may
also be involved. For example, one possible mechanism by which AR
stimulation inhibits T cell proliferation is by influencing the
assembly of cytoskeletal elements. Under normal conditions, activation
of T cells by anti-CD3 antibody resulted in the conversion of globular
(G)-actin to filamentous (F)-actin to facilitate TCR activation,
costimulation, and cell proliferation (Parsey and Lewis, 1993).
However, stimulation of the AR on anti-CD3-activated T cells
inhibited F-actin assembly that occurred within the first hour of
activation (Selliah et al., 1995). This effect of AR stimulation on
cytoskeletal elements was proposed to be cAMP-dependent, because
similar results were observed in cells exposed to either forskolin or
db-cAMP. Thus, one mechanism by which early elevations in intracellular
cAMP may inhibit activation-induced T cell proliferation is via
disruption of cytoskeletal events leading to cell division.
Therefore, studies conducted over the past 30 years suggest that
stimulation of the 2AR decreases CD4+ T cell
proliferation via a mechanism that may involve elevations in the
intracellular concentration of cAMP, increased PKA activation, and
possible effects on cytoskeletal elements. Future studies are necessary
to further determine whether the method of T cell activation or signals
originating from other surface receptors may influence the effect of
2AR-induced elevations in cAMP on the rate of T cell proliferation.
Finally, other mechanisms may also contribute to the effect of 2AR
stimulation on T cell proliferation, such as 2AR-induced alterations
in both cytokine production by T cells and cytokine receptor expression
on T cells; however, this subject will be discussed in later sections.
In addition to proliferation, the process of cellular differentiation
critically influences T cell function. Importantly, the cytokine
microenvironment is one of the fundamental criteria that determine the
fate of a differentiating CD4+ T cell. For
example, naive CD4+ T cells receiving either
antigen-presenting cells and antigen or anti-CD3 stimulation alone in
the presence of IL-12 differentiate preferentially into Th1 cells
(Seder et al., 1993), whereas the same naive CD4+
T cells stimulated in the presence of IL-4 antibody will differentiate preferentially into Th2 effector cells (Hsieh et al., 1992; Seder et
al., 1992). In light of the importance of the cytokine microenvironment in determining the path of CD4+ T cell
differentiation, norepinephrine-induced changes in the cytokine profile
of antigen-presenting cells may influence whether a naive
CD4+ T cell differentiates into either a Th1 or a
Th2 effector cell.
In support of this hypothesis, a number of studies suggested that
elevated intracellular levels of cAMP may influence both the cytokine
profile and the level of cytokine secreted by antigen-presenting cells.
For example, individually, either the addition of
PGE2 exposure, or the direct addition of db-cAMP
or norepinephrine (108-106 M) to
LPS-activated monocytes each decreased the level of IL-12 but increased
the level of IL-10 produced by antigen-presenting cells (van der Pouw
Kraan et al., 1995; Elenkov et al., 1996). In a similar manner, in
vitro exposure of monocytes and dendritic cells to 2AR-selective
agonists inhibited LPS- or anti-CD40-induced IL-12 production,
respectively, but did not influence monocyte production of either
IL-1, IL-1, IL-6, or IL-10 (Panina-Bordignon et al., 1997).
Finally, one study reported no effect of norepinephrine on IL-12
production by antigen-presenting cells in culture (Swanson et al.,
2001). Thus, although there are conflicting observations concerning the
effects of 2AR stimulation and elevated levels of cAMP on the level
of IL-10 production, these studies suggest that stimulation of the
2AR on a professional antigen-presenting cell may favor the
development of Th2 cells by decreasing the level of IL-12 produced by
antigen-presenting cells that is required for Th1 cell development.
In addition to effects on cytokine production by antigen-presenting
cells, norepinephrine and 2AR stimulation may also influence CD4+ T cell differentiation via direct effects on
the naive CD4+ T cell. Recently, Swanson et al.
(2001) investigated the role of 2AR stimulation on the naive
CD4+ T cell during differentiation to a Th1 cell
and on the function of progeny effector cells. Activation of naive
CD4+ T cells by anti-CD3/28 antibody and IL-12 in
the presence of norepinephrine (106 M)
generated effector Th1 cells that produced significantly higher levels
of IFN-
per cell upon restimulation in comparison with Th1 cells
generated in the absence of norepinephrine. Importantly, the effects of
norepinephrine on Th1 cell IFN- production were 2AR- and
IL-12-dependent as demonstrated by the use of AR and AR
antagonists, a 2AR-selective antagonist, and IL-12R-deficient mice.
Therefore, these studies suggest that norepinephrine may influence Th1
cell function via stimulation of the 2AR expressed on naive
CD4+ T cell and via either augmentation or
collaboration with the IL-12R signaling pathway.
2. In Vivo Proliferation and Cell Trafficking.
When
considering the effects of norepinephrine on in vivo T cell
proliferation, one important factor is the model system being employed.
Although very few studies have investigated the effects of
norepinephrine on T cell proliferation in vivo, the findings of one
study suggested a strain-specific effect of norepinephrine depletion on
T cell proliferation. Whereas norepinephrine depletion significantly
enhanced the level of in vitro Con A-induced T cell proliferation in
spleen cells isolated from DBA/2 mice, no effect was observed on the
level of proliferation of T cells isolated from C57BL/6 mice (Lyte et
al., 1991). In addition, norepinephrine depletion seemed to also exert
differential effects on T cell proliferation, depending on the T cell
activation status. For example, when lymph node cells isolated from
norepinephrine-depleted mice were restimulated in vitro with anti-CD3
antibody, the rate of T cell proliferation was significantly lower in
comparison to lymph node cells isolated from norepinephrine-intact mice
(Madden et al., 1994). In contrast, the basal rate of lymph node cell proliferation was significantly higher in norepinephrine-depleted mice
in comparison to norepinephrine-intact mice as determined by injection
of [125I]deoxyuridine. In summary, findings
from these few in vivo norepinephrine depletion studies suggest that
norepinephrine release in lymph nodes may decrease the proliferation
rate of unstimulated T cells but enhance the proliferation rate of
activated T cells. Thus, whereas most studies suggest that
norepinephrine and 2AR stimulation decreases the rate of T cell
proliferation in vitro, regardless of the type of activation stimulus
used, exposure of T cells to norepinephrine in vivo appears to induce
both strain- and activation stimuli-dependent effects on cellular
proliferation. However, these studies measured the rate of T cell
proliferation by in vitro assays, which only provide an indication of
how the T cell may have behaved in vivo. Thus, because none of these
studies measured the effect of norepinephrine and/or 2AR stimulation on the rate of T cell proliferation in vivo, it is difficult to interpret the findings of these studies.
AR stimulation on in vivo cell trafficking. Most
of the early studies investigating the role of catecholamines in
modulating cell homing were performed with epinephrine, suggesting a
catecholamine-induced elevation in the number of circulating lymphocytes (reviewed in Benschop et al., 1996). Although a number of
early studies dating back to the early 1900s were conducted to
determine the effects of epinephrine on lymphocyte cell homing, the
adrenergic receptor subtype responsible for the action of epinephrine
was not determined until 1974. Using both AR and AR antagonists,
AR blockade alone was shown to inhibit the epinephrine-induced increase in circulating human lymphocytes, suggesting that earlier studies may have been describing the effects of AR stimulation on
lymphocyte homing (Gader, 1974). Supporting these findings, isoproterenol and salbutamol both increased the number of circulating lymphocytes in humans (Gader and Cash, 1975).
The source of the lymphocytes contributing to the catecholamine-induced
increase in circulating cell numbers was also investigated (Ernstrom
and Sandberg, 1973). Isoproterenol and norepinephrine both increased
the number of lymphocytes leaving the spleen, without any apparent
alterations in blood flow. These studies were the first to suggest that
alterations in lymphocyte adhesion molecule expression may mediate the
norepinephrine-induced increase in the number of circulating
lymphocytes, even though this was not tested directly. In contrast,
others showed that the pretreatment of fluorescently labeled
lymphocytes with isoproterenol (106 M) for 15 min prior to i.v. reconstitution increased the homing of lymphocytes to
the spleen and peripheral lymph nodes in comparison to control cells
(Carlson et al., 1997). More specifically, a significantly higher
percentage of the cells homed to the white pulp in the spleen,
particularly the T cell-containing periarterial lymphoid sheath.
In support of 2AR-mediated alterations in lymphocyte migration from
the spleen, the AR antagonist propranolol, but not the AR
antagonist phentolamine, decreased the number of lymphocytes leaving
the spleen via both blood flow-dependent and -independent mechanisms
(Rogausch et al., 1999). In contrast, others have reported that axotomy
increased both the total number and percentage of Thy-1.2-positive T
cells in the spleen, suggesting that norepinephrine depletion either
increased the migration of non-T cells out of the spleen or the homing
of T cells into the spleen (Miles et al., 1985). Thus, these studies
support the findings of earlier studies that norepinephrine may alter
the number of circulating lymphocytes and that 2AR stimulation may
differentially influence the cell trafficking of specific cell
populations. However, conflicting findings still exist concerning the
exact role of norepinephrine and 2AR stimulation in regulating
lymphocyte trafficking.
C. In Vitro and In Vivo Cell Surface Molecule Expression
In light of the apparent influence of 2AR stimulation on the
rate of CD4+ T cell proliferation, and because
stimulation of the IL-2R plays a pivotal role in stimulating
CD4+ T cell proliferation, it was possible that
2AR stimulation influenced the level of IL-2 receptor expression on
T cells.
Although a number of in vitro studies have reported that either
isoproterenol or elevations in the intracellular level of cAMP
down-regulate IL-2R expression on T cells at the protein level (Feldman
et al., 1987; Johnson et al., 1988; Rincon et al., 1988; Krause and
Deutsch, 1991; Anastassiou et al., 1992) and the mRNA level
(Anastassiou et al., 1992), there are conflicting reports concerning
which chains of the IL-2R are affected. For example, whereas one study
reported that db-cAMP or forskolin decreased the number of only the
high affinity IL-2R chain (p75 subunit) (Johnson et al., 1988), others
have reported that similar concentrations of db-cAMP decrease the
number of both the high and low affinity chains of the IL-2R on
activated T cells in vitro (Rincon et al., 1988; Krause and Deutsch,
1991). To complicate matters, one other group has reported no effect of
either similar concentrations of isoproterenol or db-cAMP on the level
of IL-2R expressed by activated T cells in vitro (Chouaib et al.,
1985). Although the source of these conflicting findings is currently unknown, the cell population being studied, the duration and type of T
cell stimulus used, and the kinetics of IL-2R expression on the T cell
should be considered. Finally, in addition to altering the level of
IL-2R expression on T cells, elevations in intracellular cAMP may also
influence signaling components associated with the IL-2R, such as JAK3,
which is an essential mediator of IL-2R signaling (reviewed in Bacon et
al., 1996). PGE2 both inhibited the up-regulation of JAK3 in naive CD4+ T cells and decreased JAK3
expression in activated CD4+ T cells (Kolenko et
al., 1999). The PGE2-induced suppression of JAK3
was also mimicked by forskolin and db-cAMP, but exaggerated by IBMX, an
inhibitor of cAMP phosphodiesterase. Thus, in addition to decreasing
the level of IL-2R expression on naive and effector CD4+ T cells in vitro, 2AR-induced elevations
in cAMP may also down-regulate signaling components of the IL-2R in T cells.
In addition to cellular proliferation, surface molecule expression on
CD4+ T cells is critical for the execution of
their effector functions. For example, CD40L is an essential molecule
that is up-regulated on the surface of activated
CD4+ T cells, allowing them to provide the
necessary "help" that B cells require to differentiate into
antibody-secreting cells. Thus, alterations in CD40L expression on the
CD4+ T cell surface influences their
ability to both initiate and modulate the level of B cell
activation. Although exogenous addition of db-cAMP to T cells alone did
not induce CD40L expression in vitro, exposure of
PMA/ionomycin-activated T cells to db-cAMP synergistically increased
both the level of CD40L mRNA and surface protein expression (Suarez et
al., 1997). Another study reported that the effect of intracellular
elevations of cAMP on the level of CD40L expression was dependent upon
the mode of T cell activation. For example, whereas cAMP elevations
inhibited TCR-induced levels of CD40L on T cells in vitro, cAMP
enhanced Ca2+-induced CD40L expression (Wingett
et al., 1999). Thus, although no studies have specifically reported
AR-induced alterations in CD40L expression on T cells, it is likely
that 2AR-induced elevations in the intracellular level of cAMP may
regulate the level of T cell CD40L expression during the course of an
immune response.
One important factor regulating T cell trafficking in vivo is adhesion
molecule expression (reviewed in D'Ambrosio et al., 2000; Davenport et
al., 2000). Multiple families of adhesion molecules have been described
which regulate T cell homing, and the expression of these molecules on
the T cell surface is regulated by a number of stimuli, such as
cytokine receptor stimulation and cellular activation. Recent studies
have investigated the effects of AR stimulation on the level of
other adhesion molecules expressed on T cells. For example, the
incubation of T cells with isoproterenol for 2 h did not alter the
level of LFA-1 or VLA-4 expression in vitro (Carlson et al.,
1996), which are the counter-receptors for ICAM-1 and VCAM-1 on
T cells (Marlin and Springer, 1987; Dustin and Springer, 1988; Carlos
et al., 1990). In agreement with these findings, others reported that
isoproterenol infusion did not alter the level of LFA-1 expression in
vivo, but significantly decreased the level of L-selectin on
CD4+ T cells (Mills et al., 2000). One
possibility was that the effects of norepinephrine and AR agonists
on lymphocyte trafficking were mediated via the stimulation of
adrenergic receptors expressed on endothelial cells, not on
lymphocytes. However, AR stimulation of endothelial cells did not
alter their level of ICAM-1 and VLA-4 expression (Carlson et al.,
1996). In light of these findings, the authors suggested that
even though adhesion molecule expression may not be altered on either
lymphocytes or endothelial cells, the affinity of the adhesion
molecules may be influenced by AR stimulation because this was shown
to occur following interaction of lymphocytes with endothelial cells
(Hourihan et al., 1993). Thus, further studies are necessary to gain a
better understanding of the mechanisms by which norepinephrine and
2AR stimulation influence lymphocyte trafficking.
In addition to adhesion molecule expression on lymphocytes, chemokines
also play an essential role in directing lymphocyte trafficking. T
cells have been reported to express a number of different chemokine
receptors, such as CXCR4, and stimulation of these receptors exerts a
significant influence on the trafficking of these cells in vivo
(reviewed in Syrbe et al., 1999). db-cAMP (104
M), forskolin (104 M), and norepinephrine
(105 M) all enhanced the constitutive level of
CXCR4 expression on CD4+ T cells and
CD19+ B cells in comparison to unexposed cells
(Cole et al., 1999). Interestingly, db-cAMP and norepinephrine
both blocked the CD3/CD28 activation-induced decrease in CXCR4
expression, suggesting that intracellular elevations of cAMP not only
elevated CXCR4 on resting CD4+ T cells, but also
maintained CXCR4 expression following activation of these cells. These
findings are important because alterations in CXCR4 expression on
lymphocytes influence their sensitivity to stromal cell-derived
factor-1 (SDF-1) and may display altered trafficking patterns.
Therefore, 2AR-mediated alterations in CXCR4 expression on
CD4+ T cells may influence their cell trafficking
in vivo.
In summary, stimulation of the 2AR may differentially influence the
trafficking of CD4+ naive T cells, Th1 cells, and
Th2 cells due to either differential 2AR expression or alterations
in the level of adhesion molecules and chemokine receptor expression.
However, future studies are necessary to further investigate the role
of norepinephrine and 2AR stimulation in directing lymphocyte
trafficking during an ongoing immune response, as well as in affecting
chemokine production by both lymphocytes and nonlymphocytes.
D. T Cell Cytokine Production
1. In Vitro Th1-Like Cytokines.
Table
3 summarizes past findings concerning the
effects of norepinephrine, 2AR stimulation, and cAMP-elevating
agents on T cell cytokine production in vitro and in vivo. A number of
reports have suggested that the level of IL-2 production by activated T
cells is affected either by agents that directly elevate the intracellular level of cAMP or 2AR-selective agonists. For example, exogenous addition of db-cAMP inhibited the level of IL-2 production by
PHA-activated T cells (Chouaib et al., 1985; Van der Pouw-Kraan et al.,
1992; Snijdewint et al., 1993). In addition, other cAMP-elevating agents such as PGE2 (Minakuchi et al., 1990; Betz
and Fox, 1991; Anastassiou et al., 1992), 8-bromo-cAMP (Anastassiou et
al., 1992), cholera toxin (Anastassiou et al., 1992), or
2AR-selective agonists (Sekut et al., 1995; Ramer-Quinn et al.,
1997; Holen and Elsayed, 1998) also decreased the level of IL-2
produced by either activated T cells or Th1 cell clones by decreasing
the rate of IL-2 gene transcription (Anastassiou et al., 1992). Lastly,
whereas most studies reported that either elevations in cAMP or 2AR
stimulation decreased the level of IL-2 production by activated T
cells, one study reported that the 2AR-selective salbutamol
(albuterol) had no effect on IL-2 production (Sekut et al., 1995).
However, most studies support the hypothesis that elevations in
intracellular cAMP or stimulation of the 2AR decreases the level of
IL-2 production regardless of the type of cAMP-elevating stimulus.
TABLE 3
The effects of norepinephrine, 2AR stimulation, and cAMP-elevating
agents on T cell cytokine production in vitro and in vivo
). Similarly, restimulation of Con A-activated CD4+ T cells 7 days later with
either PMA and ionomycin, db-cAMP, cholera toxin, or
PGE2 resulted in a dose-dependent decrease in the
level of IL-2 production (Lacour et al., 1994). Interestingly, the
observed decrease in IL-2 production correlated with a decrease in the
binding of nuclear factor of activated T cells (NF-AT) to the IL-2
promoter in these cells (Lacour et al., 1994). This finding was
supported by others (Tsuruta et al., 1995) who also observed that
elevations in the intracellular concentration of cAMP inhibited IL-2
gene transcription via both an inhibition of NF-AT binding and a
decrease in NF-B nuclear binding. Taken together, these studies
suggest that 2AR-induced elevations in cAMP may decrease the level
of IL-2 produced by CD4+ T cells by decreasing
the level of transcription factor binding to the IL-2 promoter site to
decrease IL-2 gene transcription.
To gain a better understanding of the actions of norepinephrine on
cytokine production by more physiologically relevant populations of Th1
and Th2 cells, recent studies investigated the role of 2AR
stimulation in modulating cytokine production by naive and newly
generated CD4+ T cells. Exposure of sort-purified
naive CD4+ T cells to norepinephrine
(105 M) at the time of cell activation by
anti-CD3 and anti-CD28 mAb stimulation decreased the level of IL-2
produced by naive CD4+ T cells via stimulation of
the 2AR (Ramer-Quinn et al., 2000). Implicating the role of the
2AR in these studies, terbutaline also decreased the level of IL-2
production by naive CD4+ T cells in a
concentration-dependent manner, and this effect was blocked by the
AR antagonist nadolol. Supporting these findings, another study used
the 1AR-selective antagonist metoprolol and the 2AR-selective
antagonist ICI 118,551 to determine that the norepinephrine-induced
suppression of IL-2 production by naive CD4+ T
cells was mediated via stimulation of the 2AR expressed on these
cells (Swanson et al., 2001). This was not surprising, since reverse
transcriptase-polymerase chain reaction data also showed that naive
CD4+ T cells expressed 2AR mRNA, but not
1AR or 3AR mRNA. Thus, these studies suggest a role for 2AR
stimulation on the naive CD4+ T cell in
decreasing the level of IL-2 production by these cells.
In addition to IL-2 production by naive CD4+ T
cells, the role of AR stimulation on the level of IFN- production
by T cells has also been examined. PGE2 exposure
(106 M) inhibited the level of pigeon
cytochrome c-induced IFN- production by a pigeon
cytochrome c-specific Th1 cell clone (Betz and Fox, 1991).
Other studies also reported that increased levels of intracellular cAMP
decreased the level of IFN- production by T cells (Van der Pouw-Kraan et al., 1992; Snijdewint et al., 1993). Importantly, the
2AR-selective agonist salbutamol (105 M)
decreased the level of IFN- production by PHA- and PMA-activated T
cells (Paul-Eugene et al., 1992). In support of these findings, both
the AR-nonselective agonist isoproterenol and the 2AR-selective agonist fenoterol concentration-dependently inhibited Con A-induced IFN- and IL-3 mRNA expression by human T cells (Borger et al., 1998). Furthermore, the selectivity of the effect of AR stimulation was demonstrated using a 2AR-selective antagonist ICI 118,551, the
1AR-selective antagonist atenolol, and the 3AR-selective agonist
BRL 37344, showing that only stimulation of the 2AR expressed by
these cells influenced the production of the cytokines examined. Finally, the 2AR-selective agonist terbutaline decreased the level
of IFN- by resting (Sanders et al., 1997) Th1 cell clones in a
concentration-dependent manner. Thus, the majority of past findings
suggest that norepinephrine and 2AR stimulation may decrease the
level of cytokine production by Th1 cells.
Finally, norepinephrine may exert different effects on Th1 cell
cytokine production, depending on whether the naive or effector cell is
exposed to norepinephrine. As previously discussed, naive CD4+ T cells exposed to norepinephrine during the
process of differentiation generated progeny Th1 cells that produced
higher levels of IFN- upon restimulation with antigen in comparison
to progeny Th1 cells generated in the absence of norepinephrine
(Swanson et al., 2001). Thus, not only may norepinephrine exert
activation stimulus-dependent effects on T cell cytokine production,
but, in addition, norepinephrine may also exert differential effects on
naive versus effector CD4+ T cells.
2. In Vitro Th2-Like Cytokines.
Although Th2 cells do
not appear to express the 2AR (Sanders et al., 1997), some studies
have investigated the effect of cAMP-elevating agents on the level of
cytokine production by Th2 cells. Unfortunately, studies investigating
the effect of elevated intracellular cAMP and PKA activation on IL-4
production have produced conflicting findings reporting either an
increase, a decrease, or no change in the level of IL-4 production. For
example, a number of studies have reported that cAMP-elevating agents
increase the level of IL-4 production by CD4+ T
cells. Either db-cAMP, cholera toxin, or PGE2
increased the level of IL-4 production by Con A-activated
CD4+ T cells restimulated 7 days later with PMA
and ionomycin in a concentration-dependent manner (Lacour et al.,
1994). Similarly, db-cAMP increased the level of IL-4 production by
both mitogen-activated murine lymph node CD4+ T
cells and CD4+ thymocytes in a
concentration-dependent manner
(106-104 M) (Wirth et
al., 1996). Finally, PKA activation by the cAMP phosphodiesterase
inhibitor IBMX (100-750 µM) enhanced the level of IL-4 production by
a mitogen-activated Th2 cell line in a concentration-dependent manner
(Teschendorf et al., 1996). However, others have reported that high
concentrations of db-cAMP (103 M) inhibited the
level of IL-4 mRNA expression induced by the activation of human T
cells by either Con A, anti-CD3 antibody, or anti-CD3/anti-CD28
antibody (Borger et al., 1996). Although the source of these
conflicting findings is unknown, elevations in intracellular cAMP and
PKA may increase the level of IL-4 production by T cells in a
concentration-dependent manner until a threshold concentration is
reached and these mediators begin to exert an inhibitory influence on
IL-4 production. In addition, all studies reporting a cAMP- or
PKA-induced increase in IL-4 measured cytokine production from
mitogen-activated T cells, whereas the study reporting a cAMP-induced
decrease in IL-4 production measured cytokine production by
TCR-activated T cells. Thus, the method of cell activation may
influence the role of cAMP and PKA in mediating IL-4 production by
CD4+ T cells.
2AR-selective agonist salbutamol
failed to affect the level of IL-4 production by either mature T cells or Th2 cell clones (Novak and Rothenberg, 1990; Betz and Fox, 1991;
Paul-Eugene et al., 1992; Van der Pouw-Kraan et al., 1992; Snijdewint
et al., 1993). Because Th2 cells do not express the 2AR (Sanders et
al., 1997), it is not surprising that salbutamol did not influence IL-4
production by Th2 cell clones. However, future studies are necessary to
further investigate the importance of both the type of stimulus used to
activate the T cells and the intracellular concentration of cAMP on the
level of IL-4 production by Th2 cells.
In addition to IL-4, a few other studies have investigated the effects
of cAMP-elevating agents and PKA activation on the level of IL-5 and
IL-10 production by Th2 cells. For example, PGE2
exposure (108-106 M)
enhanced IL-5 production in a concentration-dependent manner (Betz and
Fox, 1991). Similarly, db-cAMP, cholera toxin, and
PGE2 all increased the level of IL-5 production
by Con A-activated CD4+ T cells restimulated 7 days later with PMA and ionomycin in a concentration-dependent manner
(Lacour et al., 1994). Finally, PKA activation by the cAMP
phosphodiesterase inhibitor IBMX did not affect the level of IL-10
production by a Th2 cell line (Teschendorf et al., 1996).
In summary, a number of studies have reported conflicting findings
concerning the effects of cAMP-elevating agents on the level of IL-4
production by CD4+ T cells, and fewer studies
have investigated the effects of these agents on other "Th2-like"
cytokines. However, whereas a number of studies reported that
elevations in intracellular cAMP do not affect the level of IL-4
production or IL-10 production by CD4+ T cells,
others have observed a concentration-dependent enhancement in the level
of both IL-4 and IL-5 production. Importantly, if Th2 cells do not
express the 2AR, then norepinephrine may only affect Th2 cell
cytokine production indirectly via effects on other supporting cell
populations, assuming that the AR is not expressed during the
resting or activated states of the Th2 cell.
3. In Vivo Cytokine Production.
Unfortunately, there is a
significant lack of literature concerning the effects of norepinephrine
stimulation of the 2AR on cytokine production in vivo. Madden et al.
(1994) reported an organ- and cytokine-specific effect of
norepinephrine depletion on the level of cytokine production. For
example, norepinephrine depletion increased the level of IFN- but
did not affect IL-2 production by Con A-stimulated lymph node cells, in
comparison with lymph node cells isolated from norepinephrine-intact
mice. In contrast, norepinephrine depletion decreased the level of both IFN- and IL-2 production by Con A-stimulated spleen cells, in comparison to cells isolated from norepinephrine-intact mice. Thus,
findings from these studies suggest that norepinephrine depletion may
differentially affect the level of T cell cytokine production depending
on both the specific cytokines measured and the target organ
contributing the cells for study.
investigated the effects of
norepinephrine depletion on the level of cytokine production in two
strains of mice, C57BL/6J (Th1 cell-slanted strain) and BALB/c (Th2
cell-slanted strain) mice. Mice were injected once with
6-hydroxydopamine (6-OHDA), which significantly depleted norepinephrine
in both strains of mice, and then immunized 2 days later with the T
cell-dependent antigen KLH. Spleen cells isolated from
norepinephrine-depleted C57BL/6J mice 3 or 6 days after antigen-induced activation in vivo produced significantly higher levels of IL-2 and
IL-4 following restimulation in vitro, in comparison with cells
isolated from norepinephrine-intact controls. In addition, similar
findings were observed in experiments performed in Th2 cell-slanted
BALB/c mice. Therefore, norepinephrine and 2AR stimulation may
differentially affect CD4+ T cell cytokine
production, depending on the strain of mouse, the mode of T cell
activation, and the specific cytokine measured. In addition, because
the 2AR is differentially expressed by subpopulations of
CD4+ T cells, norepinephrine may also selectively
influence naive and Th1 cell cytokine production in vivo.
4. Differential Effects on Th1 versus Th2 Cytokines.
Studies
by Gajewski et al. (1990) were some of the first to propose that
alterations in intracellular cAMP may exert differential effects
on Th1 and Th2 cell cytokine production. For example, cholera toxin and
8-bromo-cAMP more significantly inhibited the level of Th1 cell
cytokine production in comparison with Th2 cell cytokine production. In
support of these findings, cholera toxin inhibited TCR-induced IL-2
production and proliferation in Th1 cell clones, but not TCR-induced
IL-4 production and proliferation in Th2 cell clones (Munoz et al.,
1990). Since cholera toxin elevates the level of adenylyl cyclase
activity by ribosylation of the G-stimulatory subunit of the G-protein,
resulting in the accumulation of cAMP (Neer and Clapham, 1988), this
differential effect of cholera toxin on Th1 and Th2 cytokine production
may be directly related to the differential expression of the 2AR on
these cell population, because 2AR-negative Th2 cells may express
lower levels of G-protein to be activated by cholera toxin. However, because forskolin, a direct activator of adenylate cyclase, also failed
to influence TCR-induced IL-4 production and proliferation in Th2 cells
(Munoz et al., 1990), the lack of a cholera toxin-induced effect on Th2
cell IL-4 production is more likely to be the result of additional
alternative mechanisms regulating the differential effect of
cAMP-elevating agents on the level of cytokine production by Th1 and
Th2 cells as opposed to a difference in G-protein expression. Finally, others suggested that the effect in intracellular cAMP may be
gene-specific, not cell type-specific. Various cAMP-elevating agents
decreased IL-2 mRNA expression by the murine thymoma EL4.E1 but had no
effect on the level of IL-4 mRNA (Novak and Rothenberg, 1990).
Therefore, intracellular cAMP may differentially affect Th1 and Th2
cell cytokine production via gene-specific mechanisms.
-hydroxylase-deficient mice (Alaniz et al., 1999). Because
dopamine -hydroxylase enzymatically converts dopamine to
norepinephrine (reviewed in Levi-Montalcini and Angeletti, 1966), mice
deficient in this enzyme are deficient in norepinephrine. T cells
isolated from the spleens of -hydroxylase-deficient mice infected
with Listeria monocytogenes produced lower levels of IFN-, TNF-, and IL-10 in comparison with cells isolated from wild-type mice. However, T cells isolated from the spleens of -hydroxylase-deficient mice infected with Mycobacterium
tuberculosis produced lower levels of IFN- and TNF-, but
increased levels of IL-10, in comparison with cells isolated from
wild-type mice. These studies suggest that norepinephrine may
differentially influence both Th1- and Th2-like cytokine production,
depending on the infection model system used. Because Th2 cells do not
express ARs, the mechanism by which norepinephrine deficiency
influences Th2 cell cytokine production is unknown. However, these
findings may involve potential effects of norepinephrine deficiency on
2AR-positive naive CD4+ T cells
differentiating into Th2 cells or effects of norepinephrine deficiency
on other cells, such as antigen-presenting cells, that influence the
level of cytokine production by both Th1 and Th2 cells.
One study investigated the mechanism by which cAMP-dependent PKA
activation inhibited the level of IL-2 production but enhanced the
level of IL-4 production by T cells (Neumann et al., 1995). They
reported a PKA-dependent increase in the level of expression, nuclear
translocation, and DNA binding activity of a number of members of the
Rel/NF-B transcription factor family, such as c-Rel, p105/p50, and
IB. In addition, although there was no observed increase in the
synthesis of the p65 subunit of NF-B, increased PKA activity
inhibited p65 nuclear translocation and DNA binding, possibly via
stabilization of IB-. This finding was correlated with
observations that forskolin inhibited mitogen-induced IL-2 promoter
activity but enhanced the level of IL-4 promoter activity, due to the
fact that the IL-2 promoter, but not the IL-4 promoter, contained an
inhibitory B binding site. Thus, the differential expression of
various enhancing and inhibitory binding sites within the promoters of
Th1 and Th2 cytokines may be another mechanism for the differential
regulation of cytokine production in Th1 and Th2 cells.
| |
V. Effects on B Lymphocytes |
|---|
|
TopPreviousNextReferences
|
|---|
A. 2-Adrenergic Receptor Signaling Components
The earliest studies investigating the effects of AR
stimulation on immune cell function reported that stimulation of the receptor increased the level of intracellular cAMP accumulation and
adenylyl cyclase activity in whole lymphocyte populations (Bourne and
Melmon, 1971; Makman, 1971; Smith et al., 1971a,b; Williams et al.,
1976; Astaldi et al., 1976; Conolly and Greenacre, 1977). Bach (1975)
first reported that isoproterenol induced an accumulation of
intracellular cAMP in murine splenic B cells. In contrast, this same
group later reported that isoproterenol did not increase cAMP
accumulation in human tonsillar B cells (Niaudet et al., 1976).
However, because tonsillar B cells tend to display an activated
phenotype in comparison to splenic B cells, which display a resting
phenotype, these findings suggested that stimulation of the 2AR on
resting, but not activated, B cells increased the level of
intracellular cAMP. Subsequently, a number of studies reported that
AR stimulation induces adenylyl cyclase activity and increased
intracellular cAMP accumulation in resting B cells (Galant et al.,
1978; Bishopric et al., 1980; Pochet and Delespesse, 1983; Blomhoff et
al., 1987; Holte et al., 1988; Kohm and Sanders, 1999).
As discussed previously, the number of AR binding sites expressed on
a B cell did not always correlate with the level of cAMP accumulation
induced by AR stimulation. A number of studies reported that whereas
B cells expressed approximately twice the number of ARs as T cells,
stimulation of the AR on T cells generated higher levels of
intracellular cAMP accumulation (Niaudet et al., 1976; Galant et al.,
1978; Bishopric et al., 1980; Pochet and Delespesse, 1983). The
mechanism responsible for the apparent inverse relationship between the
level of AR expression on T and B cells and the level of
isoproterenol-induced cAMP accumulation in these cells is currently
unknown. One possible explanation is that T and B cells have varying
levels of membrane fluidity, such that if the cell membrane is more
rigid, 2AR activation may less effectively activate membrane-bound
adenylyl cyclase. Also, a number of additional mechanisms have been
reported that influence the effectiveness of 2AR stimulation in
initiating intracellular signaling cascades, such as the receptor
phosphorylation state, and any one of these processes may be
differentially active in T and B cells (reviewed in Hein and Kobilka,
1995; Bouvier and Rousseau, 1998; Lefkowitz et al., 1998).
B. B Cell Proliferation
Similar to CD4+ T cells, B cell
proliferation is an essential component of an ongoing immune response.
Only a small percentage of the body's B cells are capable of
responding to any given protein antigen. Therefore, upon
antigen-induced B cell activation, it is critical that the small
antigen-specific cell population expand its numbers so that a suitable
number of B cells are generated to differentiate into both
antibody-secreting cells and memory B cell. Table
4 summarizes past findings concerning the
effects of 2AR stimulation and cAMP-elevating agents on B cell
proliferation in vitro.
|
Studies by Diamantstein and Ulmer (1975) were the first to investigate
the effects of intracellular cAMP levels on the rate of B cell
proliferation. Using the B cell mitogen LPS, these findings suggested
that the addition of exogenous cAMP (103 M)
blocked the LPS-induced elevation in spleen cell proliferation in
vitro. These studies were also performed using spleen cells isolated
from nude mice, which lack T cells, to enrich for the percentage of B
cells in the spleen. However, monocytes, which also expressed
adrenergic receptors on their surface and are responsive to LPS
stimulation, remained in these spleens and, thus, the exact role of
norepinephrine specifically on B cells was still unknown.
In support of these early findings, forskolin decreased the
proliferation of a B cell precursor cell line in vitro, a finding which
correlated with the down-regulation of c-myc and
c-Ha-ras expression, two known proto-oncogenes (Blomhoff et
al., 1987). Importantly, the effect of cAMP elevation on the rate of B
cell proliferation may be dependent upon certain activation signals. For example, db-cAMP (104 M) inhibited the
effects of IL-4 (known then as BSF-1) but enhanced the effects of IL-1
on anti-IgM antibody-induced B cell proliferation in vitro (Hoffmann,
1988). Similarly, db-cAMP (104 M) and IBMX
(104 M) both decreased the level of anti-IgM
antibody-induced B cell proliferation in vitro, but increased the level
of PMA/ionomycin-induced B cell proliferation (Cohen and Rothstein,
1989). Finally, one group reported that cAMP elevations influenced the
rate of B cell proliferation in a cytokine-specific manner (Vazquez et
al., 1991). For example, both forskolin and db-cAMP decreased the rate
of anti-IgM antibody-induced proliferation in the presence of IL-2 but
enhanced the level of B cell proliferation in the presence of IL-4. In
addition to cAMP-elevating agents, the role of AR agonists in
modulating B cell proliferation has also been studied. Norepinephrine
and isoproterenol both increased the rate of LPS-induced B cell
proliferation in vitro, and this effect was blocked by the AR
antagonist propranolol, but not the AR antagonist phentolamine (Li
et al., 1990).
Thus, these studies emphasize the fact that the effects of cAMP on the
level of B cell proliferation cannot be generalized, such that
elevations in cAMP may exert differing effects on B cell proliferation
in vitro, depending on the B cell activation stimulus used and the
cytokines present within the microenvironment. Unfortunately, there is
a lack of data concerning the effects of norepinephrine and 2AR
stimulation on B cell proliferation in vivo.
C. B Cell Surface Molecule Expression and Function
1. In Vitro Surface Molecule Expression and Function.
One
molecule expressed on the surface of B cells that plays an important
role in B cell function is B7-2 (CD86). B7-2 is a costimulatory
molecule that is either induced or constitutively expressed on all
types of antigen-presenting cells. B7-2 expression on the B cell
serves two functions. First, B7-2-mediated stimulation of either CD28
or CTLA-4 expressed on the surface of T cells exerts a critical
regulatory influence on T cell cytokine production and surface molecule
expression, thus indirectly influencing B cell function by modulating
the level of "help" that the cell receives from T cells. Second,
stimulation of B7-2 sends a signal directly into the B cell to
regulate the level of antibody production (Kasprowicz et al., 2000).
Thus, alterations in the level of B7-2 expression on the B cell
surface is one mechanism to both directly and indirectly regulate B
cell function.), B7-2 mRNA
expression and protein expression peak at approximately 12 and 24 h following BCR- or LPS-induced activation of B cells, respectively
(Freeman et al., 1993; Lenschow et al., 1993, 1994). Therefore, cell
activation appears to be one mechanism by which B7-2 protein
expression is up-regulated on B cells. In addition to cell activation,
a number of other stimuli are now known to enhance the level of B7-2
expression on B cells, such as cytokine receptor stimulation (reviewed
in Lenschow et al., 1996). Interestingly, stimulation of the 2AR
alone on resting B cells increased the level of B7-2 expression
(Kasprowicz et al., 2000). In addition, concomitant stimulation of both
the B cell receptor and the 2AR resulted in an additive increase in
the level of B7-2 expression on B cells, suggesting the change in
B7-2 expression may be one mechanism by which 2AR stimulation can
influence the T-dependent immune response.
2AR regulates the level of B7-2 expression on the B cell has been
investigated. One study reported two molecular mechanisms by which
stimulation of the BCR and/or 2AR may cooperate to up-regulate the
level of B7-2 surface protein and mRNA expression in B cells, i.e.,
increased mRNA stability and NF-B-dependent gene transcription (A. P. Kohm and V. M. Sanders, manuscript submitted
for publication). Importantly, the concurrent stimulation of both
receptors resulted in an additive enhancement in the level of B7-2
expression on the B cell, and this cooperative effort between the BCR
and 2AR may be one mechanism by which signals originating from the
immune and nervous system synergize to regulate immune cell function. However, future studies are necessary to further dissect the mechanism by which stimulation of the BCR and 2AR cooperate to regulate the
level of B7-2 expression on the B cell, as well as other B cell-associated molecules that may also be influenced by norepinephrine.
In addition to B7-2, stimulation of the 2AR may also alter the
level of expression of various other surface molecules on the B cell
surface. Exposure to db-cAMP (104 M) slightly
decreased the level of MHC class II expression and sIgD expression on
resting B cells but did not alter the level of these molecules on
PMA/ionomycin-activated B cells (Li et al., 1989). Similarly,
norepinephrine and isoproterenol did not influence the level of MHC
class II expression on LPS-activated B cells (Li et al., 1990). In
agreement with these findings, antigen-induced activation or IL-4
exposure enhanced the level of MHC class II expression on
antigen-specific B cells, whereas terbutaline alone did not alter the
resting level of MHC class II expression (Sanders and Powell-Oliver,
1992).
In summary, in vitro findings suggest that norepinephrine and 2AR
stimulation may cooperate with signals originating from the immune
system to regulate surface molecule expression on the B cell surface,
such as B7-2 and MHC class II. However, future studies are necessary
to translate these findings into in vivo model systems, as well as to
determine whether the expression of other surface molecules is affected
by signals originating from the sympathetic nervous system.
2. In Vivo Surface Molecule Expression.
Unfortunately, very
few studies have investigated the effects of norepinephrine and/or
2AR stimulation on surface molecule expression in vivo. However, one
important observation in respect to the previously discussed
norepinephrine depletion studies is that norepinephrine depletion in
vivo may alter the level of adrenergic receptor expression. For
example, Miles et al. (1981, 1984, 1985) investigated the effect of
6-OHDA treatment on AR expression on T and B cells. They showed that
1 week after norepinephrine depletion via 10 daily injections of
6-OHDA (100 mg/kg), there was a significant increase in the level
of AR expression on both splenic T and B lymphocytes in comparison
to cells from saline-injected animals. In contrast, others reported no
effects of 6-OHDA-mediated norepinephrine depletion on the lymphocyte
surface density of AR (Nahorski et al., 1979), even though the
norepinephrine depletion protocol in these studies was milder than that
used by Miles et al. (1984). Thus, norepinephrine depletion in vivo may
influence the level of AR expression on B cells, depending on the
model system and treatment protocol used.
D. B Cell Differentiation and Antibody Production
1. In Vitro Direct Alterations Induced by Elevations in
Intracellular cAMP.
Resting B cells become activated following
recognition of antigen by the surface immunoglobulin component of the
BCR. Upon B cell activation, these cells must first differentiate into
plasma cells prior to producing and secreting antibody. Therefore, the total amount of antibody produced in response to a specific antigen is
dependent upon the number of B cells that differentiate into antibody-secreting cells, the level of antibody secreted per plasma cell, and the function of other accessory cells that are critical to
the successful formation of a T cell-dependent antibody response. In
light of this, it is important to determine whether norepinephrine and
2AR stimulation influence B cell function by either effects on B
cell differentiation or plasma cell function. Tables
5 and 6
summarize past findings concerning the effects of cAMP-elevating agents
on B cell differentiation into antibody-secreting cells in vitro and in
vivo and the level of B cell antibody production in vitro,
respectively.
TABLE 5
The effects of cAMP-elevating agents on B cell differentiation into
antibody-secreting cells in vitro and in vivo
TABLE 6
The effects of cAMP-elevating agents on the level of B cell antibody
production in vitro
; Robison and Sutherland, 1971). However, the
concentration-dependent curves generated by these agents on B cell
differentiation were bell-shaped, such that low (0.01-0.1 ng/ml) and
high (100 ng/ml) concentrations of theophylline had no effect, whereas
medium concentrations (1-10 ng/ml) significantly enhanced the number
of antibody-secreting cells. A similar pattern was observed with
poly(A:U) treatment, except that high concentrations of poly(A:U) (100 ng/ml) suppressed the number of antibody-secreting cells. The role of
cAMP in mediating the effects of poly(A:U) treatment on the number of
antibody-secreting cells was further suggested by the potentiation of
the poly(A:U)-induced increase in the number of antibody-secreting
cells by theophylline (Braun and Ishizuka, 1971). In contrast, others
reported that varying concentrations of either db-cAMP
(105-103 M) or
theophylline (103 M) inhibited the number of
antibody-secreting cells produced in response to sRBC-induced B cell
activation (Watson et al., 1973; Melmon et al., 1974; Montgomery
et al., 1975). Finally, whereas a high concentration of db-cAMP
(103 M) inhibited the number of
antibody-secreting cells in response to LPS and sRBC stimulation, a
slightly lower concentration of db-cAMP (104 M)
enhanced the number of antibody-secreting cells (Watson, 1975). Therefore, there appear to be conflicting findings concerning the
effect of cAMP-elevating agents on the number of antibody-secreting cells.
Others have reported a biphasic effect by varying concentrations of
cAMP on the formation of antibody-secreting cells following sRBC-induced B cell activation. For example, the addition of relatively low concentrations of db-cAMP
(107-106 M) to spleen
cell cultures at the time of sRBC activation had no effect on the
number of antibody-secreting cells, but moderate concentrations of
db-cAMP (105-104 M)
increased and higher concentrations db-cAMP
(103 M) decreased the number of
antibody-secreting cells (Marchalonis and Smith, 1976). In addition,
exposure of sRBC-activated spleen cell cultures to db-cAMP did not
alter the kinetics of the ensuing antibody response, a finding that was
mimicked when spleen cells isolated from nude mice, which do not
possess peripheral T cells, were used. However, although these studies
may have ruled out the possibility that cAMP-elevating agents were
influencing B cell function indirectly via alterations in T cell
function, other cell types may also have been affected in this model
system, such as macrophages and dendritic cells, that might have
contributed to the alterations in B cell differentiation.
Importantly, elevations in the intracellular concentration of cAMP did
not always inhibit the formation of antibody-secreting cells. For
example, a biphasic effect of cAMP on the formation of
antibody-secreting cells was observed, depending on the length of
exposure to the cAMP-elevating agent. For example, if the
cAMP-elevating agents db-cAMP (103 M) or
aminophylline (103 M) were present only during
the first 24 h of B cell activation, but then washed out of the
culture, the number of antibody-secreting cells was enhanced at days 4 and 5 of culture (Teh and Paetkau, 1974). However, if the
cAMP-elevating agents were present at times later than 24 h, the
formation of antibody-secreting cells was inhibited. Thus, these
studies were some of the first to suggest that cAMP can exert
time-dependent effects on B cell function, such that early elevations
in the level of cAMP increase the number of antibody-secreting cells,
whereas later elevations in cAMP inhibit the same response.
In addition to influencing the number of B cells differentiating into
antibody-secreting cells, elevations in intracellular cAMP may also
influence the level of antibody produced. DNP-primed spleen cells
activated with either anti-Ig or DNP in the presence of db-cAMP
(103-104 M) produced
more antigen-specific IgG and total IgG in comparison to B cells
stimulated in the absence of db-cAMP (Kishimoto and Ishizaka, 1976).
However, if db-cAMP was added after 24 h of activation, the level
of IgG production was suppressed. In addition, similar results were
obtained with the cAMP phosphodiesterase inhibitor aminophylline.
Therefore, these studies and others suggested that cAMP may exert
differential effects on B cell function, such that increases in
intracellular cAMP concentrations during the early stages of T
cell-dependent B cell activation enhanced the level of antibody
production, whereas elevations in intracellular cAMP during the later
stages of cellular activation suppressed the level of antibody
production (Kishimoto et al., 1977; Cook et al., 1978; Sanford et al.,
1979; Koh et al., 1995).
Of importance, the previously discussed studies investigating the
effects of cAMP-elevating agents on the level of antibody production
used whole lymphocyte cultures. Because these cultures contained
various cell populations that may be affected by elevations in cAMP and
that may influence the level of antibody production by B cells, it was
possible that elevations in cAMP were not exerting direct effects on B
cells but, instead, were influencing the function of other accessory
cells to modulate the level of antibody production by B cells.
Using purified populations of B cells, the influence of cAMP elevation
specifically in B cells was investigated (Gilbert and Hoffmann, 1985).
This study reported that B cells activated with either sRBCs alone or
in combination with either IL-1 or cAMP (103 M)
did not differentiate into antibody-secreting cells. However, when the
sRBC-activated B cells were exposed to both IL-1 and db-cAMP
concurrently, a significant number of antibody-secreting cells formed
in comparison to B cells activated in the absence of db-cAMP.
Importantly, these studies suggested that cAMP elevations were not
sufficient to induce antibody formation, even following activation of B
cells with sRBCs. Thus, it is possible that cAMP contributed to a
signal generated by either IL-1R or BCR stimulation.
A later study further investigated the mechanism by which elevations in
the intracellular concentration of cAMP decreased the level of antibody
production during the later stages of cell activation.
PGE1-mediated increases in cAMP decreased the
level of spontaneous IgM and PMA-enhanced IgM production by transformed B cells (Patke et al., 1991). These data support earlier findings concerning the relationship between the timing of cAMP exposure and the
level of antibody produced. Since these studies utilized transformed B
cells, which spontaneously produced antibody, the cells were already in
an activated state. Thus, these studies supported previous findings
suggesting that elevations in intracellular cAMP accumulation decreased
the level of antibody production under conditions in which these
elevations occurred either at a time later than 24 h following B
cell activation or after the B cell had already differentiated into an
antibody-secreting cell.
In contrast to previous studies that measured IgM production alone,
elevations in cAMP have been reported to also influence the antibody
isotype produced by B cells. For example, PGE2
(108-106 M)
significantly enhanced the level of IgG1 and IgE
produced by LPS-activated B cells, but decreased the level of IgM and
IgG3 production, in the presence of varying
concentrations of IL-4 (100-10,000 U/ml) (Roper et al., 1990). In
addition, other cAMP-elevating agents such as cholera toxin (100 pg/ml)
and db-cAMP (104 M) exerted similar effects on
the level of antibody production. Thus, these findings suggested that
intracellular elevations in cAMP contributed to the IL-4-dependent
antibody response to increase the switching of antibody production to
"Th2-like" isotypes, such as IgG1 and IgE, or
expanded a population of cells that had already switched to
IgG1 and IgE.
Because stimulation of the B cell IL-4R has been reported to increase
the intracellular cAMP levels in B cells within 10-20 min of exposure
(Finney et al., 1990; Rigley and Callard, 1991; McKay et al., 2000), it
is possible that the level of intracellular cAMP influences the level
of B cell IgG1 and IgE production. In support of
these findings, cholera toxin-induced elevations in intracellular cAMP
synergistically enhanced the number of
IgG1-producing B cells and the level of germline
1 transcript produced in B cells exposed to both LPS and IL-4, but
decreased the number of IgM-, IgA-, and IgG3-
producing B cells (Lycke et al., 1990). These findings were extended to
show that PGE2 exposure of LPS-activated B cells
resulted in a quicker generation of germline 
transcripts, as well
as a higher level of gene transcription (Roper et al., 1995). It was
also reported that cAMP-elevating agents increased the level of IgE
production (Coqueret et al., 1996). Finally, IL-4 enhanced the effect
of cholera toxin in a concentration-dependent manner to increase the
number of IgG1-producing B cells. Thus, these
studies further supported the hypothesis that elevations in cAMP
complement the IL-4-induced effects on B cells. However, other studies
reported that elevations in the level of intracellular cAMP augmented
the IFN- receptor signaling pathway. For example, exposure of
IFN--pulsed B cells to either db-cAMP, PGE2,
or cholera toxin enhanced both the number of
IgG2a-producing B cells and the total level of
IgG2a production following LPS-induced activation (Stein and Phipps, 1991). Thus, elevations in the intracellular concentration of cAMP may influence the effects of cytokines on the
level and isotype of antibody produced by the B cell, perhaps by
altering the signaling pathways associated with cytokine receptors.
In summary, the mechanism by which cAMP contributes to the
intracellular signaling events that induce antibody production is
unknown, but several possibilities exist. For example, it is known that
the physical interaction between a T cell and a B cell results in cAMP
accumulation within the B cell (Pollok et al., 1991). In addition,
stimulation of the B cell receptor and cytokine receptors may result in
the intracellular accumulation of cAMP, and elevations in cAMP may
augment the signal transduction pathways initiated by stimulation of
these receptors to increase either the number of antibody-secreting
cells or the level of antibody produced by B cells.
2. In Vitro 2-Adrenergic Receptor Stimulation.
Tables
7 and 8
summarize past findings concerning the effects of norepinephrine and
2AR stimulation on B cell differentiation into antibody-secreting
cells and the level of B cell antibody production in vitro,
respectively. As with studies investigating the effects of elevated
intracellular cAMP levels in B cells on antibody production, early
studies investigating the effects of AR agonists on the level of
antibody production in vitro used whole splenic cell populations.
Norepinephrine
(106-103 M) and
isoproterenol (106-103
M) both dose dependently inhibited the number of antibody-secreting cells in response to the particulate antigen sRBC when added at later
times following B cell activation (Melmon et al., 1974). Importantly,
the AR agonist phenylephrine
(106-103 M) did not
significantly affect the number of antibody-secreting cells, whereas
later studies reported that 2AR stimulation decreases the level of
antibody production (Sanders and Munson, 1985b). However, because these
studies investigated the differentiation of B cells into
antibody-secreting cells using unfractionated spleen cell populations,
AR-mediated effects may have been due to the expression of these
receptors on non-B cells.
|
|
5 M) or terbutaline
(105 M) and sRBCs to spleen cell cultures
significantly enhanced the number of anti-sRBC antibody-secreting
cells, whereas addition of norepinephrine to sRBC-activated B cells at
later times following cell activation resulted in a loss of the
norepinephrine-induced increase in the number of antibody-secreting
cells (Sanders and Munson, 1984a). Thus, stimulation of the 2AR at
earlier times during the B cell response to sRBC enhanced the number of
antibody-secreting cells. These findings were supported by later
studies in which sRBC-activated spleen cells were exposed to either
norepinephrine or terbutaline at the time of activation. At varying
times following concurrent cell activation and 2AR stimulation, a
AR antagonist was added to the cultures and the number of
antibody-secreting cells was determined on day 5 of culture (Sanders
and Munson, 1984b). Addition of the AR antagonist within 6 h of
activation inhibited the maximal effect of 2AR stimulation to
increase the number of antibody-secreting cells, whereas the addition
of the AR antagonist at either 6 h or later following cell
activation did not alter the terbutaline-induced increase in the number
of antibody-secreting cells. Thus, these studies suggested that
stimulation of the B cell 2AR initiated intracellular signals during
the first 6 h of cell activation that were critical to the
2AR-induced increase in the number of antibody-secreting cells.
Others reported that norepinephrine enhanced the level of LPS-induced
antibody production by whole splenic cell cultures depleted of T cells when norepinephrine was added to the cultures at the time of LPS exposure, but not when added 2 h following cell activation, an effect that was blocked by the AR antagonist propranolol, but not by
the AR antagonist phentolamine (Kouassi et al., 1988). Taken
together, these findings suggest that stimulation of the B cell 2AR
at times early during cell activation may increase the number of
antibody-secreting cells or the level of antibody produced by
antibody-secreting cells, whereas B cell differentiation and function
may be inhibited by 2AR stimulation at later times following cell activation.
However, because the frequency of antigen-specific B cells is
relatively low in the spleen, and because other cell types are present
in these cultures that also express adrenergic receptors, later studies
elucidated the role of 2AR stimulation in modulating B cell function
using enriched populations of antigen-specific B cells and T cell
clones (Sanders and Powell-Oliver, 1992). TNP-specific B cells were
cocultured with KLH-specific CD4+ Th2 cell clones
in the presence of TNP-KLH and terbutaline (106
M). Terbutaline increased the number of anti-TNP IgM-secreting cells 3 to 5 days following the initiation of culture. In addition, terbutaline
increased both the number of anti-TNP IgM-secreting cells and the total
level of TNP-specific antibody, in a concentration-dependent manner
(109-106 M). These
effects of terbutaline were blocked by the AR antagonists nadolol
and propranolol, but not by the AR antagonist phentolamine, ensuring
the participation of 2AR stimulation in mediating the effects of
terbutaline. This study also reported a nonsignificant effect of
terbutaline on the number of IgG1-secreting cells; however, later
studies reported that terbutaline increased the total amount of IgG1
secreted by B cells, not the number of secreting cells (Kasprowicz et
al., 2000). In support of these findings, AR stimulation by either
isoproterenol or norepinephrine increased the level of IgM, IgG, and
IgA produced by LPS-activated B cells (Li et al., 1990). Importantly,
the effects of AR-stimulating agents were blocked by propranolol,
but not by phentolamine. Thus, these studies using B cells activated by
either a soluble protein antigen (TNP-KLH) or a B cell mitogen (LPS)
suggest that stimulation of the B cell-associated 2AR enhanced both
the number of antibody-secreting cells and the total level of antibody
produced by B cells.
As discussed earlier, previous studies using cAMP-elevating
agents had implicated increases in the intracellular level of cAMP in
augmenting IL-4R signaling. In addition, this hypothesis has been
supported by studies that employed 2AR-selective agonists. Exposure
of PBMC to either the 2AR-selective agonist salbutamol (1010-106 M) or
fenoterol (1010-106 M)
increased the level of IL-4-dependent IgE production which was blocked
by the AR antagonist propranolol (Paul-Eugene et al., 1992). These
findings suggested that the effect of 2AR stimulation was mediated
via the release of soluble CD23 receptors that modulated the level of B
cell activation and IgE production (reviewed in Delespesse et al.,
1989). Also, fenoterol enhanced both IL-4-induced IgE mRNA expression
and protein secretion by human PBMC, and this effect correlated with
the 2AR-induced increase in the level of intracellular cAMP
(Coqueret et al., 1996). The role of cAMP in mediating the effect of
2AR stimulation on IgE production was further supported by the
ability of PKA inhibitors, H8 (105 M) and
Rp-AMP (105 M), which compete for the
ATP-binding site of the PKA catalytic subunit, to block the effects of
2AR stimulation on IgE production. Pretreatment of PBMC with the
cyclo-oxygenase inhibitor indomethacin significantly decreased the
effects of fenoterol on the level of IgE production, suggesting that
fenoterol may also stimulate the production of prostaglandins, which
subsequently induce cAMP accumulation and PKA activation. Finally,
fenoterol also enhanced the level of CD40-induced IgE production by
purified B cells. Taken together, these studies suggest that
2AR-mediated increases in cAMP and PKA activity may contribute to
the IL-4R signaling cascade, because IL-4R signaling results in both
enhanced levels of intracellular cAMP (McKay et al., 2000) and PKA
activity (Vazquez et al., 1991).
More recently, 2AR stimulation on the B cell alone was found to
augment the level of IL-4-dependent IgG1 and IgE
produced by either the Th2 cell- or CD40L-activated B cell
(Kasprowicz et al., 2000). Pretreatment of TNP-specific B cells with
both antigen and terbutaline for 24 h prior to activation
increased the responsiveness of B cells to IL-4 and the level of
TNP-specific IgG1 and IgE produced per B cell,
but did not affect the number of antibody-secreting cells produced or
the level of IL-4R expressed. Importantly, terbutaline increased the
level of B cell IL-4 responsiveness in a concentration-dependent
manner. This effect was blocked by the AR antagonist nadolol and did
not occur if B cells were isolated from the spleens of 2AR 
/
mice. Therefore, these studies suggest that stimulation of the 2AR
increases the responsiveness of the B cell to IL-4 and the level of
IgG1 and IgE produced per cell.
In addition to influencing the level of IL-4 responsiveness, 2AR
stimulation may also influence the level of BCR-dependent B7-2
signaling to the B cell. Previous studies suggested that stimulation of
B7-2 on activated human tonsillar B cells increased the level of
IgG4 and IgE production (Jeannin et al., 1997).
Importantly, exposure of murine B cells activated by CD40 and IL-4R
stimulation produced more antigen-specific IgG1
and IgE when exposed to anti-B7-2 antibody, anti-sIg antibody (or
antigen), and terbutaline, in comparison with anti-sIg antibody and
terbutaline only (Kasprowicz et al., 2000). These studies reported not
only that B7-2 signaling to the murine B cell was dependent upon BCR
stimulation to enhance the level of IgG1 and IgE
produced per B cell but, in addition, that stimulation of the 2AR
further enhanced the level of B7-2 signaling in this model system.
Taken together, these studies support the hypothesis that stimulation
of the B cell 2AR either prior to, or at the time of, cell
activation may increase either the number of antibody-secreting cells
or the amount of antibody produced per cell. In addition, a number of
studies suggest that stimulation of the 2AR may generate intracellular signals that augment the IL-4R signaling pathway and/or
the B7-2 signaling pathway to increase the level of IL-4-dependent IgG1 and IgE produced per B cell.
3. In Vivo B Cell Differentiation and Antibody
Production.
Tables 7 and 8 summarize past findings concerning the
effects of norepinephrine and 2AR stimulation on B cell
differentiation into antibody-secreting cells and the level of B cell
antibody production in vivo, respectively. An early study investigated the role of cAMP accumulation in regulating antibody-secreting cell
formation in vivo (Braun and Ishizuka, 1971). Immunization of mice with
poly(A:U) enhanced the number of antibody-secreting cells 48 h
following sRBC immunization. A few years later, the effect of
norepinephrine-depletion on the primary T-dependent antibody response
in vivo was investigated (Kasahara et al., 1977b). Using a low dose of
6-OHDA, which selectively destroyed peripheral sympathetic nerve
terminals (reviewed in Kostrzewa and Jacobwitz, 1974), both the
hemagglutinin titer and the number of plaque-forming cells in response
to immunization with sRBC were decreased. The suppressive effect of
norepinephrine depletion on the hemagglutinin titer was measured 4 days
following immunization by this group and others (Williams et al.,
1981), but not at later time points after immunization. These findings
were extended to determine the effect of norepinephrine depletion on
the secondary (memory) response to a T-dependent antigen.
Norepinephrine depletion at the time of primary immunization did not
alter the secondary response to antigen administered 10 days following
the primary antigen challenge (Kasahara et al., 1977a). However, 6-OHDA
did inhibit the secondary antibody response in a dose-dependent manner
when administered concurrently with the secondary exposure to antigen, suggesting that the level of norepinephrine at the time of antigen administration may be important. In contrast, others have reported that
either surgical axotomy of the spleen or 6-OHDA-mediated norepinephrine
depletion increased the number of antibody-secreting cells following
immunization with sRBC (Besedovsky et al., 1979). However, it is
important to note that norepinephrine depletion was performed on
newborn animals in these studies, a procedure that not only results in
permanent peripheral norepinephrine depletion but, in addition, alters
central levels of norepinephrine as well. Thus, norepinephrine
depletion may exert varying effects on the number of antibody-producing
cells formed in response to sRBCs, depending on the mouse age,
concentration, and timing of 6-OHDA administration in relation to the
delivery of the antigen signal.
). More recently, the level of serum
TNP-specific antibody produced by dopamine -hydroxylase-deficient
mice (norepinephrine-deficient) immunized with the soluble protein
antigen TNP-KLH was significantly lower than the level of antibody
produced by B cells in wild-type mice (Alaniz et al., 1999). Thus,
these studies suggested that norepinephrine depletion suppressed the
level of antibody produced in vivo following immunization with either a
particulate (sRBC) or soluble protein (TNP-KLH) antigen. In contrast,
peripheral norepinephrine depletion in mice increased the number of
antibody-forming cells activated by T-independent antigens but did not
alter the number of antibody-secreting cells activated by T-dependent
antigens (Miles et al., 1981). In addition, it is difficult to
determine the exact effect of norepinephrine depletion on these
responses, because responses against two different antigens were
initiated in each animal, and in some cases, T-dependent and
T-independent responses were initiated in the same animal by immunizing
mice with multiple antigens concurrently.
Using immunizations of cholera toxin, Lycke et al. (1990) reported that
elevations in the intracellular level of cAMP slanted the in vivo
T-independent antibody response toward a "Th2-like" profile,
because cholera toxin-treated animals produced elevated levels of
DNP-specific IgG1, but not IgM or
IgG3. Zalcman et al. (1994) showed that exogenous
IL-2 administration enhanced the antibody response against sRBC in vivo
and that this effect was dependent upon intact splenic sympathetic
innervation. It was possible that exogenous administration of IL-2
increased hypothalamic activity (Zalcman et al., 1994) to increase the
level of peripheral norepinephrine released; and, because sympathetic
nerves express IL-2 receptors (Haugen and Letourneau, 1990), it was
possible that IL-2 increased sympathetic nerve activity and
norepinephrine release in the spleen. This hypothesis was supported by
the finding that pretreatment of norepinephrine-intact animals with the
AR antagonist propranolol blocked the IL-2-induced enhancement in antibody production, whereas the AR antagonist phentolamine had no
effect. Finally, the timing of IL-2 administration was a critical factor that influenced this response because exogenous IL-2 had to be
administered either the day before or the day of sRBC immunization to
increase the number of antibody-secreting cells. Thus, these studies
suggest that IL-2 administration may enhance the early in vivo antibody
response via activation of the sympathetic nervous system to increase
the level of norepinephrine release in lymphoid organs.
One study reported a strain-specific enhancement in antibody production
in norepinephrine-depleted C57BL/6J (Th1-slanted strain) and BALB/c
(Th2-slanted strain) mice (Kruszewska et al., 1995). For example, when
C57BL/6J were depleted of norepinephrine using a single injection of
6-OHDA (100 mg/kg) and immunized with the T-dependent antigen KLH,
serum levels of KLH-specific IgM, IgG, IgG1, and
IgG2a were enhanced in norepinephrine-depleted
animals 1 to 2 weeks post-immunization. In contrast, when similar
studies were performed in BALB/c mice, serum levels of antigen-specific IgM, IgG, and IgG2a were not significantly
different in norepinephrine-depleted and norepinephrine-intact mice.
However, norepinephrine depletion did slightly enhance the level of
KLH-specific IgG1. Interestingly, though,
norepinephrine depletion seemed to create a trend, but not significant
suppression of all isotypes in BALB/c mice during the first week. This
finding suggested that a re-examination of the approach used to address
these questions needed to be undertaken. When this was done, it became
apparent that resident lymphocytes were exposed to a burst of
norepinephrine when using 6-OHDA, because the mechanism of action of
6-OHDA is to displace norepinephrine before destruction of the
sympathetic nerve terminal. Because adrenergic receptors are expressed
by resident immune cells, it is probable that the displaced
norepinephrine might affect these cells in a manner similar to
norepinephrine released in response to antigen.
Because we now know that the spleen contains immune cells in all stages
of differentiation, and because cells in these different stages may
differentially express adrenergic receptors, a recent study addressed
this deficiency in experimental design by using a reconstitution model
system to specifically investigate the role of norepinephrine-induced
2AR stimulation on the B cell in regulating the Th2-dependent
antibody response (Kohm and Sanders, 1999). KLH-specific Th2 cell
clones (2AR-negative) and TNP-specific B cells (2AR-positive)
were adoptively transferred into norepinephrine-depleted T cell- and B
cell-deficient scid mice after the mice had been depleted of
norepinephrine by 6-OHDA. A significantly lower serum level of
TNP-specific IgM and IgG1 was measured in
response to the soluble cognate protein antigen TNP-KLH in
norepinephrine-depleted animals. Importantly, the effects of
norepinephrine depletion on the primary IgM response were reversed by
the 2AR-selective agonists terbutaline and metaproterenol in a
dose-dependent manner, suggesting that the effects of norepinephrine
depletion on the in vivo antibody response were mediated via a lack of
2AR stimulation on the B cell. In addition, whereas the level of
TNP-specific IgM returned to control levels following secondary
immunization of animals that were depleted of norepinephrine prior to
the primary immunization, serum levels of TNP-specific
IgG1 were still significantly lower. However,
memory antibody levels were only measured for 3 weeks following
secondary immunization; thus, the memory antibody response in
norepinephrine-depleted animals may have been delayed, not inhibited.
Finally, whereas norepinephrine depletion did not alter T and B cell
trafficking to the spleen in this model system, spleen cell
proliferation and germinal center formation were significantly lower in
norepinephrine-depleted animals in comparison to norepinephrine-intact controls. Thus, these data suggest that stimulation of the B cell 2AR by endogenous norepinephrine released during the course of a
T-dependent immune response (Kohm et al., 2000) is necessary to
maintain an optimal level of antibody production in vivo.
Taken together, these studies demonstrate the potential for
norepinephrine to exert varying effects on B cell function in vivo. For
example, NE depletion may exert age-dependent effects on B cell
function, because norepinephrine depletion of neonatal animals
increases the number of antibody-secreting cells, whereas norepinephrine depletion in adults decreases both the number of antibody-secreting cells and the level of antibody production by B
cells. In addition, the effects of norepinephrine depletion on the
level of antibody production were dependent upon both the strain of
mouse and the model system used. However, in addition to the these
sources of conflicting findings in vivo, additional problems arise when
comparing in vivo and in vitro effects of norepinephrine on B cell
function. As previously discussed, most studies investigating the
effects of norepinephrine on B cell function in vivo depleted normal
mice of the neurotransmitter. Therefore, although in vitro studies
could specifically study the effects of norepinephrine on B cell
function, these in vivo studies were in fact studying the effects of NE
depletion on all cell populations that both expressed adrenergic
receptors and participated in the antibody response. Finally, in vivo
studies may have also been studying the effects of norepinephrine on
cells in various states of differentiation, because NE depletion mostly likely affected both naive and effector cells in these animals. Thus,
future studies are needed to further dissect the role of norepinephrine
and 2AR stimulation in regulating B cell function both in vitro and
in vivo. These studies may be assisted by the use of additional model
systems, such as reconstituted scid mice, to investigate the
role of norepinephrine in regulating the function of each cell type
contributing to antibody production in vivo, or gene disruption of
NE-synthesizing enzymes in specific cell populations in vivo.
| |
VI. Disease- and Health-Specific Implications |
|---|
|
TopPreviousNextReferences
|
|---|
In light of the ability of 2AR stimulation to influence the
level of CD4+ Th1 cell cytokine,
CD4+ T cell, and B cell proliferation; lymphocyte
homing; B cell antibody production; and B cell costimulatory molecule
expression and signaling, it is not surprising that norepinephrine or
the stimulation of the lymphocyte 2AR has been reported to influence
both the onset and progression of various diseases or age-related
abnormalities, such as Down's syndrome (Morale et al., 1992),
rheumatoid arthritis (Felten et al., 1992; Baerwald et al., 1997;
Lombardi et al., 1999), multiple sclerosis (Zigmond et al., 1989), and
aging (Callard and Basten, 1978; Doria et al., 1980; Kohno et al.,
1986; Madden et al., 1989, 1995).
Decreases in the level of splenic innervation are present in aged
subjects and individuals with certain pathological conditions. For
example, an age-related withdrawal of sympathetic innervation was
observed in both the spleen and lymph nodes of rats, but not in the
thymus (Felten et al., 1987a,b, 1988a,b; Ackerman et al., 1991;
Bellinger et al., 1992b). This observation may explain the declining T
and B cell responses (Callard and Basten, 1978; Doria et al., 1980;
Madden et al., 1989) and cellularity of the white pulp (Cheung and
Verity, 1983; Bellinger et al., 1992a) that are associated with aging.
Finally, the level of lymphoid organ innervation appears to be related
to autoimmune disease expression, because sympathetic innervation was
decreased in mice prone to the development of autoimmune disease prior
to expression of the disease phenotype (Chelmicka-Schorr et al., 1988,
1992). Thus, alterations in the level of sympathetic innervation within
lymphoid organs of individuals with certain disease states, or during
the process of aging, may translate into alterations in the rate of
norepinephrine release during the course of the immune response.
A number of studies have investigated the effects of aging on the level
of 2AR expression and function on lymphocytes. For example, one
study reported an age-dependent decrease in both the number of ARs
expressed on the surface of spleen cells and the affinity
(Kd) of the receptors (Kohno et al.,
1986). In addition, others investigated the effects of age and
norepinephrine depletion on the T-dependent antibody response in vivo
(Madden et al., 1995). For example, norepinephrine depletion did not
significantly influence the level of KLH-specific IgM or IgG production
in young rats but significantly enhanced the level of KLH-specific IgM
and IgG in aged animals. Similarly, norepinephrine depletion increased the level of KLH-, Con A-, and LPS/dextran sulfate-induced
spleen cell proliferation in aged animals more significantly than in young animals. These findings are surprising in light of the
possibility that the level of AR affinity and expression on
lymphocytes may be decreased in aged animals. However, it is possible
that aged lymphocytes are more responsive to AR-derived signals;
thus, even though these cells express lower levels of AR on their
surface, stimulation of this receptor may still affect the function of aged cells more significantly than that of younger cells.
Others have investigated the effects of norepinephrine and 2AR
stimulation on rheumatoid arthritis and reported that that AR
antagonists delayed both the onset and progression of rheumatoid arthritis. Importantly, lymphocytes isolated from rheumatoid arthritis patients do not expressed altered levels of 2AR expression or affinity; however, the activity and expression of GRKs were lower in
patients in comparison with healthy controls (Lombardi et al., 1999).
Because GRKs function to desensitize the 2AR, the lower activity and
expression of GRKs in lymphocytes isolated from rheumatoid arthritis
patients may account for both the higher levels of intracellular cAMP
measured in these cells and the lower levels of TNF- produced by
diseased lymphocytes. In contrast, others have reported that the number
of 2AR expressed on synovial fluid lymphocytes was significantly
lower than the number of 2AR expressed on peripheral blood
lymphocytes (Baerwald et al., 1997), thus suggesting a local mechanism
for down-regulating the level of 2AR expression during arthritis.
Such a decrease in the level of 2AR expression may remove the
inhibitory influences of 2AR stimulation on T cell function, thus
leading to enhanced inflammatory cytokine production. Finally, in
support of the inhibitory role of 2AR stimulation on lymphocyte
function during rheumatoid arthritis, depletion of peripheral
norepinephrine via 6-OHDA resulted in both an earlier onset and
enhanced severity of experimentally induced arthritis (Felten et al.,
1992). Taken together, these studies suggest that norepinephrine
stimulation of the 2AR expressed by lymphocytes may inhibit the
progression of rheumatoid arthritis; however, future studies are
necessary to determine the specific cell population(s) being influenced
by norepinephrine and 2AR stimulation.
| |
Acknowledgements |
|---|
|
TopPreviousNextReferences
|
|---|
This work was supported in part by research funds from the National Institutes of Health Grants AI37326 and AI47420 (V.M.S.).
| |
Footnotes |
|---|
1 Address for correspondence: Virginia M. Sanders, Ph.D., Department of Molecular Virology, Immunology, and Medical Genetics, Ohio State University, 2078 Graves Hall, 333 West 10th St., Columbus, OH 43210-1239. E-mail: A-Kohm{at}Northwestern.edu (Adam P. Kohn) E-mail: VSANDER{at}LUMC.EDU
| |
Abbreviations |
|---|
BCR, B cell receptor;
AR, -adrenergic receptor;
Ab, antibody;
Ag, antigen;
ASC, antibody-secreting cells;
AR, beta-adrenergic receptor;
ARK, -adrenergic receptor kinase;
BBB, blood-brain barrier;
CNS, central
nervous system;
Con A, concanavalin A;
CRF, corticotropin-releasing
factor;
CT, cholera toxin;
db, dibutyryl;
DNP, dinitrophenyl;
DOPAC, 3,4-dihyroxyphenylacetic acid;
Feno, fenoterol;
GRK, G-protein
receptor kinase;
IBMX, 3-isobutyl-1-methylxanthine;
IFN, interferon;
Ig, immunoglobulin;
IL, interleukin;
Iso, isoproterenol;
KLH, keyhole
limpet hemocyanin;
LPS, lipopolysaccharide;
NE, norepinephrine;
6-OHDA, 6-hydroxydopamine;
OVA, ovalbumin;
PGE, prostaglandin E;
PHA, phytohemagglutinin;
PKA, protein kinase A;
PKC, protein kinase C;
PMA, phorbol ester;
Salb, salbutamol;
scid, severe combined
immunodeficient;
sRBC, sheep red blood cell;
TCR, T cell receptor;
Terb, terbutaline;
Th, T-helper;
TNF, tumor necrosis factor;
TNP, trinitrophenyl.
| |
References |
|---|
|
TopPrevious |
|---|
-adrenergic receptor function.
FASEB J
4:
2881-2889[Abstract]. T cell receptor transgenic system.
Proc Natl Acad Sci USA
89:
6065-6060)-[3H]dihydroalprenolol binding to intact and broken cell preparations of human peripheral blood lymphocytes.
Eur J Pharmacol
85:
185-194[Medline].)3H-alprenolol binding.
J Clin Invest
57:
149-155.
0031-6997/01/5304-487-525$3.00
PHARMACOLOGICAL REVIEWS
Copyright © 2001 by The American Society for Pharmacology and Experimental Therapeutics
This article has been cited by other articles:
|
|
 |
|
  W. Mao, S. Fukuoka, C. Iwai, J. Liu, V. K. Sharma, S.-S. Sheu, M. Fu, and C.-s. Liang Cardiomyocyte apoptosis in autoimmune cardiomyopathy: mediated via endoplasmic reticulum stress and exaggerated by norepinephrine Am J Physiol Heart Circ Physiol, September 1, 2007; 293(3): H1636 - H1645. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 A. Chamorro, X. Urra, and A. M. Planas Infection After Acute Ischemic Stroke: A Manifestation of Brain-Induced Immunodepression Stroke, March 1, 2007; 38(3): 1097 - 1103. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. Cosentino, A. M. Fietta, M. Ferrari, E. Rasini, R. Bombelli, E. Carcano, F. Saporiti, F. Meloni, F. Marino, and S. Lecchini Human CD4+CD25+ regulatory T cells selectively express tyrosine hydroxylase and contain endogenous catecholamines subserving an autocrine/paracrine inhibitory functional loop Blood, January 15, 2007; 109(2): 632 - 642. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 V. von Dossow, N. Baehr, M. Moshirzadeh, C. von Heymann, J. P. Braun, O. V. Hein, M. Sander, K.-D Wernecke, W. Konertz, and C. D. Spies Clonidine Attenuated Early Proinflammatory Response in T-Cell Subsets After Cardiac Surgery Anesth. Analg., October 1, 2006; 103(4): 809 - 814. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 N. W. Kin and V. M. Sanders It takes nerve to tell T and B cells what to do J. Leukoc. Biol., June 1, 2006; 79(6): 1093 - 1104. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. J. Loza, S. Foster, S. P. Peters, and R. B. Penn Beta-agonists modulate T-cell functions via direct actions on type 1 and type 2 cells Blood, March 1, 2006; 107(5): 2052 - 2060. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. Oostendorp, D. S. Postma, H. Volders, H. Jongepier, H. F. Kauffman, H. M. Boezen, D. A. Meyers, E. R. Bleecker, S. A. Nelemans, J. Zaagsma, et al. Differential Desensitization of Homozygous Haplotypes of the {beta}2-Adrenergic Receptor in Lymphocytes Am. J. Respir. Crit. Care Med., August 1, 2005; 172(3): 322 - 328. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 C A Hudson, T K Mondal, L Cao, J Kasten-Jolly, V C Huber, and D A Lawrence The dietary supplement ephedrine induces b-adrenergic mediated exacerbation of systemic lupus erythematosus in NZM391 mice Lupus, April 1, 2005; 14(4): 293 - 307. [Abstract] [PDF]
|
|
|
|
 |
|
 C. K. Ganta, F. Blecha, R. R. Ganta, B. G. Helwig, S. Parimi, N. Lu, R. J. Fels, T. I. Musch, and M. J. Kenney Hyperthermia-enhanced splenic cytokine gene expression is mediated by the sympathetic nervous system Physiol Genomics, October 4, 2004; 19(2): 175 - 183. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 D. Lukashev, A. Ohta, S. Apasov, J.-F. Chen, and M. Sitkovsky Cutting Edge: Physiologic Attenuation of Proinflammatory Transcription by the Gs Protein-Coupled A2A Adenosine Receptor In Vivo J. Immunol., July 1, 2004; 173(1): 21 - 24. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
T. W. Klein, C. Newton, K. Larsen, L. Lu, I. Perkins, L. Nong, and H. Friedman The cannabinoid system and immune modulation J. Leukoc. Biol., October 1, 2003; 74(4): 486 - 496. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. Bedoui, S. Miyake, Y. Lin, K. Miyamoto, S. Oki, N. Kawamura, A. Beck-Sickinger, S. von Horsten, and T. Yamamura Neuropeptide Y (NPY) Suppresses Experimental Autoimmune Encephalomyelitis: NPY1 Receptor-Specific Inhibition of Autoreactive Th1 Responses In Vivo J. Immunol., October 1, 2003; 171(7): 3451 - 3458. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
 |
 |
 |
|
 |
 |
 |
