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Vol. 52, Issue 3, 349-374, September 2000
Department of Pharmacology and Toxicology, Michigan State University, East Lansing, Michigan
Abstract
I. Introduction
II. General Mechanisms of Transendothelial Neutrophil Migration
A. Capture and Rolling
1. Leukocyte Selectin.
2. Platelet Selectin.
3. Endothelial Cell Selectin.
B. Firm Adhesion
1. Integrins.
2. Intercellular Adhesion Molecules.
C. Polymorphonuclear Leukocyte Homing to Transmigration Sites on Endothelium
D. Transmigration
E. Migration as an Ordered Sequence of Events
F. Soluble Mediators of Migration
1. Cytokines: Tumor Necrosis Factor-and Interleukin-1.
2. Chemoattractants.
a. Platelet-activating factor.
b. Leukotriene B4.
c. Complement protein C5a.
d. Formyl-methionyl-leucyl-phenylalanine.
e. Chemokines.
3. Cytokine-Chemoattractant Interaction during Migration.
G. Implications
III. Polymorphonuclear Leukocyte Migration in the Lungs
A. Marginated Pool of Polymorphonuclear Leukocytes
B. Site of Migration
C. Adhesion Molecule Requirements
1. Integrins.
2. Selectins.
D. Models of Pulmonary Polymorphonuclear Leukocyte Migration
1. Bacteria and Bacterial Products.
2. Acid Aspiration.
3. Interleukin-1.
4. Complement Protein C5a.
5. Immune-Complex Deposition.
6. Summary and Research Needs.
E. Polymorphonuclear Leukocyte Migration during Endotoxemia: A Special Case of CD18 Inhibition?
IV. Pharmacologic Intervention
A. Lipid A Analogs
B. Cytokine Blockers
C. Steroids
D. Nonsteroidal Anti-Inflammatory Drugs
E. Anti-Selectin Therapies
1. Fucoidan.
2. Glycomimetics.
F. Anti-Integrin Therapies
1. Leumedins.
2. Ligand-Based Peptide Inhibitors.
G. 3-Hydroxy-3-methyl-glutaryl Coenzyme A Reductase Inhibitors
H. Nitric Oxide Donors
V. Summary
References
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Abstract |
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Leukocyte trafficking into pulmonary tissue and airspaces is a critical component of the host defense response. Activation and migration of polymorphonuclear leukocytes (PMNs) into lungs also contribute to inflammatory tissue injury and remodeling of tissue architecture. There have been considerable advances in our understanding of the mechanisms that control PMN adhesion and transendothelial migration (TEM). Mechanisms of migration unique to the lungs have been described with regard to the profile of adhesion molecules, cytokines, and chemokines elicited during PMN emigration from blood vessels. This work reviews general mechanisms of TEM of PMNs and discusses the nature of PMN recruitment in several models of airway inflammation that illustrate how various stimuli elicit different responses. Pharmacologic manipulation of adhesive interactions between PMNs and endothelial cells is a current area of research aimed at developing pharmacologic agents to control inflammation during pulmonary and other inflammatory diseases. A summary of some of these agents and their actions is presented.
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I. Introduction |
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Knowledge of the mechanisms of leukocyte migration has expanded greatly in recent years. Whereas only in the last decade the first adhesion molecule was being characterized, today our understanding includes molecular descriptions for numerous adhesive receptor-ligand interactions, discovery of novel families of chemotaxins and cytokines, and the appreciation for the pivotal role adhesion pathways play in diseases from AIDS and atherosclerosis to cancer and inflammatory syndromes. Although these research efforts have clarified some issues, they also have illuminated new and perhaps difficult challenges. Indeed, the current literature on leukocyte transendothelial migration (TEM)3 portrays a complex phenomenon with many mechanisms and points of control that depend on the site of migration, the cell type, the initial inflammatory stimuli, and the presence and absence of several cellular and soluble mediators. The simple paradigm of rolling/adhering/diapedesis still holds true. However, the number of variables involved in our understanding of the process has increased dramatically.
This review will first describe the general mechanisms of TEM and the current understanding of the adhesion molecules, cytokines, and chemokines that drive leukocyte migration with an emphasis on polymorphonuclear leukocytes (PMNs; neutrophils). Also discussed are the interactions among soluble mediators that together determine the outcome of the adhesive/migratory process. Next, the special case of pulmonary neutrophil migration will be used to illustrate the dependence of the form of migration on the specific inflammatory stimuli. A summary of these observations in lung support the hypothesis that different adhesion molecule pathways are invoked by a qualitative difference in the inflammatory cytokines and chemokines present. Last is a brief discussion of current clinical and experimental strategies for pharmacologic interventions that target the adhesive process.
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II. General Mechanisms of Transendothelial Neutrophil Migration |
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Circulating leukocytes can migrate from vessels into tissues under
both normal and pathologic circumstances. It is well accepted that
leukocyte migration from the vasculature occurs by a multistep process,
dictated by the sequential activation of adhesive proteins and their
ligands on both leukocytes and endothelial cells (ECs) (Lawrence and
Springer, 1991
; von Andrian et al., 1991
; Konstantopoulos and McIntire,
1996
). Monocytes, lymphocytes, and PMNs all migrate by these similar,
sequence-dependent mechanisms but differ in their responses to
chemotactic and inflammatory signals, particularly in their qualitative
and quantitative expression of adhesion molecules (Dransfield et al.,
1992
; Springer, 1994
; Ager, 1996
; Li et al., 1996
). Initiation of
migration begins with the "capture" by the vessel wall of PMNs from
flowing blood, and this is followed by their "rolling" along the
vessel wall. This process of margination is a normal behavior of
circulating PMNs. Only after appropriate stimuli are present do rolling
leukocytes become firmly adhered to ECs and are thus positioned for
migration from the blood vessel into tissue parenchyma.
A. Capture and Rolling
Both the capture, or initial tethering and removal of PMNs from
the flowing blood, and their rolling along the vessel wall is due to
the reversible binding of transmembrane glycoprotein adhesive molecules
called selectins, which are found on both PMNs and ECs (Bevilacqua and
Nelson, 1993
; Albelda et al., 1994
; Luscinskas and Lawler, 1994
;
Crockett-Torabi and Fantone, 1995
; Tedder et al., 1995a
). Selectins
have a calcium-dependent lectin domain on the extracellular
NH2 terminus that is attached to an epidermal growth factor-like domain and then to a number of short
consensus sequences. A short, intracellular domain is linked to signal
transduction proteins (reviewed in Crockett-Torabi and Fantone, 1995
;
Crockett-Torabi, 1998
). Selectin-type adhesive proteins are found on
most cell types of hematopoietic origin and on ECs of blood and lymph vessels.
1. Leukocyte Selectin.
Intravital microscopic analysis of
the microvascular circulation in normal tissue offers visual evidence
of the transient "stick and release" behavior of PMNs rolling along
the vessel wall. In noninflamed tissues, the tenuous association of
PMNs within postcapillary venules can be blocked by treatment with antibodies to leukocyte selectin (L-selectin) (Mel-14, LAM-1, CD62L) on
circulating PMNs (Spertini et al., 1991
; von Andrian et al., 1991
).
Constitutive expression of L-selectin is greatest on PMNs newly
released from bone marrow compared with older, circulating PMNs
(Matsuba et al., 1997
). Loss or shedding of L-selectin from the PMN
surface is due in part to metalloprotease activity, which results in
rapid accumulation of bioactive L-selectin in the blood (Kishimoto et
al., 1995
). Autoproteolysis of L-selectin can occur after exposure to
various inflammatory mediators such as lipopolysaccharide (LPS;
endotoxin) and tumor necrosis factor-
(TNF-
), but it may also
occur from normal rolling interactions with the vessel wall. For
example, metalloprotease inhibitors significantly reduce rolling velocity and cleavage of L-selectin on PMNs in vitro, suggesting that
L-selectin is routinely shed from circulating PMNs during normal,
nonpathologic conditions (Walchek et al., 1996
). Thus, the longer a PMN
has been in the circulation and interacting with the vessel wall, the
more L-selectin it has lost to transient binding and cleavage.
Replacement of L-selectin on circulating PMNs has not been
demonstrated, and a low expression level is associated with apoptosis
and may be a signal for the removal of PMNs from the circulation
(Matsuba et al., 1997
). High plasma levels of soluble L-selectin that
can occur during infection may inhibit PMN rolling at noninflamed
sites. Bioactive, soluble L-selectin can bind to endothelial ligands
and block their interactions with PMN-borne L-selectin (Schleiffenbaum
et al., 1992
; McGill et al., 1996
; Ohno et al., 1997a
).
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2. Platelet Selectin.
At least two endothelium-bound
selectins, platelet selectin (P-selectin) and endothelial cell selectin
(E-selectin), can facilitate PMN-EC adhesions. These selectins are
expressed only when appropriate inflammatory stimuli are present.
P-selectin (granule membrane protein-140; CD62P) is stored
intracellularly in Weibel-Palade bodies of ECs and in
-granules of
platelets (Malik and Lo, 1996
). Within minutes of exposure of ECs to
inflammatory mediators such as complement products, oxygen-derived free
radicals, or various cytokines, P-selectin is mobilized to the cell
surface where it can interact with its PMN counterpart, P-selectin
glycoprotein ligand-1 (PSGL-1; CD162). Monocytes and platelets also
possess PSGL-1 and can bind to P-selectin on activated ECs. Like CD34, the PSGL-1 protein is modified with O-linked sialic acid and
other sugar groups. It consists of a disulfide-bonded homodimer on the PMN surface and is capable of binding two P-selectin ligands
simultaneously (McEver and Cummings, 1997
).
3. Endothelial Cell Selectin.
A second endothelial-borne
selectin, E-selectin (ELAM-1, CD62), is not stored but requires gene
transcription for expression. Peak expression and activity in ECs in
vitro is 4 to 6 h after exposure to inflammatory cytokines (Klein
et al., 1995
; Scholz et al., 1996
). E-selectin can support rolling and
tethering of PMNs in a fashion similar to P-selectin (Lawrence and
Springer, 1994
). Thus, its role in inflammation may be to
maintain PMN rolling after P-selectin has been down-regulated (Malik
and Lo, 1996
; Yang et al., 1999a
). Like L- and P-selectins, E-selectin
binds in vitro to a variety of sialic acid- and fucose-containing
glycoproteins, as well as to sulfated glycosaminoglycans such as
heparan sulfate. However, the in vivo ligand for E-selectin may be a
unique mucin modified with N-linked sugars that has been
characterized in murine cells called E-selectin ligand 1 (Levinowitz et
al., 1993
; Steegmaier et al., 1995
; Willmroth and Beaudet,
1999
). Another sialylated ligand for E-selectin has been described in
bovine T cells and is distinct from E-selectin ligand 1 (Walchek and
Jutila, 1993
). Human analogs for either murine or bovine ligands have
not been reported.
B. Firm Adhesion
The initiating signal for the next step of TEM, that of firm
adhesion, is postulated to be either a receptor-mediated event in
response to an inflammatory cytokine or an event propagated from
signals from activated selectins. Cytoplasmic domains of bound and
activated L-selectin and PSGL-1 are linked to signal transduction
pathways that lead to integrin activation in PMNs (Crockett-Torabi and
Fantone, 1995
; Simon et al., 1995
, 1999
; Zimmerman et al., 1996a
;
McEver and Cummings, 1997
; Williams and Solomkin, 1999
). Thus,
selectins may function to promote the orderly transition to the
adhesion process by invoking integrin expression pathways in a timely,
sequential manner that ensures successful migration.
1. Integrins.
Integrins are a group of heterodimeric
transmembrane glycoproteins found on PMNs and other hematopoietic cells
that mediate cell-cell and cell-extracellular matrix adhesions (Hynes,
1992
; Luscinskas and Lawler, 1994
). All integrins comprise one
- and one
-subunit, which together form an extracellular ligand binding site. Cytoplasmic tails of integrins provide phosphorylation sites and
linkages to cytoskeletal proteins involved in signal transduction (Alpin et al., 1998
). There are 8 different
-subunits
(
1-
8) that associate
with one of 16
-subunits to form at least 23 known receptors in a
variety of cells, including lymphocytes, leukocytes, and platelets.
Integrins are capable of mediating cell-cell binding but also are
involved in cell interactions with extracellular proteins such as
laminin, fibronectin, vitronectin, and fibrinogen, to name a few.
2- (CD18) subunits.
These are macrophage antigen-1 (Mac-1;
M
2; CD11b/CD18) and
lymphocyte-associated function antigen-1 (LFA-1;
L
2; CD11a/CD18) (Fig.
1). LFA-1 is the predominate integrin used for lymphocyte emigration
(Li et al., 1996
X
2), can promote PMN trafficking under certain conditions. A fourth member of the
2-integrin family,
D
2, has been recently
described as a critical mediator of eosinophil adhesion. This integrin
has yet to be reported on PMNs (Grayson et al., 1998
among others. Inflammatory stimuli can also promote
transcription and translation of Mac-1 genes, thus prolonging integrin
involvement during inflammation.
Even when Mac-1 is incorporated into the plasma membrane of activated
cells, only a small percentage (~10%) may be competent for ligand
binding (Diamond and Springer, 1993
-subunit called "inserted domain" (I domain) that is
inaccessible to potential ligands unless integrins are activated by
intracellular or extracellular signals (Stanley and Hogg, 1998
-subunit is a series
of seven repeated sequences that form a
-propeller domain and
possess putative calcium binding sites that modify binding affinity to
ligands (Springer, 1997
-propeller and I domains are exposed for ligand binding. An
I-like domain also exists on the
-subunit and is necessary for
binding to some ligands.
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and IL-8 decrease
intracellular cAMP and thus release inhibition of Rho (Laudanna et al.,
19962. Intercellular Adhesion Molecules.
An important
complementary endothelial ligand for Mac-1 is intercellular adhesion
molecule-1 (ICAM-1) (CD54), an Ig-like molecule that exhibits low
constitutive presentation on EC membranes but is markedly induced by
exposure of ECs to inflammatory cytokines (Gerritsen and Bloor,
1993
; Hashimoto et al., 1994
; Klein et al., 1995
; Scholz et al.,
1996
; Iigo et al., 1997
). ICAM-1 is also found on tissue epithelial
cells including pneumocytes (Barton et al., 1995
; Burke-Gaffney and
Hellewell, 1996
). LFA-1 can bind to ICAM-1, but it has higher
affinity to a related protein, ICAM-2, a ligand to which Mac-1 binds
with less affinity. Recent in vitro studies with human umbilical vein
ECs demonstrate Mac-1-mediated adhesion and TEM that is partially
independent of both ICAM-1 and ICAM-2 (Issekutz et al., 1999
).
1- instead
of
2-integrins. VCAM-1 interaction with
1-integrins is critical to migration of
monocytes and eosinophils. Recently however,
4
1-integrin [very
late antigen-4 (VLA-4)] has been identified on both activated human
and rat PMNs and can mediate VCAM-1-dependent adhesion of PMNs to
endothelium in vitro (Reinhardt et al., 1997C. Polymorphonuclear Leukocyte Homing to Transmigration Sites on Endothelium
Egress of PMNs through EC monolayers occurs preferentially at
tricellular junctions (Burns et al., 1997a
). Although the mechanism of
extravasation is imperfectly understood, it is clear that selectin involvement has been down-regulated at this time and an IgG-type adhesion molecule, platelet-endothelial cell adhesion molecule-1 (PECAM-1; CD31), becomes critical for the actual passage of PMNs between ECs (Vaporciyan et al., 1993
; Dejana et al., 1995
; Newman, 1997
).
Found on PMNs, platelets, and ECs, PECAM-1 can serve as its own ligand
and form homodimers with molecules on opposing cells. PECAM-1 is evenly
distributed over the surface of circulating PMNs and is
concentrated at intercellular junctions of unstimulated ECs (Newman,
1997
). By virtue of its junctional location, PECAM-1 is hypothesized to
be a homing receptor to locate the transendothelial portal for the
migrating PMN. Treatment of PMNs or endothelial monolayers with
an antibody to PECAM-1 blocks transmigration in vitro (Muller et
al., 1993
; Muller, 1995
), and similar antibodies have been effective in
inhibiting PMN migration in rat models of peritonitis and alveolitis
(Vaporciyan et al., 1993
; Bogen et al., 1994
; Muller, 1995
). In both
whole animal and cell systems, PECAM-1 antibody treatment does not
block adhesion.
D. Transmigration
Compared with the aforementioned processes of margination and
adhesion, there is relatively little known of the mechanisms of PMN
egress and migration through the subendothelial matrix. PMNs possess
proteases capable of digesting collagen, laminin, and other
extracellular components present in the vascular wall. Adhesion and
migration is accompanied by release of PMN-derived proteases (Wright
and Gallin, 1979
; Hanlon et al., 1991
), and both chemotaxis and
migration through artificial substrates in vitro is inhibited by
treatment with antiproteases (Lomas et al., 1995
; Delclaux et al.,
1996
). Hence, a critical role has been proposed for PMN-derived
protease activity during extravascular transit. However, protease
inhibitors are ineffective in stopping PMN migration through intact EC
monolayers and basement membrane matrices in vitro (Allport et al.,
1997a
; Mackeral et al., 1999
). Furthermore, mice deficient in
gelatinase B (a PMN-derived collagenase) have normal PMN emigration
into the lungs, peritoneum, and skin (Betsuyaku et al., 1999
). Thus,
requirements for protease release and digestion of extravascular matrix
components during adhesion and migration are not certain.
Leukocytes bind to matrix components such as collagen, vitronectin, and
laminin via
1-integrins. As mentioned above,
activated PMNs can express the
1-integrin
VLA-4, and this allows for their binding to endothelial VCAM-1 in
vitro. Interaction with
1-integrins might also
be important for transit of PMNs through the extravascular milieu. For
example, PMN migration through lung or synovial fibroblast barriers has
recently been shown to require not only CD18
(
2), but also VLA-4, VLA-5
(
5
1), VLA-6
(
6
1), and VLA-9
(
9
1) (Shang and
Issekutz, 1997
; Shang et al., 1999
). VLA-5 and VLA-6 bind fibronectin
and laminin, respectively. Like VLA-4, VLA-9 interacts with fibronectin
and VCAM-1, but it also binds to the matrix protein tenascin.
Furthermore, VLA-9 is the most highly expressed
1-integrin on PMNs and is not expressed by
either lymphocytes or monocytes (Shang et al., 1999
). Fibroblasts and
parenchymal tissues such as pulmonary epithelium and hepatocytes
express both VCAM-1 and ICAM-1. As such, VLA-9 and Mac-1 appear to be
the more critical integrins responsible for PMN migration
through the subendothelium.
Lastly, there is evidence for the involvement of
PECAM in extravascular transit of leukocytes. Unlike the
homotypic interactions between PECAM molecules that mediate homing and
diapedesis, migration through the subendothelial environment requires
heterophilic binding of leukocytic PECAM-1 to an unidentified ligand
(Liao et al., 1995
; Wakelin et al., 1996
). Antibodies to an
amino-terminal domain of PECAM-1 inhibit homophilic binding, whereas
antibodies to a membrane-proximal extracellular domain of PECAM blocks
heterophilic interactions and movement across the basement membrane
(reviewed in Muller and Randolph, 1999
).
E. Migration as an Ordered Sequence of Events
Multiple lines of evidence suggest that selectin activation and
binding is required before firm adhesion can occur via Mac-1-ICAM-1 interactions (Lawrence and Springer, 1991
; von Andrian et al., 1992
;
Ley et al., 1995
). As mentioned previously, selectin-mediated signals are incorporated with other extracellular inflammatory stimuli
to regulate the timely expression and engagement of PMN integrins
(Crockett-Torabi and Fantone, 1995
; Crockett-Torabi, 1998
; Williams and
Solomkin, 1999
). For example, studies in vitro demonstrate that
cross-linking activation of L-selectin on PMNs can lead directly to
2-integrin-mediated adhesion in both static assays (Simon et al., 1995
, 1999
; Steeber et al., 1997
) and in shear-flow models (Gopalan et al., 1997
). Furthermore, removal of
L-selectin from PMNs renders them incapable of integrin-mediated firm
adhesion to endothelial monolayers (Endemann et al., 1997
; Zouki et
al., 1997
). These results are not surprising for flow systems;
L-selectin binds more rapidly and at higher shear rates than CD18
integrins (Taylor et al., 1996
). However, the requirement for
L-selectin in static systems suggests that selectin-binding is required
for more than just physical capture of PMNs. Stimulation of leukocytes
via L-selectin can activate G-proteins, tyrosine kinases, release of
ceramide, and assembly of filamentous actin (Brenner et al.,
1997
, 1998
; Simon et al., 1999
), events that may be a required prelude
to integrin-dependent adhesion. Firm adhesion by L-selectin activation
is blocked by inhibitors of protein tyrosine kinase and protein kinase
C, suggesting that these pathways link L-selectin to Mac-1
up-regulation (Steeber et al., 1997
).
Engagement of PSGL-1 can modulate Mac-1 function. Binding of PSGL-1 to
P-selectin activates tyrosine kinase and leads to the up-regulation and
avidity of CD18 on PMNs (Hidari et al., 1997
; Blanks et al., 1998
). In
addition, integrin-activating chemoattractants cause a redistribution
of PSGL-1 on PMNs and concomitant loss of P-selectin binding (Lorant et
al., 1995
). Thus, signal transduction pathways after selectin
engagement appear to promote "bond-trading" from selectins to
integrins by down-regulating the former and inducing functional
expression of the latter.
Activation of CD18 integrins, in turn, can lead to phosphorylation of
tyrosine residues on PECAM-1 and potentially modulate the function of
these adhesive proteins during PMN transmigration (Lu et al., 1996
).
Conversely, activation of PECAM-1 on PMNs with either cross-linking
antibodies or antigen-binding fragments can increase the
activity of Mac-1 (Berman and Muller, 1995
), suggesting that cross-talk
between the molecules occurs during TEM. Because the trailing end of
the migrating PMN must detach from ECs as the leading uropod attaches,
the coordination of both Mac-1-ICAM-1 and PECAM-PECAM interactions is
critical for successful diapedesis. Engagement of PECAM up-regulates
CD18 through phosphoinositide 3-kinase, which is a different pathway
for CD18 activation from G-protein-associated activators such as fMLP,
IL-8, and C5a (Jones et al., 1998a
; Pellegatta et al., 1998
). These
studies suggest that activation of PMN adhesion molecules via
integrin-ligand or selectin-ligand binding may differ from pathways
initiated by cytokine-receptor binding.
Binding of Mac-1 to ICAM-1 can cause structural changes in endothelial
cytoskeletal proteins associated with adherens junctions without
causing EC retraction or injury to monolayers (Del Maschio et al.,
1996
; Allport et al., 1997b
). The endothelial junctional proteins
plakoglobin, cadherin, and
- and
-catenin dissociate within
minutes of PMN binding to endothelium (Del Maschio et al., 1996
). It is
unclear if this reorganization is required for TEM. After prolonged
exposure of endothelial monolayers to inflammatory cytokines, PECAM-1
becomes diffusely distributed throughout the cell membrane and away
from intercellular junctions (Romer et al., 1995
), whereas L-selectin
ligands and ICAM-1 redistribute from random expression to localization
at cellular junctions (Bradley and Pober, 1996
). It is unknown what
role this plays in PMN emigration; in each system, neither ICAM-1,
PECAM-1, nor the L-selectin ligand was bound by PMN counterparts. Taken
together, control of TEM is dictated in part by intracellular signaling
and cross-talk between adhesive proteins and may be separate from
pathways elicited by cytokines and chemoattractants that require
receptor activation.
F. Soluble Mediators of Migration
Promigratory stimuli can be classified generally as either
nonchemotactic cytokines or chemoattractants. Canonical inflammatory cytokines, such as TNF-
and IL-1 can engender expression of adhesive proteins on PMNs and ECs, but they are not by themselves chemotactic for PMNs. For example, TNF-
and IL-1, along with other inflammatory mediators, promote the firm adhesion of PMNs to endothelium in systems
in vitro (Schleimer and Rutledge, 1986
; Huber et al., 1991
;
Burke-Gaffney and Hellewell, 1996
; Komatsu et al., 1997
). However,
egress and migration of PMNs into extravascular spaces requires the
presence of chemoattractants that cause the directed migration of PMNs
through tissue. Some chemoattractants can promote expression of
adhesion molecules on PMNs similar to the responses elicited by IL-1
and TNF-
.
1. Cytokines: Tumor Necrosis Factor-
and Interleukin-1.
Two of the most important pro-adhesive cytokines that are present
during most inflammatory responses are TNF-
and IL-1 (Table 1). The macrophage/monocyte is the
primary cellular source of both TNF-
and IL-1, and LPS is perhaps
their most important inducer (reviewed in Tracey and Cerami, 1993
,
1994
; Bemelmans et al., 1996
; Di Girolamo et al., 1997
). Among many
pathophysiologic effects of TNF-
are shock, cytotoxicity, and
cachexia. Binding to two different TNF-
receptors present on each
cell induces the effects of TNF-
on PMNs and ECs. PMNs can respond
to TNF-
by activating and expressing integrins, producing
platelet-activating factor (PAF) and other mediators, and releasing
granule contents. Likewise, ECs mobilize selectins, up-regulate ICAM-1,
and activate procoagulant pathways in response to TNF-
exposure.
During inflammation and endotoxemia, PMNs release from their membranes
a soluble TNF-
receptor that can bind to and effectively inactivate
circulating TNF-
.
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, and
the two are often found together in a variety of inflammatory
scenarios. Like TNF-
, IL-1 induces selectin and ICAM-1 expression on
ECs and promotes integrin activation on PMNs. Exposure of EC monolayers
or PMNs in vitro to TNF-
(Romer et al., 1995
and IL-1
treatment in vivo induces ICAM-1 in lung and small intestine (Komatsu
et al., 1997
and IL-1 in plasma
during inflammation is likely critical for the capture and firm
adhesion of PMNs to vascular endothelium. During inflammation, PMNs and
macrophages express on their membranes a receptor antagonist (IL-1ra)
that binds to IL-1, but it is not linked to signal transduction
machinery. A similar decoy receptor has not been identified in ECs.
Such receptors likely serve to regulate the magnitude of inflammation.
Although LPS, TNF-
, and IL-1 are not themselves chemotactic for
PMNs, their exposure to ECs can elicit TEM in vitro. This phenomenon is
dependent on cytokine-stimulated production of endothelial-derived chemoattractants that can be detected in the culture medium during migration assays in vitro (Huber et al., 19912. Chemoattractants.
PMNs have at least five different
receptors for chemotactic stimuli. Unique receptors exist for PAF,
complement protein C5a, leukotriene B4
(LTB4), and bacterial peptides (e.g., fMLP)
(Table 1). In addition, a growing number of chemokines and their
specific receptors are being discovered and characterized as important players in inflammatory responses of PMNs (Furie and Randolph, 1995
).
Chemoattractants/chemokines for PMNs can be produced by a wide variety
of cells, including endothelial and epithelial cells, macrophages,
monocytes, lymphocytes, platelets, PMNs, and parenchymal cells. In
models in vitro, chemoattractants can activate PMNs or ECs to express
adhesive proteins in a manner similar to TNF-
or IL-1. Thus,
redundant pathways for adhesive and migratory processes probably occur
in vivo (Detmers et al., 1990
, 1991
; Huber et al., 1991
).
- or CXC
chemokines, the cysteines are separated by an amino acid
(X). In
- or CC chemokines, the cysteines are adjacent to one another. The structural difference is related to their ability to
elicit distinct leukocyte migration. In general, PMN responses are
invoked by
-chemokines, and mononuclear cells respond most strongly
to
-chemokines.
In addition to IL-8, at least six other CXC chemokines mediate PMN
responses in humans: neutrophil-activating peptide-2, three forms (
,
, and
) of growth-related oncogenes/melanoma growth-stimulating activity, and epithelial cell-derived neutrophil-activating peptide-78. Different roles for two PMN receptors, CXCR1 and CXCR2, have been characterized. IL-8 can bind to either receptor, but the remaining six
proteins appear to have affinity for only CXCR2.
Rodents do not have an IL-8 analog and instead possess cytokine-induced
neutrophil chemoattractants (CINCs) that are similar to growth-related
oncogene proteins (Watanabe et al., 1993
and CINC-2
proposed. All four are
released from LPS-stimulated rat macrophages in vitro and possess
similar abilities to elicit chemotaxis and degranulation of PMNs
(Shibata et al., 1995
-chemokines and PMNs show a high-affinity receptor for MIP-2
and a shared receptor for the other three CINCs (Murakami et al., 19973. Cytokine-Chemoattractant Interaction during Migration.
Although LPS, TNF-
, and IL-1 are not chemotactic for PMNs, their
exposure to cultured cells can induce the production of chemoattractants such as IL-8 and PAF (Huber et al., 1991
; Kuijpers et
al., 1992
; Smart and Casale, 1994
; Burns et al., 1997b
). For example,
IL-8 released into culture medium after TNF-
stimulation of ECs can
cause CD18 up-regulation on PMNs (Huber et al., 1991
). In addition,
EC-derived IL-8 can become localized on the cell surface where it can
activate PMNs close to the vessel wall (Kuijpers et al., 1992
; Rot,
1993
; Imaizumi et al., 1997
). Indeed, treatment of EC monolayers with
antibody to IL-8 inhibits rolling PMNs from firmly adhering to the
monolayer (Rainger et al., 1997
), suggesting that localized IL-8 is
required for adhesion. The physical nature of the IL-8-EC relationship
is unclear. Rot and coworkers (Rot et al., 1996
) described in situ
binding of radiolabeled IL-8 to vascular endothelium in lungs, liver,
and kidney. However, there is no evidence for an IL-8 receptor or
binding protein on cultured or primary ECs in vitro (Petzelbauer et
al., 1995
; Rot et al., 1996
). IL-8 likely binds to components of the
vessel wall matrix such as heparan and glycosaminoglycans (Hoogewerf et
al., 1997
). Thus, localization might not be due to an EC component but
to extracellular sites near the cell.
, IL-1, and C5a.
Modulation of TEM by cross-talk among receptors for cytokines,
chemokines, chemoattractants, and LPS is depicted in Fig.
3. Although the mechanisms of
chemoattractant-induced depression of migration are not entirely clear,
results from studies in vitro suggest that receptor desensitization may
be involved (Campbell et al., 1997
can inhibit PMN
migration in models ex vivo and in vivo (Otsuka et al., 1990
is functionally linked
to chemotactic receptors (Schleiffenbaum and Fehr, 1990
and LPS down-modulate CXC receptors by a
metalloproteinase-mediated mechanism (Khandaker et al., 1999
|
, IL-1, IL-8, C5a, and other soluble mediators that are capable
of modulating PMN function (Wagner and Roth, 1999G. Implications
TEM of PMNs is a carefully orchestrated sequence of cell
activation, adhesion molecule expression, and molecular cross-talk between receptors on both cell types. As such, there are several opportunities to modulate and modify the migratory response by both
exogenous and endogenous factors. Several lines of evidence suggest
that integrin activation needs to occur at the PMN-EC interface and not
in the circulation. Premature activation of PMN integrins and
chemotactic receptors by circulating mediators in vivo might be avoided
by temporally and spatially controlled release and concentration of
chemokines at the endothelium-blood interface as suggested by Rot and
coworkers (Rot, 1993
; Rot et al.,1996
). Knowledge of the molecular
basis of control of TEM by endogenous factors (i.e., cytokines and
chemokines, etc.) may suggest how therapeutic interventions might be
formulated to alter undesirable recruitment of inflammatory cells. In
this regard, pharmacologic manipulation of adhesion molecules by a
variety of agents has shown promise in both human and animal models of inflammation. Several therapeutic approaches are discussed in the last
section of this review, but first the special case of adhesive
interactions that occur during pulmonary inflammation is presented in
the next section.
| |
III. Polymorphonuclear Leukocyte Migration in the Lungs |
|---|
|
|
|---|
Most of the known molecular and cellular mechanisms of TEM have been elucidated from studies in vitro and in systemic vessels in the mesentery and dermis. Recent work in lung, however, suggests that PMN trafficking in the pulmonary circulation is fundamentally different from events in the systemic vasculature. In animal models tested so far, PMN behavior in lungs differs with respect to 1) the marginated pool of PMNs, 2) the site of TEM, and 3) requirements of adhesion molecules for PMN extravasation (Fig. 4). Although the basic mechanisms of TEM remain incompletely understood, these three differences suggest that different paradigms are needed to explain the kinetics of PMN migration in different vascular beds.
|
A. Marginated Pool of Polymorphonuclear Leukocytes
Margination of PMNs is a prerequisite for their escape from the
main flux of flowing blood, where they can sense and respond to
inflammatory signals present in vessel wall microenvironment. The
concentration of PMNs within pulmonary capillary blood is 35 to 100 times greater than in large vessels of the systemic circulation
(Doerschuk et al., 1987
, 1993
; Gee and Albertine, 1993
). In the
systemic circulation, margination occurs as rolling in postcapillary
venules and is mediated by L-selectin and P-selectin and their
respective ligands (Butcher, 1991
; Lawrence and Springer, 1991
;
Spertini et al., 1991
; Tozeren and Ley, 1992
). However, 97% of
pulmonary vascular PMNs are found in the capillary network where
vessels are too small to allow rolling (2-15 µm) (Doerschuk et al.,
1993
). Selectin-mediated rolling still occurs in pulmonary venules
(>15 µm), but it makes a small contribution to the total pool of
marginated PMNs in the pulmonary circulation.
Despite the lack of rolling in pulmonary capillaries, selectins may be
involved in the PMN-EC interactions. For instance, treating rabbits
with fucoidan, a polysaccharide made mostly of sulfated fucose that
inhibits binding of all selectins, decreases the frequency of PMN
stopping in capillaries by 25% and the duration of stops by 50%
(Kuebler et al., 1997
). Similar results are seen in fucoidan-treated
rats, in which PMN localization in alveolar capillaries is decreased by
15% (Yamaguchi et al., 1997
). In the same study in rats, an antibody
to P-selectin was ineffective in slowing PMN transit through the lungs,
whereas rolling on systemic venules was significantly inhibited.
Because E-selectin is not expressed in noninflamed rat lungs, all
selectin-mediated binding is therefore due to L-selectin. Furthermore,
despite the expression of ICAM-1 in normal rat pulmonary capillaries,
treatment with antibodies to ICAM-1 does not affect PMN transit times
(Yamaguchi et al., 1997
). Thus, during nonpathologic conditions,
L-selectin is the only adhesion molecule likely to be involved in
pulmonary PMN margination in rat.
Another factor to account for the dramatic extent of pulmonary PMN
margination may be the discrepancy between neutrophil and capillary
diameters (Downey et al., 1990
). PMNs that are 7 to 8 µm in diameter
must deform to an elongated shape to pass through capillaries, which
average 5 to 6 µm and can be as small as 2 µm (Hogg, 1987
;
Doerschuk et al., 1993
; Wiggs et al., 1994
; Gebb et al., 1995
). Shape
change in PMNs takes much longer than in erythrocytes, and this likely
accounts for their longer pulmonary transit time compared with
erythrocytes (2.7 versus 1.3 s) (Hogg et al., 1988
; Lien et al.,
1991
).
A dramatic increase in pulmonary margination is accompanied by
neutropenia when animals are given i.v. LPS or zymosan-activated serum
(ZAS; a source of complement activation products; C5a) (Haslett et al.,
1987
; Doerschuk, 1992
). Capillary sequestration in these models may be
due to lack of PMN deformability, inasmuch as these same agents cause
actin polymerization and PMN stiffness in vitro (Erzurum et al., 1992
).
B. Site of Migration
Given their slow transit and intimate contact with capillary
endothelium, PMNs are well positioned to respond to inflammatory signals generated within airspaces. Thus, in contrast to migration predominately from postcapillary venules in the systemic circulation, PMNs primarily extravasate from the alveolar capillary network in
rabbits, rats, and mice (Doerschuk et al., 1989
; Walker et al., 1991
;
Doerschuk, 1992
; Downey et al., 1993
). It is curious, therefore, that
P-selectin is constitutively expressed on pulmonary arterioles and
venules in rabbits and not on pulmonary capillary ECs where margination
and extravasation occurs (Mulligan et al., 1992b
). In rat, by contrast,
P-selectin is absent in the normal lung, but ICAM-1 is constitutively
expressed on capillary and venular endothelium (Yamaguchi et al.,
1997
). As discussed previously, ICAM-1 associates with CD18 to mediate
the firm adhesion of PMNs and usually requires induction for full
responses. Although ICAM-1 has a low constitutive level of expression
in most tissues, it is 30-fold higher in lung tissue (Panes et al.,
1995
). Thus, pulmonary leukostasis after infusion of LPS or ZAS may be
due to the high level of ICAM-1 in lung (Haslett et al., 1987
;
Doerschuk et al., 1989
; Doerschuk, 1992
). Both LPS and ZAS are capable
of mobilizing CD18 on isolated PMNs, and similar up-regulation of CD18
in vivo would provide ligands for lung ICAM-1 (Erzurum et al., 1992
;
Klut et al., 1997
). Therefore, by virtue of the density in adhesion molecules and slow PMN transit times, PMN adhesion and extravasation is
favored in the pulmonary capillaries over downstream venules.
C. Adhesion Molecule Requirements
1. Integrins.
Another important difference between the
pulmonary and systemic circulations is in the mechanics of PMN
diapedesis, or egress of the PMN through the vessel wall. In all models
tested so far, systemic PMN migration from postcapillary venules
requires CD18 integrins. However, pulmonary PMN migration can occur
independently of CD18 adhesion molecules. Whether or not migration
depends on CD18 varies with the intrapulmonary stimulus. IL-1, phorbol
myristate acetate, and Gram-negative bacterial stimuli, including LPS,
elicit migration via pathways predominately mediated by CD18 (Doerschuk et al., 1990
; Hellewell et al., 1994
; Qin et al., 1996
; Ramamoorthy et
al., 1997
). By contrast, Gram-positive bacteria, hydrochloric acid, and
C5a elicit pulmonary PMN recruitment that is mostly independent of
CD18. The adhesion molecules needed for CD18-independent migration in
lung are unknown.
; Freeman et al., 1996
;
Beck-Schimmer et al., 1997
). At the same time, expression of L-selectin
and CD18 is unchanged on PMNs in the vascular compartment, and only
after PMNs arrive into airspaces is L-selectin shed and CD11/CD18
expression increased (Burns and Doerschuk, 1994
). By contrast, during
CD18-independent migration to Gram-positive organisms, there is no
change in ICAM-1, yet CD11/CD18 is up-regulated on vascular PMNs before
migration and is expressed to an even greater degree on airway
PMNs. Thus, an inverse relationship exists between the requirement for
CD18 during PMN migration and CD18 expression on vascular PMNs before
migration. As described previously, PMNs can express and adhere via
1-type integrins to a variety of substrates, including cardiac myocytes, ECs, fibroblasts, fibronectin, and laminin
(Reinhardt et al., 1997
; Shang and Issekutz, 1997
). It remains
unresolved if
1-type integrins mediate
CD18-independent migration in vivo. Furthermore, studies in vitro of
CD18-independent TEM have failed to implicate
1-integrins or any other adhesion molecule
examined (Issekutz et al., 1995
).
2. Selectins.
It is unclear if selectins are responsible for
CD18-independent migration in vivo. Studies using knockout mice and
blocking antibodies that target selectins have not provided definitive answers. Mice deficient in L-, P-, or E-selectins exhibit PMN rolling
behavior that is compromised in both normal and inflamed venules in the
systemic circulation (Mayadas et al., 1993
; Johnson et al., 1995
;
Kunkel and Ley, 1996
). In a model of pneumonia using Streptococcus pneumo