Elsevier

Biochemical Pharmacology

Volume 77, Issue 4, 15 February 2009, Pages 597-607
Biochemical Pharmacology

Review
The immune phenotype of AhR null mouse mutants: Not a simple mirror of xenobiotic receptor over-activation

https://doi.org/10.1016/j.bcp.2008.10.002Get rights and content

Abstract

Intrinsic and induced cell differentiation and the cellular response to endogenous and exogenous signals are hallmarks of the immune system. Specific and common signalling cascades ensure a highly flexible and adapted response. Increasing evidence suggests that gene modulation by the aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor, is an important part of these processes. For decades the AhR has been studied mainly for its toxic effects after artificial activation by man-made chemical pollutants such as dioxins. These studies gave important, albeit to some extent skewed, evidence for a mechanistic link between the AhR and the immune system. AhR null mutants and other mutants of the AhR signalling pathway have been generated and used to analyse the physiological function of the AhR, including for the developing and antigen-responding immune system. In this review I look at the natural immunological function(s) of the AhR.

Introduction

The aryl hydrocarbon receptor (AhR) signalling pathway is evolutionarily conserved, and can act independently or in concert with other signalling pathways. Similar to steroid hormones, the AhR molecule is a ligand-activated gene transcription factor. Residing in the cytosol chaperoned by hsp90, AIP, and p23, the AhR dissociates these proteins upon ligand binding to translocate into the nucleus. In the nucleus, the AhR dimerizes with aryl hydrocarbon nuclear translocator (ARNT), and eventually binds to small conserved promoter elements called xenobiotic response elements (XREs) for transcriptional regulation in cooperation with co-factors. The AhR is then exported to the cytosol and degraded [1].

Numerous genes contain XREs in combination with other responsive elements in promoter specific patterns, thus the ligand-bound AhR regulates a plethora of genes in a cell-, tissue- and condition-specific fashion [2], [3].

Biased by its discovery as a regulator of xenobiotic metabolizing enzymes in vertebrates more than 30 years ago [4], the AhR has long been studied for its pathological activity in response to man-made environmental pollutants. In particular halogenated polycyclic aromatic hydrocarbons (PAH), such as dioxins attracted attention and raised concern. The AhR mediates toxicity, mainly through alterations of gene expression as outlined above. The toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a prototypic ligand of the AhR, and other PAHs are far-reaching and include alterations in lipid metabolism, skin physiology, tumour promotion, and embryonic development. Last, but not least, the immune system is a very sensitive target of AhR-mediated toxicity, responding at particularly low concentrations of chemical exposure [5].

As the AhR is evolutionary old, with members of the family already present in fungi, insects, or nematodes, and expressed constitutively, but tissue-specifically, a physiological role beyond responding to man-made chemicals is commonly postulated (reviewed in [6]).

Studies with (i) persistent activation of the AhR by e.g. TCDD, (ii) with AhR null mice, hypomorphs or natural low-affinity mutants, and later (iii) with strains with cell-specific conditional AhR deletions, confirmed multiple physiological roles of the AhR (reviewed in [6]). In brief, the AhR is a regulator of cell proliferation, e.g. via induction of Cdk2, or by physical interaction with the retinoblastoma protein. It cooperates and cross-talks with other signalling pathways, shown for instance for the estrogen pathway, NFκB, or cAMP [7], [8], [9]. The AhR induces oxidative stress, and may play a role for cell migration and adhesion [6], [9], [10], [11]. AhR activity is highly cell-specific and controlled at multiple levels. Receptor affinity, expression level, signalling crosstalk, feed-back inhibition by the AhR-repressor, competition for the dimerization partner ARNT, and/or competition for transcription co-factors participate in the outcome of AhR-activation [1]. The AhR is quite promiscuous and accepts chemically very different ligands [12]. The ligand determines to a considerable extent the outcome of AhR-activation, albeit only one binding site exists [13]. Subtle changes in protein conformation, or quick degradability may be reasons [14]. Last but not least, some ligands are persistent, while others have a high metabolic turnover rate. This can result in different outcomes of AhR-activation [15], [16]; in addition to the anthropogenic chemicals such as PAHs, numerous natural ligands (i.e. not anthropogenic) and endogenous ligands (made by the organism itself) have been identified and continue to be found. Interestingly, UVB radiation present in sunlight turns tryptophan into 6-formyl-[3,2b] indolo-carbazole (FICZ), a high-affinity ligand [17], [18]. Other endogenous ligands are heme metabolites, indigo derivatives, and leukotrienes [12], [19], [20], [21], [22]. Over-activation of the AhR by various ligands, and the ensuing consequences for the immune system are the topic of a review by Nancy Kerkvliet in this issue. My review focuses on AhR-deficiency, and the outcome for the immune system, in particular, I will discuss and compare the results derived from genetically engineered null mutant mice.

Section snippets

Murine mutants of the AhR signalling pathway

In the 90s, three groups generated AhR-deficient mouse mutants independently, by either deleting exon1 or exon2. In the null mutant made in the laboratory of Frank Gonzalez, exon1 is replaced from the translational start site onwards with a neomycin gene [23]. A Japanese group around Yoshiaki Fujii-Kuriyama replaced part of exon1 with the bacterial β-galactosidase gene joined to a nuclear localization signal, allowing to screen for AhR expression [24]. In the null mutant made in the laboratory

The immune system in AhR-deficient mice

The immune system is a complex organ with highly diverse functions, short- and long-distance interactions, and memory capacities. Immune cells communicate directly with each other by cell surface structures, or over considerable distances via lymphokines and chemokines. Lymphoid organs provide relevant spatial structures for direct communication of immune cells. Immune cells follow their intrinsic programmes, and/or adapt to external cues, relayed into the cells by a number of signal

An extrinsic rather than an intrinsic role of the AhR?

Immune phenotypes of naïve mice and in infection models contributed to the understanding that AhR-over-activation is only one side of the coin, yet the better studied one. AhR over-activation by environmental pollutants is of concern for public health. Insights into AhR biology point to chances for pharmacological manipulation [61], [63]. The role of the AhR in the “untouched” state appeared unexpectedly more subtle. However, does an “untouched” state really exist? Gene expression profiling for

Summary

Three null mutants of the AhR allowed tackling the two questions, namely (i) to what extent immunotoxic events after AhR ligand exposure to environmental chemicals are AhR dependent, and (ii) whether and how the AhR plays a role for a functioning immune system. Some differences in immune phenotype were noted in the null mutant mice (such as splenocyte numbers at certain ages), but these differences reported early after generation of the mice can be explained by mixed genetic background,

Conclusion and outlook

In conclusion, the AhR seems particularly relevant for the differentiation and balance of T cell subsets in ongoing immune responses, and for the decision of the immune system to tolerate or fight antigens. The AhR thus links the immune response to environmental factors, and may help control the risk of developing adverse immune reactions.

Further research will have to focus on the role of individual ligands in shaping these responses, elucidating the environmental risks for autoimmunity,

Acknowledgements

I thank Drs. Heike Weighardt, Bettina Jux, and Nancy Kerkvliet for critical reading of the manuscript. The work in my laboratory is supported through grants of the Bundesministerium für Umwelt, the Deutsche José Carreras Stiftung für Leukämieforschung, and the Deutsche Forschungsgemeinschaft (GRK1427).

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