Autoimmunity and the environment
The aryl hydrocarbon receptor in
immunity
Charlotte Esser1
, Agneta Rannug2
and Brigitta Stockinger3
1
Institut fu¨ r Umweltmedizinische Forschung, Auf’m Hennekamp 50, 40225 Du¨ sseldorf, Germany
2
Institutet fo¨ r miljo¨ medicin (Institute of Environmental Medicine), Karolinska Institutet, Box 210, 17177 Stockholm, Sweden
3
Division of Molecular Immunology, MRC National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, UK
Low-molecular-weight chemicals or xenobiotics might
contribute to the increasing prevalence of allergies and
autoimmunity. Certain chemicals can alter immune
responses via their action on the cytosolic transcription
factor aryl hydrocarbon receptor (AhR). AhR recognizes
numerous small xenobiotic and natural molecules, such
as dioxin and the tryptophan photoproduct 6-formylindolo[3,2-b]carbazole.
Although AhR is best known for
mediating dioxin toxicity, knockout studies have indicated
that AhR also plays a role in normal physiology,
including certain immune responses. In particular, Th17
cells and dendritic cells express high levels of AhR. We
review here current evidence for the physiological role of
AhR in the immune system, focussing in particular on Tcell
biology.
Introduction
The incidence of autoimmune and allergic diseases in the
developed world has been increasing over the last few
decades and a growing body of evidence indicates that
chemicals of low molecular weight (<1000 Da) are an
important contributor to this phenomenon. Polycyclic aromatic
compounds, which are formed during the combustion
of organic materials and are therefore found in cigarette
smoke, wood smoke and automobile exhaust gases, are
increasingly linked to immune-related diseases. Such lowmolecular-weight
chemicals are also ubiquitous as food
components, in life-style products (e.g. cosmetics) and as
environmental pollutants. They can form protein adducts
and haptenize antigens, expose cryptic antigens and act as
endocrine disruptors. The immune system in turn
responds to neo-antigens generated by small chemicals.
Small chemicals can also take part in normal immunological
differentiation and signaling pathways, with glucocorticoids
a well-known example. Immunotoxicology occurs
via adverse interference by small chemical xenobiotics
with the immune system, such as allergic reactions to
urushiol (the active ingredient of poison ivy), drug-induced
autoimmunity, or immunosuppression by 2,3,7,8-tetrachlorodibenzo-p-dioxin
(referred to as dioxin hereafter).
Moreover, immunopharmacologists are interested in small
chemicals in their search for immunomodulating compounds
that could be used to mimic or specifically block
immune functions.
The transcription factor aryl hydrocarbon receptor
(AhR) is a cytosolic sensor of small synthetic compounds
(called xenobiotics by toxicologists) and natural chemicals,
which act as its ligands (also called agonists). Ligand
binding induces a conformational change in AhR, thereby
exposing a nuclear translocation site. Ligands can be of
diverse chemical structure and need to meet only minimal
requirements for size and planar shape. AhR is chaperoned
by heat shock protein 90, p23 and AhR-interacting protein
in the cytosol. Both allele and species differences account
for the range of AhR ligand affinities, which can differ by
orders of magnitudes (Box 1). AhR was first discovered as a
mediator of dioxin toxicity, including immunotoxicity. Persistent
triggering of AhR results in other pathological
effects in both animals and humans [1,2].
AhR is highly conserved in evolution and is present in
many cell types, albeit at differing abundance [3–5]. The
selective forces that led to the high degree of conservation
of the AhR amino acid sequence are unknown and its
physiological function(s) are still being elucidated. At present,
it is clear that AhR has a dual role as an activator of
metabolism of small molecules and as a player in many cell
functions, including the immune system. As discussed
below, AhR might link adaptive immune responses to
environmental factors. AhR null mutant mice, mice with
a constitutively active AhR and several other mutants of
the AhR signaling pathway have been generated and used
to analyze the physiological function of AhR, including its
role in the developing immune system [6]. In parallel,
endogenous and/or exogenous natural AhR ligands – long
enigmatic – have been discovered, providing important
insights into the physiological functions of AhR. In this
review we consider AhR signaling, the endogenous lowmolecular-weight
chemicals that trigger it and its effects
on natural immunological function(s). In particular we
focus on the role of AhR in T-cell biology and its suggested
link to autoimmune diseases.
Brief overview of the biochemistry of AhR signaling
AhR is a member of the bHLH-PAS protein family found in
organisms as diverse as Caenorhabditis elegans (nematode),
Drosophila melanogaster (insect) and mammals
[3]. bHLH-PAS proteins are biological sensors for a variety
of stimuli, controlling neurogenesis, vascularization, circadian
rhythms, metabolism and stress responses to
hypoxia, among others.
Review
Corresponding author: Esser, C. (chesser@uni-duesseldorf.de).
1471-4906/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.it.2009.06.005 447
During canonical signaling, cytosolic AhR binds to a
suitable small chemical (the ligand), which facilitates AhR
translocation to the nucleus and eventually results in de
novo transcription of target genes. The promoters of AhR
target genes have the responsive element 50
-TNGCGTG-30
,
termed DRE or XRE for dioxin or xenobiotic responsive
elements. The genes for xenobiotic-metabolizing enzymes
(e.g. cytochrome P450) are well-known targets of AhR and
are often called AhR battery genes. Hundreds of other
genes also have DREs [7]. Elucidation of the biochemistry
of canonical AhR signaling revealed several parameters
that can fine-tune AhR activity. These include ligand
characteristics, adapter molecules and transcriptional
co-activators or co-repressors that regulate the extraordinary
cell-specific activity of AhR [8,9].
In recent years a alternative pathway of AhR signaling
has also been demonstrated. For instance, AhR can bind to
retinoblastoma protein, estrogen receptor (ER), the transcription
factor E2F1 and to the NFkB pathway subunits
RelA and RelB [10–13]. Evidence of AhR cross-talk with
other signaling pathways, such as via kinases (src, JNK,
p38, MAPK) or competition for transcription cofactors, has
also been reported. AhR can act as a ubiquitin ligase,
targeting the ER for proteasomal degradation [8,14,15].
In these signaling pathways, AhR and the other proteins
sometimes mutually repress each other’s function. Indeed,
bioinformatics analysis points to the existence of complex
signal cross-talk between AhR and further transcription
factors or transcription co-activators [8,14,16]. The downstream
targets of these pathways differ; for example, IL-2
can be induced via canonical signaling [17] and IL-8 via
RelB [18], but the full biological complexity has yet to be
elucidated.
Exogenous and endogenous ligands critically shape
AhR function
Dioxin is widely used as a surrogate ligand for AhR but its
use is problematic in revealing the true function of AhR
because it is not quickly metabolized in the body (its halflife
is $2 weeks in mice and several years in adult humans
[19,20]). Indeed, a proper understanding of AhR biology
must differentiate between effects triggered by toxic
ligands such as dioxin and physiological effects triggered
by endogenous ligands. Simple extrapolation of data
obtained for one ligand to all others would not correctly
reflect what is currently known about AhR biology. At the
very least, the dose-dependent and cell-specific potential of
toxic ligands to kill cells must be considered.
Although it is possible that AhR might function in the
absence of a ligand (i.e. spontaneously), overwhelming
evidence suggests that ligands are required [21]. A number
of low-molecular-weight chemicals qualify as endogenous
or physiological AhR ligands (which is not necessarily the
same), that is, they have binding dissociation constants
(Kd) and effective concentrations at the level expected for a
physiologically relevant receptor ligand [22,23]. Physical
fluid shear stress (which causes oxidation of low-density
lipoproteins), the second messengers cAMP and Ca2+
,
serum and growth medium components all activate AhR
responses [14,24,25]. This list of ligands is impressive and
striking in its diversity; however, it remains to be determined
which of these are actually relevant in a physiological
or immunotoxic sense. AhR has not yet been
crystallized, so information on ligand-dependent structural
changes is currently lacking. Ligand-protected protease
digestion studies indicated that only one binding
pocket for ligands exists [26]. Because different ligands
result in different outcomes [23], it is generally assumed
that the AhR signaling pathway uses ligands in an adaptive
fashion, depending on the tissue and situation.
A tryptophan photoproduct is a high-affinity AhR ligand
Two tryptophan-derived molecules of an indolo[3,2-b]carbazole
type were identified in 1987 [27]. They were
assumed to be endogenous AhR ligands because they could
bind to the receptor with the highest affinity of any known
compound, including dioxin. These two compounds were
detected in experiments designed to produce oxidation
products of adenine for testing as possible AhR ligands.
An unfiltered high-pressure mercury UV lamp was used in
the oxidation of adenine and tryptophan was added as a
photo-sensitizing molecule. This led to the unexpected
observation that dilute aqueous solutions containing
UV-exposed tryptophan could compete efficiently with
3
H-labeled dioxin for AhR binding. Moreover, it was later
shown that AhR-mediated expression of cytochrome P450,
family 1, subfamily A, polypeptide 1 (CYP1A1) was amplified
in human, mouse and rat cells exposed to visible or UV
light in the presence of tryptophan [28,29]. Intensive studies
to characterize the chemically active tryptophan
photoproducts identified the compounds 6-formylindolo[3,2-b]carbazole
(FICZ) and 6,12-diformylindolo[3,2b]carbazole
(dFICZ) and a procedure to synthesize these
has now been described [30]. To date, FICZ has been
detected in aged batches of tryptophan, in light-exposed
solutions of tryptophan, in cell culture media and in living
cells [31,32]. Recently, sulfate conjugates of the compound
FICZ were found to be present in human urine [33]. FICZinduced
autocatabolic breakdown via the cytochrome P450
Box 1.
Biochemistry of AhR signaling
AhR is a transcription factor that resides in the cytosol and is a
member of the evolutionarily ancient bHLH-PAS protein family. AhR
undergoes conformational change on binding of small ligands (also
called agonists), which exposes a nuclear translocation site. AhR
translocates to the nucleus and binds to its dimerization partner
ARNT (another bHLH-PAS protein) and the AhR-ARNT complex
initiates transcription of genes with promoters containing a dioxinresponsive
element (DRE) consensus sequence. Ligands only need
to meet minimal requirements for size and planar shape to fit into
the AhR binding pocket. Consequently, a broad range of lowmolecular-weight
chemicals can activate AhR, albeit at different
affinities ranging between 10À12
and 10À3
M. Many ligands have two
carbon ring systems, such as tryptophan derivatives, flavonoids and
biphenyls. The AhR system is genetically polymorphic and different
alleles influence responsiveness to AhR ligands. Inbred mouse
strains are classified into high- and low-responder strains on the
basis of their ability to induce AhR-dependent cytochrome P450 1A1
expression. For instance, the C57BL/6 strain carries the AhRb
highresponder
allele, whereas DBA/2, NOD, and 129 mice have the lowresponder
AhRd
allele. Experiments in cell lines have demonstrated
that human AhR has at least 10-fold lower dioxin sensitivity than the
known murine strains.
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448
enzymes CYP1A1, 1A2 and 1B1 is highly efficient and
seems to be limited only by the rate of diffusion of the
compound [33,34]. The high specificities and catalytic efficiencies
observed, which lead to low intracellular steadystate
levels of FICZ, support the argument that this molecule
is an endogenous AhR ligand of high importance.
Dioxin and FICZ are both lipophilic molecules that are
similar in size and planarity. They bind to AhR with very
high affinity and are efficient inducers of the AhR gene
battery, including xenobiotic-metabolizing enzymes
(XMEs). The different kinetic patterns of AhR responses
are the most important differences between the two compounds
because FICZ, in contrast to dioxin, is metabolized
by XMEs. This results in efficient autoregulatory feedback
and transient activation of AhR.
The formation of a chemical messenger molecule with
high AhR binding affinity thus explains cutaneous and
extracutaneous induction of CYP1A1 enzyme activity after
dermal exposure to UV light [32,35]. It remains to be
shown whether UV light mediates enhanced metabolism
of polycyclic aromatic hydrocarbons and other environmental
pollutants to which humans are exposed; if so, this
could play a role in rapid elimination of topical carcinogenic
substances [36]. Equally speculative, UV-generated
FICZ might also provide an explanation for the photosensitivity
observed in lupus erythematosus [37]. In the epidermis,
keratinocytes and Langerhans cells (LCs) express
AhR and are therefore targets of light-generated FICZ.
Surprisingly, in dioxin-treated LCs, canonical AhR signaling
is repressed and LCs do not express any of the
metabolic enzymes normally induced by AhR activation.
However, the biological significance of this is not clear.
AhR and the immune system
Immunotoxicological evidence
In vitro and in vivo immunotoxicological studies with
dioxin in relation to persistently activated AhR have
revealed drastic changes in thymocyte lineage decisions,
shifts in immune-cell subset frequencies, aberrant cytokine
secretion and many other effects on immune functions
(Figure 1). Among the most sensitive outcomes of dioxin
exposure in animals is strong systemic immunosuppression
of the humoral, cellular and innate immune responses.
The cause–effect relationships are largely unclear and the
fact that only a few immune cell subsets (see below)
actually express AhR (although many seem to be affected
by dioxin) suggests that the answers will be very complex.
For more details on immunotoxicology, readers are
referred to recent reviews [2,38].
Exposure routes and ligands
Environmental exposure to AhR ligands is typically via
the barrier organs of the gut, lung and skin, which are also
highly active immune sites in their own right. The oral
route accounts not only for 90% of human dioxin exposure
[39], but also for many other AhR ligands, including
normal dietary components such as flavonoids and indoles
present in fruit and vegetables. Examples of quantitatively
relevant dietary components are quercetin in apples,
resveratrol in red wine and indole-3-carbinol in many
cruciferous plants [39] (Table 1). Apart from UV-generated
FICZ, the skin comes into contact with AhR ligands
present in cosmetics and plants, as well as those generated
by the common skin-residing yeast Malassezia furfur.
Dietary ligands are often found in blood and although
Figure 1. Schematic compilation of current evidence of the influence of AhR on immune function and differentiation. The major lymphoid and myeloid differentiation lines
are shown. The red flashes indicate cells where AhR overactivation by in vivo or ex vivo exposure to dioxin has been reported to result in immunotoxic effects (e.g. biased
differentiation, lack of cytokine secretion, or altered function). It is important to note that many of these effects are presumably indirect effects because some of the cells
affected express little or no AhR. Blue asterisks indicate cells for which AhR expression has been experimentally confirmed by either RNA measurements or Western
blotting. Green flashes indicate evidence of a direct physiological role for AhR (e.g. necessary for cell-specific expression of characteristic genes, involved in maturation or
differentiation). DC, dendritic cells; DN, double negative thymocyte; DP, double positive thymocyte; HSC, hematopoietic stem cell; LC, Langerhans cell; nTreg, natural T
regulatory cell; PC, plasma cell; iTreg, induced T regulatory cell.
Review Trends in Immunology Vol.30 No.9
449
not rigorously demonstrated for all ligands, it can be
expected that most ligands circulate freely in the body,
where they can activate AhR. Surprisingly, almost no
research has been carried out on the effects of AhR ligands
(exogenous or endogenous) on mucosal lymphoid tissue,
especially the gut, which conceivably could be different
from the effects on peripheral immune cells.
Evidence from AhR-deficient mice
A good understanding of the physiological role of AhR, in
particular its role in the immune system, has still not been
achieved. Information regarding its physiological role(s),
however, has been provided by studies of mouse lines
expressing the high- AhRb
versus low-affinity AhRd
alleles,
by recombinant mouse models in which AhR has been
mutated and by the use of cre-lox conditional knockout
mice [6]. AhR knockout mice have lower life expectancy
and fecundity and exhibit a spectrum of hepatic defects and
skin abnormalities without exposure to exogenous chemicals,
suggesting important roles of AhR in many physiological
functions [38,40]. For instance, hyperproliferative,
fibrotic and inflammatory lesions were one of the original
observations made in AhR-deficient mice, in addition to
abnormalities in vascular and hematopoietic development
[41]. AhR-deficient mice also exhibit liver deformation
characterized by a failure to close the ductus venosus (a
blood vessel shunt in the liver) after birth, resulting in lifelong
impairment of blood flow through the liver [42].
AhR-deficient mice do not have an overt immunological
phenotype, although one of the knockouts generated had
decreased lymphocyte numbers in the spleen and lymphocyte
infiltration of lung, intestine and urinary tract. However,
these immunological alterations were not detected in
a second, more widely used AhR knockout mouse [43]. The
reasons underlying these discrepancies have not yet been
resolved but might be related to the general health status
of different animal facilities. However, as detailed in a
recent review [6], AhR acts as an important co-factor in
infections. For instance, AhR-deficient mice infected with
Listeria monocytogenes, an intracellular bacterium, were
more susceptible to infection but developed enhanced
resistance to re-infection [44]. Furthermore, AhR-deficient
mice have an increased leukocyte count and a diminished
capacity to maintain the hematopoietic stem cell compartment
[40]. Regarding the skin, primary LCs isolated from
mice that lack AhR poorly upregulate CD40 and CD80
after ex vivo maturation and lose the capacity to produce
tolerogenic indoleamine-2,3-dioxygenase [45]. It is conceivable
that this points to a role for AhR in LC maturation
and host immune responses.
AhR and interactions with NFkB
With regard to the role of AhR signaling in immune functions,
the non-canonical interactions between NFkB,
Stat1, Stat5 and the AhR are especially intriguing. NFkB
activators such as the inflammatory cytokines IL-1b, IL-6,
TNFa and lipopolysaccharide suppress constitutive and
AhR-induced CYP1A1, suggesting a link between inflammation,
oxidative stress and CYP expression [46]. Conversely,
AhR agonists have been shown to repress or
transactivate NFkB response genes [18,46]. Further evidence
of cross-talk between AhR and other signaling
modules has come from studies showing that AhR can
be co-immunoprecipitated with Stat1 and Stat5 [47].
Presumably, this interaction is relevant for the abrogation
of Stat1 signaling and hence Th17 differentiation [47]
(see below).
AhR and T cell subsets
Immune cell-specific AhR expression
Liver and lung exhibit high levels of AhR expression.
However, certain hematopoietic stem cells, some dendritic
cells, particular subsets of thymocytes and T-cells have
similar or even higher levels of AhR expression than the
liver [5,48,49]. Immune functions and immune cells can be
targeted directly or indirectly by AhR activity. Unfortunately,
data on AhR protein levels are not available for
many immune cell subpopulations, limiting the interpretation
of causative AhR-ligand effects. Recently, studies
using cell sorting combined with RT-PCR and Western
blotting have identified Lin-Sca+
and ScaÀ
progenitor cells
in bone marrow (BM), double negative (CD4À
CD8À
) DN4
cells in thymus, CD4+
Th17 cells, BM-derived dendritic
cells and LCs as subpopulations with high AhR levels
[40,45,50]. A careful and wide-ranging re-analysis of highly
pure immune cell subsets for AhR expression would be in
helpful in untangling the cell-specific actions of AhR sig-
naling.
AhR and T-helper cells, regulatory T-cells and dendritic
cells
Considering the wide-ranging immunosuppressive effects
of dioxins, some researchers have investigated the potential
of AhR ligands to manipulate immune responses. In
particular, two compounds with AhR-binding capacity
have been studied. VAF347, a water-soluble ligand of
AhR, suppressed graft rejection in a mouse transplantation
model [51]. The evidence presented so far suggests
that shifts in T subset frequencies, as well as effects on
dendritic cells, are relevant for these effects [52]. Another
ligand, a compound named M50367, has been tested in
Table 1. Some clinically or quantitatively relevant ligands of
AhRa
.
Endogenous
FICZ, 6-formylindolo[3,2-b]carbazole (tryptophan photoproduct)
Bilirubin (product of heme metabolism by the liver)
Lipoxin A4 (eicosanoid with anti-inflammatory properties)
ITE [2-(10
H-indole-30
-carbonyl)-thiazole-4-carboxylic acid methyl
ester] (isolated from lung tissues)
Environmental pollutants (formed during combustion of organic
material)
2,3,7,8-tetrachlorodibenzo-p-dioxin
Benz[a]pyrene
Dietary
Quercetin (present in apples and onions)
Indol-3-carbinol (present in many Brassicaceae, e.g. cabbage)
Resveratrol (present in red wine)
Curcumin (a spice frequently used in Indian cuisine)
Drugs (synthetic)
M50367 {3-[2-(2-phenylethyl) benzoimidazole-4-yl]-3-hydroxypropanoic
acid}
VAF347 {[4-(3-chloro-phenyl)-pyrimidin-2-yl]}
a
For comprehensive lists and a discussion of immunological relevance see Refs [4]
and [23].
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450
models of allergy [53]. The results suggest that inhibition
of allergy is due to suppression of Th2 differentiation from
naı¨ve T-cells. Although the findings are intriguing, it must
be pointed out that neither naı¨ve T-cells nor Th1 or Th2
cells express AhR. Moreover, attempts to manipulate the
immune system with compounds that bind to a transcription
factor that is also present in many non-immune cells
and for which systemic triggering can be highly toxic must
be viewed with the utmost caution.
AhR and Th17 cells
A very defined role for AhR in the immune system was
revealed by analysis of transcription factors expressed in
differentiated CD4 effector T-cell subsets. Th17 cells, a
relatively new addition to the group of effector T-cells,
express high levels of AhR [50] (Box 2). Although AhR
expression was also reported in regulatory T (Treg) cells
[54], the levels of expression are extremely low compared
with Th17 cells and so far no clear-cut mechanistic role for
AhR has been reported in Treg cells. AhR activation results
in expansion of Th17 cells, as well as increased cytokine
production. Moreover, in a model of experimental autoimmune
encephalomyelitis (EAE) AhR activation caused
earlier onset of the disease and more severe pathology.
Although AhR is not required for Th17 differentiation, its
activation in these cells promotes further functional differentiation.
This link between a transcription factor that
responds to a wide range of environmental pollutants and
the Th17 program opens intriguing possibilities regarding
the potential of such factors to augment autoimmune
conditions that are crucially dependent on Th17 cells, such
as EAE (or human multiple sclerosis), rheumatoid arthritis
(RA) and myocarditis. Because environmental factors
clearly contribute to autoimmune disease manifestation,
further studies of AhR involvement should provide important
clues to an understanding of disease parameters such
as relapse or induction of spinal cord inflammation in
multiple sclerosis. Box 3 lists some environmental factors
that could be related to AhR activation and autoimmune
disease. It should be noted that Th17 cells are not only
involved in certain autoimmune diseases, but also play
important and beneficial roles in defense against several
types of microbial pathogens and are probably involved in
maintaining barrier functions of the gut.
AhR-mediated induction of IL-22
In Th17 cells, AhR activation is a prerequisite for the
production of the cytokine IL-22. IL-22, a member of the
IL-10 family [55], is linked to pro-inflammatory processes
such as dermal inflammation, psoriasis, inflammatory
bowel disease and Crohn’s disease. In addition, IL-22
induces the production of antimicrobial proteins that are
needed for defense against pathogens in the skin and gut
[56,57]. It also plays a protective role in models of hepatitis,
whereby IL-22 neutralization exacerbates pathology and
IL-22 delivery prevents it [58,59]. IL-22 is linked to psoriasis
and is strongly upregulated in skin lesions from
psoriasis patients [60], together with a host of IL-22–IL-
17-dependent downstream molecules such as defensins 2
and 3, psoriasin (S100A7), MMP3 and MMP1 [57]. Liang
et al. reported that IL-22 is induced in Th17 cells by the
proinflammatory cytokine IL-23 [61], which is produced by
macrophages and dendritic cells. It is clear that IL-23 plays
a fundamental role in maintaining the survival and function
of Th17 T-cells and recent data suggest that it is
needed for terminal effector differentiation of these cells
[62]. However, the role of IL-23 in IL-22 production
appears to be downstream of AhR because Th17 cells from
AhR-deficient mice cannot produce IL-22 despite the presence
of IL-23 under inflammatory conditions such as
immunization with Freund’s complete adjuvant or injection
with heat-killed mycobacteria [50]. Interestingly, lowresponder
DBA/2 mice were resistant to EAE induction
[54] and highly susceptible to infection with Candida
albicans [63], both important features of Th17 responses.
Human AhR has at least 10-fold lower sensitivity to dioxin
as determined by exposure of various cell lines to dioxin
[64], but it remains to be seen whether AhR polymorphisms
play a role in various IL-17–IL-22-dependent immune
pathologies in humans.
Endogenous AhR ligands affect Th17 differentiation
Although AhR is most widely studied for its effects downstream
of dioxin, its evolutionary conservation suggests
there must be other, more physiological ligands that drive
its functions [22]. An interesting group of such endogenous
ligands are metabolites of tryptophan, notably the UVand
light-generated metabolite FICZ described above.
Given that tryptophan is an essential amino acid and
therefore has to be supplied either by food intake or in
culture media, it is clear that investigation of the roles of
Box 2.
Th17 cells and their development
Th17 cells are a new effector T-cell subset in the CD4 lineage. These
cells differentiate in response to IL-6 produced by antigen-presenting
cells such as dendritic cells and macrophages and to TGF-b,
which is produced by many cells types. Antigen-presenting cells
might be the most critical factor in the induction of Th17. Co-factors
that enhance differentiation are IL-1b, TNFa and IL-21. IL-23 released
by dendritic cells and macrophages is an essential component for
the maintenance but not the induction of Th17 cells. The transcription
factors RORgt and RORa define the Th17 lineage and together
with STAT3 are essential for induction of IL-17 production. By
contrast, AhR is not required for initial differentiation of Th17 cells
but promotes their expansion and is essential for their production of
IL-22. Although the discovery of Th17 cells was linked to the etiology
of autoimmune diseases such as EAE and RA, their true function is
most likely the recruitment and activation of neutrophils and
protection against certain microbial pathogens.
Box 3.
Environmental links of AhR to autoimmune diseases
Epidemiology, anecdotal evidence and mechanistic studies suggest
links between autoimmune diseases and environmental exposure to
small chemicals and/or AhR ligands or xenobiotic-metabolizing
enzyme activity. Such links include:
 Dioxin and rheumatoid arthritis [69]
 Smoking and rheumatoid arthritis [70]
 Smoking and psoriasis [72]
 UV light and systemic lupus erythematosus [71]
 Cytochrome P450 RNA levels and multiple sclerosis [73]
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451
AhR in immune responses has to take into account background
levels of endogenous ligands. This is especially
true for studies involving C57BL/6 mice, which express a
high-affinity version of AhR [65]. Such background
activity has been demonstrated in in vitro studies using
cell lines transfected with reporter constructs for CYP
expression [66]. Similarly, we recently showed that culture
medium contains AhR ligands that shape the in vitro
differentiation of Th17 cells [48]. AhR antagonists reduce
Th17 differentiation in naı¨ve C57BL/6 CD4 T-cells to
levels equivalent to those observed for CD4 T-cells from
AhR-deficient mice. Interestingly, standard culture medium
such as RPMI with suboptimal AhR ligand content
results in poor Th17 differentiation and a failure to generate
IL-22 responses from Th17 cells, highlighting the
requirement for additional modulating stimuli to drive
fully functional Th17 responses [17]. FICZ increases the
proportion of Th17 cells and their production of IL-22 by
activating the AhR [50], thus demonstrating a role for AhR
in mounting an elevated response against infections. However,
such functions were not evident in studies with
dioxin [54].
AhR and autoimmunity: a critical perspective
Except for drug-induced autoimmune reactions, most
autoimmune diseases remain idiopathic. Epidemiology,
anecdotal evidence and mechanistic studies have
suggested links between autoimmune diseases and
environmental exposure to small chemicals and/or AhR
ligands or xenobiotic-metabolizing enzyme activity. For
instance, links exist between dioxin and rheumatoid
arthritis and between smoking and psoriasis. Other
examples are given in Box 3. Autoimmunity requires
the breakdown of central and/or peripheral tolerance
mechanisms. This can have a number of causes, including
dysregulated T-cell functions, the cytokine milieu and
aberrant antigen-presentation. Figure 2 shows how AhR
can be involved in the generation of neo-antigens and the
exacerbation of autoimmune diseases for which Th17 cells
are the driving force of pathogenesis. Whether and which
environmental AhR ligands are involved in the various
autoimmune diseases are questions that remain to be
answered. A link between AhR ligands and infections
can also be envisioned.
Conclusions and future perspectives
AhR, a sensor of small chemical compounds, is abundant in
many cells of the immune system. It is involved in the
metabolism of these compounds and in regulating cell
differentiation, cell cycling and important homeostatic
processes. Recent evidence has shown that AhR enhances
Th17 differentiation and is essential for induction of IL-22.
Because Th17 cells are the driving force for some autoimmune
diseases, it is possible that AhR activation exacerbates
(rather than induces) Th17-mediated autoimmunity.
An important consideration is the involvement of AhR
ligands in mucosal immune responses that are driven by
IL-22 because this cytokine seems to be critically dependent
on AhR activation.
Several challenges remain. First, further epidemiological
studies are needed to verify links between
potential AhR ligands and autoimmune or allergic diseases
in humans. Second, AhR has not been crystallized
and it remains unclear what functional consequences
different AhR ligands have on the immune system (dietary
substances, tryptophan photoproducts, environmental
pollutants). Finally, and possibly most complex, future
research will need to define more precisely how and to
what extent small chemicals and the AhR shape immune
responses.
Figure 2. Two pathways by which direct gene induction via AhR could contribute to initiation and/or exacerbation of autoimmune diseases. (I) Low-molecular-weight
chemicals and in particular their more reactive metabolites – formation of which is under control of AhR-inducible phase I and phase II metabolizing enzymes – (xenobiotic
metabolizing enzymes, XMEs) can form protein adducts, leading to the formation of new self antigens against which no tolerance exists. For instance, the phenotype of
slow acetylation by N-acetyltransferase, a phase II enzyme, is more common in patients with drug-induced lupus caused by procainamide or hydralazine [67]. Variant alleles
of CYP1A1, which code for enzymes with higher activity, might protect against psoriasis [68]. The links between dioxin and rheumatoid arthritis (RA) and between psoriasis
and smoking are strong. Smoke contains numerous strong AhR ligands, including benz[a]pyrene dioxin and other planar aromatic hydrocarbons [69–71]. (II) AhR can
mediate induction of cytokine genes in a cell-specific manner. AhR is necessary for IL-22 secretion by Th17 cells. Where IL-22 is relevant in autoimmune disease, this might
exacerbate effects. An example is the skin, where IL-22 involvement in psoriasis is known. However, it should be noted that IL-22 has other functions; in particular, it is
essential in protection against infectious agents.
Review Trends in Immunology Vol.30 No.9
452
Acknowledgements
The research of the authors was supported by grants from
Bundesministerium fu¨r Umwelt (C.E.), the Swedish Research Council
for Environment, Agricultural Science and Spatial Planning (FORMAS)
(A.R.), and the Medical Research Council UK and an ERC Advanced
Investigator grant (G.S.).
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