Cutting edge: activation of the aryl hydrocarbon receptor by 2,3,7,8-tetrachlorodibenzo-p-dioxin generates a population of CD4+ CD25+ cells with characteristics of regulatory T cells

Environmental control of Th17 differentiation

Th17 cells participate in the control of extracellular bacteria and fungi, but dysregulated Th17 cell activity can result in immunopathology. Thus, the generation and the activity of Th17 cells should be tightly regulated, and simultaneously it should be able to respond to the challenges presented by a changing environment. In this viewpoint, we discuss some of the mechanisms by which microbes, dietary components and environmental toxins influence the Th17 response.

2,3,7,8-Tetrachlorodibenzo-p-dioxin alters the differentiation of alloreactive CD8+ T cells toward a regulatory T cell phenotype by a mechanism that is dependent on aryl hydrocarbon receptor in CD4+ T cells

Activation of aryl hydrocarbon receptor (AhR) by 2,3,7,8-tetracholordibenzo- p-dioxin (TCDD) during an acute graft-versus-host response induces a population of alloreactive donor CD4+CD25+ regulatory T (Treg)-like cells that have potent suppressive activity in vitro. In the present studies, we show that TCDD induced a similar population of donor CD8+CD25+ T-cells with suppressive activity in vitro. Like the CD4+ Treg cells, donor CD8+CD25+ T-cells also expressed higher levels of CD28, glucocorticoid-induced TNFR (GITR) and CTLA-4 along with low levels of CD62L. These TCDD-induced phenotypic changes were not observed if donor T-cells were obtained from AhR-KO mice. When CD4+ and CD8+ donor T-cells from AhR-WT and AhR-KO mice were injected in various combinations into F1 mice, the enhanced expression of CD25 on CD8+ T-cells required AhR in donor CD4+ T-cells, while down-regulation of CD62L required AhR in the donor CD8+ T-cells themselves. Changes in GITR and CTLA-4 on donor CD8+ T-cells were partially mediated by AhR in both T-cells subsets. In contrast, all phenotypic changes in donor CD4+ T-cells were dependent on the presence of AhR in the CD4+ T-cells themselves. These findings suggest that the direct effects of AhR-mediated signaling in CD8+ T-cells are more limited than the direct effects in CD4+ T-cells, and that AhR signaling in CD4+ T-cells may be a unique pathway for the induction of both CD4+ and CD8+ adaptive Treg.

Natural agonists for aryl hydrocarbon receptor in culture medium are essential for optimal differentiation of Th17 T cells

Th17 cell differentiation is dependent on interleukin (IL)-6 and transforming growth factor (TGF)-beta, and it is modulated by activation of the aryl hydrocarbon receptor (AhR). In this study, we show that differentiation of Th17 cells, but not Th1 or induced regulatory T (iT reg) cells, is increased by endogenous AhR agonists present in culture medium. Th17 development from wild-type mice is suboptimal in the presence of the AhR antagonist CH-223191, similar to the situation in AhR-deficient mice, which show attenuated IL-17 production and no IL-22 production. The presence of natural AhR agonists in culture medium is also revealed by the induction of CYP1A1, a downstream target of AhR activation. However, the most commonly used medium, RPMI, supports very low levels of Th17 polarization, whereas Iscove's modified Dulbecco's medium, a medium richer in aromatic amino acids, which give rise to AhR agonists, consistently results in higher Th17 expansion in both mouse and human cells. The relative paucity of AhR agonists in RPMI medium, coupled with the presence of factors conducive to IL-2 activation and enhanced Stat5 phosphorylation, conspire against optimal Th17 differentiation. Our data emphasize that AhR activation plays an essential part in the development of Th17 cells and provide a rational explanation for the poor in vitro polarization of Th17 cells that is reported in the majority of publications for both mouse and human cells.

AHR-mediated immunomodulation: the role of altered gene transcription

The immune system is a sensitive target for aryl hydrocarbon receptor (AHR)-mediated transcriptional regulation. Most of the cells that participate in immune responses express AHR protein, and many genes involved in their responses contain multiple DRE sequences in their promoters. However, the potential involvement of many of these candidate genes in AHR-mediated immunomodulation has never been investigated. Many obstacles to understanding the transcriptional effects of AHR activation exist, owing to the complexities of pathogen-driven inflammatory and adaptive immune responses, and to the fact that activation of AHR often influences the expression of genes that are already being regulated by other transcriptional events in responding cells. Studies with TCDD as the most potent, non-metabolized AHR ligand indicate that AHR activation alters many inflammatory signals that shape the adaptive immune response, contributing to altered differentiation of antigen-specific CD4(+) T helper (TH) cells and altered adaptive immune responses. With TCDD, most adaptive immune responses are highly suppressed, which has been recently linked to the AHR-dependent induction of CD4(+)CD25(+) regulatory T cells. However activation of AHR by certain non-TCDD ligands may result in other immune outcomes, as a result of metabolism of the ligand to active metabolites or to unknown ligand-specific effects on AHR-mediated gene transcription. Based on studies using AHR(-/-) mice, evidence for a role of endogenous AHR ligands in regulation of the immune response is growing, with bilirubin and lipoxinA4 representing two promising candidates.

The aryl hydrocarbon receptor is a modulator of anti-viral immunity

Although immune modulation by AhR ligands has been studied for many years, the impact of AhR activation on host defenses against viral infection has not, until recently, garnered much attention. The development of novel reagents and model systems, new information regarding anti-viral immunity, and a growing appreciation for the global health threat posed by viruses have invigorated interest in understanding how environmental signals affect susceptibility to and pathological consequences of viral infection. Using influenza A virus as a model of respiratory viral infection, recent studies show that AhR activation cues signaling events in both leukocytes and non-immune cells. Functional alterations include suppressed lymphocyte responses and increased inflammation in the infected lung. AhR-mediated events within and extrinsic to hematopoietic cells has been investigated using bone marrow chimeras, which show that AhR alters different elements of the immune response by affecting different tissue targets. In particular, suppressed CD8(+) T cell responses are due to deregulated events within leukocytes themselves, whereas increased neutrophil recruitment to and IFN-gamma levels in the lung result from AhR-regulated events extrinsic to bone marrow-derived cells. This latter discovery suggests that epithelial and endothelial cells are overlooked targets of AhR-mediated changes in immune function. Further support that AhR influences host cell responses to viral infection are provided by several studies demonstrating that AhR interacts directly with viral proteins and affects viral latency. While AhR clearly modulates host responses to viral infection, we still have much to understand about the complex interactions between immune cells, viruses, and the host environment.

A new cross-talk between the aryl hydrocarbon receptor and RelB, a member of the NF-kappaB family

The discovery of the new crosstalk between the aryl hydrocarbon receptor (AhR) and the NF-kappaB subunit RelB may extend our understanding of the biological functions of the AhR and at the same time raises a number of questions, which will be addressed in this review. The characteristics of this interaction differ from that of AhR with RelA in that the latter appears to be mostly negative unlike the collaborative interactions of AhR/RelB. The AhR/RelB dimer is capable of binding to DNA response elements including the dioxin response element (DRE) as well as NF-kappaB binding sites supporting the activation of target genes of the AhR as well as NF-kappaB pathway. Further studies show that AhR/RelB complexes can be found not only in lymphoid cells but also in a human hepatoma cell line (HepG2) or breast cancer cell line (MDA-MB-231). RelB has been implicated in carcinogenesis of breast cancer for instance and RelB is known to be a critical factor for the function and differentiation of dendritic cells; interestingly the participation of AhR in both processes has been suggested recently, which offers the great potential to expand the scope of the physiological roles of the AhR. There is evidence indicating that RelB may serve as a pro-survival factor, including its ability to promote "inflammation resolution" besides the association of RelB with inflammatory disorders. Based on such information, a hypothesis has been proposed in this review that AhR together with RelB functions as a coordinator of inflammatory responses.

Functional characterization and gene expression analysis of CD4+ CD25+ regulatory T cells generated in mice treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

Although the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) are mediated through binding and activation of the aryl hydrocarbon receptor (AhR), the subsequent biochemical and molecular changes that confer immune suppression are not well understood. Mice exposed to TCDD during an acute B6-into-B6D2F1 graft-vs-host response do not develop disease, and recently this has been shown to correlate with the generation of CD4(+) T cells that express CD25 and demonstrate in vitro suppressive function. The purpose of this study was to further characterize these CD4(+) cells (TCDD-CD4(+) cells) by comparing and contrasting them with both natural regulatory CD4(+) T cells (T-regs) and vehicle-treated cells. Cellular anergy, suppressive functions, and cytokine production were examined. We found that TCDD-CD4(+) cells actively proliferate in response to various stimuli but suppress IL-2 production and the proliferation of effector T cells. Like natural T-regs, TCDD-CD4(+) cells do not produce IL-2 and their suppressive function is contact dependent but abrogated by costimulation through glucocorticoid-induced TNFR (GITR). TCDD-CD4(+) cells also secrete significant amounts of IL-10 in response to both polyclonal and alloantigen stimuli. Several genes were significantly up-regulated in TCDD-CD4(+) cells including TGF-beta3, Blimp-1, and granzyme B, as well as genes associated with the IL12-Rb2 signaling pathway. TCDD-CD4(+) cells demonstrated an increased responsiveness to IL-12 as indicated by the phosphorylation levels of STAT4. Only 2% of TCDD-CD4(+) cells express Foxp3, suggesting that the AhR does not rely on Foxp3 for suppressive activity. The generation of CD4(+) cells with regulatory function mediated through activation of the AhR by TCDD may represent a novel pathway for the induction of T-regs.

Regulation of constitutive and inducible AHR signaling: complex interactions involving the AHR repressor

The AHR is well known for regulating responses to an array of environmental chemicals. A growing body of evidence supports the hypothesis that the AHR also plays perhaps an even more important role in modulating critical aspects of cell function including cell growth, death, and migration. As these and other important AHR activities continue to be elucidated, it becomes apparent that attention now must be directed towards the mechanisms through which the AHR itself is regulated. Here, we review what is known of and what biological outcomes have been attributed to the AHR repressor (AHRR), an evolutionarily conserved bHLH-PAS protein that inhibits both xenobiotic-induced and constitutively active AHR transcriptional activity in multiple species. We discuss the structure and evolution of the AHRR and the dominant paradigm of a xenobiotic-inducible negative feedback loop comprised of AHR-mediated transcriptional up-regulation of AHRR and the subsequent AHRR-mediated suppression of AHR activity. We highlight the role of the AHRR in limiting AHR activity in the absence of xenobiotic AHR ligands and the important contribution of constitutively repressive AHRR to cancer biology. In this context, we also suggest a new hypothesis proposing that, under some circumstances, constitutively active AHR may repress AHRR transcription, resulting in unbridled AHR activity. We also review the predominant hypotheses on the molecular mechanisms through which AHRR inhibits AHR as well as novel mechanisms through which the AHRR may exert AHR-independent effects. Collectively, this discussion emphasizes the importance of this understudied bHLH-PAS protein in tissue development, normal cell biology, xenobiotic responsiveness, and AHR-regulated malignancy.

Activation of the aryl hydrocarbon receptor is essential for mediating the anti-inflammatory effects of a novel low-molecular-weight compound

VAF347 is a low-molecular-weight compound that inhibits allergic lung inflammation in vivo. This effect is likely the result of a block of dendritic cell (DC) function to generate proinflammatory T-helper (Th) cells because VAF347 inhibits interleukin (IL)-6, CD86, and human leukocyte antigen (HLA)-DR expression by human monocyte-derived DC, 3 relevant molecules for Th-cell generation. Here we demonstrate that VAF347 interacts with the aryl hydrocarbon receptor (AhR) protein, resulting in activation of the AhR signaling pathway. Functional AhR is responsible for the biologic activity of VAF347 because (1) other AhR agonists display an identical activity profile in vitro, (2) gene silencing of wild-type AhR expression or forced overexpression of a trans-dominant negative AhR ablates VAF347 activity to inhibit cytokine induced IL-6 expression in a human monocytic cell line, and (3) AhR-deficient mice are resistant to the compound's ability to block allergic lung inflammation in vivo. These data identify the AhR protein as key molecular target of VAF347 and its essential role for mediating the anti-inflammatory effects of the compound in vitro and in vivo.

Aryl hydrocarbon receptor signaling mediates expression of indoleamine 2,3-dioxygenase

Aryl hydrocarbon receptor (AhR) activation by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) leads to immune suppression associated with the induction of regulatory T cells (T(reg)) expressing the transcription factor Foxp3. The immunological mechanisms of suppression are not well understood however dendritic cells (DC) are considered a key target for AhR-mediated immune suppression. Here we show that activation of AhR by TCDD induces DC indoleamine 2,3-dioxygenase 1 (IDO1) and indoleamine 2,3-dioxygenase-like protein (IDO2). Induction of IDO1 and IDO2 was also found in lung and spleen associated with an increase of the T(reg) marker Foxp3 in spleen of TCDD-treated C57BL/6 mice, which is suppressed by inhibition of IDO. These data indicate that AhR-activation is an important signaling pathway for IDO expression and suggest a critical role of IDO in the mechanism leading to the generation of T(reg) that mediates the immune suppression through activation of AhR.

Aryl hydrocarbon receptor regulates Stat1 activation and participates in the development of Th17 cells

IL-17-producing T helper cells (Th17) have been recently identified as a previously undescribed subset of helper T cells. Here, we demonstrate that aryl hydrocarbon receptor (Ahr) has an important regulatory function in the commitment of Th17 cells. Ahr was robustly induced under Th17-polarizing conditions. Ahr-deficient naïve T cells showed a considerable loss in the ability to differentiate into Th17 cells when induced by TGF-beta plus IL-6. We were able to demonstrate that Ahr interacts with Stat1 and Stat5, which negatively regulate Th17 development. Whereas Stat1 activation returned to its basal level in Ahr wild type naïve T cells 24 h after stimulation with TGF-beta plus IL-6, Stat1 remained activated in Ahr-deficient naïve T cells after stimulation. These results indicate that Ahr participates in Th17 cell differentiation through regulating Stat1 activation, a finding that constitutes additional mechanisms in the modulation of Th17 cell development.

Activation of the aryl hydrocarbon receptor promotes allograft-specific tolerance through direct and dendritic cell-mediated effects on regulatory T cells

VAF347 is a low-molecular-weight compound, which activates the aryl hydrocarbon receptor (AhR). Herein, we report that oral administration of a water-soluble derivative of VAF347 (VAG539) promotes long-term graft acceptance and active tolerance in Balb/c mice that receive a transplant of MHC-mismatched pancreatic islet allografts. In vivo VAG539 treatment results in increased frequency of splenic CD4(+) T cells expressing CD25 and Foxp3, markers associated with regulatory T (Tr) cells, and in vitro VAF347 treatment of splenic CD4(+) T cells improved CD4(+)CD25(+)Foxp3(+) T-cell survival. Interestingly, transfer of CD11c(+) dendritic cells (DCs), but not of CD4(+) T or CD19(+) B cells, from VAG539-treated long-term tolerant hosts into mice that recently underwent transplantation resulted in donor (C57Bl/6)-specific graft acceptance and in a significantly higher frequency of splenic CD4(+)CD25(+)Foxp3(+) Tr cells. Furthermore, the transfer of CD4(+)CD25(+) T cells from these mice into mice that recently underwent transplantation promoted graft acceptance. Similarly, cell therapy with in vitro VAF347-treated bone marrow-derived mature DCs prevented islet graft rejection, and reduced OVA-specific T-cell responses in OVA-immunized mice. Collectively, our data indicate that AhR activation induces islet allograft-specific tolerance through direct as well as DC-mediated effects on Tr-cell survival and function.

The aryl hydrocarbon receptor links TH17-cell-mediated autoimmunity to environmental toxins

The aryl hydrocarbon receptor (AHR) is a ligand-dependent transcription factor best known for mediating the toxicity of dioxin. Environmental factors are believed to contribute to the increased prevalence of autoimmune diseases, many of which are due to the activity of T(H)17 T cells, a new helper T-cell subset characterized by the production of the cytokine IL-17. Here we show that in the CD4+ T-cell lineage of mice AHR expression is restricted to the T(H)17 cell subset and its ligation results in the production of the T(H)17 cytokine interleukin (IL)-22. AHR is also expressed in human T(H)17 cells. Activation of AHR by a high-affinity ligand during T(H)17 cell development markedly increases the proportion of T(H)17 T cells and their production of cytokines. CD4+ T cells from AHR-deficient mice can develop T(H)17 cell responses, but when confronted with AHR ligand fail to produce IL-22 and do not show enhanced T(H)17 cell development. AHR activation during induction of experimental autoimmune encephalomyelitis causes accelerated onset and increased pathology in wild-type mice, but not AHR-deficient mice. AHR ligands may therefore represent co-factors in the development of autoimmune diseases.

Control of T(reg) and T(H)17 cell differentiation by the aryl hydrocarbon receptor

Regulatory T cells (T(reg)) expressing the transcription factor Foxp3 control the autoreactive components of the immune system. The development of T(reg) cells is reciprocally related to that of pro-inflammatory T cells producing interleukin-17 (T(H)17). Although T(reg) cell dysfunction and/or T(H)17 cell dysregulation are thought to contribute to the development of autoimmune disorders, little is known about the physiological pathways that control the generation of these cell lineages. Here we report the identification of the ligand-activated transcription factor aryl hydrocarbon receptor (AHR) as a regulator of T(reg) and T(H)17 cell differentiation in mice. AHR activation by its ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin induced functional T(reg) cells that suppressed experimental autoimmune encephalomyelitis. On the other hand, AHR activation by 6-formylindolo[3,2-b]carbazole interfered with T(reg) cell development, boosted T(H)17 cell differentiation and increased the severity of experimental autoimmune encephalomyelitis in mice. Thus, AHR regulates both T(reg) and T(H)17 cell differentiation in a ligand-specific fashion, constituting a unique target for therapeutic immunomodulation.

The aryl hydrocarbon receptor affects distinct tissue compartments during ontogeny of the immune system

There is growing evidence that prenatal and early postnatal environmental factors influence the development and programming of the immune system, causing long-lasting negative health consequences. The aryl hydrocarbon receptor (AhR) is an important modulator of the development and function of the immune system; however, the mechanism is poorly understood. Exposure to the AhR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin throughout gestation and during lactation yields adult offspring with persistent defects in their immune response to influenza virus. These functional alterations include suppressed lymphocyte responses and increased inflammation in the infected lung despite normal cellularity and anatomical development of lymphoid organs. The studies presented here were conducted to determine the critical period during immune ontogeny that is particularly sensitive to inappropriate AhR activation. We also investigated the contribution of AhR-mediated events within and extrinsic to hematopoietic cells. Our findings show that AhR activation alters different elements of the immune system at different times during development by affecting different tissue targets. In particular, diminished T-cell responses arise due to deregulated events within bone marrow-derived cells. In contrast, increased interferon gamma levels in the infected lung result from AhR-regulated events extrinsic to bone marrow-derived cells, and require AhR agonist exposure during early gestation. The persistence of AhR activation induced immune modulation was also compared, revealing that AhR activation causes long-lasting functional alterations in the developing immune system, whereas the impact on the mature immune system is transient.

The aryl hydrocarbon receptor is required for optimal resistance to Listeria monocytogenes infection in mice

The aryl hydrocarbon receptor (AhR) is part of a powerful signaling system that is triggered by xenobiotic agents such as polychlorinated hydrocarbons and polycyclic aromatic hydrocarbons. Although activation of the AhR by 2,3,7,8-tetrachlorodibenzo-p-dioxin or certain polycyclic aromatic hydrocarbons can lead to immunosuppression, there is also increasing evidence that the AhR regulates certain normal developmental processes. In this study, we asked whether the AhR plays a role in host resistance using murine listeriosis as an experimental system. Our data clearly demonstrate that AhR null C57BL/6J mice (AhR(-/-)) are more susceptible to listeriosis than AhR heterozygous (AhR(+/-)) littermates when inoculated i.v. with log-phase Listeria monocytogenes. AhR(-/-) mice exhibited greater numbers of CFU of L. monocytogenes in the spleen and liver, and greater histopathological changes in the liver than AhR(+/-) mice. Serum levels of IL-6, MCP-1, IFN-gamma, and TNF-alpha were comparable between L. monocytogenes-infected AhR(-/-) and AhR(+/-) mice. Increased levels of IL-12 and IL-10 were observed in L. monocytogenes-infected AhR(-/-) mice. No significant difference was found between AhR(+/-) and AhR(-/-) macrophages ex vivo with regard to their ability to ingest and inhibit intracellular growth of L. monocytogenes. Intracellular cytokine staining of CD4(+) and CD8(+) splenocytes for IFN-gamma and TNF-alpha revealed comparable T cell-mediated responses in AhR(-/-) and AhR(+/-) mice. Previously infected AhR(-/-) and AhR(+/-) mice both exhibited enhanced resistance to reinfection with L. monocytogenes. These data provide the first evidence that AhR is required for optimal resistance but is not essential for adaptive immune response to L. monocytogenes infection.

Exposure to Arsenic at Levels Found in U.S. Drinking Water Modifies Expression in the Mouse Lung

The mechanisms of action of drinking water arsenic in the lung and the threshold for biologic effects remain controversial. Our study utilizes Affymetrix 22,690 transcript oligonucleotide microarrays to assess the long-term effects of increasing doses of drinking water arsenic on expression levels in the mouse lung. Mice were exposed at levels commonly found in contaminated drinking water wells in the United States (0, 0.1, 1 ppb), as well as the 50 ppb former maximum contaminant level, for 5 weeks. The expression profiles revealed modification of a number of important signaling pathways, many with corroborating evidence of arsenic responsiveness. We observed statistically significant expression changes for transcripts involved in angiogenesis, lipid metabolism, oxygen transport, apoptosis, cell cycle, and immune response. Validation by reverse transcription-PCR and immunoblot assays confirmed expression changes for a subset of transcripts. These data identify arsenic-modified signaling pathways that will help guide investigations into mechanisms of arsenic's health effects and clarify the threshold for biologic effects and potential disease risk.

Microarray analysis of the AHR system: Tissue-specific flexibility in signal and target genes

Data mining published microarray experiments require that expression profiles are directly comparable. We performed linear global normalization on the data of 1967 Affymetrix U74av2 microarrays, i.e. the transcriptomes of >100 murine tissues or cell types. The mathematical transformation effectively nullifies inter-experimental or inter-laboratory differences between microarrays. The correctness of expression values was validated by quantitative RT-PCR. Using the database we analyze components of the aryl hydrocarbon receptor (AHR) signaling pathway in various tissues. We identified lineage and differentiation specific variant expression of AHR, ARNT, and HIF1alpha in the T-cell lineage and high expression of CYP1A1 in immature B cells and dendritic cells. Performing co-expression analysis we found unorthodox expression of the AHR in the absence of ARNT, particularly in stem cell populations, and can reject the hypothesis that ARNT2 takes over and is highly expressed when ARNT expression is low or absent. Furthermore the AHR shows no co-expression with any other transcript present on the chip. Analysis of differential gene expression under 308 conditions revealed 53 conditions under which the AHR is regulated, numerous conditions under which an intrinsic AHR action is modified as well as conditions activating the AHR even in the absence of known AHR ligands. Thus meta-analysis of published expression profiles is a powerful tool to gain novel insights into known and unknown systems.

An early age-related increase in the frequency of CD4+ Foxp3+ cells in BDC2.5NOD mice

The role of regulatory T cells (Treg) in maintaining tolerance to self has been intensively scrutinized, particularly since the discovery of Foxp3 as a Treg-specific transcription factor. The BDC2.5NOD transgenic mouse is an excellent model of immunoregulation because it has a very low incidence of diabetes despite a highly autoreactive T-cell repertoire. It has previously been shown that reactivity against islets decreases with age in BDC2.5NOD mice. Here we show that there is a markedly higher frequency of Foxp3(+) Treg in the CD4(+) subset of 16-20-week-old mice compared with 4- or 8-week-old mice. This phenomenon can be observed in the spleen, thymus, pancreatic draining lymph nodes and the pancreas itself. We show that this early age-related increase in the frequency of Foxp3(+) cells does not occur in wild-type NOD, BALB/c or C57BL/6 mice. Further, we show that, in contrast to some reports on Treg in wild-type NOD mice, the suppressive function of BDC2.5NOD Treg from 16- to 20-week-old mice is intact and comparable to that from 4- to 8-week-old mice both in vitro and in vivo. Our data offer insights into the long-term protection of BDC2.5NOD mice from diabetes and an explanation for the age-related decrease in anti-islet responses seen in BDC2.5NOD mice.

Cytochrome P450 gene regulation and physiological functions mediated by the aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that functions as an intracellular mediator in the xenobiotic signaling pathway. Although a number of studies have examined AhR-mediated CYP1A1 induction in detail, recent studies of AhR-null mice have revealed that AhR plays important regulatory roles in the normal homeostasis and development of animals. In this short review, we summarize the present state of knowledge about the molecular mechanisms of AhR-mediated CYP1 induction, and we also focus on recent advances in the study of the physiological functions of AhR.

Aryl Hydrocarbon Receptor Activation Impairs the Priming but Not the Recall of Influenza Virus-Specific CD8+T Cells in the Lung

The response of CD8+ T cells to influenza virus is very sensitive to modulation by aryl hydrocarbon receptor (AhR) agonists; however, the mechanism underlying AhR-mediated alterations in CD8+ T cell function remains unclear. Moreover, very little is known regarding how AhR activation affects anamnestic CD8+ T cell responses. In this study, we analyzed how AhR activation by the pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) alters the in vivo distribution and frequency of CD8+ T cells specific for three different influenza A virus epitopes during and after the resolution of a primary infection. We then determined the effects of TCDD on the expansion of virus-specific memory CD8+ T cells during recall challenge. Adoptive transfer of AhR-null CD8+ T cells into congenic AhR(+/+) recipients, and the generation of CD45.2AhR(-/-)-->CD45.1AhR(+/+) chimeric mice demonstrate that AhR-regulated events within hemopoietic cells, but not directly within CD8+ T cells, underlie suppressed expansion of virus-specific CD8+ T cells during primary infection. Using a dual-adoptive transfer approach, we directly compared the responsiveness of virus-specific memory CD8+ T cells created in the presence or absence of TCDD, which revealed that despite profound suppression of the primary response to influenza virus, the recall response of virus-specific CD8+ T cells that form in the presence of TCDD is only mildly impaired. Thus, the delayed kinetics of the recall response in TCDD-treated mice reflects the fact that there are fewer memory cells at the time of reinfection rather than an inherent defect in the responsive capacity of virus-specific memory CD8+ cells.

Regulatory T cells, a potent immunoregulatory target for CAM researchers: modulating tumor immunity, autoimmunity and alloreactive immunity (III)

Regulatory T (T(reg)) cells are the major arbiter of immune responses, mediating actions through the suppression of inflammatory and destructive immune reactions. Inappropriate T(reg) cell frequency or functionality potentiates the pathogenesis of myriad diseases with ranging magnitudes of severity. Lack of suppressive capability hinders restraint on immune responses involved in autoimmunity and alloreactivity, while excessive suppressive capacity effectively blocks processes necessary for tumor destruction. Although the etiology of dysfunctional T(reg) cell populations is under debate, the ramifications, and their mechanisms, are increasingly brought to light in the medical community. Methods that compensate for aberrant immune regulation may not address the underlying complications; however, they hold promise for the alleviation of debilitating immune system-related disorders. The dominant immunoregulatory nature of T(reg) cells, coupled with recent mechanistic knowledge of natural immunomodulatory compounds, highlights the importance of T(reg) cells to practitioners and researchers of complementary and alternative medicine (CAM).

Transcriptional signatures of immune cells in aryl hydrocarbon receptor (AHR)-proficient and AHR-deficient mice

The ligand-activated aryl hydrocarbon receptor (AHR) is known to modulate many genes in a highly cell-specific manner, either directly or indirectly via secondary effects. In contrast, little is known about the effects of AHR deficiency on gene expression balance. We compared the transcriptome of CD4 T cells from AHR-/- mice and wild-type mice; 390 genes, many of them immunotypic, were deregulated in AHR-deficient CD4 cells. TCDD-induced transcriptome changes correlated with the AHR expression level in immune cells. However, there was little overlap in AHR-dependent transcripts found in T lineage cells or dendritic cells. Our results demonstrate flexible gene accessibility for the AHR in immune cells. The idea of a universal battery of AHR-responsive genes is not tenable.