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

Guardians under Siege: Exploring Pollution's Effects on Human Immunity

Chemical pollution poses a significant threat to human health, with detrimental effects on various physiological systems, including the respiratory, cardiovascular, mental, and perinatal domains. While the impact of pollution on these systems has been extensively studied, the intricate relationship between chemical pollution and immunity remains a critical area of investigation. The focus of this study is to elucidate the relationship between chemical pollution and human immunity. To accomplish this task, this study presents a comprehensive review that encompasses in vitro, ex vivo, and in vivo studies, shedding light on the ways in which chemical pollution can modulate human immunity. Our aim is to unveil the complex mechanisms by which environmental contaminants compromise the delicate balance of the body's defense systems going beyond the well-established associations with defense systems and delving into the less-explored link between chemical exposure and various immune disorders, adding urgency to our understanding of the underlying mechanisms and their implications for public health.

Toxicogenomics of the C-C chemokine receptor type 5 (CCR5): Exploring the potential impacts of chemical-CCR5 interactions on inflammation and human health

This article explores the impact of environmental chemicals on CCR5 expression and related inflammatory responses based on curated data from the Comparative Toxicogenomics Database (CTD). A total of 143 CCR5-interacting chemicals was found, with 229 chemical interactions. Of note, 67 (29.3%) out of 229 interactions resulted in "increased expression" of CCR5 mRNA or CCR5 protein, and 42 (18.3%) chemical interactions resulted in "decreased expression". The top-5 CCR5-interacting chemicals were "Tetrachlorodibenzodioxin", "Lipopolysaccharides", "Benzo(a)pyrene", "Drugs, Chinese Herbal", and "Ethinyl Estradiol". Based on the number of interactions and importance as environmental contaminant, we then focused our analysis on Tetrachlorodibenzodioxin and Benzo(a)pyrene. There is some consistency in the data supporting an increase in CCR5 expression triggered by Tetrachlorodibenzodioxin; although data concerning CCR5-Benzo(a)pyrene interactions is limited. Considering the high linkage disequilibrium between CCR5 and CCR2 genes, we also search for chemicals that interact with both genes, which resulted in 72 interacting chemicals, representing 50.3% of the 143 CCR5-interacting chemicals and 37.5% of the 192 CCR2-interacting chemicals. In conclusion, CTD data showed that environmental contaminants indeed affect CCR5 expression, with a tendency towards increased expression. The interaction of environmental contaminants with other chemokine receptor genes may potentialize their toxic effects on the chemokine system, favoring inflammation.

Target Organ Toxicity in Rats After Subchronic Oral Exposure to Soil Extracts Containing a Complex Mixture of Contaminants

Complex mixtures of unknown contaminants present a challenge to identify toxicological risks without using large numbers of animals and labor-intensive screens of all organs. This study examined soil extracts from a legacy-contaminated pesticide packaging and blending site. HepG2 cytotoxicity was used as an initial screen of 18 soil samples; then, three extracts (A, B and C) from different locations at the study site were used for testing in animals. The first two extracts were identified as the most toxic in vitro, and the latter extract obtained from a location further from these two toxic sampling sites. Then, target organ toxicities were identified following biweekly oral gavage for one month of three soil extracts (0.1% in polyethylene glycol or PEG) compared to vehicle control in male Sprague-Dawley rats (n = 9-10/group). Exposure to extract A significantly increased neutrophils and lymphocytes compared to control. In contrast, all extracts increased plasma α-2 macroglobulin and caused mild-to-moderate lymphocytic proliferation within the spleen white pulp, all indicative of inflammation. Rats exposed to all soil extracts exhibited acute tubular necrosis. Cholinesterase activity was significantly reduced in plasma, but not brain, after exposure to extract A compared to control. Increased hepatic ethoxyresorufin-o-deethylase activity compared to control was observed following exposure to extracts A and B. Exposure to soil extract C in rats showed a prolonged QTc interval in electrocardiography as well as increased brain lipid peroxidation. Candidate contaminants are organochlorine, organophosphate/carbamate pesticides or metabolites. Overall, HepG2 cytotoxicity did not successfully predict the neurotoxicity and cardiotoxicity observed with extract C but was more successful with suspected hydrocarbon toxicities in extracts A and B. Caution should be taken when extrapolating the observation of no effects from in vitro cell culture to in vivo toxicity, and better cell culture lines or assays should be explored.

Consensus on the Key Characteristics of Immunotoxic Agents as a Basis for Hazard Identification

BACKGROUND

Key characteristics (KCs), properties of agents or exposures that confer potential hazard, have been developed for carcinogens and other toxicant classes. KCs have been used in the systematic assessment of hazards and to identify assay and data gaps that limit screening and risk assessment. Many of the mechanisms through which pharmaceuticals and occupational or environmental agents modulate immune function are well recognized. Thus KCs could be identified for immunoactive substances and applied to improve hazard assessment of immunodulatory agents.

OBJECTIVES

The goal was to generate a consensus-based synthesis of scientific evidence describing the KCs of agents known to cause immunotoxicity and potential applications, such as assays to measure the KCs.

METHODS

A committee of 18 experts with diverse specialties identified 10 KCs of immunotoxic agents, namely, 1) covalently binds to proteins to form novel antigens, 2) affects antigen processing and presentation, 3) alters immune cell signaling, 4) alters immune cell proliferation, 5) modifies cellular differentiation, 6) alters immune cell-cell communication, 7) alters effector function of specific cell types, 8) alters immune cell trafficking, 9) alters cell death processes, and 10) breaks down immune tolerance. The group considered how these KCs could influence immune processes and contribute to hypersensitivity, inappropriate enhancement, immunosuppression, or autoimmunity.

DISCUSSION

KCs can be used to improve efforts to identify agents that cause immunotoxicity via one or more mechanisms, to develop better testing and biomarker approaches to evaluate immunotoxicity, and to enable a more comprehensive and mechanistic understanding of adverse effects of exposures on the immune system. https://doi.org/10.1289/EHP10800.

Effects of perinatal TCDD exposure on colonic microbiota and metabolism in offspring and mother mice

Emerging evidence supports that exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) impacts the gut microbiota and metabolic pathways. TCDD can be transmitted from mother to child; thus, we hypothesize that maternal exposure to TCDD may affect the gut microbiota in mothers and offspring. To acquire in vivo evidence supporting this hypothesis, female C57BL/6 mice were administered with TCDD (0.1 and 10 μg/kg body weight (bw)) during pregnancy and lactation periods, and then changes of colonic microbiota in offspring and mothers were evaluated. High-throughput sequencing of the V4 regions of the 16S rRNA gene was performed. The composition and structure of the colonic microbiota in offspring and mothers were significantly influenced by 10 μg/kg bw TCDD as demonstrated by upregulation of harmful bacteria and downregulation of beneficial bacteria. Paradoxically, pathogenic bacteria and opportunistic pathogens were conversely decreased in the offspring of the low-dose TCDD treatment group. Tryptophan (Trp) metabolism exhibited a noticeable change caused by the alteration of colonic microbiota in offspring after maternal exposure to 10 μg/kg bw TCDD, which showed a linear dependence, demonstrating that pathogens or opportunistic pathogens may accelerate the dysbiosis of Trp metabolism. Trp metabolism dysregulation caused by the changed colonic microbiota may subsequently impact other intestinal segments or even living organisms. Our study provides new evidence indicating a potential influence of early TCDD exposure on the colonic microbiota and metabolism.

The Aryl Hydrocarbon Receptor Modulates T Follicular Helper Cell Responses to Influenza Virus Infection in Mice

T follicular helper (Tfh) cells support Ab responses and are a critical component of adaptive immune responses to respiratory viral infections. Tfh cells are regulated by a network of signaling pathways that are controlled, in part, by transcription factors. The aryl hydrocarbon receptor (AHR) is an environment-sensing transcription factor that modulates many aspects of adaptive immunity by binding a range of small molecules. However, the contribution of AHR signaling to Tfh cell differentiation and function is not known. In this article, we report that AHR activation by three different agonists reduced the frequency of Tfh cells during primary infection of C57BL/6 mice with influenza A virus (IAV). Further, using the high-affinity and AHR-specific agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin, we show that AHR activation reduced Tfh cell differentiation and T cell-dependent B cell responses. Using conditional AHR knockout mice, we demonstrated that alterations of Tfh cells and T cell-dependent B cell responses after AHR activation required the AHR in T cells. AHR activation reduced the number of T follicular regulatory (Tfr) cells; however, the ratio of Tfr to Tfh cells was amplified. These alterations to Tfh and Tfr cells during IAV infection corresponded with differences in expression of BCL6 and FOXP3 in CD4⁺ T cells and required the AHR to have a functional DNA-binding domain. Overall, these findings support that the AHR modulates Tfh cells during viral infection, which has broad-reaching consequences for understanding how environmental factors contribute to variation in immune defenses against infectious pathogens, such as influenza and severe acute respiratory syndrome coronavirus.

AhR promotes phosphorylation of ARNT isoform 1 in human T cell malignancies as a switch for optimal AhR activity

The aryl hydrocarbon receptor nuclear translocator (ARNT) is a transcription factor present in immune cells as a long and short isoform, referred to as isoforms 1 and 3, respectively. However, investigation into potential ARNT isoform–specific immune functions is lacking despite the well-established heterodimerization requirement of ARNT with, and for the activity of, the aryl hydrocarbon receptor (AhR), a critical mediator of immune homeostasis. Here, using global and targeted transcriptomics analyses, we show that the relative ARNT isoform 1:3 ratio in human T cell lymphoma cells dictates the amplitude and direction of AhR target gene regulation. Specifically, shifting the ARNT isoform 1:3 ratio lower by suppressing isoform 1 enhances, or higher by suppressing isoform 3 abrogates, AhR responsiveness to ligand activation through preprograming a cellular genetic background that directs explicit gene expression patterns. Moreover, the fluctuations in gene expression patterns that accompany a decrease or increase in the ARNT isoform 1:3 ratio are associated with inflammation or immunosuppression, respectively. Molecular studies identified the unique casein kinase 2 (CK2) phosphorylation site within isoform 1 as an essential parameter to the mechanism of ARNT isoform–specific regulation of AhR signaling. Notably, CK2-mediated phosphorylation of ARNT isoform 1 is dependent on ligand-induced AhR nuclear translocation and is required for optimal AhR target gene regulation. These observations reveal ARNT as a central modulator of AhR activity predicated on the status of the ARNT isoform ratio and suggest that ARNT-based therapies are a viable option for tuning the immune system to target immune disorders.

EDC-induced mechanisms of immunotoxicity: a systematic review

Endocrine-disrupting chemicals (EDCs) refer to a group of chemicals that cause adverse effects in human health, impairing hormone production and regulation, resulting in alteration of homeostasis, reproductive, and developmental, and immune system impairments. The immunotoxicity of EDCs involves many mechanisms altering gene expression that depend on the activation of nuclear receptors such as the aryl hydrocarbon receptor (AHR), the estrogen receptor (ER), and the peroxisome proliferator-activated receptor (PPAR), which also results in skin and intestinal disorders, microbiota alterations and inflammatory diseases. This systematic review aims to review different mechanisms of immunotoxicity and immunomodulation of T cells, focusing on T regulatory (Treg) and Th17 subsets, B cells, and dendritic cells (DCs) caused by specific EDCs such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), bisphenols (BPs) and polyfluoroalkyl substances (PFASs). To achieve this objective, a systematic study was conducted searching various databases including PubMed and Scopus to find in-vitro, in-vivo, and biomonitoring studies that examine EDC-dependent mechanisms of immunotoxicity. While doing the systematic review, we found species- and cell-specific outcomes and a translational gap between in-vitro and in-vivo experiments. Finally, an adverse outcome pathway (AOP) framework is proposed, which explains mechanistically toxicity endpoints emerging from different EDCs having similar key events and can help to improve our understanding of EDCs mechanisms of immunotoxicity. In conclusion, this review provides insights into the mechanisms of immunotoxicity mediated by EDCs and will help to improve human health risk assessment.

Targeting AhR as a Novel Therapeutic Modality against Inflammatory Diseases

For decades, activation of Aryl Hydrocarbon Receptor (AhR) was excluded from consideration as a therapeutic approach due to the potential toxic effects of AhR ligands and the induction of the cytochrome P450 enzyme, Cyp1a1, following AhR activation. However, it is now understood that AhR activation not only serves as an environmental sensor that regulates the effects of environmental toxins, but also as a key immunomodulator where ligands induce a variety of cellular and epigenetic mechanisms to attenuate inflammation. Thus, the emergence of further in-depth research into diverse groups of compounds capable of activating this receptor has prompted reconsideration of its use therapeutically. The aim of this review is to summarize the body of research surrounding AhR and its role in regulating inflammation. Specifically, evidence supporting the potential of targeting this receptor to modulate the immune response in inflammatory and autoimmune diseases will be highlighted. Additionally, the opportunities and challenges of developing AhR-based therapies to suppress inflammation will be discussed.

Recent advances in the development of AHR antagonists in immuno-oncology

The arylhydrocarbon receptor (AHR) is a ligand activated transcription factor that controls the expression of a number of immunosuppressive signaling molecules, including the immune checkpoint proteins PD-1/L1 and cytokine IL-10. AHR activation also stimulates the formation and recruitment of tolerogenic dendritic cells, tumor associated macrophages, and regulatory T cells in the tumor microenvironment, which restrains antitumoral immune response. Overexpression of AHR has been observed in a number of different types of cancer and suggested to contribute to immune dysfunction and cancer progression. One prominent endogenous ligand of AHR is the oncometabolite kynurenine, a product of tryptophan metabolism catalyzed by the dioxygenases IDO1 and TDO that are often aberrantly activated in cancer. AHR has gained significant interest as a drug target for the development of novel small molecule cancer immunotherapies, as evidenced by the advancement of two clinical candidates into phase 1 clinical trials in patients with advanced cancer. Discussed in this Review is a brief background of AHR in immuno-oncology and the recent progress in the discovery and development of AHR antagonists.

AhR Activation Leads to Alterations in the Gut Microbiome with Consequent Effect on Induction of Myeloid Derived Suppressor Cells in a CXCR2-Dependent Manner

Aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a potent ligand for AhR and a known carcinogen. While AhR activation by TCDD leads to significant immunosuppression, how this translates into carcinogenic signal is unclear. Recently, we demonstrated that activation of AhR by TCDD in naïve C57BL6 mice leads to massive induction of myeloid derived-suppressor cells (MDSCs). In the current study, we investigated the role of the gut microbiota in TCDD-mediated MDSC induction. TCDD caused significant alterations in the gut microbiome, such as increases in Prevotella and Lactobacillus, while decreasing Sutterella and Bacteroides. Fecal transplants from TCDD-treated donor mice into antibiotic-treated mice induced MDSCs and increased regulatory T-cells (Tregs). Injecting TCDD directly into antibiotic-treated mice also induced MDSCs, although to a lesser extent. These data suggested that TCDD-induced dysbiosis plays a critical role in MDSC induction. Interestingly, treatment with TCDD led to induction of MDSCs in the colon and undetectable levels of cysteine. MDSCs suppressed T cell proliferation while reconstitution with cysteine restored this response. Lastly, blocking CXC chemokine receptor 2 (CXCR2) impeded TCDD-mediated MDSC induction. Our data demonstrate that AhR activation by TCDD triggers dysbiosis which, in turn, regulates, at least in part, induction of MDSCs.

The aryl hydrocarbon receptor as a mediator of host-microbiota interplay

Increasing evidence suggests a significant role for microbiota dependent metabolites and co-metabolites, acting as aryl hydrocarbon receptor (AHR) ligands, to facilitate bidirectional communication between the host and the microbiota and thus modulate physiology. Such communication is particularly evident within the gastrointestinal tract. Through binding to or activating the AHR, these metabolites play fundamental roles in various physiological processes and likely contribute to the maintenance of intestinal homeostasis. In recent years, tryptophan metabolites were screened to identify physiologically relevant AHR ligands or activators. The discovery of specific microbiota-derived indole-based metabolites as AHR ligands may provide insight concerning how these metabolites affect interactions between gut microbiota and host intestinal homeostasis and how this relates to chronic GI disease and overall health. A greater understanding of the mechanisms that modulate the production of such metabolites and associated AHR activity may be utilized to effectively treat inflammatory diseases and promote human health. Here, we review microbiota-derived AHR ligands generated from tryptophan that modulate host-gut microbiota interactions and discuss possible intervention strategies for potential therapies in the future.

Baicalein Restores the Balance of Th17/Treg Cells via Aryl Hydrocarbon Receptor to Attenuate Colitis

As one of the ligands of aryl hydrocarbon receptor (AhR), baicalein, isolated from Scutellaria baicalensis Georgi, has been proved to exert potential therapeutic effects on ulcerative colitis (UC), but its therapeutic mechanism remains obscure. Authentically, ulcerative colitis can be alleviated by regulating the differentiation of naïve CD4⁺ T cells via AhR activation. So, our study planned to prove the hypothesis that baicalein protected mice against UC by regulating the balance of Th17/Treg cells via AhR activation. Immunofluorescence and western blot results showed that baicalein could promote AhR activation and induce it to transfer to the nucleus. We further determined the effect of baicalein on naïve CD4⁺ T cell differentiation in vitro by magnetic cell separation and drug intervention. The results showed that baicalein could promote Treg cell differentiation by activating AhR. In vivo study, UC mice were established by free drinking of dextran sulfate sodium (DSS) for 7 days and then were orally administrated by baicalein (10, 20, and 40 mg/kg), TCDD (AhR agonist), and CH223191 (antagonist). The results demonstrated that baicalein improved the symptoms of UC mice, regulated the balance of Th17/Treg cells, and restored the balance of proinflammatory cytokines such as IL-17, IL-6, and TNF-α; anti-inflammatory cytokines such as IL-10 and TGF-β; and epithelial protective cytokine IL-22 in UC mice, and these effects were related to AhR. Taken together, our research found that baicalein might be a potential drug for UC via regulating Treg cell differentiation and maintaining immune homeostasis and attempted to shed a light on the pivotal role of AhR in these effects.

The potential therapeutic benefit of resveratrol on Th17/Treg imbalance in immune thrombocytopenic purpura

BACKGROUND

Immune thrombocytopenic purpura (ITP) is an autoimmune disease characterized by the restrained production of new platelets and the persistent reduction of existing platelets. An imbalance between Th17 and Treg cells is associated with a decrease in platelets. However, few therapeutic strategies aim to modulate this imbalance between Th17 and Treg cells in ITP.

METHODS

ITP patients and healthy controls were enrolled in this study. Quantitative real-time PCR (qRT-PCR) and Western blotting were performed to measure the expression of the aryl hydrocarbon receptor (AhR), cytochrome P450 family 1 member A1 (CYP1A1), RAR-related orphan receptor gamma t (ROR-γt) and forkhead-box P3 (Foxp3). ELISA was employed to measure the secretion of IL-17A, IL-22 and IL-10. Flow cytometry was used to assess the proportion of Th17 and Treg cells. A 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay was performed to measure cell viability.

RESULTS

The proportion of Th17 cells and the secretion of the pro-inflammatory cytokines IL-17A and IL-22 were both elevated, whereas the proportion of Treg cells and the production of the anti-inflammatory cytokine IL-10 were both reduced in ITP patients compared to healthy controls. The ratio of Th17/Treg cells and the expression of IL-17A and IL-22 displayed a positive correlation with the severity of ITP. Low and moderate concentrations of resveratrol did not affect the viability of CD4⁺ T cells from ITP patients but repressed Th17 differentiation and promoted Treg differentiation. Moreover, resveratrol could markedly downregulate the production of IL-17A and IL-22 and upregulate the secretion of IL-10 in CD4⁺ T cells in a time- and concentration-dependent manner. Mechanistic studies revealed that resveratrol exerted its beneficial function mainly through suppressing the AhR pathway, which led to the impaired expression of ROR-γt and reduced secretion of IL-17A and IL-22, as well as enhanced expression of Foxp3 and augmented secretion of IL-10. The induction of AhR by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in CD4⁺ T cells led to a Th17/Treg imbalance and the upregulation of IL-17A and IL-22, an effect that could be reversed by resveratrol treatment.

CONCLUSION

This study revealed that resveratrol reversed the Th17/Treg imbalance by a mechanism involving the suppression of the AhR pathway. Since ITP is characterized by a Th17/Treg imbalance, resveratrol might be beneficial for the treatment of this condition.

Regulation of the Immune Response by the Aryl Hydrocarbon Receptor

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that is activated by small molecules provided by the diet, microorganisms, metabolism, and pollutants. AhR is expressed by a number of immune cells, and thus AhR signaling provides a molecular pathway that integrates the effects of the environment and metabolism on the immune response. Studies have shown that AhR signaling plays important roles in the immune system in health and disease. As its activity is regulated by small molecules, AhR also constitutes a potential target for therapeutic immunomodulation. In this review we discuss the role of AhR in the regulation of the immune response in the context of autoimmunity, infection, and cancer, as well as the potential opportunities and challenges of developing AhR-targeted therapeutics.

The Aryl Hydrocarbon Receptor and Tumor Immunity

The aryl hydrocarbon receptor (AhR) is an important cytosolic, ligand-dependent transcription factor. Emerging evidence suggests the promoting role of the AhR in the initiation, promotion, progression, invasion, and metastasis of cancer cells. Studies on various tumor types and tumor cell lines have shown high AhR expression, suggesting that AhR is activated constitutively in tumors and facilitates their growth. Interestingly, immune evasion has been recognized as an emerging hallmark feature of cancer. A connection between the AhR and immune system has been recognized, which has been suggested as an immunosuppressive effector on different types of immune cells. Certain cancers can escape immune recognition via AhR signaling pathways. This review discusses the role of the AhR in tumor immunity and its potential mechanism of action in the tumor microenvironment.

TCDD, FICZ, and Other High Affinity AhR Ligands Dose-Dependently Determine the Fate of CD4+ T Cell Differentiation

FICZ and TCDD, two high-affinity AhR ligands, are reported to have opposite effects on T cell differentiation with TCDD inducing regulatory T cells and FICZ inducing Th17 cells. This dichotomy has been attributed to ligand-intrinsic differences in AhR activation, although differences in sensitivity to metabolism complicate the issue. TCDD is resistant to AhR-induced metabolism and produces sustained AhR activation following a single dose in the μg/kg range, whereas FICZ is rapidly metabolized and AhR activation is transient. Nonetheless, prior studies comparing FICZ with TCDD have generally used the same 10-50 μg/kg dose range, and thus the two ligands would not equivalently activate AhR. We hypothesized that high-affinity AhR ligands can promote CD4+ T cell differentiation into both Th17 cells and Tregs, with fate depending on the extent and duration of AhR activation. We compared the immunosuppressive effects of TCDD and FICZ, along with two other rapidly metabolized ligands (ITE and 11-Cl-BBQ) in an acute alloresponse mouse model. The dose and timing of administration of each ligand was optimized for TCDD-equivalent Cyp1a1 induction. When optimized, all of the ligands suppressed the alloresponse in conjunction with the induction of Foxp3- Tr1 cells on day 2 and the expansion of natural Foxp3+ Tregs on day 10. In contrast, a low dose of FICZ induced transient expression of Cyp1a1 and did not induce Tregs or suppress the alloresponse but enhanced IL-17 production. Interestingly, low doses of the other ligands, including TCDD, also increased IL-17 production on day 10. These findings support the conclusion that the dose and the duration of AhR activation by high-affinity AhR ligands are the primary factors driving the fate of T cell differentiation.

AhR activation increases IL-2 production by alloreactive CD4+ T cells initiating the differentiation of mucosal-homing Tim3+ Lag3+ Tr1 cells

Activation of the aryl hydrocarbon receptor (AhR) by immunosuppressive ligands promotes the development of regulatory T (Treg) cells. Although AhR-induced Foxp3+ Treg cells have been well studied, much less is known about the development and fate of AhR-induced Type 1 Treg (AhR-Tr1) cells. In the current study, we identified the unique transcriptional and functional changes in murine CD4+ T cells that accompany the differentiation of AhR-Tr1 cells during the CD4+ T-cell-dependent phase of an allospecific cytotoxic T lymphocyte (allo-CTL) response. AhR activation increased the expression of genes involved in T-cell activation, immune regulation and chemotaxis, as well as a global downregulation of genes involved in cell cycling.  Increased IL-2 production was responsible for the early AhR-Tr1 activation phenotype previously characterized as CD25+ CTLA4+ GITR+ on day 2. The AhR-Tr1 phenotype was further defined by the coexpression of the immunoregulatory receptors Lag3 and Tim3 and non-overlapping expression of CCR4 and CCR9. Consistent with the increased expression of CCR9, real-time imaging showed enhanced migration of AhR-Tr1 cells to the lamina propria of the small intestine and colon. The discovery of mucosal imprinting of AhR-Tr1 cells provides an additional mechanism by which therapeutic AhR ligands can control immunopathology.

Modulatory Influence of Segmented Filamentous Bacteria on Transcriptomic Response of Gnotobiotic Mice Exposed to TCDD

Environmental toxicants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), an aryl hydrocarbon receptor (AhR), are known to induce host toxicity and structural shifts in the gut microbiota. Key bacterial populations with similar or opposing functional responses to AhR ligand exposure may potentially help regulate expression of genes associated with immune dysfunction. To examine this question and the mechanisms for AhR ligand-induced bacterial shifts, C57BL/6 gnotobiotic mice were colonized with and without segmented filamentous bacteria (SFB) – an immune activator. Mice were also colonized with polysaccharide A producing Bacteroides fragilis – an immune suppressor to serve as a commensal background. Following colonization, mice were administered TCDD (30 μg/kg) every 4 days for 28 days by oral gavage. Quantified with the nCounter^® mouse immunology panel, opposing responses in ileal gene expression (e.g., genes associated with T-cell differentiation via the class II major histocompatibility complex) as a result of TCDD dosing and SFB colonization were observed. Genes that responded to TCDD in the presence of SFB did not show a significant response in the absence of SFB, and vice versa. Regulatory T-cells examined in the mesenteric lymph-nodes, spleen, and blood were also less impacted by TCDD in mice colonized with SFB. TCDD-induced shifts in abundance of SFB and B. fragilis compared with previous studies in mice with a traditional gut microbiome. With regard to the mouse model colonized with individual populations, results indicate that TCDD-induced host response was significantly modulated by the presence of SFB in the gut microbiome, providing insight into therapeutic potential between AhR ligands and key commensals.

Modeling the Effect of the Aryl Hydrocarbon Receptor on Transplant Immunity

BACKGROUND

Exposure to pollutants through inhalation is a risk factor for lung diseases including cancer, asthma, and lung transplant rejection, but knowledge of the effects of inhaled pollutants on pathologies outside of the lung is limited.

METHODS

Using the minor-mismatched model of male C57BL/6J (B6) to female B6 skin grafts, recipient mice were treated with an inhaled urban dust particle sample every 3 days before and after grafting. Graft survival time was determined, and analysis of the resulting immune response was performed at time before rejection.

RESULTS

Significant prolongation of male skin grafts occurred in recipient female mice treated with urban dust particles compared with controls and was found to be dependent on aryl hydrocarbon receptor (AHR) expression in the recipient mouse. T cell responses to the male histocompatibility antigen (H-Y) Dby were not altered by exposure to pollutants. A reduction in the frequency of IFNγ-producing CD4 T cells infiltrating the graft on day 7 posttransplant was observed. Flow cytometry analysis revealed that AHR expression is upregulated in IFNγ-producing CD4 T cells during immune responses in vitro and in vivo.

CONCLUSIONS

Surprisingly, inhalation of a pollutant standard was found to prolong graft survival in a minor-mismatched skin graft model in an AHR-dependent manner. One possible mechanism may be an effect on IFNγ-producing CD4 T cells responding to donor antigen. The increased expression of AHR in this CD4 T cell subset suggests that AHR ligands within the particulate matter may be directly affecting the type 1 T helper cell response in this model.

TCDD influences reservoir of antibiotic resistance genes in murine gut microbiome

Dysbiosis of the gut microbiome via antibiotics, changes in diet and infection can select for bacterial groups that more frequently harbor antimicrobial resistance genes (ARGs) and mobile genetic elements (MGEs). However, the impact of environmental toxicants on the reservoir of ARGs in the gut microbiome has received less attention. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent aryl hydrocarbon receptor (AhR) agonist with multiple toxic health effects including immune dysfunction. The selective pressure of TCDD on the abundance of ARG and MGE-harboring gut populations was examined using C57BL/6 mice exposed to 0-30 μg/kg TCDD for 28 and 92 days with the latter having a 30-day recovery period. DNA extracted from temporally collected fecal pellets was characterized using a qPCR array with 384 assays targeting ARGs and MGEs. Fourteen genes, typically observed in Enterobacteriaceae, increased significantly within 8 days of initial dosing, persisted throughout the treatment period, and remained induced 30 days post dosing. A qPCR primer set targeting Enterobacteriaceae also showed 10- to 100-fold higher abundance in TCDD-treated groups, which was further verified using metagenomics. Results show a bloom of ARG-harboring bacterial groups in the gut due to a xenobiotic compound that is not a metal, biocide or antimicrobial.

IL-27 triggers IL-10 production in Th17 cells via a c-Maf/RORγt/Blimp-1 signal to promote the progression of endometriosis

Endometriosis is an estrogen-dependent inflammatory disease. The anti-inflammatory cytokine IL-10 is also increased in endometriosis. IL-10 production by Th17 cells is critical for limiting autoimmunity and inflammatory responses. However, the mechanism of inducing IL-10-producing Th17 cells is still largely unknown. The present study investigated the differentiation mechanism and role of IL-10-producing Th17 cells in endometriosis. Here, we report that IL-10⁺Th17 cells are significantly increased in the peritoneal fluid of women with endometriosis, along with an elevation of IL-27, IL-6 and TGF-β. Compared with peripheral CD4⁺ T cells, endometrial CD4⁺ T cells highly expressed IL-27 receptors, especially the ectopic endometrium. Under external (2,3,7,8-tetrachlorodibenzo-p-dioxin, TCDD) and local (estrogen, IL-6 and TGF-β) environmental regulation, IL-27 from macrophages and endometrial stromal cells (ESCs) induces IL-10 production in Th17 cells in vitro and in vivo. This process may be mediated through the interaction between c-musculoaponeurotic fibrosarconna (c-Maf) and retinoic acid-related orphan receptor gamma t (RORγt), and associated with the upregulation of downstream B lymphocyte-induced maturation protein-1 (Blimp-1). IL-10⁺Th17 cells, in turn, stimulate the proliferation and implantation of ectopic lesions and accelerate the progression of endometriosis. These results suggest that IL-27 is a pivotal regulator in endometriotic immune tolerance by triggering Th17 cells to produce IL-10 and promoting the rapid growth and implantation of ectopic lesions. This finding provides a scientific basis for potential therapeutic strategies aimed at preventing the development of endometriosis, especially for patients with high levels of IL-10⁺Th17 cells.

Is chronic AhR activation by rapidly metabolized ligands safe for the treatment of immune-mediated diseases?

There is a long standing perception that AhR ligands are automatically disqualified from pharmaceutical development due to their induction of Cyp1a1 as well as their potential for causing "dioxin-like" toxicities. However, recent discoveries of new AhR ligands with potential therapeutic applications have been reported, inviting reconsideration of this policy. One area of exploration is focused on the activation of AhR to promote the generation of regulatory T cells, which control the intensity and duration of immune responses. Rapidly metabolized AhR ligands (RMAhRLs), which do not bioaccumulate in the same manner as 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) have been discovered that induce Tregs and display impressive therapeutic efficacy in a broad range of preclinical models of immune-mediated diseases. Given the promise of these RMAhRLs, is the bias against AhR activators still valid? Can RMAhRLs be given chronically to maintain therapeutic levels of AhR activation without producing the same toxicity profile as dioxin-like compounds? Based on our review of the data, there is little evidence to support the indiscriminate exclusion of AhR activators/Cyp1a1 inducers from early drug developmental pipelines. We also found no evidence that short-term treatment with RMAhRLs produce "dioxin-like toxicity" and, in fact, were well tolerated. However, safety testing of individual RMAhRLs under therapeutic conditions, as performed with all promising new drugs, will be needed to reveal whether or not chronic activation of AhR leads to unacceptable adverse outcomes.

Stellate Cell Activation and Imbalanced Expression of TGF-β1/TGF-β3 in Acute Autoimmune Liver Lesions Induced by ConA in Mice

Objective. To study the pathogenic feature of liver injury, activation of hepatic stellate cells, and dynamic expression of TGF-β1/TGF-β3 to reveal their role in liver injury induced by ConA. Methods. Mice were randomly divided into control group and ConA treatment group. ConA (20 mg/kg) was injected through vena caudalis in ConA treatment group; the controls received the same volume of saline injection. After injection for 2 h, 8 h, 24 h, and 48 h, animals were terminated. Blood, liver, and spleen were harvested. Liver function and histopathology were studied. α-SMA, vimentin, TGF-β1, and TGF-β3 were detected. Results. After ConA injection, liver damage started to increase. Expression of α-SMA, vimentin, TGF-β1, and TGF-β3 was significantly enhanced; all above indicators reached peak at 8 h; but from 24 h after ConA injection, TGF-β3 expression began to decline, while the TGF-β1/TGF-β3 ratio at 48 h was significantly lower than control. Conclusion. (1) Autoimmune liver injury induced by ConA showed time-based features, in which the most serious liver lesions happened at 8 h after ConA injection. (2) Early activation of HSC and imbalance expression of TGF-β1 and TGF-β3 existed in ConA-induced acute autoimmune liver injury, which may be associated with liver dysfunction and the mechanisms of progression to fibrosis.

Environmental Factors Impacting Bone-Relevant Chemokines

Chemokines play an important role in normal bone physiology and the pathophysiology of many bone diseases. The recent increased focus on the individual roles of this class of proteins in the context of bone has shown that members of the two major chemokine subfamilies-CC and CXC-support or promote the formation of new bone and the remodeling of existing bone in response to a myriad of stimuli. These chemotactic molecules are crucial in orchestrating appropriate cellular homing, osteoblastogenesis, and osteoclastogenesis during normal bone repair. Bone healing is a complex cascade of carefully regulated processes, including inflammation, progenitor cell recruitment, differentiation, and remodeling. The extensive role of chemokines in these processes and the known links between environmental contaminants and chemokine expression/activity leaves ample opportunity for disruption of bone healing by environmental factors. However, despite increased clinical awareness, the potential impact of many of these environmental factors on bone-related chemokines is still ill defined. A great deal of focus has been placed on environmental exposure to various endocrine disruptors (bisphenol A, phthalate esters, etc.), volatile organic compounds, dioxins, and heavy metals, though mainly in other tissues. Awareness of the impact of other less well-studied bone toxicants, such as fluoride, mold and fungal toxins, asbestos, and chlorine, is also reviewed. In many cases, the literature on these toxins in osteogenic models is lacking. However, research focused on their effects in other tissues and cell lines provides clues for where future resources could be best utilized. This review aims to serve as a current and exhaustive resource detailing the known links between several classes of high-interest environmental pollutants and their interaction with the chemokines relevant to bone healing.

Environmental Immunology: Lessons Learned from Exposure to a Select Panel of Immunotoxicants

Exposure to environmental contaminants can produce profound effects on the immune system. Many classes of xenobiotics can significantly suppress or enhance immune responsiveness depending on the levels (i.e., dose) and context (i.e., timing, route) of exposure. Although defining the effects that toxicants can have on the immune system is a valuable component to improving public health, environmental immunology has greatly enhanced our understanding of how the immune system functions and has provided innovative avenues to explore new immunotherapies. This Brief Review focuses on three examples of how immunotoxicology has benefitted the field of immunology, presenting information on the aryl hydrocarbon receptor signaling pathway, the immunomodulatory effects of nanomaterials, and the impact of xenobiotic exposure on the developing immune system. Collectively, contributions from immunotoxicology have significantly enhanced public health and spurred seminal advances in both basic and applied immunology.

Transgenic Overexpression of Aryl Hydrocarbon Receptor Repressor (AhRR) and AhR-Mediated Induction of CYP1A1, Cytokines, and Acute Toxicity

BACKGROUND

The aryl hydrocarbon receptor repressor (AhRR) is known to repress aryl hydrocarbon receptor (AhR) signaling, but very little is known regarding the role of the AhRR in vivo.

OBJECTIVE

This study tested the role of AhRR in vivo in AhRR overexpressing mice on molecular and toxic end points mediated through a prototypical AhR ligand.

METHODS

We generated AhRR-transgenic mice (AhRR Tg) based on the genetic background of C57BL/6J wild type (wt) mice. We tested the effect of the prototypical AhR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the expression of cytochrome P450 (CYP)1A1 and cytokines in various tissues of mice. We next analyzed the infiltration of immune cells in adipose tissue of mice after treatment with TCDD using flow cytometry.

RESULTS

AhRR Tg mice express significantly higher levels of AhRR compared to wt mice. Activation of AhR by TCDD caused a significant increase of the inflammatory cytokines Interleukin (IL)-1β, IL-6 and IL-10, and CXCL chemokines in white epididymal adipose tissue from both wt and AhRR Tg mice. However, the expression of IL-1β, CXCL2 and CXCL3 were significantly lower in AhRR Tg versus wt mice following TCDD treatment. Exposure to TCDD caused a rapid accumulation of neutrophils and macrophages in white adipose tissue of wt and AhRR Tg mice. Furthermore we found that male AhRR Tg mice were protected from high-dose TCDD-induced lethality associated with a reduced inflammatory response and liver damage as indicated by lower levels of TCDD-induced alanine aminotransferase and hepatic triglycerides. Females from both wt and AhRR Tg mice were less sensitive than male mice to acute toxicity induced by TCDD.

CONCLUSION

In conclusion, the current study identifies AhRR as a previously uncharacterized regulator of specific inflammatory cytokines, which may protect from acute toxicity induced by TCDD.

CITATION

Vogel CF, Chang WL, Kado S, McCulloh K, Vogel H, Wu D, Haarmann-Stemmann T, Yang GX, Leung PS, Matsumura F, Gershwin ME. 2016. Transgenic overexpression of aryl hydrocarbon receptor repressor (AhRR) and AhR-mediated induction of CYP1A1, cytokines, and acute toxicity. Environ Health Perspect 124:1071-1083; http://dx.doi.org/10.1289/ehp.1510194.

A Role for Regulatory T Cells in a Murine Model of Epicutaneous Toluene Diisocyanate Sensitization

Toluene diisocyanate (TDI) is a leading cause of chemical-induced occupational asthma which impacts workers in a variety of industries worldwide. Recently, the robust regulatory potential of regulatory T cells (Tregs) has become apparent, including their functional role in the regulation of allergic disease; however, their function in TDI-induced sensitization has not been explored. To elucidate the kinetics, phenotype, and function of Tregs during TDI sensitization, BALB/c mice were dermally exposed (on each ear) to a single application of TDI (0.5-4% v/v) or acetone vehicle and endpoints were evaluated via RT-PCR and flow cytometry. The draining lymph node (dLN) Treg population expanded significantly 4, 7, and 9 days after single 4% TDI exposure. This population was identified using a variety of surface and intracellular markers and was found to be phenotypically heterogeneous based on increased expression of markers including CD103, CCR6, CTLA4, ICOS, and Neuropilin-1 during TDI sensitization. Tregs isolated from TDI-sensitized mice were significantly more suppressive compared with their control counterparts, further supporting a functional role for Tregs during TDI sensitization. Last, Tregs were depleted prior to TDI sensitization and an intensified sensitization response was observed. Collectively, these data indicate that Tregs exhibit a functional role during TDI sensitization. Because the role of Tregs in TDI sensitization has not been previously elucidated, these data contribute to the understanding of the immunologic mechanisms of chemical induced allergic disease.

Activation of the Aryl Hydrocarbon Receptor by 10-Cl-BBQ Prevents Insulitis and Effector T Cell Development Independently of Foxp3+ Regulatory T Cells in Nonobese Diabetic Mice

Aryl hydrocarbon receptor (AhR) activation by high-affinity ligands mediates immunosuppression in association with increased regulatory T cells (Tregs), making this transcription factor an attractive therapeutic target for autoimmune diseases. We recently discovered 10-chloro-7H-benzimidazo[2,1-a]benzo[de]iso-quinolin-7-one (10-Cl-BBQ), a nanomolar affinity AhR ligand with immunosuppressive activity and favorable pharmacologic properties. In this study, we tested the consequences of AhR activation in the NOD model. Oral 10-Cl-BBQ treatment prevented islet infiltration without clinical toxicity, whereas AhR-deficient NOD mice were not protected. Suppression of insulitis was associated with an increased frequency, but not total number, of Foxp3(+) Tregs in the pancreas and pancreatic lymph nodes. The requirement for Foxp3(+) cells in AhR-induced suppression of insulitis was tested using NOD.Foxp3(DTR) mice, which show extensive islet infiltration upon treatment with diphtheria toxin. AhR activation prevented the development of insulitis caused by the depletion of Foxp3(+) cells, demonstrating that Foxp3(+) cells are not required for AhR-mediated suppression and furthermore that the AhR pathway is able to compensate for the absence of Foxp3(+) Tregs, countering current dogma. Concurrently, the development of disease-associated CD4(+)Nrp1(+)Foxp3(-)RORγt(+) cells was inhibited by AhR activation. Taken together, 10-Cl-BBQ is an effective, nontoxic AhR ligand for the intervention of immune-mediated diseases that functions independently of Foxp3(+) Tregs to suppress pathogenic T cell development.

Aryl hydrocarbon receptor ligands in cancer: friend and foe

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that is best known for mediating the toxicity and tumour-promoting properties of the carcinogen 2,3,7,8-tetrachlorodibenzo-p-dioxin, commonly referred to as ‘dioxin’. AHR influences the major stages of tumorigenesis — initiation, promotion, progression and metastasis — and physiologically relevant AHR ligands are often formed during disease states or during heightened innate and adaptive immune responses. Interestingly, ligand specificity and affinity vary between rodents and humans. Studies of aggressive tumours and tumour cell lines show increased levels of AHR and constitutive localization of this receptor in the nucleus. This suggests that the AHR is chronically activated in tumours, thus facilitating tumour progression. This Review discusses the role of AHR in tumorigenesis and the potential for therapeutic modulation of its activity in tumours.

Expert panel workshop consensus statement on the role of the environment in the development of autoimmune disease

Autoimmune diseases include 80 or more complex disorders characterized by self-reactive, pathologic immune responses in which genetic susceptibility is largely insufficient to determine disease onset. In September 2010, the National Institute of Environmental Health Sciences (NIEHS) organized an expert panel workshop to evaluate the role of environmental factors in autoimmune diseases, and the state of the science regarding relevant mechanisms, animal models, and human studies. The objective of the workshop was to analyze the existing data to identify conclusions that could be drawn regarding environmental exposures and autoimmunity and to identify critical knowledge gaps and areas of uncertainty for future study. This consensus document summarizes key findings from published workshop monographs on areas in which “confident” and “likely” assessments were made, with recommendations for further research. Transcribed notes and slides were reviewed to synthesize an overview on exposure assessment and questions addressed by interdisciplinary panels. Critical advances in the field of autoimmune disease research have been made in the past decade. Collaborative translational and interdisciplinary research is needed to elucidate the role of environmental factors in autoimmune diseases. A focus on exposure assessment methodology is needed to improve the effectiveness of human studies, and more experimental studies are needed to focus on causal mechanisms underlying observed associations of environmental factors with autoimmune disease in humans.

Role of AhR/ARNT system in skin homeostasis

Aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that binds to structurally diverse synthetic and naturally occurring chemicals including dioxins, flavonoids, tryptophan photoproducts, and Malassezia metabolites. Upon binding to its ligands, cytoplasmic AhR translocates to the nucleus, heterodimerizes with aryl hydrocarbon receptor nuclear translocator (ARNT), and mediates numerous biological and toxicological effects by inducing the transcription of various AhR-responsive genes. AhR ligation controls oxidation/antioxidation, epidermal barrier function, photo-induced response, melanogenesis, and innate immunity. This review summarizes recent advances in the understanding of the regulatory mechanisms of skin homeostasis mediated by the AhR/ARNT system.

Benzimidazoisoquinolines: a new class of rapidly metabolized aryl hydrocarbon receptor (AhR) ligands that induce AhR-dependent Tregs and prevent murine graft-versus-host disease

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that plays multiple roles in regulation of immune and inflammatory responses. The ability of certain AhR ligands to induce regulatory T cells (Tregs) has generated interest in developing AhR ligands for therapeutic treatment of immune-mediated diseases. To this end, we designed a screen for novel Treg-inducing compounds based on our understanding of the mechanisms of Treg induction by the well-characterized immunosuppressive AhR ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). We screened a ChemBridge small molecule library and identified 10-chloro-7H-benzimidazo[2,1-a]benzo[de]Iso-quinolin-7-one (10-Cl-BBQ) as a potent AhR ligand that was rapidly metabolized and not cytotoxic to proliferating T cells. Like TCDD,10-Cl-BBQ altered donor CD4⁺ T cell differentiation during the early stages of a graft versus host (GVH) response resulting in expression of high levels of CD25, CTLA-4 and ICOS, as well as several genes associated with Treg function. The Treg phenotype required AhR expression in the donor CD4⁺ T cells. Foxp3 was not expressed in the AhR-induced Tregs implicating AhR as an independent transcription factor for Treg induction. Structure-activity studies showed that unsubstituted BBQ as well as 4, 11-dichloro-BBQ were capable of inducing AhR-Tregs. Other substitutions reduced activation of AhR. Daily treatment with 10-Cl-BBQ during the GVH response prevented development of GVH disease in an AhR-dependent manner with no overt toxicity. Together, our data provide strong support for development of select BBQs that activate the AhR to induce Tregs for treatment of immune-mediated diseases.

Regulation of central nervous system autoimmunity by the aryl hydrocarbon receptor

The ligand-activated transcription factor aryl hydrocarbon receptor controls the activity of several components of the immune system, many of which play an important role in neuroinflammation. This review discusses the role of AhR in T cells and dendritic cells, its relevance for the control of autoimmunity in the central nervous system, and its potential as a therapeutic target for immune-mediated disorders.

The immunotoxic effects of dual exposure to PCP and TCDD

Pentachlorophenol (PCP) was a commonly used fungicide, herbicide, insecticide, and bactericide in industrial, agricultural, and domestic settings; however, it was also contaminated with polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). It has been reported that technical grade PCP had immunosuppressive effects and that the immune system was the major target of PCDD/PCDFs toxicity. Although the immune response after exposure to PCP or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been studied, the toxic effects of exposure to both PCP and TCDD have not yet been reported. The aim of this study was to evaluate the effects on immune cells from mice intraperitoneally immunized with OVA and subsequently treated with PCP or TCDD alone or in combination by gavage. The animals were terminated on day 7 and 14, and the spleen and plasma samples were collected for immunotoxicity evaluation. The numbers and populations of splenocytes, T cell-derived cytokines produced by splenocytes, splenocyte-generated cytotoxicity and OVA-specific antibodies in plasma were investigated. Our results indicate that the spleen/body weight ratio and splenocyte number was reduced by TCDD alone; in addition, this reduction was enhanced when TCDD was combined with PCP. Exposure to TCDD alone or in conjunction with PCP suppressed many ovalbumin (OVA)-stimulated cytokines, including IL-2, IFN-γ, IL-4, IL-5, and IL-10. Furthermore, the immunoglobulins IgG and IgM were suppressed in mice administered by PCP alone, but the suppressive effects were greater in mice treated with TCDD alone or in combination with PCP. Co-exposure to PCP and TCDD resulted in an antagonistic effect on TCDD-induced suppression of IFN-γ and IL-10. Our results demonstrate that PCP alone is immunotoxic, regardless of the presence of TCDD. PCP led to mild changes in cytokine secretion, and it compromised splenocyte-generated cytotoxicity and IgM and IgG antibody production on day 7. The finding that PCP antagonizes TCDD-induced IFN-γ suppression could be due to the competitive binding of PCP to AhR (aryl hydrocarbon receptor).

Suppression of acute graft-versus-host response by TCDD is independent of the CTLA-4-IFN-γ-IDO pathway

Activation of the aryl hydrocarbon receptor (AhR) by its prototypic ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), induces potent suppression of an acute graft-versus-host (GVH) response and prevents GVH disease (GVHD). Suppression is associated with development of a regulatory population of donor CD4(+) CD25(+)T-cells that express high levels of cytotoxic T-lymphocyte antigen 4 (CTLA-4). However, a direct link between these AhR-induced Tregs (AhR-Tregs) and suppression of GVHD remains to be shown. CTLA-4 is a negative regulator of T-cell responses and is associated with the induction of tolerogenic dendritic cells (DCs) that produce indoleamine 2,3-dioxygenase (IDO). We hypothesized that AhR-Tregs mediate suppression via their enhanced expression of CTLA-4, which, in turn, induces IFN-γ and IDO in host DCs. Subsequent depletion of tryptophan by IDO leads to termination of the donor T-cell response prior to development of effector CTL. Here, we show that despite increased expression of Ifng, Irf3, Irf7, Ido1, and Ido2 in the lymph nodes of TCDD-treated host mice, inhibition of IDO enzyme activity by 1-methyl-tryptophan was unable to relieve TCDD-mediated suppression of the GVH response. Furthermore, treatment with an anti-CTLA-4 antibody that blocks CTLA-4 signaling was also unable to alleviate TCDD-mediated suppression. Alternatively, we investigated the possibility that donor-derived AhR-Tregs produce IFN-γ to suppress effector CTL development. However, suppression of GVHD by TCDD was not affected by the use of Ifng-deficient donor cells. Together, these results indicate that neither overexpression of CTLA-4 nor production of IFN-γ by AhR-Tregs plays a major role in the manifestation of their immunosuppressive function in vivo.

Role of the aryl hydrocarbon receptor in carcinogenesis and potential as a drug target

The aryl hydrocarbon receptor (AHR) is highly expressed in multiple organs and tissues, and there is increasing evidence that the AHR plays an important role in cellular homeostasis and disease. The AHR is expressed in multiple tumor types, in cancer cell lines, and in tumors from animal models, and the function of the AHR has been determined by RNA interference, overexpression, and inhibition studies. With few exceptions, knockdown of the AHR resulted in decreased proliferation and/or invasion and migration of cancer cell lines, and in vivo studies in mice overexpressing the constitutively active AHR exhibited enhanced stomach and liver cancers, suggesting a pro-oncogenic role for the AHR. In contrast, loss of the AHR in transgenic mice that spontaneously develop colonic tumors and in carcinogen-induced liver tumors resulted in increased carcinogenesis, suggesting that the receptor may exhibit antitumorigenic activity prior to tumor formation. AHR ligands also either enhanced or inhibited tumorigenesis, and these effects were highly tumor specific, demonstrating that selective AHR modulators that exhibit agonist or antagonist activities represent an important new class of anticancer agents that can be directed against multiple tumors.

Aryl hydrocarbon receptor control of adaptive immunity

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that belongs to the family of basic helix-loop-helix transcription factors. Although the AhR was initially recognized as the receptor mediating the pathologic effects of dioxins and other pollutants, the activation of AhR by endogenous and environmental factors has important physiologic effects, including the regulation of the immune response. Thus, the AhR provides a molecular pathway through which environmental factors modulate the immune response in health and disease. In this review, we discuss the role of AhR in the regulation of the immune response, the source and chemical nature of AhR ligands, factors controlling production and degradation of AhR ligands, and the potential to target the AhR for therapeutic immunomodulation.

The emerging roles of AhR in physiology and immunity

The aryl hydrocarbon receptor (AhR) is traditionally defined as a transcriptional regulator involved in adaptive xenobiotic response, however, emerging evidence supports physiological functions of AhR in normal cell development and immune response. The role of AhR in immunomodulation is multi-dimensional. On the one hand, activation of AhR by TCDD and other ligands leads to profound immunosuppression, potentially via skewed Th1/Th2 cell balance toward Th1 dominance, and boosted Treg cell differentiation. On the other hand, activation of AhR can also induce Th17 cell polarization and increase the severity of autoimmune disease. In addition to T lymphocytes, the AhR also appears to play a vital role in B cell maturation, and regulates the activity of macrophages, dendritic cells and neutrophils following lipopolysaccharide challenge or influenza virus infection. In these scenarios, activation of AhR is associated with decreased host response and reduced survival. Furthermore, gene knock out studies suggest that AhR is indispensable for the postnatal maintenance of intestinal intraepithelial lymphocytes and skin-resident dendritic epidermal gamma delta T cells, providing a potential link between AhR and gut immunity and wound healing. It is well accepted that the magnitude and the type of immune response is dependent on the local cytokine milieu and the AhR appears to be one of the key factors involved in the fine turning of this cytokine balance.

Reduction of CC-chemokine ligand 5 by aryl hydrocarbon receptor ligands

BACKGROUND

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that recognizes a large number of xenobiotics, such as polycyclic aromatic hydrocarbons (PAHs), dioxins, and some endogenous ligands. Despite numerous investigations targeting AhR ligands, the precise physiological role of AhR remains unknown.

OBJECTIVE

We explored novel AhR target genes, especially focused on inflammatory chemokine.

METHODS

We treated (1) HaCaT, a human keratinocyte cell line, (2) normal human epidermal keratinocytes (NHEKs), and (3) mouse primary keratinocytes with AhR ligands, such as 6-formylindolo[3,2-b]carbazole (FICZ; endogenous ligand) and benzo[a]pyrene (BaP; exogenous ligand). Then, we detected mRNA and protein of chemokine using quantitative RT-PCR and ELISA. We next clarified the relationship between AhR and chemokine expression using AhR siRNA. In addition, we measured serum chemokine levels in patients with Yusho disease (oil disease), who were accidentally exposed to dioxins in the past.

RESULTS

We identified CC-chemokine ligand 5 (CCL5), a key mediator in the development of inflammatory responses, as the AhR target gene. AhR ligands (FICZ and BaP) significantly reduced CCL5 mRNA and protein expression in HaCaT cells. These effects were observed in NHEKs and mouse primary keratinocytes. AhR knockdown with siRNA restored CCL5 inhibition by AhR ligands. In addition, AhR ligands exhibited a dose-dependent suppression of CCL5 production induced by Th1-derived cytokines. Finally, serum levels of CCL5 in patients with Yusho disease, were significantly lower than in controls.

CONCLUSION

Our findings indicate that CCL5 is a target gene for AhR, and might be associated with the pathology of dioxin exposure.

The Aryl hydrocarbon receptor is involved in UVR-induced immunosuppression

UVR suppresses the immune system through the induction of regulatory T cells (Tregs). UVR-induced DNA damage has been recognized as the major molecular trigger involved, as reduction of DNA damage by enhanced repair prevents the compromise to the immune system by UVR. Nevertheless, other signaling events may also be involved. The aryl hydrocarbon receptor (AhR) was identified as another target for UVR, as UVR activates the AhR and certain UVR effects were not detected in AhR-deficient cells. We studied whether the AhR is involved in UVR-induced local immunosuppression and whether similar effects can be induced by AhR agonists. The AhR antagonist 3-methoxy-4-nitroflavone reduced UVR-mediated immunosuppression and the induction of Tregs in murine contact hypersensitivity (CHS). Conversely, activation of the AhR by the agonist 4-n-nonylphenol (NP) suppressed the induction of CHS and induced antigen-specific Tregs similar to UVR. This was further confirmed in AhR knockout mice in which UVR- and NP-induced immunosuppression were significantly reduced. Together, this indicates that the AhR is involved in mediating UVR-induced immunosuppression. Activation of the AhR might represent an alternative to modulate the immune system in a similar manner like UVR but without causing the adverse effects of UVR, including DNA damage.

The evolving role of the aryl hydrocarbon receptor (AHR) in the normophysiology of hematopoiesis

In addition to its role as a toxicological signal mediator, the Aryl Hydrocarbon Receptor (AHR) is also a transcription factor known to regulate cellular responses to oxidative stress and inflammation through transcriptional regulation of molecules involved in the signaling of nucear factor-erythroid 2-related factor-2 (Nrf2), p53 (TRP53), retinoblastoma (RB1), and NFκB. Recent research suggests that AHR activation of these signaling pathways may provide the molecular basis for understanding AHR's evolving role in endogenous developmental functions during hematopoietic stem-cell maintenance and differentiation. Recent developments into the hematopoietic roles for AHR are reviewed, aiming to reconcile divergent findings as to the endogenous function of AHR in hematopoiesis. Potential mechanistic explanations for AHR's involvement in hematopoietic differentiation are discussed, focusing on its known role as a cell cycle mediator and its interactions with Hypoxia-inducible transcription factor-1 alpha (HIF1-α). Understanding the physiological mechanisms of AHR activation and signaling have far reaching implications ranging from explaining the action of various toxicological agents to providing novel ways to expand stem cell populations ex vivo for use in transplant therapies.

Use of natural AhR ligands as potential therapeutic modalities against inflammatory disorders

The aim of this review is to discuss research involving ligands for the aryl hydrocarbon receptor (AhR) and their role in immunomodulation. While activation of the AhR is well known for its ability to regulate the biochemical and toxic effects of environmental chemicals, more recently an exciting discovery has been made indicating that AhR ligation can also regulate T-cell differentiation, specifically through activation of Foxp3(+) regulatory T cells (Tregs) and downregulation of the proinflammatory Th17 cells. Such findings have opened new avenues of research on the possibility of targeting the AhR to treat inflammatory and autoimmune diseases. Specifically, this review will discuss the current research involving natural and dietary AhR ligands. In addition, evidence indicating the potential use of these ligands in regulating inflammation in various diseases will be highlighted. The importance of the AhR in immunological processes can be illustrated by expression of this receptor on a majority of immune cell types. In addition, AhR signaling pathways have been reported to influence a number of genes responsible for mediating inflammation and other immune responses. As interest in the AhR and its ligands increases, it seems prudent to consolidate current research on the contributions of these ligands to immune regulation during the course of inflammatory diseases.

Cytokines as molecular targets for aryl hydrocarbon receptor ligands: implications for toxicity and xenobiotic detoxification

INTRODUCTION

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor historically known for regulating expression of several important drug-detoxifying proteins. Besides drug metabolism pathways, cytokines have been recently recognized as targeted by the AhR signaling cascade, which may contribute to toxicity and changes in xenobiotic detoxification caused by AhR agonists.

AREAS COVERED

This article summarizes the nature of the main cytokines regulated by AhR ligands and reviews their involvement in toxic effects of AhR ligands, especially in relation with inflammation. The article also discusses the potential implications for drug detoxification pathways.

EXPERT OPINION

Even if various cytokines, including inflammatory ones, have already been demonstrated to constitute robust targets for AhR, the exact role played by AhR with respect to inflammation remains to be determined. Further studies are also required to better characterize the molecular mechanisms implicated in regulation of cytokines by AhR ligands and to determine the role that may play AhR-targeted cytokines in alteration of xenobiotic detoxification. Finally, changes in cytokine receptor expression triggered by AhR ligands have additionally to be taken into account to better and more extensively comprehend the role played by AhR in the cytokine/inflammation area.

Novel cellular targets of AhR underlie alterations in neutrophilic inflammation and inducible nitric oxide synthase expression during influenza virus infection

The underlying reasons for variable clinical outcomes from respiratory viral infections remain uncertain. Several studies suggest that environmental factors contribute to this variation, but limited knowledge of cellular and molecular targets of these agents hampers our ability to quantify or modify their contribution to disease and improve public health. The aryl hydrocarbon receptor (AhR) is an environment-sensing transcription factor that binds many anthropogenic and natural chemicals. The immunomodulatory properties of AhR ligands are best characterized with extensive studies of changes in CD4(+) T cell responses. Yet, AhR modulates other aspects of immune function. We previously showed that during influenza virus infection, AhR activation modulates neutrophil accumulation in the lung, and this contributes to increased mortality in mice. Enhanced levels of inducible NO synthase (iNOS) in infected lungs are observed during the same time frame as AhR-mediated increased pulmonary neutrophilia. In this study, we evaluated whether these two consequences of AhR activation are causally linked. Reciprocal inhibition of AhR-mediated elevations in iNOS and pulmonary neutrophilia reveal that although they are contemporaneous, they are not causally related. We show using Cre/loxP technology that elevated iNOS levels and neutrophil number in the infected lung result from separate, AhR-dependent signaling in endothelial and respiratory epithelial cells, respectively. Studies using mutant mice further reveal that AhR-mediated alterations in these innate responses to infection require a functional nuclear localization signal and DNA binding domain. Thus, gene targets of AhR in non-hematopoietic cells are important new considerations for understanding AhR-mediated changes in innate anti-viral immunity.

Expression of the aryl hydrocarbon receptor pathway and cyclooxygenase-2 in dog tumors

In humans, the aryl hydrocarbon receptor (AHR) gene battery constitutes a set of contaminant-responsive genes, which have been recently shown to be involved in the regulation of several patho-physiological conditions, including tumorigenesis. As the domestic dog represents a valuable animal model in comparative oncology, mRNA levels of cytochromes P450 1A1, 1A2 and 1B1 (CYP1A1, 1A2 and 1B1), AHR, AHR nuclear translocator (ARNT), AHR repressor (AHRR, whose partial sequence was here obtained) and cyclooxygenase-2 (COX2) were measured in dog control tissues (liver, skin, mammary gland and bone), in 47 mast cell tumors (MCTs), 32 mammary tumors (MTs), 5 osteosarcoma (OSA) and related surgical margins. Target genes were constitutively expressed in the dog, confirming the available human data. Furthermore, their pattern of expression in tumor biopsies was comparable to that already described in a variety of human cancers; in particular, both AHR and COX2 genes were up-regulated and positively correlated, while CYP1A1 and CYP1A2 mRNAs were generally poorly expressed. This work demonstrated for the first time that target mRNAs are expressed in neoplastic tissues of dogs, thereby increasing the knowledge about dog cancer biology and confirming this species as an useful animal model for comparative studies on human oncology.

Aryl Hydrocarbon Receptor-Mediated Perturbations in Gene Expression during Early Stages of CD4(+) T-cell Differentiation

Activation of the aryl hydrocarbon receptor (AhR) by its prototypic ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), mediates potent suppression of T-cell dependent immune responses. The suppressive effects of TCDD occur early during CD4(+) T-cell differentiation in the absence of effects on proliferation and have recently been associated with the induction of AhR-dependent regulatory T-cells (Treg). Since AhR functions as a ligand-activated transcription factor, changes in gene expression induced by TCDD during the early stages of CD4(+) T-cell differentiation are likely to reflect fundamental mechanisms of AhR action. A custom panel of genes associated with T-cell differentiation was used to query changes in gene expression induced by exposure to 1 nM TCDD. CD4(+) T-cells from AhR(+/+) and AhR(-/-) mice were cultured with cytokines known to polarize the differentiation of T-cells to various effector lineages. Treatment with TCDD induced the expression of Cyp1a1, Cyp1b1, and Ahrr in CD4(+) T-cells from AhR(+/+) mice under all culture conditions, validating the presence and activation of AhR in these cells. The highest levels of AhR activation occurred under Th17 conditions at 24 h and Tr1 conditions at 48 h. Unexpectedly, expression levels of most genes associated with early T-cell differentiation were unaltered by AhR activation, including lineage-specific genes that drive CD4(+) T-cell polarization. The major exception was AhR-dependent up-regulation of Il22 that was seen under all culture conditions. Independent of TCDD, AhR down-regulated the expression of Il17a and Rorc based on increased expression of these genes in AhR-deficient cells across culture conditions. These findings are consistent with a role for AhR in down-regulation of inflammatory immune responses and implicate IL-22 as a potential contributor to the immunosuppressive effects of TCDD.

Mechanisms of environmental influence on human autoimmunity: a National Institute of Environmental Health Sciences expert panel workshop

The mechanisms leading to autoimmune diseases remain largely unknown despite numerous lines of experimental inquiry and epidemiological evidence. The growing number of genome-wide association studies and the largely incomplete concordance for autoimmune diseases in monozygotic twins support the role of the environment (including infectious agents and chemicals) in the breakdown of tolerance leading to autoimmunity via numerous mechanisms. The present article reviews the major theories on the mechanisms of the environmental influence on autoimmunity by addressing the different degrees of confidence that characterize our knowledge. The theories discussed herein include (i) the role of innate immunity mediated by toll-like receptors in triggering the autoimmune adaptive response characterizing the observed pathology; (ii) changes in spleen marginal zone B cells in autoantibody production with particular focus on the B10 subpopulation; (iii) Th17 cell differentiation and T regulatory cells in the aryl hydrocarbon receptor model; (iv) self antigen changes induced by chemical and infectious agents which could break tolerance by post-translational modifications and molecular mimicry; and finally (v) epigenetic changes, particularly DNA methylation, that are induced by environmental stimuli and may contribute to autoimmunity initiation. We are convinced that these working hypotheses, in most cases supported by solid evidence, should be viewed in parallel with animal models and epidemiological observations to provide a comprehensive picture of the environmental causes of autoimmune diseases.