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

Regulation of gut microbiota on immune cell ferroptosis: A novel insight for immunotherapy against tumor

Gut microbiota contributes to the homeostasis of immune system and is related to various diseases such as tumorigenesis. Ferroptosis, a new type of cell death, is also involved in the disease pathogenesis. Recent studies have found the correlations of gut microbiota mediated ferroptosis and immune cell death. Gut microbiota derived immunosuppressive metabolites, which can promote differentiation and function of immune cells, tend to inhibit ferroptosis through their receptors, whereas inflammatory metabolites from gut microbiota also affect the differentiation and function of immune cells and their ferroptosis. Thus, it is possible for gut microbiota to regulate immune cell ferroptosis. Indeed, gut microbiota metabolite receptor aryl hydrocarbon receptor (AhR) can affect ferroptosis of intestinal intraepithelial lymphocytes, leading to disease pathogenesis. Since immune cell ferroptosis in tumor microenvironment (TME) affects the occurrence and development of tumor, the modulation of gut microbiota in these cell ferroptosis might influence on the tumorigenesis, and also immunotherapy against tumors. Here we will summarize the recent advance of ferroptosis mediated by gut microbiota metabolites, which potentially acts as regulator(s) on immune cells in TME for therapy against tumor.

Activation of the aryl hydrocarbon receptor improves allergen-specific immunotherapy of murine allergic airway inflammation: a novel adjuvant option?

Background

Allergen-specific immunotherapy (AIT) is able to restore immune tolerance to allergens in allergic patients. However, some patients do not or only poorly respond to current treatment protocols. Therefore, there is a need for deeper mechanistic insights and further improvement of treatment strategies. The relevance of the aryl hydrocarbon receptor (AhR), a ligand-dependent transcription factor, has been investigated in several inflammatory diseases, including allergic asthma. However, its potential role in AIT still needs to be addressed.

Methods

A murine model of AIT in ovalbumin-induced allergic airway inflammation was performed in AhR-deficient (AhR-/-) and wild-type mice. Furthermore, AIT was combined with the application of the high-affinity AhR agonist 10-chloro-7H-benzimidazo[2,1-a]benzo[de]iso-quinolin-7-one (10-Cl-BBQ) as an adjuvant to investigate the effects of AhR activation on therapeutic outcome.

Results

Although AhR-/- mice suffer stronger allergic responses than wild-type mice, experimental AIT is comparably effective in both. Nevertheless, combining AIT with the administration of 10-Cl-BBQ improved therapeutic effects by an AhR-dependent mechanism, resulting in decreased cell counts in the bronchoalveolar fluid, decreased pulmonary Th2 and Th17 cell levels, and lower sIgE levels.

Conclusion

This study demonstrates that the success of AIT is not dependent on the AhR. However, targeting the AhR during AIT can help to dampen inflammation and improve tolerogenic vaccination. Therefore, AhR ligands might represent promising candidates as immunomodulators to enhance the efficacy of AIT.

The Role of Endocrine Disruption Chemical-Regulated Aryl Hydrocarbon Receptor Activity in the Pathogenesis of Pancreatic Diseases and Cancer

The aryl hydrocarbon receptor (AHR) serves as a ligand-activated transcription factor crucial for regulating fundamental cellular and molecular processes, such as xenobiotic metabolism, immune responses, and cancer development. Notably, a spectrum of endocrine-disrupting chemicals (EDCs) act as agonists or antagonists of AHR, leading to the dysregulation of pivotal cellular and molecular processes and endocrine system disruption. Accumulating evidence suggests a correlation between EDC exposure and the onset of diverse pancreatic diseases, including diabetes, pancreatitis, and pancreatic cancer. Despite this association, the mechanistic role of AHR as a linchpin molecule in EDC exposure-related pathogenesis of pancreatic diseases and cancer remains unexplored. This review comprehensively examines the involvement of AHR in EDC exposure-mediated regulation of pancreatic pathogenesis, emphasizing AHR as a potential therapeutic target for the pathogenesis of pancreatic diseases and cancer.

Gut microbiota: A double-edged sword in immune checkpoint blockade immunotherapy against tumors

Tumor cells can evade immune surveillance by expressing immune checkpoint molecule ligands, resulting in effective immune cell inactivation. Immune checkpoint blockades (ICBs) have dramatically improved survival of patients with multiple types of cancers. However, responses to ICB immunotherapy are heterogeneous with lower patient response rates. The advances have established that the gut microbiota can be as a promising target to overcome resistance to ICB immunotherapy. Furthermore, some bacterial species have shown to promote improved responses to ICBs. However, gut microbiota is critical in maintaining gut and systemic immune homeostasis. It not only promotes differentiation and function of immunosuppressive immune cells but also inhibits inflammatory cells via gut microbiota derived products such as short chain fatty acids (SCFAs), tryptophan (Trp) and bile acid (BA) metabolites, which play an important role in tumor immunity. Since the gut microbiota can either inhibit or enhance immune against tumor, it should be a double-edged sword in ICBs against tumor. In this review, we discuss the effects of gut microbiota on immune cells and also tumor cells, especially enhances of gut microbiota on ICB immunotherapy. These discussions can hopefully promote the development of ICB immunotherapy.

Activation of the aryl hydrocarbon receptor inhibits neuropilin-1 upregulation on IL-2-responding CD4+ T cells

Neuropilin-1 (Nrp1), a transmembrane protein expressed on CD4+ T cells, is mostly studied in the context of regulatory T cell (Treg) function. More recently, there is increasing evidence that Nrp1 is also highly expressed on activated effector T cells and that increases in these Nrp1-expressing CD4+ T cells correspond with immunopathology across several T cell-dependent disease models. Thus, Nrp1 may be implicated in the identification and function of immunopathologic T cells. Nrp1 downregulation in CD4+ T cells is one of the strongest transcriptional changes in response to immunoregulatory compounds that act though the aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor. To better understand the link between AhR and Nrp1 expression on CD4+ T cells, Nrp1 expression was assessed in vivo and in vitro following AhR ligand treatment. In the current study, we identified that the percentage of Nrp1 expressing CD4+ T cells increases over the course of activation and proliferation in vivo. The actively dividing Nrp1+Foxp3- cells express the classic effector phenotype of CD44hiCD45RBlo, and the increase in Nrp1+Foxp3- cells is prevented by AhR activation. In contrast, Nrp1 expression is not modulated by AhR activation in non-proliferating CD4+ T cells. The downregulation of Nrp1 on CD4+ T cells was recapitulated in vitro in cells isolated from C57BL/6 and NOD (non-obese diabetic) mice. CD4+Foxp3- cells expressing CD25, stimulated with IL-2, or differentiated into Th1 cells, were particularly sensitive to AhR-mediated inhibition of Nrp1 upregulation. IL-2 was necessary for AhR-dependent downregulation of Nrp1 expression both in vitro and in vivo. Collectively, the data demonstrate that Nrp1 is a CD4+ T cell activation marker and that regulation of Nrp1 could be a previously undescribed mechanism by which AhR ligands modulate effector CD4+ T cell responses.

Activation of the aryl hydrocarbon receptor inhibits neuropilin-1 upregulation on IL-2 responding CD4 + T cells

Neuropilin-1 (Nrp1), a transmembrane protein expressed on CD4 + T cells, is mostly studied in the context of regulatory T cell (Treg) function. More recently, there is increasing evidence that Nrp1 is also highly expressed on activated effector T cells and that increases in these Nrp1-expressing CD4 + T cells correspond with immunopathology across several T cell-dependent disease models. Thus, Nrp1 may be implicated in the identification and function of immunopathologic T cells. Nrp1 downregulation in CD4 + T cells is one of the strongest transcriptional changes in response to immunoregulatory compounds that act though the aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor. To better understand the link between AhR and Nrp1 expression on CD4 + T cells, Nrp1 expression was assessed in vivo and in vitro following AhR ligand treatment. In the current study, we identified that the percentage of Nrp1 expressing CD4 + T cells increases over the course of activation and proliferation in vivo . The actively dividing Nrp1 + Foxp3 - cells express the classic effector phenotype of CD44 hi CD45RB lo , and the increase in Nrp1 + Foxp3 - cells is prevented by AhR activation. In contrast, Nrp1 expression is not modulated by AhR activation in non-proliferating CD4 + T cells. The downregulation of Nrp1 on CD4 + T cells was recapitulated in vitro in cells isolated from C57BL/6 and NOD (non-obese diabetic) mice. CD4 + Foxp3 - cells expressing CD25, stimulated with IL-2, or differentiated into Th1 cells, were particularly sensitive to AhR-mediated inhibition of Nrp1 upregulation. IL-2 was necessary for AhR-dependent downregulation of Nrp1 expression both in vitro and in vivo . Collectively, the data demonstrate that Nrp1 is a CD4 + T cell activation marker and that regulation of Nrp1 could be a previously undescribed mechanism by which AhR ligands modulate effector CD4 + T cell responses.

Suppression of Ah Receptor (AhR) increases the aggressiveness of TNBC cells and 11-Cl-BBQ-activated AhR inhibits their growth

Triple-negative breast cancer (TNBC) represents around 15% of the 2.26 million breast cancers diagnosed worldwide annually and has the worst outcome. Despite recent therapeutic advances, there remains a lack of targeted therapies for this breast cancer subtype. The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor with biological roles in regulating development, xenobiotic metabolism, cell cycle progression and cell death. AhR activation by select ligands can promote tumor suppression in multiple cancer types. AhR can negatively regulate the activity of different oncogenic signaling pathways and can directly upregulate tumor suppressor genes such as p27Kip1. To determine the role of AhR in TNBC, we generated AhR-deficient cancer cells and investigated the impact of AhR loss on TNBC cell growth phenotypes. We found that AhR-deficient MDA-MB-468 TNBC cells have increased proliferation and formed significantly more colonies compared to AhR expressing cells. These cells without AhR expression grew aggressively in vivo. To determine the molecular targets driving this phenotype, we performed transcriptomic profiling in AhR expressing and AhR knockout MDA-MB-468 cells and identified tyrosine receptor kinases, as well as other genes involved in proliferation, survival and clonogenicity that are repressed by AhR. In order to determine therapeutic targeting of AhR in TNBC, we investigated the anti-cancer effects of the novel AhR ligand 11-chloro-7H-benzimidazo[2,1-a]benzo[de]iso-quinolin-7-one (11-Cl-BBQ), which belongs to a class of high affinity, rapidly metabolized AhR ligands called benzimidazoisoquinolines (BBQs). 11-Cl-BBQ induced AhR-dependent cancer cell-selective growth inhibition and strongly inhibited colony formation in TNBC cells.

Induction of Aryl Hydrocarbon Receptor-Mediated Cancer Cell-Selective Apoptosis in Triple-Negative Breast Cancer Cells by a High-Affinity Benzimidazoisoquinoline

Triple-negative breast cancer (TNBC) remains a disease with a paucity of targeted treatment opportunities. The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that is involved in a wide range of physiological processes, including the sensing of xenobiotics, immune function, development, and differentiation. Different small-molecule AhR ligands drive strikingly varied cellular and organismal responses. In certain cancers, AhR activation by select small molecules induces cell cycle arrest or apoptosis via activation of tumor-suppressive transcriptional programs. AhR is expressed in triple-negative breast cancers, presenting a tractable therapeutic opportunity. Here, we identify a novel ligand of the aryl hydrocarbon receptor that potently and selectively induces cell death in triple-negative breast cancer cells and TNBC stem cells via the AhR. Importantly, we found that this compound, Analog 523, exhibits minimal cytotoxicity against multiple normal human primary cells. Analog 523 represents a high-affinity AhR ligand with potential for future clinical translation as an anticancer agent.

Effects of sulforaphane on breast cancer based on metabolome and microbiome

Sulforaphane (SFN) is a promising phytochemical with a wide range of antitumor activities. A comprehensive understanding of the effects of SFN on breast cancer based on the metabolome and microbiome is limited. Thus, we treated MCF-7 cell-transplanted nude mice with 50 mg/kg SFN. SFN inhibits breast cancer cell proliferation. SFN increased the levels of sulfate-related metabolites and glutathione-related metabolites and decreased tryptophan metabolites and methyl-purine metabolites in urinary metabolic profile. SFN indirectly affected the activation of aryl hydrocarbon receptor by tryptophan metabolism. The ratio of SAM to methionine was decreased by SFN while the global DNA methylation was downregulated in tumor tissue. SFN decreased the sulfate-reducing bacterium Desulfovibrio, which is related to reduced methylation capacity, and increased the genus Lactobacillus related to tryptophan metabolites with antitumor activities. In conclusion, we provide a perspective on the metabolome and microbiome to elucidate the antitumor activities of SFN.

Tumor-Suppressive Functions of the Aryl Hydrocarbon Receptor (AhR) and AhR as a Therapeutic Target in Cancer

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor involved in regulating a wide range of biological responses. A diverse array of xenobiotics and endogenous small molecules bind to the receptor and drive unique phenotypic responses. Due in part to its role in mediating toxic responses to environmental pollutants, AhR activation has not been traditionally viewed as a viable therapeutic approach. Nonetheless, the expression and activation of AhR can inhibit the proliferation, migration, and survival of cancer cells, and many clinically approved drugs transcriptionally activate AhR. Identification of novel select modulators of AhR-regulated transcription that promote tumor suppression is an active area of investigation. The development of AhR-targeted anticancer agents requires a thorough understanding of the molecular mechanisms driving tumor suppression. Here, we summarized the tumor-suppressive mechanisms regulated by AhR with an emphasis on the endogenous functions of the receptor in opposing carcinogenesis. In multiple different cancer models, the deletion of AhR promotes increased tumorigenesis, but a precise understanding of the molecular cues and the genetic targets of AhR involved in this process is lacking. The intent of this review was to synthesize the evidence supporting AhR-dependent tumor suppression and distill insights for development of AhR-targeted cancer therapeutics.

Gut-Microbiota-Derived Metabolites Maintain Gut and Systemic Immune Homeostasis

The gut microbiota, including bacteria, archaea, fungi, viruses and phages, inhabits the gastrointestinal tract. This commensal microbiota can contribute to the regulation of host immune response and homeostasis. Alterations of the gut microbiota have been found in many immune-related diseases. The metabolites generated by specific microorganisms in the gut microbiota, such as short-chain fatty acids (SCFAs), tryptophan (Trp) and bile acid (BA) metabolites, not only affect genetic and epigenetic regulation but also impact metabolism in the immune cells, including immunosuppressive and inflammatory cells. The immunosuppressive cells (such as tolerogenic macrophages (tMacs), tolerogenic dendritic cells (tDCs), myeloid-derived suppressive cells (MDSCs), regulatory T cells (Tregs), regulatory B cells (Breg) and innate lymphocytes (ILCs)) and inflammatory cells (such as inflammatory Macs (iMacs), DCs, CD4 T helper (Th)1, CD4Th2, Th17, natural killer (NK) T cells, NK cells and neutrophils) can express different receptors for SCFAs, Trp and BA metabolites from different microorganisms. Activation of these receptors not only promotes the differentiation and function of immunosuppressive cells but also inhibits inflammatory cells, causing the reprogramming of the local and systemic immune system to maintain the homeostasis of the individuals. We here will summarize the recent advances in understanding the metabolism of SCFAs, Trp and BA in the gut microbiota and the effects of SCFAs, Trp and BA metabolites on gut and systemic immune homeostasis, especially on the differentiation and functions of the immune cells.

A discovery-based proteomics approach identifies protein disulphide isomerase (PDIA1) as a biomarker of β cell stress in type 1 diabetes

BACKGROUND

Stress responses within the β cell have been linked with both increased β cell death and accelerated immune activation in type 1 diabetes (T1D). At present, information on the timing and scope of these responses as well as disease-related changes in islet β cell protein expression during T1D development is lacking.

METHODS

Data independent acquisition-mass spectrometry was performed on islets collected longitudinally from NOD mice and NOD-SCID mice rendered diabetic through T cell adoptive transfer.

FINDINGS

In islets collected from female NOD mice at 10, 12, and 14 weeks of age, we found a time-restricted upregulation of proteins involved in stress mitigation and maintenance of β cell function, followed by loss of expression of protective proteins that heralded diabetes onset. EIF2 signalling and the unfolded protein response, mTOR signalling, mitochondrial function, and oxidative phosphorylation were commonly modulated pathways in both NOD mice and NOD-SCID mice rendered acutely diabetic by T cell adoptive transfer. Protein disulphide isomerase A1 (PDIA1) was upregulated in NOD islets and pancreatic sections from human organ donors with autoantibody positivity or T1D. Moreover, PDIA1 plasma levels were increased in pre-diabetic NOD mice and in the serum of children with recent-onset T1D compared to non-diabetic controls.

INTERPRETATION

We identified a core set of modulated pathways across distinct mouse models of T1D and identified PDIA1 as a potential human biomarker of β cell stress in T1D.

FUNDING

NIH (R01DK093954, DK127308, U01DK127786, UC4DK104166, R01DK060581, R01GM118470, and 5T32DK101001-09). VA Merit Award I01BX001733. JDRF (2-SRA-2019-834-S-B, 2-SRA-2018-493-A-B, 3-PDF-20016-199-A-N, 5-CDA-2022-1176-A-N, and 3-PDF-2017-385-A-N).

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.

The key player in the pathogenesis of environmental influence of systemic lupus erythematosus: Aryl hydrocarbon receptor

The aryl hydrocarbon receptor was previously known as an environmental receptor that modulates the cellular response to external environmental changes. In essence, the aryl hydrocarbon receptor is a cytoplasmic receptor and transcription factor that is activated by binding to the corresponding ligands, and they transmit relevant information by binding to DNA, thereby activating the transcription of various genes. Therefore, we can understand the development of certain diseases and discover new therapeutic targets by studying the regulation and function of AhR. Several autoimmune diseases, including systemic lupus erythematosus (SLE), have been connected to AhR in previous studies. SLE is a classic autoimmune disease characterized by multi-organ damage and disruption of immune tolerance. We discuss here the homeostatic regulation of AhR and its ligands among various types of immune cells, pathophysiological roles, in addition to the roles of various related cytokines and signaling pathways in the occurrence and development of SLE.

The cyclin-dependent kinase inhibitor p27Kip1 interacts with the aryl hydrocarbon receptor and negatively regulates its transcriptional activity

p27^Kip1 functions to coordinate cell cycle progression through the inhibition of cyclin-dependent kinase (CDK) complexes. p27^Kip1 also exerts distinct activities beyond CDK-inhibition, including functioning as a transcriptional regulator. The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor with diverse biological roles. The regulatory inputs that control AhR-mediated transcriptional responses are an active area of investigation. AhR was previously established as a direct regulator of p27^Kip1 transcription. Here, we report the physical interaction of AhR and p27^Kip1 and show that p27^Kip1 expression negatively regulates AhR-mediated transcription. p27^Kip1 knockout cells display increased AhR nuclear localisation and significantly higher expression of AhR target genes. This work thus identifies new regulatory cross-talk between p27^Kip1 and AhR.

Gut Microbiota-Derived Tryptophan Metabolites Maintain Gut and Systemic Homeostasis

Tryptophan is an essential amino acid from dietary proteins. It can be metabolized into different metabolites in both the gut microbiota and tissue cells. Tryptophan metabolites such as indole-3-lactate (ILA), indole-3-acrylate (IAC), indole-3-propionate (IPA), indole-3-aldehyde (IAID), indoleacetic acid (IAA), indole-3-acetaldehyde and Kyn can be produced by intestinal microorganisms through direct Trp transformation and also, partly, the kynurenine (Kyn) pathway. These metabolites play a critical role in maintaining the homeostasis of the gut and systematic immunity and also potentially affect the occurrence and development of diseases such as inflammatory bowel diseases, tumors, obesity and metabolic syndrome, diseases in the nervous system, infectious diseases, vascular inflammation and cardiovascular diseases and hepatic fibrosis. They can not only promote the differentiation and function of anti-inflammatory macrophages, Treg cells, CD4⁺CD8αα⁺ regulatory cells, IL-10⁺ and/or IL-35⁺B regulatory cells but also IL-22-producing innate lymphoid cells 3 (ILC3), which are involved in maintaining the gut mucosal homeostasis. These findings have important consequences in the immunotherapy against tumor and other immune-associated diseases. We will summarize here the recent advances in understanding the generation and regulation of tryptophan metabolites in the gut microbiota, the role of gut microbiota-derived tryptophan metabolites in different immune cells, the occurrence and development of diseases and immunotherapy against immune-associated diseases.

Aryl hydrocarbon receptor: From pathogenesis to therapeutic targets in aging-related tissue fibrosis

Aging promotes chronic inflammation, which contributes to fibrosis and decreases organ function. Fibrosis, the excessive synthesis and deposition of extracellular matrix components, is the main cause of most chronic diseases including aging-related organ failure. Organ fibrosis in the heart, liver, and kidneys is the final manifestation of many chronic diseases. The aryl hydrocarbon receptor (AHR) is a cytoplasmic receptor and highly conserved transcription factor that is activated by a variety of small-molecule ligands to affect a wide array of tissue homeostasis functions. In recent years, mounting evidence has revealed that AHR plays an important role in multi-organ fibrosis initiation, progression, and therapy. In this review, we summarise the relationship between AHR and the pathogenesis of aging-related tissue fibrosis, and further discuss how AHR modulates tissue fibrosis by regulating transforming growth factor-β signalling, immune response, and mitochondrial function, which may offer novel targets for the prevention and treatment of this condition.

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.

Modulation of Intestinal ILC3 for the Treatment of Type 1 Diabetes

Gut-associated lymphoid tissue (GALT) is crucial for the maintenance of the intestinal homeostasis, but it is also the potential site of the activation of autoreactive cells and initiation/propagation of autoimmune diseases in the gut and in the distant organs. Type 3 innate lymphoid cells (ILC3) residing in the GALT integrate signals from food ingredients and gut microbiota metabolites in order to control local immunoreactivity. Notably, ILC3 secrete IL-17 and GM-CSF that activate immune cells in combating potentially pathogenic microorganisms. ILC3 also produce IL-22 that potentiates the strength and integrity of epithelial tight junctions, production of mucus and antimicrobial peptides thus enabling the proper function of the intestinal barrier. The newly discovered function of small intestine ILC3 is the secretion of IL-2 and the promotion of regulatory T cell (Treg) generation and function. Since the intestinal barrier dysfunction, together with the reduction in small intestine ILC3 and Treg numbers are associated with the pathogenesis of type 1 diabetes (T1D), the focus of this article is intestinal ILC3 modulation for the therapy of T1D. Of particular interest is free fatty acids receptor 2 (FFAR2), predominantly expressed on intestinal ILC3, that can be stimulated by available selective synthetic agonists. Thus, we propose that FFAR2-based interventions by boosting ILC3 beneficial functions may attenuate autoimmune response against pancreatic β cells during T1D. Also, it is our opinion that treatments based on ILC3 stimulation by functional foods can be used as prophylaxis in individuals that are genetically predisposed to develop T1D.

Discovery and Mechanistic Characterization of a Select Modulator of AhR-regulated Transcription (SMAhRT) with Anti-cancer Effects

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor and a member of the bHLH/PAS (basic Helix-Loop-Helix/Per-Arnt-Sim) family of proteins. The AhR was cloned and characterized for its role in mediating the toxicity of dioxins. Subsequent research has identified the role of AhR in suppression of cancer cell growth. We hypothesized that the AhR is a molecular target for therapeutic intervention in cancer, and that activation of the AhR by unique AhR ligands in cancer cells could have anti-cancer effects including induction of cell death. This study describes the discovery and characterization of a new class of anti-cancer agents targeting the AhR, that we designate as Select Modulators of AhR-regulated Transcription (SMAhRTs). We employed two independent small molecule screening approaches to identify potential SMAhRTs. We report the identification of CGS-15943 that activates AhR signaling and induces apoptosis in an AhR-dependent manner in liver and breast cancer cells. Investigation of the downstream signaling pathway of this newly identified SMAhRT revealed upregulation of Fas-ligand (FasL), which is required for AhR-mediated apoptosis. Our results provide a basis for further development of a new class of anti-cancer therapeutics targeting an underappreciated molecular target, the AhR.

Tissue Nutrient Environments and Their Effect on Regulatory T Cell Biology

Regulatory T cells (Tregs) are essential for mitigating inflammation. Tregs are found in nearly every tissue and play either beneficial or harmful roles in the host. The availability of various nutrients can either enhance or impair Treg function. Mitochondrial oxidative metabolism plays a major role in supporting Treg differentiation and fitness. While Tregs rely heavily on oxidation of fatty acids to support mitochondrial activity, they have found ways to adapt to different tissue types, such as tumors, to survive in competitive environments. In addition, metabolic by-products from commensal organisms in the gut also have a profound impact on Treg differentiation. In this review, we will focus on the core metabolic pathways engaged in Tregs, especially in the context of tissue nutrient environments, and how they can affect Treg function, stability and differentiation.

A Triple Threat? The Role of Diet, Nutrition, and the Microbiota in T1D Pathogenesis

In recent years the role of the intestinal microbiota in health and disease has come to the forefront of medical research. Alterations in the intestinal microbiota and several of its features have been linked to numerous diseases, including type 1 diabetes (T1D). To date, studies in animal models of T1D, as well as studies in human subjects, have linked several intestinal microbiota alterations with T1D pathogenesis. Features that are most often linked with T1D pathogenesis include decreased microbial diversity, the relative abundance of specific strains of individual microbes, and altered metabolite production. Alterations in these features as well as others have provided insight into T1D pathogenesis and shed light on the potential mechanism by which the microbiota plays a role in T1D pathogenesis, yet the underlying factors leading to these alterations remains unknown. One potential mechanism for alteration of the microbiota is through diet and nutrition. Previous studies have shown associations of diet with islet autoimmunity, but a direct contributing factor has yet to be identified. Diet, through introduction of antigens and alteration of the composition and function of the microbiota, may elicit the immune system to produce autoreactive responses that result in the destruction of the beta cells. Here, we review the evidence associating diet induced changes in the intestinal microbiota and their contribution to T1D pathogenesis. We further provide a roadmap for determining the effect of diet and other modifiable factors on the entire microbiota ecosystem, including its impact on both immune and beta cell function, as it relates to T1D. A greater understanding of the complex interactions between the intestinal microbiota and several interacting systems in the body (immune, intestinal integrity and function, metabolism, beta cell function, etc.) may provide scientifically rational approaches to prevent development of T1D and other childhood immune and allergic diseases and biomarkers to evaluate the efficacy of interventions.

Modelling and Phenotypic Screening of NAP-6 and 10-Cl-BBQ, AhR Ligands Displaying Selective Breast Cancer Cytotoxicity in Vitro

To exploit the interaction of the aryl hydrocarbon receptor (AhR) pathway in developing breast-cancer-specific cytotoxic compounds, we examined the breast cancer selectivity and the docking pose of the AhR ligands (Z)-2-(2-aminophenyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (NAP-6; 5) and 10-chloro-7H-benzo[de]benzo[4,5]imidazo[2,1-a]isoquinolin-7-one (10-Cl-BBQ; 6). While the breast cancer selectivity of 5 in vitro is known, we discuss the SAR around this lead and, by using phenotypic cell-line screening and the MTT assay, show for the first time that 6 also presents with breast cancer selectivity, notably in the triple-negative (TN) receptor breast cancer cell line MDA-MB-468, the ER+ breast cancer cell lines T47D, ZR-75-1 and the HER2+ breast cancer cell line SKBR3 (GI₅₀ values of 0.098, 0.97, 0.13 and 0.21 μM, respectively). Indeed, 6 is 55 times more potent in MDA-MB-468 cells than normal MCF10A breast cells (GI₅₀ of 0.098 vs 5.4 μM) and more than 130 times more potent than in cell lines derived from pancreas, brain and prostate (GI₅₀ of 0.098 vs 10-13 μM). Molecular docking poses of 5 and 6 together with analogue synthesis and phenotypic screening show the importance of the naphthalene moiety, and an ortho-disposed substituent on the N-phenyl moiety for biological activity.

Dietary Indole-3-Carbinol Activates AhR in the Gut, Alters Th17-Microbe Interactions, and Exacerbates Insulitis in NOD Mice

The diet represents one environmental risk factor controlling the progression of type 1 diabetes (T1D) in genetically susceptible individuals. Consequently, understanding which specific nutritional components promote or prevent the development of disease could be used to make dietary recommendations in prediabetic individuals. In the current study, we hypothesized that the immunoregulatory phytochemcial, indole-3-carbinol (I3C) which is found in cruciferous vegetables, will regulate the progression of T1D in nonobese diabetic (NOD) mice. During digestion, I3C is metabolized into ligands for the aryl hydrocarbon receptor (AhR), a transcription factor that when systemically activated prevents T1D. In NOD mice, an I3C-supplemented diet led to strong AhR activation in the small intestine but minimal systemic AhR activity. In the absence of this systemic response, the dietary intervention led to exacerbated insulitis. Consistent with the compartmentalization of AhR activation, dietary I3C did not alter T helper cell differentiation in the spleen or pancreatic draining lymph nodes. Instead, dietary I3C increased the percentage of CD4+RORγt+Foxp3- (Th17 cells) in the lamina propria, intraepithelial layer, and Peyer's patches of the small intestine. The immune modulation in the gut was accompanied by alterations to the intestinal microbiome, with changes in bacterial communities observed within one week of I3C supplementation. A transkingdom network was generated to predict host-microbe interactions that were influenced by dietary I3C. Within the phylum Firmicutes, several genera (Intestinimonas, Ruminiclostridium 9, and unclassified Lachnospiraceae) were negatively regulated by I3C. Using AhR knockout mice, we validated that Intestinimonas is negatively regulated by AhR. I3C-mediated microbial dysbiosis was linked to increases in CD25high Th17 cells. Collectively, these data demonstrate that site of AhR activation and subsequent interactions with the host microbiome are important considerations in developing AhR-targeted interventions for T1D.

Epigenetic Regulations of AhR in the Aspect of Immunomodulation

Environmental factors contribute to autoimmune disease manifestation, and as regarded today, AhR has become an important factor in studies of immunomodulation. Besides immunological aspects, AhR also plays a role in pharmacological, toxicological and many other physiological processes such as adaptive metabolism. In recent years, epigenetic mechanisms have provided new insight into gene regulation and reveal a new contribution to autoimmune disease pathogenesis. DNA methylation, histone modifications, chromatin alterations, microRNA and consequently non-genetic changes in phenotypes connect with environmental factors. Increasing data reveals AhR cross-roads with the most significant in immunology pathways. Although study on epigenetic modulations in autoimmune diseases is still not well understood, therefore future research will help us understand their pathophysiology and help to find new therapeutic strategies. Present literature review sheds the light on the common ground between remodeling chromatin compounds and autoimmune antibodies used in diagnostics. In the proposed review we summarize recent findings that describe epigenetic factors which regulate AhR activity and impact diverse immunological responses and pathological changes.

The AHR Signaling Attenuates Autoimmune Responses During the Development of Type 1 Diabetes

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcriptional factor widely expressed in immune cells. Its ligands range from xenobiotics and natural substances to metabolites, which renders it capable of sensing and responding to a variety of environmental cues. Although AHR signaling has long been recognized to be implicated in the pathogenesis of autoimmune disorders, such as rheumatoid arthritis (RA), colitis, and systemic lupus erythematosus (SLE), its effect on the pathogenesis of type 1 diabetes (T1D) remains less understood. In this review, we intend to summarize its potential implication in T1D pathogenesis and to sort out the related regulatory mechanisms in different types of immune cells. Emerging evidence supports that β cell destruction caused by autoimmune responses can be rectified by AHR signaling. Upon activation by its ligands, AHR not only modulates the development and functionality of immune cells, but also suppresses the expression of inflammatory cytokines, through which AHR attenuates autoimmune responses during the course of T1D development. Since AHR-initiated biological effects vary between different types of ligands, additional studies would be necessary to characterize or de novo synthesize effective and safe ligands aimed to replenish our arsenal in fighting autoimmune responses and β mass loss in a T1D setting.

Tryptophan Metabolism, Regulatory T Cells, and Inflammatory Bowel Disease: A Mini Review

Inflammatory bowel disease (IBD) is a chronic inflammatory disorder of the gastrointestinal tract resulting from the homeostasis imbalance of intestinal microenvironment, immune dysfunction, environmental and genetic factors, and so on. This disease is associated with multiple immune cells including regulatory T cells (Tregs). Tregs are a subset of T cells regulating the function of various immune cells to induce immune tolerance and maintain intestinal immune homeostasis. Tregs are correlated with the initiation and progression of IBD; therefore, strategies that affect the differentiation and function of Tregs may be promising for the prevention of IBD-associated pathology. It is worth noting that tryptophan (Trp) metabolism is effective in inducing the differentiation of Tregs through microbiota-mediated degradation and kynurenine pathway (KP), which is important for maintaining the function of Tregs. Interestingly, patients with IBD show Trp metabolism disorder in the pathological process, including changes in the concentrations of Trp and its metabolites and alteration in the activities of related catalytic enzymes. Thus, manipulation of Treg differentiation through Trp metabolism may provide a potential target for prevention of IBD. The purpose of this review is to highlight the relationship between Trp metabolism and Treg differentiation and the role of this interaction in the pathogenesis of IBD.

Chronic Kidney Disease-Associated Immune Dysfunctions: Impact of Protein-Bound Uremic Retention Solutes on Immune Cells

Regardless of the primary disease responsible for kidney failure, patients suffering from chronic kidney disease (CKD) have in common multiple impairments of both the innate and adaptive immune systems, the pathophysiology of which has long remained enigmatic. CKD-associated immune dysfunction includes chronic low-grade activation of monocytes and neutrophils, which induces endothelial damage and increases cardiovascular risk. Although innate immune effectors are activated during CKD, their anti-bacterial capacity is impaired, leading to increased susceptibility to extracellular bacterial infections. Finally, CKD patients are also characterized by profound alterations of cellular and humoral adaptive immune responses, which account for an increased risk for malignancies and viral infections. This review summarizes the recent emerging data that link the pathophysiology of CKD-associated immune dysfunctions with the accumulation of microbiota-derived metabolites, including indoxyl sulfate and p-cresyl sulfate, the two best characterized protein-bound uremic retention solutes.

The aryl hydrocarbon receptor (AhR) as a breast cancer drug target

Breast cancer is the most common cancer in women, with more than 1.7 million diagnoses worldwide per annum. Metastatic breast cancer remains incurable, and the presence of triple-negative phenotypes makes targeted treatment impossible. The aryl hydrocarbon receptor (AhR), most commonly associated with the metabolism of xenobiotic ligands, has emerged as a promising biological target for the treatment of this deadly disease. Ligands for the AhR can be classed as exogenous or endogenous and may have agonistic or antagonistic activity. It has been well reported that agonistic ligands may have potent and selective growth inhibition activity in a number of oncogenic cell lines, and one (aminoflavone) has progressed to phase I clinical trials for breast cancer sufferers. In this study, we examine the current state of the literature in this area and elucidate the promising advances that are being made in hijacking the cytosolic-to-nuclear pathway of the AhR for the possible future treatment of breast cancer.

Targeting Aryl hydrocarbon receptor for next-generation immunotherapies: Selective modulators (SAhRMs) versus rapidly metabolized ligands (RMAhRLs)

Aryl Hydrocarbon Receptor (AhR) constitutes a major network hub of genomic and non-genomic signaling pathways, connecting host's immune cells to environmental factors. It shapes innate and adaptive immune processes to environmental stimuli with species-, cell- and tissue-type dependent specificity. Although an ever increasing number of studies has thrust AhR into the limelight as attractive target for the development of next-generation immunotherapies, concerns exist on potential safety issues associated with small molecule modulation of the receptor. Selective AhR modulators (SAhRMs) and rapidly metabolized AhR ligands (RMAhRLs) are two classes of receptor agonists that are emerging as interesting lead compounds to bypass AhR-related toxicity in favor of therapeutic effects. In this article, we discuss SAhRMs and RMAhRLs reported in literature, covering concepts underlying their definitions, specific binding modes, structure-activity relationships and AhR-mediated functions.

The Aryl Hydrocarbon Receptor as an Immune-Modulator of Atmospheric Particulate Matter-Mediated Autoimmunity

This review examines the current literature on the effects of atmospheric particulate matter (PM) on autoimmune disease and proposes a new role for the aryl hydrocarbon receptor (AHR) as a modulator of T cells in PM-mediated autoimmune disease. There is a significant body of literature regarding the strong epidemiologic correlations between PM exposures and worsened autoimmune diseases. Genetic predispositions account for 30% of all autoimmune disease leaving environmental factors as major contributors. Increases in incidence and prevalence of autoimmune disease have occurred concurrently with an increase in air pollution. Currently, atmospheric PM is considered to be the greatest environmental health risk worldwide. Atmospheric PM is a complex heterogeneous mixture composed of diverse adsorbed organic compounds such as polycyclic aromatic hydrocarbons (PAHs) and dioxins, among others. Exposure to atmospheric PM has been shown to aggravate several autoimmune diseases. Despite strong correlations between exposure to atmospheric PM and worsened autoimmune disease, the mechanisms underlying aggravated disease are largely unknown. The AHR is a ligand activated transcription factor that responds to endogenous and exogenous ligands including toxicants present in PM, such as PAHs and dioxins. A few studies have investigated the effects of atmospheric PM on AHR activation and immune function and demonstrated that atmospheric PM can activate the AHR, change cytokine expression, and alter T cell differentiation. Several studies have found that the AHR modulates the balance between regulatory and effector T cell functions and drives T cell differentiation in vitro and in vivo using murine models of autoimmune disease. However, there are very few studies on the role of AHR in PM-mediated autoimmune disease. The AHR plays a critical role in the balance of effector and regulatory T cells and in autoimmune disease. With increased incidence and prevalence of autoimmune disease occurring concurrently with increases in air pollution, potential mechanisms that drive inflammatory and exacerbated disease need to be elucidated. This review focuses on the AHR as a potential mechanistic target for modulating T cell responses associated with PM-mediated autoimmune disease providing the most up-to-date literature on the role of AHR in autoreactive T cell function and autoimmune disease.

The aryl hydrocarbon receptor is required for induction of p21cip1/waf1 expression and growth inhibition by SU5416 in hepatoma cells

The aryl hydrocarbon receptor (AhR) is a potential clinical target for cancer and autoimmune dysfunction. Identifying selective AhR modulators that produce desirable clinical outcomes represents an opportunity for developing new anti-cancer agents. Repurposing clinically-used drugs with established safety profiles that activate the AhR represents a good starting place to pursue this goal. In this study, we characterized the AhR-dependent effects of SU5416 (Semaxanib) following its identification in a small-molecule library screen. SU5416 potently activated AhR-dependent reporter genes, induced AhR nuclear localization, facilitated AhR-DNA binding, and increased, expression of its endogenous target genes. SU5416 significantly inhibited proliferation of Hepa1 hepatoma cells in an AhR-dependent manner, but did not induce apoptosis. SU5416 also inhibited the growth of human HepG2 liver cancer cells. The effects of SU5416 correlated with an increased G1 population and increased expression of cell cycle inhibitor p21^cip1/waf1 at both the mRNA and protein level. Increased expression of p21^cip1/waf1 by SU5416 required expression of both AhR and Arnt. In addition, evidence for long-term activation of the AhR in vivo by a single dose of SU5416 was identified by analyzing published microarray data. Our results provide support for continued investigation of the AhR as therapeutic for cancers such as hepatocellular carcinoma. In addition, our findings raise the possibility that some of the previously observed anti-proliferative effects of SU5416 may be due to activation of the AhR.

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.

Aryl hydrocarbon receptor signaling modulates antiviral immune responses: ligand metabolism rather than chemical source is the stronger predictor of outcome

The aryl hydrocarbon receptor (AHR) offers a compelling target to modulate the immune system. AHR agonists alter adaptive immune responses, but the consequences differ across studies. We report here the comparison of four agents representing different sources of AHR ligands in mice infected with influenza A virus (IAV): TCDD, prototype exogenous AHR agonist; PCB126, pollutant with documented human exposure; ITE, novel pharmaceutical; and FICZ, degradation product of tryptophan. All four compounds diminished virus-specific IgM levels and increased the proportion of regulatory T cells. TCDD, PCB126 and ITE, but not FICZ, reduced virus-specific IgG levels and CD8⁺ T cell responses. Similarly, ITE, PCB126, and TCDD reduced Th1 and Tfh cells, whereas FICZ increased their frequency. In Cyp1a1-deficient mice, all compounds, including FICZ, reduced the response to IAV. Conditional Ahr knockout mice revealed that all four compounds require AHR within hematopoietic cells. Thus, differences in the immune response to IAV likely reflect variances in quality, magnitude, and duration of AHR signaling. This indicates that binding affinity and metabolism may be stronger predictors of immune effects than a compound’s source of origin, and that harnessing AHR will require finding a balance between dampening immune-mediated pathologies and maintaining sufficient host defenses against infection.

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.

Role of the aryl hydrocarbon receptor in carcinogenesis and potential as an anti-cancer drug target

The aryl hydrocarbon receptor (AhR) was initially identified as the receptor that binds and mediates the toxic effects induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and structurally related halogenated aromatics. Other toxic compounds including some polynuclear aromatic hydrocarbons act through the AhR; however, during the last 25 years, it has become apparent that the AhR plays an essential role in maintaining cellular homeostasis. Moreover, the scope of ligands that bind the AhR includes endogenous compounds such as multiple tryptophan metabolites, other endogenous biochemicals, pharmaceuticals and health-promoting phytochemicals including flavonoids, indole-3-carbinol and its metabolites. It has also been shown that like other receptors, the AhR is a drug target for multiple diseases including cancer, where both AhR agonists and antagonists effectively block many of the critical hallmarks of cancer in multiple tumor types. This review describes the anti-cancer activities of AhR ligands and demonstrates that it is time to separate the AhR from TCDD and exploit the potential of the AhR as a novel target for cancer chemotherapy.

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.