Aryl hydrocarbon receptor signaling regulates NF-κB RelB activation during dendritic-cell differentiation

The aryl hydrocarbon receptor: a rehabilitated target for therapeutic immune modulation

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor originally identified as the target mediating the toxic effects of environmental pollutants including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and dioxins. For years, AHR activation was actively avoided during drug development. However, the AHR was later identified as an important physiological regulator of the immune response. These findings triggered a paradigm shift that resulted in identification of the AHR as a regulator of both innate and adaptive immunity and outlined a pathway for its modulation by the diet, commensal flora and metabolism in the context of autoimmunity, cancer and infection. Moreover, the AHR was revealed as a candidate target for the therapeutic modulation of the immune response. Indeed, the first AHR-activating drug (tapinarof) was recently approved for the treatment of psoriasis. Clinical trials are underway to evaluate the effects of tapinarof and other AHR-targeting therapeutics in inflammatory diseases, cancer and infections. This Review outlines the molecular mechanism of AHR action, and describes how it regulates the immune response. We also discuss links to disease and AHR-targeting therapeutics that have been tested in past and ongoing clinical trials.

Using the Key Characteristics Framework to Unlock the Mysteries of Aryl Hydrocarbon Receptor-Mediated Effects on the Immune System

Initially discovered for its role mediating the deleterious effects of environmental contaminants, the aryl hydrocarbon receptor (AHR) is now known to be a crucial regulator of the immune system. The expanding list of AHR ligands includes synthetic and naturally derived molecules spanning pollutants, phytochemicals, pharmaceuticals, and substances derived from amino acids and microorganisms. The consequences of engaging AHR vary, depending on factors such as the AHR ligand, cell type, immune challenge, developmental state, dose, and timing of exposure relative to the immune stimulus. This review frames this complexity using the recently identified key characteristics of agents that affect immune system function (altered cell signaling, proliferation, differentiation, effector function, communication, trafficking, death, antigen presentation and processing, and tolerance). The use of these key characteristics provides a scaffold for continued discovery of how AHR and its myriad ligands influence the immune system, which will help harness the power of this enigmatic receptor to prevent or treat disease.

CD103+ dendritic cell-fibroblast crosstalk via TLR9, TDO2, and AHR signaling drives lung fibrogenesis

Idiopathic pulmonary fibrosis (IPF) is characterized by progressive scarring and loss of lung function. With limited treatment options, patients die from the disease within 2-5 years. The molecular pathogenesis underlying the immunologic changes that occur in IPF is poorly understood. We characterize noncanonical aryl-hydrocarbon receptor (ncAHR) signaling in DCs as playing a role in the production of IL-6 and increased IL-17+ cells, promoting fibrosis. TLR9 signaling in myofibroblasts is shown to regulate production of TDO2, which converts tryptophan into the endogenous AHR ligand kynurenine. Mice with augmented ncAHR signaling were created by crossing mice harboring a floxed AHR exon 2 deletion (AHRΔex2) with mice harboring a CD11c-Cre. Bleomycin (blm) was used to study fibrotic pathogenesis. Isolated CD11c+ cells and primary fibroblasts were treated ex vivo with relevant TLR agonists and AHR-modulating compounds to study how AHR signaling influenced inflammatory cytokine production. Human datasets were also interrogated. Inhibition of all AHR signaling rescued fibrosis; however, AHRΔex2 mice treated with blm developed more fibrosis, and DCs from these mice were hyperinflammatory and profibrotic upon adoptive transfer. Treatment of fibrotic fibroblasts with TLR9 agonist increased expression of TDO2, and fibrotic fibroblasts activated IL-6 production in CD103+ DCs. Study of human samples corroborated the relevance of these findings in patients with IPF. We also show, for the first time to our knowledge, that AHR exon 2 floxed mice retain the capacity for ncAHR signaling.

Immunometabolism: signaling pathways, homeostasis, and therapeutic targets

Immunometabolism plays a central role in sustaining immune system functionality and preserving physiological homeostasis within the organism. During the differentiation and activation, immune cells undergo metabolic reprogramming mediated by complex signaling pathways. Immune cells maintain homeostasis and are influenced by metabolic microenvironmental cues. A series of immunometabolic enzymes modulate immune cell function by metabolizing nutrients and accumulating metabolic products. These enzymes reverse immune cells' differentiation, disrupt intracellular signaling pathways, and regulate immune responses, thereby influencing disease progression. The huge population of immune metabolic enzymes, the ubiquity, and the complexity of metabolic regulation have kept the immune metabolic mechanisms related to many diseases from being discovered, and what has been revealed so far is only the tip of the iceberg. This review comprehensively summarized the immune metabolic enzymes' role in multiple immune cells such as T cells, macrophages, natural killer cells, and dendritic cells. By classifying and dissecting the immunometabolism mechanisms and the implications in diseases, summarizing and analyzing advancements in research and clinical applications of the inhibitors targeting these enzymes, this review is intended to provide a new perspective concerning immune metabolic enzymes for understanding the immune system, and offer novel insight into future therapeutic interventions.

Dendritic Cell - Fibroblast Crosstalk via TLR9 and AHR Signaling Drives Lung Fibrogenesis

Idiopathic pulmonary fibrosis (IPF) is characterized by progressive scarring and loss of lung function. With limited treatment options, patients succumb to the disease within 2-5 years. The molecular pathogenesis of IPF regarding the immunologic changes that occur is poorly understood. We characterize a role for non-canonical aryl-hydrocarbon receptor signaling (ncAHR) in dendritic cells (DCs) that leads to production of IL-6 and IL-17, promoting fibrosis. TLR9 signaling in myofibroblasts is shown to regulate production of TDO2 which converts tryptophan into the endogenous AHR ligand kynurenine. Mice with augmented ncAHR signaling were created by crossing floxed AHR exon-2 deletion mice (AHR Δex2 ) with mice harboring a CD11c-Cre. Bleomycin was used to study fibrotic pathogenesis. Isolated CD11c+ cells and primary fibroblasts were treated ex-vivo with relevant TLR agonists and AHR modulating compounds to study how AHR signaling influenced inflammatory cytokine production. Human datasets were also interrogated. Inhibition of all AHR signaling rescued fibrosis, however, AHR Δex2 mice treated with bleomycin developed more fibrosis and DCs from these mice were hyperinflammatory and profibrotic upon adoptive transfer. Treatment of fibrotic fibroblasts with TLR9 agonist increased expression of TDO2. Study of human samples corroborate the relevance of these findings in IPF patients. We also, for the first time, identify that AHR exon-2 floxed mice retain capacity for ncAHR signaling.

Mechanisms underlying aryl hydrocarbon receptor-driven divergent macrophage function

Macrophages play an essential role in the innate immune system by differentiating into functionally diverse subsets in order to fight infection, repair damaged tissues, and regulate inappropriate immune responses. This functional diversity stems from their ability to adapt and respond to signals in the environment, which is in part mediated through aryl hydrocarbon receptor (AHR)-signaling. AHR, an environmental sensor, can be activated by various ligands, ranging from environmental contaminants to microbially derived tryptophan metabolites. This review discusses what is currently known about how AHR-signaling influences macrophage differentiation, polarization, and function. By discussing studies that are both consistent and divergent, our goal is to highlight the need for future research on the mechanisms by which AHR acts as an immunological switch in macrophages. Ultimately, understanding the contexts in which AHR-signaling promotes and/or inhibits differentiation, proinflammatory functions, and immunoregulatory functions, will help uncover functional predictions of immunotoxicity following exposure to environmental chemicals as well as better design AHR-targeted immunotherapies.

Kynurenic Acid/AhR Signaling at the Junction of Inflammation and Cardiovascular Diseases

Persistent systemic chronic inflammatory conditions are linked with many pathologies, including cardiovascular diseases (CVDs), a leading cause of death across the globe. Among various risk factors, one of the new possible contributors to CVDs is the metabolism of essential amino acid tryptophan. Proinflammatory signals promote tryptophan metabolism via the kynurenine (KYN) pathway (KP), thereby resulting in the biosynthesis of several immunomodulatory metabolites whose biological effects are associated with the development of symptoms and progression of various inflammatory diseases. Some participants in the KP are agonists of aryl hydrocarbon receptor (AhR), a central player in a signaling pathway that, along with a regulatory influence on the metabolism of environmental xenobiotics, performs a key immunomodulatory function by triggering various cellular mechanisms with the participation of endogenous ligands to alleviate inflammation. An AhR ligand with moderate affinity is the central metabolite of the KP: KYN; one of the subsequent metabolites of KYN-kynurenic acid (KYNA)-is a more potent ligand of AhR. Understanding the role of AhR pathway-related metabolites of the KP that regulate inflammatory factors in cells of the cardiovascular system is interesting and important for achieving effective treatment of CVDs. The purpose of this review was to summarize the results of studies about the participation of the KP metabolite-KYNA-and of the AhR signaling pathway in the regulation of inflammation in pathological conditions of the heart and blood vessels and about the possible interaction of KYNA with AhR signaling in some CVDs.

Tryptophan catabolism via the kynurenine pathway regulates infection and inflammation: from mechanisms to biomarkers and therapies

BACKGROUND

L-Tryptophan (L-Trp), an essential amino acid, is the only amino acid whose level is regulated specifically by immune signals. Most proportions of Trp are catabolized via the kynurenine (Kyn) pathway (KP) which has evolved to align the food availability and environmental stimulation with the host pathophysiology and behavior. Especially, the KP plays an indispensable role in balancing the immune activation and tolerance in response to pathogens.

SCOPE OF REVIEW

In this review, we elucidate the underlying immunological regulatory network of Trp and its KP-dependent catabolites in the pathophysiological conditions by participating in multiple signaling pathways. Furthermore, the KP-based regulatory roles, biomarkers, and therapeutic strategies in pathologically immune disorders are summarized covering from acute to chronic infection and inflammation.

MAJOR CONCLUSIONS

The immunosuppressive effects dominate the functions of KP induced-Trp depletion and KP-produced metabolites during infection and inflammation. However, the extending minor branches from the KP are not confined to the immune tolerance, instead they go forward to various functions according to the specific condition. Nevertheless, persistent efforts should be made before the clinical use of KP-based strategies to monitor and cure infectious and inflammatory diseases.

Unveiling the mechanistic role of the Aryl hydrocarbon receptor in environmentally induced Breast cancer

The aryl hydrocarbon receptor (AhR) is a crucial cytosolic evolutionary conserved ligand-activated transcription factor and a pleiotropic signal transducer. The biosensor activity of the AhR is attributed to the promiscuity of its ligand-binding domain. Evidence suggests exposure to environmental toxins such as polycyclic aromatic hydrocarbons, polychlorinated biphenyls and halogenated aromatic hydrocarbons activates the AhR signaling pathway. The constitutive activation of the receptor signaling system leads to multiple health adversities and enhances the risk of several cancers, including breast cancer (BC). This review evaluates several mechanisms that integrate the tumor-inducing property of such environmental contaminants with the AhR pathway assisting in BC tumorigenesis, progress and metastasis. Intriguingly, immune evasion is identified as a prominent hallmark in BC. Several emerging pieces of evidence have identified AhR as a potent immunosuppressive effector in several cancers. Through AhR signaling pathways, some tumors can avoid immune detection. Thus the relevance of AhR in the immunomodulation of breast tumors and its putative mode of action in the breast tumor microenvironment are discussed in this review. Additionally, the work also explores BC stemness and its associated inflammation in response to several environmental cues. The review elucidates the context-dependent ambiguous behavior of AhR either as an oncogene or a tumor suppressor with respect to its ligand. Conclusively, this holistic piece of literature attempts to potentiate AhR as a promising pharmacological target in BC and updates on the therapeutic manipulation of its various exogenous and endogenous ligands.

Regulation of IDO2 by the Aryl Hydrocarbon Receptor (AhR) in Breast Cancer

Indoleamine 2,3-dioxygenase 2 (IDO2) is a tryptophan-catabolizing enzyme and a homolog of IDO1 with a distinct expression pattern compared with IDO1. In dendritic cells (DCs), IDO activity and the resulting changes in tryptophan level regulate T-cell differentiation and promote immune tolerance. Recent studies indicate that IDO2 exerts an additional, non-enzymatic function and pro-inflammatory activity, which may play an important role in diseases such as autoimmunity and cancer. Here, we investigated the impact of aryl hydrocarbon receptor (AhR) activation by endogenous compounds and environmental pollutants on the expression of IDO2. Treatment with AhR ligands induced IDO2 in MCF-7 wildtype cells but not in CRISPR-cas9 AhR-knockout MCF-7 cells. Promoter analysis with IDO2 reporter constructs revealed that the AhR-dependent induction of IDO2 involves a short-tandem repeat containing four core sequences of a xenobiotic response element (XRE) upstream of the start site of the human ido2 gene. The analysis of breast cancer datasets revealed that IDO2 expression increased in breast cancer compared with normal samples. Our findings suggest that the AhR-mediated expression of IDO2 in breast cancer could contribute to a pro-tumorigenic microenvironment in breast cancer.

Functions of the aryl hydrocarbon receptor (AHR) beyond the canonical AHR/ARNT signaling pathway

The aryl hydrocarbon receptor (AHR) is a ligand-dependent transcription factor regulating adaptive and maladaptive responses toward exogenous and endogenous signals. Research from various biomedical disciplines has provided compelling evidence that the AHR is critically involved in the pathogenesis of a variety of diseases and disorders, including autoimmunity, inflammatory diseases, endocrine disruption, premature aging and cancer. Accordingly, AHR is considered an attractive target for the development of novel preventive and therapeutic measures. However, the ligand-based targeting of AHR is considerably complicated by the fact that the receptor does not always follow the beaten track, i.e. the canonical AHR/ARNT signaling pathway. Instead, AHR might team up with other transcription factors and signaling molecules to shape gene expression patterns and associated physiological or pathophysiological functions in a ligand-, cell- and micromilieu-dependent manner. Herein, we provide an overview about some of the most important non-canonical functions of AHR, including crosstalk with major signaling pathways involved in controlling cell fate and function, immune responses, adaptation to low oxygen levels and oxidative stress, ubiquitination and proteasomal degradation. Further research on these diverse and exciting yet often ambivalent facets of AHR biology is urgently needed in order to exploit the full potential of AHR modulation for disease prevention and treatment.

Environmental exposure and the role of AhR in the tumor microenvironment of breast cancer

Activation of the aryl hydrocarbon receptor (AhR) through environmental exposure to chemicals including polycyclic aromatic hydrocarbons (PAHs) and polychlorinated dibenzo-p-dioxins (PCDDs) can lead to severe adverse health effects and increase the risk of breast cancer. This review considers several mechanisms which link the tumor promoting effects of environmental pollutants with the AhR signaling pathway, contributing to the development and progression of breast cancer. We explore AhR's function in shaping the tumor microenvironment, modifying immune tolerance, and regulating cancer stemness, driving breast cancer chemoresistance and metastasis. The complexity of AhR, with evidence for both oncogenic and tumor suppressor roles is discussed. We propose that AhR functions as a "molecular bridge", linking disproportionate toxin exposure and policies which underlie environmental injustice with tumor cell behaviors which drive poor patient outcomes.

Particulate matter 2.5 induces the skin barrier dysfunction and cutaneous inflammation via AhR- and T helper 17 cell-related genes in human skin tissue as identified via transcriptome analysis

Particulate matter (PM2.5) is an environmental pollutant causing skin inflammatory diseases via epidermal barrier damage. However, the mechanism and related gene expression induced by PM2.5 remains unclear. Our aim was to determine the effect of PM2.5 on human skin tissue ex vivo, and elucidate the mechanism of T helper 17 cell-related inflammatory cytokine and skin barrier function. We verified the expression levels of gene in PM2.5-treated human skin tissue using Quantseq (3' mRNA-Seq), and Gene Ontology (GO) terms and protein-protein interaction (PPI) networks were performed. The PM2.5 treatment significantly enhanced the expression of Th 1, 2, 17 and 22 cell-related genes (cut-off value: │1.2 │ > fold change and p < 0.05). Most of all, Th17 cell-related genes are upregulated and those genes are associated with skin epidermal barrier function and Aryl hydrocarbon receptor (AhR), a xenobiotic receptor, pathway. In human keratinocyte cell lines, AhR-regulated genes (e.g. AhRR, CYP1A1, IL6 and IL36G), Th17 cell-related genes (e.g. IL17C) and epidermal barrier-related genes (e.g. SPRR2A and KRT71) are significantly increased after PM2.5. In the protein level, the secretion of IL-6 and IL-36G was increased in human skin tissue following PM2.5 treatment, and the expression of SPRR2A and KRT71 was significantly increased. PM2.5 exposure could ruin the skin epidermal barrier function via AhR- and Th17 cell-related inflammatory pathway.

Porphyromonas gingivalis infection promotes inflammation via inhibition of the AhR signalling pathway in periodontitis

Porphyromonas gingivalis (P. gingivalis) is a key pathogen of chronic periodontitis. Aryl hydrocarbon receptor (AhR) is essential in immune homeostasis via modulation of pro-inflammatory cytokines production and indoleamine 2,3-dioxygenase (IDO). In this study, it is demonstrated that P. gingivalis may regulate AhR signalling in periodontitis, which provides a potential target for further immune regulation studies in periodontitis. Experimental periodontitis was induced in C57BL/6 mice by silk ligature and P. gingivalis oral inoculation. The alveolar bone resorption was examined using Micro-CT. Histological structures were observed and related cytokines involved in AhR signalling pathway were analysed. RAW264.7 cells were pretreated with AhR agonist (FICZ) and antagonist (CH223191) and infected with P. gingivalis subsequently. The levels of IDO, AhR and other related cytokines were measured. To demonstrate IDO activity, the concentrations of tryptophan (Trp) and kynurenine (Kyn) were assessed by HPLC. Histological analysis of periodontitis mice showed distinct alveolar bone resorption and inflammatory cell infiltration. The level of AhR and its downstream target factors were significantly decreased in inflamed gingival tissue. Furthermore, RAW 264.7 cells incubated by P. gingivalis exhibited increased pro-inflammatory cytokines production and decreased AhR, CYP1A1, CYP1B1, and IDO expression. Decreased IDO activity was observed as decreased Kyn/Trp ratio in the supernatant. Moreover, FICZ decreased the pro-inflammatory cytokines levels in P. gingivalis infected cells. It is concluded that P. gingivalis may promote inflammatory responses via inhibiting the AhR signalling pathway in periodontitis.

Polyphenols: Chemoprevention and therapeutic potentials in hematological malignancies

Polyphenols are one of the largest plant-derived natural product and they play an important role in plants' defense as well as in human health and disease. A number of them are pleiotropic molecules and have been shown to regulate signaling pathways, immune response and cell growth and proliferation which all play a role in cancer development. Hematological malignancies on the other hand, are cancers of the blood. While current therapies are efficacious, they are usually expensive and with unwanted side effects. Thus, the search for newer less toxic agents. Polyphenols have been reported to possess antineoplastic properties which include cell cycle arrest, and apoptosis via multiple mechanisms. They also have immunomodulatory activities where they enhance T cell activation and suppress regulatory T cells. They carry out these actions through such pathways as PI3K/Akt/mTOR and the kynurenine. They can also reverse cancer resistance to chemotherapy agents. In this review, i look at some of the molecular mechanism of action of polyphenols and their potential roles as therapeutic agents in hematological malignancies. Here i discuss their anti-proliferative and anti-neoplastic activities especially their abilities modulate signaling pathways as well as immune response in hematological malignancies. I also looked at clinical studies done mainly in the last 10-15 years on various polyphenol combination and how they enhance synergism. I recommend that further preclinical and clinical studies be carried out to ensure safety and efficacy before polyphenol therapies be officially moved to the clinics.

Aryl hydrocarbon receptor (AhR) reveals evidence of antagonistic pleiotropy in the regulation of the aging process

The antagonistic pleiotropy hypothesis is a well-known evolutionary theory to explain the aging process. It proposes that while a particular gene may possess beneficial effects during development, it can exert deleterious properties in the aging process. The aryl hydrocarbon receptor (AhR) has a significant role during embryogenesis, but later in life, it promotes several age-related degenerative processes. For instance, AhR factor (i) controls the pluripotency of stem cells and the stemness of cancer stem cells, (ii) it enhances the differentiation of embryonal stem cells, especially AhR signaling modulates the differentiation of hematopoietic stem cells and progenitor cells, (iii) it also stimulates the differentiation of immunosuppressive Tregs, Bregs, and M2 macrophages, and finally, (iv) AhR signaling participates in the differentiation of many peripheral tissues. On the other hand, AhR signaling is involved in many processes promoting cellular senescence and pathological processes, e.g., osteoporosis, vascular dysfunction, and the age-related remodeling of the immune system. Moreover, it inhibits autophagy and aggravates extracellular matrix degeneration. AhR signaling also stimulates oxidative stress, promotes excessive sphingolipid synthesis, and disturbs energy metabolism by catabolizing NAD⁺ degradation. The antagonistic pleiotropy of AhR signaling is based on the complex and diverse connections with major signaling pathways in a context-dependent manner. The major regulatory steps include, (i) a specific ligand-dependent activation, (ii) modulation of both genetic and non-genetic responses, (iii) a competition and crosstalk with several transcription factors, such as ARNT, HIF-1α, E2F1, and NF-κB, and (iv) the epigenetic regulation of target genes with binding partners. Thus, not only mTOR signaling but also the AhR factor demonstrates antagonistic pleiotropy in the regulation of the aging process.

The immunosuppressive role of indoleamine 2, 3-dioxygenase in glioblastoma: mechanism of action and immunotherapeutic strategies

Glioblastoma multiforme (GBM) is a fatal brain tumor in adults with a bleak diagnosis. Expansion of immunosuppressive and malignant CD4 + FoxP3 + GITR + regulatory T cells is one of the hallmarks of GBM. Importantly, most of the patients with GBM expresses the tryptophan-degrading enzyme indoleamine 2,3-dioxygenase (IDO). While IDO1 is generally not expressed at appreciable levels in the adult central nervous system, it is rapidly stimulated and highly expressed in response to ongoing immune surveillance in cancer. Increased levels of immune surveillance in cancer are thus related to higher intratumoral IDO expression levels and, as a result, a worse OS in GBM patients. Conversion of the important amino acid tryptophan into downstream catabolite known as kynurenines is the major function of IDO. Decreasing tryptophan and increasing the concentration of immunomodulatory tryptophan metabolites has been shown to induce T-cell apoptosis, increase immunosuppressive programming, and death of tumor antigen-presenting dendritic cells. This observation supported the immunotherapeutic strategy, and the targeted molecular therapy that suppresses IDO1 activity. We review the current understanding of the role of IDO1 in tumor immunological escape in brain tumors, the immunomodulatory effects of its primary catabolites, preclinical research targeting this enzymatic pathway, and various issues that need to be overcome to increase the prospective immunotherapeutic relevance in the treatment of GBM malignancy.

Aryl Hydrocarbon Receptor in Oxidative Stress as a Double Agent and Its Biological and Therapeutic Significance

The aryl hydrocarbon receptor (AhR) has long been implicated in the induction of a battery of genes involved in the metabolism of xenobiotics and endogenous compounds. AhR is a ligand-activated transcription factor necessary for the launch of transcriptional responses important in health and disease. In past decades, evidence has accumulated that AhR is associated with the cellular response to oxidative stress, and this property of AhR must be taken into account during investigations into a mechanism of action of xenobiotics that is able to activate AhR or that is susceptible to metabolic activation by enzymes encoded by the genes that are under the control of AhR. In this review, we examine various mechanisms by which AhR takes part in the oxidative-stress response, including antioxidant and prooxidant enzymes and cytochrome P450. We also show that AhR, as a participant in the redox balance and as a modulator of redox signals, is being increasingly studied as a target for a new class of therapeutic compounds and as an explanation for the pathogenesis of some disorders.

Indoleamine 2,3-dioxygenase 1 activation in mature cDC1 promotes tolerogenic education of inflammatory cDC2 via metabolic communication

Conventional dendritic cells (cDCs), cDC1 and cDC2, act both to initiate immunity and maintain self-tolerance. The tryptophan metabolic enzyme indoleamine 2,3-dioxygenase 1 (IDO1) is used by cDCs in maintaining tolerance, but its role in different subsets remains unclear. At homeostasis, only mature CCR7⁺ cDC1 expressed IDO1 that was dependent on IRF8. Lipopolysaccharide treatment induced maturation and IDO1-dependent tolerogenic activity in isolated immature cDC1, but not isolated cDC2. However, both human and mouse cDC2 could induce IDO1 and acquire tolerogenic function when co-cultured with mature cDC1 through the action of cDC1-derived l-kynurenine. Accordingly, cDC1-specific inactivation of IDO1 in vivo exacerbated disease in experimental autoimmune encephalomyelitis. This study identifies a previously unrecognized metabolic communication in which IDO1-expressing cDC1 cells extend their immunoregulatory capacity to the cDC2 subset through their production of tryptophan metabolite l-kynurenine. This metabolic axis represents a potential therapeutic target in treating autoimmune demyelinating diseases.

Role of indoleamine 2,3-dioxygenase 1 (IDO1) and kynurenine pathway in the regulation of the aging process

Indoleamine 2,3-dioxygenase 1 (IDO1) is activated in chronic inflammatory states, e.g., in the aging process and age-related diseases. IDO1 enzyme catabolizes L-tryptophan (L-Trp) into kynurenine (KYN) thus stimulating the KYN pathway. The depletion of L-Trp inhibits the proliferation of immune cells in inflamed tissues and it also reduces serotonin synthesis predisposing to psychiatric disorders. Interestingly, IDO1 protein contains two immunoreceptor tyrosine-based inhibitory motifs (ITIM) which trigger suppressive signaling through the binding of PI3K p110 and SHP-1 proteins. This immunosuppressive activity is not dependent on the catalytic activity of IDO1. KYN and its metabolite, kynurenic acid (KYNA), are potent activators of the aryl hydrocarbon receptor (AhR) which can enhance immunosuppression. IDO1-KYN-AhR signaling counteracts excessive pro-inflammatory responses in acute inflammation but in chronic inflammatory states it has many harmful effects. A chronic low-grade inflammation is associated with the aging process, a state called inflammaging. There is substantial evidence that the activation of the IDO1-KYN-AhR pathway robustly increases with the aging process. The activation of IDO1-KYN-AhR signaling does not only suppress the functions of effector immune cells, probably promoting immunosenescence, but it also impairs autophagy, induces cellular senescence, and remodels the extracellular matrix as well as enhancing the development of osteoporosis and vascular diseases. I will review the function of IDO1-KYN-AhR signaling and discuss its activation with aging as an enhancer of the aging process.

Aryl Hydrocarbon Receptor Activation by Benzo[a]pyrene Prevents Development of Septic Shock and Fatal Outcome in a Mouse Model of Systemic Salmonella enterica Infection

This study focused on immunomodulatory effects of aryl hydrocarbon receptor (AhR) activation through benzo[a]pyrene (BaP) during systemic bacterial infection. Using a well-established mouse model of systemic Salmonella enterica (S.E.) infection, we studied the influence of BaP on the cellular and humoral immune response and the outcome of disease. BaP exposure significantly reduced mortality, which is mainly caused by septic shock. Surprisingly, the bacterial burden in BaP-exposed surviving mice was significantly higher compared to non-exposed mice. During the early phase of infection (days 1-3 post-infection (p.i.)), the transcription of proinflammatory factors (i.e., IL-12, IFN-γ, TNF-α, IL-1β, IL-6, IL-18) was induced faster under BaP exposure. Moreover, BaP supported the activity of antigen-presenting cells (i.e., CD64 (FcγRI), MHC II, NO radicals, phagocytosis) at the site of infection. However, early in infection, the anti-inflammatory cytokines IL-10 and IL-22 were also locally and systemically upregulated in BaP-exposed S.E.-infected mice. BaP-exposure resulted in long-term persistence of salmonellae up to day 90 p.i., which was accompanied by significantly elevated S.E.-specific antibody responses (i.e., IgG1, IgG2c). In summary, these data suggest that BaP-induced AhR activation is capable of preventing a fatal outcome of systemic S.E. infection, but may result in long-term bacterial persistence, which, in turn, may support the development of chronic inflammation.

Aryl Hydrocarbon Receptor Signaling Synergizes with TLR/NF-κB-Signaling for Induction of IL-22 Through Canonical and Non-Canonical AhR Pathways

Interleukin 22 (IL-22) is critically involved in gut immunity and host defense and primarily produced by activated T cells. In different circumstances IL-22 may contribute to pathological conditions or act as a cancer promoting cytokine secreted by infiltrating immune cells. Here we show that bone marrow-derived macrophages (BMM) express and produce IL-22 after activation of the aryl hydrocarbon receptor (AhR) when cells are activated through the Toll-like receptor (TLR) family. The additional activation of AhR triggered a significant induction of IL-22 in TLR-activated BMM. Deletion and mutation constructs of the IL-22 promoter revealed that a consensus DRE and RelBAhRE binding element are necessary to mediate the synergistic effects of AhR and TLR ligands. Inhibitor studies and analysis of BMM derived from knockout mice confirmed that the synergistic induction of IL-22 by AhR and TLR ligands depend on the expression of AhR and Nuclear Factor-kappa B (NF-κB) member RelB. The exposure to particulate matter (PM) collected from traffic related air pollution (TRAP) and wildfires activated AhR as well as NF-κB signaling and significantly induced the expression of IL-22. In summary this study shows that simultaneous activation of the AhR and NF-κB signaling pathways leads to synergistic and prolonged induction of IL-22 by integrating signals of the canonical and non-canonical AhR pathway.

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.

Indoleamine 2,3-dioxygenase 1 is essential for sustaining durable antibody responses

Humoral immunity is essential for protection against pathogens, emphasized by the prevention of 2-3 million deaths worldwide annually by childhood immunizations. Long-term protective immunity is dependent on the continual production of neutralizing antibodies by the subset of long-lived plasma cells (LLPCs). LLPCs are not intrinsically long-lived, but require interaction with LLPC niche stromal cells for survival. However, it remains unclear which and how these interactions sustain LLPC survival and long-term humoral immunity. We now have found that the immunosuppressive enzyme indoleamine 2,3- dioxygenase 1 (IDO1) is required to sustain antibody responses and LLPC survival. Activation of IDO1 occurs upon the engagement of CD80/CD86 on the niche dendritic cells by CD28 on LLPC. Kynurenine, the product of IDO1 catabolism, activates the aryl hydrocarbon receptor in LLPC, reinforcing CD28 expression and survival signaling. These findings expand the immune function of IDO1 and uncover a novel pathway for sustaining LLPC survival and humoral immunity.

Effects of oral administration of Spirulina platensis and probiotics on serum immunity indexes, colonic immune factors, fecal odor, and fecal flora in mice

To evaluate the effects of Spirulina platensis and probiotics on growth, immunity indexes, fecal flora, and fecal odor in mice, 40 mice were randomly allotted to four groups, and each was administrated with nothing, S. platensis, probiotics, or both for 28 days, respectively. Then, many indexes were measured. The results showed that S. platensis was more effective (P < 0.001) than probiotics in improving mice's feed conversion ration (FCR). In immunity, probiotics administration increased (P < 0.042) serum IgE, IgM, IFN-γ, colonic AHR, TLR4, and NF-κB protein expression and decreased (P < 0.039) serum IL-1α, IL-21, IL-22, and colonic ARNT gene expression. However, the S. platensis showed weaker effect, which increased (P < 0.025) the serum IgE, IgM, TNF-α, and the colonic AHR and NF-κB protein expression, and decreased (P < 0.01) serum IL-21. Probiotics consumption decreased the fecal odor by decreasing (P < 0.02) fecal Escherichia coli, indole-3-acetic acid (IAA), and skatole contents, and the S. platensis decreased (P = 0.04) the IAA. These results indicated that oral administration of probiotics, S. platensis, or both of them in mice probably benefited body's immunity and reduced fecal odor. However, their mechanisms were still unclear and need further study.

Aryl hydrocarbon receptor in mesenchymal stromal cells: new frontiers in AhR biology

Mesenchymal stromal cells (MSCs) are nonhematopoietic cells that have been clinically explored as investigational cellular therapeutics for tissue injury regeneration and immune-mediated diseases. Their pharmaceutical properties arise from activation of endogenous receptors and transcription factors leading to a paracrine effect which mirror the biology of progenitors from which they arise. The aryl hydrocarbon receptor (AhR) is a transcription factor that has been extensively studied as an environmental sensor for xenobiotics, but recent findings suggest it can modulate immunological functions. Both genetic and pharmacological investigations revealed that MSCs express AhR and that it plays roles in inflammation, immunomodulation, and mesodermal plasticity of endogenous MSCs. Further, AhR has been shown to interact with key signaling cascades associated with these conditions. Therefore, AhR has potential to be an attractive target in both endogenous and culture-adapted MSCs for novel therapeutics to treat inflammation and other age-related disorders.

TLR2-mediated innate immune priming boosts lung anti-viral immunity

BACKGROUND

We assessed whether Toll-like receptor (TLR)2 activation boosts the innate immune response to rhinovirus infection, as a treatment strategy for virus-induced respiratory diseases.

METHODS

We employed treatment with a novel TLR2 agonist (INNA-X) prior to rhinovirus infection in mice, and INNA-X treatment in differentiated human bronchial epithelial cells derived from asthmatic-donors. We assessed viral load, immune cell recruitment, cytokines, type I and III interferon (IFN) production, as well as the lung tissue and epithelial cell immune transcriptome.

RESULTS

We show, in vivo, that a single INNA-X treatment induced innate immune priming characterised by low-level IFN-λ, Fas ligand, chemokine expression and airway lymphocyte recruitment. Treatment 7 days before infection significantly reduced lung viral load, increased IFN-β/λ expression and inhibited neutrophilic inflammation. Corticosteroid treatment enhanced the anti-inflammatory effects of INNA-X. Treatment 1 day before infection increased expression of 190 lung tissue immune genes. This tissue gene expression signature was absent with INNA-X treatment 7 days before infection, suggesting an alternate mechanism, potentially via establishment of immune cell-mediated mucosal innate immunity. In vitro, INNA-X treatment induced a priming response defined by upregulated IFN-λ, chemokine and anti-microbial gene expression that preceded an accelerated response to infection enriched for nuclear factor (NF)-κB-regulated genes and reduced viral loads, even in epithelial cells derived from asthmatic donors with intrinsic delayed anti-viral immune response.

CONCLUSION

Airway epithelial cell TLR2 activation induces prolonged innate immune priming, defined by early NF-κB activation, IFN-λ expression and lymphocyte recruitment. This response enhanced anti-viral innate immunity and reduced virus-induced airway inflammation.

The aryl hydrocarbon receptor suppresses immunity to oral squamous cell carcinoma through immune checkpoint regulation

Immune checkpoint inhibitors represent some of the most important cancer treatments developed in the last 20 y. However, existing immunotherapy approaches benefit only a minority of patients. Here, we provide evidence that the aryl hydrocarbon receptor (AhR) is a central player in the regulation of multiple immune checkpoints in oral squamous cell carcinoma (OSCC). Orthotopic transplant of mouse OSCC cells from which the AhR has been deleted (MOC1^AhR-KO) results, within 1 wk, in the growth of small tumors that are then completely rejected within 2 wk, concomitant with an increase in activated T cells in tumor-draining lymph nodes (tdLNs) and T cell signaling within the tumor. By 2 wk, AhR⁺ control cells (MOC1^Cas9), but not MOC1^AhR-KO cells up-regulate exhaustion pathways in the tumor-infiltrating T cells and expression of checkpoint molecules on CD4⁺ T cells (PD-1, CTLA4, Lag3, and CD39) and macrophages, dendritic cells, and Ly6G⁺ myeloid cells (PD-L1 and CD39) in tdLNs. Notably, MOC1^AhR-KO cell transplant renders mice 100% immune to later challenge with wild-type tumors. Analysis of altered signaling pathways within MOC1^AhR-KO cells shows that the AhR controls baseline and IFNγ-induced Ido and PD-L1 expression, the latter of which occurs through direct transcriptional control. These observations 1) confirm the importance of malignant cell AhR in suppression of tumor immunity, 2) demonstrate the involvement of the AhR in IFNγ control of PD-L1 and IDO expression in the cancer context, and 3) suggest that the AhR is a viable target for modulation of multiple immune checkpoints.

The emerging roles of IDO2 in cancer and its potential as a therapeutic target

During the past decades, tryptophan metabolism disorder was discovered to play a vital and complex role in the development of cancer. Indoleamine 2,3-dioxygenase 2 (IDO2) is one of the initial and rate-limiting enzymes of the kynurenine pathway of tryptophan catabolism. Increasing evidence indicates that IDO2 is upregulated in some tumors and plays a role in the development of cancer. In spite of the growing body of research, few reviews focused on the role of IDO2 in cancer. Here, we review the emerging knowledge on the roles of IDO2 in cancer and its potential as a therapeutic target. Firstly, the main biological features and regulatory mechanisms are reviewed, after which we focus on the expression and roles of IDO2 in cancer. Finally, we discuss the potential of IDO2 as a therapeutic target for cancer treatment.

Trajectory Shifts in Interdisciplinary Research of the Aryl Hydrocarbon Receptor-A Personal Perspective on Thymus and Skin

Identifying historical trajectories is a useful exercise in research, as it helps clarify important, perhaps even “paradigmatic”, shifts in thinking and moving forward in science. In this review, the development of research regarding the role of the transcription factor “aryl hydrocarbon receptor” (AHR) as a mediator of the toxicity of environmental pollution towards a link between the environment and a healthy adaptive response of the immune system and the skin is discussed. From this fascinating development, the opportunities for targeting the AHR in the therapy of many diseases become clear.

The Landscape of AhR Regulators and Coregulators to Fine-Tune AhR Functions

The aryl-hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates numerous cellular responses. Originally investigated in toxicology because of its ability to bind environmental contaminants, AhR has attracted enormous attention in the field of immunology in the last 20 years. In addition, the discovery of endogenous and plant-derived ligands points to AhR also having a crucial role in normal cell physiology. Thus, AhR is emerging as a promiscuous receptor that can mediate either toxic or physiologic effects upon sensing multiple exogenous and endogenous molecules. Within this scenario, several factors appear to contribute to the outcome of gene transcriptional regulation by AhR, including the nature of the ligand as such and its further metabolism by AhR-induced enzymes, the local tissue microenvironment, and the presence of coregulators or specific transcription factors in the cell. Here, we review the current knowledge on the array of transcription factors and coregulators that, by interacting with AhR, tune its transcriptional activity in response to endogenous and exogenous ligands.

Aryl hydrocarbon receptor (AHR), integrating energy metabolism and microbial or obesity-mediated inflammation

Aryl hydrocarbon receptor (AHR) has been characterized as multifunctional sensor, integrator and ligand-activated transcription factor of the bHLH/PAS family. Regulation of inflammatory diseases and energy metabolism are among the putative functions of AHR. Challenges in AHR research include marked species differences, and cell, tissue and context dependence of AHR functions. The commentary is focused on AHR's role in the integration between energy expenditure and microbial and non-infectious inflammation, the latter exemplified by obesity-mediated nonalcoholic fatty liver disease. One of the mechanisms controlling energy-consuming inflammation is represented by a signalsome that is involved in retinoic acid-triggered neutrophil differentiation and regulation of the NADPH oxidase complex (NOX). Established signalsome components are AHR, CD38, multiple protein kinases and adaptors. To prevent chronic inflammatory diseases, the complex interplay between a range of inflammatory responses and energy expenditure must be precisely regulated. Surviving an infection requires both pathogen clearance and tissue protection from inflammatory damage. Defenses are energy-consuming anabolic programs. Therefore, anti-inflammatory, catabolic tolerance programs by metabolic reprogramming of macrophages have evolved. Therapeutic options of AHR agonists to reduce chronic inflammatory diseases are discussed.

Intestinal microbiota-derived tryptophan metabolites are predictive of Ah receptor activity

Commensal microbiota-dependent tryptophan catabolism within the gastrointestinal tract is known to exert profound effects upon host physiology, including the maintenance of epithelial barrier and immune function. A number of abundant microbiota-derived tryptophan metabolites exhibit activation potential for the aryl hydrocarbon receptor (AHR). Gene expression facilitated by AHR activation through the presence of dietary or microbiota-generated metabolites can influence gastrointestinal homeostasis and confer protection from intestinal challenges. Utilizing untargeted mass spectrometry-based metabolomics profiling, combined with AHR activity screening assays, we identify four previously unrecognized tryptophan metabolites, present in mouse cecal contents and human stool, with the capacity to activate AHR. Using GC/MS and LC/MS platforms, quantification of these novel AHR activators, along with previously established AHR-activating tryptophan metabolites, was achieved, providing a relative order of abundance. Using physiologically relevant concentrations and quantitative gene expression analyses, the relative efficacy of these tryptophan metabolites with regard to mouse or human AHR activation potential is examined. These data reveal indole, 2-oxindole, indole-3-acetic acid and kynurenic acid as the dominant AHR activators in mouse cecal contents and human stool from participants on a controlled diet. Here we provide the first documentation of the relative abundance and AHR activation potential of a panel of microbiota-derived tryptophan metabolites. Furthermore, these data reveal the human AHR to be more sensitive, at physiologically relevant concentrations, to tryptophan metabolite activation than mouse AHR. Additionally, correlation analyses indicate a relationship linking major tryptophan metabolite abundance with AHR activity, suggesting these cecal/fecal metabolites represent biomarkers of intestinal AHR activity.

Activation of aryl hydrocarbon receptor in Langerhans cells by a microbial metabolite of tryptophan negatively regulates skin inflammation

BACKGROUND

Skin commensal bacteria play important roles in skin homeostasis. Langerhans cells (LCs) are epidermis-resident dendritic cells that sense environmental stimuli and are critical in the induction of immune tolerance to allergen and bacterial skin flora. However, response of LCs to the metabolites of the skin microbiota is not clear.

OBJECTIVE

To explore the effects of the skin microbial metabolites on LCs activation.

METHODS

LCs derived from CD34+ hematopoietic stem cells in the cord blood were treated with a microbial metabolite of tryptophan, indole-3-aldehyde (IAId). Activation aryl hydrocarbon receptor (AhR) signaling, production of IL-10, and expression of receptor activator of NF-κB (RANK) / receptor activator of NF-κB ligand (RANKL) in LCs or keratinocytes were analyzed using quantitative PCR, western blotting and flow cytometry. LCs maturation induced by IAId and CD4⁺ T cell response induced by IAId-conditioned LCs were also investigated.

RESULTS

IAId induced the production of indoleamine 2,3-dioxygenase (IDO) and IL-10 in LCs through the activation of AhR. IAId promoted the expression of RANK and RANKL on LCs and keratinocytes in an AhR-dependent manner respectively, which might result in activation of NF-κB signaling and production of IL-10. Moreover, a mature phenotype of LCs was induced by IAId, and IAId-activated LCs inhibited CD4⁺ T cell proliferation and induced IL-10 secretion.

CONCLUSIONS

Our study revealed a negatively regulatory function of a tryptophan metabolite on LCs through the activation of AhR, and the microbial metabolites could be utilized in future treatment for inflammatory skin diseases.

The therapeutic potential of targeting tryptophan catabolism in cancer

Based on its effects on both tumour cell intrinsic malignant properties as well as anti-tumour immune responses, tryptophan catabolism has emerged as an important metabolic regulator of cancer progression. Three enzymes, indoleamine-2,3-dioxygenase 1 and 2 (IDO1/2) and tryptophan-2,3-dioxygenase (TDO2), catalyse the first step of the degradation of the essential amino acid tryptophan (Trp) to kynurenine (Kyn). The notion of inhibiting IDO1 using small-molecule inhibitors elicited high hopes of a positive impact in the field of immuno-oncology, by restoring anti-tumour immune responses and synergising with other immunotherapies such as immune checkpoint inhibition. However, clinical trials with IDO1 inhibitors have yielded disappointing results, hence raising many questions. This review will discuss strategies to target Trp-degrading enzymes and possible down-stream consequences of their inhibition. We aim to provide comprehensive background information on Trp catabolic enzymes as targets in immuno-oncology and their current state of development. Details of the clinical trials with IDO1 inhibitors, including patient stratification, possible effects of the inhibitors themselves, effects of pre-treatments and the therapies the inhibitors were combined with, are discussed and mechanisms proposed that might have compensated for IDO1 inhibition. Finally, alternative approaches are suggested to circumvent these problems.

Biological characteristics of transcription factor RelB in different immune cell types: implications for the treatment of multiple sclerosis

Transcription factor RelB is a member of the nuclear factror-kappa B (NF-κB) family, which plays a crucial role in mediating immune responses. Plenty of studies have demonstrated that RelB actively contributes to lymphoid organ development, dendritic cells maturation and function and T cells differentiation, as well as B cell development and survival. RelB deficiency may cause a variety of immunological disorders in both mice and humans. Multiple sclerosis (MS) is an inflammatory and demyelinating disease of the central nervous system which involves a board of immune cell populations. Thereby, RelB may exert an impact on MS by modulating the functions of dendritic cells and the differentiation of T cells and B cells. Despite intensive research, the role of RelB in MS and its animal model, experimental autoimmune encephalomyelitis, is still unclear. Herein, we give an overview of the biological characters of RelB, summarize the updated knowledge regarding the role of RelB in different cell types that contribute to MS pathogenesis and discuss the potential RelB-targeted therapeutic implications for MS.

Aryl hydrocarbon receptor (AHR): From selected human target genes and crosstalk with transcription factors to multiple AHR functions

Accumulating evidence including studies of AHR-deficient mice and TCDD toxicity suggests multiple physiologic AHR functions. Challenges to identify responsible mechanisms are due to marked species differences and dependence upon cell type and cellular context. Transient AHR modulation is often necessary for physiologic functions whereas TCDD-mediated sustained receptor activation has been demonstrated to be responsible for toxic outcomes. To stimulate studies on responsible action mechanisms the commentary is focused on human AHR target genes and crosstalk with transcription factors. Discussed AHR functions include chemical and microbial defense, organ development, modulation of immunity and inflammation, reproduction, and NAD⁺-dependent energy metabolism. Obviously, much more work is needed to elucidate action mechanisms. In particular, studies of pathways leading to NAD⁺-dependent energy metabolism may shed light on the puzzling species differences of TCDD-mediated lethality and provide options for treatment of obesity and age-related degenerative diseases.

Genome-Wide Transcriptional Analysis Reveals Novel AhR Targets That Regulate Dendritic Cell Function during Influenza A Virus Infection

Activation of the ligand inducible aryl hydrocarbon receptor (AhR) during primary influenza A virus infection diminishes host responses by negatively regulating the ability of dendritic cells (DC) to prime naive CD8⁺ T cells, which reduces the generation of CTL. However, AhR-regulated genes and signaling pathways in DCs are not fully known. In this study, we used unbiased gene expression profiling to identify differentially expressed genes and signaling pathways in DCs that are modulated by AhR activation in vivo. Using the prototype AhR agonist TCDD, we identified the lectin receptor Cd209a (DC-SIGN) and chemokine Ccl17 as novel AhR target genes. We further show the percentage of DCs expressing CD209a on their surface was significantly decreased by AhR activation during infection. Whereas influenza A virus infection increased CCL17 protein levels in the lung and lung-draining lymph nodes, this was significantly reduced following AhR activation. Targeted excision of AhR in the hematopoietic compartment confirmed AhR is required for downregulation of CCL17 and CD209a. Loss of AhR's functional DNA-binding domain demonstrates that AhR activation alone is necessary but not sufficient to drive downregulation. AhR activation induced similar changes in gene expression in human monocyte-derived DCs. Analysis of the murine and human upstream regulatory regions of Cd209a and Ccl17 revealed a suite of potential transcription factor partners for AhR, which may coregulate these genes in vivo. This study highlights the breadth of AhR-regulated pathways within DCs, and that AhR likely interacts with other transcription factors to modulate DC functions during infection.

Role of NF-kB RelB in Aryl Hydrocarbon Receptor-Mediated Ligand Specific Effects

Here, we investigate the role of RelB in the regulation of genes which were identified to be induced in an aryl hydrocarbon receptor (AhR)-dependent manner and critically involved in regulation of immune responses. We analyzed the expression of genes of the AhR gene battery, cytokines, and immune regulatory enzymes in bone marrow-derived macrophages (BMM) and thymus of B6 wildtype (wt) mice and RelB knockout (RelB-/-) mice after treatment with various AhR ligands. The 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced expression of indoleamine 2,3-dioxygenase 1 (IDO1) and IDO2 was significantly repressed in thymus of RelB-/- mice but not in BMM derived from RelB-/- mice. Interestingly, the induced and basal expression of the cytokines interleukin (IL)-17A, IL-22, and CCL20 required the functional expression of RelB. The RelB-dependent expression of CCL20 was induced by the AhR ligands TCDD and 6-formylindolo[3,2-b]carbazole (FICZ), whereas indole-3-carbinol (I3C) suppressed CCL20 in lipopolysaccharide (LPS)-activated wt BMM. The LPS-induced expression of IL-6 and IL-10 was enhanced by TCDD and FICZ, whereas I3C significantly suppressed these cytokines in BMM. The exposure to FICZ led to higher increases of IL-17A and IL-22 mRNA compared to the effect of TCDD or I3C in thymus of wt mice. On the other hand, TCDD was the strongest inducer of CYP1A1, AhR Repressor (AhRR), and IDO2. In summary, these findings provide evidence for the important role of RelB in the transcriptional regulation of cytokines and enzymes induced by AhR ligands.

RELB: A novel prognostic marker for glioblastoma as identified by population-based analysis

Glioblastoma multiforme (GBM) is the most common and malignant type of glioma, with a poor prognosis for patients. The survival time of patients varies greatly due to the complexity of the human genome, which harbors diverse oncogenic drivers. In order to identify the specific driving factors, 325 glioma samples from the Chinese Glioma Genome Atlas (CGGA) database were analyzed in the present study. The level of RELB proto-oncogene, NF-κβ subunit (RELB) expression increased with the pathological grade progression of the gliomas, and higher expression levels were present in the mesenchymal subtype and isocitrate dehydrogenase 1 (IDH1) wild-type gliomas. This RELB expression pattern was identified in the CGGA database and observed in three large independent databases. In patients with GBM from the CGGA database, a higher RELB expression level was associated with a shorter survival time, a mesenchymal subtype and IDH1 wild-type gliomas. Kaplan-Meier survival analysis, survival nomograms and Cox analysis demonstrated an independent prognostic value for RELB expression. Moreover, biological function analysis indicated the association of RELB with the ‘immune response’, ‘cell activation’ and the ‘apoptotic process’. In addition, RELB expression levels exhibited a negative correlation with the levels of microRNA (miR)-139-5p and miR-139-3p. The present study identified the pathological and biological roles of RELB in glioma and revealed its independent prognostic effect. These results suggested that RELB may be used as a prognostic biomarker and potential therapeutic target in glioma.

Myalgic encephalomyelitis or chronic fatigue syndrome: how could the illness develop?

A model of the development and progression of chronic fatigue syndrome (myalgic encephalomyelitis), the aetiology of which is currently unknown, is put forward, starting with a consideration of the post-infection role of damage-associated molecular patterns and the development of chronic inflammatory, oxidative and nitrosative stress in genetically predisposed individuals. The consequences are detailed, including the role of increased intestinal permeability and the translocation of commensal antigens into the circulation, and the development of dysautonomia, neuroinflammation, and neurocognitive and neuroimaging abnormalities. Increasing levels of such stress and the switch to immune and metabolic downregulation are detailed next in relation to the advent of hypernitrosylation, impaired mitochondrial performance, immune suppression, cellular hibernation, endotoxin tolerance and sirtuin 1 activation. The role of chronic stress and the development of endotoxin tolerance via indoleamine 2,3-dioxygenase upregulation and the characteristics of neutrophils, monocytes, macrophages and T cells, including regulatory T cells, in endotoxin tolerance are detailed next. Finally, it is shown how the immune and metabolic abnormalities of chronic fatigue syndrome can be explained by endotoxin tolerance, thus completing the model.

Inflammatory marker and aryl hydrocarbon receptor-dependent responses in human macrophages exposed to emissions from biodiesel fuels

Biodiesel or renewable diesel fuels are alternative fuels produced from vegetable oil and animal tallow that are being considered to help reduce the use of petroleum-based fuels and emissions of air pollutants including greenhouse gases. Here, we analyzed the gene expression of inflammatory marker responses and the cytochrome P450 1A1 (CYP1A1) enzyme after exposure to diesel and biodiesel emission samples generated from an in-use heavy-duty diesel vehicle. Particulate emission samples from petroleum-based California Air Resource Board (CARB)-certified ultralow sulfur diesel (CARB ULSD), biodiesel, and renewable hydro-treated diesel all induced inflammatory markers such as cyclooxygenase-2 (COX)-2 and interleukin (IL)-8 in human U937-derived macrophages and the expression of the xenobiotic metabolizing enzyme CYP1A1. Furthermore, the results indicate that the particle emissions from CARB ULSD and the alternative diesel fuel blends activate the aryl hydrocarbon receptor (AhR) and induce CYP1A1 in a dose- and AhR-dependent manner which was supported by the AhR luciferase reporter assay and gel shift analysis. Based on a per mile emissions with the model year 2000 heavy duty vehicle tested, the effects of the alternative diesel fuel blends emissions on the expression on inflammatory markers like IL-8 and COX-2 tend to be lower than emission samples derived from CARB ULSD fuel. The results will help to assess the potential benefits and toxicity from biofuel use as alternative fuels in modern technology diesel engines.

Role of Aryl Hydrocarbon Receptor (AhR) in the Regulation of Immunity and Immunopathology During Trypanosoma cruzi Infection

Resistance to Trypanosoma cruzi infection is dependent on a rapid induction of Th1-type and CD8+ T cell responses that should be promptly balanced to prevent immunopathology. T. cruzi-infected B6 mice are able to control parasite replication but show a limited expansion of Foxp3+regulatory T (Treg) cells that results in the accumulation of effector immune cells and the development of acute liver pathology. AhR is a ligand-activated transcription factor that promotes Treg cell development and suppression of pro-inflammatory cytokine production in dendritic cells, altering the course of adaptive immune response and the development of immunopathology. Here, we used different AhR-dependent activation strategies aiming to improve the Treg response, and B6 congenic mice carrying a mutant AhR variant with low affinity for its ligands (AhRd) to evaluate the role of AhR activation by natural ligands during experimental T. cruzi infection. The outcome of TCDD or 3-HK plus ITE treatments indicated that strong or weak AhR activation before or during T. cruzi infection was effective to regulate inflammation improving the Treg cell response and regularizing the ratio between CD4+ CD25- to Treg cells. However, AhR activation shifted the host-parasite balance to the parasite replication. Weak AhR activation resulted in Treg promotion while strong activation differentially modulated the susceptibility and resistance of cell death in activated T and Treg cells and the increase in TGF-β-producing Treg cells. Of note, T. cruzi-infected AhRd mice showed low levels of Treg cells associated with strong Th1-type response, low parasite burden and absence of liver pathology. These mice developed a Treg- and Tr1-independent mechanism of Th1 constriction showing increased levels of systemic IL-10 and IL-10-secreting CD4+ splenocytes. In addition, AhR activation induced by exogenous ligands had negative effects on the development of memory CD8+ T cell subsets while the lack/very weak activation in AhRd mice showed opposite results, suggesting that AhR ligation restricts the differentiation of memory CD8+T cell subsets. We propose a model in which a threshold of AhR activation exists and may explain how activation or inhibition of AhR-derived signals by infection/inflammation-induced ligands, therapeutic interventions or exposure to pollutants can modulate infections/diseases outcomes or vaccination efficacy.

Significance of AHR nuclear translocation sequence in 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced cPLA2α activation and hydronephrosis

The aryl hydrocarbon receptor (AHR) plays a major role in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced toxicity phenotypes. TCDD bound to AHR elicits both genomic action in which target genes are transcriptionally upregulated and nongenomic action in which cytosolic phospholipase A₂α (cPLA₂α) is rapidly activated. However, how either of these actions, separately or in combination, induces toxicity phenotypes is largely unknown. In this study, we used AHR^nls/nls mice as a model in which AHR was mutated to lack nuclear translocation sequence (NLS), and AHR^d/- mice as the corresponding control. Using this model, we studied TCDD-induced alterations in cPLA₂α activation and related factors because of the pivotal roles of cPLA₂α both in AHR's nongenomic action and in regulation of causative genes of TCDD-induced hydronephrosis. Dams were orally administered TCDD at a dose of 300 µg/kg body weight on postnatal day 1, and pups subsequently exposed to TCDD via milk were examined for gene expression on PND 7 and for histological changes on PND 14. The activation of the AHR genomic action and hydronephrosis onset were observed in the control group but not in the AHR^nls/nls group. An ex vivo experiment using peritoneal macrophages exposed to 100 nM TCDD resulted in rapid activation of cPLA₂α, an indicator of the nongenomic action, only in the control group but not in the AHR^nls/nls group. These results indicated that an NLS is required for the AHR's genomic and nongenomic actions.

Oxidative Stress and Bronchopulmonary Dysplasia: Evidences From Microbiomics, Metabolomics, and Proteomics

Bronchopulmonary dysplasia is a major issue affecting morbidity and mortality of surviving premature babies. Preterm newborns are particularly susceptible to oxidative stress and infants with bronchopulmonary dysplasia have a typical oxidation pattern in the early stages of this disease, suggesting the important role of oxidative stress in its pathogenesis. Bronchopulmonary dysplasia is a complex disease where knowledge advances as new investigative tools become available. The explosion of the "omics" disciplines has recently affected BPD research. This review focuses on the new evidence coming from microbiomics, metabolomics and proteomics in relation to oxidative stress and pathogenesis of bronchopulmonary dysplasia. Since the pathogenesis is not yet completely understood, information gained in this regard would be important for planning an efficacious prevention and treatment strategy for the future.

Human AHR functions in vascular tissue: Pro- and anti-inflammatory responses of AHR agonists in atherosclerosis

Despite decades of intense research physiologic aryl hydrocarbon receptor (AHR) functions have not been elucidated. Challenges include marked species differences and dependence on cell type and cellular context. A previous commentary on human AHR functions in skin and intestine has been extended to vascular tissue. Similar functions appear to be operating in vascular tissue including microbial defense, modulation of stem/progenitor cells as well as control of immunity and inflammation. However, AHR functions are Janus faced: Detrimental AHR functions in vascular tissue are well documented, e.g., upon exposure to polycyclic aromatic hydrocarbons in cigarette smoke leading to oxidative stress and generation of oxidized LDL. Modified LDL particles accumulate in macrophages and smooth muscle-derived pro-inflammatory foam cells, the hallmark of atherosclerosis. On the other hand, numerous anti-inflammatory AHR agonists have been identified including bilirubin and quercetin. Mechanisms as to how AHR produces pro- and anti-inflammatory responses in the vascular system need further investigation.

Activation of Aryl Hydrocarbon Receptor Negatively Regulates Thymic Stromal Lymphopoietin Gene Expression via Protein Kinase Cδ-p300-NF-κB Pathway in Keratinocytes under Inflammatory Conditions

Epithelial-derived thymic stromal lymphopoietin (TSLP) plays an important role in pathogenesis in several types of dermatitis. Recently, the anti-inflammatory effects of aryl hydrocarbon receptor (AhR) have been reported in inflamed skin. In this study, keratinocytes were stimulated with tumor necrosis factor-α or flagellin in combination with AhR ligands or antagonist. TSLP gene expression and recruitment of transcriptional regulator to TSLP gene promoter were determined. The effects of AhR activation were also studied in DNFB-induced dermatitis model. We found that AhR activation suppressed upregulation of TSLP expression in keratinocytes treated with tumor necrosis factor-α or flagellin. In addition, AhR activation induced protein kinase Cδ-mediated phosphorylation of p300 at serine 89, leading to decreased acetylation and DNA binding activity of NF-κB p65 to the TSLP gene promoter. We also found that AhR activation alleviates dermatitis induced by DNFB treatment. Protein kinase Cδ depletion by small interfering RNA abolished the beneficial effect of AhR activation on dermatitis. Our study suggests that AhR activation may help to reduce inflammation in the dermatitis via downregulation of TSLP expression.

Environmental cues received during development shape dendritic cell responses later in life

Environmental signals mediated via the aryl hydrocarbon receptor (AHR) shape the developing immune system and influence immune function. Developmental exposure to AHR binding chemicals causes persistent changes in CD4+ and CD8+ T cell responses later in life, including dampened clonal expansion and differentiation during influenza A virus (IAV) infection. Naïve T cells require activation by dendritic cells (DCs), and AHR ligands modulate the function of DCs from adult organisms. Yet, the consequences of developmental AHR activation by exogenous ligands on DCs later in life has not been examined. We report here that early life activation of AHR durably reduces the ability of DC to activate naïve IAV-specific CD8+ T cells; however, activation of naïve CD4+ T cells was not impaired. Also, DCs from developmentally exposed offspring migrated more poorly than DCs from control dams in both in vivo and ex vivo assessments of DC migration. Conditional knockout mice, which lack Ahr in CD11c lineage cells, suggest that dampened DC emigration is intrinsic to DCs. Yet, levels of chemokine receptor 7 (CCR7), a key regulator of DC trafficking, were generally unaffected. Gene expression analyses reveal changes in Lrp1, Itgam, and Fcgr1 expression, and point to alterations in genes that regulate DC migration and antigen processing and presentation as being among pathways disrupted by inappropriate AHR signaling during development. These studies establish that AHR activation during development causes long-lasting changes to DCs, and provide new information regarding how early life environmental cues shape immune function later in life.

The Science behind Microgreens as an Exciting New Food for the 21st Century

Chronic diseases are a major health problem in the United States. Accumulated data suggest that consumption of vegetables can significantly reduce the risk of many chronic diseases. Dietary guidelines for 2015-2020 from the U.S. Department of Agriculture and the U.S. Department of Health and Human Services recommend 1-4 cups of vegetables per day for males and 1-3 cups of vegetables per day for females, depending on their age. However, the average intake of vegetables is below the recommended levels. Microgreens are young vegetable greens. Although they are small, microgreens have delicate textures, distinctive flavors, and various nutrients. In general, microgreens contain greater amounts of nutrients and health-promoting micronutrients than their mature counterparts. Because microgreens are rich in nutrients, smaller amounts may provide similar nutritional effects compared to larger quantities of mature vegetables. However, literature on microgreens remains limited. In this Review, we discuss chemical compositions, growing conditions, and biological efficacies of microgreens. We seek to stimulate interest in further study of microgreens as a promising dietary component for potential use in diet-based disease prevention.

Aryl Hydrocarbon Receptor Promotes IL-10 Expression in Inflammatory Macrophages Through Src-STAT3 Signaling Pathway

The aryl hydrocarbon receptor (AhR) is an important immune regulator with a role in inflammatory response. However, the role of AhR in IL-10 production by inflammatory macrophages is currently unknown. In this study, we investigated LPS-induced IL-10 expression in macrophages from AhR-KO mice and AhR-overexpressing RAW264.7 cells. AhR was highly expressed after LPS stimulation through NF-κB pathway. Loss of AhR resulted in reduced IL-10 expression in LPS-induced macrophages. Moreover, the IL-10 expression was elevated in LPS-induced AhR-overexpressing RAW264.7 cells. Maximal IL-10 expression was dependent on an AhR non-genomic pathway closely related to Src and STAT3. Furthermore, AhR-associated Src activity was responsible for tyrosine phosphorylation of STAT3 and IL-10 expression by inflammatory macrophages. Adoptive transfer of AhR-expressing macrophages protected mice against LPS-induced peritonitis associated with high IL-10 production. In conclusion, we identified the AhR-Src-STAT3-IL-10 signaling pathway as a critical pathway in the immune regulation of inflammatory macrophages, It suggests that AhR may be a potential therapeutic target in immune response.