RelB and the aryl hydrocarbon receptor: dendritic cell tolerance at the epithelial interface

The Contribution of Genetic Variation and Aberrant Methylation of Aryl Hydrocarbon Receptor Signaling Pathway Genes to Rheumatoid Arthritis

The aryl hydrocarbon receptor (AHR) signaling pathway participates in immune regulation of multiple autoimmune diseases, including rheumatoid arthritis (RA). We conducted this study to investigate the association of AHR signaling pathway genes (AHR, ARNT, AHRR) single nucleotide polymorphisms (SNPs), as well as their methylation levels, with RA susceptibility. Nine SNPs (AHR gene rs2066853, rs2158041, rs2282885, ARNT gene rs10847, rs1889740, rs11204735, AHRR gene rs2292596, rs2672725, rs349583) were genotyped via improved multiple ligase detection reaction (iMLDR) in 479 RA patients and 496 healthy controls. We used the Illumina Hiseq platform to detect methylation levels of these genes in 122 RA patients and 123 healthy controls. A significant increase in rs11204735 C allele frequency was observed in RA patients when compared to controls. Further, rs11204735 polymorphism was associated with a decreased risk of RA under the dominant model. ARNT CCC haplotype frequency was significantly increased in RA patients in comparison to controls. In the AHRR gene, rs2672725 GG genotype, G allele frequencies were significantly related to an increased risk of RA and rs2292596, rs2672725 polymorphism were significantly associated with an increased risk of RA under the dominant model, recessive model, respectively. However, no significant association was identified between AHR gene polymorphism and RA susceptibility. The AHR methylation level in RA patients was significantly higher than the controls, while AHRR methylation level was abnormally reduced in RA patients. In addition, AHRR rs2672725 genotype distribution was significantly associated with the AHRR methylation level among RA patients. In summary, ARNT rs11204735, AHRR rs2292596, and rs2672725 polymorphisms were associated with RA susceptibility and altered AHR, AHRR methylation levels were related to the risk of RA.

Hidradenitis suppurativa - The role of interleukin-17, the aryl hydrocarbon receptor and the link to a possible fungal aetiology

Hidradenitis Suppurativa (HS) is a chronic, recurrent, debilitating skin disease of the hair follicle that usually presents after puberty with painful, deep-seated, inflamed lesions in the apocrine gland bearing areas of the body, most commonly the axillae, inguinal and anogenital regions. The pathophysiology of the disease remains elusive, with newer therapies targeting various aspects of the dysregulated immune system. This presents a useful opportunity to look at the cytokine profile in HS and other inflammatory conditions that share similar patterns with the aim of teasing out less considered explanations for HS pathogenesis. It has been observed that IL-17 appears to be the most common denominator linking HS with other immune mediated diseases like Crohn, ulcerative colitis, multiple sclerosis and psoriasis. Given that IL-17 plays an important role in antifungal immunity, evidenced by the cytokine pattern in fungal disease and the bulk of data citing their potential involvement in Crohn, ulcerative colitis, multiple sclerosis and psoriasis; it is fair to suggest the need to explore the role that fungi play in the setting of HS going forward. The aryl hydrocarbon receptor (ahr) is a ubiquitous and largely conserved entity that is gaining interest in inflammatory conditions such as psoriasis and atopic dermatitis. It is well known to modulate autoimmune states. Its activation by both exogenous and endogenous agents result in secretion of IL-17 by Th17 cells. One of such agents is the tryptophan metabolite 6-formylindolo [3,2-b] carbazole (FICZ)-which can be produced by microorganisms such as fungi. It will be interesting to explore its usefulness in HS pathogenesis.

Apigenin Modulates Dendritic Cell Activities and Curbs Inflammation Via RelB Inhibition in the Context of Neuroinflammatory Diseases

Neuroinflammation leads to tissue injury causing many of the clinical symptoms of Multiple Sclerosis, an autoimmune disorder of the central nervous system (CNS). While T cells, specifically T_h1 and T_h17 cells, are the ultimate effectors of this disease, dendritic cells (DCs) mediate T cell polarization, activation, etc. In our previous study, Apigenin, a natural flavonoid, has been shown to reduce EAE disease severity through amelioration of demyelination in the CNS as well as the sequestering of DCs and other myeloid cells in the periphery. Here, we show that Apigenin exerts its effects possibly through shifting DC modulated T cell responses from T_h1 and T_h17 type towards T_reg directed responses evident through the decrease in T-bet, IFN-γ (T_h1), IL-17 (T_h17) and increase in IL-10, TGF-β and FoxP3 (T_reg) expression in cells from both normal human donors and EAE mice. RelB, an NF-κβ pathway protein is central to DC maturation, its antigen presentation capabilities and DC-mediated T cell activation. Apigenin reduced mRNA and protein levels of RelB and also reduced its nuclear translocation. Additionally, siRNA-mediated silencing of RelB further potentiated the RelB-mediated effects of Apigenin thus confirming its role in Apigenin directed regulation of DC biology. These results provide key information about the molecular events controlled by Apigenin in its regulation of DC activity marking its potential as a therapy for neuroinflammatory disease. Graphical Abstract.

NCoR1: Putting the Brakes on the Dendritic Cell Immune Tolerance

Understanding the mechanisms fine-tuning immunogenic versus tolerogenic balance in dendritic cells (DCs) is of high importance for therapeutic approaches. We found that NCoR1-mediated direct repression of the tolerogenic program in conventional DCs is essential for induction of an optimal immunogenic response. NCoR1 depletion upregulated a wide variety of tolerogenic genes in activated DCs, which consequently resulted in increased frequency of FoxP3⁺ regulatory T cells. Mechanistically, NCoR1 masks the PU.1-bound super-enhancers on major tolerogenic genes after DC activation that are subsequently bound by nuclear factor-κB. NCoR1 knockdown (KD) reduced RelA nuclear translocation and activity, whereas RelB was unaffected, providing activated DCs a tolerogenic advantage. Moreover, NCoR1^DC−/- mice depicted enhanced Tregs in draining lymph nodes with increased disease burden upon bacterial and parasitic infections. Besides, adoptive transfer of activated NCoR1 KD DCs in infected animals showed a similar phenotype. Collectively, our results demonstrated NCoR1 as a promising target to control DC-mediated immune tolerance.

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NCoR1 directly represses tolerogenic program in mouse cDCs

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Depletion of NCoR1 in cDCs enhanced Treg development ex vivo and in vivo

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NCoR1 masks PU.1-bound super-enhancers on tolerogenic genes in cDCs

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NCoR1^DC−/− animals depicted enhanced Treg frequency and infection load

The Role of the Aryl Hydrocarbon Receptor (AHR) in Immune and Inflammatory Diseases

The aryl hydrocarbon receptor (AHR) is a nuclear receptor that modulates the response to environmental stimuli. It was recognized historically for its role in toxicology but, in recent decades, it has been increasingly recognized as an important modulator of disease—especially for its role in modulating immune and inflammatory responses. AHR has been implicated in many diseases that are driven by immune/inflammatory processes, including major depressive disorder, multiple sclerosis, rheumatoid arthritis, asthma, and allergic responses, among others. The mechanisms by which AHR has been suggested to impact immune/inflammatory diseases include targeted gene expression and altered immune differentiation. It has been suggested that single nucleotide polymorphisms (SNPs) that are near AHR-regulated genes may contribute to AHR-dependent disease mechanisms/pathways. Further, we have found that SNPs that are outside of nuclear receptor binding sites (i.e., outside of AHR response elements (AHREs)) may contribute to AHR-dependent gene regulation in a SNP- and ligand-dependent manner. This review will discuss the evidence and mechanisms of AHR contributions to immune/inflammatory diseases and will consider the possibility that SNPs that are outside of AHR binding sites might contribute to AHR ligand-dependent inter-individual variation in disease pathophysiology and response to pharmacotherapeutics.

Lipopolysaccharide-primed heterotolerant dendritic cells suppress experimental autoimmune uveoretinitis by multiple mechanisms

Exposure of bone‐marrow‐derived dendritic cells (BMDC) to high‐dose ultrapure lipopolysaccharide for 24 hr (LPS‐primed BMDC) enhances their potency in preventing inter‐photoreceptor retinoid binding protein: complete Freund's adjuvant‐induced experimental autoimmune uveoretinitis (EAU). LPS‐primed BMDC are refractory to further exposure to LPS (= endotoxin tolerance), evidenced here by decreased phosphorylation of TANK‐binding kinase 1, interferon regulatory factor 3 (IRF3), c‐Jun N‐terminal kinase and p38 mitogen‐activated protein kinase as well as impaired nuclear translocation of nuclear factor κB (NF‐κB) and IRF3, resulting in reduced tumour necrosis factor‐α (TNF‐α), interleukin‐6 (IL‐6), IL‐12 and interferon‐β secretion. LPS‐primed BMDC also show reduced surface expression of Toll‐like receptor‐4 and up‐regulation of CD14, followed by increased apoptosis, mediated via nuclear factor of activated T cells (NFATc)‐2 signalling. LPS‐primed BMDC are not only homotolerant to LPS but are heterotolerant to alternative pathogen‐associated molecular pattern ligands, such as mycobacterial protein extract (Mycobacterium tuberculosis). Specifically, while M. tuberculosis protein extract induces secretion of IL‐1β, TNF‐α and IL‐6 in unprimed BMDC, LPS‐primed BMDC fail to secrete these cytokines in response to M. tuberculosis. We propose that LPS priming of BMDC, by exposure to high doses of LPS for 24 hr, stabilizes their tolerogenicity rather than promoting immunogenicity, and does so by multiple mechanisms, namely (i) generation of tolerogenic apoptotic BMDC through CD14:NFATc signalling; (ii) reduction of NF‐κB and IRF3 signalling and downstream pro‐inflammatory cytokine production; and (iii) blockade of inflammasome activation.

Endotoxin-Induced Tryptophan Degradation along the Kynurenine Pathway: The Role of Indolamine 2,3-Dioxygenase and Aryl Hydrocarbon Receptor-Mediated Immunosuppressive Effects in Endotoxin Tolerance and Cancer and Its Implications for Immunoparalysis

The degradation of tryptophan (TRP) along the kynurenine pathway plays a crucial role as a neuro- and immunomodulatory mechanism in response to inflammatory stimuli, such as lipopolysaccharides (LPS). In endotoxemia or sepsis, an enhanced activation of the rate-limiting enzyme indoleamine 2,3-dioxygenase (IDO) is associated with a higher mortality risk. It is assumed that IDO induced immunosuppressive effects provoke the development of a protracted compensatory hypoinflammatory phase up to a complete paralysis of the immune system, which is characterized by an endotoxin tolerance. However, the role of IDO activation in the development of life-threatening immunoparalysis is still poorly understood. Recent reports described the impact of inflammatory IDO activation and aryl hydrocarbon receptor- (AhR-) mediated pathways on the development of LPS tolerance and immune escape of cancer cells. These immunosuppressive mechanisms offer new insights for a better understanding of the development of cellular dysfunctions in immunoparalysis. This review provides a comprehensive update of significant biological functions of TRP metabolites along the kynurenine pathway and the complex regulation of LPS-induced IDO activation. In addition, the review focuses on the role of IDO-AhR-mediated immunosuppressive pathways in endotoxin tolerance and carcinogenesis revealing the significance of enhanced IDO activity for the establishment of life-threatening immunoparalysis in sepsis.