Final Report Summary - AHRIMMUNITY (The influence of Aryl hydrocarbon receptor ligands on protective and pathological immune responses)
The transcription factor aryl hydrocarbon receptor, also termed dioxin receptor has a clearly defined role in an inflammatory subset of CD4+ T cells, the Th17 cell subset. These cells are important in the defense against fungal infections, but they also play a pathogenic role in many autoimmune diseases. The first part of our investigations focused on the effect of several diverse AhR agonists, both chemicals as well as putative physiological ligands on Th17 cell differentiation and clearly established equivalent modes of action by all AhR ligands in promoting Th17 differentiation and inducing the cytokine IL-22. Transcriptional analysis of Th17 cells exposed to AhR ligand confirmed the induction of IL-22 and the cytochrome P450 metabolising enzymes CYP1A1 and CYP1B1 as well as the AhR repressor as top hits, but also indicated the upregulation of markers that link AhR with increased inflammation (IL-1receptor expression) and lipid metabolism (pltp). In vivo analysis of Th17 mediated autoimmunity in the central nervous system (experimental autoimmune encephalomyelitis EAE) showed, in contrast to the in vitro effect of divergent AhR ligands on T cells, that the mode of application of AhR ligands in the context of immunization for EAE determines the outcome of pathology. As described in the literature, dioxin has a strong immunosuppressive effect and systemic injection of dioxin in the context of immunization with MOG and adjuvant to induce EAE resulted in loss of Th17 cell activation and absence of any autoimmune phenotype. Surprisingly, however, also the physiological AhR ligand FICZ when given systemically caused strong reduction in EAE pathology (albeit less pronounced than dioxin). In contrast, FICZ given together with the antigen emulsion increased the Th17 response and exacerbated pathology, possibly due to a more direct action on T cells rather than on additional cell types that are affected systemically. Our current hypothesis is that prolonged signaling by AhR ligands that are not degraded by the induced metabolic enzymes (eg dioxin) exerts a negative influence on the function of antigen presenting cells, thereby preventing appropriate Th17 cell activation. Furthermore prolonged AhR signaling might interfere with the physiological functions of this transcription factor that is normally transiently activated by physiological ligands that are rapidly degraded so that excessive signaling is avoided. In order to establish the effects of AhR activation on different cell types that express this transcription factor, we have generated cell type specific deletions using the Cre-lox system so that we can investigate the functions of AhR in different cells as well as the integration of these functions in complex immune reactions. Initial data comparing inflammatory responses of wildtype mice with those of AhR deficient mice show enhanced recruitment of neutrophils and overexpression of proinflammatory cytokines and chemokines in the absence of AhR. This is seen in a model of cutaneous infection with Candida albicans as well as in the inflammatory response to a skin-sensitizing chemical (delayed type hypersensitivity reaction DTH). In imiquimod-induced skin inflammation, a mouse model resembling features of human psoriasis- the inflammatory response was highly exacerbated in AhR deficient mice. Analysing cell type specific AhR knockouts we established that AhR deficiency in antigen presenting cells (DC and macrophages) or in T and B cells was not responsible for the hyperreactive response, but instead AhR deficiency in keratinocytes recapitulated all features of the full AhR deficient mouse. Inflammatory signals such as IL-1 contributed to promoting overexpression of chemokines, cytokines and S100 proteins in keratinocytes via an AP-1 dependent pathway. Finally we exposed psoriasis patient biopsies (lesional and non-lesional) vs control biopsies to either FICZ or an AhR agonist and analysed them by RNA deep sequencing. The data clearly showed that AhR activation by physiological agonist (FICZ) ameliorated the expression of psoriasis-related overexpressed genes, whereas exposure to antagonist exacerbated the expression and induce pro-inflammatory gene expression in non-lesional skin samples.
Thus, our studies have led us beyond the initial focus on Th17 cells and established that AhR has diverse functional roles in additional cells types in the body where under physiological conditions it appears to exert restraining influences on inflammatory immune responses.
Thus, our studies have led us beyond the initial focus on Th17 cells and established that AhR has diverse functional roles in additional cells types in the body where under physiological conditions it appears to exert restraining influences on inflammatory immune responses.