AHR is essential for IL

AHR is essential for IL-22 production by T NHS Biotin and ILC3s [83]. Coexpression of AHR and RORγt by retroviral transduction in a thymoma cell line, EL4, synergistically upregulates IL-22 expression [48]. The cooperativity between AHR and RORγt has also been observed in primary T cells (L.Z., unpublished). A physical association between AHR and RORγt was detected in HEK293T cells by an overexpression study [48]. Recently, this interaction at the endogenous level has been confirmed in primary ILC3s using proximity ligation assay (PLA), an assay that allows protein–protein interaction to be detected on a per cell basis using immunofluorescence microscopy (Wang, X. and L.Z., unpublished). The exact contribution of the interaction between AHR and RORγt to the regulation of Il22 transcription remains to be determined. Nevertheless, the recruitment of AHR to the Il22 locus can only be detected by ChIP assay when the cells coexpress RORγt, while AHR binding to the Cyp1a1 locus is independent of RORγt [48,79]. These data point to an essential role for RORγt in facilitating AHR binding at the chromatin level. Facilitated by RORγt, AHR may achieve enhanced DNA-binding activity, whereby directly binding to the AhREs at the Il22 locus to induce transcription. Alternatively, AHR may indirectly bind to ROR response elements (ROREs) via an interaction with RORγt (Figure 3). Although RORγt is essential for IL-22 expression in wild-type CD4+ T cells [63,84], it becomes completely dispensable for IL-22 production by mutant T cells that express a loss-of-function form of Ikaros that lacks the DNA-binding zinc finger 4 [36]. In these Ikaros-mutant T cells, however, AHR is still absolutely required for IL-22 expression. One intriguing hypothesis is that some pioneering factors (e.g., RORγt) may create a permissive chromatin environment that allows subsequent binding of AHR to the Il22 locus. Without RORγt, the accessibility of AHR to the locus is limited, thus diminishing its potential in Il22 transcriptional promotion. However, in Ikaros-mutant cells the chromatin environment may be changed such that RORγt is no longer required for AHR binding to the locus. This model is consistent with the reported role of Ikaros in general as a chromatin repressor by interacting with chromatin modulators Mi-2 (chromodomain helicase DNA binding protein 3/CHD3) and HDAC (histone deacetylase) [85]. However, detailed structural and functional studies of RORγt and AHR at the Il22 locus will be necessary to provide insight into the mechanisms of action of AHR in promoting IL-22 expression in Th17/22 cells and ILC3s.
AHR in Treg Cell Development and Function Although AHR is expressed by Treg cells, particularly in specific subtypes with an activated phenotype [23,86,87], the precise role of AHR in Treg cell development/function remains controversial [88]. A substantial body of literature suggests that xenobiotic ligand TCDD administration in vivo suppresses immune responses [52]. This immunosuppressive effect of TCDD has been linked to the expansion or induction of Treg cells and to promotion of Treg cell function in an AHR-dependent manner in mice and in humans [23,89,90]. Consistently, TCDD has been shown to induce FOXP3 expression by activating AHR, which binds to the Foxp3 locus and regulates its epigenetic status and transcription [23,91]. However, it has also been suggested that the immunosuppressive nature of TCDD could be due to its cytotoxicity that might kill particular proinflammatory effector cells (e.g., Th17 cells) [16]. By contrast, kynurenine, a breakdown product in the IDO-dependent tryptophan degradation pathway, has recently been shown to function as an endogenous AHR ligand and to enhance Treg cell differentiation through AHR activation [27,92,93]. Another endogenous ligand of AHR [2-(1′H-indole-3′-carbonyl)-thiazole-4-carboxylic acid methyl ester, ITE] has also been shown to suppress autoimmunity by inducing Tregs [30]. Interestingly, it has been shown that activation of human intestinal lamina proprial cells, including Tregs, by AHR ligand leads to downregulation of proinflammatory cytokines (e.g., IFN-γ) [94]. It remains to be determined whether the suppression of gut inflammation by AHR ligand is mediated through AHR-expressing Treg cells in humans. Despite these exciting results, the Treg cell-intrinsic role of AHR remains to be determined. It is tempting to speculate that the expression of AHR in Treg cells may have a favorable biological outcome for immune tolerance. For example, high AHR expression in gut-associated Treg cells may render these cells readily activated by environmental cues (e.g., AHR ligands that are abundantly present in the gut) to exert their suppressive function locally and promote intestinal immune homeostasis (Figure 4).