Ahr-Foxp3-ROR?t Axis Controls Gut Homing of CD4+ T Cells by Regulating GPR15

The orphan chemoattractant receptor GPR15 is important for homing T lymphocytes to the large intestine, thereby maintaining intestinal immune homeostasis. However, the molecular mechanisms underlying the regulation of GPR15 expression remain elusive. Here we show a central role of the aryl hydrocarbon receptor (Ahr) in promoting GPR15 expression both in mice and human, thus gut homing of T lymphocytes. Mechanistically, Ahr directly binds to open chromatin regions of the Gpr15 locus to enhance its expression. Ahr transcriptional activity in directing GPR15 expression was modulated by two transcription factors, Foxp3 and ROR?t, both of which are expressed preferentially by gut Tregs in vivo. Specifically, Foxp3 interacted with Ahr and enhanced Ahr DNA binding at the Gpr15 locus, thereby promoting GPR15 expression. In contrast, ROR?t plays an inhibitory role at least in part by competing with Ahr binding to the Gpr15 locus. Our findings thus demonstrate a key role for Ahr in regulating Treg intestinal homing under the steady state and during inflammation, and the importance of Ahr-ROR?t-Foxp3 axis in regulating gut homing receptor GPR15 expression by lymphocytes. Overall design: ChIP-seq (2 replicates) analysis of in vitro differentiated Th17 and iTreg from Ahr KO and wild type mice.