Aryl hydrocarbon receptor signals in epithelial cells govern the recruitment and location of Helios+ Tregs in the gut

CD4+ Foxp3+ regulatory T cells (Tregs) are essential for homeostasis in the colon, but how local environmental cues determine the localization of colonic Tregs is unclear. We administered indigo naturalis (IN), a non-toxic phytochemical aryl hydrocarbon receptor (AhR) agonist used for treating patients with ulcerative colitis (UC) in Eastern Countries, and showed that IN increased Helios+ Tregs and MHC class II+ epithelial cells (ECs) in the colon. Interactions between Tregs and MHC class II+ ECs were mainly observed near the crypt bottom in the steady state, whereas Tregs were dramatically increased and shifted toward the crypt top following IN treatment. Moreover, the number of CD25+ T cells was increased near the surface of ECs in IN-treated UC patients compared with that in those treated with other therapies. In addition, AhR-signaling in ECs governed the induction and localization of Helios+ Tregs in the gut. We also highlighted the additional AhR signaling mechanisms in intestinal ECs that determine the accumulation and localization of Helios+ Tregs in the colon.