Extrathymically generated regulatory T cells control mucosal TH2 inflammation

A balance between pro- and anti-inflammatory mechanisms at mucosal interfaces, which are sites of constitutive exposure to microbes and non-microbial foreign substances, allows for efficient protection against pathogens yet prevents adverse inflammatory responses associated with allergy, asthma and intestinal inflammation 1 . Regulatory T (Treg) cells prevent systemic and tissuespecific autoimmunity and inflammatory lesions at mucosal interfaces. These cells are generated in the thymus (tTreg cells) and in the periphery (induced (i)Treg cells), and their dual origin implies a division of labour between tTreg and iTreg cells in immune homeostasis. Here we show that a highly selective blockage in differentiation of iTreg cells in mice did not lead to unprovoked multiorgan autoimmunity, exacerbation of induced tissue-specific autoimmune pathology, or increased pro-inflammatory responses of T helper 1 (TH1) and TH17 cells. However, mice deficient in iTreg cells spontaneously developed pronounced TH2-type pathologies at mucosal sites—in the gastrointestinal tract and lungs—with hallmarks of allergic inflammation and asthma. Furthermore, iTreg-cell deficiency altered gut microbial communities. These results suggest that whereas Treg cells generated in the thymus appear sufficient for control of systemic and tissue-specific autoimmunity, extrathymic differentiation of Treg cells affects commensal microbiota composition and serves a distinct, essential function in restraint of allergic-type inflammation at mucosal interfaces. Exquisitely balanced control mechanisms operating at mucosal sites are able to accommodate potent immune defences and the need to prevent tissue damage resulting from inflammatory responses caused by commensal microorganisms, food and environmental antigens, allergens, and noxious substances 1 .