Cutting edge: Spontaneous development of IL-17-producing γδ T cells in the thymus occurs via a TGF-β1-dependent mechanism

In naive animals, γδ T cells are innate sources of IL-17, a potent proinflammatory cytokine mediating bacterial clearance as well as autoimmunity. However, mechanisms underlying the generation of these cells in vivo remain unclear. In this study, we show that TGF-β1 plays a key role in the generation of IL-17⁺ γδ T cells and that it mainly occurs in the thymus particularly during the postnatal period. Interestingly, IL-17+ gd TCR⁺ thymocytes were mainly CD44^highCD25 ^low cells, which seem to derive from double-negative 4 γδ TCR⁺1 cells that acquired CD44 and IL-17 expression. Our findings identify a novel developmental pathway during which IL-17-competent γδ T cells arise in the thymus by a TGF-β1-dependent mechanism.