γδ T cells are highly conserved in jawed vertebrates, suggesting an essential role in the immune system. However, γδ T cell-deficient Tcrd-/- mice display surprisingly mild phenotypes. We hypothesized that the lack of γδ T cells in constitutive Tcrd-/- mice is functionally compensated by other lymphocytes taking over genuine γδ T cell functions. To test this, we generated a knock-in model for diphtheria toxin-mediated conditional γδ T cell depletion. In contrast to IFN-γ-producing γδ T cells, IL-17-producing γδ T cells (Tγδ17 cells) recovered inefficiently after depletion, and their niches were filled by expanding Th17 cells and ILC3s. Complementary genetic fate mapping further demonstrated that Tγδ17 cells are long-lived and persisting lymphocytes. Investigating the function of γδ T cells, conditional depletion but not constitutive deficiency protected from imiquimod-induced psoriasis. Together, we clarify that fetal thymus-derived Tγδ17 cells are nonredundant local effector cells in IL-17-driven skin pathology.
