Development of interleukin-17-producing γδ T cells is restricted to a functional embryonic wave

γδ T cells are an important innate source of interleukin-17 (IL-17). In contrast to T helper 17 (Th17) cell differentiation, which occurs in the periphery, IL-17-producing γδ T cells (γδT17 cells) are probably committed during thymic development. To study when γδT17 cells arise during ontogeny, we used TcrdH2BeGFP reporter mice to monitor T cell receptor (TCR) rearrangement and IL-17 production in the embryonic thymus. We observed that several populations such as innate lymphoid cells and early T cell precursors were able to produce IL-17 prior to (and thus independent of) TCR recombination. γδT17 cells were absent after transplantation of IL-17-sufficient bone marrow into mice lacking both Il17a and Il17f. Also, γδT17 cells were not generated after genetic restoration of defective Rag1 function in adult mice. Together, these data suggested that these cells developed exclusively before birth and subsequently persisted in adult mice as self-renewing, long-lived cells.