Th17 cells are not required for maintenance of IL-17A-producing γδ T cells in vivo.

γδ T cells producing interleukin-17A (γδT17) are thought to develop spontaneously in the thymus and to be maintained in the periphery. Previous studies suggested a role for T-helper 17 (Th17) cells in the maintenance of γδT17 via the expression of transforming growth factor-β1 (TGFβ1). However, we have previously found that Th17 cells were not required for expansion of γδT17 cells after lung transplant in a mouse model. Using mice deficient in signal transducer and activator of transcription 3 (STAT3) in CD4 T cells, which are unable to develop Th17 cells, we investigated the requirement for Th17 cells and TGFβ1 to maintain γδT17 cells in the lung and lymphoid tissues. At steady state, we found no defect in γδT17 cells in the thymus or periphery of these mice. Further, STAT3-deficient CD4 T cells produced significantly higher levels of TGFβ1 than wild-type CD4 T cells under Th17 differentiation conditions in vitro. To determine whether STAT3-deficient CD4 T cells could expand γδT17 cells in vivo, we used TCRβ mice, which are known to have a defect in γδT17 cells that can be rescued by Th17 cells. However, adoptive transfer of wild-type Th17 cells or bulk CD4 T cells did not expand γδT17 cells in TCRβ mice. In contrast, interferon-γ γδ T cells preferentially expanded, particularly in the lungs. Interestingly, we found in vivo and in vitro that TGFβ1 may negatively regulate the pool of γδT17 cells. Our data suggest that Th17 cells and TGFβ1 are not required for the maintenance of γδT17 cells.