Regulation of IL-17 expression by the developmental pathway of CD4 T cells in the thymus

CD4 T cells selected by MHC class II expressing thymocytes (T-CD4 T cells) have distinct effector functions compared to that of epithelial cell-selected CD4 T cells (E-CD4 T cells). T-CD4 T cells produce both Th1 and Th2 effector cytokines immediately after stimulation and also express IL-4 in addition to IFN-γ under the Th1 differentiation condition. In the present study, we investigated the capability of T-CD4 T cells to become IL-17-producing cells. We found that T-CD4 T cells express reduced IL-17 under Th17-inducing conditions. T-CD4 T cells express very low levels of receptor for TGF-β and IL-21 that are essential to induce IL-17 expression. In addition, the induction of RORγt, a key transcription factor for IL-17 gene expression, was compromised in T-CD4 T cells under Th17 skewing conditions and ectopic expression of RORγt restored IL-17 expression. The defect of IL-17 and RORγt expression in T-CD4 T cells is cell intrinsic and not due to effects of a secreted factor. Thus, the developmental pathway of CD4 T cells in the thymus plays a critical role in controlling an immune response by suppressing the generation of the Th17 lineage.