Differential Effects Of ΓC-Cytokines On Post-Selection Differentiation Of Cd8 Thymocytes.

The primary consequence of positive selection is to render thymocytes responsive to cytokines and chemokines expressed in the thymic medulla. Our main objective was to discover which cytokines could support the differentiation of positively selected thymocytes. To this end, we have developed an in vitro model suitable for high-throughput analyses of positive selection and CD8 T-cell differentiation. The model involves coculture of TCR(hi)CD5(int)CD69(-) double-positive (DP) thymocytes with peptide-pulsed OP9 cells and γc-cytokines. We report that IL-4, IL-7 and IL-21 have non-redundant effects on positively selected DP thymocytes. IL-7 signaling phosphorylates STAT5 and ERK, induces Foxo1, Klf2, and S1pr1 and supports the differentiation of classic CD8 T cells. IL-4 activates STAT6 and ERK, and supports the differentiation of CD8(int)PD-L1(hi)CD44(hi)EOMES(+) innate CD8 T cells. IL-21 is produced by thymic epithelial cells and IL-21Rα is strongly induced on DP thymocytes undergoing positive selection. IL-21 signaling phosphorylates STAT3 and STAT5, but not ERK, and does not support CD8 T-cell differentiation. However, IL-21 has a unique ability to upregulate BCL-6, to expand DP thymocytes undergoing positive selection and to increase the production of mature T cells. Our data suggest that injection of rIL-21 might enhance thymic output in subjects with age- or disease-related thymic atrophy.