Stat5a regulates T helper cell differentiation by several distinct mechanisms

We have previously shown that CD4+ T cell–mediated allergic inflammation is diminished in signal transducer and activator of transcription (Stat)5a-deficient (Stat5a−/−) mice. To determine whether Stat5a regulates T helper cell differentiation, we studied T helper (Th)1 and Th2 cell differentiation of Stat5a−/−CD4+ T cells at single-cell levels. First, Th2 cell differentiation from antigen-stimulated splenocytes was significantly decreased in Stat5a−/− mice as compared with that in wild-type mice. Further, Th2 cell differentiation was also impaired in Stat5a−/− mice even when purified CD4+ T cells were stimulated with anti-CD3 plus anti-CD28 antibodies in the presence of interleukin-4. Moreover, the retrovirus-mediated gene expression of Stat5a in Stat5a−/−CD4+ T cells restored the Th2 cell differentiation at the similar levels to that in wild-type CD4+ T cells. In addition, interleukin-4 normally phosphorylated Stat6 in CD4+ T cells from Stat5a−/− mice. Second, the development of CD4+CD25+ immunoregulatory T cells was impaired in Stat5a−/− mice, as indicated by a significant decrease in the number of CD4+CD25+ T cells in Stat5a−/− mice. Furthermore, the depletion of CD4+CD25+ T cells from wild-type splenocytes significantly decreased Th2 cell differentiation but increased Th1 cell differentiation, whereas the depletion of CD4+CD25+ T cells from Stat5a−/−splenocytes had no significant effect on the Th1 and Th2 cell differentiation. Together, these results indicate that the intrinsic expression of Stat5a in CD4+ T cells is required for Th2 cell differentiation and that Stat5a is involved in the development of CD4+CD25+ immunoregulatory T cells that modulate T helper cell differentiation toward Th2 cells.