Essential role of STAT3 in the upregulation of CD28 expression by IL-21 (IRM6P.658)

CD28 plays an essential costimulatory role in T cell activation and survival by promoting IL-2 expression. However, the regulation of CD28 expression on CD8 T cells has not been well defined. Our previous studies demonstrated that tumor antigen-specific CD8 T cells generated in the presence of IL-21 stably express high levels of CD28 on the cell surface and exhibit enhanced replicative potential and in vivo persistence following adoptive transfer. The mechanism by which IL-21, a gamma-chain receptor cytokine, known to activate STAT3, and to a lesser degree STAT1 and STAT5, regulates CD28 expression is currently not known. Here we report that IL-21 regulates expression of CD28 at the transcriptional level through pSTAT3 binding to the CD28 promoter region. Using a reported small molecule inhibitor of STAT3, we demonstrate that STAT3 is critical for IL-21-induced expansion of tumor antigen-specific T cells and upregulation of CD28 expression. In addition, using CD8 T cells from Job’s syndrome patients with a sole dominant negative stat3 mutation and T cells transfected with stat3 knockdown shRNAs, we further confirmed that STAT3 is essential for the IL-21 mediated CD28 effect on human CD8 T cells. Moreover, we discovered that STAT3 directly regulates CD28 expression by binding to consensus STAT sites in human CD28 promoter. Taken together, our study demonstrates for the first time a specific role for STAT3 in IL-21-mediated CD28 expression in activated human CD8 T cells.