Interleukin 21 (IL-21) is a potent immunomodulatory cytokine mainly produced by NKT and CD4+ T-cells, particularly the inflammatory Th17 subset and has pleiotropic effects on both innate and adaptive immune responses.? These actions include positive effects such as enhanced proliferation of natural killer (NK) cells and cytotoxic T cells that can destroy virally infected or cancerous cells and direct inhibitory effects on the antigen-presenting function of dendritic cells and can be proapoptotic for B cells and NK cells.? Recent studies have shown that IL-21 is also an autocrine cytokine that potently induces Th17 differentiation and suppresses Foxp3 expression, and serves as a target for treating inflammatory diseases.