The recently identified cytokine IL-21 augments anti-CD3 mediated CD8+ T cell activation. However, the effect of IL-21 on tumor-antigen induced CD8+ T cell responses remains unknown. The goals of this proposal are to understand the molecular and cellular mechanisms underlying the effects of IL-21 on tumor antigen-specific CD8+ T cell responses and utilize this information to generate effective T cells for adoptive cellular therapy of cancer. The preliminary characterizations suggest that IL-21 enhances activation, proliferation, differentiation and sustenance of tumor antigen specific CD8+ T cell responses in vivo. The ability of IL-21 to regulate naive CD8+ T cell responses was confirmed using an in vitro system in which TCR transgenic CD8+ T cells (OT-I) are evaluated for their molecular and cellular response to IL-21 treatment. By extending these investigations we will test the hypothesis that IL-21 treatment will generate large numbers of long-lived effector CD8+ T cells and promote adoptive immunity against cancer. Four specific aims are proposed. In aim1, we will test how IL-21 augments naive and anergized OT-I T cell activation and proliferation upon antigen stimulation, the role for STAT1, STAT3 and STAT5 in the IL-21 effect will be determined. The generation of effector functions such as type 1 cytokine expression and cytotoxicity are critical for immunological control of tumor cells. In aim 2, we will determine how IL-21 promotes differentiation in na[unreadable]ve and anergized OT-I T cells, by addressing the specific role of STAT1, STAT3 and STAT5 in OT-I effector development. Immunological memory is a desirable objective for most cancer immunotherapies. In aim 3, we will test the ability of IL-21 to generate memory in OT-I T cells and establish a role for STAT1, STAT3 and/or STAT5 in the memory formation. Finally, in aim 4, we will utilize this information to test the effectiveness of IL-21 alone or in combination with cytokines such as IL-15 and/or IL-12 in producing OT-I T cells that result inefficacy against established cancer by adoptive therapy. The insights provided by these investigations are likely to develop rational use of IL-21 alone or in combination with other cytokines for the therapy of cancer, infectious diseases, autoimmunity and transplantation.