Abstract OBJECTIVE: We have examined if decreasing Cbl-b expression in tumor-reactive CD8 T cells used for adoptive immunotherapy of progressive leukemia can enhance efficacy and bypass the requirement for IL-2 administration. METHODS: Mice expressing a transgenic receptor, TCRgag, which renders CD8 T cells reactive with FBL leukemia, were crossed with Cbl-b-deficient and CD28-deficient mice. TCRgag Cbl-b+/+ and Cbl-b-/- cells were tested for function and expanded in vitro prior to use in adoptive therapy. RESULTS: Cbl-b-/- TCRgag cells exhibited a lower threshold for T cell activation, greater IL-2 production and enhanced proliferation, expansion and survival. The enhanced response of Cbl-b-/- TCRgag cells compared to Cbl-b+/+ cells in vitro revealed both IL-2-dependent and IL-2-independent effects. In vivo, adoptively transferred Cbl-b+/+ TCRgag CD8 T cells can only prevent lethal disease in mice bearing disseminated leukemia if low dose IL-2 is subsequently administered to promote proliferation and maintain cell survival. By contrast, Cbl-b-/- TCRgag cells cured mice independent of a requirement for IL-2 administration. CONCLUSIONS: These results suggest that targeting Cbl-b in antigen-specific T cells may provide a strategy to enhance efficacy of CD8 T cell adoptive immunotherapy in humans with cancer.