One major obstacle for the success of dendritic cell (DC) vaccines is host DC-mediated immunosuppression. Cross-priming, a process which DCs activate CD8+ T cells through cross-presentation, plays a major role in generating anti-tumor CD8+ T cell immunity. Tumor antigen cross-presentation by host DCs, however, often induces CD8+ T cell tolerance instead of immunity. Thus, there is a critical need to better understand whether and how tumors modulate cross-priming to suppress CD8+ T cell immunity. The long-term goal is to develop strategies to block tumor-induced immunosuppression to augment CD8+ T cell immunity and improve cancer vaccine efficacy. The objectives in this application is to elucidate the underlying mechanisms of how tumors inhibit cross-priming through ß-catenin in DCs, and validate blocking ß-catenin signaling as a novel strategy to improve cancer vaccine efficacy. The central hypothesis is that tumors differentially regulate DCs' cytokine induction through ß-catenin to inhibit cross-priming, and blocking ß-catenin's function in cross-priming augments vaccine-induced anti-tumor CD8+ T cell immunity. This hypothesis has been formulated on the basis of preliminary data produced in the applicant's laboratory. The rationale is that once it is known how tumors suppress CD8+ T cell responses through ß-catenin, strategies targeting ß-catenin signaling can then be developed to improve cancer vaccine efficacy. Three specific aims are proposed: 1) To determine whether activation of ß-catenin in DCs suppresses anti- tumor CD8+ T cell immunity under diverse cancer vaccinations. 2) To elucidate the molecular mechanisms of how tumors inhibit cross-priming through ß-catenin in DCs. 3) To determine whether blocking ß-catenin pharmacologically improves cancer vaccine efficacy. We will carry out adoptive transfer, phenotypic and functional assays including multiple cross-priming assays, real-time PCR, in vivo killing assays and DC-targeted vaccinations with DC-specific knockout mice with either active or inactive ß-catenin. The project is innovative because: (1) Based on the findings that ß-catenin mediates tumor-induced suppression of CD8+ T cell immunity by inhibiting cross-priming, we will determine whether a novel strategy targeting DCs' function in cross-priming improves vaccine efficacy. (2) Using genetic and pharmacological approaches to alter ß-catenin expression, this proposal will elucidate the mechanisms of how tumors inhibit cross-priming through ß-catenin in DCs. As ß- catenin inhibitors have been tested in pre-clinical studies and clinical trials, this proposal will additionally provide direct evidenc to support the application of ß-catenin inhibitors in cancer vaccines. The proposed research is significant, because it addresses how cross-priming is modulated by tumors to achieve DC-mediated immunosuppression, a fundamental but unanswered question in cancer immunology and DC biology, and more importantly will validate modulating ß-catenin signaling as a novel strategy to improve cancer vaccine efficacy.