This proposal centers upon the application of a novel Fc fusion protein transfer method that we have recently developed to combination therapy for cancer. The configuration of the proposed studies has been based upon three hypotheses. First, in addition to converting tumor cells to professional APCs via T cell costimulator transfer in order to directly activate CTLs (direct priming), anti-tumor immunity may be more efficiently elicited by simultaneously promoting bystander host antigen- presenting cells (APCs), dendritic cells (DCs) in particular, to prime CTLs (indirect priming). Hence, one can begin to think beyond T cells per se and look to the potentiation of DC function in parallel. Second, in light of the short life span of DCs in vivo, anti-tumor immunity may be enhanced by promoting DC longevity in tumor microenvironment, maximizing the extent of T cell activation inducible by limited number of tumor antigen- pulsed DCs. Third, anti-tumor immunity may be further optimized by, in addition to improving DC longevity, simultaneously promoting the recruitment of DCs to tumor site. Driven by these hypotheses, our preliminary data demonstrate the feasibility of eliciting a systemic, long-term anti-tumor immunity via in situ protein transfer of a "tetra-costimulator" combination, chosen to activate T cells as well as APCs. Building upon these preliminary data, the proposed studies seek to exploit our Fc fusion protein transfer method as a means to deepen our understanding of the regulations of T cell activation and DC longevity and, in turn, use the gained insights to optimize anti- tumor immunity in vivo. The three specific aims are: 1) To assess synergy/cooperation among T cell costimulators, spatial requirements among costimulatory signals themselves as well as between costimulatory and antigenic signals, and costimulator synergy in promoting T cell mitosis; 2) To determine the quantitative interplay between DC activators and DC inhibitors in modulating DC longevity; and 3) To assess the relevance of costimulator synergy, enhanced DC longevity, and increased DC quantity to more effective anti-tumor response.