PROJECT SUMMARY Immune checkpoint blockade (ICB) is an immunotherapy that is revolutionizing cancer treatment, but is effective in a minority of patients. Across many solid tumor types, this can largely be ascribed to an insufficient number, diversity, and/or function of endogenously generated, pre-existing T cells that recognize tumor neoantigens and infiltrate tumors. Therefore, there is a critical need for new strategies to bolster the magnitude, breadth, and quality of neoantigen-specific T cells, to recruit cytotoxic CD8+ T cells to tumors, and to amplify their expansion, effector function, and persistence. Towards this goal, we propose a new strategy for neoantigen-targeted cancer immunotherapy. Our approach leverages a STING-activating nanoparticle vaccine (STAN-V) that we have designed to overcome several critical immunopharmacological barriers that limit cancer vaccine efficacy. The STAN-V platform is based on polymer nanoparticles engineered to enhance intracellular co-delivery of peptide neoantigen and agonists of stimulator of interferon genes (STING), a design that we have demonstrated stimulates potent neoantigen-specific CD8+ T cells and increases response to ICB. Our objective in this R01 application is to advance and mature STAN-V as a universal platform for neoantigen- targeted cancer vaccines. We will accomplish this through the following Specific Aims. First, we will develop and optimize a facile strategy for rapid fabrication of STAN-Vs based on spontaneous and efficient loading of neoantigenic peptides designed with optimized lipophilic domains. We will evaluate the capacity of this approach to increase the magnitude and breadth of neoantigen-specific T cell responses to physicochemically diverse neoantigens. As such, we expect these studies to advance the translational-readiness of STAN-Vs as a personalized vaccine technology. Second, we will leverage the unique morphology and properties of STAN- Vs to develop and optimize a novel adjuvant combination based on coordinated co-packaging and co-delivery of STING and TLR agonists. We will systematically explore the effect of combinatorial adjuvant delivery on innate and adaptive immunity, studies that we expect will yield an optimized adjuvant combination for stimulating antitumor cellular immunity. Third, we will devise and test a new approach for enhancing tumor homing and infiltration of T cells elicited via vaccination. This strategy will leverage systemic administration of a nanoparticle STING agonist that reshapes the tumor milieu to enhance T cell infiltration, proliferation, and function. Overall, these studies will advance STAN-Vs as an enabling and versatile technology for stimulating robust neoantigen-specific T cell responses and improving outcomes of immunotherapy across many cancers.