Project Summary Tumor immune suppression represents a major obstacle in achieving effective cancer immunotherapy. The goal of this exploratory project is to develop a novel mRNA-lipid nanoparticle (mRNA-LNP)-based immunotherapy to overcome this challenge in pancreatic cancer, one of the deadliest malignancies. Currently, few effective treatments are available for pancreatic cancer. The majority of pancreatic cancers are also resistant to immune checkpoint blockade. Thus, novel therapeutic strategies are needed to target this lethal cancer. Most pancreatic cancers display a highly immunosuppressive tumor microenvironment (TME). Tumor-infiltrating effector CD8+ T cells are critical for improved patient survival, and yet they are either absent or sparse in the majority of pancreatic cancers, indicating an intrinsic mechanism that impedes T cell infiltration and activation. We recently discovered that Stimulator of Interferon Genes (STING) is silenced in pancreatic and other cancers. Because STING function is critical for stimulating antitumor T cell responses, our finding suggests that STING silencing contributes to the immunologically “cold” TME. We found that reactivating STING upregulates cytokines/chemokines that are crucial for promoting intratumoral T cell infiltration. More importantly, reactivation of STING specifically kills STING-silenced cancer cells. Because tumor antigens released by dying cancer cells in vivo could be engulfed by antigen-presenting cells to generate systemic antitumor response and amplify tumoricidal effect, we hypothesize that targeted reactivation of STING in pancreatic cancer could invigorate the immune-dampened TME and improve tumor immunogenicity. In this project, we will develop mRNA-LNP to specifically deliver permanently active STING mutants into pancreatic cancer to bolster T cell antitumor cytotoxicity. This approach aims to overcome the limitations of traditional STING agonists, which lack tumor specificity and do not work in STING-silenced cancers. To define their efficacy in stimulating antitumor immunity, the STING mRNA-LNP will be tested in vitro and in an orthotopic syngeneic murine pancreatic cancer model, which faithfully recapitulates the immunobiologically “cold” TME of pancreatic cancer. We will also combine STING mRNA-LNP with PD-1 blockade to circumvent pancreatic cancer resistance to the immune checkpoint therapy and spur synergistic antitumoral activity. These preclinical studies have the potential for developing a novel immunotherapy to overcome immune resistance and improve treatments for a diverse array of STING-silenced cancers that are refractory to current therapies.