PROJECT SUMMARY It is now widely recognized that the cGAS-STING pathway plays a critical role in determining the efficacy of radiotherapy. Radiation-induced DNA damage can cause the leakage of nuclear dsDNA fragments into the cytoplasm, activating the cGAS-STING pathway and inducing type I interferons and antitumor immune response. However, the radiation-induced antitumor immune response is self-limiting, with many molecular mechanisms negatively regulating it. Therefore, a better understanding of the suppressive molecular mechanisms is necessary to enhance the beneficial effects of the radiation-induced antitumor immune response. In this project, we intend to study the role of PCSK9, a critical cholesterol-regulating factor, for its role in regulating the antitumor immune response. We base our project on our recently published study that reveals PCSK9’s role in binding and promoting the degradation of MHC class I on the surface of tumor cells, thereby limiting intratumoral T cell activation (Liu et al., Nature, 2020, PMC7770056). We also base our project on our unpublished preliminary data suggesting that inhibiting PCSK9 can significantly enhance the antitumor efficacy of radiotherapy. We will use genetic approaches to study the roles of PCSK9 in regulating tumor growth and the tumor immune microenvironment after radiotherapy in both transplanted and genetically induced autochthonous murine tumor models (Specific Aim 1). We will also use genetic approaches to determine how PCSK9 interacts with the cGAS- STING signaling pathway in regulating tumor growth and the tumor microenvironment after radiotherapy, with a particular emphasis on STK11/LKB1-mutant tumors, which are known to have a suppressed cGAS-STING signaling (Specific Aim 2). Finally, we will also determine if using a clinically approved PCSK9 inhibitor evolocumab can enhance radiotherapy and immune checkpoint blockade (ICB) therapy of locoregional and distant tumors. We expect our results will inform future PCSK9 inhibitor-based radiotherapy and immunotherapy clinical trials upon completing our project.