PROJECT SUMMARY/ABSTRACT High-grade serous ovarian cancer has the highest mortality rate of all gynecologic cancers. There is a critical unmet need to identify new therapeutic targets and therapeutic approaches that will improve outcomes for women with chemo-resistant ovarian cancer. Deficiencies in the DNA damage response (DDR) can drive increased tumor immunogenicity, which leads to enhanced responses to immune checkpoint inhibitor therapy. PP4 is a multi-subunit phosphatase complex that plays a central role in the response to DNA damage through the dephosphorylation of many proteins that play crucial roles in the DDR. However, the role of PP4 in cancer development, chemoresistance, and anti-tumor immunity is currently unknown. The central hypothesis is that PP4 inhibition will sensitize OC cells to chemotherapy-mediated immunogenicity leading to enhanced anti- tumor immunity, and increased response to immune checkpoint blockade. The rationale for the proposed research is that understanding of the how the immune response is activated in reaction to deficiencies in PP4- mediated DNA damage repair has the potential to translate into new therapeutic strategies that will benefit those with ovarian cancer. This hypothesis will be tested by pursuing the following two specific aims: 1) Determine the role of the PP4 complex in the immunogenicity of OC; and 2) Determine the role of PP4 in tumor-infiltrating lymphocyte recruitment, function, and response to immunotherapy. In Aim 1 the effect of PP4 knockout on chemotherapy-induced immunogenicity will be assessed using mechanistic studies of genomic instability, antigen presentation, and immunogenic cell death. Under the second Aim the effect of PP4 deficiency combined with PD-1 inhibition and chemotherapy will be evaluated in organoid cultures of primary human ovarian cancer and in two immunocompetent mouse models of ovarian cancer. Mechanisms of inflammatory signaling will be investigated. The research proposed in this application is innovative because it represents a new theoretical concept by shifting the focus on PP4 function to its role in anti-tumor immunity. The proposed research is significant because it is predicted to advance and expand our understanding of the importance of how DNA damage repair resolution contributes to anti-tumor immunity and response to immunotherapy. Ultimately, such knowledge has the potential to inform the development of new therapies benefiting many types of cancer.