Project Summary Epithelial ovarian cancer (EOC) is a leading cause of gynecologic cancer death in women, highlighting the critical clinical need for new therapeutic strategies. In 2018 approximately 22,000 women were diagnosed with EOC in the United States and the majority of them will ultimately succumb to their disease. The reason underlying the poor survival is due to poor diagnosis with 80% of patients with EOC present in advanced stage (III-IV) accounting for the poor prognosis (5-year cancer-specific survival 42% and 26%, respectively). Hyperthermic intraperitoneal chemotherapy (HIPEC) has recently emerged as a clinical regimen that prolongs overall survival for patients with advanced EOC by over 12 months compared to standard of care. Despite its proven clinical benefit, how HIPEC extends survival remains poorly understood. Is the improvement in tumor control driven by increased cytotoxic effects or alterations in the tumor microenvironment? What role does the immune system play? To overcome this limitation, we analyzed ovarian tumors at the time of the debulking surgery and immediately following HIPEC protocol. As EOC is often found to metastasize to the omental fat, we focused on this site to interrogate cellular and molecular changes. We leveraged single cell RNA sequencing to identify the major populations in the omental tumors and underlying cellular and molecular changes accompanying HIPEC. Our data demonstrated that HIPEC activates immune cells and modulates the transcriptome of epithelial and stromal cell populations. In parallel, we used a mouse EOC lines to develop an innovative hyperthermic chemotherapy model. We determined that hyperthermic chemotherapy leads to increased immune cell infiltration with a corresponding decrease in immune suppressor cells. Based on these observations, we hypothesize that the survival benefit conferred by HIPEC is due to its ability to augment immune cell infiltration. We will test this hypothesis using a complementary combination with an innovative mouse model of HIPEC and single cell analysis of human patient specimen studies. We propose two Specific Aims. In Aim 1, we will test the hypothesis that HIPEC promotes transcriptional activation of adaptive immune system. In Aim 2, we will test hypothesis that HIPEC via attenuation of immune suppression improves EOC. The goal of the study is to gain valuable information into the cellular pathways that HIPEC uses to disrupt EOC progression in order to translate these findings into adjunct therapies to further enhance the clinical benefit of HIPEC for patients with advanced EOC.