Approximately one out of 67 U.S. women will get epithelial ovarian cancer (EOC) in her lifetime. Late-stage disease accounts for 70% of cases, with a dismal five year survival rate of less than 25%. Under the current standard of care, surgery followed by chemotherapy, survival of women with EOC has changed very little over the past three decades. Novel therapies for EOC are clearly needed. Epigenetic agents are being explored as new therapies for EOC and other solid tumors. Clinical success in patients with lung cancer suggests that the actions of the DNA methyltransferase inhibitor 5-azacytidine (AZA) and the histone deacetylase inhibitor entinostat may stimulate the immune response and target immune cells to eradicate tumors. Furthermore AZA has FDA approval for treatment of acute myeloid leukemia. Cancers are characterized by repressive chromatin, including DNA methylation and deacetylation of histones, at promoters of tumor suppressor genes. AZA inhibits DNA methylation and entinostat inhibits histone deacetylases, opening up chromatin and causing transcription of repressed genes. In EOC, higher expression of immune genes and immune cell infiltration into tumors predict better prognosis. Our preliminary data from 23 EOC cell lines treated with AZA show an upregulation of genes involved in the immune response. These AZA-Induced iMmune genes (AIM genes), representing pathways of both innate and adaptive immunity, cluster EOC tumors from TCGA and indicate a group of tumors with low AIM expression. In this grant I propose to use AZA to stimulate an immune response in EOC cells. This therapy could work alone for a subset of patients but should sensitize a larger group of patients to subsequent immune therapy. I hypothesize that an AZA-induced de-repression of AIMs will lead to cancer cell death and recruitment of host immune cells to the tumor and can sensitize ovarian cancer cells to subsequent immunotherapy. The mechanism by which AZA activates an epithelial cell immune response is unknown. Demethylation by AZA could cause re-expression of immune master regulators. I observe transcription of normally silenced RNAs such as human endogenous retroviruses (ERVs) and repetitive elements upon AZA treatment, which may activate the immune (viral defense) response. Separately, AZA can induce expression of the immune checkpoint ligand PD-L1 on tumors. Thus, I propose that combining epigenetic immune modulation with inhibition of the PD-1/PD-L1 pathway may result in synergistic anti-tumor effects against EOC tumors. Aim 1: I will determine the phenotypic effects and mechanism of the AZA immune response. Aim 2: I will determine whether endogenous retrovirus and transposable element RNAs are responsible for the AZA immune response. Aim 3: I will treat with AZA to stimulate the immune response, then an anti-PD-1 antibody to block immune evasion, in immunocompetent mouse models of ovarian cancer and assess host immune cell activation and tumor cell killing. I will thus assess the efficacy of combining epigenetic with therapy to break immune tolerance in one of the deadliest human malignancies.