Abstract The goal of this project is to identify mechanisms by which immune checkpoint programmed death ligand 1 (PD-L1) regulates mRNA homeostasis of anti-apoptotic genes. As a cell surface protein, PD-L1 inhibits T cell responses, resulting in immune escape. However, PD-L1 also has important non-immune intracellular functions that are much less understood. Our recent studies show that IFNγ induces the nuclear translocation and accumulation of PD-L1 in ovarian and prostate cancer cells. In addition, our preliminary data indicate that suppression of the IFNγ-induced PD-L1 expression increases apoptosis and decreases mRNA levels of NFκB-dependent anti-apoptotic genes. Based on those findings, we will test the central hypothesis that the intracellular PD-L1 serves as a regulator of anti-apoptotic gene mRNA homeostasis. In Aim 1, we will determine whether PD-L1 increases the levels of anti-apoptotic genes by promoting their transcription, and/or whether it increases stability of their mRNAs in ovarian and prostate cancer cells. In Aim 2, we will determine whether PD-L1 directly binds to the anti-apoptotic gene promoters and facilitates their acetylation, and we will investigate whether PD-L1 binds the anti-apoptotic mRNAs and affects their subcellular localization. Although the project focuses on the NFκB-dependent anti-apoptotic genes, the results will likely reveal general mechanisms by which PD-L1 regulates synthesis and/or stability of other mRNAs. Immunotherapies targeting cell surface PD-L1 have shown impressive clinical outcomes in many cancers; however, only a fraction of patients achieves durable responses, highlighting our incomplete understanding of the mechanisms of PD-L1 functions. Findings from this study will provide the first data on the mechanisms of how PD-L1 regulates mRNA homeostasis of anti- apoptotic genes. This information will advance our understanding of the intracellular, immune- independent functions of PD-L1, and may lead to the development of novel therapies that target PD- L1 anti-apoptotic functions in cancer cells. In addition, this R16 SuRE project will enhance the research environment at St. John’s University by providing students from underrepresented backgrounds with numerous opportunities to learn the fundamentals of biomedical research.