This proposal assembles experts in tumor immunotherapy, tumor immunology, melanoma, biostatistics, pathology and mouse models to address effects of tumor intrinsic PD-L1 signals on melanomagenesis. The principal mechanism of action of αPD-L1 immunotherapy is thought to be protection of PD-1+ anti-tumor T cells from negative tumor surface-expressed PD-L1. Paradigm shifting work from our lab has now defined important immunopathogenic tumor cell-intrinsic PD-L1 signals. It is unreported whether PD-L1 in a given cancer cell of origin contributes to carcinogenesis. Our published and preliminary data on major tumor cell-intrinsic effects of tumor PD-L1, illuminated in this grant proposal, suggest that these signals could contribute to carcinogenesis. We developed a novel mouse melanoma model in which PD-L1 can be deleted in the cancer cell of origin (melanocyte). Melanomas are induced by either oncogene induction (Nras/Cdkn2a) or oncogene induction plus DNA damage from UV irradiation. We will use this unique and clinically relevant model to test our overarching hypothesis that melanocyte-intrinsic PD-L1 signals promote melanomagenesis by enhancing the oncogenic effects of Nras/Cdkn2a and/or DNA damage. We address this hypothesis in the following aims: Specific Aim 1: Define cell-intrinsic PD-L1 control of the DNA damage response (DDR) and consequent effects on the tumor microenvironment (TME). We will induce tumors in our mouse melanoma model via either tamoxifen-induced oncogene induction or oncogene induction plus DNA damage by UV irradiation. Tumor cell-intrinsic PD-L1-regulated alterations in DNA repair and subsequent TME effects will be assessed using Westerns, confocal imaging, RNA-seq, immune analyses, histologic and DNA damage studies. Our novel model will show how appearance of melanocyte PD-L1 alters DDR and other oncogenic mediators (e.g., mTOR) sculpt the stroma and immune microenvironment over time during early melanomagenesis. We will also treat melanoma-bearing mice with αPD-L1 or DDR kinase inhibitors and assess tumor intrinsic PD-L1-signaling consequence on tumor (e.g., mutational burden) and microenvironment (immune analyses). Specific Aim 2: Test the hypothesis that PD-L1 promotes initiation of oncogene and/or UV induced melanomagenesis through cell-intrinsic effects. To distinguish cell-intrinsic versus immune-dependent PD- L1 signaling consequences on carcinogenesis, melanomas will be induced in mice ± immune cell depletions with specific antibodies. Mouse experiments will be complemented with human cell studies in vitro by transfecting Nras, Cdkn2a and other candidate genes into normal human melanocytes to test effects on tumorigenesis and PD-L1 expression.