Therapeutic antibodies targeting the programmed cell death 1 (PD-1) pathway have shown remarkable efficacy in the treatment of advanced melanoma. Consequently, the United States Food and Drug Administration recently approved two PD-1 inhibitors for the treatment of patients with advanced melanoma who no longer respond to other drugs. However, anti-PD-1 therapy does not translate into long-term disease control in most patients, highlighting the need for mechanistic insights and biomarkers that predict and help optimize clinical benefit. Because PD-1 has mainly been studied in immune cells, the majority of research regarding its role in melanoma and the identification of biomarkers for predicting response to PD-1 inhibitors has focused on immune cell-associated PD-1 functions and their immunologic and molecular correlates. We have recently demonstrated that PD-1 is not only expressed by immune cells, but also by melanoma cells. Melanoma- expressed PD-1 serves as a cancer cell-intrinsic growth receptor that activates oncogenic signaling and promotes tumorigenesis. Inhibition of melanoma-PD-1 suppresses tumor growth, even in mice lacking adaptive immunity. In patients with stage IV disease, tumoral expression of melanoma-PD-1 receptor targets correlates with clinical response to PD-1 inhibition. Our preliminary studies identify novel PD-1 pathway functions in tumor growth and suggest that melanoma-PD-1 blockade might contribute to the clinical efficacy of anti-PD-1 therapy. Therefore, we propose to define the precise mechanisms through which the melanoma-PD-1 receptor promotes tumor progression and examine how the melanoma cell-intrinsic PD-1 signaling axis might be exploited for predicting and optimizing treatment response to therapeutic PD-1 antibodies. Our specific aims are to 1) define melanoma cell-intrinsic PD-1 signaling networks required for cancer progression and 2) to examine the utility of melanoma-PD-1 signaling mediators as biomarkers of clinical response to anti-PD-1 therapy. We will use several state-of-the-art gain- and loss-of-function approaches, including mutagenesis of melanoma-PD-1 signaling motifs, knockdown, overexpression, antibody-mediated and pharmacologic inhibition or activation of PD-1 pathway members in melanoma and immune cells, as well as PD-1 and PD-1 ligand knockout mice, with or without functional immunity, to define the immunologic, molecular, and cellular factors required for melanoma-PD-1-dependent tumorigenesis. Our initiative will also implement tumor biospecimens obtained from melanoma patients before, during, and/or after anti-PD-1 therapy, to examine whether expression of melanoma-PD-1 signaling effectors correlates with clinical outcome. These studies will enhance our basic understanding of the mechanisms underlying melanoma progression and provide insight into how melanoma-PD-1 signaling interactions may be effectively manipulated for improved tumor therapy.