Project Abstract Immune cells infiltrate the tumor microenvironment (TME) and secrete inflammatory cytokines such as interferons (IFNs) in order to drive anti-tumor responses and tumor clearance(1). Type I and II IFNs have long been known to reduce tumor growth and to have significant anti-proliferative effects. However, recent evidence suggests that tumor cells can also harness IFNs in order to enhance growth and tumor burden(2, 5). Our preliminary data indicate that one outcome of IFNγ signaling is nicotinamide phosphoribosyltransferase (Nampt) upregulation. Nampt is the rate-limiting enzyme in NAD+ synthesis(6). All cells utilize Nampt, and melanoma cells exhibit increased Nampt expression compared to healthy cells. Nampt inhibition in melanoma cells significantly reduces proliferation(7-9). Our preliminary data indicate that IFNγ activation upregulates Nampt in both human and mouse melanoma, and we show that IFNγ-inducible Nampt is vital for melanoma cell growth in vivo. Additionally, a Type I IFN, IFNβ, can also upregulate Nampt in melanoma cells. Stimulator of IFN Genes (STING) agonists promote IFNβ expression and immune cell infiltration into the TME to improve anti-tumor responses during immunotherapies. However, they may also promote tumor growth by promoting tumor NAMPT expression. Lastly, Nampt can regulate cellular processes through sirtuin 1 (SIRT1) and mammalian Target of Rapamycin (mTOR), and we will investigate whether these pathways mediate the proliferative effect of IFN- inducible Nampt. Our working hypothesis is that Nampt mediates tumor resistance to the anti-proliferative effects of Interferons (IFNs) and IFN-based immunotherapies such as STING agonist treatment by promoting SIRT1 and mTOR signaling. We will investigate our hypothesis by completing the following aims: Aim 1: Understand the mechanisms by which IFN-inducible Nampt promotes melanoma tumor growth Aim 2: Investigate the role of two downstream targets of IFN-inducible Nampt, SIRT1 and mTOR, on melanoma growth The objective of this grant is to elucidate mechanisms of immune evasion driven by IFN-inducible Nampt in tumor cells. This proposal will pave the way for improving immunotherapy outcomes by combining compounds such as Nampt inhibitors, STING agonists, and other downstream immune modulators. Collectively the proposed work will, for the first time, give us insight into the pro-tumorigenic role of the metabolic gene, Nampt, in mouse and human melanoma tumor growth in vivo in the context of immunotherapy.