Project Summary/Abstract Chronic alcohol (i.e. ethanol) has been linked to the etiology of several cancers1-3, however, to date there is almost no knowledge regarding the effects of alcohol exposure on responses to immunotherapy. Ethanol abuse disrupts immune function, which could specifically impact immunotherapy responses in several cancers. This prompted the American Society of Clinical Oncology to call for studies into the impact of chronic ethanol on immunotherapy4. We find that chronic ethanol reduces anti-PD-1 efficacy and causes intestinal dysbiosis during tumor growth in a manner that may reduce responses to anti-PD-1. Our central hypothesis is that ethanol impairs efficacy of anti-PD-1 therapy in multiple tumor types via dysregulation of T cell activation and tumor ingress. We hypothesize ethanol-induced intestinal dysbiosis contributes to this dysfunction. PD-1 receptor targeted immunotherapy has emerged as an effective treatment for a variety of advanced malignancies in a subset of responsive patients5. Tumor-induced PD-1 ligands suppress T-cell responses as a means of immune invasion to promote tumor growth5-7. Anti-PD-1 monoclonal antibodies (αPD-1 mAbs) can reverse PD-1 immunosuppression to facilitate tumor clearance in responsive patients. However, despite these exciting clinical results, only a minority of patients respond to anti-PD-1 therapy. Anti-PD-1 efficacy can be influenced by immunological factors such as T cell invasion into the tumor8 and environmental factors such as intestinal microbiota composition. Chronic ethanol use increases gut permeability, alters peripheral cytokines9, alters T-cell numbers and function10, and causes intestinal dysbiosis11-13, with multiple bacterial species depleted in ethanol models involved in response to αPD-1 mAbs11,12,14-18. Most of these bacteria are short-chain fatty acid (SCFA) producers that promote T-cell responses19,20 21,22. We assessed the effect of chronic ethanol on PD-1 immunotherapy in our established anti-PD-1 responsive murine immunocompetent BBN963 cancer model23. We found that chronic ethanol-treated mice (5 weeks) had substantially increased tumor growth and higher mortality than αPD-1 mAb treatment alone (100% survival in vehicle + anti-PD-1 vs 33% survival in Ethanol + anti-PD-1), with no effect of ethanol on tumor growth without αPD-1. Ethanol-treated mice showed alterations in peripheral and tumoral T-cell populations. Further, we found that during tumor growth chronic ethanol depletes Bifidobacterium, which is associated with positive responses to anti-PD-1 immunotherapy15. We hypothesize that ethanol impairs anti-PD-1 efficacy by disrupting T cell responses to tumor growth that involves alterations in intestinal microbiome composition. To test this hypothesis we propose to investigate the effect of ethanol on T- cell tumor infiltration and tumoricidal activity (Aim 1) and to study the role ethanol-induced intestinal dysbiosis in the loss of anti-PD-1 efficacy (Aim...