RESEARCH SUMMARY Immune checkpoint blockade (ICB) therapies—including ipilimumab (IPI; developed against cytotoxic T lymphocyte-antigen 4), nivolumab (NIVO; anti-programmed death 1 antibody) and their combination (IPI/NIVO)—have demonstrated durable survival benefits in melanoma. Despite high response rates, >50% of patients do not respond to these treatments. In addition, patients often develop immune-related adverse events with severe morbidity, substantially reducing quality of life. Efforts to identify biomarkers of ICB outcomes have mainly centered on the tumor microenvironment because anti-tumor T cell immunity is the primary target of ICB, the focus has been predominantly on tumor T-cell infiltration. While promising tumor-based surrogates of ICB have been proposed, none of these markers alone or in combination fully explains variability in ICB outcome. Hence, there is a continuing need to identify more powerful biomarkers of ICB outcomes that would also serve as potential novel targets for more effective and less toxic treatments. We propose a novel hypothesis that ICB outcomes are strongly impacted by host immunity, shown in recent reports to be influenced by underlying inherited factors. It was demonstrated that phenotypic variation in T-cell subsets, including CD8+ T cells, is attributed to germline genetic variation. In a recent study, we showed that this inherited component maps to the non-coding regulatory genome, impacting transcriptional regulation of T-cell differentiation and function. Based on these data, we hypothesize that circulating CD8+ T cells, a primary target of NIVO and IPI/NIVO therapies, are controlled by germline genetic variation in the CD8+ non-coding regulatory genome (regulome), and that this genetic variability modulates ICB efficacy and toxicity. The goal of the proposed study is to discover inherited signatures of the CD8+ T cell regulome that predict ICB efficacy and toxicity. Using samples from 600 melanoma patients from a clinical trial of NIVO and IPI/NIVO, we will perform a comprehensive analysis of whole-genome sequencing (WGS) and a whole-transcriptome analysis on peripheral blood pre-treatment CD8+ T cells to identify non-coding transcriptome signatures that predict ICB efficacy (Aim 1). We will use the genetic information from WGS to comprehensively assess open chromatin states in pre-treatment CD8+ T cells from the same 600 patients to identify epigenetic signatures controlled by inherited genetic variation, predicting ICB response and immune-related toxicity (Aim 2). Our preliminary data have revealed novel genomic imprints in the non-coding regulome that predict ICB response with high clinical accuracy, thus substantially supporting our hypotheses and design. For the first time, our study will elucidate the effect of inherited anti-tumor host immunity on ICB outcomes. As we suggest, besides imminent applicability to personalized prediction of ICB treatment benefits, the integration of genomic info...