PROJECT SUMMARY ABSTRACT: Project 2 Immunotherapy with anti-PD1 monoclonal antibodies (mAbs) is associated with improved response and survival rates in multiple solid tumors, including melanoma1. Anti-PD1 mAbs (anti-PD1) produces durable clinical responses in 33-40% of melanoma patients with minimal toxicity. While dual anti-PD1/anti-CTLA4 blockade produces higher response rates (58%) and 5-year PFS (36%), this is associated with 55% grade 3/4 adverse events. Therefore, there is a critical need for novel combinatorial immunotherapy to improve the efficacy of anti-PD1 while mitigating the rate of serious adverse events in advanced melanoma. One major barrier limiting the efficacy of anti-PD1 is the lack of spontaneous tumor-infiltrating T cells (TILs) and defective IFNa production in tumor microenvironment (TME) in so-called “cold” or non-inflamed tumors. One promising therapeutic approach to overcome this hurdle is via toll-like receptor 9 (TLR9) agonists. TLR9 is predominantly expressed by plasmacytoid dendritic cells (pDCs) and B cells and binds to agonists including unmethylated cytosine guanosine oligodinucleotides (CpG). We have recently performed the first-in-human trial of neoadjuvant intratumoral CMP, a novel type A CpG, and Nivolumab in PD1-naïve high-risk resectable melanoma (NCT03618641). In 30 evaluable melanoma patients, we have observed 60% major pathologic responses with increased CD8+ TILs and peritumoral CD303+ pDCs in injected tumors, and higher frequency circulating PD1+Ki67+CD8+ T cells in responders. Therapy with intratumoral CMP and Nivolumab (CMP/Nivolumab) has also shown clinical efficacy in PD1 refractory melanoma with responses in non-injected tumors, supporting the occurrence of systemic antitumor immunity beyond the injected tumors. To further our understanding of the mechanisms of responses or resistance to CMP/Nivolumab in injected and non-injected tumors, we will take advantage of a substudy of 60 melanoma patients included in the randomized phase II/III clinical trial evaluating CMP/nivolumab vs. nivolumab in PD1 naïve metastatic melanoma with accessible tumors for intratumoral CMP. Based on our preliminary findings, we will investigate whether CMP/Nivolumab :1) increases pDC activation and recruitment into injected tumors to promote CD8+TIL expansion and functions in injected and non-injected tumors; 2) induces melanoma cell death, primes potent neoepitope-specific CD8+T cells in injected tumors, and epitope spreading to melanoma-associated antigens; and 3) fails to induce potent T cell responses because of melanoma cell-extrinsic or melanoma cell-intrinsic mechanisms. Collectively, the findings in this application will improve our understanding of the mechanisms of response and resistance to CMP/Nivolumab in melanoma. They will further support novel combinatorial immunotherapies to further enhance the immunogenicity and clinical activity of CMP/Nivolumab in injected and non-injected tumors of advanced melanoma.