Background: PD-1 antibodynivolumab was administered with/without a multi-peptide vaccine to 126 patients (pts) with unresectable melanoma that failed at least one regimen and were IPI naïve (34), or progressed after IPI (92). We assess its toxicity especially in those with prior dose limiting immune related adverse events (irAEs) to IPI, update survival and response duration data, and characterize myeloid derived suppressor cell (MDSC) and T cell subsets in IPI refractory pts. Methods: Pts refractory to IPI received NIVO at 3 mg/kg: two cohorts of pts were A*0201 positive and had either grade 2 or less IPI-related irAE (10 pts), or grade 3-4 dose limiting irAE (21 pts); 61 pts had grade 2 or less irAE, were not HLA restricted and received NIVO alone. Pre- and 12 week post-treatment peripheral blood was analyzed. Results: Median follow-up for IPI refractory pts was 18.7 months (mos);the response ratewas 29% by mWHO; 44% had clinical benefit with confirmed partial and complete response or stable disease at 24 weeks (CR+PR+SD).Median duration of response was 14.3 mos. Median progression-free survival (PFS) was 5.4 mos, and estimated median overall survival (OS) was 20.1 mos (95% CI: 17.0, not reached) with 1 and 2 year OS of 69.2% (95% CI: 57.9-78.0%) and 39.1% (95% CI 26.0-52.0%). Of 14 pts that have completed all therapy or stopped due to toxicity while stable or in response, all remain in remission. Of 21 pts with prior IPI-induced grades 3-4 irAEs, only 2 had a subsequent dose limiting (and different) irAE with NIVO, with 8 PR and 5 SD seen; all 8 PR and 3 SD are without progression. Biomarker studies showed that circulating pre-treatment HLA-DR lo/CD14+/CD11b+ myeloid-derived suppressor cells (MDSC) were associated with progression and worse OS (p = 0.0001 and 0.0009). Pre-treatment, MDSC suppressed T cell reactivity (p = 0.006) which was overcome by PD-1 blockade ex vivo, and had high levels of VISTA, CD244 and BTLA. Low PD-L1 and Tim3 expression on MDSC was associated with response. Conclusions: This is the first survival assessment in IPI refractory NIVO treated melanoma pts, with median OS of 20.1 mos and PFS of 5.4 mos. Prior irAEs to IPI were not replicated with NIVO. Novel biomarkers of outcome were found on circulating MDSC. Clinical trial information: NCT01176461