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Introduction: WT1 often presents on the surface of diffuse pleural mesotheliomas (DPMs) and is an ideal therapeutic target. Galinpepimut-S (GPS), a tetravalent, non–human leukocyte antigen–restricted, heteroclitic WT1–specific peptide vaccine was safe and effective in early phase clinical trials and upregulates T-cell suppressive programmed death-ligand 1 in the tumor microenvironment of other malignancies. A randomized phase 2 study of adjuvant GPS in patients with DPM trended toward improved median overall survival. Methods: To further enhance immunogenicity, we combined GPS with nivolumab, an anti-PD1 monoclonal antibody, in an open-label, single-center phase 1 study, examining tolerability and immunogenicity in patients with previously treated DPM. We enrolled patients with progressive or recurrent DPM treated with at least one course of pemetrexed-based chemotherapy. Patients received two doses of GPS followed by six doses of GPS with intravenous nivolumab every 2 weeks, and up to six additional cycles until disease progression or unacceptable toxicity. Results: Ten patients were treated; 70% experienced mostly mild treatment-related adverse events; two experienced a grade 3 or higher adverse event. Three of the 10 patients (30%) reported vaccine-specific T-cell responses. There were no partial responses; three patients had prolonged stable disease with up to 17% decrease in tumor volume. Median progression-free survival was 3.9 months and the median overall survival was 7.4 months. Conclusions: Coadministration of GPS and nivolumab reported a tolerable toxicity profile and induced immune responses in a subset of patients, but initial response and survival benefit were limited possibly owing to the small sample size.