The purpose of this study is to evaluate the safety of an investigational DNA vaccine, pTVG-HP, a plasmid DNA encoding human prostatic acid phosphatase (PAP), in combination with nivolumab, and the efficacy of this combination in decreasing serum Prostate-Specific Antigen (PSA) in patients with non-metastatic, non-castrate prostate cancer (clinical stage D0/M0).