Patient description: The patient is a 55 year old man visiting his primary care physician for lower urinary tract symptoms including frequency, urgency, weak stream, incomplete emptying and intermittent flow for the past 9 months. Further evaluation revealed: IPSS score : 15 Post-void residual: 70 mL Prostate volume (TRUS): 60 mL Prostate-specific antigen (PSA) level: 3.2 ng/mL 10 mL/sec of maximum flow rate when urine volume was 130 mL He is otherwise healthy only using Vit D 1000 units daily. His recent blood chemistry (3 days ago) was normal: Hgb: 13.5 g/dl WBC: 135000 /mm3 Plt: 350000 /ml PT: 11 second PTT: 35 second INR: 0.9 Creatinine: 0.5 mg/dl BUN: 10 mg/dl U/A: Color: yellow Appearance: cloudy PH: 5.3 Specific gravity: 1.010 Glc: 100 Nitrite: negative Ketone: none Leukocyte esterase: negative RBC: negative WBC: 2 WBCs/hpf U/C: negative

Clinical trial eligibility criteria: eligible ages (years): 18.0-999.0,  Prostate Cancer Recurrent Prostate Cancer Willing and able to provide signed informed consent and HIPAA authorization for the release of personal health information Males aged 18 years and above Prior local therapy with prostatectomy or EBRT/brachytherapy is required Prior salvage or adjuvant radiation therapy is allowed but not mandated. Radiation therapy must have been completed for at least 6 months Absolute PSA >=1.0 ng/mL at screening Must have at least one of the following genetic alterations identified using archival tissue (i.e. prostate needle biopsy prior to radiation therapy or prostatectomy specimen) Microsatellite instability (MSI-high) status by clinical grade testing MMR protein loss (MSH2, MSH6, MLH1, PMS2) by immunohistochemistry Inactivating mutation of MSH2, MSH6, MLH1 or PSM2 by clinical grade genomic testing Tumor mutational burden >= 20 mutations/megabase (TMB >=20 muts/Mb) by clinical grade testing  Metastatic disease or currently active second malignancy Prior androgen deprivation therapy (ADT) in the past 6 months. Prior ADT in context of neoadjuvant/adjuvant primary; prior ADT for biochemical recurrence is allowed, as long as no ADT has been administered in past 6 months and testosterone has recovered (>150 ng/dL) Prior oral anti-androgen (e.g. bicalutamide, nilutamide, enzalutamide, apalutamide), or androgen synthesis inhibitor (e.g. abiraterone, orteronel) within the past 2 weeks is not permitted. 5-alpha reductase inhibitor therapy (e.g. finasteride, dutasteride) is allowed, as long as subject has been stable on medication for past 6 months Involvement in the planning and/or conduct of the study (applies to both BMS staff and/or staff at the study site) Participation in another clinical study with an investigational product during the last 4 weeks/28 days Patients should be excluded if they have had prior systemic treatment with an anti-PD-1, anti-PD-L1, anti-PD-L2, anti-CTLA-4 antibody, or any other antibody or drug specifically targeting T-cell costimulation or immune checkpoint pathways Patients should be excluded if they have an active, known or suspected autoimmune disease (e.g. inflammatory bowel disease, rheumatoid arthritis, autoimmune hepatitis, lupus, celiac disease). Subjects are permitted to enroll if they have vitiligo, type I diabetes mellitus, residual hypothyroidism due to autoimmune condition only requiring hormone replacement, psoriasis not requiring systemic treatment, or conditions not expected to recure in the absence of an external trigger Patients should be excluded if they have a condition requiring systemic treatment with either corticosteroids (>10 mg daily prednisone daily equivalents) or other immunosuppressive medications within 14 days of study drug administration. Inhaled or topical steroids and adrenal replacement doses >10mg daily prednisone equivalents are permitted in the absence of active autoimmune disease Permitted therapies topical, ocular, intra-articular, intranasal, and inhalational corticosteroids (with minimal systemic absorption). Physiologic replacement doses of systemic corticosteroids are permitted, even if > 10 mg/day prednisone equivalents. A brief course of corticosteroids for prophylaxis (e.g. contrast dye allergy) or for treatment of nonautoimmune conditions (e.g. delayed-type hypersensitivity reaction caused by contact allergen) is permitted As there is potential for hepatic toxicity with nivolumab, drugs with a predisposition to hepatotoxicity should be used with caution in patients treated with nivolumab-containing regimen