Exclusion criteria

Prior PD-(L)1-directed therapy Prior radiotherapy with or without radiosensitizing chemotherapy within 28 days of cycle 1 day 1 [or 14 days for palliative radiotherapy (30 Gy or less) that is not directed to the pelvic region] Receiving a live vaccine within 28 days of cycle 1 day 1 Known additional malignancy that is progressing or requires active treatment, or history of other malignancy within 3 years of study entry Active autoimmune disease requiring systemic immunosuppression in excess of physiologic maintenance doses of corticosteroids (>10 mg of prednisone or equivalent) Known active central nervous system metastases and/or carcinomatous meningitis Known active hepatitis A, B, or C virus infection Active infections requiring systemic therapy Impaired cardiac function or clinically significant cardiac disease

ART/HAART, antiretroviral therapy/highly active antiretroviral therapy; ECOG, Eastern Cooperative Oncology Group; HIV, human immunodeficiency virus; PD-(L)1, programmed cell death protein (ligand) 1; RECIST, Response Evaluation Criteria in Solid Tumors; SCAC, squamous cell carcinoma of the anal canal.