Inclusion Criteria:  Subjects with ECOG 0,1,2,3 Subjects with pathologically confirmed PCNSL with lesion in brain who progressed after or did not respond to at least 1 line of systemic therapy. PCNSL prior therapy may include HD-MTX, HD-MTX-based regimen, high-dose cytarabine, radiation therapy alone as treatment or as part of consolidation therapy, high-dose therapy with autologous stem cell transplant as part of consolidation therapy, and/or intraocular MTX alone or as part of consolidation therapy Subject with sufficient function of liver, kidney, heart, lung and hematopoiesis Subjects with relapsed PCNSL and are able to receive biopsy  Exclusion Criteria:  Subjects that are not DLBCL type pathologically Intraocular PCNSL without evidence of brain disease Subjects who cannot undergo MRI assessments Relapsed PCNSL patients who cannot undergo biopsy Biopsy suggests no significant tumor-infiltration T cell or poor PD-1 expression Subjects with other malignancy Subjects with history of any inflammatory CNS diseases Subjects with an active, known, or suspected autoimmune disease Subjects with a condition requiring systemic treatment with either corticosteroids or other immunosuppressive medications within 14 days of start of study treatment Prior treatment with an anti-PD-1, anti-PD-L1, anti-PD-L2, anti-CD137, or anti-CTLA-4 antibody, or any other antibody or drug specifically targeting T-cell co-stimulation or checkpoint pathways  Other protocol defined inclusion/exclusion criteria could apply