Additional Study Information: The purpose of this study is to test the effectiveness (how well the drug works), safety, and tolerability of an investigational drug called nivolumab (also known as BMS-936558). This study will see if treatment with nivolumab increases the chance you will have a positive response (complete or partial) in treating your primary central nervous system lymphoma (PCNSL) or primary testicular lymphoma (PTL). It will also see if nivolumab improves your chances of survival and at delaying the worsening of your PCNSL or PTL (called progression).