Inclusion Criteria:

          -  Age 18 years or older and willing and able to provide informed consent;

          -  Histologically confirmed relapsed or refractory CD20+ DLBCL, transformed indolent
             lymphoma (follicular or other), or primary mediastinal large B-cell lymphoma;

          -  Received prior treatment with a standard anthracycline and therapeutic anti-CD20
             monoclonal antibody-based regimen;

          -  For transplant-ineligible patients, salvage therapy just prior to MDV9300 treatment
             must have resulted in a PR or stable disease;

          -  For post autologous stem cell transplant (ASCT) patients, salvage therapy plus ASCT
             just prior to MDV9300 treatment must have resulted in a PR or stable disease;

          -  Adequate bone marrow reserve as defined per protocol;

          -  Eastern Cooperative Oncology Group (ECOG) performance status ≤ 1. Patients with stable
             ECOG scores of 2 may be allowed with medical monitor approval.

        Exclusion Criteria:

          -  Burkitt, mantle cell, follicular, or mucosa-associated lymphoid tissue lymphoma

          -  History of serious autoimmune disease;

          -  History of central nervous system involvement of lymphoma;

          -  Prior therapy with agents targeting immune coinhibitory receptors.