Epstein-Barr Virus-positive Diffuse Large B-cell Lymphoma

This research study is evaluating the combination of drugs, pembrolizumab with chimeric antigen receptor (CAR) T-cell therapy, as a possible treatment for primary mediastinal B-cell lymphoma that has recurred after prior treatment. The names of the study drugs involved in this study are: \- Pembrolizumab Standard treatment will include: * CAR T-cell therapy (either axicabtagene-ciloleucel or lisocabtagene maraleucel) * Cyclophosphamide * Fludarabine