Bortezomib Plus Rituximab for EBV+ PTLD

* Both rituximab and bortezomib will be given to participants intravenously. Each cycle of treatment will consist of 21 days. Rituximab will be given on Days 1, 8 and 15 of Cycle 1 and on Day 1 of subsequent cycles. Bortezomib will be given on Days 1, 4, 8 and 11 of every cycle. Participants will receive a maximum of 4 cycles. * The following study procedures will be performed during each cycle throughout the study: Medical history review; Physical exam; Performance Status; Questionnaire; Blood draws and; PET/CT scans (After cycles 2, 4 and 6 only). * After Cycle 4, if the study doctor feels the participant has had a complete response to treatment, then they will continue onto the Post-Treatment Surveillance period, which will consist of regular clinic visits over two years. * However, if the study doctor feels the participant has had a partial response to treatment and that they may benefit from continuing, they will receive an additional two cycles of bortezomib and be given daily tablets of the antiviral drug valganciclovir to help further target EBV.