RecruitingDrug trialPolatuzumab Vedotin (Pola) Plus Rituximab (R) in Patients With Post-transplant Lymphoproliferative Disorder (PTLD)This study will test polatuzumab vedotin in combination with rituximab in patients with treatment-naïve CD20-positive post-transplant lymphoproliferative disorder (PTLD) based on the established efficacy of polatuzumab vedotin in B-cell lymphomas and the inadequate response rate of PTLD to single-agent rituximab. The hypothesis is that this combination therapy will be safe, well-tolerated, and effective. If so, patients with PTLD will be able to be spared the toxicity of anthracycline-based chemotherapy. Additionally, the role of the tumor microenvironment and the role of anellovirus, a non-human pathogen virus, will be explored as prognostic markers in PTLD.