Dynamics of Immune Reconstitution after Unrelated Allogeneic Transplantation with Post-Transplantation Cyclophosphamide Compared to Classical Immunosuppression with ATG

Introduction: Post-transplant cyclophosphamide (PTCY), in hematopoietic stem cell transplantation (HSCT), is responsible for the success of haploidentical stem cell transplantation and is also being used in matched transplants such as unrelated transplants, with promising outcomes in graft versus host disease (GVHD). However, we lack data on immune reconstitution with this type of immunosuppression, especially in unrelated transplantation. Immune reconstitution is directly involved with post-transplant morbidity and mortality, being related to GVHD, relapses and infections, such as cytomegalovirus (CMV). Objectives: To evaluate global and CMV-specific immune reconstitution after unrelated donor HSCT, comparing the use of thymoglobulin (ATG) or PTCY protocol. Methods: We prospectively included patients undergoing first unrelated donor HSCT with ATG, methotrexate and tacrolimus or PTCY (50mg/Kg D+3 and D+4), mycophenolate mofetil (MMF) and tacrolimus with patient and/or donor CMV IgG positive. Immune reconstitution analyzes were performed by flow cytometry and CMV-specific recovery by ELISPot technique on days +30, +60, +90 and +180 post HSCT. Results: Thirty-six patients were included in this study, 20 in the ATG group and 16 in the PTCY group (table 1). Only 2 patients were 9x10 match, and the others were 10x10 match. All patients were submitted to preemptive strategy of CMV treatment, without the use of letermovir. Regarding clinical outcomes, neutrophilic engraftment was earlier with ATG (median of 12 days), compared to PTCY (median of 16 days) (p=0.0001). There was no difference between the groups in terms of length of hospital stay (median 33 days in both groups), CMV reactivation (70% in ATG and 75% PTCY, p=0.98), acute GVHD grades II to IV (p=0.26) or grades III-IV (p=0.56), and chronic GVHD (p=0.45), as well as relapse (p=0.3), non relapse mortality (p=0.77) and overall survival (p=0.4). Regarding immune reconstitution we observed an initial expansion of monocytes and NK lymphocytes in both groups and a higher increase in CD8+ T lymphocytes was evidenced, with an inversion of the CD4/CD8 ratio (CD4 14.9% vs 15.5% p=0.9 and CD8 49.7% vs 54.5%p=0.5 in ATG and PTCY respectively). We identified a CD4+/CD8+ double-positive T cell (DP) population, which was higher in PTCY group compared to ATG on days +30 (3.5% vs 10.2%, p=0.009), +90 (3.1% vs 12% p= 0.01) and +180 (5.2% vs 13%, p=0.007, in ATG and PTCY respectively ). On day +30 there were higher proport