Complete Donor T-Cell Chimerism Predicts Lower Relapse Incidence after Standard Double Umbilical Cord Blood Reduced Intensity Conditioning Regimen Allogeneic Transplantation in Adults

Double umbilical cord blood (dUCB) allogeneic transplantation following low dose TBI, cyclophosphamide and Fludarabine (TCF regimen)-based reduced-intensity conditioning regimen (RIC) is increasingly used in adults lacking a suitable related or unrelated donor. Currently, there is little data regarding the impact on long-term outcome of CD3+ T cell chimerism (TCC) in this particular setting. Thirty-six adults with various hematological diseases and who receieved dUCB conditioned with TCF were included in this retrospective study. Peripheral blood CD3+ TCC was considered until day +100 post-tranplant in order to determine the impact of full versus mixed chimerism on long-term outcomes. Twenty-nine and 7 patients were documented with full and mixed CD3+ TCC, respectively, within the first 100 days post-transplant. With a median follow-up of 36 months, 3 year-OS, DFS, and relapse incidence were 61%, (95% CI 43-75); 50% (95% CI 32.5-66) and 28% (95% CI 16-44), respectively. In univariate analysis, a full CD3+ TCC was associated with a better 3-year DFS : 59% (95% CI 39-75.5) versus 14% (95% CI 7-46), (HR=0.24 [0.09-0.65], p=0.005) and a lower cumulative incidence of relapse : 24% (95% CI 21.5-57) versus 78% (95% CI 52-99), (HR=0.18 [0.05-0.5], p=0.004). In multivariate analysis, a full CD3+ TCC remained associated with a lower incidence or relapse (HR=0.17, 95% CI 0.028-0.99, p=0.049). CD3+ TCC has no impact on GVHD and NRM in this study. In conclusion, in our study, full CD3+ TCC was independently associated with a lower risk of relapse after dUCB TCF RIC allogeneic transplant in adults, highlighting the need to develop immunotherapy approaches allowing for early conversion to full chimerism after dUCB.