Kinetics of Chimerism and Unit Predominance After Double Umbilical Cord Blood Transplantation

Abstract 225 Double umbilical cord blood (dUCB) transplantation (dUCBT) is a strategy to overcome dose limitation in adult recipients. It is established that after dUCBT, one unit will predominate by day +100 after transplant in >95%. While in some studies order of infusion has been associated with unit predominance, this has not been reproduced in an analysis at our center. However, significant differences in UCB infusion between these two analysis were present. In particular, at our center, unit order of infusion is random and the second infusion is within minutes of the first, while this prior study separated infusion time by 6 hours. Between 2001 and 2009, 262 patients with hematologic malignancies underwent a dUCBT and engrafted. Of these, 233 were >18 years of age with 39% conditioned with cyclophosphamide (CY) 60 mg/kg, fludarabine (FLU) 75 mg/m2 and total body irradiation (TBI) 1320–1375 cGy and 61% with CY 50 mg/kg, FLU 200 mg/m2 and TBI 200 cGy with 1/3 also receiving antithymocyte globulin (ATG); 100% received cyclosporine and mycophenolate mofetil posttransplant immunosuppression. Median recipient weight was 78 kg and median follow-up was 2.7 years (0.5-7.2). The following factors were considered in the logistic regression model: total nucleated cell (TNC), CD34+ and CD3+ cell and colony forming units-granulocyte macrophage (CFU-GM) doses, HLA match, sex and ABO-match, CXCR4 expression on CD34+ cells, order of infusion and cell viability. Cell viability, infused TNC, CD34+ and CD3+ cell and CFU-GM doses were remarkably similar between the predominating and non-predominating unit. By day 21, the predominating unit (i.e., representing >70% of hematopoiesis) was achieved in 73 of 90 (81%) patients after MA conditioning and in 88 of 145 (61%) patients after reduced dose conditioning (p<0.01). Subsequently, predominant unit chimerism in the bone marrow between MA and NMA was similar by day 100 (95% vs. 97%, p=0.35), day 180 (97% vs. 100%, p=0.3), day 365 (97% vs. 98%, p=0.84) and day 730 (94% vs. 93%, p=0.81). Notably, CD3+ cell dose and HLA were strongly associated with unit predominance. In the MA setting, CD3+ cell dose was the most significant factor that predicted unit predominance [OR 4.4 (95% CI, 1.8–10.6, p<0.01)]; while CD3+ cell dose [OR 2.1 (95%CI, 1.0–4.2, p=0.05)] and HLA-match [OR 3.4 (95%CI 1.0–11.4, p=0.05)] were independent predictors in the reduced intensity setting. In summary, immunological graft-graft interactions are likely responsible for unit predominance. While the combined CD34 dose and CFU-GM dose from the two UCB units are critical for rate of neutrophil recovery (data previously reported), CD3 dose and HLA match after reduced intensity conditioning are important in determining which unit will ultimately predominate. These findings have potential implications in the algorithm of graft selection. Disclosures: No relevant conflicts of interest to declare.