Cord Blood Units Containing Lower CD34+ Cells (0.5 - 1.0 x 105 /kg) Could be Alternative Donor Candidates for Single-Unit Cord Blood Transplantation for Adults: A Retrospective Study of 421 Patients in a Single Institute

BACKGROUND: Cord blood is an established alternative donor cell source for allogeneic hematopoietic cell transplantation. However, engraftment failure is still a major concern after transplantation, especially for patients transplanted lower doses of donor cells. Higher CD34+ cell dose leads to a secure and fast engraftment, and the units of cord blood which contain CD34+ cells of 1.0 - 1.7 x 105 /kg at freezing or 1.0 - 1.2 x 105 /kg at thawing are recommended (Thomas' Hematopoietic Cell Transplantation, 5th Edition). Actually, most adult patients cannot obtain such sufficient cell dose-containing cord bloods and the feasibility of single-unit cord blood transplantation (CBT) containing lower CD34+ cell dose than 1.0 x 105 /kg is unclear. METHODS: To investigate the lower threshold of CD34+ cell dose, we studied the patients who received single-unit CBT as the first transplantation between 2009 and 2017. The patients whose ECOG performance status was 0 or 1, and who do not have donor-specific anti-HLA antibody (DSA) were analyzed. Institutional review board of Toranomon Hospital approved the study (research number #1666). RESULTS: A total of 421 patients were studied. The median age and body weight of patients was 57 years (range, 16 - 74) and 56.4 kg (32.2 - 94.6), respectively. Myeloid diseases accounted for 78% of the patients, and 83% were not in remission. Myeloablative conditioning regimens were used in 80% of the patients. All patients used Tac (26%) or Tac plus MMF (74%) as GVHD prevention. The median numbers of total nucleated cells and CD34+ cells were 2.61 x107 /kg (range, 1.57 - 5.85) and 0.86 x 105 /kg (0.29 - 3.77) at freezing, respectively. The cumulative incidence of neutrophil engraftment was 90.7% at 60 days after transplantation (95% confidence interval, 87.5 - 93.1). The median day of neutrophil engraftment was day 21 (range, 5 - 45). Multivariate analysis identified higher CD34+ cell dose, less HLA mismatch, and lymphoid disease as significant favorable factors for neutrophil engraftment (p < 0.05), and CD34+ cell dose was most significant among the following pre-transplant factors (HR 1.57, p < 0.00001): age (≤ 57 vs. >57 years), body weight (≤ 56.4 vs. > 56.4 kg), ECOG performance status (0 vs. 1), disease (myeloid vs. lymphoid), disease status (in CR vs. not in CR), anti-HLA antibody (not DSA) (positive vs. negative), total nucleated cell dose (≤ 2.61 vs. > 2.61 x 107 /kg), CD34+ cell dose (≤ 0.86 vs. >0.86 x 105 /kg), HLA antigen