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 match (≤4/6 vs. ≥5/6), ABO match (match vs. mismatch), sex match (match vs. mismatch), GVHD prevention (Tac vs. TAC plus MMF), and the intensity of conditioning regimen (MAC vs. RIC). Then, we compared the cumulative incidence of neutrophil engraftment between 4 groups as follows: 90.2% for group A (> 1.5 x 10⁵ /kg, n = 41); 91.7% for group B (1.0 - 1.5 x 10⁵ /kg, n = 109); 91.4% for group C (0.5 - 1.0 x 10⁵ /kg, n = 255); 75.0% for group D (< 0.5 x 10⁵ /kg, n = 16) (p < 0.01). The median day of neutrophil engraftment was faster for the patients transplanted more CD34⁺ cell doses: day 17 for group A; day 19 for B; day 21 for C; day 26.5 for D (p < 0.0001). Next, we focused on group C transplanted lower CD34⁺ cell dose than the recommendation (0.5 - 1.0 x 10⁵ /kg, n = 255).The patients were divided into 5 groups according to their CD34⁺ cell doses, and we compared their cumulative incidence of neutrophil engraftment as follows: 96.0% for group C1 (0.9 - 1.0 x 10⁵ /kg, n = 50); 89.7% for group C2 (0.8 - 0.9 x 10⁵ /kg, n = 39); 88.1% for group C3 (0.7 - 0.8 x 10⁵ /kg, n = 67); 92.2% for group C4 (0.6 - 0.7 x 10⁵ /kg, n = 51); 91.7% for group C5 (0.5 - 0.6 x 10⁵ /kg, n = 48) (p = 0.03). The median day of neutrophil engraftment was significantly faster for the patients transplanted more CD34⁺ cell dose: day 20 for group C1; day 21 for C2; day 21 for C3; day 23 for C4; day 24 for C5 (p < 0.01). Overall survival was not significantly different between group A vs. B vs. C vs. D, nor group C1 vs. C2 vs. C3 vs. C4 vs. C5, respectively.

DISCUSSION & CONCLUSION: Significantly faster neutrophil engraftment was demonstrated for patients transplanted more CD34⁺ cells after single-unit CBT. On the other hand, the cumulative incidences of neutrophil engraftment at day 60 were comparable among the patients who used > 0.5 x 10⁵ /kg of CD34⁺ cells to be around 90%. The cord blood units containing 0.5 - 1.0 x 10⁵ /kg at freezing could be alternative donor candidates for cord blood selection, if delayed engraftment was clinically acceptable for recipients.