Subsets of CD34+ and Engraftment Kinetics in Allogeneic Peripheral Stem Cell Transplantation.

Engraftment kinetics in allogeneic peripheral blood stem cell transplantation (alloPBSCT) depend on the number and efficiency of the stem cells in the graft, the conditioning regimen and GvHD prophylaxis. Currently, stem cell evaluation is performed by counting CD34+ cells; however, CD34+ cells are a heterogeneous population including the early uncommitted fraction as well as different subsets committed to one or the other lineage; hence, defining the CD34+ subset most predictive of engraftment and its threshold value would be of the utmost importance.

This study aimed to identify which graft product subset of CD34+ cells might be the most predictive of early hematopoietic recovery following alloPBSCT. The relationships between the number of “mature” subsets of CD34+ cells (CD34+/CD33+, CD34+/CD38+, CD34+/DR+ and CD34+/CD133) and “immature” subsets of CD34+ cells (CD34+/CD33−, CD34+/CD38−, CD34+/DR− and CD34+/CD133+) and early neutrophil and platelet engraftment were studied in a homogeneous series (for disease, pre-transplant chemotherapy, conditioning regimen GvHD prophylaxis) of 30 acute myeloid leukemia (AML) patients after alloPBSCT from HLA-identical siblings. All patients received the BU-CY regimen consisting of busulfan 4 mg/kg/day for 4 consecutive days followed by cyclophosphamide 60 mg/kg/day for 2 consecutive days; GvHD prophylaxis included cyclosporin and methotrexate. The CD34+ dose infused ranged from 2.9 to 8.8 × 10⁶/Kg (median 4.6); the percentage of immature CD34+ cells was 36% for CD34+/CD33−, 60% for CD34+/CD38−, 5% for CD34+/DR− and 70% for CD34+/CD133+; this translates into a median dose of 1.6 × 10⁶/Kg (range 0.3–5) for CD34+/CD33−, 2.6 × 10⁶/Kg (range 0.1–6.2) for CD34+/CD38−, 0.4 × 10⁶/Kg (range 0.1–2.3) for CD34+/DR− and 0.95 ×10⁶/Kg (range 0.6–2.3) × 10⁶/Kg for CD34+/CD133+. Median time to achieve engraftment of neutrophils and platelets was 13 days (range 10–16) and 15 days (range 13–19), respectively. In our experience the total CD34+/CD133+ cell number was inversely correlated with the days required for recovery of 0.5 × 10⁹/L neutrophils (r = −0.76, p<0.05) and 100 × 10⁹/L platelets (r = −0.71, p<0.05); this correlation was better than the total CD34+ cells dose and neutrophil (r = −0.71, p<0.05) and platelets engraftment (r = −0.68, p = 0.06). No correlation was found between the other CD34+ subsets and neutrophil and platelets engraftment. With regard to the threshold dose for early neutrophil engraftment, all 14 patients who received more than 1 × 10⁶/Kg of CD34+/CD133+ had a neutrophil count higher than 1.0 × 10⁹/L at 12 days. We suggest that a high number of CD34+/CD133+ peripheral blood stem cells may be associated with faster neutrophils and platelets recovery; these findings may help to predict the repopulating capacity of PBSC in patients after allogeneic PBSCT, especially when a relatively low number of CD34+ cells is infused.