Background and objective: While the minimum number of CD34+ cells required for complete and long-lasting engraftment is quite well established, there is not general agreement about the optimal number of CD34+ per kg needed in order to obtain engraftment as rapidly as possible. In the present study we assess factors affecting hemopoietic recovery and the optimal peripheral blood progenitor cell (PBPC) number for rapid engraftment in patients treated with high-dose therapy.
Design and methods: We enrolled 80 consecutive patients affected by hematologic and non-hematologic malignancies treated with a median of 10 chemotherapy courses (range 3-38). PBPC collection was performed after mobilization with high-dose chemotherapy and G-CSF 5 micrograms/kg/day. The circulating and harvested CD34+ cells were recognized in the cytofluorimetric CD45+/CD14- lymphocyte gate. After myeloablative therapy, PBPC infusion was followed by G-CSF 5 micrograms/kg/day from day +5 until WBC > or = 5.0 x 10(9)/L. Univariate and multivariate Cox analyses were performed to investigate factors affecting hemopoietic recovery. The Kaplan-Meier probabilities of hemopoietic reconstitution were compared by log-rank test to assess the optimal CD34+ cell number for rapid engraftment.
Results: We performed a median of two apheresis (range 1-4) per patient and we infused a median of 6.1 x 10(6) CD34+ cells/kg (range 0.5-30.5). Absolute neutrophil count (ANC) > 0.5 x 10(9)/L was reached after 11 days (range 8-15). The only factor affecting granulocyte recovery proved to be the CD34+ cell number; 5.0 to 7.8 x 10(6) CD34+ cells/kg allowed a significantly faster granulocyte recovery than < 2.5 x 10(6) CD34+ cells/kg (p = 0.0312). Platelet transfusion independence (> 20 x 10(9)/L) and 50 x 10(9)/L platelets were reached after 12 (range 8-24) and 15 days (range 9-40), respectively. The CD34+ cell number was also the only factor affecting platelet recovery; the number of 5.0 to 7.8 CD34+ cells/kg allowed a significantly faster platelet recovery than the lower dose, whereas a higher number did not. No late graft failures were observed. Patients receiving 5.0 to 7.8 x 10(9) CD34+ cells/kg had a significantly shorter duration of neutropenia, fewer platelet transfusions and less time spent in hospital than those receiving lower number did, whereas patients transplanted with a higher number had no advantage.
Interpretation and conclusions: When G-CSF is employed both for PBPC mobilization and after PBPC transplantation, the CD34+ cell number is the only factor that affects hemopoietic recovery. Moreover, > 5.0 x 10(6) CD34+ cells/kg is the optimal number for obtaining rapid platelet recovery and reducing the costs of HDT but there is no advantage exceeding the threshold of 7.8 x 10(6) CD34+ cells/kg.