CHARACTERIZATION OF CD34+ CELLS MOBILIZED TO THE PERIPHERAL-BLOOD DURING THE RECOVERY FROM CYCLOPHOSPHAMIDE CHEMOTHERAPY

Twenty two patients were treated with cyclophosphamide therapy to mobilize progenitors into the blood. Progenitor cells were quantitated in peripheral blood or leukapheresis products using colony assays and flow cytometric measurement of CD34+ cells. Prompt engraftment of > 500 granulocytes/ul at a median of 13 days was observed in all patients reconstituted with mobilized cells. In four patients where complete sets of serial samples were obtained, the appearance of CD34+ cells preceded the increase in CFU-GM by 24 to 48 hours. Peak levels of CD34+ cells ranged from 0.6-5% and coincided with the peak increase in CFU-GM. Mobilized CD34+ cells were predominantly CD33+, CD13+, CD45R+, CD38+, HLA-DR+ and CD41+. In contrast to bone marrow CD34+ cells, few mobilized CD34+ cells expressed CD71, CD19 or CD10. Long term culture initiating cells (LTC-IC), capable of reconstituting hematopoiesis in vitro on irradiated stromal layers, were also measured in serial samples from a single patient with high peak levels of CD34 (approximately 5%). LTC-IC were present in all samples and the ratio of LTC-IC to CD34+ cells remained similar throughout the post cyclophosphamide recovery phase. In this patient, preferential mobilization of LTC-IC earlier in the recovery phase was not observed suggesting that during recovery both primitive and committed progenitors are mobilized together. These data indicate that mobilized CD34+ cells represent subpopulations of CD34+ cells with a myeloid phenotype and that the presence of primitive progenitors among these cells confirms clinical reports that they are capable of long term reconstitution.