Early hemopoietic progenitors in the peripheral blood of patients with severe aplastic anemia (SAA) after treatment with antilymphocyte globulin (ALG), cyclosporin-A and G-CSF

Background and Objective. We previously reported that patients with acquired severe aplastic anemia (SAA) treated with antilymphocyte globulin (ALG), 6-methylprednisolone, cyclosporin A (CyA) and granulocyte colony- stimulating factor (G-CSF) can mobilize peripheral blood hemopoietic progenitors (PBHP). The aim of the present study was to assess phenotypic and functional properties of these PBHP. Methods. We studied seven patients who underwent 43 leukophereses (median 5) between day +30 and +80 following ALG, while in treatment with CyA and G-CSF. Mobilized peripheral blood hemopoietic progenitors were analyzed using surface markers conventional assays for clonogenic cells (CFU-GM, BFU-E, CFU-GEMM) as well as the recently developed assay for long-term culture initiating cells (LTC-ICs). Results. The proportion of CD34⁺ cells ranged between 0% and 5.4% median 0.3%) CD34⁺DR^- between 0% and 3.5% (median 0.1%) and CD8⁺ cells between 3.3% and 56% (median 31%). When light density mononuclear cells (MNC) were plated in vitro, we could grow colony-forming units-granulo-macrophage (CFU-GM) (range 0-45/10⁵ MNC; norma controls 21-200/10⁵ MNC), burst-forming units-erythroid (BFU-E)(range 0-5/10⁵ MNC; normal controls 0-6/10⁵ MNC), multipotent colonies (CFU-GEMM)(range 0-3/10⁵ MNC; normal controls 0-6/10⁵ MNC) and high proliferative potential colony-forming cells (HPP-CFC) (range 03.4/10⁵ MNC). We studied long-term culture-initiating cells (LTC-ICs) in 18 leukophereses from 4 patients; in 7/18 samples LTC-ICs were grown at low frequency (range 0.4-2/10⁶ MNC) (normal controls 5-130/10⁶ MNC), and in one patient in the absence of CFU-GM growth. The total yield of LTC-ICs in two patients was 7.64 and 10.5x10²/kg of body weight. Interpretation and Conclusions. This study suggests that cells with the phenotype and in vitro function of early hemopoietic progenitors are found, though in small numbers, in the peripheral blood of patients with SAA after treatment with immunosuppressants and prolonged G-CSF administration. Whether G-CSF- mobilized progenitors contribute to hemopoietic recovery in these patients remains to be determined.