PERIPHERAL-BLOOD SELL HARVESTS YIELD PRIMITIVE MULTILINEAGE PROGENITOR CELLS IN THE CD34+ 33- FRACTION/

The presence of primitive hematopoietic progenitor cells or stem cells in peripheral blood (PBSC's) harvests was investigated in a single ce ll culturing assay and compared with the results obtained in aspirates of normal bone marrow. Based on the presence of CD33, rather differen tiated progenitor cells (CD34+/33+) were distinguished from more primi tive cells (CD34+/33-). The growth potential of CD34+/33+ and CD34+/33 - cells have been studied. Single cell sorting was performed from peri pheral blood harvests, obtained from three patients with multiple myel oma during hematopoietic recovery after treatment with high dose cyclo phosphamide and rhu-GM-CSF. To test the effect of ''stem cell recruiti ng factors'' the cells were sorted in 96-well plates, prefilled with l iquid medium both in the presence of IL-3 + G-CSF + GM-CSF + Epo and t he same growth factors supplemented with SCF + IL-6. Addition of SCF a nd IL-6 to the culturing medium enhanced the plating efficiency of CD3 4+/33- cells considerably more than that of CD34+/33+ cells. This was observed in harvests of peripheral blood as well as in aspirates of no rmal bone marrow. The differences between CD34+/33+ and CD34+/33- were even more pronounced when only the large colonies (>500 cells/well) w ere taken into consideration. Assuming that IL-6 and SCF are ''stem ce ll recuiting factors'' the CD34+/33- fraction contains more clonogenic cells than the CD34+/33+ fraction. In all three patients the first CD 34+ cells appearing in the peripheral blood (PB) after cytoreductive t reatment were predominantly CD34+/33- (>80%). At later stages when the leukocyte counts had reached higher values the CD34+/33+ cells predom inated. We conclude that peripheral blood stem cell harvests contain p rimitive multipotential clonogenic progenitor cells (of the CD34+/33- phenotype).