ISOLATION OF SMALL, PRIMITIVE HUMAN HEMATOPOIETIC STEM-CELLS - DISTRIBUTION OF CELL-SURFACE CYTOKINE RECEPTORS AND GROWTH IN SCID-HU MICE

Human CD34(+) cells were subfractionated into three size classes using counterflow centrifugal elutriation followed by immunoadsorption to p olystyrene cell separation devices. The three CD34(+) cell fractions ( Fr), Fr 25/29, Fr 33/37, and Fr RO, had mean sizes of 8.5, 9.3 and 13. 5 mu m, respectively. The majority of cells in the large Fr RO CD34(+) cell population expressed the committed stage antigens CD33, CD19, CD 38, or HLA-DR and contained the majority of granulocyte-macrophage col ony-forming units (CFU-GM), burst-forming units-erythroid (BFU-E), and CFU-mixed lineage (GEMM). In contrast, the small Fr 25/29 CD34(+) cel ls were devoid of committed cell surface antigens and lacked colony-fo rming activity. When seeded to allogeneic stroma, fr RO CD34(+) cells produced few CFU-GM at week 5, whereas cells from the Fr 25/29 CD34(+) cell population showed a 30- to 55-fold expansion of myeloid progenit ors at this same time point. Furthermore, CD34(+) cells from each size fraction supported ontogeny of T cells in human thymus/liver grafts i n severe combined immunodeficient (SCID) mice. Upon cell cycle analyse s, greater than 97% of the Fr 25/29 CD34(+) cells were in G(0)/G(1) ph ase, whereas greater proportions of the two larger CD34(+) cell fracti ons were in active cell cycle. Binding of the cytokines interleukin (I L)-1 alpha, IL-3, IL-6, stem cell factor (SCF), macrophage inhibitory protein (MIP)-1 alpha, granulocyte colony-stimulating factor (G-CSF), and granulocyte-macrophage (GM)-CSF to these CD34(+) cell populations was also analyzed by flow cytometry. As compared with the larger CD34( +) cell fractions, cells in the small Fr 25/29 CD34(+) cell population possessed the highest numbers of receptors for SCF, MIP1 alpha, and I L-1 alpha. Collectively, these results indicate that the Fr 25/29 CD34 (+) cell is a very primitive, quiescent progenitor cell population pos sessing a high number of receptors for SCF and MIP1 alpha and capable of yielding both myeloid and lymphoid lineages when placed in appropri ate in vitro or in vivo culture conditions. (C) 1995 by The American S ociety of Hematology.