SURFACE-MARKERS AND GROWTH-FACTOR RECEPTORS OF IMMATURE HEMATOPOIETICSTEM-CELL SUBSETS

The regenerative potential of bone marrow following exposure to relati vely high doses of ionizing radiation, as well as the efficacy of hemo poietic growth factor treatment, are dependent on the residual number of hemopoietic stem cells. From studies in mice in particular, evidenc e has been obtained that immature hemopoietic stem cells are heterogen ous with respect to repopulating capacity, with one subset being capab le of short-term, transient hemopoietic reconstitution and another sub set of sustained reconstitution. In rhesus monkeys, CD34(+), RhLA-DR(d ull) cells were identified as the small fraction of a bone marrow cell that contains reconstituting hemopoietic stem cells. The growth facto r receptor phenotype of this immature cell fraction has been determine d for granulocyte-macrophage colony-stimulating factor (GM-CSF), inter leukin 3 (IL-3), and IL-6 as well as for kit-ligand, making c-kit an e specially strong growth factor receptor marker for reconstituting stem cells. In addition, it is demonstrated that CD34(+) cells appear in p eripheral blood after exposure to radiation and are correlated to numb ers of CD34(+) cells in bone marrow. This finding suggests that circul ating CD34(+) cells may be used as a cellular marker with prognostic s ignificance for both the number of residual stem cells as well as rege neration of immature hemopoietic cells in bone marrow.