It is known that the majority of primitive hematopoietic progenitors are in a noncycling quiescent state. In addition, normal hematopoietic progenitors and progenitor cell lines show an absolute dependence on growth factors for their survival in vitro, yet the effect of growth factors on progenitor cell survival has not been separated from effects on both proliferation and differentiation. Using an in vitro assay system, we examined whether growth factors could promote the survival of stem cells in culture in the absence of cell division. These studies show that steel factor (SLF) and, to a lesser extent, interleukin-3 (IL- 3) directly promoted the survival of elutriated bone marrow progenitor cells (countercurrent centrifugal elutriation [CCE]-27) that are enriched for primitive hematopoietic progenitors that respond to the combination of SLF plus IL-3. Furthermore, SLF promoted the survival of short-term reconstituting cells (STRC), and long-term reconstituting cells (LTRC) with trilineage reconstitution potential in vivo. In comparison, granulocyte colony-stimulating factor (G-CSF), IL-6, leukemia inhibitory factor, IL-11, IL-1, granulocyte macrophage CSF (GM- CSF), and macrophage CSF (M-CSF) had no effect on the survival of these cells. In the presence of mitotic inhibitors (nocodazole or aphidicolin), SLF promoted the survival of CCE-27 progenitor cells that respond to the combination of SLF plus IL-3 in vitro and STRCs and LTRCs that are detected in vivo. Taken together, these data show that SLF can directly promote the survival of hematopoietic progenitor cells in the absence of cell division.

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