Human primitive hematopoietic progenitor cells are more enriched in KIT^low cells than in KIT^high cells

To clarify the phenotypes of various classes of human hematopoietic progenitor cells, we used a multicolor staining protocol in conjunction with CD34 and a newly developed mouse antihuman c-kit proto-oncogene product (KIT) monoclonal antibody (MoAb). We characterized three cell fractions in CD34⁺ cells that express KIT^low and KIT^high cells in addition to KIT^- cells. A clonogenic assay showed that most granulocyte-macrophage colony-forming cells (GM-CFC) were present in CD34⁺ KIT^high populations, whereas erythroid burst-forming cells (BFU-E) were detected mainly in the CD34⁺ KIT^low population. CD34⁺-KIT^- fraction contained a small number of BFU-E. Morphologic analysis showed that blast-like cells were more enriched in the CD34⁺ KIT^low fraction. KIT^low cells contained CD34⁺ CD38^- cells that were considered to be very primitive progenitor cells, as determined by a replating assay. To clarify the biologic differences between both fractions, we examined the more primitive progenitor cell functions by assessing long-term culture-initiating cells (LTC-IC) on the stromal cells. At week 2, more CFC recovered from the culture in the fraction initiated with a CD34⁺ KIT^high population. However, more LTC-IC were present during weeks 5 to 9 in the CD34⁺ KIT^low population. These results indicate that primitive progenitors are more enriched in the KIT^low population and that the KIT^high population contains many GM-committed progenitor cells. We also showed that anti-KIT MoAb inhibited the ability of CD34⁺ cells to generate CFC on the stromal layer in the LTC system. This suppressive effect was more evident in the generation of BFU-E by CD34⁺ KIT^low cells. Moreover, we confirmed that CD34⁺ KIT^high cells emerged from CD34⁺ KIT^low cells during coculture with allogeneic stromal cells or from liquid culture in the presence of stem cell factor (SCF), interleukin-6, and erythropoietin. These results emphasize the pivotal role of the KIT and SCF interaction in hematopoiesis and indicate that KIT^low cells are more primitive than KIT^high cells.