Induction of c‐kit Molecules on Human CD34 + /c‐kit <low Cells: Evidence for CD34 + /c‐kit <low Cells as Primitive Hematopoietic Stem Cells

c‐kit, a receptor for stem cell factor, has been widely accepted as a distinctive marker for hematopoietic stem cells. However, the level of c‐kit expression on pluripotent hematopoietic stem cells is still controversial in mice and humans. We purified CD34 + /c‐kit <low cells (phenotypically c‐kit‐negative but only detectable at the message level) from human cord blood and examined their maturational steps in relation to the expression of c‐kit molecules. When the CD34 + /c‐kit <low cells were cultured with cytokines (flt 3 ligand, interleukin 6 and interleukin 7) plus immobilized anti‐CD34 monoclonal antibody (to crosslink CD34 molecules), c‐kit molecules were clearly induced within 24 h. The c‐kit expression gradually increased until day 8. When CD34 + /c‐kit low or CD34 + /c‐kit + cells that had been induced from CD34 + /c‐kit <low cells were resorted and recultured using a methylcellulose culture system, they showed the same colony‐forming ability as the freshly isolated CD34 + /c‐kit low or CD34 + /c‐kit + cells, respectively. Furthermore, CD34 + /c‐kit <low cells have a similar hematopoietic potential to CD34 + /c‐kit low cells in assays for long‐term culture initiating cell and colony‐forming unit culture generated from long‐term cultures. These findings suggest that CD34 + /c‐kit <low cells mature into CD34 + /c‐kit low and CD34 + /c‐kit + cells, and acquire the reactivity to various humoral hematopoietic stimuli. Moreover, CD34 + /c‐kit <low cells showed a low level of rhodamine 123 retention, suggesting that CD34 + /c‐kit <low cells have multidrug resistance. Therefore, the CD34 + /c‐kit <low cells without colony‐forming unit‐granulocyte‐erythroid‐macrophage‐megakaryocyte activity are also a pluripotent hematopoietic stem cell population, and the expression of c‐kit on c‐kit <low cells is the first maturational step of hematopoiesis.