INDUCTION OF C-KIT MOLECULES ON HUMAN CD34+ C-KIT(LESS-THAN-LOW) CELLS - EVIDENCE FOR CD34+/C-KIT(LESS-THAN-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 con troversial 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 relat ion to the expression of c-kit molecules, When the CD34(+)/c-kit(<low) cells were cultured with cytokines (flt 3 ligand, interleukin 6 and i nterleukin 7) plus immobilized anti-CD34 monoclonal antibody (to cross link 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 methylcellulos e culture system, they showed the same colony-forming ability as the f reshly isolated CD34(+)/c-kit(low) or CD34(+)/c-kit(+) cells, respecti vely, Furthermore, CD34(+)/c-kit(<low) cells have a similar hematopoie tic potential to CD34(+)/c-kit(low) cells in assays for long-term cult ure initiating cell and colony-forming unit culture generated from lon g-term cultures, These findings suggest that CD34(+)/c-kit(<low) cells mature into CD34(+)/c-kit(low) and CD34(+)/c-kit(+) cells, and acquir e the reactivity to various humoral hematopoietic stimuli, Moreover, C D34(+)/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 nit-g ranulocyte-erythroid-macrophage-megakaryocyte activity are also a plur ipotent hematopoietic stem cell population, and the expression of c-ki t on c-kit(<low) cells is the first maturational step of hematopoiesis .