FLT3 LIGAND INDUCES PROLIFERATION OF QUIESCENT HUMAN BONE-MARROW CD34(-) CELLS AND MAINTAINS PROGENITOR CELLS IN-VITRO()CD38()

Flt3 is a class III tyrosine kinase receptor expressed on primitive hu man and murine hematopoietic progenitor cells (HPC). In previous studi es using stroma-free short term assays, Flt3 ligand (FL) has been show n to induce proliferation of HPC at proportions similar to or less tha n c-kit ligand (steel factor, SF). Using long term stromal cocultivati on assays, we studied the effects of FL on proliferation and different iation of a highly primitive and cytokine nonresponsive subpopulation of human HPC, CD34(+)CD38(-) cells. Cell proliferation was significant ly greater with FL than with SF, when used individually or in combinat ions with interleukin-3 (IL-3) and/or IL-6. The effect of FL was great er on bone marrow (BM) CD34(+)CD38(-) cells than the more cytokine res ponsive cord blood CD34(+)CD38(-) cells. Little or no effect was seen with FL on more mature CD34(+)CD38(+) cells from either BM or cord blo od. The frequency of colony-forming units (CFU) and high proliferative potential-colony forming cells (HPP-CFC) during early culture (less t han or equal to 30 days) was increased by both SF and FL to similar le vels. However, in theLTC-IC period (35 to 60 days) and extended long-t erm culture initiating cell (ELTC-IC) period (>60 days), the frequency of CFU and HPP-CFC was significantly greater in cultures containing F L than those without FL (P < .0025). Fluorescence-activated cell sorte r analysis of cultures after 21 days showed a significantly higher per centage of cells remained CD34(+) in the combination of FL, IL-3, IL-6 , and SF (F/3/6/S) than in 3/6/S (0.78% +/- 0.52% v 0.21% +/- 0.29% re spectively, mean +/- SD). Cloning efficiency of BM CD34(+)CD38(-) cell s was significantly increased by the addition of FL to the combination of 3/6/S (mean 11.7% v 0.5%, P < .0001). These data show that FL is a ble to induce proliferation of CD34(+)CD38(-) cells that are nonrespon sive to other early acting cytokines and to improve the maintainence o f progenitors in vitro. (C) 1996 by The American Society of Hematology .