Flt3 ligand induces proliferation of quiescent human bone marrow CD34(+)CD38(-) cells and maintains progenitor cells in vitro

Flt3 is a class III tyrosine kinase receptor expressed on primitive human and murine hematopoietic progenitor cells (HPC). In previous studies using stroma-free short term assays, Flt3 ligand (FL) has been shown to induce proliferation of HPC at proportions similar to or less than c-kit ligand (steel factor, SF). Using long term stromal cocultivation assays, we studied the effects of FL on proliferation and differentiation of a highly primitive and cytokine nonresponsive subpopulation of human HPC, CD34(+)CD38(-) cells. Cell proliferation was significantly greater with FL than with SF, when used individually or in combinations with interleukin-3 (IL-3) and/or IL-6. The effect of FL was greater on bone marrow (BM) CD34(+)CD38(-) cells than the more cytokine responsive cord blood CD34(+)CD38(-) cells. Little or no effect was seen with FL on more mature CD34(+)CD38(+) cells from either BM or cord blood. The frequency of colony-forming units (CFU) and high proliferative potential-colony forming cells (HPP-CFC) during early culture (less than or equal to 30 days) was increased by both SF and FL to similar levels. However, in theLTC-IC period (35 to 60 days) and extended long-term culture initiating cell (ELTC-IC) period (>60 days), the frequency of CFU and HPP-CFC was significantly greater in cultures containing FL than those without FL (P < .0025). Fluorescence-activated cell sorter analysis of cultures after 21 days showed a significantly higher percentage 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% respectively, mean +/- SD). Cloning efficiency of BM CD34(+)CD38(-) cells 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 able to induce proliferation of CD34(+)CD38(-) cells that are nonresponsive to other early acting cytokines and to improve the maintainence of progenitors in vitro. (C) 1996 by The American Society of Hematology.