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

Abstract

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 (≤30 days) was increased by both SF and FL to similar levels. However, in the LTC-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 < .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 maintenance of progenitors in vitro.