FLK-2/FLT-3 LIGAND REGULATES THE GROWTH OF EARLY MYELOID PROGENITORS ISOLATED FROM HUMAN FETAL LIVER

The effects of the recently identified FLK-2/FLT-3 ligand (FL) on the growth of purified human fetal liver progenitors were investigated under serum-deprived culture conditions. FL alone was found to stimulate modest proliferation in shortterm cultures of CD34(++) CD38(+) lineage (Lin)(-) light-density fetal liver (LDFL) cells and the more primitive CD34(++) CD38(-)Lin(-) LDFL cells. However, the low levels of growth induced by FL were insufficient for colony formation in clonal cultures. Synergism between FL and either granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3) or KIT ligand (KL) was observed in promoting the growth of high-proliferative potential (HPP) colony-forming cells (CFC) and/or low-proliferative potential (LPP)-CFC in cultures of CD34(++) CD38(+) Lin(-) and CD34(++) CD38(-) Lin(-) LDFL-cells. FL, alone or in combination with other cytokines, was not found to affect the growth of CD34(+) Lin(-) LDFL cells, the most mature subpopulation of fetal liver progenitors investigated. The growth of the most primitive subset of progenitors studied, CD34(++) CD38(-) Lin(-) LDFL cells, required the interactions of at least two cytokines, because only very low levels of growth were observed in response to either FL, GM-CSF, IL-3 or KL alone. However, the results of delayed cytokine-addition experiments suggested that individually these cytokines did promote the survival of this early population of progenitors. Although two-factor combinations of FL, KL, and GM-CSF were observed to promote the growth of early progenitors in a synergistic manner, neither of these factors was found to make fetal liver progenitors more responsive to suboptimal concentrations of a second cytokine. Only myeloid cells were recovered from liquid cultures of CD34(++) CD38(-) Lin(-) LDFL cells grown in the presence of combinations of FL, KL, and GM-CSF. These results indicate that FL is part of a network of growth factors that regulate the growth and survival of early hematopoietic progenitors. (C) 1995 by The American Society of Hematology.