THE FLK2 FLT3 LIGAND SYNERGIZES WITH INTERLEUKIN-7 IN PROMOTING STROMAL-CELL-INDEPENDENT EXPANSION AND DIFFERENTIATION OF HUMAN FETAL PRO-BCELLS IN-VITRO/

The effects of a novel cytokine FLK2/FLT3 ligand (FL) on human fetal b one marrow-derived CD34(+)CD19(+) pro-B cells were analyzed in a strom al-cell-independent, serum-deprived culture system. FL, like interleuk in-3 (IL-3), synergized with IL-7 in promoting pro-B cell growth, and differentiation of these cells into CD34(-)CD19(+)clgM(+)slgM(-) pre-B cells, whereas a small proportion of these cells even differentiate i nto more mature slgM(+) B cells. In contrast, KIT ligand (KL) and gran ulocyte-macrophage colony-stimulating factor (GM-CSF) were ineffective in promoting IL-7-dependent pro-B cell growth and differentiation. Ma ximal levels of pro-B cell expansion, generally resulting in 15- to 30 -fold increases in cellularity, were obtained in cultures supplemented with optimal doses of FL + IL-7 + IL-3. The addition of mouse bone ma rrow stromal cells further enhanced the proliferation and differentiat ion of pro-B cells obtained in the presence of these three cytokines. Under these conditions, cultures could be maintained for more than 4 w eeks, and in general 40- to 50-fold increases in cell numbers were obs erved by 3 weeks of culture, The percentages of clgM(+) and slgM(+) B cells increased 1.5- to 3-fold and 2-fold, respectively, suggesting th at stromal cells may provide additional costimulatory signals for huma n B-cell growth and differentiation that are different from IL-7, IL-3 , and FL. Collectively, our results indicate that FL, in contrast to K L, strongly promotes long-term expansion and differentiation of human pro-B cells in the presence of IL-7 or in combination of IL-7 and IL-3 , which is a novel property of this hematopoietic growth factor. (C) 1 996 by The American Society of Hematology.