Multiple cytokines can synergize to stimulate the in vitro proliferation and exclusive myeloid differentiation of multipotent bone marrow progenitor cells. The ligand for c-kit (stem cell factor [SCF]) plays a key role in stimulating myeloid and erythroid cell production of primitive hematopoietic progenitors. SCF in combination with interleukin-7 (IL-7) can also stimulate the combined myeloid and B-cell differentiation of uncommitted hematopoietic progenitor cells as well as the growth of early B-cell progenitor cells, although the involvement of c-kit in early B lymphopoiesis remains controversial. In the present study, the flt3-ligand (FL), which, in combination with other cytokines, has overlapping activities with SCF on myeloid cell production from uncommitted progenitors, was investigated for its ability to induce selective stroma-independent B-cell commitment from uncommitted Lin-Sca-li bone marrow progenitor cells. IL-7 alone did not induce any clonal growth and FL alone gave rise to a few clusters (<50 cells) but no colonies (>50 cells), whereas the combined stimulation with FL and IL-7 resulted in clonal growth of 10% of Lin(-)Sca-1(+) bone marrow cells. After 12 days of incubation of Lin(-)Sca-1(+) cells in FL + IL-7, an almost 400-fold increase in cell production was observed. Phenotyping showed that greater than 99% expressed 8220, but not cell surface markers specific for myeloid, erythroid, or T-cell lineages. furthermore, the cells did not express cytoplasmic mu-heavy chain (c mu) or surface IgM, but were positive for CD24 (heat stable antigen [HSA]) and CD43 (leukosialin), suggesting that the cells produced were blocked at a late pro-B-cell stage. Interestingly, although all FL + IL-7-responsive Lin(-)Sca-1(+) progenitor cells and the resulting pro-B cells expressed c-kit, FL + IL-7 was much more potent (62-fold) than SCF + IL-7 in stimulating production of cells of the B-cell lineage. in addition, whereas FL + IL-7 selectively stimulated the production of pro-B cells, SCF + IL-7 predominantly stimulated the production of mature granulocytes. Replating studies showed that FL + IL 7-responsive Lin-Sca-1(+) progenitors were not committed to the 8-cell lineage, because after 2 days of incubation in FL + IL-7, 80% of the progenitors retained a myeloid potential. As much as 27% of the FL + IL-7-responsive progenitors remained uncommitted after 7 days of incubation, but all had committed to the B-cell lineage after 10 days of incubation in FL + IL-7. These results show that FL much more potently and selectively than SCF synergizes with IL-7 to enhance B-cell commitment and development from uncommitted progenitor cells. (C) 1996 by The American Society of Hematology.
