Combined signaling through interleukin-7 receptors and flt3 but not c-kit potently and selectively promotes B-cell commitment and differentiation from uncommitted murine bone marrow progenitor cells.

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-1+ 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% of the cells produced were of the B-cell lineage, in that they expressed B220, but not cell surface markers specific for myeloid, erythroid, or T-cell lineages. Furthermore, the cells did not express cytoplasmic mu-heavy chain (cmu) 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 B-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.