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 proliferati on and exclusive myeloid differentiation of multipotent bone marrow pr ogenitor cells. The ligand for c-kit (stem cell factor [SCF]) plays a key role in stimulating myeloid and erythroid cell production of primi tive hematopoietic progenitors. SCF in combination with interleukin-7 (IL-7) can also stimulate the combined myeloid and B-cell differentiat ion of uncommitted hematopoietic progenitor cells as well as the growt h 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 uncomm itted progenitors, was investigated for its ability to induce selectiv e stroma-independent B-cell commitment from uncommitted Lin-Sca-li bon e 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 resulte d in clonal growth of 10% of Lin(-)Sca-1(+) bone marrow cells. After 1 2 days of incubation of Lin(-)Sca-1(+) cells in FL + IL-7, an almost 4 00-fold increase in cell production was observed. Phenotyping showed t hat greater than 99% expressed 8220, but not cell surface markers spec ific for myeloid, erythroid, or T-cell lineages. furthermore, the cell s did not express cytoplasmic mu-heavy chain (c mu) or surface IgM, bu t were positive for CD24 (heat stable antigen [HSA]) and CD43 (leukosi alin), suggesting that the cells produced were blocked at a late pro-B -cell stage. Interestingly, although all FL + IL-7-responsive Lin(-)Sc a-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 stimulati ng 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. Repl ating studies showed that FL + IL 7-responsive Lin-Sca-1(+) progenitor s were not committed to the 8-cell lineage, because after 2 days of in cubation in FL + IL-7, 80% of the progenitors retained a myeloid poten tial. 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 wi th IL-7 to enhance B-cell commitment and development from uncommitted progenitor cells. (C) 1996 by The American Society of Hematology.