Thrombopoietin, Steel Factor and the Ligand for Flt3/flk2 Interact to Stimulate the Proliferation of Human Hematopoietic Progenitors in Culture

We have tested the effects of steel factor (SF) the ligand for flt3/flk2 (FL) and thrombopoietin (TPO, Mpl ligand), on the proliferation of primitive human bone marrow progenitors in serum-deprived culture. Varying combinations of SF, FL and TPO supported formation of only few colonies from CD34+/c-Kit(low)/CD38neg/low cells. However, the addition of interleukin 3 (IL-3) to the three cytokines significantly increased the number of colonies. When this population of cells was tested in suspension culture for one week for production of colony-forming cells there was synergism among SF, FL and TPO. Addition of IL-3 to the three cytokines further increased the number of erythroid colony-forming cells. The effects of these four factors on CD34+/c-Kit(low)/CD38high cells were merely additive. Studies of individual CD34+/c-Kit(low)/CD38neg/low cells demonstrated the direct effects of SF, FL and TPO. In the presence of SF, FL and TPO, approximately half of the individual CD34+/c-Kit(low)/CD38neg/low cells proliferated in seven day suspension culture. Addition of IL-3 to the combination of SF, FL and TPO did not increase the frequencies of proliferating clones, but increased the size of individual clones. These observations suggest that SF, FL and TPO play important roles in survival and proliferation of primitive human hematopoietic progenitors.