THE EFFECTS OF HUMAN FLT3 LIGAND ON IN-VITRO HUMAN MEGAKARYOCYTOPOIESIS

The human homolog of the murine flt3/flk2 gene product is a tyrosine k inase receptor that plays a role in regulating the proliferation and d ifferentiation of cells in the hematopoietic system. Using a plasma-cl ot clonal assay and a long-term bone marrow culture (LTBMC) system, we studied the effects of the recently cloned human flt3 ligand (FL) alo ne and in combination with granulocyte-macrophage colony-stimulating f actor (GM-CSF), interleukin-3 (IL-3), or stem cell factor (c-kit ligan d [KL]) on human megakaryocytopoiesis. The effects of FL on the primit ive megakaryocyte (MK) progenitor cell, the burst-forming unit-megakar yocyte (BFU-MK), and the more differentiated colony-forming unit-megak aryocyte (CFU-MK) were determined. FL alone had no megakaryocytic colo ny-stimulating activity (MK-CSA), but was capable of augmenting the MK -CSA of both GM-CSF and IL-3. FL synergized with IL-3 at the level of both CFU-MK and BFU-MK and with GM-CSF and KL at the level of CRI-MK. Although FL alone exhibited a limited potential in sustaining long-ter m megakaryocytopoiesis in vitro, it synergistically augmented the abil ity of IL-3 and KL, alone or in association, to promote long-term mega karyocytopoiesis. These data indicate that multiple cytokines are nece ssary to optimally stimulate the proliferation of both classes of MK p rogenitor cells and that FL plays a significant role in this process b y amplifying the MK-CSA of GM-CSF, IL-3, and KL.