RECOMBINANT HUMAN C-MPL LIGAND IS NOT A DIRECT STIMULATOR OF PROPLATELET FORMATION IN MATURE HUMAN MEGAKARYOCYTES

To evaluate the effect of the c-Mpl ligand on platelet production by m egakaryocytes, we investigated proplatelet formation in isolated human megakaryocytes cultured in serum-free medium, with or without the c-M pl ligand, interleukin-6 and erythropoietin. When interleukin-6 was ad ded to the culture medium, the percentage of megakaryocytes displaying proplatelets was approximately 1.5-fold the control value; whereas, i n the presence of the c-Mpl ligand, the percentage of megakaryocytes d isplaying proplatelets decreased in a dose-dependent manner and did no t increase compared to control values at any dose tested. However, the viability of megakaryocytes after a 4 d culture in the presence of th e c-Mpl ligand was significantly higher than that of the cells culture d without it. The c-Mpl ligand did not stimulate the proplatelet forma tion in megakaryocytes in vitro.