Compared effects of Mpl ligand and other cytokines on human MK differentiation

The discovery of the Mpl ligand (Mpl-L), also called thrombopoietin (TPO), has facilitated in vitro investigation of human megakaryocytopoiesis. By confocal microscopy, endomitosis appeared as abortive mitosis skipping late stages of mitosis. No telophase and cytokinesis were observed. A spherical multipolar spindle which limits chromatid segregation was observed. The nuclear envelope subsequently reformed isolating all chromatids in a single nucleus. Platelet shedding was ultrastructurally studied. Platelet release occurred after formation of long cytoplasmic extensions (proplatelet formation), constriction areas delineating platelet territories. Heterogeneity in platelet size may be determined by the length of these extensions. Pegylated-recombinant human megakaryocyte growth and development factor, a truncated form of Mpl-L, was the most efficient cytokine to produce proplatelet-bearing megakaryocytes (MKs) and platelets in vitro. However, functional platelets with a normal ultrastructure could be produced in the presence of a combination of other cytokines. Finally, we investigated whether the induction of MK differentiation by the MS-5 stromal cell lines is due to Mpl-L. MS-5 cells synthesized Mpl-L transcripts and a biologically active protein. When human CD34+ cells were grown in contact or noncontact cultures with MS-5 cells, MK differentiation was observed. Soluble Mpl receptor (sMpl-Fc) addition inhibited MK growth, suggesting that the MK-promoting activity was due to Mpl-L production. Marrow stromal cell lines derived from TPO-/- mice were also able to sustain MK growth. Despite the absence of any production of Mpl-L, the sMpl-Fc continued to inhibit MK differentiation. This result suggests that the sMpl has a direct inhibitory effect and may explain the divergent results in the literature concerning the precise role of Mpl-L on the MK terminal differentiation.