The Mpl-ligand is involved in the growth-promoting activity of the murine stromal cell line MS-5 on ES cell-derived hematopoiesis.

We investigated the ability of the murine stromal cell line MS-5 to enhance the hematopoietic potential of embryonic stem (ES) cells. The presence of increasing concentrations of MS-5 cells during the differentiation of ES cells into embryoid bodies (EBs) resulted in a positive dose effect on the efficiency of EB development. Moreover, the number of myeloid progenitors derived from EBs at days 6 and 10 of differentiation significantly increased. This increase resulted from an elevation of both the proportions of positive EBs (EBs containing at least one progenitor each) and the progenitor cell content per positive EB. The stimulatory activity of MS-5 cells affected all types of myeloid progenitors except erythroid progenitors, which were depressed. However, the relative numbers of ES-derived granulocyte-macrophage progenitors (colony-forming units granulocyte/macrophage [CFU-GM], -macrophage [CFU-M], and -granulocyte [CFU-G]) and of mixed cell colonies were unchanged. In contrast, the incidence of megakaryocytic progenitors (colony-forming units-megakaryocyte [CFU-MK]) was significantly increased, that of erythroid progenitors (burst-forming units-erythroid [BFU-E]) was concomitantly decreased, and the total numbers of both progenitor types remained constant. Addition of Mpl-ligand (Mpl-L; thrombopoietin) during the growth of EBs was found to mimic the effect of the MS-5 cell line on the output of progenitor cells. No effect of Mpl-L on the efficiency of EB formation was observed. In addition, supplementation of cultures with sufficient soluble Mpl to abrogate Mpl-L activity resulted in the reversion of the quantitative and qualitative effects of MS-5 cells on progenitor cell formation but not on the efficiency of EB formation. Together, these data indicate two major effects and two levels of action of the MS-5 cell line on hematopoietic differentiation of ES cells. First, the cell line acts before hematopoietic determination, promoting the plating efficiency of ES cells via mechanisms that remain to be clarified. Second, at a later stage of differentiation, the MS-5 cells promote hematopoiesis within EBs. Mpl-L appears to be one of the components that confer this latter ability on the MS-5 cell line.