We have studied the effects of recombinant human thrombopoietin (TPO, Mpl ligand) on human hematopoiesis in vitro. TPO alone did not support erythroid burst formation but, in the presence of erythropoietin, it enhanced erythroid burst formation from CD34+ bone marrow and cord blood cells. The burst- promoting activity (BPA) was stronger under 5% serum than 30% serum conditions. The direct nature of BPA effects was documented by replating studies of early erythroid bursts. The BPA of TPO was less than that of interleukin-3 but was comparable with that of granulocyte/macrophage colony- stimulating factor and steel factor. The soluble form of Mpl receptor inhibited burst enhancing effects of TPO, suggesting that the BPA effects of TPO are mediated through the Mpl receptor. These results further delineate the physiologic roles of TPO and may aid in the determination of the clinical usages of TPO.
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