An agonist murine monoclonal antibody to the human c-Mpl receptor stimulates megakaryocytopoiesis.

Thrombopoietin (TPO) is a hematopoietic growth factor that stimulates megakaryocytopoiesis and platelet production in vivo and promotes the development of identifiable megakaryocytes in vitro. We have developed a murine monoclonal antibody, BAH-1, raised against human megakaryocytic cells, which specifically recognizes the c-Mpl receptor and shows agonist activity by stimulating megakaryocytopoiesis in vitro. BAH-1 antibody specifically binds to platelets and to recombinant c-Mpl with high affinity. Similar to TPO, BAH-1 alone supported the formation of colony-forming unit-megakaryocyte (CFU-MK) colonies. The combination of BAH-1 plus interleukin-3 or of BAH-1 plus human TPO significantly increased the number of human CFU-MK colonies. In addition, BAH-1 monoclonal antibody stimulated the proliferation and maturation of primary bone marrow megakaryocytes in a dynamic heterogeneous liquid culture system. Individual large megakaryocytes as well as small megakaryocytic cells were observed in cultures of CD34(+) CD41(+) cells in the presence of BAH-1 antibodies. Similar to TPO, BAH-1 antibody induced a significant response of murine immature megakaryocytes as observed by an increase in the detectable numbers of acetylcholinesterase-positive megakaryocytes. No effects of BAH-1 antibody were observed on colony-forming unit-granulocyte-macrophage, burst-forming unit-erythroid, or colony-forming unit-erythroid colonies. In vivo studies showed that BAH-1, alone or in combination with TPO, expands the numbers of megakaryocytic progenitor cells in myelosuppressed mice. This antibody should prove useful in understanding the structure-function aspects of the c-Mpl receptor as well as in evaluating the effects of the sustained activation of this receptor in preclinical models of severe thrombocytopenia.