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 megakaryocy tic cells, which specifically recognizes the c-Mpl receptor and shows agonist activity by stimulating megakaryocytopoiesis in vitro. BAH-1 a ntibody 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 o f BAH-1 plus interleukin-1 or of BAH-1 plus human TPO significantly in creased the number of human CFU-MK colonies. In addition, BAH-1 monocl onal antibody stimulated the proliferation and maturation of primary b one marrow megakaryocytes in a dynamic heterogeneous liquid culture sy stem. Individual large megakaryocytes as well as small megakaryocytic cells were observed in cultures of CD34(+) CD41(+) cells in the presen ce of BAH-1 antibodies. Similar to TPO, BAH-1 antibody induced a signi ficant response of murine immature megakaryocytes as observed by an in crease in the detectable numbers of acetylcholinesterase-positive mega karyocytes. No effects of BAH-1 antibody were observed on colony-formi ng unit-granulocyte-macrophage, burst-forming unit-erythroid, or colon y-forming unit-erythroid colonies, In vivo studies showed that BAH-1, alone or in combination with TPO, expands the numbers of megakaryocyti c progenitor cells in myelosuppressed mice. This antibody should prove useful in understanding the structure-function aspects of the c-Mpl r eceptor as well as in evaluating the effects of the sustained activati on of this receptor in preclinical models of severe thrombocytopenia. (C) 1998 by The American Society of Hematology.