THE MPI RECEPTOR IS EXPRESSED IN THE MEGAKARYOCYTIC LINEAGE FROM LATEPROGENITORS TO PLATELETS

The Mpl receptor (Mpl-R) is a cytokine receptor belonging to the hemat opoietin receptor superfamily for which a ligand has been recently cha racterized. To study the lineage distribution of Mpl-R in normal hemat opoietic cells, we developed a monoclonal antibody (designated M1 MoAb ) by immunizing mice with a soluble form of the human Mpl-R protein. W ith few exceptions, Mpl-R was detected by indirect immunofluorescent a nalysis on all human leukemic hematopoietic cell lines with pluripoten tial and megakaryocytic phenotypes, but not on other cell lines. By im munoprecipitation and immunoblotting, M1 MoAb recognized a band at 82 to 84 kD corresponding to the expected size of the glycosylated recept or. Among normal hematopoietic cells, M1 MoAb strongly stained megakar yocytes (MK) and Mpl-R was detected on platelets by indirect immunoflu orescence staining or immunoblotting. On purified CD34(+) cells, less than 2% of the population was stained, but the labeling was weak and j ust above the threshold of detection. However, dual-labeling with the M1 and antiplatetet glycoprotein MoAbs showed that most Mpl-R(+)/CD34( +) cells coexpressed CD41a, CD61, or CD42a, suggesting that cell surfa ce appearance of Mpl-R and platelet glycoproteins could be coordinated . M1-positive and M1-negative subsets were sorted from purified CD34() cell populations. Colony assays showed that the absolute number of h ematopoietic progenitors was extremely low and no primitive progenitor s were present in the CD34(+)/ Mpl-R(+) fraction. However, this cell f raction was significantly enriched in low proliferative colony-forming units-MK. When the CD34(+)/Mpl-R(+) fraction was grown in liquid cult ure containing human aplastic serum and a combination of growth factor s, mature MK were seen as early as day 4, whereas the predominant cell population was erythroblasts on day 8. Similar data were also obtaine d with the CD34(+)/Mpl-R(-)fraction with, however, a delay in the time of appearance of both MK and erythroblasts. In conclusion, Mpl-R is a cytokine receptor restricted to the MK cell lineage. Its expression i s low on CD34(+) cells and these cells mainly correspond to late MK pr ogenitors and transitional cells. These data indicate that the action of the Mpl-R ligand might predominate during the late stages of human MK differentiation. (C) 1995 by The American Society of Hematology.