MB1, a quail leukocyte-endothelium antigen: partial characterization of the cell surface and secreted forms in cultured endothelial cells.

We describe here conditions allowing the selective growth in culture of embryonic capillary endothelial cells from quail yolk sac. Such cultures were set up to characterize an antigen present on the endothelial cell surface and to study whether it was secreted in the culture medium. This antigen, MB1, was previously evidenced by a monoclonal antibody raised to quail IgM heavy chain. It is present at the surface of all endothelial and hemopoietic cells (except mature erythrocytes) starting from the hemangioblast, the early mesodermal precursor of blood and vascular endothelial cells. The MB1 epitope is also found on quail plasma molecules of 80 and 125-200 kDa. By immunoprecipitation of either surface or metabolically labeled endothelial cellular material, we have chemically characterized MB1-bearing components as glycoproteins of apparent molecular mass ranging from 80 to 200 kDa and provided evidence for their release into the culture medium. This is consistent with the hypothesis that, in the quail, vascular endothelium participates in the secretion of the alpha-MB1-positive plasmatic components.