MB1, a quail leukocyte-endothelium antigen: further characterization of soluble and cell-associated forms

A monoclonal antibody (anti-MB1), raised in response to the quail immunoglobulin mu chain, binds to a surface marker on hemopoietic and endothelial cells that is expressed throughout ontogeny and adult life. Besides its cellular localization, the MB1 epitope, which is glycosylated, is present on plasma components, which have been identified as the mu chain of immunoglobulin and the proteinase inhibitor alpha 2-macroglobulin (alpha 2-M). From previous studies, it was established that the MB1 material from the surface of lymphocytes and endothelial cells comprises a set of acidic glycoproteins of Mr ranging from 80,000 to 240,000, and that cultured endothelial cells secrete a component with a molecular weight under reducing conditions close to that of the alpha 2-M subunit. In the present study the cell-associated, surface and secreted MB1 components have been further characterized biochemically and compared with serum alpha 2-M. We present evidence for the absence of a structural relationship between the different MB1 glycoproteins expressed by a given cell type as well as between cell-associated and serum material. Conversely, it appears that at least one of the cell surface components, of Mr 100-110,000 is common to T lymphocytes, B lymphocytes and endothelial cells.