Human p85 glycoprotein bears three distinct epitopes defined by several monoclonal antibodies

A glycoprotein of apparent mol. wt 85,000 isolated from human cells of B lineage by affinity to 50B4-IgG immunoadsorbent was shown previously to express two spatially distinct epitopes identified with monoclonal antibodies 50B4 and 50E6 [Letarte et al. (1985) Molec. Immun. 22, 113-124]. It is now demonstrated that the p85 glycoprotein is homologous to the F10-44-2 antigen, defined initially as a T-lymphocyte-granulocyte-brain antigen [Dalchau et al. (1980) Eur. J. Immun. 10, 745-749], the A1G3 human medullary thymocyte antigen [Haynes et al. (1983) J. Immun. 131, 1195-1200] and the A3D8 antigen defined on erythrocytes [Telen et al. (1983) J. clin. Invest. 71, 1878-1886]. The 50B4 antigen purified either from a B lymphoblastoid cell line or a leukemic cell line, at a concn ranging from 0.25 to 2.0 micrograms protein/ml, could block the binding of either F10-44-2 and F-10-62-1 (a related antibody) or A1G3 and A3D8 antibodies to human leukemic cells. All of these antibodies immunoprecipitated, from the purified antigenic preparation, a single glycosylated polypeptide chain of apparent mol. wt 85,000. Competitive binding studies indicated that these antibodies define at least three epitopes. The F10-44-2 and A3D8 antibodies blocked the reactivity of monoclonal 50E6-IgG with human leukemic cells and thus bind to an epitope identical or spatially related to the 50E6 epitope. Antibody F10-62-1 competed for the binding of 50B4-IgG to leukemic cells and thus recognizes the 50B4-like epitope. The A1G3 antibody blocked 30% of the binding of 50E6-IgG but did not inhibit the binding of 50B4-IgG: it is thus reacting with a third epitope on the p85 glycoprotein distinct from but close to the 50E6 epitope.