Identification of an additional class of C3-binding membrane proteins of human peripheral blood leukocytes and cell lines.

Abstract

Proteins binding the 3rd component of complement (C3) were isolated by affinity chromatography from surface-labeled solubilized membranes of human peripheral blood cells and cell lines. The isolated molecules were subjected to sodium dodecyl sulfate polyacrylamide gel electrophoresis and autoradiographs of these gels indicated that C3-binding proteins could be divided into 3 groups based on MW: (i) gp200, an .apprxeq. 200,000 MW molecule previously identified as the C3b/C4b receptor or CR1; (ii) gp140, and .apprxeq. 140,000 MW molecule previously identified as the C3d receptor or CR2; and (iii) gp45-70, a heretofore unrecognized group of 45,000-70,000 MW C3-binding molecules. The cell distribution, MW, antigenic cross-reactivity, and specificity of gp45-70 were examined. Erythrocytes have no detectable gp45-70, but all leukocyte populations examined possess this group of molecules. On neutrophils and mononuclear phagocytes, CR1 is the predominant C3-binding glycoprotein, but gp45-70 is present on both cell populations and on macrophage and neutrophil cell lines. B plus null cells, chronic lymphocytic leukemia cells, and an Epstein-Barr virus-transformed B-cell line possess CR1, CR2 and gp45-70. On T cells and T-cell lines gp45-70 is the predominant or, in some cases, the only C3-binding protein isolated. gp45-70 is structurally characterized as a broad band or doublet with a mean MW that is slightly different for each cell population. gp45-70 binds iC3, C3b, and C4b, but not C3d, indicating that the binding region is probably within the C3c portion of C3b. A polyclonal antibody to CR1 and monoclonal antibodies to CR1 and CR2 do not immunoprecipitate gp45-70. While gp45-70 has not been previously characterized on human cells, a C3b-binding glycoprotein of similar MW is present on rabbit alveolar macrophages. Evidently, gp45-70 is an additional group of membrane proteins present on human leukocytes that possess ligand-binding activity for C3b.