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

Proteins binding the third 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 NaDodSO₄/PAGE, and autoradiographs of these gels indicated that C3-binding proteins could be divided into three groups based on M(r): (i) gp200, an ~ 200,000 M(r) molecule previously identified as the C3b/C4b receptor or CR1; (ii) gp140, an ~ 140,000 M(r) molecule previously identified as the C3d receptor or CR2; and (iii) gp45-70, a heretofore unrecognized group of 45,000-70,000 M(r) C3-binding molecules. The cell distribution, M(r), 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 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 M(r) 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 M(r) is present on rabbit alveolar macrophages. We conclude that gp45-70 is an additional group of membrane proteins present on human leukocytes that possess ligand-binding activity for C3b.