title: C1q and C4b bind simultaneously to CR1 and additively support erythrocyte adhesion.
abstract: Previously, we showed that soluble C1q bound specifically to CR1 on transfected cells. If the CR1-C1q interaction were to participate in immune complex clearance, then this interaction should support E adhesion. Using a tip plate adhesion assay, we found that immobilized C1q mediated adhesion of human E. E binding to C1q was specifically inhibited by polyclonal anti-CR1 Fab fragments. Intact C1 was not efficient as an adherence ligand until it was treated with EDTA or the C1 inhibitor to remove the C1r2C1s2 complex from C1, leaving C1q. Titration of C1q alone, C4b alone, and C1q + C4b indicated that the two complement ligands were additive in their ability to support CR1-mediated adhesion of E. Analysis of binding to immobilized CR1 using a BIAcore instrument documented that C1q, C4b, and C3b binding were independent events. Additionally, C1q-dependent binding of immune complexes and heat-aggregated IgG to E was documented. These experiments confirm that the immune adherence receptor in humans, CR1, is the single receptor for all of the opsonic ligands of complement, provide evidence for a single C1q binding site on LHR-D of CR1, and suggest that C1q may participate in immune clearance.
authors.affiliation: Department of Medicine, Harvard Medical School, Charles A. Dana Research Institute, Beth Israel Deaconess Medical Center, Boston, MA 02215, USA.
authors.forename: S W
authors.initials: SW
authors.lastname: Tas
authors.forename: L B
authors.initials: LB
authors.lastname: Klickstein
authors.forename: S F
authors.initials: SF
authors.lastname: Barbashov
authors.forename: A
authors.initials: A
authors.lastname: Nicholson-Weller
mesh_terms: D000936:Antigen-Antibody Complex / Q000378:metabolism; D002448:Cell Adhesion / Q000187:drug effects / Q000276:immunology; D003094:Collagen / Q000378:metabolism; D003172:Complement C1 / Q000378:metabolism; D003174:Complement C1 Inactivator Proteins / Q000494:pharmacology; D015922:Complement C1q / Q000378:metabolism*; D015935:Complement C4b / Q000378:metabolism*; D003165:Complement System Proteins / Q000378:metabolism; D004492:Edetic Acid / Q000494:pharmacology; D004912:Erythrocytes / Q000276:immunology* / Q000378:metabolism*; D006358:Hot Temperature; D006801:Humans; D007140:Immunoglobulin Fab Fragments / Q000494:pharmacology; D007074:Immunoglobulin G / Q000378:metabolism; D007700:Kinetics; D008024:Ligands; D010446:Peptide Fragments / Q000276:immunology / Q000378:metabolism; D011485:Protein Binding / Q000276:immunology; D017463:Receptors, Complement 3b / Q000276:immunology / Q000378:metabolism*; D013745:Tetanus Toxoid / Q000378:metabolism
chemical_list: D000936:Antigen-Antibody Complex; D003172:Complement C1; D003174:Complement C1 Inactivator Proteins; D007140:Immunoglobulin Fab Fragments; D007074:Immunoglobulin G; D008024:Ligands; D010446:Peptide Fragments; D017463:Receptors, Complement 3b; D013745:Tetanus Toxoid; D015922:Complement C1q; D015935:Complement C4b; D003094:Collagen; D003165:Complement System Proteins; D004492:Edetic Acid