C1q and C4b bind simultaneously to CR1 and additively support erythrocyte adhesion

Previously, we showed that soluble C1q bound specifically to CR1 on transfe cted cells. If the CR1-C1q interaction were to participate in immune comple x clearance, then this interaction should support E adhesion. Using a tip p late adhesion assay, we found that immobilized C1q mediated adhesion of hum an E, E binding to C1q was specifically inhibited by polyclonal anti-CR1 Fa b fragments, Intact C1 was not efficient as an adherence ligand until it wa s treated with EDTA or the C1 inhibitor to remove the C1r(2)C1s(2) complex from C1, leaving C1q, Titration of Clq alone, C4b alone, and C1q + C4b indi cated that the two complement ligands were additive in their ability to sup port CR1-mediated adhesion of E. Analysis of binding to Immobilized CR1 usi ng a BIAcore instrument documented that C1q, C4b, and C3b binding were inde pendent events. Additionally, C1q-dependent binding of immune complexes and heat-aggregated IgG to E was documented. These experiments confirm that th e immune adherence receptor in humans, CR1, is the single receptor for all of the opsonic ligands of complement, provide evidence for a single C1q bin ding site on LHR-D of CR1, and suggest that C1q may participate in immune c learance.