Complement activation by IgG containing immune complexes regulates the interaction of C1q with its ligands

Classical pathway activation of the compl ement system is initiated by the binding of the globular head domains of glycoprotein C1q to its corresponding ligand leading to both C1 activation and C3 convertase formation. However, the whereabouts and function of C1q after complement activation have only been marginally investigated. This report presents two mechanisms of action that remove bound C1q from a complement activating IgG immune complex in concentrated serum. The first mechanism details that sequential activation of the classical and alternative pathways releases bound C1q from an immune complex and that the dissociated C1q is subsequently found in complex with complement fragment C3c. The second mechanism is the displacement of C1q from an immune complex by the addition of near physiologic concentrations of purified or serum C1q. This activity can also be demonstrated using serum depleted of C3, normal serum chelated in EDTA, or purified C1. Fresh C1q in C3-depleted serum was found to replace dissociated C1q on the immune complex. C1q dissociated from immune complexes by the mechanism of C1q displacement is able to bind B and T lymphoblastoid cells that express receptors and ligands for both the collagen like region and the globular head domains of C1q. C1q dissociated from immune complexes by the mechanism of C3 activation do not bind these cells. This result suggests that C3 bound to C1q during complement activation and dissociation interferes with the ability of released C1q to access C1q receptors and ligands, particularly receptors for the globular head domains. These underlying mechanisms that regulate the interaction of C1q with its ligands reveal a novel function for complement activation during the immune response.