title: The distortive mechanism for the activation of complement component C1 supported by studies with a monoclonal antibody against the "arms" of C1q.
abstract: A mouse monoclonal antibody (IgG1 isotype) against human C1q (MAb 130) is presented that activates C1 in serum through its antigen-binding sites at an optimal molar ratio of 3 MAbs:1 C1q. The antibody does not inhibit binding of C1q to IgG. Experiments with pepsin- and collagenase-digested C1q showed that MAb 130 binds to the fibril-like strands (arms) of C1q, close to the globular heads. Bivalency of MAb 130 was a requirement for C1-activation, but not for binding to C1q. Increasing the segmental flexibility of the intact antibody by reduction and alkylation destroyed its capacity to activate C1. A MAb against the globular heads of C1q completely inhibited C1-activation by aggregated IgG (AHG), but did not prevent activation by MAb 130. C1, reconstituted by adding C1q-stalks that lack the globular heads to C1q-depleted serum was not activated by AHG, whereas activation by MAb 130 was not affected. Activation of serum-C1 by AHG and MAb 130 was inhibited by addition of excess purified C1-inhibitor in a comparable and dose-dependent manner. Sucrose-gradient analysis indicated a predominance of stable complexes of a single C1q-molecule with three MAbs at the optimal activating ratio. When isolated and added to C1q-depleted serum, these complexes activated C1 efficiently. A mechanism for activation by MAb 130 is proposed that supports the "distortive" model of C1-activation.
authors.affiliation: Central Laboratory, Netherlands Red Cross Blood Transfusion Service, University of Amsterdam.
authors.forename: R
authors.initials: R
authors.lastname: Hoekzema
authors.forename: M
authors.initials: M
authors.lastname: Martens
authors.forename: M C
authors.initials: MC
authors.lastname: Brouwer
authors.forename: C E
authors.initials: CE
authors.lastname: Hack
mesh_terms: D000911:Antibodies, Monoclonal / Q000276:immunology; D002499:Centrifugation, Density Gradient; D003166:Complement Activating Enzymes / Q000276:immunology; D003167:Complement Activation; D003172:Complement C1 / Q000276:immunology / Q000378:metabolism*; D015922:Complement C1q; D003171:Complement Pathway, Classical; D006801:Humans; D007140:Immunoglobulin Fab Fragments / Q000276:immunology; D007074:Immunoglobulin G / Q000276:immunology; D011489:Protein Denaturation; D013329:Structure-Activity Relationship
chemical_list: D000911:Antibodies, Monoclonal; D003172:Complement C1; D007140:Immunoglobulin Fab Fragments; D007074:Immunoglobulin G; D015922:Complement C1q; D003166:Complement Activating Enzymes