Structures of C1-IgG1 provide insights into how danger pattern recognition activates complement

• Published in: Science (American Association for the Advancement of Science) Vol. 359; no. 6377; pp. 794 - 797
• Main Authors: Ugurlar, Deniz, Howes, Stuart C, de Kreuk, Bart-Jan, Koning, Roman I, de Jong, Rob N, Beurskens, Frank J, Schuurman, Janine, Koster, Abraham J, Sharp, Thomas H, Parren, Paul W H I, Gros, Piet
• Format: Journal Article
• Language: English
• Published: United States The American Association for the Advancement of Science 16.02.2018
• Subjects: Alarmins - chemistry; Alarmins - ultrastructure; Antibodies, Monoclonal - chemistry; Antibodies, Monoclonal - ultrastructure; Avidity; Binding Sites; Complement; Complement Activation; Complement C1 - chemistry; Complement C1 - ultrastructure; Complement component C1; Complement component C1q; Cryoelectron Microscopy; Damage; Electron microscopy; Hazards; Hexamers; Humans; Immune clearance; Immune response; Immune system; Immunoglobulin G; Immunoglobulin G - chemistry; Immunoglobulin G - genetics; Immunoglobulin G - ultrastructure; Immunoglobulins; Initiation complex; Innate immunity; Microscopy; Monoclonal antibodies; Mutagenesis; Pattern recognition
• ISSN: 0036-8075; 1095-9203
• DOI: 10.1126/science.aao4988

Danger patterns on microbes or damaged host cells bind and activate C1, inducing innate immune responses and clearance through the complement cascade. How these patterns trigger complement initiation remains elusive. Here, we present cryo-electron microscopy analyses of C1 bound to monoclonal antibodies in which we observed heterogeneous structures of single and clustered C1-immunoglobulin G1 (IgG1) hexamer complexes. Distinct C1q binding sites are observed on the two Fc-CH2 domains of each IgG molecule. These are consistent with known interactions and also reveal additional interactions, which are supported by functional IgG1-mutant analysis. Upon antibody binding, the C1q arms condense, inducing rearrangements of the C1r s proteases and tilting C1q's cone-shaped stalk. The data suggest that C1r may activate C1s within single, strained C1 complexes or between neighboring C1 complexes on surfaces.