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

Deniz Ugurlar1, Stuart C Howes2, Bart-Jan de Kreuk3

  • 1Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Department of Chemistry, Faculty of Science, Utrecht University, Padualaan 8, 3584 CH Utrecht, Netherlands.

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This study reveals how the C1 complex initiates the complement cascade by binding to antibodies. Cryo-electron microscopy shows C1q arm condensation and protease rearrangement upon antibody binding, clarifying immune response triggers.

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Area of Science:

  • Immunology
  • Structural Biology
  • Biochemistry

Background:

  • The complement cascade is crucial for innate immunity and pathogen clearance.
  • Initiation of the complement cascade by the C1 complex is triggered by microbial or host cell damage patterns.
  • The precise mechanism of complement initiation by C1 remains incompletely understood.

Purpose of the Study:

  • To elucidate the structural mechanisms by which the C1 complex initiates the complement cascade.
  • To investigate the interactions between C1 and antibodies, specifically immunoglobulin G1 (IgG1).

Main Methods:

  • Cryo-electron microscopy (cryo-EM) was used to analyze C1 complex structures bound to monoclonal antibodies.
  • Functional analysis of IgG1 mutants was employed to validate observed interactions.

Main Results:

  • Heterogeneous structures of single and clustered C1-IgG1 hexamer complexes were observed.
  • Specific binding sites of C1q on IgG1 Fc-CH2 domains were identified, including novel interactions.
  • Antibody binding induced C1q arm condensation, rearrangements of C1r2s2 proteases, and tilting of the C1q stalk.
  • Data suggest C1r activation of C1s can occur within single strained C1 complexes or between adjacent complexes.

Conclusions:

  • Antibody binding to C1 induces conformational changes that likely lead to complement cascade initiation.
  • The findings provide a structural basis for understanding how C1 activation occurs on surfaces.
  • This work clarifies a critical step in innate immune response activation via the classical complement pathway.