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Related Experiment Video

Updated: Nov 18, 2025

Purification and Aggregation of the Amyloid Precursor Protein Intracellular Domain
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Staphylococcal protein A inhibits complement activation by interfering with IgG hexamer formation.

Ana Rita Cruz1, Maurits A den Boer2, Jürgen Strasser3

  • 1Medical Microbiology, University Medical Center Utrecht, Utrecht University, 3584 CX Utrecht, The Netherlands.

Proceedings of the National Academy of Sciences of the United States of America
|February 10, 2021
PubMed
Summary

Staphylococcal protein A (SpA) blocks essential immunoglobulin G (IgG) hexamerization, a key step in bacterial defense. This immune evasion mechanism prevents complement activation, except for IgG3 antibodies which remain effective against Staphylococcus aureus.

Keywords:
IgG hexamerizationStaphylococcus aureusantibodiescomplementstaphylococcal protein A

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Area of Science:

  • Immunology
  • Microbiology
  • Structural Biology

Background:

  • Immunoglobulin G (IgG) is crucial for combating bacterial infections.
  • IgG-mediated immunity relies on complement activation, triggered by IgG hexamer formation.
  • Staphylococcal protein A (SpA) is a virulence factor of Staphylococcus aureus.

Purpose of the Study:

  • To investigate the mechanism by which SpA interferes with IgG function.
  • To determine if SpA blocks IgG hexamerization and subsequent complement activation.
  • To assess the efficacy of IgG3 antibodies against S. aureus in the presence of SpA.

Main Methods:

  • Native mass spectrometry
  • High-speed atomic force microscopy
  • Analysis of (IgG)6:C1q complex formation
  • Assessment of complement activation on S. aureus

Main Results:

  • SpA inhibits IgG hexamerization by binding to the Fc-Fc interaction interface.
  • SpA prevents the formation of (IgG)6:C1q complexes, blocking complement activation.
  • IgG3 antibodies effectively induce complement activation and opsonophagocytic killing despite SpA presence.

Conclusions:

  • SpA is identified as an immune evasion protein that specifically targets IgG hexamerization.
  • SpA's mechanism involves competitive binding, disrupting a critical step in antibody-dependent immunity.
  • IgG3 antibodies represent a potential therapeutic strategy against S. aureus infections, overcoming SpA-mediated immune evasion.