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Ionic binding of C3 to the human pathogen Moraxella catarrhalis is a unique mechanism for combating innate immunity

Therése Nordström1, Anna M Blom, Thuan Tong Tan

  • 1Medical Microbiology, Lund University, Malmo University Hospital, Malmo, Sweden.

Journal of Immunology (Baltimore, Md. : 1950)
|September 9, 2005
PubMed

Insights

Moraxella catarrhalis surface proteins UspA1 and UspA2 uniquely bind and neutralize complement component C3, inhibiting the innate immune system. This mechanism helps the bacteria evade complement-mediated destruction by human serum.

Area of Science:

  • Immunology
  • Microbiology
  • Bacteriology

Background:

  • Moraxella catarrhalis is a human pathogen that can cause respiratory infections.
  • The complement system is a crucial part of the innate immune system that defends against pathogens.
  • Ubiquitous surface proteins A1 and A2 (UspA1/A2) of M. catarrhalis are known to interfere with complement activation.

Purpose of the Study:

  • To investigate the interaction between M. catarrhalis UspA1/A2 and the third component of complement (C3).
  • To determine if this interaction is unique to M. catarrhalis and its role in immune evasion.

Main Methods:

  • Binding assays using recombinant UspA1/A2 proteins and M. catarrhalis mutants.
  • Experiments with EDTA-treated and methylamine-treated serum to assess C3 binding.
  • Complement activation assays and serum bactericidal assays.

Main Results:

  • M. catarrhalis UspA1 and UspA2 bind C3 noncovalently and in a dose-dependent manner.
  • This C3 binding neutralizes complement activation, with UspA2 being primarily responsible.
  • Other Moraxella subspecies and pathogenic bacteria tested did not exhibit C3 binding.

Conclusions:

  • M. catarrhalis possesses a unique mechanism to evade innate immunity by binding and neutralizing C3 via UspA1/A2.
  • UspA2 plays a significant role in M. catarrhalis's resistance to complement-mediated lysis.
  • This finding highlights a novel immune evasion strategy employed by M. catarrhalis.

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