Pseudomonas aeruginosa alkaline protease blocks complement activation via the classical and lectin pathways

J Immunol. 2012 Jan 1;188(1):386-93. doi: 10.4049/jimmunol.1102162. Epub 2011 Nov 30.


The complement system rapidly detects and kills Gram-negative bacteria and supports bacterial killing by phagocytes. However, bacterial pathogens exploit several strategies to evade detection by the complement system. The alkaline protease (AprA) of Pseudomonas aeruginosa has been associated with bacterial virulence and is known to interfere with complement-mediated lysis of erythrocytes, but its exact role in bacterial complement escape is unknown. In this study, we analyzed how AprA interferes with complement activation and whether it could block complement-dependent neutrophil functions. We found that AprA potently blocked phagocytosis and killing of Pseudomonas by human neutrophils. Furthermore, AprA inhibited opsonization of bacteria with C3b and the formation of the chemotactic agent C5a. AprA specifically blocked C3b deposition via the classical and lectin pathways, whereas the alternative pathway was not affected. Serum degradation assays revealed that AprA degrades both human C1s and C2. However, repletion assays demonstrated that the mechanism of action for complement inhibition is cleavage of C2. In summary, we showed that P. aeruginosa AprA interferes with classical and lectin pathway-mediated complement activation via cleavage of C2.

MeSH terms

  • Bacterial Proteins / immunology*
  • Bacterial Proteins / metabolism
  • Complement C2 / immunology
  • Complement C2 / metabolism
  • Complement C3b / immunology
  • Complement C3b / metabolism
  • Complement C5a / immunology
  • Complement C5a / metabolism
  • Complement Pathway, Mannose-Binding Lectin
  • Exopeptidases / immunology*
  • Exopeptidases / metabolism
  • Humans
  • Immune Evasion
  • Neutrophils / immunology*
  • Neutrophils / metabolism
  • Phagocytosis / immunology
  • Pseudomonas Infections / enzymology
  • Pseudomonas Infections / immunology*
  • Pseudomonas aeruginosa / enzymology
  • Pseudomonas aeruginosa / immunology*
  • Pseudomonas aeruginosa / pathogenicity*
  • Virulence Factors / immunology*
  • Virulence Factors / metabolism


  • Bacterial Proteins
  • Complement C2
  • Virulence Factors
  • Complement C3b
  • Complement C5a
  • Exopeptidases