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

Area of Science:

• Immunology
• Microbiology
• Biochemistry

Background:

• The complement system is crucial for innate immunity against Gram-negative bacteria like Pseudomonas aeruginosa.
• Bacterial pathogens often develop mechanisms to evade complement-mediated immune responses.
• The Pseudomonas aeruginosa alkaline protease (AprA) is implicated in virulence but its role in complement evasion is unclear.

Purpose of the Study:

• To investigate how AprA influences complement activation and bacterial evasion.
• To determine if AprA inhibits complement-dependent neutrophil functions.
• To elucidate the specific complement pathways targeted by AprA.

Main Methods:

• Assessing AprA's effect on phagocytosis and bacterial killing by human neutrophils.
• Measuring C3b opsonization and C5a generation in the presence of AprA.
• Analyzing complement pathway activation (classical, lectin, alternative) with AprA.
• Conducting serum degradation and complement repletion assays to identify AprA's target.

Main Results:

• AprA significantly inhibited phagocytosis and killing of Pseudomonas by neutrophils.
• AprA blocked C3b deposition and C5a formation, crucial for immune cell recruitment and bacterial clearance.
• AprA specifically impaired the classical and lectin complement pathways, leaving the alternative pathway intact.
• AprA was found to degrade C1s and C2, with C2 cleavage identified as the primary mechanism of complement inhibition.

Conclusions:

• Pseudomonas aeruginosa AprA is a potent inhibitor of complement activation.
• AprA interferes with the classical and lectin pathways by cleaving the C2 component.
• This mechanism allows Pseudomonas aeruginosa to evade crucial innate immune defenses mediated by complement and neutrophils.