<i>Pseudomonas aeruginosa</i> Alkaline Protease Blocks Complement Activation via the Classical and Lectin Pathways

TLDR

The complement system rapidly detects and kills Gram‑negative bacteria, yet pathogens such as *Pseudomonas aeruginosa* evade this defense, and its alkaline protease AprA is known to interfere with complement‑mediated lysis but its precise role remains unclear. The study investigates how AprA interferes with complement activation and its potential to block complement‑dependent neutrophil functions. The authors examined AprA’s effects on complement activation pathways and neutrophil functions, focusing on its interaction with classical, lectin, and alternative pathways. AprA potently blocks neutrophil phagocytosis and killing, inhibits C3b opsonization and C5a generation, selectively suppresses classical and lectin pathway C3b deposition, degrades C1s and C2, and its complement‑inhibitory activity is mediated by cleavage of C2.

Abstract

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.