The alternative activation pathway and complement component C3 are critical for a protective immune response against...

Area of Science:

• Immunology
• Microbiology
• Pulmonary Medicine

Background:

• Pseudomonas aeruginosa is a significant pathogen causing hospital-acquired pneumonia and infecting most cystic fibrosis patients.
• The role of the complement system and its activation pathways in host defense against P. aeruginosa pulmonary infection requires further elucidation.

Purpose of the Study:

• To investigate the role of complement activation pathways in host defense against P. aeruginosa pulmonary infection.
• To determine the specific contribution of the alternative pathway to survival during P. aeruginosa infection.

Main Methods:

• Intranasal inoculation of P. aeruginosa in C3-, C4-, and factor B-deficient mice.
• Mortality rates and bacterial loads in lungs and blood were assessed.
• In vitro phagocytosis assays using C3-deficient serum were performed.
• Inflammatory cytokine and chemokine levels in lung tissue were measured.

Main Results:

• C3-deficient mice exhibited significantly higher mortality rates and bacterial loads compared to wild-type mice.
• Factor B-deficient mice, but not C4-deficient mice, showed mortality rates similar to C3-deficient mice.
• In vitro studies demonstrated impaired P. aeruginosa binding and ingestion by phagocytic cells in the absence of C3.
• C3-deficient mice had increased neutrophils and elevated levels of inflammatory cytokines (IL-1beta, IL-6, IL-10) and chemokines (KC, MIP-2) in the lungs.

Conclusions:

• The alternative pathway of complement activation is essential for host defense against P. aeruginosa pulmonary infection.
• Complement component 3 (C3) plays a critical role, primarily through C3-mediated opsonization and phagocytosis, in protecting against P. aeruginosa.
• These findings highlight the importance of the complement system in managing bacterial lung infections.