Complement fragments, alveolar macrophages, and alveolitis

Area of Science:

• Immunology
• Pulmonary Medicine
• Cell Biology

Background:

• Neutrophil infiltration into alveoli is a critical process in lung inflammation.
• The role of complement system activation in this process requires further elucidation.

Purpose of the Study:

• To investigate the mechanisms of neutrophil infiltration into rabbit alveoli.
• To determine the specific roles of complement C5 and its fragments in inducing alveolitis.

Main Methods:

• Investigating complement activation effects on pulmonary vascular margination and alveolar infiltration in rabbits.
• Administering C5 fragments directly into airways to assess neutrophil attraction.
• Analyzing the production of neutrophil chemoattractant factors by pulmonary macrophages.
• Detecting active C5 in lavage fluids and identifying macrophage-derived C5 cleaving enzymes.

Main Results:

• Circulating complement activation caused pulmonary vascular margination but minimal alveolar infiltration.
• Instillation of C5 fragments into airways significantly attracted neutrophils into alveolar spaces.
• The C5 fragment C5a des Arg was more potent than C5a in neutrophil attraction.
• C5 fragments stimulated pulmonary macrophages to produce neutrophil chemoattractants.
• Active C5 was present in lavage fluids, and macrophage enzymes capable of cleaving C5 were identified.

Conclusions:

• Complement C5 fragments, particularly C5a des Arg, play a direct role in attracting neutrophils to the alveoli.
• Pulmonary macrophages may act indirectly by producing chemoattractants in response to C5 fragments.
• A proposed pathway for neutrophil infiltration involves migration through the alveolar capillary wall and subsequent penetration of the alveolar epithelium.