Alternative complement pathway activation by Salmonella O polysaccharide as a virulence determinant in the mouse

Area of Science:

• Immunology
• Microbiology
• Bacteriology

Background:

• Salmonella O polysaccharide (O antigen) quality is a known virulence factor in mouse salmonellosis.
• The O antigen's role in phagocytosis and complement activation is inversely proportional to virulence.
• The precise mechanism by which complement activation influences bacterial fate remains unclear.

Purpose of the Study:

• To elucidate the role of complement activation rate in Salmonella virulence.
• To investigate the mechanism of Salmonella killing by macrophages in the peritoneal cavity.
• To correlate O antigen structure with complement deposition and bacterial survival.

Main Methods:

• Comparative analysis of Salmonella O-6,7 (least virulent) and O-4,12 (more virulent) strains in mouse models.
• Electron microscopy to visualize bacterial-macrophage interactions and surface deposits.
• In vitro serum incubation assays to assess complement deposition.
• Assessment of bacterial killing in the peritoneal cavity post-serum preincubation.

Main Results:

• Least virulent O-6,7 Salmonellae, despite multiplying in liver macrophages, were rapidly killed by peritoneal macrophages.
• A uniform electron-dense deposit (35 nm thick) was observed on bacteria associated with macrophages.
• This deposit, similar to serum-induced precipitate, involved complement component C3 and heat-labile serum factors.
• O-6,7 Salmonellae showed faster complement acquisition and deposition at lower serum concentrations than O-4,12 strains.
• Opsonization of O-4,12 Salmonellae via complement deposition enhanced their killing in the peritoneal cavity.

Conclusions:

• The rate of alternative complement pathway activation, influenced by O antigen quality, is a critical determinant of Salmonella virulence.
• Complement deposition on Salmonellae facilitates their opsonization and subsequent killing by peritoneal macrophages.
• This study demonstrates a direct link between complement activation kinetics and Salmonella survival in vivo.