Alternative complement pathway activation by Salmonella O polysaccharide as a virulence determinant in the mouse
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1National Public Health Institute, Helsinki, Finland.
Abstract:
The quality of Salmonella O polysaccharide (the O antigen) is a virulence factor in mouse salmonellosis. It affects the rate by which these bacteria are phagocytosed and by which they activate the alternative complement pathway in a manner inversely proportional to their virulence, suggesting that the rate of complement activation is crucial for the fate of the bacteria in the mouse. The effector mechanism has, however, remained open since Salmonellae survive and multiply in the macrophages of the mouse. We show in this study that although the least virulent O-6,7 Salmonellae multiply in the liver macrophages they are rapidly killed in the peritoneal cavity by the local resident macrophages. Electron microscopy showed a striking morphological feature--a 35 nm thick homogenous electron-dense deposit--on all the bacteria found in association with the macrophages but absent from all non-cell-associated bacteria. A similar precipitate was formed by incubating the bacteria in fresh mouse serum and was dependent on heat-labile serum components and bound anti-C3. The least virulent O-6,7 bacteria acquired this deposit more rapidly and in a lower concentration of serum than the more virulent O-4,12 bacteria consistent with the previously demonstrated difference between these bacteria in their rate of complement activation via the alternative pathway. Preincubation of the O-4,12 bacteria in fresh mouse serum leading to complement deposition on 80% of the bacteria effectively opsonized them for rapid killing in the peritoneal cavity. These data for the first time demonstrate how the rate of complement activation determines the virulence of Salmonellae.
Insights
Salmonella's O antigen quality impacts virulence by influencing complement activation. Rapid complement deposition leads to bacterial killing, revealing a key mechanism in Salmonella infection.
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.