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Granulation in livers of mice infected with Salmonella typhimurium is caused by superoxide released from host
K Umezawa1, N Ohnishi, K Tanaka
1Department of Infectious Diseases, Tokai University School of Medicine, Kanagawa, Japan.
Abstract:
The pathophysiological roles of superoxide (O2.-) at the site of infection of facultative intracellular bacteria were examined in this study. To evaluate the actual in vivo generation of the superoxide, an ex vivo chemiluminescence assay was newly developed. When ICR mice were infected with a sublethal dose (8 x 10(4) CFU) of Salmonella typhimurium, the number of bacteria in the liver reached its peak at 5 days after infection (10(5.05) CFU/g of liver) and decreased thereafter. At 21 days after infection, the bacteria became undetectable. On the other hand, phorbol myristate 13-acetate-stimulated O2.- generation reached a maximum at 7 days after infection (mean photon count, 1,249 cps versus 28.8 cps before infection; n = 4) and decreased thereafter to a level similar to that before infection at 21 days after infection (28.8 cps). Histological examinations revealed that the total area of the lesions reached a peak at 7 days after infection (7.2 x 10(4) microns 2/10 visual fields). In the early phase, a microabscess with infiltration of polymorphonuclear cells was noted, and then, in the late stage, the lesion was replaced by granulation with mononuclear cell infiltration. When microscopic lesions were measured histologically, a significant correlation between the area of the lesions and phorbol myristate 13-acetate-stimulated O2.- generation was observed, which suggested that superoxide was responsible for the generation of the lesions. Modified superoxide dismutase, i.e., alpha-4-([6-(N-maleimido)hexanoyloxymethyl] cumyl)half-butyl-esterified poly(stylrene-co-malelic acid)-conjugated superoxide dismutase (SM-SOD), was then applied. When SM-SOD was administered to suppress the O2.- generation in vivo, the number of bacteria increased (10(6.1) CFU). However, the lesion formation was inhibited (total lesion area, 0.3 x 10(4) microns 2). These results suggest that the establishment of the microabscess and granuloma formation after S. typhimurium infection is not due to the bacteria per se but rather to the O2.- from the host's phagocytes. Two aspects of the O2.-, i.e., the bactericidal role and the tissue-injurious effect, were clearly demonstrated in this study. Therefore, the information obtained from these results is useful in designing treatment strategy for similar kinds of infection.
Insights
Superoxide (O2.-) generated by host phagocytes causes tissue damage during Salmonella typhimurium infection, not the bacteria itself. Inhibiting superoxide reduced lesions but increased bacterial load, highlighting its dual role.
Area of Science:
- Immunology
- Microbiology
- Pathophysiology
Background:
- Facultative intracellular bacteria infections involve complex host-pathogen interactions.
- Superoxide (O2.-) is a reactive oxygen species produced by phagocytes, with roles in both host defense and tissue damage.
- Understanding superoxide's specific role in bacterial infection is crucial for developing targeted therapies.
Purpose of the Study:
- To investigate the pathophysiological roles of superoxide (O2.-) at the site of facultative intracellular bacterial infection.
- To evaluate the in vivo generation of superoxide during Salmonella typhimurium infection.
- To elucidate the contribution of host-derived superoxide to lesion formation and bacterial control.
Main Methods:
- Developed a novel ex vivo chemiluminescence assay to measure in vivo superoxide generation.
- Infected ICR mice with Salmonella typhimurium and monitored bacterial load, superoxide generation, and lesion development over time.
- Administered modified superoxide dismutase (SM-SOD) to inhibit superoxide production and assessed its effects on bacterial burden and lesion formation.
Main Results:
- Superoxide generation and lesion area peaked around 7 days post-infection, correlating significantly with each other.
- Inhibition of superoxide production using SM-SOD led to increased bacterial numbers but significantly reduced lesion formation.
- Histological analysis showed microabscesses with polymorphonuclear cells in early stages, transitioning to granulation with mononuclear cells in later stages.
Conclusions:
- Host-derived superoxide (O2.-) from phagocytes, rather than the bacteria itself, is responsible for microabscess and granuloma formation during S. typhimurium infection.
- Superoxide exhibits a dual role: bactericidal activity and tissue-injurious effects.
- These findings provide valuable insights for designing treatment strategies for infections involving similar pathophysiological mechanisms.