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Interactions of virulent and avirulent Legionella pneumophila with human monocytes
J T Summersgill1, M J Raff, R D Miller
1Department of Microbiology and Immunology, University of Louisville School of Medicine, Kentucky.
Abstract:
Four pairs of virulent/avirulent strains of Legionella pneumophila were examined for their adherence/uptake and activation of human monocytes. Oxidative metabolic responses of monocytes were quantitated by measuring intracellular hydrogen peroxide generation using flow cytometry and by assessment of superoxide dismutase-inhibitable superoxide anion generation. All L. pneumophila strains induced less of a response than did Escherichia coli. Within each pair of isolates, virulent strains of L. pneumophila stimulated the oxidative response of monocytes less than avirulent variants. To determine effects of complement fixation by each strain on their adherence to monocytes, a phagocytic index (PI) was determined under various conditions. In autologous donor serum (AS), all L. pneumophila strains had a PI in the range of 2.1-3.1 bacteria per monocyte, with E. coli having a PI of 9.1. No significant differences were observed between virulent L. pneumophila strains and their avirulent variants. In the presence of heat-inactivated AS, all PI fell to 0.13-0.20 for the L. pneumophila strains, and to 2.16 for E. coli. Using heat-inactivated AS reconstituted with exogenous human complement as a source of opsonization, levels of PI were indistinguishable from their respective levels in AS. This suggests that complement fixation plays an important role in the adherence of virulent and avirulent L. pneumophila to human monocytes.
Insights
Virulent Legionella pneumophila strains showed less monocyte activation than avirulent ones. Complement fixation significantly enhances the adherence of both virulent and avirulent L. pneumophila to human monocytes.
Area of Science:
- Immunology
- Microbiology
Background:
- Legionella pneumophila is a bacterium that can cause severe pneumonia.
- Human monocytes play a crucial role in the innate immune response against bacterial infections.
Purpose of the Study:
- To investigate the adherence and activation of human monocytes by virulent and avirulent strains of Legionella pneumophila.
- To determine the role of complement fixation in the interaction between L. pneumophila and human monocytes.
Main Methods:
- Quantification of monocyte oxidative metabolic responses (hydrogen peroxide and superoxide anion generation) using flow cytometry.
- Determination of phagocytic index (PI) under various serum conditions (autologous, heat-inactivated, and complement-reconstituted).
Main Results:
- All L. pneumophila strains induced lower monocyte oxidative responses compared to Escherichia coli.
- Virulent L. pneumophila strains stimulated less monocyte oxidative response than their avirulent counterparts.
- Complement fixation was found to be critical for the adherence of both virulent and avirulent L. pneumophila to human monocytes, as indicated by phagocytic index measurements.
Conclusions:
- Legionella pneumophila modulates the oxidative response of human monocytes, with virulent strains being less stimulatory than avirulent ones.
- Complement fixation is a key mechanism mediating the adherence of L. pneumophila to human monocytes, irrespective of strain virulence.