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Pneumocystis carinii stimulates tumor necrosis factor-alpha release from alveolar macrophages through a
O A Hoffman1, J E Standing, A H Limper
1Division of Thoracic Diseases and Internal Medicine, Mayo Clinic and Foundation, Rochester, MN 55905.
Abstract:
Recent studies suggest that TNF-alpha plays a central role in host defenses during Pneumocystis carinii pneumonia. To determine whether P. carinii directly stimulates TNF-alpha secretion, rat alveolar macrophages were cultured in the presence of purified P. carinii. Whereas unstimulated alveolar macrophages released only 13.0 +/- 2.7 pg/ml of TNF-alpha into the medium, macrophages stimulated with P. carinii released 108.2 +/- 20.4 pg/ml of TNF-alpha after overnight culture (p = 0.0001). Maximal TNF-alpha release was observed after 8 h of incubation and required a P. carinii: macrophage ratio of at least 2.5:1. Autoclaved P. carinii were also able to trigger TNF-alpha release from macrophages albeit at a reduced level. In view of recent evidence that P. carinii is phylogenetically related to the fungi and contains a beta-glucan-rich cell wall, we hypothesized that TNF-alpha release might in part be mediated by this cell wall component. Preincubation of macrophages with particulate beta-glucan derived from Baker's yeast resulted in complete inhibition of TNF-alpha release in response to P. carinii. In addition, digestion of P. carinii with zymolyase, a preparation containing predominantly beta-glucanase activity, substantially reduced the ability of P. carinii to cause TNF-alpha release from macrophages. In a similar manner, macrophages incubated with P. carinii in the presence of laminariheptaose, an oligosaccharide that binds to macrophage beta-glucan receptors, also displayed decreased TNF-alpha release. Interestingly, TNF-alpha release may not be completely linked to the adherence of the organism to macrophages. Particulate beta-glucan significantly reduced P. carinii adherence to macrophages and also impaired TNF-alpha release. However, yeast mannan also significantly reduced P. carinii adherence to macrophages and also impaired TNF-alpha release. However, yeast mannan also significantly reduced P. carinii adherence but had no effect on TNF-alpha release. These data demonstrate that P. carinii can directly stimulate the secretion of TNF-alpha from alveolar macrophages and that this effect is largely mediated by beta-glucan components of the P. carinii cell wall.
Insights
Pneumocystis carinii directly stimulates tumor necrosis factor-alpha (TNF-alpha) secretion from rat alveolar macrophages. This response is primarily mediated by beta-glucan in the P. carinii cell wall.
Area of Science:
- Immunology
- Microbiology
- Cell Biology
Background:
- Tumor necrosis factor-alpha (TNF-alpha) is crucial in host defense against Pneumocystis carinii pneumonia (PCP).
- The direct interaction between P. carinii and immune cells in stimulating TNF-alpha is not fully understood.
Purpose of the Study:
- To investigate whether P. carinii directly stimulates TNF-alpha secretion from alveolar macrophages.
- To identify the P. carinii components responsible for triggering this immune response.
Main Methods:
- Cultured rat alveolar macrophages with purified P. carinii.
- Quantified TNF-alpha release using ELISA.
- Investigated the role of beta-glucan by pre-incubating macrophages with beta-glucan, treating P. carinii with zymolyase, and using laminariheptaose.
Main Results:
- P. carinii significantly increased TNF-alpha secretion from macrophages compared to unstimulated controls.
- Beta-glucan components of P. carinii cell wall were identified as the primary mediators of TNF-alpha release.
- Inhibition of beta-glucan binding or digestion of P. carinii with beta-glucanase substantially reduced TNF-alpha secretion.
Conclusions:
- P. carinii directly stimulates TNF-alpha secretion from alveolar macrophages.
- The beta-glucan in the P. carinii cell wall is a major factor in inducing this TNF-alpha response.
- Understanding this interaction is key to developing targeted therapies for P. carinii infections.