Pneumocystis carinii stimulates tumor necrosis factor-alpha release from alveolar macrophages through a beta-glucan-mediated mechanism.
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.
Hoffman
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OA
Division of Thoracic Diseases and Internal Medicine, Mayo Clinic and Foundation, Rochester, MN 55905.
Standing
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JE
Limper
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AH
D000818:Animals; D002478:Cells, Cultured; D005260:Female; D005936:Glucans / Q000494:pharmacology*; D037181:Lectins, C-Type; D016676:Macrophages, Alveolar / Q000276:immunology*; D000090323:Mannose Receptor; D037241:Mannose-Binding Lectins; D011010:Pneumocystis / Q000276:immunology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D011956:Receptors, Cell Surface / Q000502:physiology*; D011971:Receptors, Immunologic / Q000502:physiology; D014409:Tumor Necrosis Factor-alpha / Q000378:metabolism*
D005936:Glucans; D037181:Lectins, C-Type; D000090323:Mannose Receptor; D037241:Mannose-Binding Lectins; D011956:Receptors, Cell Surface; D011971:Receptors, Immunologic; D014409:Tumor Necrosis Factor-alpha; C045268:beta-glucan receptor