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Histoplasma capsulatum fails to trigger release of superoxide from macrophages
Abstract:
The yeast form of the dimorphic fungus Histoplasma capsulatum survives within macrophages after phagocytosis. To do so, it must avoid, inhibit, or resist a variety of toxic oxygen metabolites. Using ferricytochrome c reduction to assay superoxide release, we examined the response of mouse macrophages to the yeast form of various H. capsulatum strains. Doses of zymosan as low as 20 particles per macrophage elicited superoxide, whereas H. capsulatum failed to induce superoxide even at 160 yeast cells per macrophage. This phenomenon was observed with two virulent strains of H. capsulatum (G217B and G186A) and with an avirulent variant of G186A. Over a 15- to 150-min observation period, zymosan stimulated increasing reduction of ferricytochrome c, but H. capsulatum did not. When added concurrently with zymosan, H. capsulatum had no effect on superoxide production. Therefore, H. capsulatum was unable either to inactivate the oxygen radical or inhibit host cell superoxide response to other competent stimuli. Enzymatically generated superoxide reduced ferricytochrome c even in the presence of H. capsulatum, again implying that the organism does not readily inactivate superoxide. This experiment also demonstrated that the yeast did not interfere with the assay used. Thus, rather than inhibiting superoxide generation or inactivating the anion, H. capsulatum yeast cells appear to avoid the toxic effects of superoxide by failing to trigger its release.
Insights
Histoplasma capsulatum yeast cells evade toxic oxygen metabolites by failing to trigger their release from macrophages. This mechanism allows fungal survival, as H. capsulatum does not inhibit or inactivate superoxide production.
Area of Science:
- Mycology
- Immunology
- Cell Biology
Background:
- Histoplasma capsulatum is a dimorphic fungus that survives within host macrophages.
- Fungal survival necessitates evasion of toxic oxygen metabolites produced by macrophages.
Purpose of the Study:
- To investigate the mechanism by which Histoplasma capsulatum yeast cells avoid toxic oxygen metabolites, specifically superoxide, produced by macrophages.
Main Methods:
- Assessed superoxide release from mouse macrophages using ferricytochrome c reduction.
- Compared macrophage response to H. capsulatum yeast cells versus zymosan (a known superoxide inducer).
- Evaluated the effect of H. capsulatum on zymosan-induced superoxide production and on enzymatically generated superoxide.
Main Results:
- H. capsulatum failed to induce superoxide release from macrophages, even at high concentrations.
- H. capsulatum did not inhibit or inactivate superoxide production when coincubated with zymosan.
- The fungus did not interfere with the ferricytochrome c assay for superoxide.
Conclusions:
- H. capsulatum yeast cells avoid toxic effects of superoxide by preventing its release from macrophages.
- The fungus does not actively inhibit or neutralize superoxide, but rather evades detection by the host cell's oxidative burst mechanism.