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Cryptococcus neoformans Neutralizes Macrophage and Astrocyte Derived Nitric Oxide without Interfering with Inducible Nitric Oxide Synthase Induction or Catalytic Activity — Possible Involvement of Nitric Oxide Consumption

The effect of Cryptococcus neoformans on the accumulation of nitrite, an indicator of nitric oxide (NO) synthesis, was investigated in cytokine (interferon‐γ [IFN‐γ] and interleukin [IL]‐1)‐stimulated cultures of rat peritoneal macrophages and C6 astrocytoma cells. Cytokine‐induced nitrite generation in cultures of both cell types was inhibited in a dose‐dependent manner by live C. neoformans, but not by heat‐killed cryptococcal cells or conditioned medium from yeast cultures. C. neoformans‐mediated reduction of nitrite formation coincided with impairment of NO‐dependent macrophage tumoricidal activity. Cytokine‐triggered induction of inducible NO synthase (iNOS) was unaffected in C6 cells, and only marginally reduced in macrophages. When cells were pretreated with cytokines for 24 h to induce iNOS, and any further induction was prevented by inhibition of protein synthesis, C. neoformans was still able to reduce nitrite accumulation in cultures of both cell types. Finally, live C. neoformans, but not heat‐killed yeast cells or yeast culture supernatant, significantly reduced nitrite production in a culture solution of NO‐releasing compound S‐nitrosoglutathione (GSNO). Thus, it appears that cryptococcal reduction of nitrite formation in macrophage and C6 cultures was caused by the consumption of NO by some yeast molecule, rather than by the inhibition of cellular NO synthesis.