Role of superoxide anion in the fungicidal activity of murine peritoneal exudate macrophages against Penicillium marneffei

Penicillium marneffei is an important opportunistic fungal pathogen. The mechanisms of host defense against P. marneffei are not fully understood. In the present study, we, for the first time, investigated the role of superoxide anion (O₂^-) in the killing of two forms of P. marneffei, yeast cells and conidia, and the role of this killing mediator in the fungicidal activity of IFN-γ-stimulated murine peritoneal macrophages. P. marneffei yeast cells were susceptible to the killing effect of activated macrophages and chemically generated O₂^-, while conidia were not. These results suggested that O₂^- played some role in the fungicidal activity of macrophages. However, an oxygen radical scavenger, superoxide dismutase (SOD), did not suppress, but rather enhanced the fungicidal activity of IFN- γ-stimulated macrophages against P. marneffei yeast cells. This inconsistency was explained by the release of insufficient concentrations of O₂^- by activated macrophages as compared with the amount of O₂^- necessary for the killing of yeast cells, which was predicted in a chemical generating system. On the other hand, SOD enhanced the production of nitric oxide (NO) by IFN-γ-activated macrophages, and their increased fungicidal activity was significantly inhibited by N(G)-monomethyl-L-arginine (L-NMMA), a competitive inhibitor of NO synthase. Our results suggested that O₂^- does not function as the killing mediator of macrophages against P. marneffei, but rather plays an important role in the regulation of the NO-mediated killing system by suppressing NO production.