2) revealed the presence of an oxidative response in the interface between the melanin-free fungi and macrophages. These experiments also showed that the presence of control-melanin (either free in the media or adhered to the fungal cell) decreased NO levels

as revealed by its direct correlation to the detected nitrite levels. Further, TC-treatment of F. pedrosoi conidia resulted in at least an 80% increase in the amount of nitrite detected after the first 24 h of interaction compared to samples with only macrophages. These data indicate that the inhibition of the melanin pathway, and consequently, the absence of melanin exposed on the cell wall of the fungus, could stimulate the production of NO by activated macrophages. Fungal glucans, the Trichostatin A major component of the fungal cell wall, were previously described to activate macrophages (which express glucan receptors) and promote the synthesis and release of NO [31]. Nimrichter Ku-0059436 et al. [32] suggested that the removal of melanin from the F. pedrosoi cell wall exposes antigens, such as glucans, that were previously masked by melanin. We conclude that the increase of the macrophages’ oxidative response after interaction with TC-treated F. pedrosoi was probably due to the unmasking of antigens/glucans in the fungal cell wall. The inhibition of i-NOS expression by pathogens has been reported in other microorganisms, e.g., Toxoplasma gondii [33]. Bocca el al.

[34] suggested that melanin from F. pedrosoi could inhibit NO production in macrophages. However, our experiments suggest that the reduction of nitrite levels after the interaction of macrophages and control conidia was not due the inhibition of i-NOS expression, since its expression was detected in all tested conditions in immunofluorescence experiments. We propose

that F. pedrosoi melanin acts as a scavenger of oxidative radicals, masking the detection of NO in some systems. The conversion of L-arginine by i-NOS in the presence of NO requires calcium ions and Fe(III)(in an heme group). Melanin participates in the storage of calcium and iron in F. pedrosoi, and therefore it might reduce the Fedratinib availability of such ions in the interaction microenvironment [11, 35]. In addition, NO reversibly reacts with both Fe(III) and Fe(II), leaving an electron that could remain trapped isometheptene in the quinone groups of melanin [8, 36]. The assays with the NO donor SNAP and H2O2 revealed that untreated F. pedrosoi grew more than TC-treated F. pedrosoi; this suggests a protective function for melanin. In these experiments, our only variables were the F. pedrosoi conidia and the oxidative agent. Consequently, in these systems, no other mechanism can occur to inhibit i-NOS production. Conclusions Our data suggest a protective role for F. pedrosoi melanin by its direct interaction with NO; the fungal melanin acts as a trap for the unpaired electron of NO, protecting the fungus against oxidative damage.