Inhibitory effect of melanin on the interaction of Fonsecaea pedrosoi with mammalian cells in vitro.

Conidial forms of Fonsecaea pedrosoi, grown under conditions where melanin was or was not synthesized, were allowed to interact with normal and cytochalasin treated macrophages. Melanin-free conidia were more infective to the macrophages. Treatment of macrophages with either cytochalasin B or D before the interaction decreased, but did not totally prevent their infection by the fungi. This inhibitory effect was higher (approximately 90%) if F. pedrosoi was grown under conditions where melanin was not synthesized. When melanin-containing conidia were used, the inhibitory effect of the cytochalasin on the infection was lower (approximately 50%). At least two mechanisms of infection of the host cell were observed: typical phagocytosis and another process in which the fungi played a more active role. Infection by F.pedrosoi was also observed in the non-professional phagocytic MDCK epithelial cell line. Two types of cytoplasmic vacuoles which contained parasites were seen in thin sections of host cells infected with F.pedrosoi: a 'tight' type and a 'loose' type. At least 200 conidia-containing vacuoles were analysed by transmission electron microscopy. The 'tight' type was observed in 75% of the vacuoles of non-treated macrophages, suggesting an association with classical phagocytosis. On the other hand, the 'loose' type vacuole was seen in 75% of the vacuoles present in cytochalasin treated macrophages and seemed to be related to induced phagocytosis or active penetration by the fungi.