Interactions of human phagocytes with moulds Fusarium spp. and Verticillium nigrescens possessing different pathogenicity.

Fusarium spp. are emerging as important causes of invasive fungal infections. They tend to have decreased susceptibility to antifungal agents, making host defences very important. The ability of human phagocytes to cause damage to hyphae of Fusarium solani, F. oxysporum and Verticillium nigrescens, a mould with very low pathogenicity, was assessed using the 2,3-bis[2-methoxy-4-nitro-5-sulphophenyl]2H-tetrazolium-5-carboxanilide (XTT) metabolic assay. The oxidative burst, evidenced as superoxide anion (O2-) production, of phagocytes in response to hyphae was also investigated, as well as phagocytosis of conidia by monocyte (MNC)-derived macrophages (MDM). Hyphal damage by polymorphonuclear leukocytes (PMNL) and MNC showed a linear trend increasing with effector cell:target cell (E:T) ratio. Although no significant differences were observed for PMNL, MNC-induced damage to F. solani hyphae was lower than that seen with F. oxysporum hyphae at an E:T ratio of 20:1 and with V. nigrescens hyphae at ratios of 1:1, 5:1 and 20:1 (P < 0.05). In contrast, levels of O2- production by phagocytes in response to F. oxysporum were lower than those induced in response to the other fungi (P < 0.01). The average number of V. nigrescens conidia ingested by MDM was higher than that of conidia of the other fungi (P < 0.01). Phagocytes respond to the test fungi differentially, with F. solani being the least susceptible to damage by MNC. This may correlate with the observation that, compared to the other fungi studied, it causes a relatively high incidence of infections in neutropenic patients.