Effects of the Aspergillus fumigatus siderophore systems on the regulation of macrophage immune effector pathways and iron homeostasis.

The saprophytic fungus Aspergillus fumigatus is the most prevalent airborne fungal pathogen, which is responsible for invasive aspergillosis in immunocompromised patients. Iron plays an essential role for the growth and proliferation of A. fumigatus. This fungus synthesizes three major siderophores. It excretes triacetylfusarinine C to capture iron, while it accumulates ferricrocin and hydroxyferricrocin for hyphal and conidial iron storage, respectively. Herein, we investigated the role of the siderophore system of A. fumigatus in the modulation of immune effector pathways and iron homeostasis in macrophages. We set up a co-culture system consisting of the murine macrophage cell line RAW264.7 and either A. fumigatus wild type or a siderophore-deficient mutant (DeltasidA). We used real-time quantitative RT-PCR and Western blot analyses to study the expression of macrophage iron metabolism and innate immune response genes in response to pathogen challenge. Infection of macrophages with A. fumigatus wild type, but not with the DeltasidA mutant, induced expression of TNF and phagocyte oxidase subunit 47 at the transcriptional level. Moreover, infection with A. fumigatus wild type, but not with the DeltasidA mutant, compromised macrophage iron homeostasis. Infection with wild-type A. fumigatus decreased expression of the two cellular iron importers, the divalent metal transporter-1 and the transferrin receptor, and the only known iron exporter ferroportin. At the same time, it increased macrophage iron retention and ferritin synthesis. These data indicate that A. fumigatus affects the regulation of macrophage iron homeostasis and innate immune effector pathways via its siderophore system. The changes in immune response may be a consequence of macrophage iron restriction.