The crucial role of the Aspergillus fumigatus siderophore system in interaction with alveolar macrophages

Abstract

Iron plays a central role in manifestation of infections for a variety of pathogens. To ensure an adequate supply with iron, Aspergillus fumigatus employs extra- and intracellular siderophores (low-molecular mass iron chelators), which are of importance for fungal growth in particular during iron starvation. Here we show that the lack of extracellular siderophores, and especially, the lack of the entire siderophore system cause in immunosuppressed mice in vivo (i) a reduced extracellular growth rate, (ii) a reduced intracellular growth rate in alveolar macrophages, and (iii) an increased susceptibility to conidial growth inhibition by alveolar macrophages. These data underline the crucial role of the fungal siderophore system not only for extracellular growth but also in the interaction with the host immune cells. Moreover, the hyphal growth rate within alveolar macrophages compared to extracellular lavage fluid was significantly decreased indicating that, besides elimination of fungal conidia, inhibition of pathogenic growth is a function of macrophages.

Fig. 1

Analysis of in vivo conidial growth inhibition by AMs 24 h (A) and 48 h (B) after infection. A. fumigatus conidia were harvested via bronchoalveolar lavage and the ability to germinate and grow was determined microscopically. Percentage of non-germinated to the total number of conidia was bar-plotted. Values represent means of three mice per strain per time point including standard deviation. Given p-values of sidA and sidF mutant strains indicate the statistical significance of the difference to wt.

Fig. 2

Hyphal length of A. fumigatus wt, ΔsidF, ΔsidD, and ΔsidC in vivo. Length of hyphae was determined inside AMs (A) and outside AMs (B). Significant growth reduction is indicated by p-values beside the bar. P-values were determined using an unpaired, two-tailed T-test. Data are derived from three independently infected animals. Error bars represent standard deviations.

Fig. 3

Histopathological analysis of murine lung sections infected with A. fumigatus. Sections were prepared after 48 h of infection, fixed in 4% v/v formaldehyde and stained using Grocotts Methenamine Silver.