Aspergillus fumigatus conidia survive and germinate in acidic organelles of A549 epithelial cells

Aspergillus fumigatus is an environmental mould that can cause invasive disease in the immunocompromised host. Previous work has shown that conidia can be internalized by lung epithelial cells (A549) and murine macrophages (J774) in vitro. Therefore, the purpose of this study was to determine the fate of A. fumigatus conidia within the endosomal network of these cells. Co-localization of conidia expressing green fluorescent protein with proteins present in the early endosomal (CD71) and lysosomal (CD63, LAMP-1) membrane was assessed using confocal microscopy. In J774 cells, 75 % of internalized conidia were found in phagosomes containing LAMP-1 120 minutes post-infection. In A549 cells, 55 % and 58 % of internalized conidia were found to co-localize with LAMP-1 and CD63 by 24 hours. Cathepsin D also co-localized with internalized conidia in A549 cells. Phagosomes containing conidia were shown to be acidified in both cell types. Less than 1% of the initial inoculum survived in J774 cells by 12 hours post-infection. After 24 hours, 3 % of internalized conidia survived in A549 cells and 34% of these had germinated. By 36 hours, the germlings were able to escape the phagosome and form extracellular hyphae without lysis of the host cell.