Mechanisms of adhesion to and invasion of cultured cells by the fungal pathogen, Aspergillus fumigatus

Aspergillus fumigatus is a common environmental mold that can cause an invasive lung disease known as aspergillosis in the immunocompromised host. The mechanisms by which A. fumigatus causes invasive pulmonary disease are not well known, but are thought to require two steps: (1) adhesion of the conidiospores (the infectious particles) to lung epithelial cells and/or extracellular matrix (ECM) proteins in the basal lamina and (2) invasion of epithelial tissue and penetration of underlying basal lamina. To investigate both aspects of pathogenicity, we studied the adhesion of A. fumigatus conidia to ECM proteins and the internalization and fate of conidia within cultured lung cells. The adhesion studies demonstrated that A. fumigatus conidia bound significantly better than other Aspergillus species to both fibronectin (an ECM protein) and to intact basal lamina. The finding that both glycosaminoglycans and high ionic strength buffers inhibited binding of A. fumigatus conidia to fibronectin and intact basal lamina suggested that negatively-charged sugars on the conidial surface may mediate adhesion to the ECM. Further support for this hypothesis was provided from competitive binding assays which showed that sialic acid-containing glycoproteins could inhibit the binding of conidia to fibronectin. FACS analysis demonstrated that A. fumigatus possessed greater amounts of sialic acid on the conidial surface than three other non-pathogenic strains. Therefore, conidial sialic acid may mediate adhesion of the fungus to fibronectin.; To determine whether A. fumigatus could enter eukaryotic cells, we studied the uptake of A. fumigatus conidia into A549 lung epithelial cells, human umbilical vein endothelial (HUVE) cells, and J774 murine macrophages in vitro. A549 cells internalized 30% of the bound conidia whereas HUVE and J774 cells internalized 50% and 90% respectively after 3 hours. Phagosomes with internalized conidia fused quickly with lysosomes in J774 and A549 cells as determined by co-localization of conidia with proteins present in the early endosomal and lysosomal membrane. After 12 hours, the conidia within J774 cells were killed; however, the conidia inside A549 cells survived and germinated. By 36 hours, the germlings escaped the A549 phagosome and formed extracellular hyphae. Internalization of conidia by nonprofessional phagocytes may be important in the development of aspergillosis in vivo, as sequestration inside these cells may allow the conidia to escape the immune response of the host.; These studies provided an increased understanding of A. fumigatus and its molecular interactions with host lung proteins and cells in vitro. Further knowledge about the biology of this organism and its interaction with the host will aid in the development of new treatments for invasive aspergillosis which are urgently needed.