Background: In tissue-invasive lung infections caused by the mould Aspergillus fumigatus the molecular basis of host damage remains unclear. It has long been hypothesised that the secretion of proteolytic enzymes by invading A. fumigatus hyphae provides a mechanism by which epithelial damage is mediated. However, in whole animal studies of disease it has not been possible to substantiate an important role of fungal proteases since A. fumigatus mutants lacking individual or multiple enzyme functions retain the ability to cause fatal infections. One of the first cellular lines of defence against A. fumigatus infection is the monolayer of epithelial cells which line the mammalian airway. Epithelial cells provide a physical barrier against endothelial invasion and initiate an inflammatory immune response upon contact with A. fumigatus spores. Here we show that the A. fumigatus pH-responsive transcription factor, PacC, which governs expression of secreted proteases and secondary metabolism genes, is required for invasion of the murine pulmonary epithelium, and pathogenicity. Results: We determined, via murine and epithelial infection assays, that DpacC mutants are defective in elicitation of early-phase host damage which occurs, in wild type isolates, via a novel contact-dependent mechanism. Transcriptomic analyses of murine aspergillosis revealed aberrant cell wall biosynthesis in infecting DpacC isolates, suggesting a novel role for the A. fumigatus cell wall in pathogen-mediated host damage. Concordant with these findings PacC null mutants were shown to have signficiantly heightened chitin content in the fungal cell wall and were hypersensitive to cell wall perturbing agents, including caspofungin. The mechanistic relevance of cell wall-mediated host damage was verified by comparative analysis of damage elicited by cell wall extracts and heat-killed hyphae from wild type and DpacC isolates. Conclusion: A. fumigatus elicits host damage in a biphasic manner, initally via a novel contact-dependent mechanism involving cell wall components, and later via soluble mediators. A. fumigatus mutants deficient in the pH-responsive transcription factor PacC suffer deficits in both mechanisms. On the basis of this functional transcriptomic analysis we propose a new model of biphasic host damage during A. fumigatus infections.
Full conference title:
27th Fungal Genetics Conference
- Fungal Genetics Conference 27th (2013)