Mechanism of EGFR activation by a novel fungal pore-forming toxin

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The mucosal (oral, vaginal, gastrointestinal) epithelium is of immense importance in protecting humans against a multitude of infections as it is the initial tissue encountered by the majority of microbes. This microbial-epithelial encounter results in either quiescence in the case of harmless 'commensal' microbes, or activation of immunity and damage protection mechanisms in the case of disease-causing 'pathogenic' microbes. When these normal host responses are disrupted, a plethora of complications can arise that have implications beyond infection, as conditions including cancer and autoimmune disease are associated with abnormal interactions between a microbe and its host. Given the importance of these interactions, epithelial cells possess distinct mechanisms enabling mucosal tissues to distinguish between commensal and pathogenic microbes.The fungus Candida albicans is an example of a microbe that can exist as a commensal in healthy people but becomes a dangerous pathogen causing severe and potentially fatal disease in unhealthy people. Indeed, Candida infections are now the third most common hospital-acquired bloodstream infection and are a serious burden to healthcare provision. Given that the vast majority of C. albicans infections are acquired through mucosal surfaces, it is of paramount importance to understand how epithelial tissues at these surfaces detect and restrict this medically-important pathogen to the commensal state. Identifying these mechanisms is of considerable value not only for fungal infections but also for controlling other microbes at mucosal surfaces. Our recent work has been instrumental in understanding the mechanisms by which epithelial cells identify disease-causing C. albicans and how this causes damage induction and immune activation in the host. We previously showed that C. albicans infection leads to epithelial damage and the secretion of immune activators to induce local mucosal immunity to protect against C. albicans infection. Recently, we found that to activate this protective immune process, epithelial cells target a specific protein (Ece1p) on the 'invasive' form of C. albicans known as hyphae. We now demonstrate that a specific region of the Ece1p protein (named Ece1-III) acts as a novel pore-forming toxin (PFT) when present at high concentration. This is the first PFT identified in a human pathogenic fungus. The pore forming abilities of Ece1-III enable C. albicans to cause epithelial damage and are recognised by epithelial cells to activate immunity. Furthermore, we show that this newly identified fungal PFT activates a specific epithelial surface receptor (EGFR), which induces epithelial immunity and the protective responses that protect against fungal-induced damage. We have demonstrated that immune activation occurs as a result of direct interaction between Ece1-III and EGFR. However, we found that Ece1-III is also able to activate EGFR indirectly through a complex series of events that take place at the epithelial cell surface as part of a response to cell damage. The nature of these surface events leading indirectly to EGFR activation is unknown, although pore formation seems to be essential for their activation. Given that EGFR is the key receptor mediating both epithelial immunity and damage protection, the aim of this proposal is to determine precisely how pore-formation by this novel fungal PFT indirectly activates EGFR and to demonstrate the importance of these activation events during C. albicans mucosal infection in vivo.This project will provide new insights into the mechanism of action of fungal PFTs and to our understanding of epithelial tissue responses to 'pathogenic' fungi during mucosal immunity. Further, it will identify novel host-microbe interactions that can be targeted not only for new, more effective antifungal therapies, but also for defining innovative approaches to combat the action of bacterial PFTs and mucosal infections in general.