Identification of a critical receptor mediating fungal recognition, epithelial activation and induction of mucosal innate immunity

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 no response in the case of harmless 'commensal' microbes or activation of immunity in the case of disease-causing 'pathogenic' microbes. When these normal responses are disrupted a plethora of complications arise, which have implications beyond infection as conditions including cancer and autoimmune disease are associated with abnormal interactions between host and microbe. 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 normally restrict this medically-important pathogen to the commensal state at these surfaces. Identifying these mechanisms is of considerable value not just for C. albicans infections but also for controlling other microbes at mucosal surfaces. Presently, we have little knowledge of the fungal components promoting mucosal infections or the host epithelial proteins (receptors) recognising C. albicans and orchestrating immunity.

Our recent work has been instrumental in understanding the mechanisms by which epithelial cells identify disease-causing C. albicans and how this results in immune protection. We previously showed that C. albicans infection leads to the secretion of immune activators (cytokines and chemokines) that recruit a specific immune cell (neutrophil) to the site of infection, which then act together with epithelial cells 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. The fundamentally important feature that is currently unknown is the epithelial receptor that recognises C. albicans Ece1p and triggers epithelial activation in the first instance. Identifying this receptor is key to understanding how epithelial cells respond to this pathogenic fungus and orchestrate protective mucosal immunity. If we identify the receptor, drugs can be developed to target this receptor for use in treating not only fungal infections but other mucosal diseases also. The aims of this proposal are to determine the role of Ece1p in C. albicans infection, to identify the epithelial receptor that recognises C. albicans Ece1p, and to determine the importance of the C. albicans Ece1p-epithelial receptor interaction in activating immune protection against C. albicans mucosal infection.

This project explores some of the most fundamental and challenging issues in infection, namely how mucosal surfaces discriminate between commensal and pathogenic microbes and how epithelial tissues initiate protective immune responses. This work has strong medical importance as it will not only have implications for developing new immune-based treatments against fungal infections and more effective antifungal therapies, but also for defining new approaches to combat other mucosal diseases and to manipulate host immunity for patient benefit.

The ability of mucosal tissues to discriminate between commensal and pathogenic microbes is essential to human health. Recently, we identified an epithelial signalling mechanism instrumental in identifying the commensal-pathogen switch of Candida albicans, a major life-threatening fungal pathogen of humans. This activation mechanism targets the hyphal form of C. albicans, resulting in induction of proinflammatory cytokines and subsequent activation of neutrophils, leading to protection against mucosal fungal infection. We have now identified the hyphal protein (Ece1p) targeted for epithelial recognition of the commensal-pathogen switch and provide strong evidence that Ece1p activates this recognition system via a novel surface epithelial receptor. This proposal will (i) determine the role of Ece1p in C. albicans infection using a novel murine mucosal model, (ii) identify the epithelial receptor that recognises Ece1p and demonstrate functional importance of the Ece1p-receptor interaction in epithelial activation using several proteomic-based approaches, RNA interference, receptor blocking/inhibition and overexpression assays, and (iii) demonstrate the role of the Ece1p-receptor interaction in controlling fungal colonisation and infection in vivo and in mediating protective innate immune responses. This project will provide substantial new insights to our understanding of epithelial-microbial interactions, and will identify new microbial and epithelial targets for the development of new drugs, immune-based treatments, vaccines and adjuvants to prevent mucosal diseases and new diagnostic tests and biomarkers for mucosal infections.

Mucosal diseases continue to be major causes of death, suffering and economic loss and have serious impacts on the welfare of global communities. The impact of mucosal-acquired fungal infections on healthcare and economic expenditures is large and of growing concern. Candida species are the most common fungal pathogens of humans giving rise to severe morbidity in millions of individuals worldwide. Vaginal candidiasis affects ~75% of women during fertile age, equating to ~20 million infections/year (2x more than tuberculosis and 5x more than HIV: WHO 2007). Candida infections are also the third most common hospital-acquired bloodstream infection and can thus be considered more medically-important than most bacterial infections including Enterococci and Pseudomonas spp. Systemic candidiasis usually results from translocation of Candida across the mucosae and is fatal (~40% mortality) equating to an estimated 100,000 deaths/year. Furthermore, 50% of HIV+ and 90% AIDS patients suffer from oral candidiasis, equating to ~2 million cases/year. Yearly healthcare costs in the USA for fungal infections are ~$2.6 billion, of which Candida infections account for $1.8 billion. EU healthcare costs are estimated to be similar. Therefore, Candida pathogens and C. albicans in particular (accounts for ~75% of Candida infections) carry an immense health burden and represent a serious socio-economic challenge for worldwide communities. Given this, identification of the host and fungal mechanisms that can be targeted to prevent or control disease is of high priority for global healthcare provision. Therefore, a major long term impact goal of this project is commercial exploitability of our data. As such, the epithelial receptor, the signalling circuits it activates and Ece1p can all be targeted for the development of more effective immune-based antifungal treatments and for future therapies to combat mucosal infections in general. For example, Ece1p possesses unique properties that impart a dual role for exploitation; firstly as a novel vaccine target (given its role in pathogenesis) to induce protective mucosal immunity against C. albicans infections and, secondly, as a novel mucosal adjuvant given its ability to activate epithelial cells and mucosal immunity without causing damage. Furthermore, the epithelial receptor and signalling circuits can be exploited to develop strategies to stimulate protective immunity against a multitude of mucosal pathogens. The project will also impact on the diagnostics industry, since Ece1p is the first fungal marker identified to date that enables the host to discriminate between commensal and pathogenic C. albicans. This could be exploited in new clinical diagnostic tests that would target Ece1p as an identifiable 'risk factor' of pathogenesis and infection. In summary, this project will have immediate impact on multiple academic fields but has significant potential in the longer term to impact on pharmaceutical and commercial industries with specific interests in developing new drugs, vaccines and adjuvants to prevent mucosal diseases and in designing new diagnostic tests and biomarkers for mucosal infections. This will have a substantial impact on clinical medicine and disease management, which will ultimately improve the economic competitiveness of the UK and improve human health and quality of life on a global scale.