Biological function of Candida albicans Ece1 and role in host-microbe interactions

粘膜（口腔、阴道、胃肠道）上皮是大多数微生物接触的初始组织，在保护人类免受多种感染方面具有极其重要的作用。在健康情况下，这种微生物与上皮的接触，要么导致无害的“共生”微生物的静止，要么导致致病的“致病”微生物的免疫激活。当这些正常反应被破坏时，就会出现大量的并发症，其影响超出了感染，因为包括癌症和自身免疫性疾病在内的疾病都与宿主和微生物之间的异常相互作用有关。鉴于这些相互作用的重要性，上皮细胞具有独特的机制，使粘膜组织能够区分共生微生物和致病微生物。真菌白色念珠菌（C.alb）是一种微生物的例子，它可以在健康人群中作为共生菌存在，但在不健康人群中成为一种危险的病原体，导致严重和可能致命的疾病。事实上，念珠菌感染经常被发现是第三或第四大最常见的医院获得性血液感染，导致约40%的个体死亡，因此是医疗保健提供的严重负担。鉴于绝大多数血液中白血细胞感染是通过粘膜表面获得的，因此了解白血细胞在健康情况下与上皮组织相互作用的机制以及确定在正常反应被破坏时促进感染和疾病的真菌因素至关重要。目前，我们对促进粘膜感染的真菌因素知之甚少。我们最近的工作有助于理解上皮细胞识别致病C.alb的机制以及这如何导致免疫保护。我们之前的研究表明，白血细胞感染导致免疫激活因子（细胞因子和趋化因子）的分泌，这些激活因子招募特异性免疫细胞来保护机体免受白血细胞感染。最近，我们发现，为了激活这种保护性免疫过程，上皮细胞识别丝状、“侵入”形式的白球菌（称为菌丝）上的一种特定蛋白（CaEce1p）。CaEce1p是一种独特的菌丝蛋白，由8个相同大小的重复片段组成，由CaKex2p酶单独处理。我们提供了强有力的证据，证明CaEce1p（或其单个片段）是告知健康宿主存在已变得危险的微生物的关键真菌蛋白。本研究旨在证明CaEce1p对C.alb的功能，CaEce1p是如何定位到菌丝表面的，最重要的是，CaEce1p的哪些片段激活了上皮细胞。这将为基本的真菌生物学和在健康中支持白芽孢杆菌与宿主相互作用的机制提供必要的新认识。该项目将确定CaEce1p作为一个新的靶点，不仅用于开发新的抗真菌药物，而且用于开发新的疫苗、佐剂、诊断测试和真菌感染的生物标志物。另外令人感兴趣的是CaEce1p和黑穗病菌（Ustilago maydis）菌丝特异性Rep1蛋白的结构相似性，黑穗病菌是一种引起玉米黑穗病的致病植物真菌。因此，利用CaEce1p获得的功能数据也可能为进一步研究植物真菌的发病机制提供有价值的基础。

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. In health, this microbial-epithelial encounter results in either quiescence 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 (C.alb) 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 regularly found to be the third or fourth most common hospital-acquired bloodstream infection, causing death in ~40% of individuals, and are thus a serious burden to healthcare provision. Given that the vast majority of bloodstream C.alb infections are acquired through mucosal surfaces, it is of paramount importance to understand the mechanisms by which C.alb interacts with epithelial tissues in health and to identify the fungal factors that promote infection and disease when normal responses are disrupted. Presently, we have little knowledge of the fungal factors that promote mucosal infections.Our recent work has been instrumental in understanding the mechanisms by which epithelial cells identify disease-causing C.alb and how this results in immune protection. We previously showed that C.alb infection leads to the secretion of immune activators (cytokines and chemokines) that recruit specific immune cells that protect against C.alb infection. Recently, we found that to activate this protective immune process epithelial cells recognise a specific protein (CaEce1p) on the filamentous, 'invasive' form of C.alb known as hyphae. CaEce1p is a unique hyphal protein consisting of eight repeated fragments of the same size that are individually processed by the enzyme CaKex2p. We provide strong evidence demonstrating that CaEce1p (or its individual fragments) is the critical fungal protein that informs the healthy host of the presence of a microbe that has become dangerous. This proposal aims to demonstrate the function of CaEce1p for C.alb, how CaEce1p is localised to the hyphal surface and, crucially, which fragments of CaEce1p activate epithelial cells. This will provide essential new understanding to basic fungal biology and the mechanisms that underpin C.alb-host interactions in health.This project will identify CaEce1p as a new target not only for the development of new antifungal drugs but also for the development of new vaccines, adjuvants, diagnostic tests and biomarkers for fungal infections. Of additional interest is the structural similarity between CaEce1p and the hypha-specific Rep1 protein of Ustilago maydis, which is a pathogenic plant fungus that causes smut disease in corn. Thus, the functional data obtained with CaEce1p may also provide valuable groundwork for future studies investigating plant fungal pathogenesis.