Different models have been developed to analyse the adhesion and invasion, especially of C. albicans, to oral and oesophageal epithelia in vitro. Human primary oral epithelial cells represent a simple in vitro model. Either scraping buccal mucosa (BEC) from healthy volunteers with sterile files (Jones et al., 1997; Jones et al., 1995; McCarron et al., 2004; Zhao et al., 2004) or collecting whole unstimulated saliva from healthy volunteers and enriching the population of epithelial cells by membrane retention have been reported (Steele et al., 2000).
A widely used three-dimensional model is the reconstituted human oral epithelium (RHOE). This model is constituted of human keratinocytes derived from the cutaneous carcinoma cell line TR146 and is commercialized by Skinethic Laboratory (Nice, France). The RHOE model resembles normal human oral epithelium and the pathological changes caused by Candida invasion are similar to the processes in human disease (Schaller et al., 1998). C. albicans has been found to grow intracellularly or traversing gap junctions of the epithelium, both in a similar proportion (Jayatilake et al., 2005). Using this epithelial cell model and a computer-assisted image analysis system the invasion of different Candida species was quantified, indicating that C. albicans is more invasive than others non-albi-cans Candida species (Jayatilake et al., 2006). Epithelial cells have mechanisms to sense the invasion by Candida spp. and respond by producing antimicrobial peptides (Lu et al., 2006). The RHOE model has been used to study this defensive response generated by oral epithelium against different Candida species which include the production of P-defensins 1, 2, and 3. Another host cell response to infection is the induction of cytokine expression. The pattern of cytokines and chemokines produced in presence of different Candida spp. correlates with their virulence potential as determined by quantitative reverse transcription-PCR (Q-RT-PCR) and enzyme-linked immunosorbent assay (ELISA) (Schaller et al., 2002).
A further improvement of the RHOE model in order to mimic the in vivo situation more closely is the supplement with polymorphonuclear leukocytes (PMNs), which generates simple aspects of the immune system (Schaller et al., 2004). Epithelial cells in contact with C. albicans induce a strong immune response that attracts PMNs to the infection site. Q-RT-PCR was employed to measure the immune response generated concluding that the extension of the system provides a useful tool for studying the immunological interaction between keratinocytes and C. albicans, as well as the role of PMNs in C. albicans pathogenesis.
Rouabhia and Deslauriers (2002) have developed an engineered human oral mucosa (EHOM). This model is a well-organized and stratified tissue in which epithelial cells are arranged to analyse pathogen-oral mucosa in vitro. The comparative study of infection in the in vitro EHOM and the RHOE model with C. albicans mutants of the manosyltransferase family (Pmt) allowed an assessment of the different functions for the individual Pmt isoforms in the different host niche-specific models (Rouabhia et al., 2005).
Alternatives to the RHOE and the EHOM are the EpiOral GIN-100 oral mucosa model based on primary oral keratinocytes (purchaser MaTek Corporation, Ashland, MA, USA) and a novel three-dimensional model, which uses the immortalized OKF6/TERT-2 oral epithelial cell line (Dongari-Bagtzoglou & Kashleva, 2006).
Skinethic Laboratory also provides a human reconstituted oesophageal epithelium derived from the immortalized cell line Kyse 510. These cells cultured in vitro develop a stratum corneum, resembling the outer cell layer of human oesophagus. This epithelial cell model was compared to the human oesophageal cell monolayer HET1-A to analyse the adherence and penetration of clinical, commensal, and reference collection C. albicans strains (Bernhardt et al., 2001). Clinical isolates showed an enhanced capability to adhere to HET1-A and to adhere and invade reconstituted oesophageal epithelium compared to commensal and collection strains (Bernhardt et al., 2001).
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