Mechanism of adherence of Candida albicans to epithelial cells

The research in this thesis was aimed at determining the chemical nature of the surface structures on Candida albicans and on host epithelial cells that are involved in yeast-epithelial adhesion. From previous studies it was known that C. albicans from active infections (I strains) were more adherent to buccal and vaginal epithelial cells after growth in medium containing 500 mM galactose than after growth on 50 mM glucose. Extracellular polymeric material (EPM) was isolated from culture supernates of C. albicans after growth in medium containing 500 mM galactose. EPM was composed of carbohydrate (70%), protein (10%) and phosphorus (0.05%) and was therefore considered to be glycoprotein. When used to pretreat buccal epithelial cells, EPM inhibited adherence, which suggested that it contains an adhesin that binds to, and blocks, epithelial-cell receptors. Fractionation of EPM by gel filtration and ion-exchange chromatography gave five components, all of which were capable of inhibiting adherence but to different extentso An index, the adherence inhibition index (All) was devised to compare activities of the various components. Fractionation of EPM by affinity chromatography on Con A-Sepharose and application of Con A-bound material to a DEAE-cellulose column yielded material which was 30 times more active as an inhibitor than crude EPM. Attempts to resolve EPM by SDS-polyacrylamide gel electrophoresis proved unsuccessful even though a variety of buffer and staining procedures was investigated. Chemical and enzymic treatment of EPM indicated that the protein portion of the (postulated) glycoprotein was more important than the carbohydrate at inhibiting adherence. Proteolytic enzymes, mild acid and heat treatment of EPM abolished its ability to inhibit adherence. Periodate and a-mannosidase had no effect, whereas papain and mild alkaline treatment of EPM enhanced its ability to inhibit adherence. The nature of the epithelial cell receptor for C. albicans was investigated with potential receptor analogues such as sugars and lectins and inhibitors. The adhesion of C. albicans GDH 2346 to buccal, cells was inhibited by L-fucose and pretreatment of the buccal cells with winged-pea lectin (which is specific for L-fucose), However, the adhesion of another C. albicans strain (GDH 2023) to buccal cells was inhibited by N-acetyl-D-glucosamine and by pretreatment of the buccal cells with wheat-germ agglutinin (which is specific for N-acetyl-D-glucosamine). These experiments indicated that glycosides function as epithelial cell receptors for C. albicans, but with the sugar residue varying between strains. Samples of EPM from various strains of C. albicans bound to affinity columns of Sepharose containing coupled sugars. The bound fractions were capable of inhibiting adherence, suggesting that they contained an adhesin component. These results indicate that 'lectin-like' interactions may be involved in yeast adherence to host epithelial cells