Role of secretory IgA in Candida albicans adhesion

Interactions between Candida albicans, saliva and saliva-coated oral surfaces are initial events in the colonisation of oral surfaces by this commensal yeast, which can also cause mucosal and systemic infections in immuno-compromised people. Objectives: Our goal is to develop new approaches to reducing C. albicans colonisation and disease, and in particular, to investigate the application of passive immunity strategies. Previous studies investigating the role of saliva in C. albicans adherence show conflicting results, which may reflect the presence in saliva of multiple factors promoting and inhibiting adherence. Methods: Our approach has been to study the interactions of C. albicans yeast cells with IgA and its component proteins. Yeast cells were incubated with epithelial cells monolayers in the presence and absence of saliva or bovine milk containing anti-C. albicans IgA, and following washing, the adhered yeast cells were enumerated. In order to identify salivary proteins, bound to yeast, that promoted adherence, C. albicans cells were incubated with biotinylated whole saliva. Biotinylated salivary polypeptides were detected in extracts from washed yeast cells by polyacrylamide gel electrophoresis (PAGE), electro-blotting and development of the blots using peroxidase-labeled avidin. Proteins were identified by protein micro-sequencing and immunoblotting using specific antibodies. Results: The presence of anti-C. albicans IgA in saliva (either endogenous, or as added bovine immune IgA) inhibited C. albicans adherence to cultured epithelial cells. Four biotinylated salivary polypeptide bands that bound to yeast cells were identified as components of secretory IgA (sIgA), and included secretory component (SC). Using non-denaturing PAGE we demonstrated that SC bound to yeast cells from the free form in saliva, and partially purified SC promoted adherence of yeast cells to cultured epithelial monolayers. Conclusion: Although immune IgA specific to C. albicans can inhibit adherence, IgA components, in particular SC, can also promote binding in a non-immune manner.