Cryptococcus neoformans is a pathogenic fungus causing life threatening infections in humans. The present in vitro study aimed to investigate the opsonizing properties of a well characterized serum ficolin (rat ficolin-A), a member of carbohydrate-recognition molecules of the innate immune system, in the defence against this fungal pathogen. Using flow cytometric analysis we have been able to demonstrate that ficolin-A readily binds to two different acapsular C. neoformans serotypes (representative of the primary infectious form of this fungus) whereas the encapsulated forms are not being recognized. The ficolin-A binding was concentration dependent and inhibited by the acetylated sugars N-acetyleglucosamine and N-acetylegalactosamine but less so by galactose, glucose and mannan. The binding was enhanced at acidic pHs (5.7 and 4.7) compared to physiological pH (7.4) which may indicate that the carbohydrate recognizing fibrinogen-like domains of ficolins undergo conformational changes providing more efficient binding at sites of inflammation where the pH is much lower than normal. We further assessed the biological consequence of the ficolin-A recognition of acapsular C. neoformans by investigating their interaction with lung epithelial cells (type II pneumocytes cell line A549). Flow cytometric analysis demonstrated that ficolin-A opsonized acapsular C. neoformans showed significantly increased adherence to A549 cells when exposed to acidic conditions compared to the unopsonized controls (p=0.04). We conclude that ficolin-A binds acapsular C. neoformans via their carbohydrate recognizing fibrinogen-like domains leading to enhanced uptake by lung epithelial cells in vitro.
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