Nanoscopic cell wall architecture of an immunogenic ligand in Candida albicans during antifungal drug treatment.
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The cell wall of Candida albicans is composed largely of polysaccharides. Here we focus on β-glucan, an immunogenic cell wall polysaccharide whose surface exposure is often restricted, or "masked", from immune recognition by Dectin-1 on dendritic cells (DCs) and other innate immune cells. Previous research suggested that the physical presentation geometry of β-glucan may determine whether or not it can be recognized by Dectin-1. We used direct Stochastic Optical Reconstruction Microscopy (dSTORM) to explore the fine structure of β-glucan exposed on C. albicans cell walls before and after treatment with the antimycotic drug caspofungin, which alters glucan exposure. Most surface-accessible glucan on C. albicans yeast and hyphae are limited to isolated Dectin-1 binding sites. Caspofungin induced unmasking caused an ∼4-7 fold increase in total glucan exposure, accompanied by increased phagocytosis efficiency of DCs for unmasked yeasts. Nanoscopic imaging of caspofungin-unmasked C. albicans cell walls revealed that the increase in glucan exposure is due to increased density of glucan exposures and increased multi-glucan exposure sizes. These findings reveal that glucan exhibits significant nano-structure, which is a previously unknown physical component of the host-Candida interaction that may change during antifungal chemotherapy and impact innate immune activation.© 2016 by The American Society for Cell Biology.