Identification of Botanicals that Unmask β-Glucan from the Cell Surface of an Opportunistic Fungal Pathogen

Abstract

Dectin-1 expressed on host immune cells recognizes β-glucans within the cell walls of fungal pathogens and plays an important role in the clearance of fungal infections. However, because β-glucan is masked by an outer layer of mannoproteins, fungal pathogens can evade detection by host immune cells. In this study, a microplate-based screen was developed to identify β-glucan unmasking activity exhibited by botanicals. This screen measures the activity of a reporter gene in response to the transcriptional activation of NF-κB due to the interaction between β-glucan on the fungal cell surface and Dectin-1 present on host immune cells. In this proof-of-concept study, we screened a collection of botanicals (10 plants and some of their reported pure compound actives) used in traditional medicine for their antifungal properties. Several hits were identified in samples that unmasked β-glucan at sub-inhibitory concentrations. The hit samples were confirmed by fluorescent staining with a β-glucan antibody, verifying that the samples identified in the screen did indeed unmask β-glucan. These results indicate that the purported antifungal activities attributed to some botanicals may be due, at least in part, to the presence of compounds that exhibit β-glucan unmasking activity. Enhanced exposure of cell wall β-glucans would allow the host to build resilience against fungal infections by helping the immune system to detect the pathogen and mount a more effective clearance mechanism. This screen, together with direct killing/growth inhibition assays, may therefore serve as a valuable tool for substantiating the use of botanicals in preventing and/or treating fungal infections.