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.

Keywords:

• antifungal
• black walnut
• dectin-1
• β-glucan unmasking
• juglone
• pseudolaric acid B

Acknowledgments

Research reported in this publication was supported by a grant from the USDA, Agricultural Research Service Specific Cooperative Agreement No. 58-6060-6-015.

Disclosure statement

All authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability statement

All data and materials are available upon request by contacting Dr. Nirmal Pugh (email: [email protected]).

Additional information

Notes on contributors

Jin Zhang

Jin Zhang, is a Research Scientist at the National Center for Natural Products Research, School of Pharmacy, University of Mississippi, with extensive research experience in natural products/biological chemistry. Jin Zhang’s interests include immune enhancing, anticancer, cell signaling, high throughput screening, isolation and structure elucidation of biologically active compounds in natural products.

Ameeta K. Agarwal

Ameeta Agarwal has a PhD in Biology, and has expertise in the areas of microbiology, developmental biology, and genomics. Her research interests include infectious diseases, natural products, and drug discovery.

Qin Feng

Qin Feng is a Principal R&D Biologist at the National Center for Natural Products Research.

Siddharth K. Tripathi

Siddharth Tripathi has a PhD in Botany and has a research focus on drug discovery and mechanistic evaluations of natural products.

Ikhlas A. Khan

Ikhlas Khan, is Director and Research Professor of Pharmacognosy at the National Center for Natural Products Research. Main areas of research include phytomedicine, drug discovery, medicinal plants research, analytical fingerprinting for standardization of herbal products, bio-analytical approaches to improvement of product quality and safety, development of analytical methods for phytochemical characterization of biologically active natural products.

Nirmal D. Pugh

Nirmal Pugh, is a Principal Scientist at the National Center for Natural Products Research with a background in pharmacognosy and performs research on botanicals.