Antiviral potential and mode of action of Metarhizium pinghaense 15R-derived polysaccharide against baculovirus

Polysaccharides have been reported to function as antiviral agents against various viruses. In this study, a polysaccharide extracted from Metarhizium pinghaense 15R demonstrated inhibitory effects against Bombyx mori nucleopolyhedrovirus (BmNPV). When Bm5 cells were co-inoculated with the M. pinghaense 15R -derived polysaccharide and eGFP-cloned BmNPV budded virus (BV), eGFP expression was significantly reduced compared to the virus-only control group. Additionally, the polysaccharide-treated group showed greater cell viability and proliferation than the ribavirin-treated group. Further investigation into the antiviral mechanism revealed that the polysaccharide inhibited both viral attachment and secondary infection of BmNPV. This antiviral activity is likely due to its high sulfate content (18 %), which is widely associated with enhanced viral inhibition. A silkworm larva bioassay demonstrated that when the occlusion body (OB) and polysaccharide were combined and administered via feeding, survival rates on Day 7 were significantly higher than those observed in the OB-only treatment group. Similarly, when the polysaccharide was injected along with BV, a significant increase in survival was observed on Day 8 when compared to the BV-only treatment group. Furthermore, the co-injection of polysaccharide and BV resulted in the upregulation of immune-related genes associated with antimicrobial peptides (AMPs), such as gloverin 2, lebocin, hemolin, defensin, and beta-glucan recognition protein (βGRP), in B. mori within 48 h. These findings suggest that the polysaccharide enhances the immune response in B. mori and has potential as a natural antiviral agent for insect viruses.