Anti-HSV-1 effect of dihydromyricetin from Ampelopsis grossedentata via the TLR9-dependent anti-inflammatory pathway

•High estimated HSV-1 infection rate among the adult population globally.•Dihydromyricetin (DHM) shows effective anti-HSV activity without significant cytotoxic effects.•Inhibitory effect on HSV-1 plaque formation, generation of progeny virus and expression of genes in all stages.•DHM establishes a Toll-like receptor 9 (TLR9)-dependent anti-inflammatory response in the anti-HSV-1 process. Herpes simplex virus 1 (HSV-1) is one of the most prevalent viruses in humans worldwide. Owing to limited therapeutic options mainly with acyclovir (ACV) and analogues and the emergence of ACV-resistant strains, new drugs with different modes of action and low toxicity are required. The aim of this study was to determine the anti-HSV-1 effect and mechanism of action of the flavonoid compound dihydromyricetin (DHM) from Ampelopsis grossedentata. The HSV-1 inhibitory effect of DHM was evaluated by measuring plaque formation and generation of progeny virus as well as expression of HSV-1-related genes in Vero cells. The molecular mechanism of the antiviral activity of DHM against HSV-1 was explored by real-time quantitative PCR and ELISA. DHM presented a significant inhibitory effect on HSV-1 plaque formation and generation of progeny virus, with an EC50 (50% effective concentration) of 12.56 μM in Vero cells. Furthermore, expression of HSV-1 immediate-early genes (ICP4 and ICP22), early genes (ICP8 and UL42) and late genes (gB, VP1/2) was decreased by DHM at concentrations of 16 μM and 32 μM. DHM specifically suppressed mRNA levels of Toll-like receptor 9 (TLR9), leading to inhibition of the inflammatory transcriptional factor NFκB and a decrease in TNFα. These findings indicate that the effective inhibitory activity of DHM was achieved by suppressing TNFα production in a TLR9-dependent manner. Although further studies are needed to better characterise the activity of DHM in vivo, the results suggest this extract as a promising new anti-HSV-1 agent.