Human noroviruses are the causative agents of non-bacterial
gastroenteritis worldwide. The rapid onset and resolution of
disease symptoms suggest that innate immune responses are
critical for controlling norovirus infection; however, no effective
antivirals are yet available. The present study was conducted
to examine the antiviral activities of Schizonepeta
tenuifolia Briquet extract (STE) against noroviruses. Treatment
of human norovirus replicon-bearing HG23 cells with
STE at 5 and 10 mg/ml concentrations resulted in the reduction
in the viral RNA levels by 77.2% and 85.9%, respectively.
STE had no cytotoxic effects on HG23 cells. Treatment of
RAW 264.7 cells infected with murine norovirus 1 (MNV-1),
a surrogate virus of human noroviruses, with STE at 10 and
20 μg/ml concentrations resulted in the reduction of viral
replication by 58.5% and 84.9%, respectively. STE treatment
induced the expression of mRNAs for type I and type II interferons
in HG23 cells and upregulated the transcription of
interferon-β in infected RAW 264.7 cells via increased phosphorylation
of interferon regulatory factor 3, a critical transcription
regulator for type I interferon production. These results suggest that STE inhibits norovirus replication through
the induction of antiviral interferon production during virus
replication and may serve as a candidate antiviral substance
for treatment against noroviruses.