Isatis indigotica Inhibits Influenza A Virus (H1N1) Virulent Protein Production and Autophagosome Accumulation

Isatis indigotica Fort. (I. indigotica) has a long history of use in the treatment of respiratory tract infections. Here, we investigated the anti-influenza effects exerted by I. indigotica on human alveolar epithelial cells via altering virulent protein production and disrupting autophagy. I. indigotica water extract was analysed by HPLC with epigoitrin specified as the fingerprint. Cellular viability was detected via XTT assay. Hemagglutination assay, plaque assay, and quantitative reverse transcription PCR were used to evaluate the inhibitory effects on H1N1 viral infection. The initial step of the viral infection was detected by attachment and penetration assays. Western blot was applied to measure the matrix-1 (M1), matrix-2 (M2), nucleoprotein, light chain 3 beta, and Beclin-1 protein levels. The extraction rate of the I. indigotica was 12.48%. The results of the hemagglutination assay, plaque assay, and quantitative reverse transcription PCR demonstrated that I. indigotica can effectively impede viral propagation. In addition, I. indigotica inhibited the production of the M1, M2, and nucleoprotein proteins, most notably in the M2 and M1 proteins, thereby hindering viral attachment, internalisation, and penetration. After treatment with I. indigotica, light chain 3 beta II levels decreased significantly, while we noted no marked change in the production of Beclin-1. This study demonstrates that I. indigotica water extract can rescue host cells from H1N1 influenza infection by inhibiting viral attachment, penetration, and propagation. This is achieved by significantly reducing virulent protein production and disrupting the process of autophagy. Collectively, our findings indicate that I. indigotica may provide an effective alternative treatment for patients infected with the H1N1 influenza virus.