Inhibitory effect of (+)-catechin on the growth of influenza A/PR/8 virus in MDCK cells.

We investigated whether (+)-catechin, a building block of tannins contained in the extract of Ephedrae herba (EHext), exerts an inhibitory effect on the acidification of intracellular compartments such as endosomes and lysosomes (referred to as ELS), and thereby inhibits the growth of influenza A PR/8/34 (PR8) virus (H1N1 subtype) in Madin-Darby canine kidney cells. The vital fluorescence microscopic study with acridine orange showed that 1-h treatment with (+)-catechin inhibited the acidification of ELS in a concentration-dependent manner (1.0-10.0 mM). Moreover, the growth of PR8 virus was inhibited markedly when the cells were treated with (+)-catechin (1.25-10.0 mM) for 1 h immediately after infection, or treated within as little as 5 to 10 min after infection. Conversely, virus growth resumed within 3 h concomitantly with the reappearance of acidified ELS after removal of (+)-catechin. Similar to EHext, (+)-catechin inhibited both the acidification of ELS and the influenza virus growth. It suggests that (+)-catechin is one of the active components in EHext.