Inhibition of influenza virus internalization by (−)-epigallocatechin-3-gallate

•EGCG inhibits the entry of influenza A and B viruses into cells with EC50 values between 5.7 and 17.3μM.•EGCG has no inhibitory effects on the viral HA and polymerase activities.•EGCG marginally inhibits the NA activity derived from influenza virus as well as from nonviral C. perfringens.•Upon EGCG treatment, influenza viral particles became resistant to NA and tightly fixed on the surface of cells.•EGCG damages influenza virus particle integrity, rendering viral membranes permeable and resulting in particle shrinkage. (−)-Epigallocatechin-3-gallate (EGCG), one of the major flavonoid components of green tea, is known to have a broad antiviral activity against several enveloped viruses, including the influenza virus. However, its mode of action and the mechanism that allows it to target influenza virus molecules have not been fully elucidated. Thus, this study investigated the molecular mechanism by which EGCG suppresses influenza virus infections. EGCG was found to block an early step in the influenza viral life cycle, but it did not affect viral adsorption to target cells or viral RNA replication. However, EGCG inhibited hemifusion events between virus particles and the cellular membrane by reducing the viral membrane integrity, thereby resulting in the loss of the cell penetration capacity of the influenza virus. EGCG also marginally suppressed the viral and nonviral neuraminidase (NA) activity in an enzyme-based assay system. In conclusion, it is suggested that the anti-influenza viral efficacy of EGCG is attributable to damage to the physical properties of the viral envelope and partial inhibition of the NA surface glycoprotein. These results may facilitate future investigations of the antiviral activity of EGCG against other enveloped viruses as well as influenza virus.