Aim of the study: Extracts from the aerial parts of the South African resurrection plant Myrothamnus flabellifolia Welw. have been used traditionally against infections of the upper respiratory tract and skin diseases. A polyphenol-enriched extract was investigated for potential antiviral effects against herpes simplex virus type 1 (HSV-1) and adenovirus, and the underlying mode of action was to be studied.
Materials and methods: Antiviral effects of an acetone-water extract (MF) from Myrothamnus flabellifolia on HSV-1 and adenovirus type 3 were tested in infected Vero cells by plaque reduction assay, MTT test and immunofluorescence. The influence of the extract on the HSV-1 envelope glycoprotein D was shown by Western blot. Organotypic full thickness skin models consisting of multilayer skin equivalents were used for the investigation of MF effects on HSV-1 replication.
Results: MF exhibited strong antiviral activity against HSV-1. The HSV-1-specific inhibitory concentration (IC(50)) was determined as 0.4 μg/mL and the cytotoxic concentration (CC(50)) against Vero cells as 50 μg/mL. A selectivity index (SI) (ratio of CC(50) to IC(50)) of approximately 120 was calculated when MF was added to the virus inoculum for 1h at 37°C prior to infection. The replication of adenovirus 3 was not affected by MF. MF abolished virus entry into the host cell by blocking viral attachment to the cell surface. When added after attachment at a concentration of >6 μg/mL, the extract also inhibited penetration of HSV-1 into the host cell. Polyphenolic compounds from MF directly interacted with viral particles, leading to the oligomerisation of envelope proteins as demonstrated for the essential viral glycoprotein D (gD). Using organotypic full thickness tissue cultures, it was shown that treatment of HSV-1 infected cultures with the MF resulted in reduced viral spread.
Conclusions: A polyphenol-enriched extract from Myrothamnus flabellifolia strongly acts against HSV-1 by blocking viral entry into the cells.
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