Cell-cell fusion assay

Cell-cell fusion assay was performed according to Ou et al. [13]. Briefly, A549 cells transduced with eGFP-luciferase-SARS-CoV-2 spike S1 lentivirus vector (GenScript, Piscataway, NJ) were detached with 1 mM EDTA, treated with indicated concentrations of selected polyphenols, for 1h at 37°C, and overlaid on 80–95% confluent human A549 lung epithelial cells overexpressing hACE2. After 4h incubation at 37°C, images of syncytia were captured with a Zeiss AxioObserver A1 fluorescence microscope (Carl Zeiss Meditec, Dublin, CA). The positive control was 20 μg/ml anti-ACE2 antibody. Control was 0.025% DMSO. Results are expressed as a percentage of polyphenol-free control (mean +/- SD, n = 3).