Vero E6 cells (CRL-1586, American Type Culture Collection [ATCC]) and 293FT/hACE2+TMPRSS (18 (link)) were cultured in high glucose Dulbecco’s Modified Eagle medium (DMEM, Thermo Fisher Scientific) supplemented with 10% fetal bovine serum (FBS, Thermo Fisher Scientific) at 37°C with 5% CO2. Rat lung epithelial cells L2 (CCL-149, ATCC) were cultured in F-12K Medium (ATCC) supplemented with 10% FBS at 37°C with 5% CO2. Rat primary tracheal epithelial cells (Cell Biologics) were grown on culture flasks or plates pre-coated with gelatin-based coating solution (Cell Biologics) in Complete Epithelial Cell Medium (Cell Biologics) at 37°C with 5% CO2.