Cell lines CHO-K1 and Vero E6 were purchased from American Type Cell Collection (Manassas, VA). Vero-E6 cells were cultured in Dulbecco's modified Eagle medium supplemented with 10% decomplemented fetal-calf serum, 2 mM l-glutamine, 100 U/mL penicillin, and 100 μg/mL streptomycin (Gibco, Paisley, UK).