Cell culture and transfection
HEK-293SF cells were maintained in serum-free suspension as previously described (D. M. Sharon et al., 2020). All transfections were performed at a cell density of 106 cell/ml using linear polyethylenimine with a mean molecular weight of 25000Da (Polysciences) complexed with plasmid DNA at a 1:2 ratio. The final concentration of plasmid DNA in all cases was 1ug/ml.