RNA extraction and quantitative RT-PCR
RNA was extracted from cell culture supernatants and cells lysates using the ExtractRNA Reagent (Eurogen, Moscow, Russia) following the manufacturer’s instructions. Reverse transcription and PCR amplification were carried out with a one-step SARS-CoV-2 FRT kit (N.F. Gamaleya NRCEM, Moscow, Russia) according to the manufacturer’s instructions. The primers and probes were designed to target the gene coding NSP1 (Leader Protein) of SARS-CoV-2 (Fig.1d). The conditions of the one-step RT-qPCR reaction were as follows: 50°C for 15 min, 95°C for 5 min, followed by 45 cycles of 95°C for 10 s and 55°C for 1 min. The number of copies of viral RNA was calculated using a standard curve generated by amplification of plasmid DNA template.