Abstract

Efforts to mitigate COVID-19 include screening of existing antiviral molecules that could be re-purposed to treat SARS-CoV-2 infections. Although SARS-CoV-2 propagates efficiently in African green monkey kidney (Vero) cells, antivirals such as nucleos(t)ide analogs (nucs) often exhibit decreased activity in these cells due to inefficient metabolization. Limited SARS-CoV-2 replication and propagation occurs in human cells, which are the most relevant testing platforms. By performing serial passages of a SARS-CoV-2 isolate in the human hepatoma cell line clone Huh7.5, we selected viral populations with improved viability in human cells. Culture adaptation led to the emergence of a significant number of high frequency changes (>90% of the viral population) in the region coding for the spike glycoprotein, including a deletion of nine amino acids in the N-terminal domain and 3 amino acid changes (E484D, P812R, and Q954H). We demonstrated that the Huh7.5-adapted virus exhibited a >3-Log10 increase in infectivity titers (TCID50) in Huh7.5 cells, with titers of ~8 Log10TCID50/mL, and >2-Log10 increase in the human lung cancer cell line Calu-1, with titers of ~6 Log10TCID50/mL. Culture adaptation in Huh7.5 cells further permitted efficient infection of the otherwise SARS-CoV-2 refractory human lung cancer cell line A549, with titers of ~6 Log10TCID50/mL. The enhanced ability of the virus to replicate and propagate in human cells permitted screening of a panel of nine nucs, including broad-spectrum compounds. Remdesivir, EIDD-2801 and to a limited extent galidesivir showed antiviral effect across these human cell lines, whereas sofosbuvir, uprifosbuvir, valopicitabine, mericitabine, ribavirin, and favipiravir had no apparent activity. ImportanceThe cell culture adapted variant of the SARS-CoV-2 virus obtained in the present study, showed significantly enhanced replication and propagation in various human cell lines, including lung derived cells otherwise refractory for infection with the original virus. This SARS-CoV-2 variant will be a valuable tool permitting investigations across human cell types, and studies of identified mutations could contribute to our understanding of viral pathogenesis. In particular, the adapted virus can be a good model for investigations of viral entry and cell tropism for SARS-CoV-2, in which the spike glycoprotein plays a central role. Further, as shown here with the use of remdesivir and EIDD-2801, two nucs with significant inhibitory effect against SARS-CoV-2, large differences in the antiviral activity are observed depending on the cell line. Thus, it is essential to select the most relevant target cells for pre-clinical screenings of antiviral compounds, facilitated by using a virus with broader tropism.