SARS-CoV-2, the causative agent of
COVID-19, emerged at the end of 2019 and by mid-June 2020, the virus has spread to at least 215 countries, caused more than 8,000,000 confirmed
infections and over 450,000 deaths, and overwhelmed healthcare systems worldwide. Like SARS-CoV, which emerged in 2002 and caused a similar disease, SARS-CoV-2 is a betacoronavirus. Both viruses use human
angiotensin-converting enzyme 2 (hACE2) as a receptor to enter cells. However, the SARS-CoV-2 spike (S)
glycoprotein has a novel insertion that generates a putative
furin cleavage signal and this has been postulated to expand the host range. Two low passage (P) strains of SARS-CoV-2 (Wash1: P4 and Munich: P1) were cultured twice in Vero-E6 cells and characterized virologically. Sanger and MinION sequencing demonstrated significant deletions in the
furin cleavage signal of Wash1: P6 and minor variants in the Munich: P3 strain. Cleavage of the S
glycoprotein in SARS-CoV-2-infected Vero-E6 cell lysates was inefficient even when an intact
furin cleavage signal was present. Indirect immunofluorescence demonstrated the S
glycoprotein reached the cell surface. Since the S
protein is a major antigenic target for the development of
neutralizing antibodies we investigated the development of
neutralizing antibody titers in serial serum samples obtained from
COVID-19 human patients. These were comparable regardless of the presence of an intact or deleted
furin cleavage signal. These studies illustrate the need to characterize virus stocks meticulously prior to performing either in vitro or in vivo pathogenesis studies.