Development of a safe neutralization assay for SARS-CoV and characterization of S-glycoprotein

The etiological agent of severe acute respiratory syndrome (SARS) has been identified as a novel coronavirus SARS-CoV. Similar to other coronaviruses, spike (S)-glycoprotein of the virus interacts with a cellular receptor and mediates membrane fusion to allow viral entry into susceptible target cells. Accordingly, S-protein plays an important role in virus infection cycle and is the primary target of neutralizing antibodies. To begin to understand its biochemical and immunological properties, we expressed both full-length and ectodomain of the protein in various primate cells. Our results show that the protein has an electrophoretic mobility of about 160-170 kDa. The protein is glycosylated with high mannose and/or hybrid oligosaccharides, which account for approximately 30 kDa of the apparent protein mass. The detection of S-protein by immunoassays was difficult using human convalescent sera, suggesting that the protein may not elicit strong humoral immune response in virus-infected patients. We were able to pseudotype murine leukemia virus particles with S-protein and produce SARS pseudoviruses. Pseudoviruses infected Vero E6 cells in a pH-independent manner and the infection could be specifically inhibited by convalescent sera. Consistent with low levels of antibodies against S-protein, neutralizing activity was weak with 50% neutralization titers ranging between 1:15 to 1:25. To facilitate quantifying pseudovirus-infected cells, which are stained blue with X-Gal, we devised an automated procedure using an ELISPOT analyzer. The high-throughput capacity of this procedure and the safety of using SARS pseudoviruses should make possible large-scale analyses of neutralizing antibody responses against SARS-CoV.