gms | German Medical Science

The SARS coronavirus (SCoV) spike (S) protein is the virus major surface antigen responsible for receptor binding and the generation of neutralising antibody. To investigate SCoV S protein, full length and individual domains of S have been expressed on the surface of insect cells and in secreted form using recombinant baculoviruses and characterised with respect to cleavability and reactivity with SARS convalescent sera and a panel of monoclonal antibodies. We found that S protein could be cleaved by trypsin, added exogenously, but not by co-expressed furin suggesting the protein is not normally processed to S1 and S2 during infection. Reactivity with patient sera was evident in both flow cytometry and western blot assays but the pattern of reactivity varied with both assay and the particular fragment of S antigen expressed. There was no reaction between SCoV S protein and human coronavirus 229E convalescent sera but limited reactivity with an OC43 serum. Epitope mapping of a panel of monoclonal antibodies revealed at least 8 distinct epitopes across the molecule. No MAb was found to inhibit binding of S to the receptor ACE2 in either cell surface or soluble phase binding assays. We conclude that the natural antibody response to SCoV S protein involves antibodies to both linear and conformational epitopes with linear epitopes concentrated toward the carboxyl domain of S and conformational epitopes associated with the amino terminal domain. S contains a number of distinct epitopes and appears not to generate high titres of neutralising antibody. Recombinant S is a suitable antigen for the development of efficient and sensitive diagnostic tests of infection but our data suggest that assay format and choice of S fragment are important considerations.