Immunogenicity of a receptor-binding domain of SARS coronavirus spike protein in mice: Implications for a subunit vaccine

We studied the immunogenicity of an anti-SARS subunit vaccine comprised of the fragment of the SARS coronavirus (SARS-CoV) spike protein amino acids 318–510 (S318–510) containing the receptor-binding domain. The S protein fragment was purified from the culture supernatant of stably transformed HEK293T cells secreting a tagged version of the protein. The vaccine was given subcutaneously to 129S6/SvEv mice in saline, with alum adjuvant or with alum plus CpG oligodeoxynucleotides (ODN). Mice immunized with the adjuvanted antigen elicited strong antibody and cellular immune responses; furthermore, adding the CpG ODN to the alum resulted in increased IgG2a antibody titers and a higher number of INF-γ-secreting murine splenocytes. Mice vaccinated with S318–510 deglycosylated by PNGase F (dgS318–510) showed a lower neutralizing antibody response but had similar numbers of INF-γ-producing cells in the spleen. This finding suggests that carbohydrate is important for the immunogenicity of the S318–510 protein fragment and provide useful information for designing an effective and safe SARS subunit vaccine.