Profiles of IgG antibodies to nucleocapsid and spike proteins of the SARS-associated coronavirus in SARS patients.

To evaluate humoral immunity against the SARS-associated coronavirus (SARS-CoV), we studied the profiles of IgG antibodies to the nucleocapsid (N) and spike (S) proteins of SARS-CoV. Serum specimens from 10 SARS patients were analyzed by Western blotting and an enzyme-linked immunosorbent assay (ELISA) using purified recombinant N and truncated S (S1, S2, and S3) proteins as antigens. Western blotting results demonstrated that 100% of the SARS patients tested positive for N protein-specific antibodies, 50% for S1 protein-specific antibodies, 30% for S2 protein-specific antibodies, and 70% for S3 protein-specific antibodies. The ELISA results, which showed positive rates of IgG reactivity against recombinant proteins N, S1, S2, and S3, were, respectively, 28.57, 14.29, 14.29, and 14.29% at week 1, 77.78, 55.56, 44.44, and 66.67% at week 2, 100, 75, 75, and 87.5% at week 3, and 100, 77.78, 77.78, and 88.89% after 3 weeks. The average titers of IgG against recombinant proteins N, S1, S2, and S3 were, respectively, 691, 56, 38, and 84 after 3 weeks. These results suggest that the recombinant proteins N and S3 are potentially useful antigens for a serological diagnosis of SARS. In consideration of possible cross-reactivity among N proteins of SARS-CoV and other coronaviruses, immunoassays using recombinant N protein in combination with S3 as antigens might improve the specificity of SARS diagnoses.