Evaluation of antibody responses against SARS coronaviral nucleocapsid or spike proteins by immunoblotting or ELISA.

Abstract

Severe acute respiratory syndrome (SARS)‐CoV is a newly emerging virus that causes SARS with high mortality rate in infected people. To study the humoral responses against SARS‐CoV, we evaluated nucleocapsid (N) and spike (S) proteins‐specific antibodies in patients' sera by Western blotting and enzyme‐linked immunosorbent assay (ELISA). Recombinant N and S proteins of SARS‐CoV were purified from transformed E. coli. Serum specimens from 40 SARS‐CoV‐infected patients in the convalescent phase were analyzed by Western blotting using the purified antigens. Serial serum specimens from 12 RT‐PCR‐confirmed SARS patients were assayed by ELISA using the recombinant N protein as coated antigen. By Western blotting, 97.5% of the SARS patients were positive for N protein‐specific antibodies whereas only 47.5% of the samples were positive for S protein‐specific antibodies. Using N protein‐based ELISA, 10 out of the 12 patients were positive for N protein‐specific antibodies and 6 of them showed seroconversion at mean of 16 days after onset of fever. Immunoblotting was useful for detecting the humoral immune response after SARS‐CoV infection. Antibodies against SARS‐CoV N protein appear at the early stage of infection, therefore, N protein‐based ELISA could serve as a simple, sensitive, and specific test for diagnosing SARS‐CoV infection. J. Med. Virol. 73:338–346, 2004. © 2004 Wiley‐Liss, Inc.