Amino terminus of the SARS coronavirus protein 3a elicits strong, potentially protective humoral responses in infected patients.

The 3a protein of severe acute respiratory syndrome (SARS)-associated coronavirus is expressed and transported to the plasma membrane in tissue cells of infected patients. Its short N-terminal ectodomain was found to elicit strong humoral responses in half of the patients who had recovered from SARS. The ectodomain-specific antibodies from the convalescent-phase plasma readily recognized and induced destruction of 3a-expressing cells in the presence of the human complement system, demonstrating their potential ability to provide immune protection by recognizing and eliminating SARS coronavirus-infected cells that express the target protein. In addition, when coupled to a carrier protein, the ectodomain peptide elicited 3a-specific antibodies in mice and rabbit at high titres. These results showed that the N terminus of the 3a protein is highly immunogenic and elicits potentially protective humoral responses in infected patients. Therefore, the short extracellular domain may be a valuable immunogen in the development of a vaccine for infectious SARS.