Severe acute respiratory syndrome coronavirus and viral mimicry.

Sir—Viruses have evolved various strategies to counteract host-cell processes, notably our immune responses toward their invasion. Oleszak and colleagues reported that the spike proteins of coronaviruses, particularly the mouse hepatitis virus, bovine coronavirus, and transmissible gastroenteritis virus, display Fc gamma receptor activity. Such molecular mimicry of components of our own immune system may even have a role in viral pathogenicity. This phenomenon, if shown to be true with the severe acute respiratory syndrome (SARS) virus, could help to explain why seemingly healthy individuals with active immune responses make up a large proportion of the patients who die from SARS. This mimicking of the Fc receptor might result in binding of non-specific IgG to the mimic receptor, expressed on virus or virus-infected cells, which would then inhibit neutralisation by virus-specific IgG because of steric hindrance. Such mimicry might also protect infected cells from antibody-dependent cellmediated cytotoxicity and complement neutralisation. Antibody-dependent enhancement of infection has been described for certain viruses, including the feline infectious peritonitis virus (FIPV), which is a coronavirus. Such enhancement involves the binding of virus-antibody complexes to Fc or complement receptors on the surface of monocytes or macrophages, resulting in virus uptake via receptor-mediated endocytosis, instead of neutralising the infection. With FIPV, antibodydependent enhancement is mediated by specific sites on the spike protein. Whether the SARS virus exhibits such properties needs to be investigated. Viruses affect the occurrence or course of certain autoimmune diseases. Talbot and colleagues have suggested that molecular mimicry between part of the human coronavirus (HCV) 229E and myelin basic protein of the central nervous system might contribute to the pathogenesis of multiple sclerosis. Autoreactive T cells specific to myelin components are cross-reactive with HCV 229E, and coronavirus-like particles have been isolated from patients with multiple sclerosis. We need to be vigilant as to whether the SARS virus (and vaccine) would also have such mimicry.