Stabilized coronavirus spike stem elicits a broadly protective antibody

Current coronavirus (CoV) vaccines primarily target immunodominant epitopes in the S1 subunit, which are poorly conserved and susceptible to escape mutations, thus threatening vaccine efficacy. Here, we use structure-guided protein engineering to remove the S1 subunit from the Middle East respiratory syndrome (MERS)-CoV spike (S) glycoprotein and develop stabilized stem (SS) antigens. Vaccination with MERS SS elicits cross-reactive β-CoV antibody responses and protects mice against lethal MERS-CoV challenge. High-throughput screening of antibody-secreting cells from MERS SS-immunized mice led to the discovery of a panel of cross-reactive monoclonal antibodies. Among them, antibody IgG22 binds with high affinity to both MERS-CoV and severe acute respiratory syndrome (SARS)-CoV-2 S proteins, and a combination of electron microscopy and crystal structures localizes the epitope to a conserved coiled-coil region in the S2 subunit. Passive transfer of IgG22 protects mice against both MERS-CoV and SARS-CoV-2 challenge. Collectively, these results provide a proof of principle for cross-reactive CoV antibodies and inform the development of pan-CoV vaccines and therapeutic antibodies.

Tracing the SARS-coronavirus

Four coronaviruses (HCoV-229E, HCoV-OC43, HCoV-NL63, HCoV-HKU1) are endemic in humans and mainly associated with mild respiratory illnesses; whereas the other two coronaviruses [Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) and Middle East Respiratory Syndrome Coronavirus (MERS-CoV)] present as emerging infections causing severe respiratory syndrome. Coronaviruses evolve by accumulation of point mutations and recombination of genomes among different strains or species. Mammalian coronaviruses including those infect humans are evolved from bat coronaviruses. While SARS-CoV and MERS-CoV are genetically closely related to bat coronaviruses, intermediate host(s) is (are) likely to be involved in the emergence and cross-species transmission of these novel human viruses. High prevalence of SARS-like coronaviruses have been found from masked palm civet cats and raccoon dogs collected from markets around the time of outbreaks in humans, but these animals are likely to be a transient accidental host rather than a persisting reservoir. More research is needed to elucidate the ecology of coronaviruses. Vigilance and surveillance should be maintained to promptly identify newly emerged coronaviruses.