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. [Display omitted] •Structure-guided design generates MERS-CoV spike stabilized stem (SS) antigens•Cross-reactive CoV-spike-stem-specific monoclonal antibodies•Structures of the Fab22-spike complexes reveal a conserved, protective epitope•Passive transfer of IgG22 protects mice against MERS-CoV and SARS-CoV-2 challenge Hsieh et al. generate MERS-CoV spike stabilized stem (SS) antigens using a structure-guided approach. Mice immunized with MERS SS are protected against MERS-CoV challenge and used for isolation of cross-reactive monoclonal antibodies, including IgG22, which protects mice against MERS-CoV and SARS-CoV-2 challenges. Structures of Fab22-spike complexes reveal a conserved epitope.