Heterologous betacoronavirus spike immunization in non-human primates elicits antibodies that neutralize both sarbeco- and merbecoviruses

Summary In anticipation of future coronavirus (CoV) pandemics, developing vaccines that elicit broadly neutralizing antibodies (bnAbs) against diverse CoVs is critical. Here, we vaccinated rhesus macaques with the SARS-CoV-2 spike (S)-protein, then boosted with heterologous β-CoV S-proteins to focus responses to common conserved S2 bnAb epitopes. Initial SARS-CoV-2 priming elicited receptor-binding domain (RBD)-focused responses, while MERS-CoV boosting redirected responses toward the S2 region, including the stem-helix bnAb site. Although S2-directed serum cross-neutralization was undetectable and most isolated cross-reactive monoclonal antibodies (mAbs) targeted non-neutralizing epitopes, two S2 stem-helix mAbs were identified from memory B cells. These bnAbs neutralized diverse sarbeco- and merbecoviruses, including MERS-CoV, and conferred robust in vivo protection against SARS-CoV-2 challenge. Structural studies reveal that these macaque bnAbs closely mimic human S2 stem bnAbs induced by infection. These findings provide proof-of-principle for vaccination strategies that elicit broadly protective β-coronavirus responses and highlight non-human primates as a translational model for evaluating S2-targeted immunogens.