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Broadly neutralizing antibodies (bnAbs) to coronaviruses (CoVs) are valuable in their own right as prophylactic and therapeutic reagents to treat diverse CoVs and, importantly, as templates for rational pan-CoV vaccine design. We recently described a bnAb, CC40.8, from a coronavirus disease 2019 (COVID-19)-convalescent donor that exhibits broad reactivity with human beta-coronaviruses (β-CoVs). Here, we showed that CC40.8 targets the conserved S2 stem-helix region of the coronavirus spike fusion machinery. We determined a crystal structure of CC40.8 Fab with a SARS-CoV-2 S2 stem-peptide at 1.6 Å resolution and found that the peptide adopted a mainly helical structure. Conserved residues in β-CoVs interacted with CC40.8 antibody, thereby providing a molecular basis for its broad reactivity. CC40.8 exhibited in vivo protective efficacy against SARS-CoV-2 challenge in two animal models. In both models, CC40.8-treated animals exhibited less weight loss and reduced lung viral titers compared to controls. Furthermore, we noted CC40.8-like bnAbs are relatively rare in human COVID-19 infection and therefore their elicitation may require rational structure-based vaccine design strategies. Overall, our study describes a target on β-CoV spike proteins for protective antibodies that may facilitate the development of pan-β-CoV vaccines.

We recently described CC40.8 bnAb from a COVID-19 donor that exhibits broad reactivity with human β-CoVs. Here, we show that CC40.8 targets the conserved S2 stem-helix region of the coronavirus spike fusion machinery. We determined a crystal structure of CC40.8 Fab with a SARS-CoV-2 S2 stem-peptide at 1.6 A resolution and found that the peptide adopts a mainly helical structure. Conserved residues in β-CoVs interact with the antibody, thereby providing a molecular basis for its broad reactivity. CC40.8 exhibits in vivo protective efficacy against SARS-CoV-2 challenge in a hamster model with reduction in weight loss and lung viral titers. Furthermore, we noted CC40.8-like bnAbs are relatively rare in human COVID-19 infection and therefore their elicitation may require rational vaccine strategies. Overall, our study describes a new target on CoV spikes for protective antibodies that may facilitate the development of pan-β-CoV vaccines.

Our data provide new insights into the specific antibody repertoires and the molecular determinants of neutralization during natural MERS-CoV infection in humans. This finding supports additional efforts to design and develop novel therapies to combat MERS-CoV infections in humans.

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A major challenge in understanding SARS-CoV-2 evolution is interpreting the antigenic and functional effects of emerging mutations in the viral spike protein. Here we describe a new deep mutational scanning platform based on non-replicative pseudotyped lentiviruses that directly quantifies how large numbers of spike mutations impact antibody neutralization and pseudovirus infection. We demonstrate this new platform by making libraries of the Omicron BA.1 and Delta spikes. These libraries each contain ~7000 distinct amino-acid mutations in the context of up to ~135,000 unique mutation combinations. We use these libraries to map escape mutations from neutralizing antibodies targeting the receptor binding domain, N-terminal domain, and S2 subunit of spike. Overall, this work establishes a high-throughput and safe approach to measure how ~'10 to the 5' combinations of mutations affect antibody neutralization and spike-mediated infection. Notably, the platform described here can be extended to the entry proteins of many other viruses.

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The major mechanism of antibody-mediated neutralization of the Middle East respiratory syndrome coronavirus (MERS-CoV) involves competition with the cellular receptor dipeptidyl peptidase 4 (DPP4) for binding to the receptor-binding domain (RBD) of the spike (S) glycoprotein. Here, we report a unique epitope and unusual neutralizing mechanism of the isolated human antibody MERS-4. Structurally, MERS-4 approached the RBD from the outside of the RBD-DPP4 binding interface. Such binding resulted in the folding of the β5-β6 loop toward a shallow groove on the RBD interface critical for accommodating DPP4. The key residues for binding are identified through site-directed mutagenesis. Structural modeling revealed that MERS-4 binds to RBD only in the "up" position in the S trimer. Furthermore, MERS-4 demonstrated synergy with several reported antibodies. These results indicate that MERS-4 neutralizes MERS-CoV by indirect rather than direct competition with DPP4. This mechanism provides a valuable addition for the combined use of antibodies against MERS-CoV infection.