An ultra-long heavy chain bovine antibody neutralizes SARS-CoV-2 and reacts broadly with sarbecoviruses

The threat of emergence of further SARS-CoV-2 variants, and the future spillover potential of other sarbecoviruses has prompted continued efforts to isolate broadly reactive monoclonal antibodies for therapeutic use. In this study, we generated monoclonal antibodies from immunised cattle, primarily because of their ability to produce antibodies with ultra-long heavy chain complementarity determining region 3 (CDRH3) domains. Such antibodies have been shown to have potent and cross-reactive neutralisation phenotypes in other virus infections. Following extended immunisation with different forms of spike protein and using single B-cell sorting and phage display techniques, we isolated 33 mAbs, including 10 with ultra-long CDRH3s (>50 amino acids). Of these, mAbs P7 and 99 exhibited remarkable neutralisation breadth and potency. Notably, mAb P7, which possessed an ultra-long CDRH3, neutralised all tested variants, including SARS-CoV-1, with IC ₅₀ values ranging from 0.01 µg/mL to 1.06 µg/mL. This antibody was also cross-reactive against a panel of RBDs from diverse sarbecovirus species. Structural studies revealed that mAb 99 targets a region of the receptor-binding domain (RBD) of the spike protein that overlaps with the ACE2 binding site. Although the structure of the P7 Fab-RBD complex was not resolvable, data suggest P7 induces trimer dissociation by binding to an occluded RBD epitope, likely mediated by the extended CDRH3 structure. Syrian hamster challenge experiments, using several VOCs, showed that mAbs P7 and 99 significantly reduced lung viral load. These findings highlight the potential of bovine-derived, especially those possessing ultra-long CDRH3s, as effective therapeutics against current and future sarbecovirus threats.