Broad cross-reactivity across sarbecoviruses exhibited by a subset of COVID-19 donor-derived neutralizing antibodies

Jette, Claudia A.; Cohen, Alexander A.; Gnanapragasam, Priyanthi N.P.; Muecksch, Frauke; Lee, Yu E.; Huey-Tubman, Kathryn E.; Schmidt, Fabian; Hatziioannou, Theodora; Bieniasz, Paul D.; Nussenzweig, Michel C.; West, Anthony P.; Keeffe, Jennifer R.; Bjorkman, Pamela J.; Barnes, Christopher O.

Many anti-SARS-CoV-2 neutralizing antibodies target the ACE2-binding site on viral spike receptor-binding domains (RBDs). Potent antibodies recognize exposed variable epitopes, often rendering them ineffective against other sarbecoviruses and SARS-CoV-2 variants. Class 4 anti-RBD antibodies against a less-exposed, but more-conserved, cryptic epitope could recognize newly-emergent zoonotic sarbecoviruses and variants, but usually show only weak neutralization potencies. Here, we characterize two class 4 anti-RBD antibodies derived from COVID-19 donors that exhibit breadth and potent neutralization of zoonotic coronaviruses and SARS-CoV-2 variants. C118-RBD and C022-RBD structures reveal orientations that extend from the cryptic epitope to occlude ACE2 binding, and CDRH3-RBD mainchain H-bond interactions that extend an RBD β-sheet, thus reducing sensitivity to RBD sidechain changes. A C118-spike trimer structure reveals rotated RBDs that allow access to the cryptic epitope and the potential for intra-spike crosslinking to increase avidity. These studies facilitate vaccine design and illustrate potential advantages of class 4 RBD-binding antibody therapeutics.