Structural basis of Omicron neutralization by affinity-matured public antibodies

The SARS-CoV-2 Omicron1 Variant of Concern (B.1.1.529) has spread rapidly in many countries. With a spike that is highly diverged from that of the pandemic founder, it escapes most available monoclonal antibody therapeutics2,3 and erodes vaccine protection4. A public class of IGHV3-53-using SARS-CoV-2 neutralizing antibodies5,6 typically fails to neutralize variants carrying mutations in the receptor-binding motif7-11, including Omicron. As antibodies from this class are likely elicited in most people following SARS-CoV-2 infection or vaccination, their subsequent affinity maturation is of particular interest. Here, we isolated IGHV3-53-using antibodies from an individual seven months after infection and identified several antibodies capable of broad and potent SARS-CoV-2 neutralization, extending to Omicron without loss of potency. By introducing select somatic hypermutations into a germline-reverted form of one such antibody, CAB-A17, we demonstrate the potential for commonly elicited antibodies to develop broad cross-neutralization through affinity maturation. Further, we resolved the structure of CAB-A17 Fab in complex with Omicron spike at an overall resolution of 2.6 [A] by cryo-electron microscopy and defined the structural basis for this breadth. Thus, public SARS-CoV-2 neutralizing antibodies can, without modified spike vaccines, mature to cross-neutralize exceptionally antigenically diverged SARS-CoV-2 variants.