Clonotype-specific convergent somatic hypermutations in the light chain play a key role in affinity maturation of a public antibody 4373

Abstract Description Public antibodies represent multiple instances of a particular antibody clonotype elicited in multiple individuals and thus provide an opportunity to study how affinity maturation pathways converge across individuals. We discovered a SARS-CoV-2 antibody, M15, that was elicited in response to monovalent XBB.1.5-mRNA COVID-19 vaccination and bound to the spike S2 domain. This antibody matched to 147 antibody sequences uniquely elicited upon SARS-CoV-2 exposure, identified through identical VJ gene annotations and >70% CDR3 similarity in both chains, in a second cohort of SARS-CoV-2 mRNA vaccinees and two databases. We then searched for convergent somatic hypermutations (SHM) in both heavy and light chains of these 147 M15-like antibodies. Strikingly, four convergent mutations in the light chain were highly specific to M15-like antibodies. When three of these mutations (S32I, G51A and T57I) were reverted concomitantly to their germline residues, we observed a significant decrease in binding affinity. Finally, we determined a high-resolution structure of M15 in complex with S2 using single-particle cryo-EM and showed that M15 binds to a novel epitope on S2. The epitope is buried in a pre-fusion conformation of spike but M15 binding seems to disrupt the trimer to fully gain access to the epitope. Overall, our results demonstrate convergence and specificity as key features of SHM that drive affinity maturation of M15, and provide structural insights into the public nature of M15. Funding Sources Supported by NIH FIRST U54CA267776; R01 AI168178; NIAID P01 AI172531; Human Immunology Project Consortium (HIPC) U19 AI168631 under award numbers S10OD026880 and S10OD030463 Topic Categories Lymphocyte Differentiation and Peripheral Maintenance (LYM)