Highlights d Few naturally acquired mutations are required for the function of HIV bnAb DH270.6 d Acquired improbable mutations are critical for neutralization d Immunogens to elicit lineage Abs containing key DH270.6 mutations are identified d More probable evolution pathways to induce DH270.6-like bnAbs likely exist

SummaryElicitation of broadly neutralizing antibodies (bnAbs) by an HIV vaccine will involve priming the immune system to activate antibody precursors, followed by boosting immunizations to select for antibodies with functional features required for neutralization breadth. The higher the number of mutations necessary for function, the more convoluted are the antibody developmental pathways. HIV bnAbs acquire a large number of somatic mutations, but not all mutations are functionally important. Here we identified a minimal subset of mutations sufficient for the function of to the naturally occurring, more mutated V3-glycan bnAb DH270.6. Using antibody library screening, candidate envelope immunogens that interacted with DH270.6-like antibodies containing this set of key mutations were identified and selected in vitro. Our results demonstrate that less complex B cell evolutionary pathways than those naturally observed exist for the induction of HIV bnAbs by vaccination, and establish rational approaches to identify boosting sequential envelope candidate immunogens.

Immune responses to SARS-CoV-2 primarily target the receptor binding domain of the spike protein, which can readily mutate to escape acquired immunity. Other regions in the spike S2 subunit, such as the fusion peptide and the stem helix, are highly conserved across sarbecoviruses and recognized by broadly reactive antibodies, providing hope that targeting these epitopes by vaccination could offer protection against both current and emergent viruses. Here we employed computational modeling to design epitope scaffolds that display the fusion peptide and the stem helix epitopes. The engineered proteins bound both mature and germline versions of multiple broad and protective human antibodies with high affinity. Binding specificity was confirmed both biochemically and via high resolution crystal structures. Finally, the epitope scaffolds showed potent engagement of antibodies and memory B-cells from subjects previously exposed to SARS-CoV2, illustrating their potential to elicit antibodies against the fusion peptide and the stem helix by vaccination.

B cell lineages that are the current focus of vaccine development efforts against HIV-1, influenza or coronaviruses, often contain rare features, such as long heavy chain complementarity determining regions (CDRH3) loops. These unusual characteristics may limit the number of available B cells in the natural immunoglobulin repertoire that can respond to pathogen vaccinations. To measure the ability of a given immunogen to engage naturally occurring B cell receptors of interest, here we describe a mixed experimental and bioinformatic approach for determining the frequency and sequence of CDRH3 loops in the immune repertoire that can be recognized by a vaccine candidate. By combining deep mutational scanning and B cell receptor database analysis, CDRH3 loops were found that can be engaged by two HIV-1 germline-targeting immunogens, thus illustrating how the methods described here can be used to evaluate candidate immunogens based on their ability to engage diverse B cell lineage precursors.

Elicitation of broadly neutralizing antibodies (bnAbs) by an HIV vaccine will involve priming the immune system to activate antibody precursors, followed by boosting immunizations to select for antibodies with functional features required for neutralization breadth. The higher the number of acquired mutations necessary for function, the more convoluted are the antibody developmental pathways. HIV bnAbs acquire a large number of somatic mutations, but not all mutations are functionally important. In this study, we identify a minimal subset of mutations sufficient for the function of the naturally occurring V3-glycan bnAb DH270.6. Using antibody library screening, candidate envelope immunogens that interact with DH270.6-like antibodies containing this set of key mutations are identified and selected in vitro. Our results demonstrate that less complex B cell evolutionary pathways than those naturally observed exist for the induction of HIV bnAbs by vaccination, and they establish rational approaches to identify boosting candidate immunogens.