Cross-reactive SARS-CoV-2 epitope targeted across donors informs immunogen design.

The emergence of the antigenically distinct and highly transmissible Omicron variant highlights the possibility of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) immune escape due to viral evolution. This continued evolution, along with the possible introduction of new sarbecoviruses from zoonotic reservoirs, may evade host immunity elicited by current SARS-CoV-2 vaccines. Identifying cross-reactive antibodies and defining their epitope(s) can provide templates for rational immunogen design strategies for next-generation vaccines. Here, we characterize the receptor-binding-domain-directed, cross-reactive humoral repertoire across 10 human vaccinated donors. We identify cross-reactive antibodies from diverse gene rearrangements targeting two conserved receptor-binding domain epitopes. An engineered immunogen enriches antibody responses to one of these conserved epitopes in mice with pre-existing SARS-CoV-2 immunity; elicited responses neutralize SARS-CoV-2, variants, and related sarbecoviruses. These data show how immune focusing to a conserved epitope targeted by human cross-reactive antibodies may guide pan-sarbecovirus vaccine development, providing a template for identifying such epitopes and translating to immunogen design.

Humoral responses to the SARS-CoV-2 spike and receptor binding domain in context of pre-existing immunity confer broad sarbecovirus neutralization.

Since the emergence of SARS-CoV-2 (SARS-2), multiple vaccine candidates were developed and studied both preclinically and clinically. Nearly all are based on the SARS-2 spike glycoprotein or its receptor binding domain (RBD). Studies of these vaccine candidates have largely been in a SARS-2 naïve context. However, pre-existing immunity to SARS-2 acquired through infection or vaccination continues to increase. Evaluating future vaccine candidates in context of this pre-existing immunity is necessary to understand how immune responses are subsequently influenced. Here, we evaluated the serum and IgG+ B cell responses to the SARS-2 RBD in context of pre-existing immunity elicited by the full SARS-2 spike, and we compared this to boosting with the full SARS-2 spike. Boosting with the SARS-2 RBD resulted in increased reactivity to RBD epitopes, but both immunization regimens resulted in similarly broad neutralization across diverse sarbecoviruses. These findings may inform comparison among SARS-2 RBD-based vaccine candidates to currently approved spike-based candidates.

Rationally designed immunogens enable immune focusing following SARS-CoV-2 spike imprinting.

Eliciting antibodies to surface-exposed viral glycoproteins can generate protective responses that control and prevent future infections. Targeting conserved sites may reduce the likelihood of viral escape and limit the spread of related viruses with pandemic potential. Here we leverage rational immunogen design to focus humoral responses on conserved epitopes. Using glycan engineering and epitope scaffolding in boosting immunogens, we focus murine serum antibody responses to conserved receptor binding motif (RBM) and receptor binding domain (RBD) epitopes following severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike imprinting. Although all engineered immunogens elicit a robust SARS-CoV-2-neutralizing serum response, RBM-focusing immunogens exhibit increased potency against related sarbecoviruses, SARS-CoV, WIV1-CoV, RaTG13-CoV, and SHC014-CoV; structural characterization of representative antibodies defines a conserved epitope. RBM-focused sera confer protection against SARS-CoV-2 challenge. Thus, RBM focusing is a promising strategy to elicit breadth across emerging sarbecoviruses without compromising SARS-CoV-2 protection. These engineering strategies are adaptable to other viral glycoproteins for targeting conserved epitopes.

Rationally designed immunogens enable immune focusing following SARS-CoV-2 spike imprinting

Hauser et al. use structure-guided design to engineer SARS-2 immunogens that direct immune responses to conserved viral sites in context of preexisting immunity. In mice, these immunogens elicit antibodies that potently neutralize related coronaviruses, including those of potential pandemic concern. Structural characterization of selected antibodies explains this observation.

Naive human B cells engage the receptor binding domain of SARS-CoV-2, variants of concern, and related sarbecoviruses.

Initial exposure to a pathogen elicits an adaptive immune response to control and eradicate the threat. Interrogating the abundance and specificity of the naive B cell repertoire drives understanding of how to mount protective responses. Here, we isolated naive B cells from eight seronegative human donors targeting the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) receptor binding domain (RBD). Single-cell B cell receptor (BCR) sequencing identified diverse gene usage and no restriction on complementarity determining region length. A subset of recombinant antibodies produced by naive B cell precursors bound to SARS-CoV-2 RBD and engaged circulating variants including B.1.1.7, B.1.351, and B.1.617.2, as well as preemergent bat-derived coronaviruses RaTG13, SHC104, and WIV1. By structural characterization of a naive antibody in complex with SARS-CoV-2 spike, we identified a conserved mode of recognition shared with infection-induced antibodies. We found that representative naive antibodies could signal in a B cell activation assay, and by using directed evolution, we could select for a higher-affinity RBD interaction, conferred by a single amino acid change. The minimally mutated, affinity-matured antibodies also potently neutralized SARS-CoV-2. Understanding the SARS-CoV-2 RBD­specific naive repertoire may inform potential responses capable of recognizing future SARS-CoV-2 variants or emerging coronaviruses, enabling the development of pan-coronavirus vaccines aimed at engaging protective germline responses.

The persistence of interleukin-6 is regulated by a blood buffer system derived from dendritic cells.

The interleukin-6 (IL-6) membrane receptor and its circulating soluble form, sIL-6R, can be targeted by antibody therapy to reduce deleterious immune signaling caused by chronic overexpression of the pro-inflammatory cytokine IL-6. This strategy may also hold promise for treating acute hyperinflammation, such as observed in coronavirus disease 2019 (COVID-19), highlighting a need to define regulators of IL-6 homeostasis. We found that conventional dendritic cells (cDCs), defined in mice via expression of the transcription factor Zbtb46, were a major source of circulating sIL-6R and, thus, systemically regulated IL-6 signaling. This was uncovered through identification of a cDC-dependent but T cell-independent modality that naturally adjuvants plasma cell differentiation and antibody responses to protein antigens. This pathway was then revealed as part of a broader biological buffer system in which cDC-derived sIL-6R set the in-solution persistence of IL-6. This control axis may further inform the development of therapeutic agents to modulate pro-inflammatory immune reactions.