Mosaic RBD nanoparticles protect against challenge by diverse sarbecoviruses in animal models

To combat future severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants and spillovers of SARS-like betacoronaviruses (sarbecoviruses) threatening global health, we designed mosaic nanoparticles that present randomly arranged sarbecovirus spike receptor-binding domains (RBDs) to elicit antibodies against epitopes that are conserved and relatively occluded rather than variable, immunodominant, and exposed. We compared immune responses elicited by mosaic-8 (SARS-CoV-2 and seven animal sarbecoviruses) and homotypic (only SARS-CoV-2) RBD nanoparticles in mice and macaques and observed stronger responses elicited by mosaic-8 to mismatched (not on nanoparticles) strains, including SARS-CoV and animal sarbecoviruses. Mosaic-8 immunization showed equivalent neutralization of SARS-CoV-2 variants, including Omicrons, and protected from SARS-CoV-2 and SARS-CoV challenges, whereas homotypic SARS-CoV-2 immunization protected only from SARS-CoV-2 challenge. Epitope mapping demonstrated increased targeting of conserved epitopes after mosaic-8 immunization. Together, these results suggest that mosaic-8 RBD nanoparticles could protect against SARS-CoV-2 variants and future sarbecovirus spillovers. Description A mosaic approach to protection The COVID-19 pandemic has been ongoing for more than 2 years now, and new variants such as Omicron are less susceptible to the vaccines developed against earlier lineages of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). In addition, there is continued risk of spillovers of other animal sarbecoviruses into humans. There is thus a need for vaccines that will give broader protection. Cohen et al. developed mosaic nanoparticles that display the receptor-binding domains (RBDs) from SARS-CoV-2 and seven other animal sarbecoviruses. Mosaic nanoparticles protected against both SARS-CoV-2 and SARS-CoV challenges in animal models even though the SARS-CoV RBD was not present on the mosaic-8 RBD nanoparticles. By contrast, a homotypic SARS-CoV-2 RBD nanoparticle (presenting only SARS-CoV-2 RBDs) only protected against a SARS-CoV-2 challenge. —VV A mosaic sarbecovirus nanoparticle protects against SARS-2 and SARS-1, whereas a SARS-2 nanoparticle only protects against SARS-2. INTRODUCTION Two animal coronaviruses from the severe acute respiratory syndrome (SARS)–like betacoronavirus (sarbecovirus) lineage, SARS coronavirus (SARS-CoV) and SARS-CoV-2, have caused epidemics or pandemics in humans in the past 20 years. SARS-CoV-2 triggered the COVID-19 pandemic that has been ongoing for more than 2 years despite rapid development of effective vaccines. Unfortunately, new SARS-CoV-2 variants, including multiple heavily mutated Omicron variants, have prolonged the COVID-19 pandemic. In addition, the discovery of diverse sarbecoviruses in bats raises the possibility of another coronavirus pandemic. Hence, there is an urgent need to develop vaccines and therapeutics to protect against both SARS-CoV-2 variants and zoonotic sarbecoviruses with the potential to infect humans. RATIONALE To combat future SARS-CoV-2 variants and spillovers of sarbecoviruses threatening global health, we designed nanoparticles that present 60 randomly arranged spike receptor-binding domains (RBDs) derived from the spike trimers of eight different sarbecoviruses (mosaic-8 RBD nanoparticles) to elicit antibodies against conserved and relatively occluded—rather than variable, immunodominant, and exposed—epitopes. The probability of two adjacent RBDs being the same is low for mosaic-8 RBD nanoparticles, a feature chosen to favor interactions with B cells whose bivalent receptors can cross-link between adjacent RBDs to use avidity effects to favor recognition of conserved, but sterically occluded, RBD epitopes. By contrast, nanoparticles that present 60 copies of SARS-CoV-2 RBDs (homotypic RBD nanoparticles) are theoretically more likely to engage B cells with receptors that recognize immunodominant and sterically accessible, but less conserved, RBD epitopes. RESULTS We compared immune responses elicited by mosaic-8 (SARS-CoV-2 RBD plus seven animal sarbecoviruses RBDs) and homotypic (only SARS-CoV-2 RBDs) nanoparticles in mice and macaques and observed stronger responses elicited by mosaic-8 to mismatched (not represented with an RBD on nanoparticles) strains, including SARS-CoV and animal sarbecoviruses. Mosaic-8 immunization produced antisera that showed equivalent neutralization of SARS-CoV-2 variants, including Omicron variants, and protected from both SARS-CoV-2 and SARS-CoV challenges in mice and nonhuman primates (NHPs), whereas homotypic SARS-CoV-2 immunization protected from SARS-CoV-2 challenge but not from SARS-CoV challenge in mice. Epitope mapping of polyclonal antisera by using deep mutational scanning of RBDs demonstrated targeting of conserved epitopes after immunization with mosaic-8 RBD nanoparticles, in contrast with targeting of variable epitopes after homotypic SARS-CoV-2 RBD nanoparticle immunization, which supports the hypothesized mechanism by which mosaic RBD nanoparticle immunization can overcome immunodominance effects to direct production of antibodies against conserved RBD epitopes. Given the recent plethora of SARS-CoV-2 variants that may be arising at least in part because of antibody pressure, a relevant concern is whether more conserved RBD epitopes might be subject to substitutions that would render vaccines and/or monoclonal antibodies targeting these regions ineffective. This scenario seems unlikely because RBD regions conserved between sarbecoviruses and SARS-CoV-2 variants are generally involved in contacts with other regions of spike trimer and therefore less likely to tolerate selection-induced substitutions. CONCLUSION Together, these results suggest that mosaic-8 RBD nanoparticles could protect against SARS-CoV-2 variants and future sarbecovirus spillovers—in particular, highlighting the potential for a mosaic nanoparticle approach to elicit more broadly protective antibody responses than those with homotypic nanoparticle approaches. Mosaic RBD nanoparticle vaccination protects and elicits antibodies against conserved epitopes. Mosaic-8 elicited broader cross-reactive responses than those of homotypic nanoparticles. In a stringent infection model [K18-human angiotensin-converting enzyme 2 (K18-hACE2)], both protected against matched challenge (SARS-CoV-2 Beta), but only mosaic-8 also protected against a mismatch (SARS-CoV). Mosaic-8–immunized NHPs were protected against mismatched SARS-CoV-2 (Delta) and SARS-CoV. Mosaic-8–elicited antibodies predominantly bound conserved epitopes, whereas homotypic-elicited antibodies predominantly bound variable epitopes.