Immune-focused RBD nanoparticles induce cross-reactive, RBS-directed responses capable of variant-resistant SARS-CoV-2 neutralization

New SARS-CoV-2 variants pose an ongoing threat due to persistent immune escape of natural and vaccine-induced immunity. The emergence of BA.1 (Omicron) produced a large antigenic shift in the spike protein, rendering many antibodies ineffective with concomitant loss of Emergency Use Authorization (EUA) status. While strains have evolved far from BA.1, re-emergence of variants from branches closer to BA.1 are of recent concern. Here, we engineered a self-assembling nanoparticle displaying RBD 4mut g5.1, an immunogen developed using structure-guided design to focus antibody responses to the receptor binding site (RBS) epitope and promote cross-reactivity by inclusion of four rationally selected BA.1 mutations in the RBS. Unlike multi-component RBD approaches, we demonstrate a single, rationally designed component is sufficient for generating broad immunity. We demonstrate that in both naïve and antigen-experienced mice, RBD 4mut g5.1 nanoparticle induced cross-reactive and durable antibody responses capable of potent neutralization of ancestral SARS-CoV-2 and many Omicron variants. RBD 4mut g5.1 provided heterologous protection at a memory timepoint. By showcasing how subtle changes in an epitope can trigger a diversified antibody response, this study offers a promising new avenue for developing vaccines that can more effectively tackle the ever-evolving threat of immune escape, not only against SARS-CoV-2 but potentially against a range of variable pathogens.