An Engineered HR1-Stem Helix-HR2 Trimeric Platform for Developing Broad-Spectrum Coronavirus Vaccines.

The rapid spread of immune-evasive viral variants, exemplified by SARS-CoV-2, highlights the urgent need for broad-spectrum coronavirus vaccines. Although multimeric display of the receptor-binding domain (RBD) using exogenous scaffolds improve immune responses, such approaches may elicit off-target immune responses against the scaffolds themselves and remain limited by rapid RBD antigenic drift. Here, we report the rational design of a self-assembling trimeric subunit vaccine, termed RBD-heptad repeat 1 (HR1)-stem helix (SH)-heptad repeat 2 (HR2) (RHS), which integrates the JN.1 RBD with a highly conserved SH epitope from SARS-CoV-2 within a native HR1-HR2 trimeric scaffold via optimized linkers. RHS exhibits high structural stability, efficient trimerization, and enhanced antigen presentation. In mice, RHS elicits robust humoral and cellular immune responses, including potent cross- neutralizing antibodies against diverse SARS-CoV-2 variants and pan-betacoronavirus SH-specific antibodies. In K18-hACE2 mice, RHS confers strong protection against both antigen-matched and antigen-mismatched Omicron variants, while intranasal immunization induces potent mucosal IgA and IgG responses. Furthermore, the modular RHS platform is readily adaptable to antigens from SARS-CoV and MERS-CoV, underscoring its versatility for pan-coronavirus vaccine development. Together, these findings establish RHS as a generalizable strategy for overcoming viral immune evasion and advancing next-generation vaccine design.