Current SARS-CoV-2 vaccines effectively prevent severe disease, but the immune protection they provide lacks the durability and breadth needed to stop transmission of highly divergent new variants, necessitating booster updates for longer-lasting immunity. Our study introduces a novel mRNA vaccine candidate, 3RBD, which focuses immunity on the receptor binding domain (RBD) of the spike protein, the primary target of neutralising antibodies, by presenting a polyvalent antigen that links three different RBDs via a flexible linker.
The 3RBD antigen, which includes RBDs from Beta, Delta, and BA.1 variants, conjugated to a C-terminal transmembrane domain, was expressed via optimised in vitro transcribed mRNA and delivered in a lipid nanoparticle (LNP) formulation. C7BL/6 mice were immunised intramuscularly with three doses of 1 µg, 3 µg, or 10 µg and subsequently challenged with a mouse-adapted strain of SARS-CoV-2 not represented in the vaccine. The 3RBD mRNA vaccine was also evaluated as a booster following two doses of full-length wildtype Spike mRNA vaccine.
Optimal antibody responses in naïve mice were achieved with three doses of 10 µg, as shown by ELISA binding and neutralising antibody assays. Mice elicited broad, polyspecific RBD antibodies against each variant in the 3RBD antigen, with increased responses observed following booster vaccination. Neutralisation assays detected neutralising antibodies against both the included variants and divergent RBDs. Notably, as a booster vaccine, mice were protected from infection upon challenge with a recently isolated mouse-infectious clinical strain, BA.2.75.
Our study highlights that an mRNA vaccine expressing a membrane-tethered 3RBD antigen can induce robust, polyspecific neutralising antibody responses against multiple variants of SARS-CoV-2, including those not represented in the vaccine. Thus, the 3RBD mRNA vaccine emerges as a promising concept for broad-spectrum protection against evolving SARS-CoV-2 variants.