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Abstract

The majority of SARS-CoV-2 vaccines in use or advanced developmentare based on the viral spike protein (S) as their immunogen. S is present on virionsas prefusion trimers in which the receptor binding domain (RBD) is stochasticallyopen or closed. Neutralizing antibodies have been described against both open andclosed conformations. The long-term success of vaccination strategies depends uponinducing antibodies that provide long-lasting broad immunity against evolvingSARS-CoV-2 strains. Here, we have assessed the results of immunization in a mousemodel using an S protein trimer stabilized in the closed state to prevent full expo-sure of the receptor binding site and therefore interaction with the receptor. Wecompared this with other modified S protein constructs, including representativesused in current vaccines. We found that all trimeric S proteins induced a T cellresponse and long-lived, strongly neutralizing antibody responses against 2019SARS-CoV-2 and variants of concern P.1 and B.1.351. Notably, the protein bindingproperties of sera induced by the closed spike differed from those induced bystandard S protein constructs. Closed S proteins induced more potent neutralizingresponses than expected based on the degree to which they inhibit interactionsbetween the RBD and ACE2. These observations suggest that closed spikes recruitdifferent, but equally potent, immune responses than open spikes and that this islikely to include neutralizing antibodies against conformational epitopes present inthe closed conformation. We suggest that closed spikes, together with their improved sta-bility and storage properties, may be a valuable component of refined, next-generationvaccines.