Broad protective RBD heterotrimer vaccines neutralize SARS-CoV-2 including Omicron sub-variants XBB/BQ.1.1/BF.7.

SARS-CoV-2 variants with severe immune evasion are a major challenge for COVID-19 prevention, especially the circulating Omicron XBB/BQ.1.1/BF.7 strains. Thus, the next-generation of broad-spectrum vaccines are urgently needed. Previously, we developed a COVID-19 protein subunit vaccine, ZF2001, based on the RBD-homodimer as the immunogen. To adapt SARS-CoV-2 variants, we developed chimeric RBD-heterodimers to induce broad immune responses. In this study, we further explored the concept of tandem RBD homotrimer and heterotrimer. Prototype SARS-CoV-2 RBD-homotrimer, prototype-Delta-BA.1 (PDO) RBD-heterotrimer and Delta-BA.2-BA.5 (DBA2BA5) RBD-heterotrimer were designed. Biochemical and cryo-EM structural characterization demonstrated total epitope exposure of the RBD-trimers. In mouse experiments, PDO and DBA2BA5 elicited broad SARS-CoV-2 neutralization. Potent protection against SARS-CoV-2 variants was observed in challenge assays and was correlated with neutralizing antibody titer. This study validated the design strategy of tandem RBD-heterotrimers as multivalent immunogens and presented a promising vaccine candidate, DBA2BA5, eliciting broad-spectrum immune responses, including against the circulating XBB/BF.7/BQ.1.1. Author summary: SARS-CoV-2 variants continue to emerge and circulate. Omicron sub-variants show severe evasion of the immune response induced by early-approved vaccines. Bivalent mRNA vaccines (prototype and Omicron BA. 5) have been approved for use in the United States and other countries. However, the co-circulation of SARS-CoV-2 variants (such as BA. 5.2, BF. 7, BQ. 1.1 and XBB) poses a challenge to disease control. The next-generation of COVID-19 vaccines need to induce potent and broad-spectrum immune responses. Here, we demonstrate a design strategy for trivalent SARS-CoV-2 vaccines with RBD-heterotrimers, which are single protein molecules without any linker sequences or trimerization domain. Biochemical and cryo-EM structural characterization demonstrated the total epitope exposure of RBD-trimers. Animal experiments showed that RBD-heterotrimers induce broad-spectrum protective immune responses. As a proof-of-concept, this trivalent immunogen design strategy is promising and practical for preventing the continuous circulation of SARS-CoV-2. This study provides guidance for future vaccine design against coronavirus and other viruses such as the influenza and dengue viruses. [ABSTRACT FROM AUTHOR]