Cell Host Microbe | A Trivalent Mucosal Vaccine Based on Ancestral RBD Sequences Provides Broad Sarbecovirus Protection in Mice

This study computationally inferred ancestral RBD sequences from multiple Sarbecovirus evolutionary branches and designed a multivalent adenovirus-vectored vaccine that provides protection against various Sarbecoviruses in a mouse model, including SARS-CoV, SARS-CoV-2 and its variants, as well as SHC014 and pangolin/GD coronavirus strains that have not yet emerged in humans. The study also demonstrates that mucosal booster vaccination can effectively enhance respiratory immune protection even in the context of pre-existing SARS-CoV-2 immunity.

Literature Overview
The article, 'A trivalent mucosal vaccine encoding phylogenetically inferred ancestral RBD sequences confers pan-Sarbecovirus protection in mice,' published in Cell Host Microbe, reviews and summarizes the broad protective capabilities of Sarbecovirus vaccines. The study inferred ancestral RBD sequences using computational methods and incorporated them into an adenovirus-vectored vaccine, which effectively prevented infection by multiple coronavirus strains in mouse models, including SARS-CoV-2 variants and SARS-CoV. It further revealed that the protective immune response induced by the vaccine primarily relies on CD4+ and CD8+ T cell recall responses rather than neutralizing antibodies, and prior mRNA vaccination or infection history does not compromise vaccine efficacy.

Background Knowledge
The SARS-CoV-2 pandemic has driven vaccine development toward broad protection to address antigenic drift and the risk of future coronavirus spillovers. The Sarbecovirus subgenus includes SARS-CoV, SARS-CoV-2, and naturally circulating coronaviruses in bats and pangolins. The receptor-binding domain (RBD) of the spike protein in these viruses binds to the human ACE2 receptor and serves as a critical target for vaccine design. Current mRNA vaccines primarily induce antibodies specific to certain variants but show reduced efficacy against emerging immune-escape variants. This study employed ancestral RBD sequences in vaccine design to elicit cross-Sarbecovirus T cell immune responses, aiming to provide long-lasting and broad-spectrum protection. Experiments were conducted in multiple mouse models, including K18-hACE2 mice expressing human ACE2, to evaluate vaccine protection against respiratory infections and to test its boosting effect in pre-immunized conditions.

Research Methods and Experiments
The research team inferred ancestral RBD sequences for three major Sarbecovirus clades (Clade 1, Clade 2, Clade 3) using evolutionary analysis and cloned these sequences into replication-defective adenovirus vectors (ChAd). They then evaluated the vaccine’s immunogenicity and protective efficacy in wild-type C57BL/6J mice and K18-hACE2 mice through intramuscular (IM) and intranasal (IN) routes. The study compared different immunization strategies (IM-IN and IN-IN), and immune mechanisms were assessed using gene knockout mice and T cell depletion antibodies. The boosting potential of the vaccine was also evaluated in mice with pre-existing immunity from prior SARS-CoV-2 infection or mRNA vaccination.

Key Conclusions and Perspectives

• The trivalent ChAd-RBD vaccine (Sarbeco RBD Mix) induced broad-spectrum IgG and IgA antibodies in mice, effectively recognizing RBD proteins from multiple Sarbecoviruses, including SARS-CoV-2, SARS-CoV, SHC014, and pangolin/GD.
• Although the vaccine elicited weak neutralizing antibody activity in serum, Sarbeco RBD Mix provided significant protection in the respiratory tracts of mice, reducing viral RNA levels and preventing lung replication.
• The protective immunity primarily depended on CD8+ T cell–mediated recall responses, with a notable increase in tissue-resident memory T cells (TRM) in respiratory tissues, particularly in the nasal cavity and lungs.
• In mice with pre-existing SARS-CoV-2 immunity, intranasal boosting with Sarbeco RBD Mix significantly enhanced protection, suggesting its potential use in heterologous boosting regimens regardless of prior immune status.
• The T cell epitopes induced by the vaccine in mice showed extensive coverage across human HLA alleles, indicating the potential for broad T cell responses in humans.

Conclusion
In summary, this study demonstrates the feasibility of a trivalent adenovirus-vectored vaccine based on ancestral RBD sequences to confer broad protection against Sarbecoviruses in mice. The vaccine provides respiratory protection primarily through CD8+ and CD4+ T cell recall responses, and its efficacy is not diminished by pre-existing SARS-CoV-2 immunity. This vaccine design offers a promising approach for combating future coronavirus pandemics and lays the groundwork for developing T cell–targeted vaccines against multiple respiratory pathogens. Although further preclinical studies are needed, the vaccine’s immunogenicity, protective effects, and safety profile in mice suggest its translational potential for human applications.

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