The COVID-19 pandemic caused by the SARS-CoV-2 virus has constituted an unparalleled threat to humanity, with over 224 million infections and 4.62 million deaths globally. The rapid development of multiple effective treatments has allowed some control of the pandemic. More than 300 vaccines, including live, inactivated, viral vector, nucleic acid, and protein subunit vaccines, have been created in record time.
Around 26 of these treatments have already been approved for emergency use and are being adopted widely. Despite this remarkable achievement, global mass vaccination campaigns are still suffering from a supply-demand imbalance. A new high-yield vaccination candidate with outstanding safety and effectiveness profiles is needed.
The researchers, in their new study published in the journal Cell Discovery, have created a SARS-CoV-2 spike receptor-binding domain (RBD)-homodimer as a protein subunit vaccine and assessed its immunogenicity and protective effectiveness in rodents and non-human primates in the current study.
The RBD-homodimer was created by disulfide bonding interdomain cysteine residues at position 538. The homodimer was produced in high-yielding cells and then purified for further investigation. The purified product’s thermal stability investigation demonstrated that the RBD-homodimer was stable at 25 °C for at least 8 weeks. Furthermore, the homodimer had a strong affinity for human angiotensin-converting enzyme 2 (ACE2), the SARS-CoV-2 host cell entrance receptor.
To assess immunogenicity, mice were inoculated with three doses of RBD-homodimer at two-week intervals. After the third inoculation, the vaccine had the maximum efficacy in eliciting robust anti-RBD binding and neutralizing antibodies. In terms of cellular immunological response, the vaccine demonstrated a great ability to induce RBD-specific T cells to secrete IL-4, IFN-, and IL-2.
In a seperate set of test, human ACE2-expressing mice were immunised first and then intranasally challenged with SARS-CoV-2. In vivo testing demonstrated that the vaccination gradually decreases viral loads to undetectable levels by day 5 post-infection. Similarly, no infectious virus was found in vaccinated mice on the fifth post-infection day. Furthermore, at days 2 and 5 post-infection, histological investigation demonstrated that the vaccinated mice had only minor pulmonary pathological symptoms with essentially no lung infiltration of inflammatory cells. Unvaccinated mice, on the other hand, developed mild to severe pulmonary diseases after infection.
Then, rhesus macaques were given three doses of the vaccine intramuscularly, followed by a virus challenge 42 days later. All vaccinated monkeys demonstrated strong antibody responses, similar to vaccinated mice. In monkeys, the cellular immunological response to the vaccine was comparable to that in mice.
None of the vaccinated monkeys had detectable levels of viral RNA in respiratory swabs on day 7 post-infection. Likewise, no viral RNA was found in anal swabs. Vaccinated monkeys had much less severe pulmonary lesions than uninfected monkeys.
RBD homodimer-induced antibodies were evaluated for cross-reactivity against nine RBD variants with mutations at K417N, N439K, Y453F, S477N, T478I, G485S, F490S, S494P, and N501Y. The antibodies bind to all of the mutants examined, with slightly reduced reactivity against the K417N, N439K, and T478I mutants. Furthermore, the results of the ACE2 – RBD blocking experiment demonstrated that vaccine-induced antibodies are capable of inhibiting ACE2 – RBD interactions and neutralizing tested RBD mutants. However, the N501Y and K417N mutants had the highest and lowest amounts of antibody-mediated neutralization, respectively.
The neutralization assay against the wildtype SARS-CoV-2 and the beta variant (originally discovered in South Africa) demonstrated that vaccine-induced antibodies have a 1-fold weaker neutralizing efficiency against the beta variant than against the wildtype virus.
The findings show that the novel RBD-homodimer vaccine candidate has good immunogenicity and protective effectiveness against severe COVID-19. Importantly, the vaccine can stimulate cross-reactive antibodies against more lethal SARS-CoV-2 strains.
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