Researchers report development of a self-assembled trimeric RBD subunit vaccine able to prevent SARS-CoV-2 infection

Since the emergence of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), the causal agent of the coronavirus disease 2019 (COVID-19) pandemic, several genomic mutations have been detected. For instance, the currently dominantly circulating SARS-CoV-2 Omicron variant contains many mutations responsible for its enhanced virulence and transmissibility compared to the original SARS-CoV-2 strain.

Background

The characterization of the Omicron strain revealed the presence of many amino acid substitutions, insertions, and deletions in the spike (S) region. This finding raised concerns related to the reduced efficacy of the available COVID-19 vaccines. This is because all the available vaccines have been designed based on the S protein of the ancestral strain. 

Unsurprisingly, this variant revealed extensive resistance to COVID-19 vaccines, resulting in a large number of breakthrough infections in the vaccinated population. Additionally, individuals recovered from COVID-19 were found to be re-infected by this SARS-CoV-2 strain.

In the Omicron genome, the presence of mutations, such as G446S, S371L, K417N, Q493R N440K, and E484A, have been associated with its decreased antibody neutralization capacity. Additionally, a reduced antigenicity was observed in the Omicron receptor binding domain (RBD) protein due to impaired serological responses. The booster vaccination strategy also failed to elicit high titers of neutralizing antibodies effective against this strain. Hence, it is important to design a new generation SARS-CoV-2 vaccine that could be effective against the updated Omicron variant.

Although spike RBD of SARS-CoV-2 represents a prominent target for vaccine development, multiple strategies associated with antigen multimerization targeting RBD have been applied to improve the antigenicity. Nevertheless, RBD multimerization approaches exhibited significant complications linked to clinical application.

The peptides present in the spike heptad-repeat sequences 1 and 2 (HR1 and HR2) possess self-assembly properties, which could be used to enhance antigenicity. These peptides can automatically assemble into a 6-helix bundle structure and trimerize RBD, thereby increasing its antigenicity. This concept was explored in a recent Nature Communications study that used a baculovirus expression system to express various targeted proteins. Previously this technology was used to develop vaccines against influenza and human papillomavirus.

About the study

The present study reported the development of a new self-assembled trimeric RBD subunit vaccine that could effectively prevent infection from SARS-CoV-2 variants, including the currently dominantly circulating Omicron variant. The vaccine antigen was developed by combining RBD with HR, and the resultant RBD-HR protein was assembled into trimers in solution.

A mice model was used to determine the immunogenicity of RBD-HR/trimer protein with MF59-like adjuvant, and the antibody response elicited by the protein was analyzed. The mice were intramuscularly immunized with different doses of the newly formulated RBD-HR/trimeric vaccine, following a prime-boost vaccination regimen spaced 21 days apart. The control group was injected with phosphate-buffered saline (PBS) or 10 μg RBD-HR/trimer protein without adjuvant. The serum samples were collected on the 7th, 14th, 56th, and 100th days since vaccination.

Key findings

The mice that received a single dose of the RBD-HR/trimer vaccine elicited an elevated RBD-specific IgG level on day 7. In the early immune stage, the binding of antibody response exhibited dose-dependency, i.e., higher anti-RBD IgG titers were observed in mice injected with increased adjuvant-formulated RBD-HR/trimer vaccine dose. 

Interestingly, even a low dosage of vaccine with adjuvant was able to induce high levels of anti-RBD IgG after two vaccination doses. Additionally, the adjuvant-free group, i.e., mice that received RBD-HR/trimer vaccination, produced robust IgG antibody responses after the final immunization. A comparable endpoint titer of anti-RBD antibodies following three-dose vaccination was estimated. 

In this study, 10 μg RBD-HR/trimer formulated with adjuvant was used for all experiments. This vaccine formulation exhibited high levels of neutralizing antibody production that was effective against the prototype, Beta, Delta, Alpha, and Omicron strains in multiple animal models.

The current study revealed the HR-fused RBD protein to contain L452R and T478K mutations that could self-assemble into trimers and, thereby, induce high levels of neutralizing antibodies. These antibodies can effectively prevent infection by SARS-CoV-2 variants, including the Omicron strain.

Conclusions

The newly formulated RBD-HR/trimer protein with adjuvant was able to induce robust T-cell and sustained humoral immune responses. Utilization of the self-assembly feature of HR significantly improved the immunogenicity of RBD through trimerization. Researchers did not have to introduce non-SARS-CoV-2 viral sequences for this purpose. Since HR is highly conserved among SARS-CoV-2 variants, this vaccine formulation could remain effective not only against the broad spectrum of existing SARS-CoV-2 variants but also future variants.

Written by

Dr. Priyom Bose

Priyom holds a Ph.D. in Plant Biology and Biotechnology from the University of Madras, India. She is an active researcher and an experienced science writer. Priyom has also co-authored several original research articles that have been published in reputed peer-reviewed journals. She is also an avid reader and an amateur photographer.