New mRNA vaccine platform provides stronger protection against SARS-CoV-2 delta variant

Since its discovery in December 2019, coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has spread swiftly and resulted in a major global pandemic. In response to the COVID-19 pandemic, several vaccines based on various platforms, including DNA, mRNA, viral vectors, protein subunit, and inactivated vaccines, have been developed, with the two mRNA vaccines and the Ad26-vectored vaccine showing high efficacy in late-stage clinical trials and thus being licensed or receiving emergency use authorization (EUA) in the United States and many other parts of the world.

Study: Combinatorial mRNA vaccination enhances protection against SARS-CoV-2 delta variant. Image Credit: thodonal88/Shutterstock

Current COVID-19 vaccines primarily target the viral spike protein (S) or its receptor-binding domain (RBD) to elicit a strong neutralizing antibody response. It is believed that vaccines targeting a more conserved viral protein in addition to the S protein would likely give greater protection, particularly against variants of concern (VOCs). The nucleoprotein (N), which is more conserved across multiple SARS-CoV-2 variants and different coronaviruses than the S protein, is another major antigen that triggers the host immune response.

A group of researchers from various institutions developed a nucleoside-modified mRNA vaccine that encodes the SARS-CoV-2 N protein (mRNA-N) and is formulated in lipid nanoparticles for this investigation (LNP). The researchers discovered that mRNA-N was highly immunogenic, eliciting strong N-specific T-cell responses and binding IgG.

The mRNA-N vaccination alone did not generate any neutralizing antibodies, as expected. It was found that mRNA-N alone had only a minor, but the substantial effect on the mouse-adapted SARS-CoV-2 strain and the delta strain (B.1.617.2) infection in mice and hamsters challenged with SARS-CoV-2.

The authors further evaluated the protective efficacy of the combinatorial mRNA-S+N vaccination with the clinically licensed S-expressing mRNA vaccine (mRNA-S-2P) alone on immunological control of the SARS-CoV-2 delta strain in the lungs and upper respiratory tract in the hamster model.

A preprint version of the study is available on the bioRxiv* server, while the article undergoes peer review.

The study

In wild-type (WT) Balb/c mice, the immunogenicity of mRNA-N was tested. PBS (mock) or mRNA-N (1g) were given to two groups of mice (8 per group). The mRNA dose was chosen based on previous mouse experiments. The vaccine was administered intramuscularly (i.m.) at weeks 0 (prime) and 3 (booster). Blood/sera were taken three weeks after prime immunization (on the day of the booster) for antibody response analysis; two weeks following booster vaccination (week 5), animals were killed, and vaccine-induced humoral and cellular immune responses were analyzed.

Flow cytometry was used to look at T-cell immunity in splenocytes first. Based on CD44 expression, the authors found that mRNA-N immunization activated total CD4+ and CD8+ T cells in the spleen when compared to mock controls. Intracellular cytokine staining (ICS) and flow cytometry were used to investigate the vaccine-induced N-specific T-cell response.

mRNA-N vaccination resulted in a significant increase in N-specific CD4+ and CD8+ T cell responses in the spleen compared to sham controls. Tumor necrosis factor (TNF) was found to be the most abundant protein expressed by N-specific T cells, followed by interferon (IFN-) and interleukin-2 (IL-2). IFN-ELISPOT was used to assess the mRNA-N vaccine-induced T-cell response, and it was found that, compared to the mock control, the mRNA-N vaccine generated high amounts of N-specific T cells in the spleen.

Both mRNA-S and mRNA-S+N triggered robust activation of CD4+ and CD8+ T cells in mice, which did not differ significantly between the mRNA-S and mRNA-S+N groups, according to flow cytometric measurement of total T-cell activation (based on CD44+). ICS was used to identify vaccine-induced S- and N-specific T cells based on cytokine expression (IFN-, TNF-, IL-2). Splenocytes were re-stimulated with S or N peptide pools, and ICS was used to identify vaccine-induced S- and N-specific T cells based on cytokine expression (IFN-, TNF-, IL-2). According to the findings, combinatorial immunization produced significant S-specific and N-specific CD4+ and CD8+ T-cell responses in mice. TNF- was the most significantly expressed cytokine by S- and N-specific T cells, followed by IFN- and IL-2.

Combinatorial mRNA-S+N vaccination appears to have synergistic effects and augments the S-specific CD8+ T-cell response (TNF-+ or IFN-+) compared to mRNA-S alone. The mRNA-S alone group showed very little or no N-specific T-cell response as a control. IFN-ELISPOT demonstrated the induction of both S- and N-specific T-cell responses by combinatorial mRNA-S+N vaccination compared to mRNA-S alone.

Implications

These findings show that combinatorial mRNA vaccination significantly improves viral control of SARS-CoV-2, including the delta variant, demonstrating that a vaccine targeting both the viral S protein and a conserved region of the virus is feasible and could induce stronger and broader protection against VOCs. This vaccine method can help limit the COVID-19 pandemic and deserves more research and development.

*Important notice

bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.