New single-chain mRNA vaccines offer enhanced immunity and protection against mpox

By Tarun Sai LomteReviewed by Lily Ramsey, LLMOct 22 2024

Groundbreaking study reveals new single-chain mRNA vaccines provide 100% protection against lethal mpox challenge in mice, far outperforming traditional vaccines.

Study: Single-chain A35R-M1R-B6R trivalent mRNA vaccines protect mice against both mpox virus and vaccinia virus. Image Credit: Pormezz/Shutterstock.com

In a recent study published in eBioMedicine, researchers developed single-chain trivalent messenger ribonucleic acid (mRNA) vaccines against both vaccinia (VACV) and mpox viruses (MPXV).

Background

Currently, three live-attenuated VACV-based vaccines are available for mpox: ACAM2000, JYNNEOS, and LC16. These were originally smallpox vaccines but were later approved for mpox.

Although the replication-deficient JYNNEOS vaccine is safer than replication-competent VACV-based vaccines, including in immunocompromised patients, a study found that only 63% of JYNNEOS vaccinees developed neutralizing antibodies against MPXV.

The efficacy and safety of mpox vaccines could be enhanced by developing new-generation vaccines against specific antigens based on mRNA technology. MPXV has two infectious forms: extracellular enveloped virus (EEV) and intracellular mature virus (IMV).

Studies have provided candidates for MPXV antigens, such as A29L and M1R in IMV and B6R and A35R in EEV. These antigens have exhibited various levels of protection against poxvirus in mice.

The study and findings

In the present study, researchers developed single-chain trivalent mRNA vaccines against MPXV and VACV. First, they determined the essential antigens required in the vaccine for optimal immunogenicity and efficacy.

Single-gene mRNA vaccines were synthesized with the MPXV B6R, M1R, or A35R gene. mRNA vaccines with their soluble forms (sB6R, sM1R, and sA35R) were also synthesized.

Mice were immunized with vaccines or cocktails of two/three vaccines. Mouse groups vaccinated with A35R, sB6R, or sM1R developed antibodies against these antigens by day 27, but the recipients of sA35R, B6R, or M1R did not.

Mice immunized with the cocktail of all three soluble antigens developed the most broad-spectrum antibodies bound to both sB6R and sM1R. Two-antigen cocktails induced antibodies that bound to only sM1R or sA35R.

In addition, the plaque reduction neutralization assay indicated that mice immunized with sM1R developed neutralizing antibodies against VACV IMVs. Further, splenocytes from each group were stimulated with overlapping peptide pools of the three antigens. Enzyme-linked immunospot (ELISpot) assay showed that immunization with either vaccines or cocktails induced robust intracellular interferon (IFN)-γ responses.

Next, the efficacy of vaccines/cocktails was assessed based on weight change and survival probability following a lethal challenge with the VACV western reserve (WR) strain.

For each antigen, groups with strong antibody responses were well protected, with 100% protection in the A35R vaccine group. Notably, all sA35R recipients who lacked detectable antibodies succumbed within five days.

Further, although B6R did not induce antigen-specific antibodies, it stimulated a specific T-cell response. B6R vaccination also conferred partial protection against VACV. The A35R, B6R, and M1R cocktail also conferred 100% protection. Next, the team designed three mpox vaccines (AMAB-wt, AMAB-C140S, and AMB-C140S) with single-chain chimeric immunogens containing sB6R, sA35R, and sM1R.

AMAB-C140S and AMAB-wt contained two copies of sA35R to boost A35R-specific immunogenicity since sA35R alone did not induce antibodies. Structure prediction analyses indicated that the epitopes for neutralizing antibodies in these immunogens were exposed and that these immunogens could induce antigen-specific immunoglobulin G (IgG) antibodies.

As such, the team evaluated the immunogenicity of the three single-chain vaccines with the live-attenuated VACV Tian Tan (VTT) vaccine as the control.

All three vaccines induced high levels of specific IgG antibodies by day 27; the two AMAB vaccines induced significantly greater levels of A35R-specific IgG than the control or AMB-C140S vaccine. Further, all single-chain vaccines elicited neutralizing antibodies against MPXV and VACV-WR IMVs.

An ELISpot assay also confirmed that the three single-chain vaccines induced robust cellular immunogenicity. Cellular responses were more potent after stimulation with the A35R peptide pool, suggesting that sA35R may contain potent epitopes for T-cell responses.

 Finally, the efficacy of the single-chain vaccines was evaluated by challenging mice with MPXV or VACV-WR. Mice that received empty lipid nanoparticles (LNPs) died within four days of the VACV-WR challenge.

By contrast, mice receiving any single-chain vaccine exhibited 100% protection following the VACV-WR challenge, with minimal loss in body weight.

Further, mice receiving empty LNPs showed over 5% weight loss following the MPXV challenge, whereas vaccine groups did not. In addition, viral load in the spleen, ovaries, and lungs of vaccinated mice was lower by several orders of magnitude compared to LNP recipients.

Conclusions

In sum, the study developed and evaluated the safety and immunogenicity of three trivalent mRNA vaccines containing the soluble B6R, M1R, and A35R antigens of MPXV in a single polypeptide chain.

These vaccines showed effective neutralization against both VACV and MPXV, providing complete protection against the lethal VACV-WR challenge and significantly reducing viral loads after the MPXV or VACV-WR challenge.