Novel mRNA vaccines encoding Monkeypox virus proteins show promise in preclinical studies

By Neha MathurNov 22 2022Reviewed by Danielle Ellis, B.Sc.

In a recent study posted to the bioRxiv* preprint server, researchers in China demonstrated the efficacy of three messenger ribonucleic acid (mRNA) technology-based Monkeypox virus (MPXV) vaccines in combating a lethal Vaccinia virus (VACV) challenge in mice models.

Study: Novel mRNA vaccines encoding Monkeypox virus M1R and A35R protect mice from a lethal virus challenge. ​​​​​​​Image Credit: NIAID

*Important notice: bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Background

MPXV belongs to the Poxviridae family, which also has the Variola virus (smallpox) and VACV. In the 1980s, live virus preparations of the infectious Vaccinia viruses globally eradicated smallpox. Though replication-attenuated, live virus vaccines, like all approved MPXV vaccines, express many viral proteins, thus, raising safety concerns.

Extracellular enveloped virus (EEV) and intracellular mature virus (IMV) are the two infectious forms of MPXV; however, subunit vaccines using these have shown better safety profiles than the live virus vaccines in small animal models. For instance, Lai et al. showed that vaccination with E. coli expressed A27L, a truncated IMV surface protein, protected mice from the lethal challenge of VACV. Thus, there is a need to explore more MPXV antigens and combination strategies for better vaccines against MPXV.

Plus, there is a need for more MPXV vaccines with higher efficacy since MPXV was declared a public health emergency by the World Health Organization (WHO) in July 2022. mRNA vaccine technology showed promise against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), with all vaccines based on mRNA technology demonstrating high efficacy and safety.

About the study

Taking cues from the success of mRNA vaccines against SARS-CoV-2, researchers in the present study developed three mRNA vaccines, VGPox1, VGPox2, and VGPox3, against MPXV. These vaccines expressed MPXV EEV protein A35R and an IMV protein M1R, homologs of A33R and L1R in VACV, respectively. While VGPox 1 and 2 were single mRNA molecules coding a fusion protein composed of A35R EEV and a full-length M1R, VGPox 3 was a mixture of mRNA-lipid nanoparticle (LNP) complexes encoding the A35R and M1R, respectively. The only difference between VGPox 1 and 2 was that the latter lacked the A35R stalk region.

The researchers tested these three vaccines for their humoral and cellular anti-VACV immunity and protection conferred by them against the lethal viral infection in mice. The team inoculated each mouse intranasally with 1x10⁶ plaque-forming units (PFU) VACV-WR virus after 36 days of vaccination.

They measured body weight and monitored their symptoms daily until sacrificed. They sacrificed animals on Day 9 post the virus challenge or when they lost more than 15% of their body weight. They harvested animal lungs and ground them in a tissue homogenizer followed by three times freezing-melting to release the virus from cells. Then, they added supernatants with different dilutions to VeroE6 cells for plaque assay.

Study findings

The study results showed that all three mRNA vaccines elicited similar levels of anti-A35R antibodies, but only VGPox 1- and 2 elicited higher antibody levels against M1R. Thus, vaccinated sera from only VGPox 1 and 2 could neutralize live viruses at early time points. As expected, VGPox 3 vaccine was ineffective in in vitro neutralization assay. Interestingly, VGPox 2 showed higher levels of total immunoglobulins G (IgG) against M1R than VGPox 1 and VGPox 3 at all-time points. Also, VGPox1 had a lower protein expression level than VGPox 2 in T cells. However, the researchers could not determine how the difference in protein expression levels contributed to the IgG levels induced by the two vaccines.

Conclusions

The mRNA vaccines coding for the fusion forms of A35R and M1R (VGPox 1 and VGPox 2) effectively induced high levels of both A35R and M1R IgGs and neutralized live virus in cell cultures at all time points. However, the mixture of these two mRNAs (VGPox 3) could not attain the same results as VGPox 3-induced M1R-specific antibodies much later. Nevertheless, all three mRNA vaccines tested in the study conferred 100% protection during the virus challenge assay. Perhaps, when all test animals were challenged at Day 36 with the live virus, both anti-A35R and anti-M1R neutralizing antibodies had been elicited by all three vaccines.

Neutralizing antibodies against EEV and IMV might confer protection against live virus challenge. Thus, it remains undetermined how VGPox3 protected mice during a lethal virus challenge, despite the late induction of anti-M1R antibodies. To conclude, given the high homology of Vaccinia and MPXV, both VGPox 1 and 2 could be potent mRNA vaccines against MPXV as they fully protected mice during the lethal Vaccinia virus challenge.

*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.