Even as over a score of vaccines against the coronavirus disease 2019 (COVID-19) pandemic have been rolled out following their emergence use authorization, others continue to be researched to fill the global need for effective deployment the world over.
A new study, published in Nano Letters, describes an effective new vaccine candidate based on a messenger ribonucleic acid (mRNA) platform.
The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) attaches to host cells via its spike protein, causing the cell membrane to fuse with the virus to accomplish viral entry via endocytosis. The spike interacts with the host cell receptor, the human angiotensin-converting enzyme 2 (ACE2), at the receptor-binding domain (RBD), which forms the basis of the current vaccine candidate.
The mRNA platform
With over 3.5 million cases the world over, intensive research culminated in the rollout of the first two vaccines to be authorized for use in December 2020. Researchers have focused on live attenuated virus vaccines, inactivated vaccines, recombinant viral vectors, recombinant viral antigen, DNA vaccines and mRNA vaccines.
The mRNA platform has garnered much interest because of the ease of stabilizing mRNA with current technological advances and the development of efficient delivery systems that allow the mRNA to reach the host cells intact before they are degraded by the host.
The mRNA strand is designed to mimic host mRNA to avoid detection by the host. It does not require to be integrated into the host genome, unlike viral vectored vaccines. Moreover, its production is both rapid and convenient.
The ease and speed of commercial production is a particularly important advantage, which has been demonstrated in the current pandemic, where Moderna developed its mRNA vaccine and administered it to the first human in a clinical trial within 66 days from the selection of the virus sequence.
The disadvantage of using mRNA has been the rapid breaking down of this molecule when exposed to the host ribonucleases (RNases), which are present in all tissues. Secondly, mRNA is negatively charged and thus resists entry into cells on their own. This mandates the use of a carrier molecule to protect it and transport it across the negatively charged cell membrane.
The use of lipid nanoparticles (LNPs) to encode the viral mRNA encoding the vaccine antigen is a striking advance, and has been described for the formulation of many vaccines against the human immunodeficiency virus (HIV), rabies, Zika and influenza viruses.
Not only do LNPs promote the encapsulation of the mRNA and reduce the strength of repulsion from the negatively charged cell membrane, but may even facilitate the escape of the mRNA from the endosomes into the cytoplasm of the host cell, which is key to the translation of the vaccine antigen protein.
LNPs are made up of ionizable lipids (which encourage the self-assembly of the LNPs), cholesterol to stabilize the particles, a phospholipid to support the double layer of the lipid structure, and a polyethylene glycol (PEG)-lipid which prolongs the half-life of the structure.
In the vaccine formulation described in the current study from Tel Aviv University, the mRNA encodes the recombinant spike RBD gene in combination with a human antibody fragment called the Fc (crystallizable fragment) portion. This is responsible for antibody attachment to the spike antigen. Since the human Fc portion has two binding sites, each hybrid molecule contained two RBD domains.
The mRNA was then mixed with an ionizable lipid mixture to prepare lipid nanoparticles (LNPs) that encapsulate the RBD-Fc mRNA. Following vaccine administration, the mRNA is carried to the host cells, which then express the RBD antigen to elicit an immune reaction.
Earlier, the immunogenicity of this formulation had been tested in BALB/c mice. When administered intramuscularly, high antibody titers, including a strong neutralizing response, and a Th1-skewed T cell response, were detected.
After confirming their expression within cells in culture by assessing their binding with purified IgG specific to the SARS-CoV-2 spike protein, the bivalent RBD-Fc-containing LNP preparation was inoculated into female hACE2-expressing mice.
These animals were chosen to express signs of severe illness, unlike hamsters and monkeys, which only show mild illness.
In the current study, four groups of animals were inoculated with the vaccine, two being administered either plain LNP-encapsulated RBD mRNA and two the LNP-RBD-Fc preparation.
In each paired set, one group was given a single dose, and one administered both priming and booster doses. Controls received neither.
Serum samples were collected on day 23 and day 46 for the two-dose group, and the antibody response to the vaccine RBD was measured. Secondly, the animals were exposed to the SARS-CoV-2 virus by intranasal inoculation, and signs of disease were evaluated. Animals that showed signs of severe disease were euthanized.
The study showed that mice developed a strong antibody response to the RBD-Fc mRNA antigen. The antibodies bound to the hACE2 receptors with high affinity, and neutralized viral infection in the vaccinated mice following exposure to the virus subsequently.
In the vaccinated group, 70% showed protection against a lethal dose of the virus, compared to 100% mortality in the control group.
What are the implications?
The study reports the immunogenicity and protective efficacy of an LNP-based RBD-Fc mRNA vaccine in an ACE2-expressing mouse model.
To the best of our knowledge, this is the first nonreplicating mRNA vaccine study reporting protection of K18-hACE2 against a lethal SARS-CoV-2 infection,” conclude the researchers.