New mRNA COVID-19 vaccine candidate shows promise in vivo

Researchers report that a new lipid nanoparticle mRNA coronavirus disease 2019 (COVID-19) vaccine candidate protected 70% of mice expressing the human angiotensin-converting enzyme 2 (ACE2), while all the non-vaccinated mice died.

To combat the COVID-19 pandemic, several vaccines have now been approved. The first vaccines to be approved were based on mRNA technology, for example, the Pfizer BNT162b2 and the Moderna mRNA-1273. This technology has shown high efficacy, has a high safety profile and has a quick manufacturing process.

One of the challenges of mRNA technology is delivering it efficiently to target cells. The most frequently used and most advanced carrier platform for mRNA delivery are lipid nanoparticles, which are also used in the current vaccines.

These lipid nanoparticles comprise cholesterol, a phospholipid, polyethylene glycol-linked lipid, and an ionizable lipid. The ionizable lipid is key in the effective delivery and translation of the mRNA.

In a new study published on the bioRxiv* preprint server, researchers from Israel report the efficacy of an mRNA vaccine comprising an ionizable lipid that was previously shown to elicit a potent immune response in vaccinated mice.

Study: A lipid nanoparticle RBD-hFc mRNA vaccine protects hACE2 transgenic mice against lethal SARS-CoV-2 infection. Image Credit: LookerStudio / Shutterstock

Characterizing new vaccine candidate

Mice animal models, which are usually used to test vaccine efficacy, cannot be used as is for SARS-CoV-2 as the mouse angiotensin-converting enzyme 2 (ACE2) cannot bind to the virus. Hence, the researchers used mice expressing the human ACE2 (hACE2), a receptor for SARS-CoV-2 in humans. This model has been shown to lead to severe disease and death upon infection with the virus.

Fc-conjugated receptor-binding domain (RBD) mRNA and lipid nanoparticles were mixed together to make the nanoparticle encapsulated mRNA. The vaccine candidate was injected intramuscularly in 6–8 week old female mice. Some animals also got a booster dose after 23 days. As controls, animals were given only the lipid nanoparticles or recombinant RBD-hFc (rRBD).

The team found that two doses of the vaccine candidate provided 70% protection against SARS-CoV-2, injected nasally. All untreated animals died.

Based on a previous screening of lipid formulations, the team chose one that showed the best immune response in mice, and vaccine candidates were prepared using this lipid formulation. Dynamic light scattering and cryogenic electron microscopy indicated the particles were small, about 55 nm, and uniformly distributed.

Western blot tests of transfected HEK293 cells revealed the effective translation of the mRNA into a functional protein. ELISA tests showed binding of the nanoparticle mRNA to hACE2, confirming the functionality of the translated RBD-hFc. Thus, the vaccine candidate can also elicit binding antibodies.

Testing in mice

To test how the lipid nanoparticle vaccine candidate behaves in vivo, the team injected mice with 5 μg of the nanoparticle mRNA or rRBD intramuscularly. A single dose of the vaccine candidate did not show a strong immune response in either case. However, a strong antibody response was seen after the second dose. The antibody levels produced were much higher for the nanoparticle mRNA than for the rRBD.

A plaque reduction neutralization test confirmed the formation of a strong neutralizing antibody response in vitro. Although the nanoparticle mRNA injection elicited a stronger response than the rRNA, the difference was not statistically significant.

After 23 days of the last injection, the mice were challenged with SARS-CoV-2 administered intranasally. All the animals lost weight starting five days after infection and reached a maximum weight loss after eight days.

Control animals lost almost a fourth of their weight, while those that received the vaccine candidate lost about 10% weight. All the control animals died, while 70% of the vaccinated animals who received two doses survived. Only 40% of the animals administered rRBD survived. Only 16% of the animals which received only a single dose of the mRNA vaccine candidate survived.

Previous studies on vaccines have used animals susceptible to SARS-CoV-2 infection but which did not lead to animal death, where the vaccine efficacy was determined by its ability to reduce viral replication. In contrast, this study used a mouse model expressing hACE2, which has been used less in studies. The study also showed that the mRNA vaccine candidate afforded 70% protection against another lethal dose of SARS-CoV-2 that killed animals that were not immunized.

*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.

Written by

Lakshmi Supriya

Lakshmi Supriya got her BSc in Industrial Chemistry from IIT Kharagpur (India) and a Ph.D. in Polymer Science and Engineering from Virginia Tech (USA).