The emergence of the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) triggered the ongoing pandemic of coronavirus disease 2019 (COVID-19). With over 176.7 cases and more than 3.8 million deaths, since the virus was first detected in Wuhan, China, in December 2019, the virus has stimulated rapid and intensive research into vaccines that prevent infection. This has culminated in the emergency use authorization (EUA) of several vaccine candidates in many parts of the world, including the Pfizer/BioNTech and Moderna vaccines, among others, in just one year.
A new study, released as a preprint on the bioRxiv* server, describes the late-stage clinical trial efficacy of a new vaccine candidate based on a deoxyribonucleic acid (DNA) plasmid platform expressing the receptor-binding domain (RBD) of the SARS-CoV-2 spike protein. Among different constructs of the vaccine plasmid, the RBD was found to produce the most potent and broad immune response.
Study: COVID-eVax, an electroporated plasmid DNA vaccine candidate encoding the SARS-CoV-2 Receptor Binding Domain, elicits protective immune responses in animal models of COVID-19. Image Credit: LookerStudio / Shutterstock
Most current vaccines are based on the spike antigen of the virus, expressed via messenger ribonucleic acid (mRNA). The first two vaccines to be approved (Pfizer/BioNTech and Moderna) were quickly followed by vaccines based on two adenovirus-based platforms: the Oxford/AstraZeneca and the Johnson & Johnson vaccines. Many other vaccines are being deployed in countries other than the US and those within Europe.
Most of these vaccines elicit antibodies to the spike antigen, which mediates virus-host cell receptor attachment as well as entry into the host cell. Among the most promising platforms is the DNA-based vaccine.
Advantages of DNA vaccines
DNA-based vaccines are both easy to produce and safe for clinical use. Synthetic DNA is shelf-stable without the need for a cold chain, increasing its logistical advantages.
The simplicity of the production, testing and regulatory pipeline is another attraction, since this favors rapid development protocols. In the current study, the use of electroporation to introduce the DNA vaccine increased the antigen expression and the overall immune response.
Electroporation (EP) is a process, also called electro-genetransfer, where the vaccine antigen DNA, in the form of a plasmid, is injected into the skeletal muscle cell. This results in a boost to DNA uptake by several hundred-fold, as well as a higher level of antigen expression in the tissues at the injection site. This is accompanied by local inflammation at the site, with short-lived tissue damage and cytokine production.
DNA vaccine candidate elicits antibodies to VOCs
The DNA plasmid used here encodes a secreted form of the monomeric SARS-CoV-2 RBD. Called COVID-eVax, it elicits antibodies with potent neutralizing capacity to the circulating variants of concern (VOCs) of this virus as well as to the parental strain. This is an important finding since the VOCs show greater transmissibility and/or resistance to neutralizing antibodies elicited by first-generation vaccines or natural infection with the parental strain.
In vivo, COVID-e-Vax protected mouse and ferret models against the infection when challenged with the virus.
The immunized mice produced antibodies that bound mostly to the common regions of the RBD rather than those that were variable between the VOCs, such as the N501K, K417N, S477, E484K and L452 mutations. This could mean that the vaccine is effective against these spike variants as well.
Neutralizing antibody and T cell responses
Specific antibodies targeting the RBD were found in the lung tissue of the vaccinated animals as well as in the blood. A strong neutralizing antibody response was observed in a dose-dependent manner up to 10 μg of the vaccine in these animals.
The anti-RBD antibodies continued to be detectable for up to six months from vaccination.
COVID-eVax also strongly induced IFNγ- or TNFα-producing CD8+ and CD4+ T cells, specifically targeting the RBD in a dose-dependent manner. The cytokine profile fitted that of a T helper cell type 1 (Th1)-skewed response, without Th2 or Th17 responses.
Other animal models show similar findings
In rats, the vaccine proved safe and well-tolerated. No serious adverse effects were noted in any animals, with all injection site lesions recovering almost completely within four weeks. These animals showed a strong dose-dependent antibody response with high neutralizing antibody titers proportional to the total antibody titers.
Pseudoviruses with three variants of the SARS-CoV-2 spike (B.1.1.7, B.1.351 and P.1) were alike neutralized by the rat antiserum. The neutralization efficacy was similar to that for the earlier D614G strain.
The same robust immune protective response was seen with mice and ferrets when given a post-vaccination challenge with the virus intranasally. Not only did the immunized mice show much milder changes in lung function, but the viral RNA, infectious virus load and virus nucleocapsid protein amount, were alike decreased markedly in the lungs and the brain of these animals compared to controls.
This was linked to the appearance of specific CD4+ T cells producing RBD-specific IFNγ/TNFα, or both, and of CD8+ T cells producing RBD-specific IFNγ, indicating strong adaptive immune cellular responses.
Additionally, by expressing RBD, the vaccine may lead to the occupation of the receptor’s binding site, and so competitively block virus-receptor binding. This potential mechanism needs further study.
What are the implications?
DNA vaccines display multiple advantages, both in terms of the ability to design them for high antigen expression and immune response, but also for large-scale manufacturing at high speed, using simple formulations unlike lipid nanoparticles and the like, as required for peptide or mRNA vaccines.
The use of electroporation improves DNA uptake by the cells at the injection site by five hundred-fold.
The choice of the RBD antigen is based 1) on the robust specific neutralizing antibody response against wildtype and several VOCs of the SARS-CoV-2, as well as 2) a strong T cell response, that is important in preventing severe COVID-19. 3) The use of the RBD reduces non-specific antibody response, thus minimizing the risk of antibody-dependent enhancement (ADE) of disease following vaccination.
Thus, multiple doses can be used to improve the immune response without fear of immunizing the patient against the vaccine itself, an advantage not seen with viral vector-based vaccines. This could even be used as a booster following an adenovirus, mRNA or protein vaccine regimen.
In summary, this study identifies COVID-eVax as an ideal COVID-19 vaccine candidate suitable for clinical development,” write the scientists.
Several DNA COVID-19 vaccines have successfully entered late-stage testing, having demonstrated their safety and immunogenic potential. The successful results obtained in several animal models encourage the further development of COVID-e-Vax, which is being tested in a Phase I/II clinical trial in Italy for safety and immunogenicity in humans.
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.
- Conforti, A. et al. (2021). COVID-eVax, an electroporated plasmid DNA vaccine candidate encoding the SARS-CoV-2 Receptor Binding Domain, elicits protective immune responses in animal models of COVID-19. bioRxiv preprint doi: https://doi.org/10.1101/2021.06.14.448343. https://www.biorxiv.org/content/10.1101/2021.06.14.448343v1.