The coronavirus disease 2019 (COVID-19) pandemic is one of only three successive outbreaks caused by highly pathogenic coronaviruses. The virus responsible for COVID-19 is the severe acute respiratory syndrome Coronavirus 2 (SARS-CoV-2), first reported in Wuhan, China.
In the absence of effective therapeutics or preventives, vaccines have become the focus of attention. Though over 170 vaccines are in preclinical and about 60 in clinical development, very few are based on plasmid DNA technology. However, a new preprint research paper posted to the bioRxiv* server reports the immunogenicity of a DNA vaccine candidate.
The reported efficacy for the vaccines that have been currently rolled out, including the mRNA vaccines from Pfizer and Moderna, and the chimpanzee adenovirus vector-based candidate from AstraZeneca, is about 95% for the mRNA vaccines and 70% for the adenovirus vaccine, respectively, based on phase III efficacy.
Both live attenuated and inactivated or killed viral vaccines elicit antibodies. This is because the antigen in the vaccine is taken up by phagocytosis or endocytosis into host cells to be processed via the major histocompatibility complex class I (MHC I), activating antibody-producing B cells, for the main part.
The DNA plasmid vaccine is a relatively new vaccine approach intended to generate both humoral and cell-mediated immunity. To induce cell-mediated immunity, antigens are required that are safe but are processed via the endogenous pathway, thus leading to the activation of both T and B cells. The activated T cells will destroy the cell infected by the virus.
Mechanism of immunity
Plasmid DNA-based vaccines introduce the genes that encode the antigens for interest. The plasmid will enter and remain in the nucleus without integrating with the host DNA. It will initiate the translation of the encoded antigenic protein in the cytoplasm of the host cell.
Following vaccination, the host immune system then carries out surveillance for these antigens as they are presented by either or both MHC I and MHC II proteins. The type of MHC molecules determines which type of immune response follows the recognition of these antigens.
Advantages of DNA vaccines
Plasmid DNA vaccines thus generate long-term, durable immunity against the viral spike antigen. The production of the antigens by the host cell will ensure the protein is folded correctly. The signal peptide simultaneously expressed will ensure the antigen is trafficked to the cell membrane.
The antigen-presenting cells (APCs) recognize the viral antigen on the infected cells' surface and thus trigger antibody production, including neutralizing antibodies.
The plasmid DNA vector also contains unmethylated cytidine phosphate guanosine (CpG) motifs that serve to enhance cell-mediated immune responses, along with humoral immunity. Immunity is thus induced with a single dose.
These vaccines also continue to stimulate immunity over the long term and do not require adjuvants. DNA vaccines also enjoy the advantages of having simple, inexpensive, and easily scalable manufacturing processes, with a long shelf life at room temperature. These factors ease the distribution of such vaccines, as cold-chain maintenance is not necessary. These are also safe even in immunocompromised patients, relative to some live vaccines.
DNA vaccines will also avoid inducing immunity to the viral vector or the potential risk of oncogene activation or other safety issues related to viral vectors.
Earlier studies have shown that the DNA vaccine candidates developed against SARS and MERS, the past coronavirus outbreaks, were immunogenic and protective. The current study examines the preclinical performance of a SARS-CoV-2 DNA plasmid vector encoding the viral spike protein and the IgE signal peptide.
The researchers first demonstrated the ZyCoV-D vaccine's capacity to express the spike protein robustly in mammalian cells and induce antibodies that bound the target antigen strongly. Their results indicate long-term storage of the vaccine at 2–8 °C, and 25°C over a few months. This is critical to rapid deployment in the setting of the current pandemic.
The vector used in the development of this vaccine is the pVAX-1 vector, used in several earlier DNA vaccines, with an excellent safety profile.
In a range of animal models, namely, mice, guinea pigs, and rabbits, immunized by intradermal injection at three dosages (25, 100 and 500μg), they found that the vaccination is followed by antibody production within two weeks after the second dose, peaking at two weeks from the third dose. The IgG (immunoglobulin G) antibody levels remain detectable at three months from the last dose, indicating both durable immunity and a secondary anamnestic response on re-exposure.
Following immunization, two assays showed a robust neutralizing response, which protected the animal against viral challenge. Future studies will be needed to determine the relevance of neutralizing antibody titers as correlates of protection, no matter which vaccine type is in use, in both animals, and in humans. This will allow benchmarking of this measure during the clinical development of future vaccines.
The neutralizing antibody response indicates that the immunity thus induced will allow viral clearance and reduction in the severity of COVID-19 as well.
The successful induction of a T cell response was also seen in mice, confirming that the vaccine activates both MHC I and MHC II pathways of antigen processing. The vaccine is loaded on both MHC I and II molecules, from viral proteins processed within the cell and viral antigens within endosomes.
Not only is cell-mediated immunity achieved, but a balanced Th1/Th2 response is observed, as shown by elevated IFN (interferon)-γ levels. This is essential to avoid the risk of vaccine-associated enhanced respiratory disease (VAERD), seen with a Th2-biased response. Conversely, a Th1 response is associated with asymptomatic and mild SARS-CoV-2 infection.
Needle-free injection system
The study also reports the efficiency of a spring-powered, needle-free injection system (NFIS) in the rabbit study, as it reduces costs and avoids needlestick injuries as well as avoiding the disposal of sharp needles. The use of a spring instead of an external source of power is another advantage. The end result is a jet of DNA that penetrates the skin to achieve even spread, resulting in a higher uptake of the DNA within the skin cells.
Plasmid DNA clearance
The plasmid levels were observed to become undetectable at 14 days from injection from all tissues, except the injection site, where clearance took 28 days.
The researchers have thus developed the ZyCoV-D candidate vaccine, and demonstrated its ability to induce both humoral and cell-mediated immunity in various animals by intradermal injection over a range of dosages.
“Immunogenicity of this DNA vaccine candidate targeting the SARS-CoV-2 S protein in animal model supports further clinical development of this candidate in response to the current COVID-19 pandemic situation.”
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.