DNA vaccine
A DNA vaccine is a type of vaccine that transfects a specific antigen-coding DNA sequence into the cells of an organism as a mechanism to induce an immune response.
DNA vaccines work by injecting genetically engineered plasmid containing the DNA sequence encoding the antigen against which an immune response is sought, so the cells directly produce the antigen, thus causing a protective immunological response. DNA vaccines have theoretical advantages over conventional vaccines, including the "ability to induce a wider range of types of immune response". Several DNA vaccines have been tested for veterinary use. In some cases, protection from disease in animals has been obtained, in others not. Research is ongoing over the approach for viral, bacterial and parasitic diseases in humans, as well as for cancers. In August 2021, Indian authorities gave emergency approval to ZyCoV-D. Developed by Cadila Healthcare, it is the first DNA vaccine approved for humans.
History
Conventional vaccines contain either specific antigens from a pathogen, or attenuated viruses which stimulate an immune response in the vaccinated organism. DNA vaccines are members of the genetic vaccines, because they contain a genetic information that codes for the cellular production of an antigen. DNA vaccines contain DNA that codes for specific antigens from a pathogen. The DNA is injected into the body and taken up by cells, whose normal metabolic processes synthesize proteins based on the genetic code in the plasmid that they have taken up. Because these proteins contain regions of amino acid sequences that are characteristic of bacteria or viruses, they are recognized as foreign and when they are processed by the host cells and displayed on their surface, the immune system is alerted, which then triggers immune responses. Alternatively, the DNA may be encapsulated in protein to facilitate cell entry. If this capsid protein is included in the DNA, the resulting vaccine can combine the potency of a live vaccine without reversion risks.In 1983, Enzo Paoletti and Dennis Panicali at the New York Department of Health devised a strategy to produce recombinant DNA vaccines by using genetic engineering to transform ordinary smallpox vaccine into vaccines that may be able to prevent other diseases. They altered the DNA of cowpox virus by inserting a gene from other viruses. In 1993, Jeffrey Ulmer and co-workers at Merck Research Laboratories demonstrated that direct injection of mice with plasmid DNA encoding a flu antigen protected the animals against subsequent experimental infection with influenza virus. In 2016 a DNA vaccine for the Zika virus began testing in humans at the National Institutes of Health. The study was planned to involve up to 120 subjects aged between 18 and 35. Separately, Inovio Pharmaceuticals and began tests of a different DNA vaccine against Zika in Miami. The NIH vaccine is injected into the upper arm under high pressure. Manufacturing the vaccines in volume remained unsolved as of August 2016. Clinical trials for DNA vaccines to prevent HIV are underway.
In August 2021, Indian authorities gave emergency approval to ZyCoV-D. Developed by Cadila Healthcare, it is the first DNA vaccine against COVID-19.
Applications
no DNA vaccines have been approved for human use in the United States. Few experimental trials have evoked a response strong enough to protect against disease and the technique's usefulness remains to be proven in humans.A veterinary DNA vaccine to protect horses from West Nile virus has been approved. Another West Nile virus vaccine has been tested successfully on American robins.
DNA immunization is also being investigated as a means of developing antivenom sera. DNA immunization can be used as a technology platform for monoclonal antibody induction.
Advantages
- No risk for infections
- Antigen presentation by both MHC class I and class II molecules
- Polarise T-cell response toward type 1 or type 2
- Immune response focused on the antigen of interest
- Ease of development and production
- Stability for storage and shipping
- Cost-effectiveness
- Obviates need for peptide synthesis, expression and purification of recombinant proteins and use of toxic adjuvants
- Long-term persistence of immunogen
- In vivo expression ensures protein more closely resembles normal eukaryotic structure, with accompanying post-translational modifications
Disadvantages
- Limited to protein immunogens
- Potential for atypical processing of bacterial and parasite proteins
- Potential when using nasal spray administration of plasmid DNA nanoparticles to transfect non-target cells, such as brain cells
- Cross-contamination when manufacturing different types of live vaccines in same facility
Plasmid vectors
Vector design
DNA vaccines elicit the best immune response when high-expression vectors are used. These are plasmids that usually consist of a strong viral promoter to drive the in vivo transcription and translation of the gene of interest. Intron A may sometimes be included to improve mRNA stability and hence increase protein expression. Plasmids also include a strong polyadenylation/transcriptional termination signal, such as bovine growth hormone or rabbit beta-globulin polyadenylation sequences. Polycistronic vectors are sometimes constructed to express more than one immunogen, or to express an immunogen and an immunostimulatory protein.Because the plasmidcarrying relatively small genetic code up to about 200 Kbpis the "vehicle" from which the immunogen is expressed, optimising vector design for maximal protein expression is essential. One way of enhancing protein expression is by optimising the codon usage of pathogenic mRNAs for eukaryotic cells. Pathogens often have different AT-contents than the target species, so altering the gene sequence of the immunogen to reflect the codons more commonly used in the target species may improve its expression.
Another consideration is the choice of promoter. The SV40 promoter was conventionally used until research showed that vectors driven by the Rous Sarcoma Virus promoter had much higher expression rates. More recently, expression and immunogenicity have been further increased in model systems by the use of the cytomegalovirus immediate early promoter, and a retroviral cis-acting transcriptional element. Additional modifications to improve expression rates include the insertion of enhancer sequences, synthetic introns, adenovirus tripartite leader sequences and modifications to the polyadenylation and transcriptional termination sequences. An example of DNA vaccine plasmid is pVAC, which uses SV40 promoter.
Structural instability phenomena are of particular concern for plasmid manufacture, DNA vaccination and gene therapy. Accessory regions pertaining to the plasmid backbone may engage in a wide range of structural instability phenomena. Well-known catalysts of genetic instability include direct, inverted and tandem repeats, which are conspicuous in many commercially available cloning and expression vectors. Therefore, the reduction or complete elimination of extraneous noncoding backbone sequences would pointedly reduce the propensity for such events to take place and consequently the overall plasmid's recombinogenic potential.
Mechanism of plasmids
Once the plasmid inserts itself into the transfected cell nucleus, it codes for a peptide string of a foreign antigen. On its surface the cell displays the foreign antigen with both histocompatibility complex classes I and class II molecules. The antigen-presenting cell then travels to the lymph nodes and presents the antigen peptide and costimulatory molecule signalling to T-cell, initiating the immune response.Vaccine insert design
Immunogens can be targeted to various cellular compartments to improve antibody or cytotoxic T-cell responses. Secreted or plasma membrane-bound antigens are more effective at inducing antibody responses than cytosolic antigens, while cytotoxic T-cell responses can be improved by targeting antigens for cytoplasmic degradation and subsequent entry into the major histocompatibility complex class I pathway. This is usually accomplished by the addition of N-terminal ubiquitin signals.The conformation of the protein can also affect antibody responses. "Ordered" structures are more effective than unordered structures. Strings of minigenes from different pathogens raise cytotoxic T-cell responses to some pathogens, especially if a TH epitope is also included.