DNA vaccine

A DNA vaccine is a form of genetic vaccine in which a plasmid containing a specific antigen-encoding DNA sequence is introduced into host cells to induce an immune response. The plasmid directs the cellular machinery to synthesise the antigen internally, allowing the immune system to recognise it as foreign and mount a protective response. DNA vaccines therefore rely on transfection rather than administration of purified antigens or attenuated pathogens. They have attracted considerable interest because of their potential to trigger broad immune responses involving both humoral and cellular pathways, and because of their relative stability and ease of manufacture.

Historical development

Conventional vaccines utilise attenuated pathogens or isolated antigenic components to provoke immunity. DNA vaccines emerged in the 1980s within the broader class of genetic vaccines, which also includes RNA vaccines. In 1983, researchers developed a method to engineer recombinant vaccinia viruses by inserting genes from pathogens such as herpes simplex virus, hepatitis B virus, or influenza into the cowpox genome. This approach demonstrated that genetic modification of viral backbones could yield tailored antigen expression.
In 1993, studies at Merck showed that intramuscular injection of plasmid DNA encoding influenza antigens protected mice against viral challenge, establishing proof of concept for non-viral DNA immunisation. Subsequent research explored DNA vaccination against a wide range of pathogens, including HIV and Zika virus. Human trials for Zika DNA vaccines began in 2016. Despite promising immunogenicity, large-scale manufacture and delivery remained challenging.
A landmark approval occurred in August 2021, when Indian regulators authorised ZyCoV-D, developed by Cadila Healthcare, as the first DNA vaccine approved for human use. It was licensed for emergency use against COVID-19, marking an important step in the translation of DNA vaccine technology from experimental platforms to clinical deployment.

Mechanism of action

DNA vaccines deliver plasmid DNA into host tissues, usually via intramuscular or intradermal injection, sometimes enhanced by electroporation or pressure-assisted devices. Once taken up by cells, the plasmid enters the nucleus and directs transcription and translation of the antigen. The expressed protein undergoes intracellular processing and is presented on the cell surface by both major histocompatibility complex (MHC) class I and class II molecules.
Antigen-presenting cells displaying the foreign antigen migrate to lymphoid tissues, engage T cells through peptide–MHC complexes, and provide costimulatory signals required for activation. These events generate cytotoxic T-cell responses, helper T-cell responses, and antibody production. Because the antigen is produced endogenously, its structure more closely resembles naturally expressed viral or microbial proteins, complete with post-translational modifications.

Plasmid vector design

Successful DNA vaccination depends heavily on vector optimisation. High-expression plasmids typically incorporate strong viral promoters such as the cytomegalovirus (CMV) immediate-early promoter or the Rous sarcoma virus (RSV) promoter to drive robust transcription. Enhancer elements, synthetic introns, and adenoviral tripartite leader sequences may be included to increase mRNA stability and translation efficiency. Polyadenylation signals, often derived from bovine growth hormone or rabbit beta-globin, facilitate proper transcriptional termination.
Cistronic vectors can encode multiple antigens or combine an immunogen with an immunostimulatory protein. Codon optimisation adjusts the gene sequence to reflect host cell codon usage, improving protein expression. To enhance cytotoxic T-cell responses, ubiquitin tags can be added to target antigens for rapid cytoplasmic degradation and entry into the MHC class I pathway.
Structural stability of plasmids is an important manufacturing consideration. Repeats and noncoding regions in plasmid backbones can predispose vectors to recombination, so minimising extraneous sequences reduces instability and improves production yields.

Vaccine insert strategies

Antigen localisation affects immune response quality. Secreted or membrane-bound antigens generally induce strong antibody responses, whereas cytosolic antigens favour cytotoxic T-cell activation. Some designs incorporate structured antigen forms, such as virus-like particles, which elicit stronger humoral responses than unstructured peptides. Multi-epitope “minigene” constructs permit simultaneous targeting of different pathogen components, potentially broadening immune coverage.

Applications

DNA vaccines have shown promise in veterinary medicine. A vaccine against West Nile virus has been licensed for horses, and another has been tested in American robins. Research also explores DNA immunisation for producing monoclonal antibodies in vivo and for inducing protective responses against snake venom components.
Human applications remain under active investigation. Clinical trials have evaluated DNA vaccines for HIV, Zika virus, and various cancers. Despite substantial research, only ZyCoV-D has received approval for human use, and in many cases immune responses in trials have been insufficiently strong to confer full protection. Work continues to refine delivery methods, vector systems, and adjuvant strategies.

Advantages

DNA vaccines offer several potential advantages:

• They cannot cause infection because they contain no live pathogen.
• Antigens are presented on both MHC class I and II pathways, generating broad immune responses.
• They allow polarisation of T-cell responses toward type 1 or type 2 immunity.
• Antigen expression is focused and consistent, without the need for complex protein purification.
• Plasmids are stable, inexpensive to produce, and easy to store or transport.
• In vivo expression promotes proper post-translational modifications.
• Immunogens can persist for extended periods, enhancing immune memory.

Disadvantages

Limitations include:

• DNA vaccines are restricted to protein antigens; polysaccharides and other non-protein molecules cannot be encoded.
• Some antigens may undergo atypical processing, reducing immunogenicity.
• Delivery routes such as intranasal administration risk transfecting non-target tissues.
• Manufacturing multiple live vector vaccines in the same facility can risk cross-contamination.
• Structural instability in plasmids may complicate large-scale production.

Functional overview of plasmids

After nuclear entry, plasmids encode the antigenic peptide, which is processed and presented by MHC molecules. Antigen-presenting cells then activate T cells within lymph nodes, initiating adaptive immune responses. This mechanism underpins the flexibility of DNA vaccines in directing either antibody-mediated or cytotoxic immunity.

Broader perspectives

As a platform, DNA vaccination continues to evolve, supported by advances in molecular biology, delivery technologies, and immunological understanding. By enabling internal production of pathogen-specific antigens, DNA vaccines offer a modular and rapidly adaptable approach to immunisation. Their development has contributed to larger trends in genetic vaccines, forming a conceptual foundation for RNA vaccines and other nucleic acid-based immunotherapies. Continued optimisation may extend their application to infectious diseases, oncology, and therapeutic antibody generation, strengthening their role in modern biomedical science