DNA vaccine

"DNA vaccine -Third-generation vaccine".

An Indian company Cadila Healthcare has successfully undertaken trials for Covid-19 vaccine using plasmid DNA technology which will the first DNA vaccine for human use.

1.A DNA vaccine is a type of vaccine that transfects a specific antigen-coding DNA sequence onto the cells of an immunized species.

2.DNA vaccines work by injecting genetically engineered plasmid containing the DNA sequence encoding the antigen(s) against which an immune response is sought, so the cells directly produce the antigen, thus causing a protective immunological response.

3.DNA vaccines have theoretical advantages over conventional vaccines, including the ability to induce a wider range of immune response types.

4.Two approaches for vaccine development :-

- Introducing specific antigens against which the immune system reacts directly.

- Introducing live attenuated infectious agents that replicate within the host without causing disease [and that can] synthesize the antigens that subsequently prime the immune system.

5.DNA vaccines contain DNA that codes for specific proteins (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.

6.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.

7.Advantages of DNA Vaccine:-

-No risk for infection

-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

8.Disadvantages of DNA Vaccine:-

- Limited to protein immunogens (not useful for non-protein based antigens such as bacterial polysaccharides)

-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

9.DNA vaccines elicit the best immune response when highly active expression vectors are used.

10.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 (MHC) classes I and class II molecules. The antigen-presenting cell then travels to the lymph nodes and presents the antigen peptide and costimulatory molecule signaled by T-cell, initiating the immune response.

10.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 (MHC) class I pathway.

11.The conformation of the protein can also affect antibody responses. “Ordered” structures (such as viral particles) are more effective than unordered structures.

12.DNA vaccine and Gene therapy techniques are similar.

13.Alternatives included aerosol instillation of naked DNA on mucosal surfaces, such as the nasal and lung mucosa, and topical administration of pDNA to the eye and vaginal mucosa.

14.By using expression library immunization (ELI) technique potentially all the genes from a pathogen can be delivered at one time, which may be useful for pathogens that are difficult to attenuate or culture.ELI can be used to identify which genes induce a protective response.

15.DNA vaccine can be administered without any special delivery mechanism.

16.By using Gene gun technique DNA can be bombarded directly into cells by using small amounts DNA.

17.By using Jet injection technique DNA can be delivered to cells mm to cm below skin surface.

18.By using Jet liposome-mediated delivery

high levels of immune response can be generated.Intravenously delivered liposome-DNA complexes can potentially transfect all tissues.

19.Intranasally delivered liposome-DNA complexes can result in expression in distal mucosa as well as nasal muscosa and the generation of IgA antibodies.

20.Delivery method defines the dose required to raise an effective immune response. Saline injections require variable amounts of DNA, from 10 μg to 1 mg, whereas gene gun deliveries require 100 to 1000 times less.Saline injections require more DNA because the DNA is delivered to the extracellular spaces of the target tissue (normally muscle), where it has to overcome physical barriers (such as the basal lamina and large amounts of connective tissue) before it is taken up by the cells, while gene gun deliveries drive/force DNA directly into the cells, resulting in less “wastage”.

21.Antigen presentation stimulates T cells to become either "cytotoxic" CD8+ cells or "helper" CD4+ cells. Cytotoxic cells directly attack other cells carrying certain foreign or abnormal molecules on their surfaces. Helper T cells, or Th cells, coordinate immune responses by communicating with other cells. In most cases, T cells only recognize an antigen if it is carried on the surface of a cell by one of the body's own MHC, or major histocompatibility complex, molecules.

22.DNA immunization can raise multiple TH responses, including lymphoproliferation and the generation of a variety of cytokine profiles. A major advantage of DNA vaccines is the ease with which they can be manipulated to bias the type of T-cell help towards a TH1 or TH2 response.Each type has distinctive patterns of lymphokine and chemokine expression, specific types of immunoglobulins, patterns of lymphocyte trafficking and types of innate immune responses.

23.The type of T-cell help raised is influenced by the delivery method and the type of immunogen expressed, as well as the targeting of different lymphoid compartments.Generally, saline needle injections (either IM or ID) tend to induce TH1 responses, while gene gun delivery raises TH2 responses.This is true for intracellular and plasma membrane-bound antigens, but not for secreted antigens, which seem to generate TH2 responses, regardless of the method of delivery.

24.Generally the type of T-cell help raised is stable over time, and does not change when challenged or after subsequent immunizations that would normally have raised the opposite type of response in a naïve specimen.

25.One of the advantages of DNA vaccines is that they are able to induce cytotoxic T lymphocytes (CTL) without the inherent risk associated with live vaccines. CTL responses can be raised against immunodominant and immunorecessive CTL epitopes, as well as subdominant CTL epitopes,in a manner that appears to mimic natural infection. This may prove to be a useful tool in assessing CTL epitopes and their role in providing immunity.

26.Targeting gene products directly to the ER (by the addition of an amino-terminal insertion sequence) should thus enhance CTL responses.