Science

DNA VACCINE – AN OVERVIEW

DNA vaccine technology is showing increasing promise in the treatment of human and animal diseases, and should offer immunizations that are both safer and cheaper than conventional vaccines. The DNA coding for a specific component of a disease causing organism is injected into the body. DNA vaccines are tantalizingly giving hope of a third vaccine revolution.

What is a DNA vaccine?

Ø      Essentially, a DNA sequence fused in a plasmid encoding a protein of infectious antigen of a pathogen, used as vaccine.

Ø      When it is introduced into the host cell it produces the protein and raises immune response against the pathogen.

DNA vaccine technology is showing increasing promise in the treatment of human and animal diseases, and should offer immunizations that are both safer and cheaper than conventional vaccines. Conventional vaccines have prevented many millions of cases of killer diseases such as small pox and polio. But some pathogens, such as malaria, have proven to be a considerable challenge to vaccine developers. It is in such cases that DNA vaccines may prove useful. Indeed, a promising DNA vaccine candidate has been developed for malaria. DNA vaccines are also currently being developed for over 15 other human illnesses including AIDS, herpes, tuberculosis and rotavirus, a common cause of childhood diarrhoea.

Traditional vaccination methods use either a weakened or killed version of the disease-causing organism or a component of the organism, such as inactivated toxins or proteins. These component vaccines can either be purified from the organism itself or genetically engineered.  The injection or oral administration of these non disease-causing mimics mobilizes the immune system to protect the host from the disease.

DNA vaccination differs from traditional vaccines in that just the DNA coding for a specific component of a disease-causing organism is injected into the body. The DNA can be administered either in a saline solution injected through a hypodermic needle or on DNA-coated gold beads propelled into the body using gene guns. The actual production of the immunizing protein takes place in the vaccinated host.  This eliminates any risk of infection associated with some live and attenuated virus vaccines.

With DNA vaccines, the subject is not injected with the antigen but with DNA encoding the antigen. The DNA is incorporated in a plasmid containing

• DNA sequences encoding one or more protein antigens or, often, simply epitopes of the complete antigen(s);
• DNA sequences incorporating a promoter that will enable the DNA to be efficiently transcribed in the human cells.
• Sometimes DNA sequences encoding
• Costimulatory molecules
• Sequences that target the expressed protein to specific intracellular locations (e.g., endoplasmic reticulum) are included as well. The DNA vaccine can be injected into a muscle just as conventional vaccines are.

In contrast to conventional vaccines, DNA vaccines elicit cell-mediated as well as antibody-mediated immune responses.

The cell-mediated response

• The plasmid is taken up by an antigen-presenting cell (APC) like a dendritic cell.
• The gene(s) encoding the various components are transcribed and translated.
• The protein products are degraded into peptides.
• These are exposed at the cell surface nestled in class I histocompatibility molecules where they serve as a powerful stimulant for the development of cell-mediated immunity.

The antibody-mediated response

• If the plasmid is taken up by other cells (e.g. muscle cells),
• The proteins synthesized are released and can be engulfed by antigen-presenting cells (including B cells).
• In this case, the proteins are degraded in the class II pathway and presented to helper T cells.
• These secrete lymphokines that aid B cells to produce antibodies.

The third generation vaccine

•           DNA vaccines are tantalizingly giving hope of a third vaccine revolution. Ever since the serendipitous discovery of naked DNA injection into the muscle of mice led to expression of the encoded marker protein, there has been a surge to use this approach to generate DNA vaccines against a variety of infectious diseases.

History

In 1990, the direct gene transfer of plasmid DNA into mouse muscle in vivo without the need for a special delivery system was demonstrated. Furthermore, intramuscular inoculation with plasmid DNA encoding reporter genes induced protein expression within the muscle cells. This study provided evidence for the idea that naked DNA could be delivered in vivo to direct protein expression. Subsequently, a further study reported the gene expression a year or more after intramuscular injection of plasmid DNA. Since these initial studies, many more experiments have been carried out to evaluate different factors that determine the efficiency of gene transfer and immunogenicity of plasmid DNA. Furthermore, plasmid DNA has been used to immunise against a variety of diseases (known as DNA vaccination). Alternatively, plasmid DNA has been used to treat genetic diseases and similar factors may affect the efficacy of this gene therapy.

DNA vaccines usually consist of plasmid vectors (derived from bacteria) that contain heterologous genes (transgenes) inserted under the control of a eukaryotic promoter, allowing protein expression in mammalian cells. An important consideration when optimising the efficacy of DNA vaccines is the appropriate choice of plasmid vector. The basic requirements for the backbone of a plasmid DNA vector are a eukaryotic promoter, a cloning site, a polyadenylation sequence, a selectable marker and a bacterial origin of replication. A strong promoter may be required for optimal expression in mammalian cells. For this, some promoters derived from viruses such as cytomegalovirus (CMV) or simian virus 40 (SV40) have been used. A cloning site downstream of the promoter should be provided for insertion of heterologous genes, and inclusion of a polyadenylation (polyA) sequence such as the bovine growth hormone (BGH) or SV40 polyadenylation sequence provides stabilisation of mRNA transcripts. The most commonly used selectable markers are bacterial antibiotic resistance genes, such as the ampicillin resistance gene. However, since the ampicillin resistance gene is precluded for use in humans, a kanamycin resistance gene is often used. Finally, the Escherichia coli ColE1 origin of replication, which is found in plasmids such as those in the pUC series, is most often used in DNA vaccines because it provides high plasmid copy numbers in bacteria enabling high yields of plasmid DNA on purification.

COMPONENTS OF DNA VACCINE

Viral

Ø      Recombinant viral vectors

             Vaccinia virus, Adenovirus etc.

   Non-viral

Ø      Electroporation

Ø      Liposomes

Ø      Nasal drops

Ø      Needle injection

Ø      Gene gun

HOW DNA VACCINES WORK?

·         Immunity is conferred by two critical arms

1.      Humoral arm-Acts on pathogens outside the cell

2.      Cellular arm-spearheaded by cytotoxic T-cells that eradicates pathogens that colonize the cells

·         Leads to creation of memory cells that can repel same pathogen in future

However, most of the work on DNA vaccines has been done in mice where they have proved able to protect them against tuberculosis, SARS, smallpox, and other intracellular pathogens. In addition, more than a dozen different DNA vaccines against HIV-1, the cause of AIDS are in clinical trials.