Research

Wait and watch

Though scientists and pharma companies are aware of the potential that DNA vaccines hold, they have still not arrived at any conclusion. Sachin Jagdale reviews the developments

Its been more than two hundred years since the discovery of first vaccine by Edward Jenner. Jenner's discovery of smallpox vaccine resulted in almost complete eradication of smallpox. Last two centuries witnessed the evolution of vaccines in many different ways. Initially, vaccines used to have live attenuated infectious material that was supposed to generate immune response inside the host body; even today this is an often employed concept. Now, vaccines have come of age. Advances in bacteriology and virology have led to more advanced form of vaccines. One such vaccine that has caught the eye of medical fraternity is the DNA vaccine. The health conscious world that has survived for a long time on a traditional form of vaccine is now giving serious thought to DNA vaccines.

What is DNA Vaccination?

DNA vaccination is a technique wherein an organism will be protected by injecting it with genetically-engineered DNA to produce an immunological response. DNA vaccines are also known as nucleic acid vaccines. However, these vaccines are still experimental, and have been applied to a number of bacterial, viral and parasitic models of disease, as well as to numerous tumour models.

Although DNA vaccine is a highly controversial issue, genetic material has been used for therapeutic purpose since the last fifty years. Scientists like Griffith had transferred DNA into cells of living animals in the early 1930. In 1943, Oswald Avery proved that DNA carries genetic information. Post 1950, experiments were conducted using purified genetic material. Such experiments provided the solid evidence that direct injection of DNA gene results in the expression of the inoculated gene in the host and this too in the absence of vector. As far as DNA vaccine is concerned, it was accidently discovered by scientists Tang and Johnson.

"The 'foreign' DNA from the DNA vaccines could become integrated into the chromosomes of the host cells." - Sanjeev Saxena Chair man and CEO Actis Biologics

"DNA vaccines are non-infectious, economical to produce in large amounts and easy to purify using simple and inexpensive procedures" - P N Rangarajan Associate Professor, Department of Biochemistry Indian Institute of Science

Construction of DNA Vaccine

DNA vaccine is made up of double stranded circular DNA called plasmid. This plasmid is genetically engineered and is capable of producing specific protein (antigens). Plasmids used in DNA based vaccines are usually composed of an expression unit and a production unit. Expression unit has promoter sequences that are followed by antigen encoding and polyadenylation sequences, while production unit is composed of bacterial sequences, which would amplify plasmid. Vaccine inserts are fitted into the bacterial plasmid by using recombinant DNA technology. Once this unit is ready, vaccine plasmid is introduced into bacteria where bacterial growth produces multiple plasmid copies. The purified form of this plasmid DNA acts as a vaccine.

Why DNA vaccine?

Genetic engineers do not make new genes, they rearrange existing ones. The headway in genetics has proved critical in finding out remedies to many kinds of diseases. DNA vaccine is one such weapon of genetic engineering. Renowned American author George F Will has rightly said that a disquieting era of genetic manipulation is in the offing, that may revolutionise human capacities, and notions of health. When for more than 150 years, conventional vaccines are successfully treating the masses, up to the level of satisfaction, why do we need DNA vaccine? PN Rangarajan, Associate Professor, Department of Biochemistry, Indian Institute of Science, Bangalore justifies the need by pointing out the advantages of this vaccine. "In case of DNA vaccination, since the pathogenic protein is synthesised in its native form inside the host cell, the chances of vaccine failure are minimal. In addition, DNA vaccines are non-infectious, economical to produce in large amounts and easy to purify using simple and inexpensive procedures. Moreover, all DNA vaccines can be produced using similar fermentation, purification and validation techniques, " he says.

Benefits of DNA vaccines are many and they are far from over. Rangarajan briefs about the cost effectiveness of DNA vaccines. "DNA vaccines do not require cold-chain and it can be stored either dry or in an aqueous solution at room temperature. Cold-chain is the series of refrigerators required to maintain the viability of a vaccine during distribution. Currently, maintaining the cold-chain represents nearly 80 per cent of the cost of vaccinating individuals in developing nations," informs Rangarajan.

Better but not Best

DNA vaccines are undoubtedly more effective and economical than conventional vaccines but they are not devoid of side effects. Sanjeev Saxena, Chairman and CEO, Actis Biologics says, "The 'foreign' DNA from the DNA vaccines could become integrated into the chromosomes of the host cells. The effect of such integration of DNA into a chromosome could range from no effect whatsoever to potentially catastrophic results such as inducing cancer through alteration of normal DNA. The effect of continuous antigen production in situ may also cause a hyperactive immune response leading to anaphylactic shock, systemic organ failure and death."

The list of worries does not end here. The mechanism of DNA vaccine action is still not well understood. There are still many questions on the use of DNA vaccines. Rangarajan raises some key queries. He asks, "Does DNA vaccination obey classical rules of inducing immune responses or are DNA-transfected muscle cells or skin cells, and not lymphoid cells, presenting the antigen? If lymphoid cells present the antigen, how do they obtain the antigen? Does DNA directly transfect these cells, or do they acquire protein from transfected skin or muscle cells?" Rangarajan adds, "Since expression of the plasmid encoded antigen appears to persist for a long time, there is a concern that unresponsiveness might result, rather than protective immunity. Another major concern is the induction of anti-DNA antibodies by plasmid DNA. Antibodies to DNA can cause disease."

What is Next?

Amidst enormous hope and suspicion, it will be interesting to see how long it takes to use DNA vaccines for human purpose. Scientists are keen on reducing the drawbacks of DNA vaccines but it continues to remain a 'do-not-access' vaccine. Saxena opines, "While DNA vaccines will be yet another weapon in the armamentarium of vaccines to prevent and treat disease in addition to the so-called conventional or traditional vaccines, it cannot be replaced entirely."

If Rangarajan is to be believed, unrealistic expectations and hype could lead to backlash, if hope fails to become reality in a timely fashion. Thus, we have no choice but to wait and watch until the results are more reliable.

Glossary of terms

Plasmid: A plasmid is an extrachromosomal DNA molecule, separate from the chromosomal DNA and capable of autonomous replication.

Vector: In molecular biology and genetic engineering a vector is a vehicle for transferring genetic material into a cell.

Recombinant DNA: It is a form of artificial DNA that is engineered through the combination or insertion of one or more DNA strands, thereby combining DNA sequences that would not normally occur together.

Antigen: An antigen (from antibody generating) or immunogen is a molecule that sometimes stimulates an immune response.

Antibodies: Antibodies or immunoglobulin are proteins that are found in blood or other bodily fluids of vertebrates, and are used by the immune system to identify and neutralise foreign objects, such as bacteria and viruses.

sachin.jagdale@expressindia.com