A new era of vaccine technology

WHAT are DNA vaccines, currently being heralded as a major breakthrough in vaccinology and immunisation? DNA vaccines conceptually belong to a class different from vaccines based on whole pathogens, killed (polio, rabies, cholera) or live-attenuated (polio, measles, mumps, rubella, tuberculosis), and sub-unit vaccines composed of just the pathogen's active proteins (tetanus, diphtheria, pertussis, hepatitis B, influenza) or polysaccharide antigens in pure form (haemophilus influenza B, pneumococcus, meningococcus).

DNA or deoxyribonucleic acid, is the basic material that encodes all the genetic information with regard to any organism. In a DNA vaccine the idea is to use the DNA that carries the code for the microbial antigen itself as the vaccine instead of the microbe or the antigen produced in vitro as the vaccine. That is, instead of administering the microbe or the antigenic protein in an appropriate medium, the genetic information for the protein is injected and the host becomes a factory for the production of the (alien) gene product. The principle is simple. Yet a complete understanding of how it may work and the revolutionary development of potential DNA vaccines and related technology have been possible only in the last few years. The Tenth International Congress of Immunology provided ample evidence of the remarkable progress that has been achieved on this front.

Significantly, the foreign DNA does not integrate with the host chromosome, which would otherwise have serious safety implications for such vaccines. H.L. Robinson of Emery University, United States, explained at the symposium on DNA vaccines that a plasmid (a circular piece of DNA that is capable of autonomously replicating and expressing outside the chromosome in the cytoplasm of cells) encoding a microbial protein is injected into the host. Appropriate promoter/enhancer genes to control the expression of the inserted protein gene are also inserted into the plasmid. A bacterial plasmid is used to code the protein which directly translates in the living cell - the relevant gene is transcribed by the host RNA (ribonucleic acid) in the cell nucleus and translated to protein. It turns out that the cells of the immune system, the antigen presenting cells (APC) or the dendritic cells in the skin are particularly good for this purpose. The good news from research of the last decade is that this factory seems to work well and does generate strong immune responses in the host, both humoral and cellular, which are long-lasting and protective. The host thus gets immunised against a foreign protein produced by the host's own cells.

So far DNA vaccines have shown little side effects. Any genomic integration with the host could be mutagenic or potentially carcinogenic. However, the carrier being a bacterial plasmid, the genes that control its replication have no homology or similarity with the genetic sequences in a eucaryotic host's DNA. This reduces to almost zero the chance of recombination and integration with the host DNA. It has been found that the plasmid DNA itself disappears from the host system within a week (essentially because the skin dendritic cells have a life span of about a week) but expresses the protein for a sufficiently long period to generate a long-term immune response.

Contrary to what scientists believed years ago - and that is why DNA was never thought of as a good antigen carrier for a vaccine - DNA is not easily degraded by the enzymes of the host body system. "Unlike other vectors, the nice thing about DNA is that it goes away after about a week or so," said Dr. Fritz Melchers of the Basel Institute for Immunology. The exact mechanism by which the DNA gets destroyed is not clear. This does not rule out random integration but the chances for this have been demonstrated to be very low. Analysis using very sensitive detection techniques have not been able to detect one copy of the plasmid in 150,000 cellular nuclei in animal model systems that have been studied. At the same time, the host immune system seems to retain memory of the foreign protein and mount an immune response whenever challenged. The exact mechanism of how this long-term memory works is an area of continuing research.

It is interesting to note that it suffices to inject the "naked" DNA itself. This was demonstrated in 1990. Earlier it was thought that a protective lipid coat over the plasmid DNA or an adjuvant may be required. Bacterial DNA itself has inherent adjuvant properties, achieved by triggering the production of chemicals called cytokines which stimulate the cellular immune responses. Once this was demonstrated, other means of delivering the vaccine have been developed, most notably by coating gold particles with the plasmid DNA and delivering it directly into the skin by a helium-pressure-loaded gene gun. In principle, even a mere rubbing of the DNA on the skin should work. Remarkably, it has been found that mice could be protected against different strains of the influenza virus using DNA encoded with a protein that is highly conserved across the various virus strains.

During the last five years, efficacious DNA vaccines in various animal models have been reported for a variety of viral, parasitic and bacterial pathogens. Promising results have been reported in Phase I and Phase II trials for malaria, tuberculosis and HIV, the three major diseases that have defied the development of effective vaccines. Other diseases for which DNA vaccines are undergoing pre-clinical trials include hepatitis B, herpes and influenza. The true worth of these vaccines will be known in the next few years as these enter clinical trials.

'Genetic immunisation', as this new method of vaccine delivery has come to be called, promises to be a potent tool that can generate strong and long-term immune responses. A great advantage of the DNA vaccine concept, particularly from the perspective of developing countries, is the relative ease with which DNA vaccines can be manufactured, without sophisticated protein production and purification technology. Also, being inherently stable, these vaccines would not require a cold chain for their preservation. DNA vaccines have clearly opened up a new area of research in immunology and a new era of vaccine technology which could be a boon to developing countries.