Importance and use of DNA vaccine

DNA vaccine brings a remarkable revolution in the history of medical science. Many infectious diseases have been brought under control, thanks to several vaccines, which are known to offer protection against them. However, vaccines cannot protect us from all diseases. Microbes that cause deadly diseases like AIDS, malaria, leprosy, herpes and hepatitis C are still victorious in their vicious attempt to flourish unchecked in the body of a human host. Owing to the inherent limitations of the existing vaccines, search is now on for new and better vaccines the DNA vaccines that may offer a sure shot solutions to combat the hitherto untamed deadly pathogens.

So far the rationale for designing a vaccine has been to utilize the body’s own defense mechanism to destroy the infecting pathogen. When a killed or weakened form of the very pathogen that causes the diseases is introduced into the body it induces the body to produce antibodies that would recognize and destroy the same pathogen in case of a later infection. In contrast, DNA vaccines are crafted from the genetic material of a particular pathogen. The crux of designing a DNA vaccine is to inject into the human body the specific genes of a pathogen, which code for those antigens against which the body raises a strong protective response. This prompts the defense machinery to attack that pathogen if it invades the body.

But how exactly are DNA vaccines made ? To start with one or more genes of a pathogen are isolated which encode proteins that act as strong antigens. These genes are then incorporated into small rings of double standard DNA called ‘plasmids’, which are originally derived from bacteria. The resultant ‘recombinant’ plasmids basically constitute the DNA vaccine.

A DNA vaccine can be administered by injecting the tailored plasmid into the muscle, thus putting genes directly into muscle cells. Alternatively, the vaccine could be coated onto gold particles and delivered into the human body by a gene gun, which propels the plasmids into skin cells and mucous membranes. Yet another way of delivering a DNA vaccine into human body is by using weakened strains of bacteria, such as Salmonella typhimurium (which causes typhoid), Shigella flexneri and Listeria monocytogenes. Weakened forms of theses bacterial species do not cause diseases but can help in carrying the genetically engineered plasmid to special cells of the immune system which instantly engulf them. Theses bacteria break down inside these cells and release the plasmid molecules. Once the DNA vaccine is delivered into human cells, the plasmids make their way into the cell’s nucleus. The pathogen’s genes carried by the plasmid then begin to encode their protein products. As these antigenic proteins are foreign to the human host, the defense machinery is alerted to raise a counter attack.

But how are genetic vaccines superior in performance as compared to the existing vaccines ? the answer to this question lies in understanding the way a vaccine works by gearing up the human defense machinery to combat an invader. A vaccine preparation could activate one or both the arms known as the cellular and humeral arms of the body’s immune system, by altering a brigade of which blood cells called the helper T-cells. The cellular arm of the immune system is spearheaded by cytotoxic T-cells, also called the killer cells, which simply destroy the infected cells and the invaders present within them. On the other hand, the B-cells commands the humoral arm of the immune system. Theses cells basically act on the pathogen’s that are outside cells. They wage a war against them by secreting specific molecules called antibodies. Acting like missiles targeted against the invader, antibodies latch onto them firmly destroying them thereafter. Conventional vaccines, which consist of killed pathogens or those comparing the select antigens of a pathogen, cannot make their way into cells. Therefore, they elicit only humoral response by which specific antibodies are produced to kill the pathogen. The cellular arm of the immune machinery does not get activated at all and the killer T cells are not put into action. In other words, conventional vaccines do not generate a complete immune response and are infective against many pathogens that infiltrate into cells. Moreover, as these vaccines comprising killed pathogens or their select antigens are actually non-living preparations, their effect to trigger the immune system wears off after effect to trigger the immune system wears off after a time. Periodic booster shouts of such vaccines are therefore necessary.