Recently, encouraging results are reported for DNA vaccines where by DNA coding for the foreign antigen is directly injected into the animal, so that the foreign antigen is directly produced by the host cells.
In theory these vaccines would be extremely safe and devoid of side effects since the foreign antigens would be directly produced by the host animal.
In addition, DNA is relatively inexpensive and easier to produce than conventional vaccines and thus this technology may one day increase the availability of vaccines to developing countries.
Moreover, the time for development is relatively short, which may enable timely immunization against emerging infectious diseases.
In addition, DNA vaccines can theoretically result in more long-term production of an antigenic protein when introduced into a relatively non dividing tissue, such as muscle.
Indeed some observers have already dubbed the new technology the “third revolution” in vaccine development on par with Pasteur’s ground-breaking work with whole organisms and the development of subunit vaccines.
The first clinical trials using injections of DNA to stimulate an immune response against a foreign protein began for HIV in 1995. Four other clinical trials using DNA vaccines against influenza, herpes simplex virus, T-cell lymphoma, and an additional trial for HIV were started in 1996.
The technique that is being tested in humans involves the direct injection of plasmids – (aditional DNA in bacteria) that contain genes for proteins to be produced by the organism being targeted for immunity. Once injected into the host’s muscle tissue, the DNA is taken up by host cells, which then start expressing the foreign protein. The protein serves as an antigen that stimulates an immune responses and protective immunological memory.
Enthusiasm for DNA vaccination in humans is tempered by the fact that delivery of the DNA to cells is still not optimal, particularly in larger animals.
Another concern is the possibility, which exists with all gene therapy that is the vaccine’s DNA may be integrated into host chromosomes and turn on oncogenes or turn off tumor suppressor genes.
Another potential drawback is that extended immune stimulation by the foreign antigen could in theory provoke chronic inflammation or autoantibody production.
I. Limited to protein immunogens (not useful for non-protein based antigens such as bacterial polysaccharides).
II. Risk of affecting genes, controlling cell growth.
III. Possibility of inducing antibody production against DNA.
IV. Possibility of tolerance to the antigen (protein) produced.
V. Potential for atypical processing of bacterial and parasite proteins.