The use of a vaccine constructed of a protein has traditionally been to induce the formation of an antibody to the particular protein. Antibodies are crucial to an or ganism's attempt to stop an infection caused by a microorganism.
In the early 1990s, scientists observed that plasmid DNA (DNA that is present in bacteria that is not part of the main body of DNA) could affect test animals. Work began on constructing vaccines that were made of DNA instead of protein.
Instead of injecting a protein into the body to induce antibody formation, the stretch of DNA that codes for the antigen is injected into the body. When the DNA is expressed in the body, the antigen is produced and antibody formation occurs.
Another approach that can be taken with DNA vaccines is the use DNA that codes for a vital component of the disease-causing microorganism. Antibodies that form to that protein will also attack the infecting microorganisms.
The DNA can be injected in a salt solution using a hypodermic syringe. Alternatively, the DNA can be coated onto gold beads, which are then propelled at high speed into the body using an apparatus dubbed the gene gun. The production of the actual protein that stimulates antibody formation inside the body eliminates the chances of infection, as can occur with some viral vaccines that use intact and living viruses.
DNA vaccines have several advantages over the traditional vaccine methods. The immune response produced is very long, as the inserted DNA continues to be expressed and code for the production of protein. So, the use of booster injections to maintain immunity is not required. Second, a single injection can contain multiple DNA sequences, providing multiple vaccines. This advantage is especially attractive for childhood vaccinations, which currently require up to 18 visits to the physician over a decade. DNA vaccines are stable, even after a long time at room temperature. Finally, as the proteins that are crucial to diseases are deciphered, the genes for those proteins can be used in DNA vaccines.
The injection of DNA does not damage the host's genetic material. So far, the presence of the added DNA has not stimulated an immune response of a host against its own components. This reaction, termed an autoimmune response, could be possible if some host DNA coded for a protein that was very similar to the protein coded for by the added DNA.
As encouraging as these results are, the technology does have limitations. For example, so far immunity can develop only to protein. Many infections in the body occur within a covering of sugary material, which makes the underlying protein components of the infecting microbe almost invisible to the immune system.
Despite the limitations, DNA vaccines against diseases show promise. As of 2002, a vaccine against infectious hematopoietic necrosis virus in salmon and trout is being tested. The viral infection causes the deaths of large numbers of commercially raised salmon and trout, and is an economic problem for commercial fisheries. Humans have displayed immune responses to diarrhea-causing viruses, malarial parasites and tuberculosis using DNA vaccines. Indeed, it is hoped that a DNA based malaria vaccine will be available by 2005. Recently, DNA vaccines against measles and rabies have been shown to be protective against the two microbiological diseases. Protection of monkeys against HIV has been achieved, and human trials were scheduled to begin by 2003 in Nairobi on a DNA AIDS vaccine.
See also Nucleic acid.
Paterson, Y. Intracellular Bacterial Vaccine Vectors: Immunology, Cell Biology, and Genetics. New York: Wiley-Liss, 1999.
Snyder, L., and W. Champness. Molecular Genetics of Bacteria. 2nd ed. Washington, DC: American Society for Microbiology Press, 2002.
Arvin, A.M., "Measles Vaccine—a Positive Step Toward Eradicating a Negative Strand." Nature Medicine 6 (July 2000): 744.
Epstein, S.L., T.M. Tumpey, J.A. Misplon, et al. "DNA Vaccine Expressing Conserved Influenzae Virus Proteins Protective Against H5N1 Challenge Infection in Mice." Emerging Infectious Diseases 8 (August 2002): 796–801.