Traditional procedures of vaccination have used purified components of an infectious organism, viruses or dead or attenuated intact cells to elicit production of specific antibodies that can provide individuals with active immunity. Such vaccines have successfully provided protection against diseases such as polio, smallpox, whooping cough, typhoid fever and diphtheria. However, the development of immunizations against pathogens such as HIV and Malaria has proven to be difficult. The enormous worldwide impact of these pathogens, the potential bioterror uses of others, and the worsening problem of antibiotic resistance have prompted the efforts to produce new vaccines.

A recent approach to immunization has used DNA containing a sequence of the pathogen’s genome. This DNA is typically a bacterial plasmid engineered to include the sequence of an antigenic protein from the pathogen. This DNA can enter a number of cell types, and it can be expressed there using cellular transcription and translation machinery. In this respect, DNA vaccines act much like viruses. How ever, these DNAs contain only a vey limited amount of genetic information and can not become infectious. The mechanism of uptake and induction of the immune response are not yet clear. However, promising results have been observed against viruses, bacteria and parasitic microorganisms. This approach holds great promise for development of effective vaccines against intractable diseases including HIV/AIDS, tuberculosis, and malaria.

DNA vaccines may also be useful for vaccination against cancers. While the antigens presented by tumor cells are only weakly immunogenic, model studies using plasmid DNA have shown promising results. Mice vaccinated by oral delivery of plasmids grown within attenuated salmonella bacteria were able to slow or completely stop the growth of a lethal dose of carcinoma cells. Death of the bacteria presumably releases large numbers of the plasmids that are taken up by antigen-presenting cells of immune system. Extension of this work to humans is currently under investigations.