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Vaccines represent the most beneficial and cost effective public health measure currently known. However, as the understanding of neoplasias and infectious diseases grows, it has become apparent that traditional vaccine strategies may not be completely effective. Traditional vaccines have employed killed or attenuated organisms or antigen subunits in order to elicit immunity in an animal. A limit with these approaches, especially with killed or subunit vaccines, is that the immune response is primarily humoral in nature, and therefore not effective in combating intracellular organism or tumors that require cell mediated immunity for their destruction. Similarly, attenuated or inactivated bacteria often only induce immunization for a short period of time and immunity is limited to a humoral response. Further, traditional attenuated or inactivated bacterial vaccines do not elicit the cytotoxic T-lymphocyte (CTL) immune response necessary for the lysis of tumor cells and cells infected with intracellular pathogens.
