Targeting the mucosa: genetically engineered vaccines and mucosal immune responses.

The discovery that inoculation of DNA leads to strong and long lasting immune responses generated enthusiasm to assess the efficacy of various genetically engineered vaccines against mucosally acquired infections. Various techniques have been used to generate the most suitable DNA vaccines, ranging from immunization with naked DNA to utilizing genetically engineered recombinant viruses and bacteria to deliver the DNA. Different DNA vaccine modalities and mucosal immune responses to them have been discussed. It has been shown that even though intramuscular and intradermal immunization with these vaccines generates strong systemic responses, mucosal responses are not induced. It has been proposed that the site of immunization determines mucosal immune responses and that primed lymphocytes preferentially accumulate at sites where they have been induced thus generating the strongest cellular and antibody responses at the site of vaccination. The impact of the site of induction on mucosal immune responses to vaccines is discussed. It is possible to enhance desired vaccine effects in the mucosa and to modify the undesirable side effects. Cytokines such as IL-2, IL-12, IL-15 and IL-18 have been used to enhance CTL activity while IL-5, IL-6 and the chemokine MIP-1 alpha have shown the capacity to increase IgA responses to vaccines.