Transcutaneous immunization: T cell responses and boosting of existing immunity.

Transcutaneous immunization (TCI) is a novel immunization strategy by which antigen and adjuvant are applied topically to intact, hydrated skin to induce potent antibody and cell-mediated immune responses specific for both the antigen and the adjuvant. Using tetanus toxoid as a model antigen, we examined the T cell response to tetanus toxoid after topical immunization with a variety of adjuvants. TCI readily induced systemic antigen specific T cell responses with a mixed Th1/Th2 phenotype but with a Th2 bias. We also investigated whether priming by the intramuscular route, which is known to induce T cell memory, could be followed by a boosting immunization on the skin to induce secondary responses. TCI could augment existing immunity, but interestingly, this strategy induced potent responses only if the antibody titer was low at the time of TCI boosting. These and previous observations suggest that TCI follows known immunological principles that govern other routes of vaccine delivery. Furthermore, booster immunization using tetanus toxoid may provide a useful model for further development of important patch and formulation concepts for TCI, and act as an early candidate for validating product feasibility of TCI in humans.