Naked DNA when co-administered intranasally with heat-labile enterotoxin of Escherichia coli primes effectively for systemic B- and T-cell responses to the encoded antigen.

In this study a novel prime-boost immunisation strategy was evaluated. Priming of BALB/c mice by the intranasal route with plasmid DNA encoding beta-galactosidase (LacZ) with or without heat-labile enterotoxin (LT) of Escherichia coli as a mucosal adjuvant, resulted in the induction of weak serum antibody and proliferative T-cell responses. However, following an intraperitoneal booster injection with the beta-galactosidase protein (beta-gal), strong antibody and proliferative T-cell responses were induced in all the mice. These responses were highest in mice primed intranasally with a mixture of LacZ+LT as compared to those mice primed with DNA (LacZ) or protein (beta-gal) alone. Moreover, LacZ+LT primed mice produced high avidity antibodies and the subclasses of serum antibodies were IgG1 and IgG2a, suggesting a mixed Th1/Th2-type response. Priming of mice with either protein (beta-gal) or DNA (LacZ) alone, produced predominantly IgG1 antibodies, suggesting a Th2-type response. These findings suggest that the use of a heterologous DNA-prime, protein-boost immunisation scheme combining different routes of administration, might be an advantageous strategy for the induction of accelerated immune responses.