Transcutaneous immunization with bacterial ADP-ribosylating exotoxins, subunits, and unrelated adjuvants

We have recently described a needle-free method of vaccination, transcutane ous immunization, consisting of the topical application of vaccine antigens to intact skin. While most proteins themselves are poor immunogens on the skin, we have shown that the addition of cholera toxin (CT), a mucosal adju vant, results in cellular and humoral immune responses to the adjuvant and coadministered antigens. The present study explores the breadth of adjuvant s that have activity on the skin, using diphtheria toroid (DTx) and tetanus toroid as model antigens. Heat-labile enterotoxin (LT) displayed adjuvant properties similar to those of CT when used on the skin and induced protect ive immune responses against tetanus toxin challenge when applied topically at doses as low as 1 mu g. Interestingly, enterotoxin derivatives LTR192G, LTK63, and LTR72 and the recombinant CT B subunit also exhibited adjuvant properties on the skin. Consistent with the latter finding, non-ADP-ribosyl ating exotoxins, including an oligonucleotide DNA sequence, as well as seve ral cytokines (interleukin-1 beta [IL-1 beta] fragment, IL-2, IL-12, and tu mor necrosis factor alpha) and lipopolysaccharide also elicited detectable anti-DTx immunoglobulin G titers in the immunized mice. These results indic ate that enhancement of the immune response to topical immunization is not restricted to CT or the ADP-ribosylating exotoxins as adjuvants. This study also reinforces earlier findings that addition of an adjuvant is important for the induction of robust immune responses to vaccine antigens delivered by topical application.