Mucosal immunization of mice using CpG DNA and/or mutants of the heat-labile enterotoxin of Escherichia coli as adjuvants

Cholera toxin (CT) and the Escherichia coli heat-labile enterotoxin (LT) are potent mucosal adjuvants in animals associated, at least in part, with their ability to induce cAMP. While toxicity generally precludes their use in humans, a number of different subunit or genetically detoxified mutants of CT and LT have been developed. Another type of adjuvant that has been shown to be effective at mucosal surfaces comprises synthetic oligodeoxynucleotides (ODN) containing immunostimulatory CpG motifs (CpG ODN). We have previously demonstrated a synergy between CpG ODN and native toxins after intranasal (IN) administration to mice, and herein have examined whether this synergy is linked to the cAMP activity. The adjuvanticity of CpG ODN was evaluated with IN and oral delivery of tetanus toroid or the hepatitis B surface antigen, relative to and in combination with native LT holotoxin (LTh), three active site mutants (LTS61F, LTA69G, LTE112K), a protease site mutant (LTR192G), and the B subunit of LT (LTB). At an equivalent dose, the adjuvants could generally be divided into two groups: one that included CPG ODN, LTh, LTR192G, and LTA69G which acted as strong adjuvants; and the second which comprised LTB, LTS61F, and LTE112K, which produced significantly weaker immune responses. When CpG ODN was co-administered with bacterial toxin-derivatives, in most cases, no synergy between CpG and the LT derivatives was found for strength of the humoral response. Nevertheless, for both routes and antigens, CpG ODN combined with any LT derivative induced a more Type I-like response than LT derivative alone. These results suggest that while the synergy seen previously with native toxins may have been due in part to inherent cAMP activity, it may have also depended on the particular antigen used and the route of immunization. (C) 2001 Elsevier Science Ltd. All rights reserved.