The role of cAMP in mucosal adjuvanticity of Escherichia coli heat-labile enterotoxin (LT)

Heat-labile enterotoxin (LT) produced by enterotoxigenic Escherichia coli ( ETEC) and cholera toxin (CT) produced by Vibrio cholerae have been shown to function as potent mucosal adjuvants. A number of studies have examined th e effects of different mutations at either the active site or the protease site of LT and CT and the influence of those mutations on toxicity and adju vanticity. However, different observations reported by various groups using a variety of animal models with different antigens or different routes of immunization have provided contradictory findings and evoked many questions regarding the underlying mechanisms of mucosal adjuvanticity of LT and CT. In this study, the role of cAMP in mucosal adjuvanticity was examined by c omparing three LT active site mutants (S61F, A69G, E112K), a protease site mutant (R192G) and recombinant LT-B for toxicity, cAMP activity and mucosal adjuvanticity using tetanus toroid (TT) as a model antigen. While all mutants examined showed reduced toxicity, the effects of each mut ation on its ability to function as an adjuvant varied. Following intranasa l immunization, native LT as well as protease and active site mutants of LT induced serum anti-TT IgG and their responses were virtually indistinguish able from one another. In addition, LT-B was also able to enhance productio n of serum anti-TT IgG, though at a level significantly lower than that ach ieved by native LT and mutants. Following oral immunization. the best serum anti-TT IgG responses were obtained with native LT and mutants that retain ed the ability to induce accumulation of cAMP. Despite the nearly identical serum anti-TT IgG responses following intranasal immunization, there was a strong correlation between the ability to induce accumulation of cAMP in c ultured Caco-2 cells and the ability to elicit production of antigen-specif ic Th1 or Th2 cytokines. (C) 1999 Elsevier Science Ltd. All rights reserved .