In vivo and in vitro adjuvant activities of the B subunit of Type IIb heat-labile enterotoxin (LT-IIb-B₅) from Escherichia coli

The pentameric B subunit of the Escherichia coli LT-IIb enterotoxin (LT-IIb-B₅) activates TLR2 signaling in macrophages. Herein we demonstrate that LT-IIb-B₅, in contrast to a TLR2-nonbinding point mutant, induces functional activation of bone marrow-derived dendritic cells and stimulates CD4⁺ T cell proliferation, activities which suggested that LT-IIb-B₅ might function as an adjuvant in vivo. Indeed, in an intranasal mouse immunization model, LT-IIb-B₅ augmented specific mucosal and serum antibody responses to a co-administered immunogen, at levels which were almost comparable to those induced by intact LT-IIb holotoxin, a potent but toxic adjuvant. Therefore, LT-IIb-B₅ displays useful adjuvant properties which, combined with lack of enterotoxicity and relative stability against degradation, may find application in mucosal vaccines.