Role of GM1 binding in the mucosal immunogenicity and adjuvant activity of the Escherichia coli heat-labile enterotoxin and its B subunit

Escherichia coli (E. coli) heat-labile toxin (LT) is a potent mucosal immunogen and immunoadjuvant towards co-administered antigens. LT is composed of one copy of the A subunit, which has ADP-ribosylation activity, and a homopentamer of B subunits, which has affinity for the toxin receptor, the ganglioside G(M1). Both the ADP-ribosylation activity of LTA and G(M1) binding of LTB have been proposed to be involved in immune stimulation. We investigated the roles of these activities in the immunogenicity of recombinant LT or LTB upon intranasal immunization of mice using LT/LTB mutants, lacking either ADP-ribosylation activity, G(M1)-binding affinity, or both. Likewise, the adjuvant properties of these LT/LTB variants towards influenza virus subunit antigen were investigated. With respect to the immunogenicity of LT and LTB, we found that G(M1)-binding activity is essential for effective induction of anti-LTB antibodies. On the other hand, an LT mutant lacking ADP-ribosylation activity retained the immunogenic properties of the native toxin, indicating that ADP ribosylation is not critically involved. Whereas adjuvanticity of LTB was found to be directly related to G(M1)-binding activity, adjuvanticity of LT was found to be independent of G(M1)-binding affinity. Moreover, a mutant lacking both G(M1)-binding and ADP-ribosylation activity, also retained adjuvanticity. These results demonstrate that neither ADP-ribosylation activity nor G(M1) binding are essential for adjuvanticity of LT, and suggest an ADP-ribosylation-independent adjuvant effect of the A subunit.