Mutational analysis of the role of ADP-ribosylation activity and G(M1)-binding activity in the adjuvant properties of the Escherichia coli heat-labile enterotoxin towards intranasally administered keyhole limpet hemocyanin

Abstract

The Escherichia coli heat-labile enterotoxin (LT) is known for its potent mucosal immunoadjuvant activity towards co-administered antigens. LT is composed of one A subunit, which has ADP-ribosylation activity, and a homopentameric B subunit, which has high affinity for the toxin receptor, ganglioside G(M1). In previous studies, we have investigated the role of the LTA and LTB subunits in the adjuvanticity of LT towards influenza virus hemagglutinin (HA), administered intranasally to mice. We now studied the adjuvant properties of LT and LT variants towards keyhole limpet hemocyanin (KLH), which, in contrast to HA, does not bind specifically to mucosal surfaces. It is demonstrated that LT mutants without ADP-ribosylation activity, as well as LTB, retain mucosal immunoadjuvant activity when administered intranasally to mice in conjunction with KLH. As with influenza HA, adjuvanticity of LTB required G(M1)-binding activity, whereas G(M1)-binding was not essential for adjuvant activity of LT. Furthermore, we found that also recombinant LTA alone acts as a potent mucosal adjuvant, and that this adjuvanticity is independent of ADP-ribosylation activity. It is concluded that binding of the antigen to mucosal surfaces does not play an essential role in the immunostimulation by LT and LT variants, and that both recombinant LTA and LTB represent powerful nontoxic mucosal adjuvants.