Adjuvant effect of enterotoxigenic Escherichia coli (ETEC) double-mutant heat-labile toxin (dmLT) on systemic immunogenicity induced by the CFA/I/II/IV MEFA ETEC vaccine: Dose-related enhancement of antibody responses to seven ETEC adhesins (CFA/I, CS1-CS6)

Hyesuk Seo,Ti Lu,Sachin Mani,A Louis Bourgeois,Richard Walker,David A Sack,Weiping Zhang

doi:10.1080/21645515.2019.1649555

Abstract

ABSTRACTDouble-mutant heat-labile toxin (dmLT, LTR192G/L211A) of enterotoxigenic Escherichia coli (ETEC) is an effective mucosal adjuvant. Recent studies have shown that dmLT also exhibits adjuvanticity for antigens administered parenterally. In this study, we subcutaneously (SC) immunized mice with the ETEC adhesin-based vaccine, CFA/I/II/IV MEFA (multiepitope fusion antigen), adjuvanted with dmLT and examined the impact of dmLT on antibody responses specific to the seven adhesins in the vaccine construction [CFA/I, CFA/II (CS1, CS2, CS3) and CFA/IV (CS4, CS5, CS6)]. Mice were immunized with a fixed dose of CFA/I/II/IV MEFA and ascending doses of dmLT adjuvant (0, 0.05, 0.1, 0.5 or 1.0 µg) to assess the potential dmLT dose response relationship. Data showed that dmLT enhanced systemic antibody responses to all seven antigens (CFA/I, CS1-CS6) targeted by MEFA in a dose-dependent way. The adjuvant effect of dmLT on the MEFA construct plateaued at a dose of 0.1 µg. Results also indicated that dmLT is an effective parenteral adjuvant when given by the SC route with the ETEC adhesin MEFA vaccine and that antibody enhancement was achieved with relatively low doses. These observations suggest the potential usefulness of dmLT for parenteral ETEC vaccine candidates and also perhaps for vaccines against other pathogens.

Highlights

ADP-ribosylating bacterial toxins, heat-labile toxin (LT) of enterotoxigenic Escherichia coli (ETEC) and cholera toxin (CT) of Vibrio cholerae, are effective adjuvants for mucosal vaccines.[1–4] LT is a typical bacterial AB5 holotoxin, consisting of a ribosylating A subunit and five B subunits which form a pentameric ring to bind to host GM gangliosides receptors.[5]
Mice SC immunized with CFA/I/II/IV MEFA with 1 μg dmLT had significantly greater anti-adhesin IgG titers detected in serum samples, compared to mice immunized with CFA/I/II/ IV MEFA without dmLT adjuvant (p < .01) (Figure 1)
AntiCFA/I, -CS1, -CS2, -CS3, -CS4, -CS5 and anti-CS6 IgG titers were detected at 4.3 ± 0.33, 4.7 ± 0.31, 4.2 ± 0.42, 4.4 ± 0.22, 4.3 ± 0.33, 4.7 ± 0.44, and 4.1 ± 0.40, respectively, in the serum samples of mice SC immunized with CFA/I/II/IV MEFA adjuvanted with 1 μg of dmLT

Summary

Introduction

ADP-ribosylating bacterial toxins, heat-labile toxin (LT) of enterotoxigenic Escherichia coli (ETEC) and cholera toxin (CT) of Vibrio cholerae, are effective adjuvants for mucosal vaccines.[1–4] LT (and CT) is a typical bacterial AB5 holotoxin, consisting of a ribosylating A subunit and five B subunits which form a pentameric ring to bind to host GM gangliosides receptors.[5]. ADP-ribosylating bacterial toxins, heat-labile toxin (LT) of enterotoxigenic Escherichia coli (ETEC) and cholera toxin (CT) of Vibrio cholerae, are effective adjuvants for mucosal vaccines.[1–4]. LT (and CT) is a typical bacterial AB5 holotoxin, consisting of a ribosylating A subunit and five B subunits which form a pentameric ring to bind to host GM gangliosides receptors.[5]. It is thought that LT binding to host GM receptors facilitates antigen uptake across mucosal membranes and likely gains access to follicle-associated epithelium and the Peyer’s patches, up-immunoregulating antigens and enhancing (as an adjuvant) stimulation of antigen-specific immunity.[6,7]. While enzymatic activity for induction of intracellular cyclic GMP was further reduced, dmLT was found to retain LT adjuvanticity and was demonstrated to immunoregulate antigen-specific mucosal immune responses in oral, intragastric, sublingual and intranasal immunization studies.[14–21]

Methods

Results

Conclusion