Construction of an inactivated typhoid vaccine candidate expressing Escherichia coli heat-labile enterotoxin B subunit and evaluation of its immunogenicity in a murine model.

Purpose. Typhoid fever caused by Salmonella enterica serovar Typhi has contributed to the global public health burden, particularly in developing countries. In this study, an S. Typhi ghost was developed and its capacity as a vaccine candidate against typhoid fever was assessed.Methodology. An asd + plasmid pJHL187 harbouring a ghost cassette comprising the PhiX 174 Elysis gene tightly controlled under the convergent promotor system was transformed into an asd gene-deleted mutant S.Typhi strain (STG). The eltB gene encoding the E. coli heat-labile enterotoxin (LTB) protein was subcloned into a foreign antigen delivery cassette of pJHL187 to increase mucosal immunity.Results. The stringent repression and expression of the lethal E lysis gene in the system allowed stable production of the ghost strain and secretion of LTB, which was confirmed by immune blot analysis. The level of IgG and sIgA was significantly increased in the mice subcutaneously immunized with STG-LTB compared to the non-immunized mice (P<0.05). The CD3+CD4+ T cell subpopulation was augmented in the immunized group (P<0.05) and showed the increment of immunomodulatory cytokines IL-2, IL-6, IL-12, IL-17 and IFN-γ in in vitro restimulated splenocytes isolated from the inoculated mice. The serum bactericidal activity of antibodies generated in the rabbits injected with STG-LTB was proved by the elimination of approximately 87.5 % of wild-type S. Typhi in the presence of exogenous complement.Conclusion. The results demonstrated that the STG-LTB ghost effectively enhanced the immunological responses, meaning that STG-LTB is potentially available as a vaccine candidate against typhoid fever.