RESUMEN
Multiple TonB dependent transporters (TBDTs) contribute to bacterial virulence due to the importance roles that their substrates play in bacterial growth, and possess vaccine potential. A putative TBDT, YncD, had been identified as one of in vivo induced antigens during human infection of typhoid fever, and is required for the pathogenicity of Salmonella enterica Serovar Typhi. The present study was aimed to determine the function and immunogenicity of YncD. Homologous recombination method was used to construct an yncD-deletion mutant and cirA-iroN-fepA-deletion mutant from the wild-type S. Typhi Ty2. The growth of mutants and the wild-type strain were assessed in iron-deficient medium, as well as in human macrophage cells. Recombinant YncD protein was expressed and purified using Ni-NTA affinity chromatography and anion exchange. A mouse model was then used to evaluate the immunogenicity and protection efficacy of the recombinant YncD. Antibody levels, serum bactericidal efficiency, passive immune protection, opsonophagocysis were assayed to analyse the immunoprotection mechanism of the recombinant YncD. Our results showed that YncD is associated with the iron-uptake of S. Typhi. The yncD-deletion mutant displayed impaired growth in iron-deficient medium, comparable to that the cirA-iroN-fepA-deletion mutant did. The mutation of yncD markedly decreased bacterial growth within human macrophage cells. Moreover, subcutaneous immunization of mice with recombinant YncD elicited high levels of specific anti-YncD IgG, IgG1 and IgG2a, which protected the immunized mice against the intraperitoneal challenge of S. Typhi, and decreased bacterial burdens in the livers and spleens of the infected mice. Passive immunization using the immunized sera also efficiently protected the mice from the challenge of S. Typhi. Moreover, the immunized sera enhanced in vitro bactericidal activity of complement, and opsonophagocytosis. Our results showed that YncD displays a role in the iron-uptake of S. Typhi and possesses immunogenicity.