Immunogenicity of Vi-Polysaccharide and Recombinant Outer Membrane Protein A (r-OmpA) Conjugate of S.Typhi In Mice

Background: According to WHO, 21 million new cases of typhoid fever occur every year with 216,000 deaths globally. Traditionally typhoid fever considered to be a disease of adolescence, an increasing evidence of high rates in young children <5 years of age. Here, a conjugate of Vi-polysaccharide with outer membrane protein A (OmpA) of S.Typhi induces an adequate antibody response in mice.  

Objectives: Development of Vi-OmpA conjugate (Vi-conjugate) candidate vaccine against typhoid fever effective in all age group including children <5 years

Methods: OmpA of S.Typhi was cloned and expressed in E. coli. Recombinant OmpA was recovered and purified under denaturing condition by Ni⁺-NTA resin. r-OmpA was conjugated with Vi-polysaccharide of S.Typhi using carbodiimide (EDAC) and ADH as linker. Mice were immunized subcutaneously with Vi-OmpA, Vi-alone, r-OmpA-alone and Saline on days 0, 14 and 21. The antibody against Vi-polysaccharide and OmpA was measured by ELISA.

Results: Amino acid and nucleotide sequencing identified the protein to be OmpA of S.Typhi origin. Different batches of Vi-OmpA conjugates had a polysaccharide/protein ratio between 0.62–0.71. Vi-alone elicited very low levels of Vi-antibody and did not show any booster effect. Vi-OmpA evoked a strong and more efficient immune response than Vi-alone. The conjugate also elicited a strong booster response which persisted for longer duration. Further, adequate level of IgG antibodies was induced only by the conjugate. Both Vi-OmpA and r-OmpA-alone immunization induced similar levels of OmpA antibody.  

Conclusions: It is expected that the Vi-OmpA conjugate will induce protective antibodies in children <5 years of age including younger than 2 years also. The OmpA as a conjugate partner with Vi-polysaccharide may prove to be better than traditional proteins like diphtheria toxin etc., because the protective effect of OmpA antibody may be additive to that provided by Vi-antibodies. The OmpA being conserved in all Salmonellae, the OmpA antibodies may provide some protection against these infections also.