Protection in a mouse peritonitis model mediated by iron-regulated outer-membrane protein of Salmonella typhi coupled to its Vi antigen

Vi polysaccharide and iron-regulated outer-membrane proteins (IROMPs) were extracted and purified from the standard strain of Salmonella typhi, Ty2. These were then conjugated by chemical coupling using the carbodimide method. Vi-IROMP conjugate was tested for its ability to protect against colonization by S. typhi in different organs. Mice immunized with 2.5 microg Vi-IROMP conjugate showed the most protection, as the least bacterial colonization of spleen, liver and Peyer's patches was observed. Peritoneal macrophages obtained from conjugate-treated mice phagocytosed bacteria efficiently. Circulating antibodies and the delayed type hypersensitivity response elucidated by mouse foot-pad swelling was significantly higher in conjugate-treated animals. These results clearly demonstrate that an IROMP and polysaccharide conjugate of S. typhi prepared from the same strain has the potential to protect animals against challenge.