Salmonella enterica serotype Typhi Vi capsular polysaccharide (Vi antigen) (Vi antigen)

The Salmonella enterica serotype Typhi Vi capsular polysaccharide is a critical virulence factor and a primary target for typhoid fever vaccines [WHO, 2018]. It is a linear homopolymer composed of alpha-1,4-linked N-acetylgalactosaminuronic acid that is variably O-acetylated at the C-3 position [Frontiers in Cellular and Infection Microbiology, 2017]. This polysaccharide forms a capsule around the bacterium, which masks underlying surface structures like the O-antigen, thereby preventing complement-mediated lysis and phagocytosis by host immune cells [Infection and Immunity, 2005]. By inhibiting the host's innate immune response, the Vi antigen allows S. Typhi to survive and replicate within the bloodstream and reticuloendothelial system [Frontiers in Cellular and Infection Microbiology, 2017]. Therapeutic intervention primarily involves the use of Vi-based vaccines, including unconjugated polysaccharide vaccines and newer Vi-conjugate vaccines (TCVs) [WHO, 2018; The Lancet, 2017]. These vaccines stimulate the production of protective antibodies that facilitate bacterial clearance and provide long-term immunity against typhoid fever [The Lancet, 2017].

Other names

Vi capsular polysaccharideVirulence antigenVi polysaccharidePoly-alpha-(1->4)-D-GalNAcASalmonella Typhi Vi antigenVi capsular antigen

Vaccines targeting the Vi capsular polysaccharide induce the production of specific anti-Vi antibodies (primarily IgG), which opsonize the bacteria for phagocytosis and neutralize the pathogen's ability to evade the host immune system [WHO, 2018; The Lancet, 2017].