Salmonella enterica serovar Typhi Vi capsular polysaccharide (Vi polysaccharide) (Vi polysaccharide)

The Vi polysaccharide is a linear homopolymer of alpha-1,4-linked N-acetylgalactosaminuronic acid, variably O-acetylated at the C-3 position, which forms the outermost capsule of Salmonella enterica serovar Typhi (S. Typhi) [PMID: 28838621]. It serves as a critical virulence factor by masking surface antigens like the O-antigen from host recognition and protecting the bacterium from complement-mediated killing and phagocytosis by neutrophils [PMID: 24511111]. As a major surface-exposed molecule, it is the primary target for typhoid fever vaccines, including both plain polysaccharide and protein-conjugated formulations [WHO Position Paper, 2018]. While plain polysaccharide vaccines are effective in adults, conjugate vaccines (TCVs) link the Vi antigen to a carrier protein to elicit a T-cell dependent response, providing longer-lasting immunity and efficacy in infants [PMID: 30595437]. Targeting this polysaccharide is essential for preventing systemic infection and reducing the global burden of typhoid fever. The degree of O-acetylation is a key determinant of its immunogenicity, making it a focal point for quality control in vaccine manufacturing [PMID: 28838621].

Other names

Vi antigenVirulence antigenVi capsular polysaccharidePoly-alpha-1,4-GalNAcA

Active immunization to induce protective anti-Vi IgG antibodies that facilitate opsonophagocytosis and complement-mediated killing of Salmonella Typhi [PMID: 24511111].