Salmonella enterica subsp. enterica serovar Typhi Vi capsular polysaccharide antigen (Vi antigen)

The Salmonella enterica subsp. enterica serovar Typhi Vi capsular polysaccharide antigen, commonly known as the Vi antigen, is a linear homopolymer of α-1,4-linked N-acetylgalactosaminuronic acid (GalNAcA) with variable O-acetylation at the C-3 position and a unique reducing terminal glycolipid anchored by two β-hydroxy acyl chains. This capsule serves as a major virulence factor in typhoid fever by forming a protective coat around the bacterium, shielding it from innate immune detection, reducing IL-8 chemokine production in epithelial and macrophage-like cells, and attenuating Toll-like receptor (TLR4/TLR5) responses. It inhibits neutrophil chemotaxis, complement deposition, and phagocytosis while paradoxically promoting uptake by human macrophages, aiding systemic spread. The antigen suppresses host actin rearrangements via interaction with membrane prohibitin and Rho GTPases like Rac1/Cdc42, further dampening inflammation. Encoded by the viaB locus on pathogenicity island SPI-7, Vi antigen is the basis for vaccines like plain polysaccharide (Typhim Vi) and conjugates (Vi-TT), which elicit rapid T-independent IgM/IgG antibodies from B1b cells, conferring opsonophagocytic protection without evident major safety issues in clinical use.

Other names

Vi capsular polysaccharideVi polysaccharide antigenSalmonella Typhi Vi capsule

Induces IgG1/IgG2 antibodies and Fc effector functions against Vi antigen, elicits T cell-independent B1b cell antibody response impairing bacterial infection, and opsonizes bacteria for phagocytosis via anti-Vi antibodies.