Vi capsular polysaccharide antigen (Vi antigen) (Vi antigen)

The Vi capsular polysaccharide is a linear homopolymer of α-1,4-linked N-acetylgalactosaminuronate that constitutes the outermost layer of Salmonella enterica serovar Typhi (Source: NIH, 2016). It functions as a critical virulence factor by shielding the bacterium from the host's innate immune system, specifically by inhibiting complement activation and preventing neutrophil-mediated phagocytosis (Source: Wikipedia, 2024). Interestingly, the Vi antigen can also facilitate bacterial entry into macrophages by interacting with receptors such as DC-SIGN and prohibitin, which supports the pathogen's intracellular lifestyle (Source: NIH, 2022). The expression of this antigen is regulated by environmental conditions such as osmolarity, allowing the bacteria to adapt to different host environments (Source: NIH, 2016). As a therapeutic target, the Vi antigen is the basis for several typhoid vaccines, including purified polysaccharide and protein-conjugate formulations (Source: WHO, 2018). While unconjugated polysaccharide vaccines are effective in adults, they fail to induce a T-cell-dependent response, making them unsuitable for infants (Source: Frontiers in Immunology, 2021). Conjugate vaccines, which link the Vi antigen to a carrier protein, overcome this limitation by providing long-lasting immunity and efficacy in young children (Source: NIH, 2015). These vaccines work by eliciting anti-Vi antibodies that neutralize the capsule's protective properties, thereby enabling the host immune system to clear the infection (Source: Frontiers in Immunology, 2021).

Other names

Vi polysaccharideVirulence antigenSalmonella Typhi Vi antigenCapsular polysaccharideViCPS

Induction of protective anti-Vi antibodies (IgG and IgM) that opsonize the bacteria, neutralizing the capsule's antiphagocytic effects and facilitating complement-mediated killing and phagocytosis by host immune cells.