Typhoid fever is an acute, systemic infection of the reticuloendothelial system caused by Salmonella enterica
serotype Typhi that is annually responsible for an estimated 16 million illnesses and 600,000 deaths worldwide (6
). Our knowledge of the pathogenesis of S.
Typhi is limited because it infects only humans, resulting in the absence of in vivo models to study host-pathogen interactions. Many studies have relied on a murine model of human typhoid that uses S. enterica
serotype Typhimurium, which causes a typhoid-like illness in mice. As a consequence, most of what is known about the pathogenicity of S.
Typhi has been extrapolated from S.
Typhimurium infections in mice. A significant limitation to using this murine model is that S.
Typhimurium does not cause typhoid fever in humans but instead causes a localized gastroenteritis resulting in diarrhea.
Whole-genome sequencing has revealed that the S.
Typhi genome contains 601 genes on 82 genetic islands that are absent from the S.
Typhimurium genome (34
). The largest of these islands, termed the Salmonella
pathogenicity island 7 (SPI-7), contains 134 kb of S.
Typhi-specific DNA and carries biosynthesis genes (viaB
locus) for the production of the Vi capsular antigen, a linear polymer of α-1,4 2-deoxy-2-N
-acetylgalacturonic acid that is variably O acetylated at the C-3 position (16
). The Vi capsular antigen is a significant virulence factor for typhoid fever, as strains positive for Vi production have higher rates of infection (20
), and it continues to be the focus for improvement in current treatment and prophylaxis for this disease.
The two-component-positive regulatory systems RcsBC and OmpR EnvZ, in addition to the promoter of the viaB
region, located upstream of tviA
, have been identified as contributors to Vi expression and are modulated by osmolarity. Under low-osmolarity conditions, the production of Sip proteins, flagellin, and Vi antigen is differentially modulated by the RcsB-RcsC regulatory system. The transcription of the iagA
, and sipB
genes is negatively controlled by the RcsB regulator (2
). The TviA protein functions as a positive regulator in cotranscribing the tviA
). In addition, the TviA protein may act in concert with the RcsB protein at the tviA
promoter to activate transcription of the genes involved in Vi synthesis (49
Typhi strains harboring ompR
deletions no longer agglutinate with Vi antiserum (35
). The rpoS
gene, which is a master regulator in stress response and required for survival under extreme conditions in S.
Typhimurium, has been implicated as another regulator of Vi synthesis of S.
). In conclusion, the regulation of Vi expression in vitro has been extensively characterized. However, gene regulation in an infected host may differ markedly from the predictions generated through in vitro and cell culture work (3
). These considerations illustrate the need for in vivo studies to fully understand regulation of capsule expression in S.
Understanding the role of the Vi antigen during pathogenesis is further complicated because the location of capsule expression in the host remains a matter of debate. One hypothesis predicts that osmolarity may represent an important signal for the transition of S.
Typhi from the lumen into intestinal tissue (36
). In the intestinal lumen, the osmolarity is high, with values considered to be equivalent to 300 mM NaCl and greater (13
). Once S
. Typhi invades the intestinal barrier, it encounters a lower-osmolarity condition equivalent to ~150 mM (29
), which has been reported to be the osmolarity of blood and plasma. It is thought that the hyperosmotic conditions within the intestinal lumen may promote a hyperinvasive phenotype while suppressing Vi expression. After invasion of epithelial cells, the expression of the Vi capsular antigen is predicted to be activated, while expression of the type III secretion system 1 (T3SS-1) is downregulated (38
An alternative interpretation of the available data proposes that the capsule may inhibit bacterial adhesion and invasion of the intestinal epithelium (2
), suggesting that the Vi antigen may be produced in the intestinal lumen. Naturally occurring typhoid fever infections as well as live S.
Typhi vaccines provoke a poor host protective immune response, even though Vi is a good antigen (44
), which may be due to the inactivation of Vi antigen expression once the pathogen invades the intestinal epithelium and resides in macrophages. This concept has been proposed to be a possible explanation for the reduced Vi antibody responses, as decreased amounts of antigen may be presented to the host's immune system for processing (35
). In conclusion, it remains unclear in which host compartment(s) the Vi capsular antigen is expressed in vivo. The purpose of this study was to investigate the expression of the Vi antigen during transition of bacteria from the intestinal lumen into mucosal tissue. We employed state-of-the-art models to study this transition in vitro, using polarized human intestinal model epithelia (an in vitro model), and in vivo, using bovine ligated ileal loops.