Analysis of Salmonella typhi-host cell interaction

PhD Student
Amita Sharma
Neha Subramaniam

Salmonella typhi causes typhoid fever in humans. The host-pathogen interaction during infection with this organism is poorly understood primarily due to non-availability of a suitable animal model. Our current understanding of how this bacterium interacts with host cells comes largely from studies carried out with S. typhimurium which in mice causes an analogous disease commonly referred to as murine typhoid. These studies have given important insights into how Salmonella species in general invade host cells, induce cellular cytotoxicity and ensure their survival in the hostile intracellular environment of a macrophage. The molecules which have been assigned a role in these functions, are very similar in different Salmonella serovars. However, inspite of this similarity, many Salmonella species exhibit a remarkable degree of host specificity. S. typhi causes a generalised systemic infection exclusively in humans whereas S. typhimurium produces a relatively localised gastroenteritis. Recent studies suggest that there might be significant differences in the way these two closely related pathogens interact with host cells. These differences could be either due to differences in the cell wall entities involved in initial contact with host cells or due to different effector molecules produced by the organims upon contact with host cells. This project is aimed at deciphering S. typhi-specific host-pathogen interactions.

The results discussed above demonstrated specific interaction of Vi with cell surface receptors on intestinal epithelial cells. To further establish the specificity of this interaction and examine the role of acetyl groups in this interaction, binding of Vi to cells was carried out in the presence of excess of commercially available polygalacturonic acid. The latter differs from Vi in the polymer size and in the absence of O-acetyl and N-acetyl groups at positions C-2 and C-3 respectively. No significant inhibition was observed in the binding of Vi to the human intestinal epithelial cell line, Caco-2, in the presence of polygalacturonic acid suggesting that acetyl groups were required for the binding of Vi to cells. Further, O-acetylated polygalacturonic acid prepared from polygalacturonic acid by treatment with acetic anhydride and pyridine also did not significantly block binding, indicating that N-acetyl at position C-2 may play a crucial role in this interaction (O-acetylation was confirmed by reactivity with an anti-Vi monoclonal antibody previously generated in the laboratory). The specificity of Vi-host cell interaction was also demonstrated by the inability of lipopolysaccharide (LPS) derived from Salmonella enteritidis, to inhibit binding of Vi to Caco-2 cells. Consistent with this analysis, the adhesion of Vi positive S. typhi to intestinal epithelial cells was significantly blocked by prior incubation of these cells with purified Vi.

To analyse the role of the capsule in the induction of proinflammatory responses during infection with S. typhi, we determined IL-8 secretion following infection of Caco-2 cells with Vi positive or Vi negative bacteria. IL-8 was detected by an ELISA using commercially available antibodies. The invasion of Caco-2 cells by Vi negative S. typhi resulted in secretion of significantly higher amounts of IL-8 as compared to Vi positive S. typhi. This may be consistent with a recently proposed model for Salmonella infection based on studies carried out with many Salmonella serovars. The model proposes that IL-8 secretion in the intestine may play a more crucial role during enteritis caused by many Salmonella species than during enteric fever produced by S. typhi. It may be mentioned that more than 95% S. typhi isolates from typhoid patients are Vi positive. We are now investigating the basis of this differential response produced by the two S. typhi strains. Preliminary data suggests that entry of S. typhi into cells may not be required for the induction of IL-8 secretion.

Our results with Vi suggest that engagement of this polysaccharide may be an important early interaction event during encounter of S. typhi with host cells. Clearly, however, there must be additional molecules which play a role in host-pathogen interaction during infection with S. typhi. Currently, it is believed that the invasion of intestinal epithelial cells by Salmonella is mediated by molecules of the type III secretion system. These molecules are produced by the bacteria following contact with host cells. The role of many of these molecules in invasion, induction of apoptosis and intracellular survival has been studied in some detail. What, however, remains elusive is the nature of the molecule(s) on host cells that makes the first crucial contact with the pathogen, and the nature of the corresponding ligand on the pathogen. We have looked at the ability of S. typhi to secrete proinflammatory molecule(s) following contact with intestinal epithelial cells or with serum. Our data, though preliminary, suggest that the signal to produce such a molecule(s) by the bacterium may come partly from contact with host cells and partly through interaction with one or more components of the serum. We were also able to demonstrate production of flagellin by bacteria in response to interaction with one of these stimuli; flagellin is a potent inducer of IL-8 secretion by intestinal epithelial cells.

Publications

Original peer reviewed articles

1.     ¶Qadri A and Ward ES (2001) Activation of a T cell hybridoma by an alloligand results in differential effects on IL-2 secretion and activation induced cell death. Eur J Immunol 31:3825-3832 (¶on deputation/work done elsewhere).