Investigating Host Responses to the Typhoid Toxin of Salmonella Enterica

The typhoid toxin is an important virulence factor secreted by S. Typhi and is believed to play a role in promoting infection (Miller et al., 2018). The toxin induces DNA damages and triggers the innate immune response (Ibler et al., 2019). Part of this is activation of type I interferon signalling which induces hundreds of ISGs, including ISG15. This protein plays an important role in combatting viral infection (Alphonse et al., 2021), and has an emerging role in defending against bacterial infections (Radoshevich and Cossart., 2018, Wu et al., 2024).

This thesis shows that the typhoid toxin caused DNA damage, cell cycle arrest and cell death in different cell types. Toxin-induced DNA damage led to increased expression of ISG15 in HT1080 human fibroblasts. Further investigation found ISG15 played no role in toxin-induced DNA damage as both wild-type and ISG15-deficient A549 cells underwent cell cycle arrest and senescence. In parallel however, marked ISG15-deficient A549 cells treated with IFN resulted in apparent cell death. This was supported by RNA sequencing of IFN-treated ISG15-deficient A549 cells, which revealed expression of the tumour necrosis factor pathway implicated in apoptosis. In contrast, wild-type A549 cells exhibited increased expression of interferon-stimulated gene (ISGs) pathways associated with defence against viral infections. IFN induced apoptosis in ISG15 deficient cells due to loss of USP18, which was shown to be driven by ISG15 in a IFN-dependent manner. Finally, ISG15 was found to be important in containing intracellular Salmonella. In the absence of ISG15, IFN-treated cells underwent cell death thereby releasing the Salmonella into the extracellular environment. This could increase dissemination and Salmonella invasion of sterile sites such as the bloodstream in ISG15-deficient humans. The thesis supports the view that ISG15 plays a role in defending the host against intracellular bacterial pathogens, which now includes Salmonella.