Cellular senescence is an innate physiological mechanism crucial in development, wound healing and tumour suppression. It is characterised by an irreversible cell cycle arrest, resistance to apoptosis, and an inflammatory secretome that remodels bystander cells, referred to as senescence-associated secretory phenotype (SASP). SASP acts in an autocrine or paracrine manner to reinforce senescence, transmit senescence to naive cells and activate immunosurveillance to remove cells of pathological potential. In ageing organisms, senescent cells accumulate, resulting in SASP-driven chronic inflammation, aberrant tissue homeostasis and immune functions, and age-related pathologies such as cancer. Ageing also comes with an increased susceptibility to infectious diseases but the cellular basis for this is unclear.
Salmonella Typhi causes more than 11 million cases of drug-resistant typhoid fever cases each year that are spreading intercontinentally, making it a global health concern. The typhoid toxin of Salmonella Typhi causes DNA damage, cell cycle arrest and cell distension, which are indicative of senescence. Toxin-induced DNA damage is associated with mortality, typhoid fever symptoms and chronic Salmonella carriage in animal models, which aids in pathogen shedding and disease transmission. Therefore, dissecting the typhoid toxin virulence mechanism is of utmost importance.
Using molecular and cellular biology, this thesis substantiates evidence that the typhoid toxin induces senescence in vitro marked by persistent DNA damage responses, increased senescence associated β-galactosidase, p21 activity, diminished lamin B1, and paracrine senescence via toxin-induced SASP (toxSASP). Interestingly, previous work in our lab demonstrated that toxSASP uniquely promoted Salmonella infection, while aphidicolin (APH), i.e. another senescence inducer, did not. This finding indicates that factors in the host secretome increase the susceptibility of bystander cells to Salmonella infection but the identity of the SASP factors are unknown. Using unbiased LC-MS/MS and GeneChip microarray transcriptomics, this thesis reveals toxSASP constituents which were divergent from other senescence inducers, namely APHSASP and ETPSASP. Additionally, in vitro experiments implicate potential crosstalk between TGFβ and Wnt5a signalling pathway in toxin-induced senescence phenotypes. Indeed, purified Activin A, a TGFβ ligand, and Wnt5A contributed to toxSASP paracrine senescence. Furthermore, TGFβ receptors knockdown via siRNAs ameliorated Salmonella invasion induced by toxSASP.
In conclusion, this study reveals the first proteomic characterisation of a SASP induced by a bacterial toxin. The study represents a Salmonella hijacking mechanism via the TGFβ signalling pathway, which represents a novel host-pathogen interaction that may be of significance to invasive infections underlying typhoid fever and chronic carriage. This thesis is of broad significance as it reveals a way by which bacterial pathogens can reprogramme multicellular infection niches by hijacking the host secretome through DNA damage responses.
