Programmed cell death processes are a key mechanism by which the human innate immune system restricts the growth and dissemination of gastrointestinal pathogens such as Salmonella. Non-typhoidal serovars infect the small intestine and underlying innate immune cells to cause gastroenteritis with symptoms of diarrhoea, nausea, vomiting, and fever, and can induce systemic infection if left unchecked in immunocompromised individuals. By activating cell death mechanisms such as pyroptosis, apoptosis and necroptosis, infected macrophages may halt intracellular bacterial replication, and enable the extracellular release of inflammatory cytokines and danger signals. However, Salmonellae interrupt these innate immune processes by using two specialised Type III Secretion Systems (T3SSs) to introduce bacterial effector proteins directly into the host cell cytosol; thus manipulating the cellular environment to promote bacterial survival. As such, characterisation of novel effector protein functions is crucial to understanding the success of these pathogens.
Our research discovered that wild type S. Typhimurium induces the degradation of cellular inhibitor of apoptosis protein 1 (cIAP1), an important host cell adaptor of tumour necrosis factor receptor 1 (TNFR1) signalling and inhibitor of apoptotic cell death. We observed strong association between cIAP1 loss and increased cellular cytotoxicity, with corresponding caspase -8 and -3 activation. Depletion of cIAP1 was associated with functional Salmonella Pathogenicity Island 1 (SPI-1) T3SS effector translocation, and was not prevented by pan-caspase, proteasomal or lysosomal inhibitors. Anti-cIAP1 immunoblot detected a low molecular weight peptide following S. Typhimurium infection, suggesting that SPI-1 effector/s may cleave cIAP1 during infection. Current work combines several molecular and in vitro techniques to explore the cIAP cleavage mechanism and determine the responsible SPI-1 effector protein, with transfection screens suggesting several key candidates. Future work will assess the involvement of cIAP proteins in overall susceptibility to Salmonella infection in vivo. This finding suggests a new role for Salmonella effector proteins in activating, rather than preventing, host cell death, which we hypothesise may promote dissemination of the bacteria.