Programmed cell death (PCD) assures the removal of infected cells and is therefore crucial for the host defence against intracellular pathogens. It has become apparent that the PCD pathways pyroptosis, apoptosis and necroptosis are tightly interconnected and regulated by a remarkable level of redundancy. The generation of mice lacking caspases-1/-11/-12/-8 and receptor-interacting protein kinase 3 (RIPK3) in various combinations allowed us to study the individual roles of different caspases in PCD processes under conditions where other key caspases required for the host response are absent and to define new mechanisms of apoptosis induction during Salmonella Typhimurium infection.
To investigate the importance of caspase-2 in apoptosis regulation during Salmonella Typhimurium infection, we compared wild-type with caspase-2 deficient and caspases-1/-11/-12/-8/-2/RIPK3 deficient mice to prevent the compensatory effects of other caspases. Our findings revealed that the absence of caspase-2 caused no major impairments in infection control and therefore argue against a significant role for caspase-2 in PCD induction and bacterial clearance during Salmonella infections.
We next analysed if apoptosis of infected cells is induced via IFN-γ-driven mechanisms, because Salmonella stimulates a robust IFN-γ-producing CD4 T cells host response. Our findings that IFN-γ and interferon regulatory factor 1 (IRF1) deficient mice have comparably elevated bacterial titres suggest that IRF1 is the main transcription factor downstream of IFN-γ signalling during Salmonella infections. In line with this, the observed impaired infection control in mice lacking the IRF1-regulated factors TNF-α, Fas ligand and TNF-related apoptosis-inducing ligand (TRAIL) indicates their importance for apoptosis induction. Additionally, T cell depletion in pyroptosis deficient mice, which rely on apoptotic PCD pathways to control intracellular pathogens, resulted in significantly elevated bacterial titres highlighting the major role of CD4 T cells in apoptosis induction and Salmonella control.
Based on our findings we hypothesise that IFN-γ production of T cells activated by cytokines released during pyroptotic cell death can sensitise macrophages via IRF1-driven upregulation of TNF receptor superfamily members to CD4 T cell-induced extrinsic apoptosis.