Caspases play central roles in the removal of unwanted cells through the control, initiation and execution of diverse forms of cell death, including apoptosis, pyroptosis and necroptosis and thereby contribute to the host’s ability to control intracellular pathogens by removing their replicative niche. However, how the diverse forms of cell death are coordinated, inter-connected and provide back-up for each other upon infection of the cell with an intracellular pathogen remains unclear.
To explore this systematically, we ablated key components of all relevant cell death pathways genetically both in vivo and in vitro and infected mice and macrophages with Salmonella. Individual loss of pyroptosis, caspase-8-driven apoptosis, or necroptosis had minor impact on Salmonella control and cell death. However, combined deficiency of these cell death pathways caused loss of bacterial control in mice and their macrophages, demonstrating that host defense can employ varying components of several cell death pathways to limit intracellular infections. This flexible use of distinct cell death pathways involved extensive cross-talk between initiators and effectors of pyroptosis and apoptosis, and CRISPR/Cas9 whole genome screening identified that caspases-1 and -8 also functioned as executioners when all known effectors of cell death were absent. We are now employing novel CRISPR/Cas activator tools to further unravel the molecular control mechanisms underlying this highly coordinated and flexible cell death system with in-built fail-safe processes that protect the host from intracellular infections.