Evolutionary convergence of a Pyroptosis-Apoptosis crosstalk drives non-canonical inflammasome signalling.

The coordination of efficient immune responses to infections is essential to enable pathogen clearance and host survival. Cell death modalities are a crucial component of the innate immune system and are key to controlling intracellular and extracellular infections. Human caspase-4 and -5 are cysteine proteases activated upon intracellular detection of bacterial lipopolysaccharide (LPS) and triggers pyroptosis, a lytic and pro-inflammatory form of cell death. However, the interaction between these caspases and other cell death modalities remains poorly understood. Here, we build on evidence that caspase-4 can engage in crosstalk with the apoptotic executioner caspases-3 and -7. Our work validated caspase-3 and -7 as direct substrates of caspase-4 and -5, and demonstrated that this specificity was regulated by exosite interactions as opposed to the tetrapeptide. Using AlphaFold, we generated an interaction model between caspase-4 and executioner caspases-3 and -7. From this, we experimentally validated the importance of a hydrophobic interface on caspase-3 and -7 that engaged the caspase-4 exosite, enabling their recognition and cleavage. Importantly, this interface is not used by the apoptotic initiator caspase-8, which cleaved caspase-3 in a tetrapeptide-dependent manner. Our work highlights that inflammatory caspases have evolved a novel mechanism to coordinate crosstalk between pyroptotic and apoptotic signalling, and suggests that these pathways may synergise for the efficient amplification of cell death pathways.