Inflammasome

Inflammasomes are cytosolic multiprotein oligomers of the innate immune system responsible for the activation of inflammatory responses. Activation and assembly of the inflammasome promotes proteolytic cleavage, maturation and secretion of pro-inflammatory cytokines interleukin 1β (IL-1β) and interleukin 18 (IL-18), as well as cleavage of gasdermin D. The N-terminal fragment resulting from this cleavage induces a pro-inflammatory form of programmed cell death distinct from apoptosis, referred to as pyroptosis, and is responsible for secretion of the mature cytokines, presumably through the formation of pores in the plasma membrane. Additionally, inflammasomes can be incorporated into larger cell death-inducing complexes called PANoptosomes, which drive another distinct form of pro-inflammatory cell death called PANoptosis. Inflammasome activation is initiated by different kinds of cytosolic pattern recognition receptors (PRRs) that respond to either microbe-derived pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns (DAMPs) generated by the host cell. PRRs involved in inflammasomes comprise NLRs (nucleotide-binding oligomerization domain and leucine-rich repeat-containing receptors) as well as AIM2 (absent in melanoma 2), IFI16 (IFN-inducible protein 16) as well as pyrin. Through their caspase activation and recruitment domain (CARD) or pyrin domain (PYD), the inflammasome receptors interact with the adaptor protein ASC, which then recruits pro-caspase-1 via its CARD domain and activates the effector caspase through proteolytic cleavage. The activated caspase-1 finally cleaves the immature pro-inflammatory cytokines pro-IL-1β and pro-IL-18, as well as gasdermin D, which are responsible for inflammatory signaling and pyroptotic cell death, respectively. In addition to these so-called canonical inflammasomes, different studies also described non-canonical inflammasome complexes that act independently of caspase-1.