A new report in Cell Host & Microbe identifies the protein cryopyrin (also known as NLRP3, NALP3 or CIAS1) as an important host mediator of a type of pathogen-induced necrotic-like cell death.

Cryopyrin is a member of a family of NLR (nucleotide-binding domain, leucine-rich-repeat-containing family) proteins, which are also known as nucleotide-binding oligomerization domain (NOD)-like receptors or CATERPILLER proteins. It is part of a cytosolic protein complex called the inflammasome, which also includes ASC (apoptosis-associated speck-like protein containing a caspase-recruitment domain (CARD); also known as PYCARD, TMS1 or CARD5) and which is responsible for processing caspase-1 to its active form; this, in turn, leads to the production of mature interleukin-1β (IL-1β), a potent pro-inflammatory cytokine, in response to bacterial, viral and other pro-inflammatory stimuli. But might cryopyrin have additional biological functions in the containment of pathogens? Macrophages deficient in cryopyrin are more resistant to cell death induced by certain Gram-positive bacteria, suggesting that cryopyrin can participate in the initiation of necrosis, but the nature of this involvement has not been defined.

Mutations in CIAS1, the gene that encodes cryopyrin, are associated with autoinflammatory periodic-fever syndromes, which are characterized by excessive production of IL-1β. The authors first confirmed that the presence of the disease-associated mutations in CIAS1 results in necrosis-like cell death in a monocytic cell line, then examined the mechanism by which cell death is induced. Cells that expressed the mutant cryopyrin released substantially more IL-1β than cells that expressed wild-type cryopyrin, however, cell death was shown to occur independently of caspase-1 activity and IL-1β-mediated signalling (and thus independent of inflammasome formation). Cell death induced by the mutant cryopyrin was also shown to be ASC dependent, and resulted in the release of high-mobility group box 1 (HMGB1), a protein that acts as a potent pro-inflammatory factor when released from cells that are undergoing necrosis. Similarly, excessive cell death was shown in monocytes from patients with disease-associated CIAS1 mutations, in response to lipopolysaccharide challenge, by a process that was dependent on ASC and cathepsin B, and which resulted in the production of HMGB1.

Because disease-associated cryopyrin mutants are considered to be gain-of-function mutants, wild-type cryopyrin should exhibit similar properties to mutant cryopyrin, but at a reduced level or following stimulation. The authors thus considered whether wild-type cryopyrin has a role in cell necrosis that is associated with bacterial pathogenesis — specifically, with the intracellular bacterium Shigella flexneri, which is thought to induce necrosis-like cell death. S. flexneri-induced necrosis of mouse macrophages was shown to require cryopyrin and ASC, but not caspase-1. It was also independent of IL-1β and was associated with the release of HMGB1. Therefore, S. flexneri-induced cell death exploits the same host cell signalling pathway as is induced by disease-associated cryopyrin.

So, S. flexneri infection of monocytes induces a caspase-1-independent and ASC- and cryopyrin-dependent form of necrotic cell death, which the authors called pyronecrosis, that is identical to that induced by disease-associated cryopyrin. This suggests that the inflammatory response that is normally stimulated by pathogenic bacteria is propagated in patients with gain-of-function CIAS1 mutations.