The inflammasome is a complex of proteins involved in the activation of the innate immune system, an evolutionarily ancient antimicrobial defence found in most multicelled animals. When activated the inflammasome sets in motion a cascade of events that leads to the production of active molecules including interleukins. Three papers in this issue report the identification of endogenous danger signals and bacterial components that activate inflammasomes containing cryopyrin (also known as NALP3). Mariathasan et al. show that cryopyrin activates the inflammasome in response to bacterial toxins and to ATP. Kanneganti et al. show that cryopyrin is activated by bacterial RNA and by the immune response modifiers R837 and R848. And Martinon et al. show that gout-associated uric acid crystals have a similar effect. In sum these results show that cryopyrin has a vital role in host antibacterial defences and may act as a sensor of cellular stress. In addition, this work provides insight into the mechanisms of autoinflammatory disorders in which abnormalities in the innate immune system have been implicated. Missense mutations in the CIAS1 gene cause three autoinflammatory disorders: familial cold autoinflammatory syndrome, Muckle–Wells syndrome and neonatal-onset multiple-system inflammatory disease1. Cryopyrin (also called Nalp3), the product of CIAS1, is a member of the NOD-LRR protein family that has been linked to the activation of intracellular host defence signalling pathways2,3. Cryopyrin forms a multi-protein complex termed ‘the inflammasome’, which contains the apoptosis-associated speck-like protein (ASC) and caspase-1, and promotes caspase-1 activation and processing of pro-interleukin (IL)-1β (ref. 4). Here we show the effect of cryopyrin deficiency on inflammasome function and immune responses. Cryopyrin and ASC are essential for caspase-1 activation and IL-1β and IL-18 production in response to bacterial RNA and the imidazoquinoline compounds R837 and R848. In contrast, secretion of tumour-necrosis factor-α and IL-6, as well as activation of NF-κB and mitogen-activated protein kinases (MAPKs) were unaffected by cryopyrin deficiency. Furthermore, we show that Toll-like receptors and cryopyrin control the secretion of IL-1β and IL-18 through different intracellular pathways. These results reveal a critical role for cryopyrin in host defence through bacterial RNA-mediated activation of caspase-1, and provide insights regarding the pathogenesis of autoinflammatory syndromes.

Legionella pneumophila is an intracellular bacterium that causes an acute form of pneumonia called Legionnaires' disease. After infection of human macrophages, the Legionella-containing phagosome (LCP) avoids fusion with the lysosome allowing intracellular replication of the bacterium. In macrophages derived from most mouse strains, the LCP is delivered to the lysosome resulting in Legionella degradation and restricted bacterial growth. Mouse macrophages lacking the NLR protein Ipaf or its downstream effector caspase-1 are permissive to intracellular Legionella replication. However, the mechanism by which Ipaf restricts Legionella replication is not well understood. Here we demonstrate that the presence of flagellin and a competent type IV secretion system are critical for Legionella to activate caspase-1 in macrophages. Activation of caspase-1 in response to Legionella infection also required host Ipaf, but not TLR5. In the absence of Ipaf or caspase-1 activation, the LCP acquired endoplasmic reticulum-derived vesicles, avoided fusion with the lysosome, and allowed Legionella replication. Accordingly a Legionella mutant lacking flagellin did not activate caspase-1, avoided degradation, and replicated in wild-type macrophages. The regulation of phagosome maturation by Ipaf occurred within 2 h after infection and was independent of macrophage cell death. In vivo studies confirmed that flagellin and Ipaf play an important role in the control of Legionella clearance. These results reveal that Ipaf restricts Legionella replication through the regulation of phagosome maturation, providing a novel function for NLR proteins in host defense against an intracellular bacterium.

Type I cells have been defined to be independent of mitochondria for the induction of Fas death receptor-mediated apoptosis, whereas Type II cells are mitochondria-dependent. Knock-out studies in mice show that thymocytes are Type I and liver cells are Type II. We have previously shown that primary human hepatocytes and HCT116 human colon carcinoma cells behave like Type II cells because TRAIL-induced apoptosis can be blocked by the caspase 9 inhibitor, Z-LEHD-FMK. On the other hand, caspase 9 inhibition does not allow survival of TRAIL-treated SW480 colon cancer cells, which is predicted for Type I cells. Investigating the differences in TRAIL-induced apoptotic pathways in HCT116 and SW480 cells revealed that although FADD, BID, and procaspase 3 protein levels are higher in SW480 cells, and although procaspase 8 and FLIP processing is more efficient at the TRAIL-DISC of SW480 cells, BID, procaspase 3, XIAP, and PARP cleavages occur more rapidly in HCT116, despite the higher levels of BCL-2 and HSP70. Cytochrome c release from the mitochondria to the cytoplasm is more efficient in HCT116 cells. These results suggest BID cleavage as a possible limiting factor in the involvement of mitochondria in TRAIL-induced cell death. Thus, regulation of BID cleavage may define if a cell is mitochondria-dependent or -independent in response to TRAIL death receptor-induced apoptosis.