Mutations within the NALP3/cryopyrin/CIAS1 gene are responsible for three autoinflammatory disorders: Muckle-Wells syndrome, familial cold autoinflammatory syndrome, and CINCA. The NALP3 protein is homologous to NALP1, which is a component of the inflammasome, a molecular platform that activates the proinflammatory caspases-1 and -5. NALP3 (and other members of the NALP family) lacks the C-terminal, CARD-containing sequence of NALP1, and its role in caspase activation is unclear. Here, we report that NALP2 and NALP3 associate with ASC, the CARD-containing protein Cardinal, and caspase-1 (but not caspase-5), thereby forming an inflammasome with high proIL-1beta-processing activity. Macrophages from Muckle-Wells patients spontaneously secrete active IL-1beta. Increased inflammasome activity is therefore likely to be the molecular basis of the symptoms associated with NALP3-dependent autoinflammatory disorders.

Activation of caspase-1 and subsequent processing and secretion of the pro-inflammatory cytokine IL-1beta is triggered upon assembly of the inflammasome complex. It is generally believed that bacterial lipopolysaccharides (LPS) are activators of the inflammasome through stimulation of Toll-like receptor 4 (TLR4). Like TLRs, NALP3/Cryopyrin, which is a key component of the inflammasome, contains Leucine-Rich-Repeats (LRRs). LRRs are frequently used to sense bacterial components, thus raising the possibility that bacteria directly activate the inflammasome. Here, we show that bacterial peptidoglycans (PGN), but surprisingly not LPS, induce NALP3-mediated activation of caspase-1 and maturation of proIL-1beta. Activation is independent of TLRs because the PGN degradation product muramyl dipeptide (MDP), which is not sensed by TLRs, is the minimal-activating structure. Macrophages from a patient with Muckle-Wells syndrome, an autoinflammatory disease associated with mutations in the NALP3/Cryopyrin gene, show increased IL-1beta secretion in the presence of MDP. The activation of the NALP3-inflammasome by MDP may be the basis of the potent adjuvant activity of MDP.

Several autoinflammatory disorders such as Muckle-Wells syndrome are characterized by mutations in the NALP3/cryopyrin gene. NALP3 and NALP1 proteins can assemble to inflammasomes that activate caspase-1, resulting in the processing of pro-inflammatory cytokines IL-1beta and IL-18. The present study was designed to determine which cells and tissues express NALP1 and NALP3. Monoclonal antibodies were developed and their use revealed distinct distribution profiles of NALP1 and NALP3. Granulocytes, monocytes (very weakly), dendritic cells, and B and T cells all express NALP1 and NALP3. Highest levels of NALP1 are found in T cells and Langerhans cells. Furthermore, NALP1 is present in glandular epithelial structures such as stomach, gut, lung, and, surprisingly, in neurons and testis. In contrast to NALP1, NALP3 shows a more restricted tissue distribution with expression mainly in non-keratinizing epithelia in the oropharynx, esophagus, and ectocervix. Moreover, NALP3 expression is found in the urothelial layer in the bladder. Likewise, a difference in subcellular distribution between NALP1 and NALP3 is observed because NALP1 is localized mainly in the nucleus, whereas NALP3 is predominantly cytoplasmic. We propose that the presence of NALP3 in epithelial cells lining the oral and genital tracts allows the rapid sensing of invading pathogens, thereby triggering an innate immune response.

The autoinflammatory disorders Muckle-Wells syndrome, familial cold urtecaria and chronic infantile neurological cutaneous and articular syndrome are associated with mutations in the NALP3 (Cryopyrin) gene, which is the central platform of the proinflammatory caspase-1 activating complex, named the inflammasome. In patients with another autoinflammatory disorder, familial Mediterranean fever (FMF), mutations in the SPRY domain of the Pyrin protein are frequently found. Recent evidence suggests that Pyrin associates with ASC, an inflammasome component, via its Pyrin domain, thereby halting the inflammatory response. This interaction, however, does not explain the effects of mutations of the SPRY domain found in FMF patients. Here we show that the Pyrin SPRY domain not only interacts with NALP3, but also with caspase-1 and its substrate pro-interleukin(IL)-1b. Whereas a Pyrin knockdown results in increased caspase-1 activation and IL-1b secretion, overexpression of the SPRY domain alone blocks these processes. Thus Pyrin binds to several inflammasome components thereby modulating their activity.

Interleukin-1 receptor (IL-1RI) is a master regulator of inflammation and innate immunity. When triggered by IL-1beta, IL-1RI aggregates with IL-1R-associated protein (IL-1RAcP) and forms a membrane proximal signalosome that potently activates downstream signaling cascades. IL-1beta also rapidly triggers endocytosis of IL-1RI. Although internalization of IL-1RI significantly impacts signaling, very little is known about trafficking of IL-1RI and therefore about precisely how endocytosis modulates the overall cellular response to IL-1beta. Upon internalization, activated receptors are often sorted through endosomes and delivered to lysosomes for degradation. This is a highly regulated process that requires ubiquitination of cargo proteins as well as protein-sorting complexes that specifically recognize ubiquitinated cargo. Here, we show that IL-1beta induces ubiquitination of IL-1RI and that via these attached ubiquitin groups, IL-1RI interacts with the ubiquitin-binding protein Tollip. By using an assay to follow trafficking of IL-1RI from the cell surface to late endosomes and lysosomes, we demonstrate that Tollip is required for sorting of IL-1RI at late endosomes. In Tollip-deficient cells and cells expressing only mutated Tollip (incapable of binding IL-1RI and ubiquitin), IL-1RI accumulates on late endosomes and is not efficiently degraded. Furthermore, we show that IL-1RI interacts with Tom1, an ubiquitin-, clathrin-, and Tollip-binding protein, and that Tom1 knockdown also results in the accumulation of IL-1RI at late endosomes. Our findings suggest that Tollip functions as an endosomal adaptor linking IL-1RI, via Tom1, to the endosomal degradation machinery.