Several pathogenic bacteria including Staphylococcus aureus, Klebsiella pneumonia and Streptococcus pyogenes also activate caspase-1 via NLRP3 46–48. Exotoxins acting as pore-forming or membrane-damaging factors are important in mediating activation of the NLRP3 inflammasome 49, 50. For example, S. aureus hemolysins and NVP-BGJ398 clinical trial S. pyogenes streptolysin O are critical for NLRP3 activation 46, 47. Although TLR stimulation contributes to NLRP3 activation via priming, S. aureus and S. pyogenes can activate caspase-1 independently of MyD88/TRIF, the critical adaptors required for all TLR signaling 46, 47. One possibility

is that pathogenic bacteria induce priming of the NLRP3 inflammasome via TLR-independent mechanisms. Alternatively, exotoxins may mediate the delivery of microbial molecules for NLRP3 activation. Unlike that triggered by TLR ligands, NLRP3 activation induced by bacterial or fungal infection is independent of the P2X7R 46, 47. Thus, the role of ATP-induced P2X7R signaling in microbial

activation of the NLRP3 inflammasome in vivo is unclear. Recent studies suggest a model of NLRP3 activation that is mediated by two signals. The first, signal one, is provided by microbial molecules such as TLR ligands or by certain cytokines that induce priming of the inflammasome at least in part by NF-κB and NLRP3 induction (Fig. 1) 29, 30. The second signal AZD8055 directly triggers caspase-1 activation, and can be mediated by at least four separate pathways that include ATP-P2X7R-pannexin-1, Syk signaling,

Metalloexopeptidase lysosomal membrane rupture and bacterial exotoxins (Fig. 1). It is likely that these different pathways culminate in a common step that leads to NLRP3 activation. However, the identification of a unifying mechanism of NLRP3 activation remains elusive. The mechanisms regulating NLRP3 activation are discussed in more detail in accompanying articles of this issue 51, 52. A possible common link is provided by the ROS because NLRP3 activation is blocked by ROS inhibitors 27. However, most of these studies rely on pharmacological inhibitors that are used at high concentrations and exhibit variable effects or RNA interference, which is artifact prone. Nonetheless, Tschopp and colleagues have identified thioredoxin-interacting protein (TXNIP) as an NLRP3-interacting protein 53. Although, it remains to be determined whether TXNIP is an essential activator or just a regulator of the NLRP3 inflammasome. There has been a remarkable growth in our knowledge about the regulation, activation and biological role of the inflammasome. However, many important questions remain. They include identifying the link between microbial stimulation and inflammasome activation given that recognition of NLRC4/NLRP3 appears indirect. The identification of TXNIP as a possible link between ROS and NLRP3 is important, but more work is needed to understand its precise role in inflammasome activation.