The NLRP3 inflammasome is activated by a range of stimuli of microbial, endogenous and exogenous origins including several viruses, bacterial pore forming toxins (e.g. Craven et al. 2009), and various irritants that form crystalline or particulate structures (see Cassel et al. 2009). Multiple studies have shown that phagocytosis of particulate elicitors is necessary for activation (e.g. Hornung et al. 2008) but not for the response to ATP, which is mediated by the P2X7 receptor (Kahlenberg & Dubyak, 2004) and appears to involve the pannexin membrane channel (Pellegrin & Suprenenant 2006), which is also involved in the response to nigericin and maitotoxin (Pellegrin & Suprenenant 2007). Direct binding of elicitors to NLRP3 has not been demonstrated and the exact process of activation is unclear, though speculated to involve changes in conformation that make available the NACHT domain for oligomerization (Inohara & Nunez 2001, 2003).
Three overlapping mechanisms are believed to be involved in NLRP3 activation. ATP stimulates the P2X7 ATP-gated ion channel leading to K+ efflux which appears necessary for NLRP3 inflammasome activation (Kahlenberg & Dubyak 2004, Dostert et al. 2008), and is believed to induce formation of pannexin-1 membrane pores. These pores give direct access of NLPR3 agonists to the cytosol. A second mechanism is the endocytosis of crystalline or particulate structures, leading to damaged lysosomes which release their contents (Hornung et al. 2008, Halle et al. 2008). The third element is the generation of reactive oxygen species (ROS) which activate NLRP3, shown to be a critical step for the activation of caspase-1 following ATP stimulation (Cruz et al. 2007). The source of the ROS is unclear.