Gout-associated uric acid crystals activate the NALP3 inflammasome

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Abstract
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. Development of the acute and chronic inflammatory responses known as gout and pseudogout are associated with the deposition of monosodium urate (MSU) or calcium pyrophosphate dihydrate (CPPD) crystals, respectively, in joints and periarticular tissues. Although MSU crystals were first identified as the aetiological agent of gout in the eighteenth century1 and more recently as a ‘danger signal’ released from dying cells2, little is known about the molecular mechanisms underlying MSU- or CPPD-induced inflammation. Here we show that MSU and CPPD engage the caspase-1-activating NALP3 (also called cryopyrin) inflammasome, resulting in the production of active interleukin (IL)-1β and IL-18. Macrophages from mice deficient in various components of the inflammasome such as caspase-1, ASC and NALP3 are defective in crystal-induced IL-1β activation. Moreover, an impaired neutrophil influx is found in an in vivo model of crystal-induced peritonitis in inflammasome-deficient mice or mice deficient in the IL-1β receptor (IL-1R). These findings provide insight into the molecular processes underlying the inflammatory conditions of gout and pseudogout, and further support a pivotal role of the inflammasome in several autoinflammatory diseases.