In contrast to NOD1/2 some NLRPs function as large macromolecular complexes called 'Inflammasomes'. These multiprotein platforms control activation of the cysteinyl aspartate protease caspase-1 and thereby the subsequent cleavage of pro-interleukin 1B (pro-IL1B) into the active proinflammatory cytokine IL1B. Activation of caspase-1 is essential for production of IL1B and IL18, which respectively bind and activate the IL1 receptor (IL1R) and IL18 receptor (IL18R) complexes. IL1R and IL18R activate NFkappaB and other signaling cascades.
As the activation of inflammasomes leads to caspase-1 activation, inflammasomes can be considered an upstream step of the IL1R and IL18R signaling cascades, linking intracellular pathogen sensing to immune response pathways mediated by Toll-Like Receptors (TLRs). Monocytes and macrophages do not express pro-IL1B until stimulated, typically by TLRs (Franchi et al. 2009). The resulting pro-IL1B is not converted to IL1B unless a second stimulus activates an inflammasome. This requirement for two distinct stimuli allows tight regulation of IL1B/IL18 production, necessary because excessive IL-1B production is associated with numerous inflammatory diseases such as gout and rheumatoid arthritis (Masters et al. 2009).
There are at least four subtypes of the inflammasome, characterized by the NLRP. In addition the protein AIM2 can form an inflammasome. All activate caspase-1. NLRP1 (NALP1), NLRP3 (Cryopyrin, NALP3), IPAF (CARD12, NLRC4) and AIM2 inflammasomes all have clear physiological roles in vivo. NLRP2, NLRP6, NLRP7, NLRP10 and NLRP12 have been demonstrated to modulate caspase-1 activity in vitro but the significance of this is unclear (Mariathasan and Monack, 2007).
NLRP3 and AIM2 bind the protein 'apoptosis-associated speck-like protein containing a CARD' (ASC, also called PYCARD), via a PYD-PYD domain interaction. This in turn recruits procaspase-1 through a CARD-CARD interaction. NLRP1 and IPAF contain CARD domains and can bind procaspase-1 directly, though both are stimulated by ASC. Oligomerization of NLRPs is believed to bring procaspases into close proximity, leading to 'induced proximity' auto-activation (Boatright et al. 2003). This leads to formation of the active caspase tetramer. NLRPs are generally considered to be cytoplasmic proteins, but there is evidence for cytoplasmic-nuclear shuttling of the family member CIITA (LeibundGut-Landmann et al. 2004) and tissue/cell dependent NALP1 expression in the nucleus of neurons and lymphocytes (Kummer et al. 2007); the significance of this remains unclear.