## 2 The Nucleotide-Binding Domain-Like Receptor Protein 3 (NLRP3) Inflammatory  
Innate immune function relies upon immune cell detection of pathogens by pattern-recognition receptors (PRRs). Activation of PRRs by highly conserved pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs) then leads to downstream effects with production of the signalling proteins interferon-\(\alpha\) and \(\beta\), and pro-inflammatory cytokines such as interleukin [21]. Inflammasomes form a critical part of this signalling cascade. Inflammasomes are large, multimeric protein complexes that assemble in the cell cytosol after sensing PAMPs or DAMPs [21; 22]. Although there are several described inflammasomes, the most well-studied of these is the NLRP3 inflammasome [21; 22]. The NLRP3 inflammasome comprises the NLRP3 sensor protein, the apoptosis-associated speck-like (ASC) adaptor protein, and the pro-caspase-1 effector protein. The inflammasome can be activated through both canonical and non-canonical pathways that then lead to the processing and release of interleukin (IL)-\(\beta\) and IL-18 [21; 22]. Canonical activation of the NLRP3 inflammasome is a multi-step process that requires priming and then activation of the inflammasome [21; 22]. The first signal comes when PRRs are activated by PAMPs and DAMPs. The toll-like receptor (TLR) family plays a significant role in downstream signalling, for instance, TLR4 activation, which occurs in response to the common Gram-negative bacterial cell wall molecule lipopolysaccharide (LPS) [24]. Transcriptional activation via the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB) pathway leads to increased expression of NLRP3 and pro-interleukin 1\(\beta\) expression [21; 22]. Deubiquitination of NLRP3 mediated by the BRCA1/BRCA2-containing complex, subunit 3 (BRCC-3), also appears to be a critical step in priming to allow for the oligomerisation of NLRP3 with the other subunits of the inflammasome [24; 25], and a complex series of other regulatory steps involving the ubiquitination and phosphorylation of both NLRP3 and ASC is not yet fully understood [26]. After priming, inflammasome activation requires a second signal that may be triggered in response to a wide range of stimuli. A recent review proposed the term homeostasis-altering molecular processes (HAMPs) to describe a number of these signals that include altered membrane potential due to potassium efflux or calcium influx, extracellular adenosine triphosphate (ATP) and oxidised deoxyribonucleic acid (DNA), lysosomal degradation products, mitochondrial damage due to reactive oxygen species (ROS), altered mitochondrial membrane potential, and oxidised mitochondrial DNA [27]. Once the inflammasome is activated, the conformational changes induced by oligomerisation activate caspase 1, which then cleaves pro-IL 1\(\beta\) and 18 to release their active cytokine forms [21; 22]. Caspase 1 is also able to cleave gasdermin D (GSDMD), the activated form of which translocates to the membrane creating pores that can mediate pyroptosis--a lytic and inflammatory type of programmed cell death--and allow for the release of IL-1\(\beta\) and 18 into the surrounding environment [28; 29; 30]. This process is outlined in Figure 2.  
Noncanonical NLRP3 inflammasome activation can occur in the absence of a priming signal through the activation of murine caspase 11 (or the human equivalents caspase 4 or 5) [32]. Interferon signalling increases the expression of pro-caspase-11 that subsequently activates the NLRP3 inflammasome and pyroptosis. Gram-negative bacterial products such as lipopolysaccharideaccharide  
Figure 2: The canonical inflammasome activation pathway. (**a**) Signal 1 involves the activation of cell-surface PRRs by PAMPs and DAMPs. This activates downstream signalling with translocation of NF-kB to the nucleus and upregulation of the expression of NLRP3, pro-IL-1\(\beta\), pro-IL-18, and pro-GSDMD.