Inflammasomes: Threat-Assessment Organelles of the Innate Immune System
Authors: Charles L. Evavold, Jonathan C. Kagan
Journal: Immunity (2019)

Inflammasomes are supramolecular organizing centers that operate to drive interleukin-1 (IL-1)-dependent inflammation. Depending on context, inflammatory caspases act upstream or downstream of inflammasome assembly, serving as the principal enzymes that control activities of these organelles. In this review, we discuss mechanisms of inflammasome assembly and signaling. We posit that upstream regulatory proteins, classically known as pattern-recognition receptors, operate to assess infectious and non-infectious threats to the host. Threat assessment is achieved through two general strategies: (1) direct binding of receptors to microbial or host-derived ligands or (2) indirect detection of changes in cellular homeostasis. Upon activation, these upstream regulatory factors seed the assembly of inflammasomes, leading to IL-1 family cytokine release from living (hyperactive) or dead (pyroptotic) cells. The molecular and physiological consequences of these distinct cell fate decisions are discussed. Inflammasomes are supramolecular organizing centers that operate to drive interleukin-1 (IL-1)-dependent inflammation. Depending on context, inflammatory caspases act upstream or downstream of inflammasome assembly, serving as the principal enzymes that control activities of these organelles. In this review, we discuss mechanisms of inflammasome assembly and signaling. We posit that upstream regulatory proteins, classically known as pattern-recognition receptors, operate to assess infectious and non-infectious threats to the host. Threat assessment is achieved through two general strategies: (1) direct binding of receptors to microbial or host-derived ligands or (2) indirect detection of changes in cellular homeostasis. Upon activation, these upstream regulatory factors seed the assembly of inflammasomes, leading to IL-1 family cytokine release from living (hyperactive) or dead (pyroptotic) cells. The molecular and physiological consequences of these distinct cell fate decisions are discussed. All organisms must interact with their environment to survive. Fundamental to these interactions is the ability of the host to discern its surroundings so that adaptations to environmental changes can be made. The specific changes that occur in the environment are countless, but they can be grouped into two broad categories. The first category encompasses beneficial changes, such as friendly encounters or exposure to favorable temperatures, salinity, or nutrients. The second category includes detrimental changes, such as dangerous encounters or conditions that cause susceptibility to infection and tissue damage. Determination of potential threat to the host is a task that is crucial to adapting to any new environment. At the cellular level, research over the last several years has revealed a variety of responses to environmental changes, which range from neuronal circuits that detect pain and temperature alterations to immune pathways that detect microbial encounters and tissue injury (Iwasaki and Medzhitov, 2010Iwasaki A. Medzhitov R. Regulation of adaptive immunity by the innate immune system.Science. 2010; 327: 291-295Crossref PubMed Scopus (1159) Google Scholar, Koch et al., 2018Koch S.C. Acton D. Goulding M. Spinal Circuits for Touch, Pain, and Itch.Annu. Rev. Physiol. 2018; 80: 189-217Crossref PubMed Scopus (15) Google Scholar). Although these discoveries have provided a clearer view of the cells and genes important for host adaptation to the environment, there is a lack of broad-based hypotheses to explain the logic underlying cell sensory networks. Herein, we discuss recent developments into how mammalian cells detect environmental change, with a specific focus on detrimental changes to the host. We discuss how these threats can be grouped into two major classes of molecular signals, which are defined by classical terms: pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) (Janeway, 1989Janeway Jr., C.A. Approaching the asymptote? Evolution and revolution in immunology.Cold Spring Harb. Symp. Quant. Biol. 1989; 54: 1-13Crossref PubMed Google Scholar, Matzinger, 1994Matzinger P. Tolerance, danger, and the extended family.Annu. Rev. Immunol. 1994; 12: 991-1045Crossref PubMed Google Scholar). The former class, when detected by the host, indicates encounters with microbes and their products. The latter class indicates encounters with environmental conditions that caused a homeostatic disruption, perhaps resulting in cellular or subcellular injury. Thus, the fundamental distinction between DAMPs and PAMPs is the source of the molecule, not their chemical structures. A growing body of evidence supports the idea that a common sensory system is used to detect DAMPs and PAMPs. This sensory system consists of a diverse family of receptors, defined by Charles Janeway, Jr., as pattern-recognition receptors (PRRs) (Janeway, 1989Janeway Jr., C.A. Approaching the asymptote? Evolution and revolution in immunology.Cold Spring Harb. Symp. Quant. Biol. 1989; 54: 1-13Crossref PubMed Google Scholar). PRRs have been viewed as sentinels of the microbial world around us. However, the ability of PRRs to serve as exclusive indicators of microbial encounters through detection of PAMPs is seemingly compromised by the increasing acceptance that these receptors also detect DAMPs. Thus, PRRs can no longer be considered to have evolved for the exclusive purpose of detecting microbes. An alternative view of the innate immune system is that PAMP detection is but one function of PRRs and that the detection of DAMPs is as important to the host as the detection of PAMPs in terms of adapting to environmental changes. Consistent with this idea are the activities of tissue-resident macrophages. These cells have been historically viewed as antimicrobial cells that use PRRs to detect infection, but recent work has extended their functions to include regulation of tissue homeostasis and detection of non-infectious threats (Okabe and Medzhitov, 2016Okabe Y. Medzhitov R. Tissue biology perspective on macrophages.Nat. Immunol. 2016; 17: 9-17Crossref PubMed Scopus (177) Google Scholar). The innate immune system may therefore be best viewed not simply as a microbe-detection system but rather a threat-assessment computer, whose function is to detect many types of potentially deleterious changes in the environment. Viewing the innate immune system as a threat-assessment computer may explain genetic and clinical evidence of the role of PRRs in diseases with an infectious component and diseases in which no microbe has been implicated. In both cases, PRRs and the cells that encode them are critical for disease development and resolution. Moreover, from an evolutionary perspective, this view may explain the long-recognized roles of one of the first-identified PRRs, the Drosophila Toll receptor, in antimicrobial defense and development. In the next sections, we explore these concepts with a specific focus on PRRs that link diverse environmental threats to the assembly and activation of inflammasomes. We have chosen inflammasome-activating PRRs as a centerpiece for this discussion, because examples of these PRRs responding to various environmental threats have been identified. For example, individual inflammasome-activating PRRs directly detect PAMPs, DAMPs, or changes in cellular processes indicative of altered homeostasis. The collective knowledge that PRRs detect each of these classes of threats supports our thesis. Low-risk environmental threats include PAMPs in the extracellular environment of barrier tissues in the organism (Figure 1). PAMPs in these environments do not necessarily indicate an encounter with a pathogen. These extracellular PAMPs induce inflammatory responses to eliminate the possibility of infection. Because infection is only a possibility, the host response may not be activated maximally. The responses induced under these conditions include the expression of inflammatory chemokines, cytokines, and other factors that ensure induction of adaptive immunity (Iwasaki and Medzhitov, 2010Iwasaki A. Medzhitov R. Regulation of adaptive immunity by the innate immune system.Science. 2010; 327: 291-295Crossref PubMed Scopus (1159) Google Scholar). These responses are often induced through the Toll-like receptor (TLR) family of PRRs. TLRs survey external environments through receptor localization at the plasma and endosomal membranes, with their ligand binding domains exposed to the extracellular space or endosomal lumen (Brubaker et al., 2015Brubaker S.W. Bonham K.S. Zanoni I. Kagan J.C. Innate immune pattern recognition: a cell biological perspective.Annu. Rev. Immunol. 2015; 33: 257-290Crossref PubMed Scopus (385) Google Scholar). The mechanisms of TLR signaling have been reviewed elsewhere (Bryant et al., 2015Bryant C.E. Gay N.J. Heymans S. Sacre S. Schaefer L. Midwood K.S. Advances in Toll-like receptor biology: Modes of activation by diverse stimuli.Crit. Rev. Biochem. Mol. Biol. 2015; 50: 359-379Crossref PubMed Scopus (35) Google Scholar, Pandey et al., 2014Pandey S. Kawai T. Akira S. Microbial sensing by Toll-like receptors and intracellular nucleic acid sensors.Cold Spring Harb. Perspect. Biol. 2014; 7: a016246PubMed Google Scholar), and we will only summarize their mechanisms of action here. Upon PAMP detection, TLRs dimerize and seed the formation of helical oligomeric complexes of proteins in the cytosol. These complexes, known as supramolecular organizing centers (SMOCs), represent the signaling organelles of the innate immune system (Kagan et al., 2014Kagan J.C. Magupalli V.G. Wu H. SMOCs: supramolecular organizing centres that control innate immunity.Nat. Rev. Immunol. 2014; 14: 821-826Crossref PubMed Scopus (69) Google Scholar). TLR-associated SMOCs concentrate enzymes that drive downstream signaling events that augment metabolism, transcription, and translation (Tan and Kagan, 2019Tan Y. Kagan J.C. Innate Immune Signaling Organelles Display Natural and Programmable Signaling Flexibility.Cell. 2019; 177: 384-398.e11Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar). These responses are best understood to drive inflammatory responses and stimulate adaptive immunity (Iwasaki and Medzhitov, 2010Iwasaki A. Medzhitov R. Regulation of adaptive immunity by the innate immune system.Science. 2010; 327: 291-295Crossref PubMed Scopus (1159) Google Scholar). Detection of extracellular PAMPs would be a relatively low-level threat because commensal and other non-pathogenic microbes might shed ligands (Figure 2). These detection events thus result in a proportionate signaling outcome in which the responding cells do not sacrifice themselves by committing to a cell death program. Commitment to a death program is restricted to threats that represent a clear and present danger to the host. One danger to the host occurs during encounters with microbial pathogens. All pathogens manipulate the host cells they encounter to promote their own replication. This process presents a life-threatening risk to the host, which must be addressed by proportionally high defensive responses. A common defensive response induced upon pathogenic encounters is to kill the infected cells to prevent intracellular microbial replication. This terminal cell fate decision by the host is only made when the threat is high; non-pathogenic encounters should not result in host cell death. Mechanistically, virulence assessment occurs by the detection of PAMPs in specific intra- or extracellular compartments. In contrast to extracellular PAMPs, which may not necessarily indicate an encounter with a virulent pathogen (or even a living microbe), cytosolic PAMPs almost always coincide with direct exposure to living and perhaps virulent microbes. The reason for this trend is that pathogenic manipulation of host cells is accomplished through access by the pathogen (or its virulence factors) to the host cytosolic environment (Figure 2). For example, bacteria can encode cell-associated secretion systems that inject virulence factors into the host cytosol. Fungal pathogens are increasingly recognized for their ability to similarly manipulate the host, and viruses must enter the host cytosol to replicate. Because non-infectious organisms do not encode virulence determinants, they cannot intentionally access the host cytosol. Only infectious agents deliver PAMPs to the cytosol. Thus, cytosolic PRRs are uniquely positioned to identify pathogenic encounters. Consequently, these PRRs induce a heightened state of inflammation and the host-determined demise of the infected cell. Cytosolic PRRs include members of the nucleotide-binding domain, leucine-rich-containing (NLR) family, and the proteins AIM2 and Pyrin. The SMOC induced by these receptors—the inflammasome—is the focus of this review. Inflammasomes are protein complexes that represent the site of caspase-1 activity. Caspase-1 is best known as an interleukin-1 (IL-1)-converting enzyme with the capacity to cleave biologically inert pro-IL-1β into its active form: IL-1β (Garlanda et al., 2013Garlanda C. Dinarello C.A. Mantovani A. The interleukin-1 family: back to the future.Immunity. 2013; 39: 1003-1018Abstract Full Text Full Text PDF PubMed Scopus (757) Google Scholar). IL-1 was the first cytokine identified and represents one of the most potent pyrogens produced by mammals. Perhaps because of its potent induction of local and systemic inflammation, the regulation of IL-1α and IL-1β release from cells is more tightly controlled than that of other cytokines. Whereas all other well-characterized cytokines, chemokines, and interferons (IFNs) are secreted upon synthesis, the secretion and inflammatory activity of the IL-1 family is dissociated from its synthesis (Garlanda et al., 2013Garlanda C. Dinarello C.A. Mantovani A. The interleukin-1 family: back to the future.Immunity. 2013; 39: 1003-1018Abstract Full Text Full Text PDF PubMed Scopus (757) Google Scholar). This dissociation is due to the lack of an N-terminal secretion signal in many members of the IL-1 family. Thus, several IL-1 family members, such as IL-1β, IL-18, and IL-37, are produced in the cytosol as latent proteins that cannot achieve bioactivity until they are cleaved by caspase-1 and released from cells. One notable exception to this rule relates to the anti-inflammatory IL-1 receptor antagonist (IL-1Ra), which contains a signal peptide for secretion through the biosynthetic pathway. Inflammasome-associated caspase-1 is responsible for generating bioactive IL-1β and IL-18 and promoting their release into the extracellular space. A common mechanism through which inflammasomes promote IL-1β release is the induction of pyroptosis, a lytic form of cell death that non-specifically releases cytosolic content from cells. Because pyroptosis is a terminal event, the choice of a cell to commit to this process must be tightly regulated. It stands to reason that such a commitment to cellular suicide would only be justified in instances of high-threat encounters, which is supported by our current knowledge that only cytosolic PRRs can induce pyroptosis. Based on these principles, extracellular PAMPs are classified as low-threat and resolvable problems, whereas cytosolic PAMPs are contextualized as high-threat problems that may induce drastic cell fate decisions such as cell death. Several examples of individual PAMPs drive transcriptional responses via TLRs and cell death responses from inflammasome-associated PRRs. These PAMPs include bacterial lipopolysaccharide (LPS) that can be sensed by TLR4 or caspase-11; bacterial flagellin that can be sensed by TLR5, NAIP5, or NAIP6; and DNA that can be sensed by TLR9 or AIM2. Although the idea that pathogens and non-pathogens may be viewed differently by the innate immune system has been offered previously (Blander and Sander, 2012Blander J.M. Sander L.E. Beyond pattern recognition: five immune checkpoints for scaling the microbial threat.Nat. Rev. Immunol. 2012; 12: 215-225Crossref PubMed Scopus (162) Google Scholar, Vance et al., 2009Vance R.E. Isberg R.R. Portnoy D.A. Patterns of pathogenesis: discrimination of pathogenic and nonpathogenic microbes by the innate immune system.Cell Host Microbe. 2009; 6: 10-21Abstract Full Text Full Text PDF PubMed Scopus (291) Google Scholar), these earlier commentaries preceded the identification of molecular explanations by which threat assessment is accomplished. Threat-level determination by inflammasome-stimulating PRRs may be beneficial during infection by stimulating adaptive immune responses through heightened inflammatory responses and cell death of infected cells (Evavold and Kagan, 2018Evavold C.L. Kagan J.C. How Inflammasomes Inform Adaptive Immunity.J. Mol. Biol. 2018; 430: 217-237Crossref PubMed Scopus (4) Google Scholar). Dysregulation of inflammasome signaling may be deleterious to the host during sepsis (Kayagaki et al., 2011Kayagaki N. Warming S. Lamkanfi M. Vande Walle L. Louie S. Dong J. Newton K. Qu Y. Liu J. Heldens S. et al.Non-canonical inflammasome activation targets caspase-11.Nature. 2011; 479: 117-121Crossref PubMed Scopus (948) Google Scholar) or through host reaction to gout crystals and amyloid plaques (Heneka et al., 2013Heneka M.T. Kummer M.P. Stutz A. Delekate A. Schwartz S. Vieira-Saecker A. Griep A. Axt D. Remus A. Tzeng T.C. et al.NLRP3 is activated in Alzheimer's disease and contributes to pathology in APP/PS1 mice.Nature. 2013; 493: 674-678Crossref PubMed Scopus (750) Google Scholar, Martinon et al., 2006Martinon F. Pétrilli V. Mayor A. Tardivel A. Tschopp J. Gout-associated uric acid crystals activate the NALP3 inflammasome.Nature. 2006; 440: 237-241Crossref PubMed Scopus (2713) Google Scholar). These latter scenarios represent chronic exposure to high concentrations of PAMPs and DAMPs. In subsequent sections, we discuss mechanisms by which PRRs drive inflammasome assembly and activity and additional context-dependent considerations made by cells to gauge threat. Like all SMOCs, inflammasomes are composed of two abundant functional units. The first unit consists of an adaptor protein or proteins with oligomerization potential, and the second unit is an effector enzyme whose activity drives inflammatory responses (Figure 3). Oligomerization potential is converted into action by an upstream seed protein, usually a PRR, which drives adaptor oligomerization and stimulates effector enzyme activity (Cai et al., 2014Cai X. Chen J. Xu H. Liu S. Jiang Q.X. Halfmann R. Chen Z.J. Prion-like polymerization underlies signal transduction in antiviral immune defense and inflammasome activation.Cell. 2014; 156: 1207-1222Abstract Full Text Full Text PDF PubMed Scopus (251) Google Scholar, Lu et al., 2014Lu A. Magupalli V.G. Ruan J. Yin Q. Atianand M.K. Vos M.R. Schröder G.F. Fitzgerald K.A. Wu H. Egelman E.H. Unified polymerization mechanism for the assembly of ASC-dependent inflammasomes.Cell. 2014; 156: 1193-1206Abstract Full Text Full Text PDF PubMed Scopus (392) Google Scholar). This simple two-unit principle enables diverse seed proteins to assemble SMOCs with distinct effector functions and likely explains their prevalence in nature. Moreover, the simplicity of SMOC architecture ensures flexibility of design and has enabled synthetic organizing centers to be engineered to induce user-defined responses (Tan and Kagan, 2019Tan Y. Kagan J.C. Innate Immune Signaling Organelles Display Natural and Programmable Signaling Flexibility.Cell. 2019; 177: 384-398.e11Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar). In this section, we describe the mechanisms that drive SMOC formation, with a focus on how different proteins operate by the two-unit SMOC design principle to mediate signal transduction to disparate inputs. A large class of inflammasomes uses the protein ASC as its oligomerization unit (Cai et al., 2014Cai X. Chen J. Xu H. Liu S. Jiang Q.X. Halfmann R. Chen Z.J. Prion-like polymerization underlies signal transduction in antiviral immune defense and inflammasome activation.Cell. 2014; 156: 1207-1222Abstract Full Text Full Text PDF PubMed Scopus (251) Google Scholar, Lu et al., 2014Lu A. Magupalli V.G. Ruan J. Yin Q. Atianand M.K. Vos M.R. Schröder G.F. Fitzgerald K.A. Wu H. Egelman E.H. Unified polymerization mechanism for the assembly of ASC-dependent inflammasomes.Cell. 2014; 156: 1193-1206Abstract Full Text Full Text PDF PubMed Scopus (392) Google Scholar). The PRRs NLRP1, NLRP3, NLRP6, AIM2, and Pyrin engage ASC through interactions between the pyrin domain on the receptor and the pyrin domain of ASC, resulting in oligomerization (Chae et al., 2011Chae J.J. Cho Y.H. Lee G.S. Cheng J. Liu P.P. Feigenbaum L. Katz S.I. Kastner D.L. 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It is thought that the induced proximity and increased local concentration of the p45 species of pro-caspase-1 within inflammasomes allows for caspase self-cleavage at various aspartic acid residues, leading to an active but transient species recently defined as a p33/p10 heterodimer (Boucher et al., 2018Boucher D. Monteleone M. Coll R.C. Chen K.W. Ross C.M. Teo J.L. Gomez G.A. Holley C.L. Bierschenk D. Stacey K.J. et al.Caspase-1 self-cleavage is an intrinsic mechanism to terminate inflammasome activity.J. Exp. Med. 2018; 215: 827-840Crossref PubMed Scopus (31) Google Scholar). Many studies have noted that the p20/p10 heterodimer is active at high concentrations, but recent work demonstrates that the p20/p10 heterodimers may represent downregulation of caspase activity at later stages of inflammasome signaling (Boucher et al., 2018Boucher D. Monteleone M. Coll R.C. Chen K.W. Ross C.M. Teo J.L. Gomez G.A. Holley C.L. Bierschenk D. Stacey K.J. et al.Caspase-1 self-cleavage is an intrinsic mechanism to terminate inflammasome activity.J. Exp. Med. 2018; 215: 827-840Crossref PubMed Scopus (31) Google Scholar). Indeed, these p20/p10 heterodimers are no longer tethered to the inflammasome; they can be released from the cell and are often used as biomarkers to indicate earlier caspase activation (Boucher et al., 2018Boucher D. Monteleone M. Coll R.C. Chen K.W. Ross C.M. Teo J.L. Gomez G.A. Holley C.L. Bierschenk D. Stacey K.J. et al.Caspase-1 self-cleavage is an intrinsic mechanism to terminate inflammasome activity.J. Exp. Med. 2018; 215: 827-840Crossref PubMed Scopus (31) Google Scholar, Keller et al., 2008Keller M. Rüegg A. Werner S. Beer H.D. Active caspase-1 is a regulator of unconventional protein secretion.Cell. 2008; 132: 818-831Abstract Full Text Full Text PDF PubMed Scopus (575) Google Scholar). PRRs of the NAIP family also stimulate inflammasome formation, but they may not use ASC as the core oligomerization unit (Hu et al., 2015Hu Z. Zhou Q. Zhang C. Fan S. Cheng W. Zhao Y. Shao F. Wang H.W. Sui S.F. Chai J. Structural and biochemical basis for induced self-propagation of NLRC4.Science. 2015; 350: 399-404Crossref PubMed Scopus (115) Google Scholar, Zhang et al., 2015Zhang L. Chen S. Ruan J. Wu J. Tong A.B. Yin Q. Li Y. David L. Lu A. Wang W.L. et al.Cryo-EM structure of the activated NAIP2-NLRC4 inflammasome reveals nucleated polymerization.Science. 2015; 350: 404-409Crossref PubMed Google Scholar). Rather, NAIPs engage the adaptor NLRC4 through NACHT domain interactions in the PRR and adaptor. Akin to oligomerized ASC, NLRC4 recruits pro-caspase-1 into the assembled inflammasome through CARD-CARD interactions between the adaptor and the effector. ASC is likely recruited to these inflammasomes to augment caspase-1 activity (Case and Roy, 2011Case C.L. Roy C.R. Asc modulates the function of NLRC4 in response to infection of macrophages by Legionella pneumophila..MBio. 2011; 2 (e00117–11)Crossref PubMed Scopus (44) Google Scholar). In fact, macrophages stimulated with NAIP agonists can induce pyroptosis in an ASC-independent, caspase-1-dependent manner, but these cells fail to efficiently cleave other abundant caspase-1 substrates, such as pro-IL-1β, in the absence of ASC (Broz et al., 2010Broz P. von Moltke J. Jones J.W. Vance R.E. Monack D.M. Differential requirement for Caspase-1 autoproteolysis in pathogen-induced cell death and cytokine processing.Cell Host Microbe. 2010; 8: 471-483Abstract Full Text Full Text PDF PubMed Scopus (289) Google Scholar). The precise biochemical relationship between ASC-dependent and ASC-independent activities of the NAIP-NLRC4 inflammasome remains to be defined. In the examples described above, caspases operate as the effector component of the assembled SMOC. A notable exception to this idea has emerged with the identification that murine caspase-11 and its human counterparts (caspase-4 and caspase-5) are PRRs that form high-affinity interactions with LPS (Shi et al., 2014Shi J. Zhao Y. Wang Y. Gao W. Ding J. Li P. Hu L. Shao F. Inflammatory caspases are innate immune receptors for intracellular LPS.Nature. 2014; 514: 187-192Crossref PubMed Scopus (33) Google Scholar). LPS binds the CARD of these inflammatory caspases and stimulates their latent protease activity, which leads to the assembly of the NLRP3 inflammasome (Shi et al., 2014Shi J. Zhao Y. Wang Y. Gao W. Ding J. Li P. Hu L. Shao F. Inflammatory caspases are innate immune receptors for intracellular LPS.Nature. 2014; 514: 187-192Crossref PubMed Scopus (33) Google Scholar). Notably, the link between caspase-11 activation and NLRP3 inflammasome assembly is indirect and is mediated by the actions of a recently identified caspase substrate known as gasdermin D (GSDMD) (Kayagaki et al., 2015Kayagaki N. Stowe I.B. Lee B.L. O'Rourke K. Anderson K. Warming S. Cuellar T. Haley B. Roose-Girma M. Phung Q.T. et al.Caspase-11 cleaves gasdermin D for non-canonical inflammasome signalling.Nature. 2015; 526: 666-671Crossref PubMed Scopus (602) Google Scholar, Rühl and Broz, 2015Rühl S. Broz P. Caspase-11 activates a canonical NLRP3 inflammasome by promoting K(+) efflux.Eur. J. Immunol. 2015; 45: 2927-2936Crossref PubMed Scopus (101) Google Scholar, Shi et al., 2015Shi J. Zhao Y. Wang K. Shi X. Wang Y. Huang H. Zhuang Y. Cai T. Wang F. Shao F. Cleavage of GSDMD by inflammatory caspases determines pyroptotic cell death.Nature. 2015; 526: 660-665Crossref PubMed Scopus (716) Google Scholar). The details of this process will be described later. Increasing attention has been paid to the biochemical and cell biological aspects of inflammasome components and regulators. This aspect of inflammasome biology is in its infancy, with no clear understanding of the cell biological principles that explain pathway activity. Despite this lack of knowledge, a consensus is emerging whereby the movement of PRRs or their downstream components from the site of recognition to the site of signaling controls inflammasome assembly and activity. This general conclusion is consistent with work done on other PRR-regulated pathways, where the PAMP-inducible transport of receptors and downstream signaling machinery is critical to initiate inflammatory activity (Tan and Kagan, 2017Tan Y. Kagan J.C. Microbe-inducible trafficking pathways that control Toll-like receptor signaling.Traffic. 2017; 18: 6-17Crossref PubMed Scopus (0) Google Scholar). In support of active transport processes being necessary for inflammasome activity, NLRP3 and Pyrin