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This article is a note for:
[https://doi.org/10.1051/medsci/20132911013]

Issue
Med Sci (Paris)
Volume 29, Number 11, Novembre 2013
Page(s) 975 - 984
Section Traduction
Published online 08 October 2014

English translation of Med Sci (Paris) 2013 ; 29 : 975–984

© 2013 médecine/sciences – Inserm

There has been renewed interest in the innate immune system since toll-like receptors (TLRs) were discovered around 20 years ago [49]. The subsequent discovery of other innate immune receptors (RIG-I-like receptors or RLRs, C-type lectin receptors or CLRs, and nucleotide-binding domain and leucine-rich repeat containing receptors or NLRs) provided a key to decoding pathogen detection mechanisms and the rapid recognition of conserved microbial patterns (PAMPs). Since the majority of microbial patterns are conserved in non-pathogenic species, they are also called MAMPs (microbe-associated molecular patterns) [1]. The receptors which recognise these PAMPs are known as PRRs (pattern recognition receptors). PRRs can also be activated by danger signals, known collectively as DAMPs (damage-associated molecular patterns) [2]. This pattern recognition allows a limited number of PRRs to recognise a wide range of pathogens. This extensive recognition capacity is increased by the diversity of the PRRs’ locations. For example, TLRs and CLRs monitor the extracellular environment and endosomal compartments, while RLRs and NLRs recognise microbial patterns or danger signals within cellular cytoplasm. In 2002, Jurg Tschopp et al. [3] discovered a molecular macrocomplex containing a new type of PRRs, which they named the inflammasome. Unlike previously identified PRRs, which induce a transcriptional response via the activation of transcription factors such as NF-κB or IRF3/7 (IFN regulatory factor 3/7) and the synthesis of hundreds of proteins, the action of the inflammasome specifically leads to activation of caspase-1, an inflammatory caspase (Inset 1). Caspase-1 plays a dual role, first enabling proteolytic cleavage of inflammatory procytokines, namely IL-1β and IL-18, allowing them to mature and be secreted and secondly triggering pyroptosis, a hyperinflammatory form of cell death [4].

Inflammasomes can therefore be defined as caspase-1-activating platforms. Each inflammasome is named after the specific PRR involved. This paper describes the different inflammasomes that have been identified, their activation mechanisms and their role during infectious diseases.

Description of inflammasomes

Inflammasomes are macromolecular complexes formed by the oligomerisation of a receptor, an adaptor and caspase-1, the effector (Figure 1). These partners are brought together through homotypical domain interactions.

thumbnail Figure 1.

Structure of the inflammasome. A. Structure of the different proteins involved in inflammasome formation. B. The inflammasome NLRP3 comprises an NLRP3 receptor, an ASC adaptor, and procaspase-1. PYD: pyrin domain; NBD: nucleotide binding domain (allows ATP-dependent oligomerisation of NLRs); LRR: leucine-rich repeats; FIIND: function to find domain; CARD: caspase activation and recruitment domain; RD: repression domain.

Inflammatory caspases and apoptotic caspases

Caspases are cysteine proteases. They are classed as either apoptotic caspases, which trigger apoptosis (a largely silent type of cell death from the immunological point of view), or inflammatory caspases, which trigger a necrotic form of cell death often accompanied by the release of cytokines. In human beings, the inflammatory caspases are caspases-1, -4, -5 and -12. The role of caspase-1 in inflammation has been widely described; however our knowledge of the roles played by 4 and 5 is less complete and is extrapolated from the mouse caspase homologue caspase-11. Caspase-11 can activate caspase-1 indirectly or trigger pyroptosis independently of caspase-1. In most human beings, caspase-12 is non-functional, except in certain sub-Saharan African populations, who synthesise a long form of caspase-12, which paradoxically plays an anti-inflammatory role.

Receptors

To date, three families of receptors have been described as the principal triggers of inflammasome activation: NLRs, ALRs (AIM2 [absent in melanoma2]-like receptors) and RLRs. These receptors and the inflammasome cascades they induce have primarily been described in macrophages and dendritic cells, but they are also present in other cell types, such as polynuclear neutrophils, keratinocytes, and intestinal epithelial cells.

NLR receptors

In human beings, 22 receptors have been identified as belonging to the NLR family [5].

As in TLRs, the carboxy-terminal end has a leucine-rich repeat (LRR) domain consisting of a characteristic repetition of 20 to 30 leucine-rich amino acids. This domain appears to be the site of receptor-ligand interaction. The central section shared by all NLRs is a NBD (nucleotide binding domain). This domain allows ATP-dependent oligomerisation of NLRs to create hexamers or heptamers, which give inflammasomes their characteristic doughnut shape (Figure 2). The LRR domain plays a self-regulation role. When no stimulation is present, its conformational fold conceals the oligomerisation domain. Activation of the receptor entails a conformational change which exposes the NBD domain, and this then sets off the cascade of reactions leading to formation of the inflammasome.

thumbnail Figure 2.

Overview of NLRP3 functioning. Following a pre-activation stage, danger signals (DAMPs) and/or microbial patterns (PAMPs) induce the formation of the inflammasome through interaction between the receptor NLRP3, the adaptor ASC, and the effector procaspase-1. The resulting activation of caspase-1 induces cleavage of the pro-(IL)-1β prodomain, transforming it into its bioactive form (IL)-1β, which is then secreted into the extracellular environment.

The N-terminal end carries the NLR’s effector domain responsible for signal transduction. The new NLR nomenclature categorises these receptors into sub-groups according to their effector domain [5]. They are classified as :

  • NLRPs, which have a pyrin domain (PYD),

  • NLRCs, comprising NOD1 (nucleotide-binding oligomerization domain-containing protein 1), NOD2, and NLRC4/IPAF, which have a caspase activation and recruitment domain (CARD),

  • NLRB/NAIPs (NLR family, apoptosis inhibitory protein), which have a baculoviral inhibitor of apoptosis protein repeat (BIR) domain.

To date, seven NLRs have been identified as inducer of inflammasome activity. The most widely studied of these, NLRP3 (or cryopyrin), is involved in the detection of various danger signals. Intrinsic defects in this receptor are responsible for diseases in human, namely cryopyrinopathies [6–8]. NLRP1, NLRP7, and NLRB/NLRC4 complexes play a more specialised role in the detection of PAMPs. In mice, NLRP1 detects the anthrax letal toxin, whilst in humans NLRP1 is thought to detect a bacterial peptidoglycan fragment. NLRP7 detects a Mycoplasma lipopeptide. NLRB/NLRC4 complex detects cytoplasmic flagellin and type III secretion systems of pathogenic bacteria. NLRP6, NLRP12, and NOD2 (another receptor which detects peptidoglycan fragments) can activate the inflammasome, but at present we do not have a complete picture of their roles.

ALR receptors

In human, four receptors belong to the ALR family: AIM2, IFI16 (interferon γ-inducible protein 16), MNDA (myeloid cell nuclear differentiation antigen) and IFIX (interferon inducible protein X) [9]. The C-terminal end of ALRs contains a double-stranded DNA binding domain called HIN200. The N-terminal end contains a PYD domain that interacts with the PYD domains of other proteins (such as ASC [apoptosis-associated Speck-like protein]) to form macrocomplexes that contribute to inflammation and cell death. Only AIM2 and IFI16 have been described as inducing inflammasomes, upon detection of DNA in the cytoplasm and nucleus respectively [10, 11].

RLR receptors

RIG-I (retinoic acid inducible gene I) -like cytoplasmic receptor has only recently been recognised as an inflammasome receptor. There are three receptors in the RLR family: RIG-I, MDA5 (melanoma differentiation-associated protein 5), and LGP2. RLRs have a central helicase domain responsible for binding to RNA and a regulator domain at the C-terminal end. RIG-I and MDA5 [50, 51] also have two CARD domains at their N-terminal end, whilst LGP2 has no CARD domain. LGP2 may, in fact, be a regulator of

the other two RLRs [12]. Despite their similar structures, RIG-I and MDA5 do not recognise the same viral RNA and do not induce transduction of the same signals. Whilst both receptors are able to activate the transcription factors IRF (interferon regulatory factor) and NF-κB and to secondarily induce transcription of the gene coding for type-1 interferon (IFN), only RIG-I appears to be capable of CARD-CARD interactions with caspase-1. However, its role as an inflammasome receptor needs to be confirmed, in particular its NLRP3-independent activation [13].

Since the pioneering discovery of the NLRP1 inflammasome, more and more receptors that act as caspase-1 activators have been identified. In addition to the NLRP family, which contains 14 potential inflammasome activators in humans, and to the 4 ALRs and 3 RLRs, other receptors such as pyrin and CLRs may also be direct inflammasome activators. Their biological importance has yet to be demonstrated, however.

Receptor ligands and activators

The list of inflammasome activators has grown considerably over the last few years, as a result of the excitement caused among researchers by inflammasomes throughout the world. Table 1 shows the main stimuli which activate inflammasomes.

Table I.

Principal inflammasome activators.

Activation of NLRP3

Activation of the canonical inflammasome NLRP3 requires two signals to operate in conjunction. The first, which is often produced by the engagement of a Toll-like receptor, induces pre-activation of NLRP3 via post-translational modifications (deubiquitination) [14], increased transcription of the corresponding gene and transcription of the gene coding for pro-IL-1β. The second signal is produced by several activators (Table I).

The NLRP3 inflammasome is activated, for example, in response to various danger signals associated with membrane damage or changes in the cellular ion or metabolic homeostasis:

  • cytoplasmic molecules that are released after cell lysis can act as danger signals (DAMPs). For example, ATP is an extracellular messenger detected by the purinergic receptor P2X7 [52]. Activation of this membrane receptor triggers the opening of a membrane channel (pannexin) and a potassium efflux from the cell. The ensuing reduced intracellular potassium level induces a conformational change in NLRP3 and activation of the NLRP3 inflammasome [15–17]. Similarly, changes in calcium ion homeostasis may contribute to detection of membrane damage via NLRP3 activation [18].

  • Another NLRP3 activation pathway has been suggested in the case of particles such as asbestos or silica [7, 19], uric acid crystals [8] (responsible for gout), cholesterol [6], haemozoin (a crystal derived from haemoglobin which forms during infection caused by Plasmodium falciparum [54]) (→), or lastly β-amyloid protein (Alzheimer’s disease) [20]. These crystals and particles can lead to rupture of lysosomes or phagolysosomes and a release of their contents into the cytoplasm. In particular, the release of cathepsins (lysosomal proteases) into the cytoplasm is thought to be involved in NLRP3 activation [21]. Toxin-producing bacteria that form pores in the cell membrane, such as Staphylococcus aureus, are thought to activate NLRP3 via one of these two membrane damage detection pathways [22, 23].

(→) See La Nouvelle by K.G. Pellé et al., on page 960 of this issue

  • Lastly, the mitochondrion appears to be at the intersection of several NLPR3 activating stimuli. The mitochondrion can produce reactive oxygen species (ROS) in response to several types of cellular stress. These ROS may constitute the initial signal for NLRP3 activation [21, 24]. They may also activate NRP3 directly through modification of the ROS-sensitive protein thioredoxin, by releasing its interacting partner TXNIP (thioredoxin-interacting protein), which may then interact with NLRP3 and activate the related inflammasome [25]. Together with other cellular stress signals, these ROS also induce mitochondrial lysis, leading to the release of oxidised mitochondrial DNA into the cytoplasm. This then can be detected directly by NLRP3 [26]. The mitochondrion integrates metabolic cellular stress (for example the cellular energy failure which activates the NLRP3 inflammasome) and may constitute a platform for assembly of the NLRP3 inflammasome.

Signal transduction and caspase-1 activation

Activation of NLRs leads to their oligomerisation via their NBD domains, with apposition of their PYD domains. The protein ASC (apoptosis-associated speck-like protein containing a CARD) is then recruited through PYD-PYD interaction. ASC is an adaptor protein, as it has a PYD at its N-terminal end (which interacts with most receptors) and a CARD at its C-terminal end. The latter is able to interact with the CARD of procaspase-1. ASC also forms complexes with AIM2 and IFI16 receptors when they are activated.

NLRC4 and RIG-I interact directly with procaspase-1. In the case of NLRC4, and depending on the stimulus, the strength of activation of caspase-1 may also require interaction with ASC [27–29].

The apposition of two procaspase-1 within the complex leads to their proteolytic autocleavage and self-activation. In its active form, caspase-1 is a heterotetramer comprising two subunit pairs, p10 and p20. Other caspases may be activated by non-canonical inflammasome complexes (Inset 2). Caspase-1 is a cysteine protease whose main roles are to convert pro-IL-1 β and pro-IL 18 into their active forms through cleavage of the C-terminal prodomains and triggering of pyroptosis.

Pro-inflammatory interleukins

Interleukin-1β

Previously known as an endogenous pyrogen, IL-1β is one of the major cytokines involved in the inflammatory response [30, 31]. It plays various roles, such as inducing fever and stimulating hematopoiesis in the bone marrow which causes hyperleukocytosis and thrombocytosis. IL-1β can also directly activate lymphocytes, epithelial cells, and endothelial cells, leading to the recruitment of polynuclear neutrophils and inflammatory cells at the site of an infection. It can also induce its own expression and the expression of genes coding for other cytokines (tumour necrosis factor-α [TNF-α] or IL-6), creating a full-blown inflammatory cascade.

Because of its potent pro-inflammatory capacity and potentially harmful effects, the synthesis, secretion, and activity of IL-1β are very closely regulated by a number of processes:

  • the need for an activating signal (priming) before NFκB-dependent transcription of an inactive form of IL-1β is triggered;

  • the regulation of its cleavage by the inflammasome;

  • the absence of a signal peptide, preventing its secretion via the classic exocytosis pathway;

  • the extracellular competition with a natural antagonist (IL-1Ra) and the extracellular binding to the IL-1R2 receptor, which has no cytoplasmic domain to use for signal transduction.

As a result of these various mechanisms, the levels of transcription of pro-IL-1β do not correlate with the levels of secretion of its active form.

Non-canonical inflammasomes

The canonical inflammasome is defined as the inflammasome which induces activation of caspase-1. Two other signalling pathways which involve inflammatory caspases or inflammasome components but do not lead to caspase-1 activation have recently been identified and are referred to as non-canonical inflammasomes. The first of these has been described in mice and involves caspase-11. Little is still known about the mechanisms behind the activation of this inflammasome, but they are thought to involve the interferon pathway and unknown receptors able to detect intracytoplasmic bacteria.

Interleukin-18

Unlike IL-1β, IL-18 is constitutively expressed in macrophages and therefore does not require a priming stage [31]. One of the main roles of IL-18 is to induce a Th1 immune response, via activation of natural killer (NK) cells and secretion of IFN-γ. IL-18 also promotes the secretion of other proinflammatory cytokines such as TNF-α, IL-1β, IL-8 and GM-CSF (granulocyte-macrophage colony-stimulating factor). In doing so, IL-18 induces the recruitment, activation, and expansion of polynuclear neutrophils and macrophages during an infection. IL-18 also increases cytotoxic activity and proliferation of CD8+ lymphocytes and NK cells. The protective role played by IL-18 during bacterial and fungal infections is primarily a result of its capacity to simulate production of the cytokine IFN-γ, which induces the microbicidal activity of

neutrophils and macrophages via several different anti-bacterial mechanisms (such as nitric oxide production).

Pyroptosis

In addition to its proinflammatory-interleukin activating role, caspase-1 is also able to cause pyroptosis, an inflammatory programmed cell death. Pyroptosis is triggered in response to several different PAMPs and DAMPs. This cell death is fast and involves the rupture of the plasma membrane, leading to the release of proinflammatory intracellular substances. Activated caspase-1 induces the formation of pores in the cell membrane, leading to increased osmotic pressure, which in turn leads to a flow of water into the cell and swelling of the cell. These stages culminate in osmotic lysis of the cell and the release of inflammatory components into the extracellular compartment. This process is different from apoptosis in that it entails fragmentation of non-oligonucleosomal DNA, condensation of the nucleus without loss of integrity, and destruction of the actin cytoskeleton.

The importance of this process becomes particularly apparent in infections caused by intracellular bacteria such as Legionella pneumophila. Once L. pneumophila has invaded the cell, it establishes itself inside an intracytoplasmic replication vacuole where it is protected from the host’s immune defences and can therefore replicate. Detection of Legionella flagellin by NLRC4 in macrophages induces inflammasome formation. The activation of caspase-1 and the ensuing pyroptosis not only destroy the bacterium’s replication site early on (cell death) but also induce a local inflammatory response (recruitment of other immune system cells), which helps to confine the infection [33].

Inflammasomes in infectious diseases

The importance of inflammasome activation in combating infections derives from the three activities of caspase-1: (1) pyroptosis, which removes the bacteria’s intracellular replication niches; (2) IL-1β production, leading to the recruitment of inflammatory cells at the infection site, and (3) IL-18 production, which induces the IFN-γ cascade and the activation of cellular bactericidal mechanisms. Tables II and III above illustrate the inflammasome activation mechanisms in different bacterial and viral infections, respectively.

Bacterial infections

In human, the NLRP1 inflammasome is activated in response to muramyl dipeptide, a fragment of the peptidoglycan present in the cell walls of many bacteria, both Gram positive and Gram negative [34]. In mice, NLRP1 also recognises lethal factor, one of the components of the anthrax bacillus toxin (Bacillus anthracis) [35]. The inflammasome plays a key role in combating B. anthracis in mouse models, but may also be responsible for inflammatory lung damage when lethal toxin is inhaled [36]. This paradoxical action illustrates why both the inflammasome and inflammation in general must be controlled, in order to defend against microbes without damaging the host’s tissues.

NLRP3 is activated by various bacteria in response to danger signals, rather than being activated by direct PAMP-receptor interaction. In the above paragraphs, we have described the different NLRP3 activation pathways that can be involved. The list of bacteria, which activate NLRP3, is shown in Table II, together with their activating patterns. For example, during S. aureus skin infections, activation of the NLRP3 inflammasome induced by detection of the membrane damage caused by pore-forming toxins is a crucial factor for the early recruitment of neutrophils to the infection site, via the secretion of IL-1β.

Table II.

Bacteria which activate the different inflammasomes and their activating patterns. ESX-1: ESAT-6 secretion system 1; ESAT-6: 6-kDa early secreted antigenic target; YopJ: Yersinia outer protein J; Mxil: Max-interactor 1.

Table III.

Inflammasome-activating viruses.

The role of the NLRP6 inflammasome in antibacterial immunity has recently been reported, but the findings are contradictory and only apply to mouse models. In one instance, NLRP6-deficient mice were found to have changes in their gut microflora and a predisposition to colitis [37]. In the other, the deficient mice seemed to have resistance to Listeria monocytogenes, Salmonella, and Escherichia coli infections [38]. On the whole, little is still known about the activator(s) of NLRP6 and their role(s) have yet to be described.

The antibacterial roles of NLRP7 and NLRP12 have been described. NLRP7 appears to be involved in recognising bacterial lipopeptides from Mycoplasma species [39], but it has not been fully described as it is absent in mouse models. NLRP12, on the other hand, forms an inflammasome in response to Yersinia infections; however this effect has only been observed in a mouse model and it is not clear what pattern is detected [40].

NLRC4 is specifically activated by bacterial secretion systems types III and IV, such as those of Yersinia and Shigella, or by the flagellin of bacteria such as Salmonella, Legionella, and Pseudomonas [27, 33]. In mouse models, the selectiveness of the NLRC4 inflammasome in response to one of its activators seems to be due to prior binding of the activator to one of the NAIP proteins [29]. However, in human, only one NLRB (NAIP homologue) is present. While it appears to be able to recognise a simple pattern common to Shigella, Salmonella, and Pseudomonas, further studies are needed in order to confirm the role of NLRB. In mice, the NLRB/NLRC4 complex plays a key role in early detection of bacterial invasion in the intestinal mucous membrane and in triggering a strong inflammatory action to control the infection.

AIM2 specifically recognises double-stranded DNA in the cytoplasm. It is activated during bacterial or viral infections when the pathogen is outside the phagosome, in particular in Listeria monocytogenes infections and in tularemia, (caused by Francisella tularensis). AIM2-deficient mice are highly susceptible to Francisella infections [41].

Interestingly, the AIM2 receptor detects a substance common to viruses and all living beings including humans: DNA. The danger signal is not sent on detection of the substance per se, but on detection of the substance in the wrong location. AIM2 detects DNA present in the cytoplasm, since in healthy cells it is only found in the nucleus.

Viral infections

The role played by the proinflammatory cytokines IL-1β and IL-18 in antiviral immunity has been demonstrated in mouse models. For example, mortality from influenza virus infection is higher among mice that are deficient in IL-1R or IL-18 than in wild-type mice [42, 43]. In the case of herpes virus 1 (HSV1) infection in IL-1-deficient mice, there is a higher viral load in the blood, a higher rate of HSV1 encephalitis and a generally reduced inflammatory response. Administration of IL-18 at an early stage in the HSV1 infection improves the animals’ survival.

An interesting model of NLRP3 inflammasome activity during influenza A virus infection has recently been proposed. It is thought that recognition of the viral RNA by the receptor TLR7 in pulmonary macrophages produces the initial signal leading to the synthesis of IL-1β. Insertion of the viral protein M2 (an ion channel) in the trans-Golgi network, leading to disruption of the intracellular ions homeostasis, is then thought to constitute the second signal. Another possible activator may be the lysosome damage caused by penetration of the virus, as this leads to an efflux of potassium and production of ROS, which both trigger NLRP3 activation [44].

There are very few data on activation of the AIM2 inflammasome during viral infections and the data available relate primarily to mouse models. AIM2 recognises murine cytomegalovirus, a DNA virus from the Herpesviridae family [45, 46]. However, the Herpesviridae viruses HSV1 and VZV (varicella-zoster virus) activate either NLRP3 (both HSV1 and VZV) or IFI16 (HSV1) [47, 48].

The intranuclear location of IFI16 explains why it is activated by the DNA of KSHV (Kaposi’s sarcoma- associated herpes virus) and HSV1 viruses, as they replicate inside the nucleus.

To our knowledge, none of the data currently available allow us to infer whether pyroptotis occurs in response to viral infections.

Conclusion

Ten years on from the discovery of the inflammasome, despite major progress in our understanding of this innate immune system complex, we are far from having fully described either its activation and regulation mechanisms or its functions, in particular those relating to cells belonging to the adaptive immune system. It is also likely that new inflammasomes are still to be discovered. Although our understanding of the molecular mechanisms of inflammasomes is improving constantly, their role in pathophysiology remains a mystery. Whilst the inflammasome has been amply demonstrated to play a protective role during infections caused by several different bacteria, its role in viral, fungal, and parasitic infections has yet to be confirmed.

In addition, the inflammasome is an extremely potent inducer of inflammatory responses, which can have devastating effects. These inflammasome-related damages are especially marked in patients suffering from cryopyrin-associated periodic syndromes (CAPS), which are inherited autoinflammatory diseases caused by mutations on the NLRP3 gene. These paediatric diseases include familial cold urticaria, Muckle-Wells syndrome, and chronic infantile neurological cutaneous and articular syndrome (CINCA) (known in the USA as neonatal onset multisystem inflammatory disease, or NOMID). In patients who carry these mutations, deregulation of the inflammasome caused by synthesis of a spontaneously activated NLRP3 receptor causes overproduction of IL-1β and a recurring hyperinflammatory state which manifests in symptoms ranging from simple rashes to life-threatening conditions. Treatment with IL-1 antagonists, especially the IL-1Ra, anakinra, has revolutionised the management of these diseases. Inflammasome deregulation may also be implicated in other diseases with similar symptoms to CAPS, such as childhood or adult-onset Still’s disease, and Schnitzler’s syndrome. Thus, these patients may therefore also benefit from treatments targeting inflammasome pathways. Both present and future research should enable us to fully decode the inflammasome’s pathways and pathophysiological functions and hence to develop targeted treatments for use in many infectious and autoinflammatory disorders and possibly even cancer. (→) [53].

(→) See Review by J. Garaude on page 985 of this issue

Declarations of interest

The authors declare that they have no interests associated with the information published in this article.

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All Tables

Table I.

Principal inflammasome activators.

In the text
Table II.

Bacteria which activate the different inflammasomes and their activating patterns. ESX-1: ESAT-6 secretion system 1; ESAT-6: 6-kDa early secreted antigenic target; YopJ: Yersinia outer protein J; Mxil: Max-interactor 1.

In the text
Table III.

Inflammasome-activating viruses.

In the text

All Figures

thumbnail Figure 1.

Structure of the inflammasome. A. Structure of the different proteins involved in inflammasome formation. B. The inflammasome NLRP3 comprises an NLRP3 receptor, an ASC adaptor, and procaspase-1. PYD: pyrin domain; NBD: nucleotide binding domain (allows ATP-dependent oligomerisation of NLRs); LRR: leucine-rich repeats; FIIND: function to find domain; CARD: caspase activation and recruitment domain; RD: repression domain.
In the text
thumbnail Figure 2.

Overview of NLRP3 functioning. Following a pre-activation stage, danger signals (DAMPs) and/or microbial patterns (PAMPs) induce the formation of the inflammasome through interaction between the receptor NLRP3, the adaptor ASC, and the effector procaspase-1. The resulting activation of caspase-1 induces cleavage of the pro-(IL)-1β prodomain, transforming it into its bioactive form (IL)-1β, which is then secreted into the extracellular environment.
In the text

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