The PYD, also referred to as PAAD or DAPIN, is a protein binding domain belonging to the death domain superfamily (12). The structure of several PYDs has been determined, which revealed a bundle of 5- to 6-α-helices. PYDs display distinct negatively and positively charged surface patches, which are indicative of electrostatic interactions to occur during PYD-PYD interactions, reminiscent to other death domain folds (13–18). NLRPs and ALRs both encode an N-terminal PYD, but while NLRPs are further composed of a central nucleotide binding NACHT domain and varying copies of C-terminal leucine-rich repeats, ALRs rather contain one or two copies of the oligonucleotide binding HIN-200 domain at the C-terminus. The PYD is the effector domain required for downstream signaling, while evidence supports a role of the LRR and HIN-200 domain in ligand recognition (19–21). The current model for both PRR families is that ligand recognition promotes a conformational change (15, 21–23), which allows nucleotide binding by the NACHT domain and consequently, enables NLRP oligomerization (24–27), while ALRs cluster alongside the DNA staircase (21). Ultimately, this exposes the PYD in NLRPs and ALRs, thus enabling the recruitment of ASC by homotypic PYD–PYD interactions and clustering of ASC. In the context of inflammasomes, the recruitment and clustering of ASC then triggers its interaction with pro-caspases-1 (3, 28) and -8 (29, 30) and their activation by induced proximity. The precise order of events is still elusive and a recent model proposed spontaneous self-oligomerization of the ASC-PYD, which subsequently facilitates its interaction with NLRP3 and potentially also other PYD-containing PRRs (31). Hence, this model suggests that PYDs contain a dual binding interface (31). The influence of NLRPs on other signaling pathways is even less well understood, but might also occur through these adaptors (32, 33). In contrast to ASC-mediated inflammasome activation in response to KSHV (34), the ALR IFI16 promotes induction of IFN-β through connecting to the common pathway leading to IRF-3 phosphorylation through the adaptor STING (stimulator of IFN genes) (35).

Only 14 NLRPs and 4 ALRs are encoded in humans, while both families are amplified to 34 and 13 members, respectively, in mice (Figures 1A,B). However, the precise function of most family members is still unknown (36–39). Besides NLRPs and ALRs, the PYD is also present in the inflammasome adaptor protein ASC, the regulatory PYD-only proteins (POPs) and Pyrin (Figure 1C) (12). Below, we will specifically discuss the mechanism of activation and function of NLRPs and ALRs, and how defects within these proteins are involved in immune-related disorders.

NLRP1 (Figure 1A) is also known as NALP1, NAC, DEFCAP, CARD7, and CLR17.1 and has initially been linked to caspase-9 activation within the apoptosome (40).

Inflammasomes were first discovered in 2002 with the initial observation that NLRP1 is able to assemble in an ASC, caspase-1, and caspase-5-containing large inducible protein complex responsible for the autocatalytic activation of caspase-1 in THP-1 cells (1). However, caspase-5 is not recruited to other inflammasomes (25, 41), which is likely due to the unique domain structure of NLRP1. In addition to the common tripartite domain organization of NLRPs, NLRP1 also encodes a C-terminal function to find (FIIND) domain and a CARD, which enables direct caspase-5 recruitment (Figure 1A). Despite its early identification, the in vivo function of NLRP1 however remains largely elusive, at least partially due to several key differences between mice and human, which limits the relevance of in vivo mouse models. In contrast to human NLRP1, mouse NLRP1 lacks the PYD and exists in three tandem paralog genes (Nlrp1a, Nlrp1b, and Nlrp1c) (Figure 1A). While the PYD is crucial for the recruitment of ASC and subsequently of caspase-1, the C-terminal CARD directly recruits caspase-5, which is necessary for full caspase-1 activation in human cells (1). However, analysis of the first in vitro-reconstituted inflammasome with purified recombinant human proteins demonstrated that the core inflammasome components NLRP1 and caspase-1 are sufficient for promoting caspase-1 activation in the presence of NTPs and MDP as a specific agonist (25). In this context, ASC was not necessary, but addition of ASC increased the efficiency of caspase-1 activation. Similar results have also been observed in vivo for murine NLRP1b (22). In contrast, a recent analysis suggested that caspase-1 is directly recruited to the C-terminal CARD of NLRP1 and that the PYD is dispensable for inflammasome activation (42). This model could therefore explain NLRP1 inflammasome activation of human and mouse NLRP1, in spite of mouse NLRP1 lacking the PYD. Although the role of the PYD in human NLRP1 is still elusive, the presence of ASC, facilitated by PYD–PYD interaction, could enable an increase in NLRP1-mediated caspase-1 activation in addition to CARD mediated caspase-1/5 recruitment. Additional insights into the molecular mechanism of NLRP1 inflammasome activation came from studies showing that the FIIND domain resembles the autoproteolytic ZU5-like domain found in PIDD, which contains a LRR and a death domain and is part of the caspase-2-activating PIDDosome (43). Accordingly, the FIIND domain in NLRP1 also undergoes autoproteolytic cleavage, which is required for inflammasome activation and congruently, NLRP1b^V988D, which disrupts the protein conformation required for autoproteolysis, or NLRP1b^S984A, which disrupts the catalytic serine residue, results in deficient caspase-1 activation without impairing NLRP1 oligomerization (42–44). This step is further regulated by splicing, since an alternative transcript lacking exon 14, which contains the FIIND cleavage site, is deficient in autoproteolytic processing (42). Moreover, rat NLRP1 activation by the Bacillus anthracis virulence factor lethal toxin (LTx), a metalloproteinase composed of the pore-forming antigen (PA) and a lethal factor (LF), also cleaves NLRP1, but within the N-terminal domain, suggesting that NLRP1 potentially has several protease cleavage sites (45, 46). Both steps appear necessary for caspase-1 activation, and a possibility could be that the FIIND has partial autoproteolytic activity, and cleavage of NLRP1 by LT might increase this activity (46). Accordingly, a C-terminal fragment of NLRP1b containing the CARD and 56 adjacent amino acids is sufficient for caspase-1 activation with the 56 adjacent amino acids being required for oligomerization (47). However, LTx-mediated cleavage of NLRP1b is still controversial, since another study failed to observe LTx-mediated cleavage of NLRP1b, although LTx was required for autoproteolysis (44).

A first glimpse into the functional importance of NLRP1 was discovered, when genetic mapping identified NLRP1b as the gene responsible for LTx sensitivity in mice. In mice, only NLRP1b, and none of the two other paralogs (NLRP1a and NLRP1c), confer susceptibility to LTx (Figure 2) (48). The exact role of LTx in this context during B. anthracis infection is, however, a matter of controversy, since in vitro cell death and in vivo end-stage LTx-induced disease and death appear to not be linked (49). Furthermore, different mechanisms have been reported for LTx and spores, with the latter promoting an inflammasome response in LTx susceptible and resistant macrophages (50). A similar protective response has also been reported in response to Toxoplasma gondii infection, where NLRP1b activation ensured selective elimination of the niche for pathogen proliferation, cytokine release, and effective spreading of danger signals to neighboring cells (51).

Several studies observed NLRP1/NLRP1b sensing of MDP (22, 25, 52). However, while the recently generated NLRP1b deficient mice demonstrated impaired inflammasome response to LTx, the response to MDP was intact and rather NLRP3-dependent (53). Furthermore, NLRP1b has been suggested to sense energy stress in fibroblasts, as a consequence of starvation (54). In particular, NLRP1b senses the reduction of intracellular ATP levels and the subsequent activation of the AMP-activated protein kinase (AMPK). Congruently, a mutation of the ATP binding pocket within the NACHT of NLRP1b yielded a constitutively active inflammasome, suggesting that ATP binding might inhibit, rather than activate NLRP1b, in contrast to what has been reported for human NLRP1 (25, 55).

Underlining its functional importance, further control mechanisms besides RNA splicing may regulate the activity of the NLRP1 inflammasome. The anti-apoptotic proteins Bcl-2 and Bcl-X_L were reported to specifically inhibit NLRP1 activation by blocking ATP binding (52, 55). Both proteins appear to bind to the LRR of NLRP1 with their loop region, suggesting that different domains are responsible for their NLRP1 inflammasome-suppressing activity compared to their apoptosis-suppressing activity. Furthermore, recent evidence suggests that NLRP1 may provide a more effective immune response by associating with NOD2 (22). Finally, there is evidence that the anti-inflammatory omega-3 (ω-3) polyunsaturated fatty acids attenuate NLRP1b through interaction of NLRP1b with β-arrestin-2, the downstream scaffold for GPR120 and GPR40 (56).

Although NLRP2 (Figure 1A), also known as PYPAF2, NALP2, PAN1, and CLR 19.9 failed to affect activation of NF-κB or caspase-1 in initial in vitro studies (57), it was later shown to inhibit cytokine-induced NF-κB activation. Subsequently, it was shown that PYD-mediated interaction of NLRP2 with ASC resulted in the abrogation of the expression of NF-κB target genes in the monocytic THP-1 cell line (58). Highly expressed in T-cells, NLRP2 was also found to inhibit NFAT and AP-1, in addition to NF-κB, following TCR activation by anti-CD3 and anti-CD28 antibodies or PMA/ionomycin (59). Besides its transcriptional regulation, biochemical studies in THP-1 cells, suggesting that NLRP2 could assemble into an ASC and caspase-1-containing inflammasome (41). NLRP2 does not contain a FIIND domain, but CARD8 (also known as Cardinal and TUCAN), which is the only other FIIND domain-containing protein besides NLRP1, is recruited to NLRP2 via its NACHT (41). In a manner similar to NLRP1, the FIIND domain of CARD8 is also autoproteolytically cleaved, potentially to promote downstream signaling (43). The in vivo function of CARD8 and its role in inflammasome activation, however, is still poorly defined, since CARD8 is does not exist in mice (60). NLRP2 is highly expressed in human astrocytes within the central nervous system and, similar to NLRP3, appears to assemble in an ASC- and caspase-1-containing inflammasome in response to exogenous ATP, as shown by gene silencing (61). In this context, NLRP2 may directly interact with the P2X₇R and pannexin-1, suggesting a direct effect on the NLRP2 inflammasome, rather than the indirect effect that is observed for NLRP3. However, these findings will need further corroboration, in particular in vivo.

NLRP3 (Figure 1A), also known as Cryopyrin, NALP3, PYPAF1, CIAS1, CLR1.1, is the best-studied member of the NLRP family. It was initially discovered by positional cloning in the search for the genetic cause of a group of auto-inflammatory diseases, now referred to as Cryopyrinopathies or Cryopyrin-associated periodic syndromes (CAPS) (62). While initial overexpression studies suggested that NLRP3 affects NF-κB activation, NLRP3-deficient mice displayed defects restricted to inflammasome activation (63–66). In contrast to other Nod-like receptors (NLRs), NLRP3 is activated by, and responds to a diverse set of stimuli originating from microbes pathogen-associated molecular patterns (PAMPs) (Figure 3) or from environmental and endogenous danger signals danger-associated molecular patterns (DAMPs), which can be of either soluble or particulate matter (Figure 4). Microbial activators include various Gram-positive and -negative bacteria (Listeria monocytogenes, Staphylococcus aureus, Vibrio cholera, Neisseria gonorrhoeae, and others) (64, 67–71), fungi (Candida albicans, Saccharomyces cerevisiae) (72), RNA and DNA viruses (adenovirus, influenza virus, Sendai virus, MCMV) (73–75), as well as protozoa (Plasmodium malariae) (76–78). The fact that NLRP3 also senses sterile environmental and endogenous stress signals, and promotes inflammatory responses further expands the repertoire of NLRP3 reactivity. Environmental triggers include the particulates alum (79–83), asbestos (84, 85), silica (81, 84, 85), skin irritants (trinitrochlorobenzene, trinitrophenylchloride, and dinitrofluorobenzene) (66, 86), and even UVB radiation (87). An increasing complexity of endogenous danger signals is now also known to activate NLRP3, since the discovery that monosodium urate crystals (MSU) and pyrophosphate dihydrate (CPPD) crystals are able to activate NLRP3 (65). Other known NLRP3-inducing crystals are cholesterol, amyloid deposits (88, 89), hydroxyapatite crystals (90), and hyaluronan (91). In addition to these crystalline danger signals, NLRP3 also senses non-crystalline stress signals, including ATP (64), high glucose (92), and saturated fatty acids (93). The mechanism that causes NLRP3 activation in response to so many different stimuli is still controversial and more discussed below.

The function of NLRP4 (also known as NALP4, PAN2, PYPAF4, RNH2, and CLR19.5) (Figure 1A) in innate immunity is still poorly understood. It has not been linked to inflammasome activation, but overexpression studies indicated that NLRP4 modulates NF-κB activation in response to pro-inflammatory cytokines, including TNFα and IL-1β (139). Recently, NLRP4 has been proposed to modulate type I IFN signaling and autophagy, based on gene silencing and overexpression (140, 141). In response to Group A Streptococcus (GAS) infection, NLRP4 inhibits the initiation of autophagy through interaction with beclin-1. Interestingly, all other tested NLRs, including NLRC4, NLRP3, and NLRP10 also interacted with beclin-1, potentially indicating this is a more common mechanism of NLRs (140). NLRP4 further interacted with the class C vacuolar protein-sorting complex to inhibit phagolysosomal maturation (140), suggesting that NLRP4 and possibly other NLR family members play a role in autophagosome maturation following bacterial infection. Yet, during viral infection, NLRP4 has been proposed to play a regulatory role within the type 1 IFN pathway in response to dsDNA and dsRNA (141). NLRP4 targets the central type IFN signaling component TBK1 for K48-linked polyubiquitination and degradation, through recruiting the E3 ubiquitin ligase DTX4 to TBK1, resulting in loss of IRF-3 activity. Only the NACHT of NLRP4 is required for this activity. While humans encode only NLRP4, mice encode seven paralog genes, NLRP4a-g, with at least NLRP4b and NLRP4g also inhibiting type I IFN production (141).

Initial overexpression studies suggested that NLRP6 (also known as NALP6, PAN3, PYPAF5, CLR11.4) (Figure 1A) mediates activation of NF-κB and caspase-1 in the presence of ASC (57). A subsequent study hinted at a function of NLRP6 within the intestinal epithelium, based on transcriptional profiling (142), and it is now evident that NLRP6 might function differently in myeloid cells and in intestinal epithelial cells. Three recent studies in NLRP6-deficient mice confirmed a role for NLRP6 in the regulation of intestinal host-microbiota (Figure 5) (143–145). NLRP6-deficient mice develop an increased sensitivity to DSS-induced colitis and colitis-induced tumorigenesis, suggesting a protective role of NLRP6 against intestinal inflammation and inflammation-induced cancer (143, 145). Although, it was previously suggested that NLRP6 is mostly expressed in the non-hematopoietic compartment, bone marrow chimera demonstrated the requirement of hematopoietic cells for this function (143). These studies further elute to a function of NLRP6 in intestinal epithelium self-renewal during steady state and during repair after inflammation through suppressing inflammation and associated colorectal carcinogenesis (143, 145, 146). NLRP6 is essential in regulating the interplay between host and gut microflora. Mice deficient in the NLRP6, or a potential NLRP6 inflammasome, although the latter is only based on overexpression data, develop a transferable colitogenic intestinal microbiota due to failure to produce IL-18, a necessary cytokine for the restriction of Prevotellaceae and TM7 species in the steady state and upon DSS treatment through induction of CCL5 and IL-6 (144, 146). These results support the idea that NLRP6-driven IL-18 production from the epithelium is the major contributor to prevent the development of the colitogenic phenotype, as opposed to IL-18 secreted from the hematopoietic compartment. IL-18 is at least partially responsible for the down-regulation of IL-22BP during inflammation, allowing IL-22 to improve epithelial cell repair, while IL-22BP increases again at the end of regeneration with the decrease of IL-18 (147). In addition to restricting colitogenic microbiota species, NLRP6 also functions downstream of TLR signaling to dampen anti-microbial host defense. Rather than contributing to elimination of infections, NLRP6 has a deleterious role within the hematopoietic and the non-hematopoietic compartments and, accordingly, NLRP6^−/− mice show increased resistance to infection by extracellular E. coli, intracellular L. monocytogenes and S. typhimurium, and display increased circulatory monocytes and neutrophils upon infection (148). Mechanistically, NLRP6 acts as an inhibitor of MAPK and the canonical NF-κB pathway activated by TLR, but not NLR ligation (148). A potential explanation could be that the full extent of the immune response is required to defend against systemic infection, whereas a more controlled immune response might be required in the case of localized inflammation in the gut. Thus, NLRP6 may play a regulatory role in both scenarios by providing protection against chronic inflammation, but consequently being deleterious during acute infections.

NLRP7 (also known as NALP7, PAN7, PYPAF3, NOD12, CLR19.4, HYDM) (Figure 1A) is one of four NLRPs, which exist in humans, but not in mice. Although, earlier overexpression studies NLRP7 failed to observe effects on NF-κB and caspase-1 activation (57), several studies since then reported modulation of these pathways by NLRP7. However, conflicting reports describe NLRP7 as either an activator or inhibitor of caspase-1 (Figure 6). NLRP7 has been proposed to directly interact with pro-caspase-1 and pro-IL-1β, without affecting NF-κB (149). It was also proposed that NLRP7 affects secretion of IL-1β and TNFα in PBMCs isolated from patients harboring NLRP7 mutations, which affected its localization to the microtubule-organizing center and the Golgi apparatus, and was associated with a down-regulation of intracellular pro- and mature IL-1β (150). NLRP7 also interacts with FAF-1, which also interacts with several other NLRPs and promotes apoptosis and inhibits NF-κB activation (151). However, modulation of NF-κB was not observed following NLRP7 over expression nor on endogenous level following NLRP7 silencing (57, 71). Overall, there are several potential mechanisms by which NLRP7 could negatively regulate release of inflammatory cytokines (152). In contrast, there is also evidence for a pro-inflammatory role of NLRP7 through the formation of an ASC-containing inflammasome that is triggered in response to bacterial acylated lipoproteins (71). NLRP7 collaborates with NLRP3 and TLR2 in restricting intracellular growth of S. aureus and L. monocytogenes in human macrophages (71). Similar to NLRP3, NLRP7 also functions downstream of lysosomal damage, with the difference that NLRP7 appears to be only partially sensitive to K⁺ efflux (71). Thus, NLRP7 might contribute to pro- as well as anti-inflammatory processes (152).

NLRP10 (also known as NALP10, PAN5, NOD8, PYNOD, CLR11.1) (Figure 1A) is the other structurally atypical NLRP besides NLRP1, since it lacks the typical C-terminal LRR. The LRR is essential for NLRP3 activation in response to specific agonists, such as MSU (19), while deletion of the LRR reliefs autoinhibition and renders the NLR active in several in vitro studies. Thus, one may predict that NLRP10 might not respond in a stimuli-dependent manner. Over expression studies proposed that NLRP10 oligomerizes with ASC and inhibits ASC-mediated NF-κB activation and apoptosis, as well as caspase-1-dependent IL-1β release (153). Direct caspase-1 inhibition only requires the NACHT domain of NLRP10, but inhibiting ASC-mediated apoptosis, NF-κB and caspase-1 activation required the PYD (154). In contrast to human NLRP10, mouse NLRP10 failed to reduce self-aggregation of ASC, which is required for inflammasome activation. However, transgenic mice ubiquitously expressing high levels of mouse NLRP10 recapitulated the inhibitory effects observed in vitro, and mice were more resistant to endotoxic shock in vivo (154). In contrast, NLRP10 contributes to host defense to Shigella flexneri in epithelial cells and fibroblasts by promoting secretion of IL-6 and IL-8 through induction of NF-κB and p38 signaling pathways, without affecting IL-18 release, arguing against an inflammasome role by gene silencing. This response required the ATPase activity and the PYD of NLRP10 (155). Mechanistically, NLRP10 interacts with signaling components of the Nodosome, including NOD1, RIP2, TAK, and IKKγ in response to S. flexneri infection (155). However, NLRP10^−/− mice revealed a pronounced defect in mounting adaptive immune responses in the Th1/Th17-dependent experimental autoimmune encephalomyelitis (EAE) mouse model and Th2-dependent OVA- and Alum-driven asthma models (156). These defective Th cell responses were caused by a defective emigration of activated DCs from sites of inflammation to draining lymph nodes, loss of antigen transport, and subsequent priming of CD4⁺ T-cell, though their activation profile remained unaffected (Figure 5). Similar results were obtained in a C. albicans dissemination model, in which NLRP10^−/− mice displayed increased susceptibility marked by defective Th1 and Th17 responses (157). In both studies, NLRP10^−/− macrophages and DCs did not reveal any impact on inflammasome-dependent pathways, and thus above described observations might be caused from overexpression (156, 157). Although hereditary mutations in NLRP3, found in CAPS patients, have been shown to affect Th17 polarization in mice (158, 159), and since CAPS itself is a purely innate immune-driven disease, this is thus the first demonstration of a profound effect of an NLRP on adaptive immunity.

NLRP12 (also known as NALP12, PYPAF7, RNO2, PAN6, Monarch-1, CLR19.3) (Figure 1A) associates with ASC to form an inflammasome and to promote NF-κB activation, when overexpressed (160). It also enhances expression of the non-classical and classical MHC Class I genes (161). However, NLRP12 also antagonizes signals originating from TLRs and TNF receptor superfamily members upstream of IκBα within the canonical NF-κB signaling pathway by binding to IRAK-1 via its NACHT domain (162) and the non-canonical NF-κB signaling pathway by binding to NIK and inducing its proteasomal degradation (163). Like several other NLRPs, also NLRP12 is an ATPase, and ATP binding/hydrolysis is critical for its function (27). Similar to NLRP3, the interaction of NLRP12 with HSP90 is also important for its stability (164). NLRP12^−/− mice recently revealed immune defects. NLRP12 is predominately expressed in DCs and neutrophils, and mice lacking NLRP12 display less severe inflammation in models of contact hypersensitivity (165). However, in contrast to in vitro studies, this effect was independent of inflammasome activation and antigen presentation and did not alter inflammatory cytokine levels (165). Similar to NLRP10^−/− mice, NLRP12^−/− mice also display defects in the migration of peripheral DCs and neutrophils to draining lymph nodes due to an impaired chemokine response (Figure 5) (165). In agreement with in vitro data showing that NLRP12 antagonizes NF-κB signaling pathways, NLRP12^−/− mice were more susceptible to intestinal inflammation, colitis and the associated colorectal tumorigenesis, due to a failure to resolve pro-inflammatory non-canonical NF-κB, ERK, and AKT signaling, which resulted in elevated levels of pro-inflammatory cytokines and chemokines (Figure 5). Overall, this suggests an important role for NLRP12 in maintaining intestinal homeostasis (166, 167). Although these functions are uncoupled from inflammasome activation, NLRP12 and NLRP3 inflammasomes do contribute to the host defense against Yersinia pestis through IL-18 and subsequent IFN-γ production. Surprisingly, NF-κB activation was not affected in this study (168). Thus, dependent on the context and cell type, NLRP12 either promotes or antagonizes immune and inflammatory responses, which has also been observed for several other NLRPs.

AIM2 or PYHIN4 was initially identified in a human malignant melanoma cell line, where the absence of AIM2 caused increased cell growth and has subsequently been mostly studied in the context of cancer (173). However, a connection between AIM2 and innate immune responses was made when AIM2 was found to recruit ASC to form an inflammasome (Figure 7) (170, 171, 174, 175). The DNA binding capability of the OB-fold within the HIN-200 domain of AIM2 (20) was confirmed to recognize synthetic cytoplasmic dsDNA as well as dsDNA from various pathogens including Francisella tularensis (174, 176–178), L. monocytogenes (178), Vaccinia virus (170, 174, 178), and MCMV (178), but not DNA from herpes simplex virus type I (HSV-1) and S. typhimurium (178). Reminiscent to NLRs, AIM2 activation relies on promoting clustering of ASC and consequently, also caspase-1, but in the case of AIM2, requires the presence of dsDNA (174). Structural analyses proposed that simultaneous binding of multiple AIM2 molecules through electrostatic interaction to the sugar-phosphate backbone of the DNA helix may facilitate the assembly of AIM2 inflammasomes along the DNA staircase (21). In vivo experiments also confirmed the importance of AIM2 in host defense, since AIM2^−/− mice are unable to limit F. tularensis replication, similar to caspase-1^−/− mice, and thereby failed to control F. tularensis infections (176, 177). AIM2 was also crucial for innate immune responses to MCMV in vivo, since the serum levels of IL-18 and the linked production of IFN-γ by NK cells was significantly reduced in the absence of AIM2, which, however, caused an increased splenic virus titer (178). Interestingly, even though cytosolic DNA and some cytosolic bacteria such as F. tularensis and L. monocytogenes induce an IFN-β response and AIM2 expression is induced by type I IFN, IFN-β signaling is still intact in AIM2^−/− macrophages, where it is even enhanced (176, 177, 179, 180). Moreover, type I IFN priming is essential for AIM2-dependent activation of caspase-1, inflammasome-mediated cell death and the release of IL-1β and IL-18 (176, 179). The HIN-200 protein, p202, negatively regulates AIM2 through competition for DNA binding in mice, but due to lacking a PYD, it cannot form an inflammasome (152), but since this protein does not exist in human, alternative regulatory mechanisms may exist. The anti-microbial cathelicidin peptide LL-37 can compete with AIM2 for DNA binding in psoriatic lesions (181).

IFI16 or PYHIN2 was the first human IFN-inducible PYHIN protein identified in myeloid cells (182). Of the three IFI16 isoforms (A, B, and C), the B form is most abundantly expressed (183). IFI16 is also able to bind and recognize DNA to promote transcriptional regulation of genes involved in innate immunity, including type I IFN. Cytosolic DNA recognition promotes recruitment of STING to IFI16 and subsequent NF-κB and TBK-1-dependent IRF-3 activation (Figure 7) (21, 35). Besides this transcriptional response, IFI16 also recruits ASC to form an inflammasome upon recognition of latent viral DNA in the nucleus (34, 172), as well as in the cytoplasm (35, 184) (Figure 7). Curiously, in the steady state, IFI16 localizes mostly to the nucleus, but IFI16 is able to efficiently launch an immune response in the presence of both, nuclear and cytoplasmic DNA. The subcellular localization of IFI16 might determine its function as an IFN-β inducer in the cytoplasm, or an inflammasome-activating PRR in the nucleus. Thus, the immune response following DNA exposure may depend on the cellular or tissue micro-environment, since the function of IFI16 can shift from a transcriptional activator leading to IFN expression to a PRR that causes caspase-1 dependent IL-1β and IL-18 processing in inflammasomes (185). Moreover, one could predict the existence of a regulatory mechanism that restrains IFI16 and AIM2 inflammasome activation in the cytosol upon contact with self-DNA during cell division, since during this process nucleic acids are temporarily exposed to the cytoplasm.

Auto-inflammatory diseases are generally characterized by recurrent episodes of inflammation and fever in spite of lack of an apparent stimulus and involvement of autoantibodies and autoreactive T cells, causing widespread systemic inflammation which affects multiple tissues and organs (186).

Although inflammasome activation is closely linked to innate immune responses, there is now increasing evidence for a role of inflammasomes in adaptive immunity. Although, IL-1β and IL-18 are prototypical cytokines produced by innate immune cells, both are also important for maintaining the Th1-Th17 vs. Th2 balance. Thus, inflammasomes play a role in initiating inflammatory events, but also in the perpetuation of autoimmune diseases characterized by a defect in the T-cell balance.