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. Author manuscript; available in PMC: 2018 Apr 11.
Published in final edited form as: Curr Opin Immunol. 2016 Nov 11;44:7–13. doi: 10.1016/j.coi.2016.10.007

Alternative inflammasome activation enables IL-1β release from living cells

Moritz M Gaidt 1, Veit Hornung 1
PMCID: PMC5894802  EMSID: EMS77038  PMID: 27842238

Abstract

Classical modes of NLRP3 activation entail a priming step that enables its activation (signal 1) and a potassium efflux-dependent activation signal (signal 2) that triggers pyroptosome formation and pyroptosis, a lytic cell death necessary for IL-1β release. Opposing to that, human monocytes engage an alternative NLRP3 inflammasome pathway in response to LPS that proceeds in the absence of signal 2 activation and enables IL-1β secretion without pyroptosis. This specifically relies on Caspase-8 to propagate signaling to NLRP3, leading to inflammasome activation in absence of pyroptosome formation. Here, we summarize the current knowledge about alternative inflammasome activation, discuss potential extensions of this signaling entity beyond LPS-dependent activation, speculate about its role in tissue homeostasis and sterile inflammation and highlight the implications of pyroptosis-independent IL-1β secretion.

Introduction

The pleotropic cytokine IL-1β serves instrumental functions in initiation and orchestration of innate and adaptive immune responses. Its receptor IL-1R shares the intracellular TIR (Toll IL-1 receptor) signaling-domain with the well-characterized PRR-family of TLRs, thereby enabling TLR-like transcriptional responses in cells not expressing TLRs. This homology in signaling explains why the biological effects of IL-1β closely resemble activation of the innate immune system. IL-1β is a cytokine of numerous functions, exerting diverse effects on multiple cells types to culminate in broadly inflammatory events. Systemic consequences of IL-1β signaling include fever, vasodilatation, hypotension and acute phase response. Locally, IL-1 signaling enables mesenchymal and endothelial cells to express adhesion molecules facilitating recruitment of lymphocytes and myeloid cells. Consequently, the infiltrating immune cells are activated and triggered to further amplify inflammation. Besides its crucial roles in the induction and orchestration of innate immune responses, IL-1β has been implicated in adaptive responses by influencing Th1 and Th17-mediated immune activation. Aberrant IL-1β signaling has been associated with rare heritable monogenetic auto-inflammatory diseases like Cryopyrin-Associated Periodic Syndromes (CAPS) or Familial Mediterranean Fever (FMF), yet also with common multifactorial diseases, such as Type 2 diabetes or gout [1–3].

IL-1β belongs to the family of IL-1 cytokines, of which several members, including IL-1β and IL-18, are synthesized as inactive precursors within the cytosol, where they await proteolytic cleavage to gain bioactivity. This maturation step is most commonly executed by the cysteine protease Caspase-1, whose activity is controlled by a cytosolic activation platform called the inflammasome (Figure 1). At the same time, other enzymes, such as Caspase-8 or neutrophil proteases have been described to mature IL-1β. Inflammasome signaling entails a sensor molecule (e.g. NLRP1, NLRP3, NLRC4 or AIM2) that seeds the assembly of the adapter molecule ASC into helically structured filaments forming a high-molecular weight complex that is called the pyroptosome [4]. In turn, Caspase-1 becomes recruited to the pyroptosome to also form helical structures, which enable its proximity-induced proteolytical auto-activation. Once Caspase-1 has become matured into the active p102:p202 hetero-tetramer it is licensed to cleave the cytokine-precursors pro-IL-1β/pro-IL-18. Besides cytokine maturation, induction of a lytic type of cell death called pyroptosis constitutes a hallmark of inflammasome activation. This programmed cell death is mediated by Caspase-1-dependent cleavage of the pyroptosis-effector molecule GSDMD (Gasdermin-D). Caspase-1 maturation of GSDMD abolishes the intra-molecular auto-inhibition of the C-terminal domain [5••,6••,7••]. Subsequently, phospholipid binding specificities of the matured N-terminal fragment of GSDMD enable its translocation to the inner leaflet of the plasma membrane, where it forms round, pore-like structures of approximately 15 nm in diameter [8,9,10•,11]. Consequently, breakdown of ion gradients over the plasma membrane leads to water influx driven by oncotic pressure, inducing the characteristic cell swelling and rupture of the plasma membrane. Although soluble cytosolic proteins are released during pyroptotic lysis, it has been shown that so called pore-induced intracellular traps (PITs) maintain larger fragments of the pyroptosed cell (including previously phagocytosed bacteria) within the cellular debris [12]. The previously noted correlation of pyroptosis and IL-1β release [13,14] has been genetically supported by the defect in IL-1β secretion but not in maturation in murine macrophages deficient for GSDMD [5••,6••,7••]. To this end, the picture emerges that IL-1β secretion critically depends on GSDMD in the course of classical inflammasome pathways, either directly through the nascent pore or in the course of plasma membrane disintegration.

Figure 1. The alternative inflammasome comprises a signaling entity distinct from classical inflammasome activation.

Figure 1

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Classical activation of the NLRP3 inflammasome involves a two-step activation model (left side of figure): a PRR-derived signal (signal 1) is required to license NLRP3 activation via upregulation of its expression at the transcriptional level and/or its posttranslational modification. NLRP3 can now detect perturbations of cellular integrity by recognizing potassium efflux (signal 2). Consequently, Nek7–NLRP3 interaction leads to pyroptosome assembly and Caspase-1 activation to mature IL-1β and GSDMD. Pore formation of GSDMD induces pyroptosis, which is critically required for IL-1β secretion. Opposing to that alternative inflammasome activation can be triggered by one signal only, for example, LPS or an iNKT cell derived signal (right side of figure): LPS sensing induces a TLR4-TRIF-RIPK1-FADD-CASP8 signaling axis, resulting in activation of NLRP3 by cleavage of an unknown Caspase-8 substrate independently of potassium efflux. The alternative NLRP3 activation complex is different from the pyroptosome and possibly displays stoichiometric composition. Although Caspase-1 becomes matured and cleaves IL-1β, pyroptosis is not induced and IL-1β is secreted by an unconventional mechanism that functions independently of GSDMD.

The NLRP3 inflammasome

NLRP3 is the most studied inflammasome sensor that has attracted much attention due to its role in anti-microbial defense, but also in sterile inflammatory conditions. Initially identified by its association with monogenetic, IL-1-mediated auto-inflammatory diseases (CAPS) [15], it has also been shown to play a central role in multi-factorial, widespread diseases like Type 2 diabetes, Alzheimer's disease, gout and atherosclerosis. A recently identified pharmacological inhibitor of NLRP3 [16] has proven its efficacy in several murine inflammation models, including exacerbated inflammation caused by influenza infection and myocardial infarction [17–19]. Opposing to other inflammasome sensors, whose ligands have been identified (e.g. AIM2 or NLRC4/NAIP), the molecular mechanism of NLRP3 activation remains elusive. The current dogma proposes a two-step activation model, in which a first signal mediates transcriptional and post-translational priming of NLRP3. Only after signal 1-induced licensing, murine macrophages become sensitive to signal 2-dependent inflammasome activation. The plethora and diverse physiochemical nature of signal 2 agonists of NLRP3 reason against direct activation of NLRP3 by any of these components. Instead, efflux of potassium along its chemical gradient across the plasma membrane has been identified as the common denominator of most known NLRP3 signal 2 agonists [20,21]. This establishes NLRP3 as a sensor of plasma membrane integrity, whose perturbation results in inflammasome activation, thereby alerting the immune system. The molecular mechanisms of potassium efflux being sensed by NLRP3 have not been understood completely, yet recent reports have implicated the cell cycle kinase Nek7 to exert a kinase-independent function in NLRP3 activation [22–24]. As such it has been proposed that potassium efflux enables NLRP3-Nek7 interaction to drive inflammasome activation [23]. Apart from that, both mitochondrial and phagosomal ROS production have been proposed to be causally connected to NLRP3 activation. Since the relationship to potassium efflux has not always been investigated in these reports, the relevance of these models remains to be determined. Recently, the other pro-inflammatory Caspases, Caspase-4, Caspase-5 and Caspase-11, have been implicated in the so called non-canonical pathway of NLRP3-inflammasome activation [25,26]. In fact, it has been demonstrated that recognition of cytosolic LPS by the CARD of respective Caspases leads to their activation [27]. Although these Caspases are capable of maturing GSDMD to induce pyroptosis, they are not able to directly mature IL-1β. Instead, pyroptosis-linked efflux of potassium has been found to drive non-canoncial NLRP3 signaling to activate Caspase-1 [28–30], thereby enabling cytokine maturation.

LPS triggers alternative inflammasome activation in human monocytes in the absence of pyroptosis

In the course of inflammasome activation, the induction of pyroptosis and IL-1β secretion is strictly correlated and the requirement of lytic cell death for IL-1β secretion is well supported by genetic means [5••,6••,7••]. Apart from that, the proposed two-step model of NLRP3 activation has mainly been inferred from studies in murine macrophages that critically depend on signal 2 agonists for NLRP3 signaling. To this effect, not much attention has been paid to the fact that human monocytes can secrete mature IL-1β solely in response to LPS without the requirement of classical signal 2 activation, a feature that had been reported early after the discovery of IL-1 cytokines. We have recently characterized this response as a unique NLRP3 signaling entity that substantially differs from signal 2-mediated activation. To clearly discriminate this signaling modality from ‘classical inflammasome activation’ (canonical + non-canonical), we have coined the term ‘alternative inflammasome activation’ to define this signaling entity as such [31••]. The major differences discriminating both pathways are the absence of pyroptosis, the absence of pyroptosome formation and the independency of potassium efflux as an upstream activation mechanism in the course of alternative inflammasome activation. Instead, signaling relies on TLR4-TRIF-RIPK1-FADD-CASP8 to propagate activation to NLRP3, while the exact molecular mechanism of Caspase-8 activating NLRP3 remains unknown. In concordance with the absence of pyroptosis, the release of IL-1β during alternative inflammasome activation does not require the pyroptosis effector GSDMD (own unpublished observations). Of note, LPS-mediated alternative inflammasome activation constitutes a species-specific response that is absent from murine but present in human and porcine PBMCs [31••]. Interestingly, Caspase-8 mediated alternative inflammasome activation in human monocytes can also be induced in vitro by co-culture with iNKT-cells utilizing an unknown upstream signaling pathway [32•]. It is tempting to speculate that a membrane-borne TNFR-family ligand mediates activation is this setting, leading to TRIF-independent Caspase-8 activation to drive the alternative inflammasome. Importantly, the role for Caspase-8 upstream of NLRP3 during alternative inflammasome activation is distinct from roles for Caspase-8 in classical inflammasome activation and IL-1β secretion. To this end, Caspase-8 exerts ill-defined roles in the NFκB-signaling-pathway [33,34] and thus critically contributes to signal 1-mediated inflammasome priming [35–37]. Furthermore, in absence of Caspase-1, recruitment of Caspase-8 to the pyroptosome can result in apoptotic cell death and IL-1β maturation, a reaction that involves heterotypic PYD-DED interaction [38,39]. Moreover, Caspase-8 can also function as a bona fide IL-1β maturing enzyme, whose activation in multiple (pro-apoptotic) circumstances leads to release of mature IL-1β [40]. Lastly, NLRP3 can be activated in the course of Caspase-8-mediated cell death induced by IAP depletion [41]. Mechanistically ill defined, it remains to be determined if classical or alternative inflammasome is at play, however the absence of pro-apoptotic activation of Caspase-8 in the course of alternative inflammasome activation might argue that classical NLRP3 activation in response to perturbed cellular integrity is at play during IAP depletion. In summary the upstream engagement of Caspase-8 during alternative inflammasome activation is different from reported role for Caspase-8 in IL-1β secretion.

Broadening the concept of alternative inflammasome signaling

Notably, two additional studies have characterized NLRP3 activation patterns with striking similarities to alternative inflammasome activation, as observed in LPS-stimulated monocytes [42••,43••] (Table 1). The first study investigated the role of the known immune-modulator oxPAPC, an endogenous oxidized membrane lipid, in inflammasome activation. In TLR-primed murine dendritic cells (DCs) oxPAPC is sufficient to activate a Caspase-11-NLRP3 inflammasome pathway that drives IL-1β secretion. For this, oxPAPC binds to the catalytic domain of Pro-Caspase-11, which does not lead to its auto-processing not even does it require its catalytic activity, yet it results in IL-1β secretion. Of note, oxPAPC also binds to Caspase-1, indicating that the signaling epistasis has to be further clarified. Although its genetic association with Caspase-11 theoretically marks this pathway as non-canonical, the fact that the catalytic activity of Caspase-11 is not required for the former clearly discriminates the two. Indeed, oxPAPC-stimulated DCs activate the NLRP3 inflammasome independently of potassium efflux and independently of pytoptosis. Nevertheless, this pathway involves pyroptosome formation [42••]. The second report investigates the relationship of proximal metabolic perturbation of glycolysis and subsequent inflammasome activation. Inhibition of hexokinase and subsequent translocation into the cytoplasm is sufficient to activate the NLRP3 inflammasome. The phenotypic characteristics linking this pathway to alternative inflammasome activation include independence of potassium efflux, absence of pyroptosome formation and absence of pyroptosis. However, at this point it is not clear if the detachment of hexokinase from the mitochondria or its presence in the cytosol triggers inflammasome activation and in addition, insights into the molecular mechanism of NLRP3 activation are missing [43••].

Table 1. Key characteristics of classical and alternative inflammasome activation.

Type of inflammasome activation Classical (canoncial + non-canoncial) Alternative Presumably alternative
Description Multiple groups Gaidt et al. [31••] Wolf et al. [43••] Zanoni et al. [42••]
NLRP3-ASC-Caspase 1 signaling Yes Yes Yes Yes
Caspase-1, IL-1β maturation and secretion Yes Yes Yes Yes
Potassium efflux requirement Yes No No No
Pyroptososme formation Yes No No Yes
Pyroptosis Yes No No No
Upstream mechanism Nek7 Caspase-8 Hexokinase detachment Pro-Caspase-11
GSDMD requirement Yes No Not known Not known
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Although the upstream signaling pathways of LPS, oxPAPC and hexokinase detachment-mediated NLRP3 activation significantly differ, distal signaling events display striking similarities. Since the molecular details of the respective activation pathways are not known, it remains speculative whether all three pathways converge on a presumably common platform of alternative inflammasome activation. In this scenario, multiple, at least three, different upstream signaling events are able to activate the alternative NLRP3 inflammasome to induce IL-1β secretion without the requirement for potassium efflux and pyroptosis. Addressing the role of Caspase-8 in metabolic disturbance-mediated inflammasome activation and the translocation of hexokinase in LPS-induced inflammasome activation may help clarify convergence or divergence of upstream signaling cascades. To this effect, it is noteworthy that both Caspase-8 and hexokinase detachment have been associated with apoptosis [44,45]. Since no apoptotic signaling cascade has been observed in LPS-stimulated monocytes, sub-apoptotic activation may comprise a common trigger of the alternative inflammasome.

In summary, the alternative inflammasome comprises a distinct signaling entity that engages the hardwiring of the classical NLRP3 inflammasome pathway to mature and secrete IL-1β, yet does not engage pyroptosis. As outlined above, the upstream activation pathways of alternative inflammasome signaling are likely to be diverse, while phenotypic criteria are not stringent enough to define alternative inflammasome signaling as such. Therefore, we would currently consider GSDMD-independent, inflammasome-mediated IL-1β secretion in pyroptosis-competent cells as the most appropriate definition for alternative inflammasome activation. Nonetheless, identification of additional molecules that specifically regulate alternative inflammasome activation may lead to better, non-exclusion-based genetic criteria. This could involve both upstream regulators that are required for Caspase-8 to activate NLRP3 or downstream mediators involved in GSDMD-independent IL-1β release.

Implications of alternative inflammasome activation

Ever since its discovery, the unconventional mode of secretion of IL-1β has been subject of intense research efforts. The notion that IL-1β is synthesized as an inactive precursor in the cytosol had already implied Golgi-independent secretion mechanisms. With the discovery of GSDMD it became clear that passive release either directly through the GSDMD pore or in the course of plasma membrane rupture constitutes the main mechanism of unconventional secretion of IL-1β during classical inflammasome activation. However, the absence of pyroptosis from alternative inflammasome activation argues for another, GSDMD-independent, active secretion pathway for IL-1β. Several mechanisms have already been suggested to explain the unconventional secretion of IL-1β, including autophagy-mediated engulfment into vesicles and subsequent exocytosis as well as shedding of mature IL-1β in vesicles from the plasma membrane [46]. Although some of these models have been proposed in the course of studying LPS-stimulated monocytes (and thus alternative inflammasome activation), others arose from characterizing classical inflammasome activation and thus have been falsified by our current knowledge of GSDMD biology. Most appealingly, autophagy-assisted entry into the exocytotic pathway may constitute a key mechanistic feature of IL-1β secretion, as suggested from studies in LPS-stimulated monocytes [47] and pyroptosis-incompetent non-myeloid cells [48•].

Interestingly, the uncoupling of inflammasome activation and pyroptosis has also been reported in neutrophils. Indeed, this cell type appears to harbor a general pyroptosis defect in response to AIM2 and NLRC4 activation, despite normal IL-1β secretion [49]. The protection from pyroptosis in the course of the alternative inflammasome is likely to differ both mechanistically and conceptually. In neutrophils a broad defect in the whole pyroptotic pathway may be due to perturbed GSDMD expression or function. However, in the course of alternative inflammasome activation, the precise omission of GSDMD-dependent pyroptosis alludes to the existence of a specific signaling event inhibiting or not inducing pyroptosis. It has been reasoned before that disabled neutrophilic pyroptosis may be beneficial to the host since most intracellular bacteria are not replication competent within neutrophils [50] and ‘premature’ pyroptosis may prevent neutrophilic anti-pathogenic functions. Opposing to that, the alternative inflammasome pathway is operational in potentially long-lived myeloid cells of the immune system, such as macrophages and dendritic cells. These cells may engage the alternative inflammasome in order to omit pyroptosis but to enable induction of IL-1β dependent immune responses in circumstances where pathogen sensing and/or effector functions require temporal and or spatial separation. Indeed, alternative inflammasome activation may be instrumental in both antigen transport to secondary lymphoid organs and tissue homeostasis by resident myeloid cells. Notably, oxPAPC has already proven its efficacy in a vaccination setting [42••]. With regards to tissue homeostasis, it has to be noted the alternative inflammasome has mainly been studied in peripheral blood monocytes [31••,32•]. However, LPS-mediated IL-1β secretion and thus alternative inflammasome activation has also been documented in human tissue macrophages [51,52], the immune cell type most instrumental for tissue homeostasis. Indeed, it is conceivable that alternative inflammasome activation is a major mediator of homeostatic, sterile inflammation. As such, activation of TLR4 and NLRP3 by endogenous sterile ligands is believed to play an important role in various sterile disease conditions. In this model TLR4-ligating DAMPs like HMGB1 [53], HSPs [54], Hyaluronan [55], Biglycan [56], Heparan sulphate [57] or S100-proteins [58] are thought to provide signal 1-mediated inflammasome priming to enable responsiveness to DAMPs such as ATP, uric acid or cholesterol crystals that activate the classical NLRP3 inflammasome via signal 2 [59]. With the epistatic relationship of TLR4 functioning directly upstream of NLRP3 in the alternative inflammasome pathway, DAMPs previously considered to provide only priming signals should be sufficient to drive alternative inflammasome activation. Considering that TLR-4-mediated alternative inflammasome activation constitutes a species-specific response, the importance of signal 2-stimuli for IL-1 driven sterile disease conditions should be revisited in appropriate conditions or models like the human or porcine system.

In summary the alternative inflammasome pathway constitutes a distinct signaling entity in the inflammasome repertoire of human monocytes and macrophages. Although mechanistic details of upstream signaling events remain ill-defined, Caspase-8 activation in response to LPS sensing or iNKT stimulation is integrated to activate the alternative NLRP3 inflammasome without the requirement for potassium efflux. Subsequent signaling engages the inflammasome components ASC and Caspase-1 without inducing pyroptosome formation to drive IL-1β secretion in absence of pyroptosis. This may enable spatio-temporal separation of sensing and effector function and may be instrumental in tissue homeostasis and sterile inflammation.

Acknowledgements

Funding: This work was supported by the European Research Council (ERC-2014-CoG - 647858 – GENESIS) and the German Research Foundation (SFB 704).

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

• of special interest

•• of outstanding interest

  • 1.Dinarello CA. Immunological and inflammatory functions of the interleukin-1 family. Annu Rev Immunol. 2009;27:519–550. doi: 10.1146/annurev.immunol.021908.132612. [DOI] [PubMed] [Google Scholar]
  • 2.Dinarello CA. A clinical perspective of IL-1b as the gatekeeper of inflammation. Eur J Immunol. 2011;41:1203–1217. doi: 10.1002/eji.201141550. [DOI] [PubMed] [Google Scholar]
  • 3.Garlanda C, Dinarello CA, Mantovani A. The Interleukin-1 Family: Back to the Future. Immunity. 2013;39:1003–1018. doi: 10.1016/j.immuni.2013.11.010. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 4.Fernandes-Alnemri T, Wu J, Yu J-W, Datta P, Miller B, Jankowski W, Rosenberg S, Zhang J, Alnemri ES. The pyroptosome: a supramolecular assembly of ASC dimers mediating inflammatory cell death via caspase-1 activation. Cell Death Differ. 2007;14:1590–1604. doi: 10.1038/sj.cdd.4402194. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 5.Shi 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;526:660–665. doi: 10.1038/nature15514. [•• This is one of three reports identifying a crucial role for GSDMD in pyroptosis regulated by proteolytic cleavage by inflammatory caspases.] [DOI] [PubMed] [Google Scholar]
  • 6.Kayagaki N, Stowe IB, Lee BL, O’Rourke K, Anderson K, Warming S, Cuellar T, Haley B, Roose-Girma M, Phung QT, et al. Caspase-11 cleaves gasdermin D for non-canonical inflammasome signaling. Nature. 2015;526:666–671. doi: 10.1038/nature15541. [•• This is one of three reports identifying a crucial role for GSDMD in pyroptosis regulated by proteolytic cleavage by inflammatory caspases.] [DOI] [PubMed] [Google Scholar]
  • 7.He W, Wan H, Hu L, Chen P, Wang X, Huang Z, Yang Z-H, Zhong C-Q, Han J. Gasdermin D is an executor of pyroptosis and required for interleukin-1β secretion. Cell Res. 2015;25:1285–1298. doi: 10.1038/cr.2015.139. [•• This is one of three reports identifying a crucial role for GSDMD in pyroptosis regulated by proteolytic cleavage by inflammatory caspases.] [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 8.Sborgi L, Rühl S, Mulvihill E, Pipercevic J, Heilig R. GSDMD pore formation in the plasma membrane constitutes the mechanism of pyroptotic cell death. EMBO J. 2016 doi: 10.15252/embj.201694696. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 9.Aglietti RA, Estevez A, Gupta A, Ramirez MG, Liu PS, Kayagaki N, Ciferri C, Dixit VM, Dueber EC. GsdmD p30 elicited by caspase-11 during pyroptosis forms pores in membranes. Proc Natl Acad Sci. 2016;113:7858–7863. doi: 10.1073/pnas.1607769113. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 10.Ding J, Wang K, Liu W, She Y, Sun Q, Shi J, Sun H, Wang D-C, Shao F. Pore-forming activity and structural autoinhibition of the gasdermin family. Nature. 2016;535:111–116. doi: 10.1038/nature18590. [• This paper identifies pore formation by GSDMD as the effector mechanism of pyroptosis.] [DOI] [PubMed] [Google Scholar]
  • 11.Liu X, Zhang Z, Ruan J, Pan Y, Magupalli VG, Wu H, Lieberman J. Inflammasome-activated gasdermin D causes pyroptosis by forming membrane pores. Nature. 2016;535:153–158. doi: 10.1038/nature18629. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 12.Jorgensen I, Zhang Y, Krantz BA, Miao EA. Pyroptosis triggers pore-induced intracellular traps (PITs) that capture bacteria and lead to their clearance by efferocytosis. J Exp Med. 2016 doi: 10.1084/jem.20151613. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 13.Martín-sánchez F, Diamond C, Zeitler M, Gomez-sanchez A, Peñalver M, Paszek P, Steringer JP, Nickel W. Inflammasome-dependent IL-1 β release depends upon membrane permeabilisation. Cell Death Differ. 2016 doi: 10.1038/cdd.2015.176. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 14.Liu T, Yamaguchi Y, Shirasaki Y, Shikada K, Yamagishi M, Hoshino K, Kaisho T, Takemoto K, Suzuki T, Kuranaga E, et al. Single-Cell Imaging of Caspase-1 Dynamics Reveals an All-or-None Inflammasome Signaling Response. Cell Rep. 2013;8:974–982. doi: 10.1016/j.celrep.2014.07.012. [DOI] [PubMed] [Google Scholar]
  • 15.Hoffman HM, Mueller JL, Broide DH, Wanderer AA, Kolodner RD. Mutation of a new gene encoding a putative pyrin-like protein causes familial cold autoinflammatory syndrome and Muckle-Wells syndrome. Nat Genet. 2001;29:301–5. doi: 10.1038/ng756. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 16.Coll RC, Robertson AAB, Chae JJ, Higgins SC, Muñoz-Planillo R, Inserra MC, Vetter I, Dungan LS, Monks BG, Stutz A, et al. A small-molecule inhibitor of the NLRP3 inflammasome for the treatment of inflammatory diseases. Nat Med. 2015;21:248–255. doi: 10.1038/nm.3806. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 17.van Hout GPJ, Bosch L, Ellenbroek GHJM, de Haan JJ, van Solinge WW, Cooper MA, Arslan F, de Jager SCA, Robertson AAB, Pasterkamp G, et al. The selective NLRP3-inflammasome inhibitor MCC950 reduces infarct size and preserves cardiac function in a pig model of myocardial infarction. Eur Heart J. 2016 doi: 10.1093/eurheartj/ehw247. [DOI] [PubMed] [Google Scholar]
  • 18.Primiano MJ, Lefker BA, Bowman MR, Bree AG, Hubeau C, Bonin PD, Mangan M, Dower K, Monks BG, Cushing L, et al. Efficacy and Pharmacology of the NLRP3 Inflammasome Inhibitor CP-456,773 (CRID3) in Murine Models of Dermal and Pulmonary Inflammation. J Immunol. 2016;773 doi: 10.4049/jimmunol.1600035. [DOI] [PubMed] [Google Scholar]
  • 19.Tate MD, Ong JDH, Dowling JK, McAuley JL, Robertson AB, Latz E, Drummond GR, Cooper MA, Hertzog PJ, Mansell A. Reassessing the role of the NLRP3 inflammasome during pathogenic influenza A virus infection via temporal inhibition. Sci Rep. 2016;6:27912. doi: 10.1038/srep27912. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 20.Pétrilli V, Papin S, Dostert C, Mayor a, Martinon F, Tschopp J. Activation of the NALP3 inflammasome is triggered by low intracellular potassium concentration. Cell Death Differ. 2007;14:1583–1589. doi: 10.1038/sj.cdd.4402195. [DOI] [PubMed] [Google Scholar]
  • 21.Muñoz-Planillo R, Kuffa P, Martínez-Colón G, Smith BL, Rajendiran TM, Núñez G. K+ efflux is the common trigger of NLRP3 inflammasome activation by bacterial toxins and particulate matter. Immunity. 2013;38:1142–53. doi: 10.1016/j.immuni.2013.05.016. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 22.Schmid-Burgk JL, Chauhan D, Schmidt T, Ebert TS, Reinhardt J, Endl E, Hornung V. A genome-wide CRISPR screen identifies NEK7 as an essential component of NLRP3 inflammasome activation. J Biol Chem. 2016;291:103–109. doi: 10.1074/jbc.C115.700492. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 23.He Y, Zeng MY, Yang D, Motro B, Núñez G. Nek7 is an essential mediator of NLRP3 oligomerization and inflammasome activation downstream of potassium efflux. Nature. 2016;530:354–357. doi: 10.1038/nature16959. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 24.Shi H, Wang Y, Li X, Zhan X, Tang M, Fina M, Su L, Pratt D, Bu CH, Hildebrand S, et al. NLRP3 activation and mitosis are mutually exclusive events coordinated by NEK7, a new inflammasome component. Nat Immunol. 2015;17:1–12. doi: 10.1038/ni.3333. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 25.Kayagaki N. Noncanonical Inflammasome Activation by Intracellular LPS Independent of TLR4 Nobuhiko Kayagaki. 2014;1246 doi: 10.1126/science.1240248. [DOI] [PubMed] [Google Scholar]
  • 26.Hagar JA, Powell DA, Aachoui Y, Ernst RK, Miao EA. Cytoplasmic LPS activates caspase-11: implications in TLR4-independent endotoxic shock. Science. 2013;341:1250–3. doi: 10.1126/science.1240988. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 27.Shi 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;514:187–192. doi: 10.1038/nature13683. [DOI] [PubMed] [Google Scholar]
  • 28.Schmid-Burgk JL, Gaidt MM, Schmidt T, Ebert TS, Bartok E, Hornung V. Caspase-4 mediates non-canonical activation of the NLRP3 inflammasome in human myeloid cells. Eur J Immunol. 2015;45:2911–2917. doi: 10.1002/eji.201545523. [DOI] [PubMed] [Google Scholar]
  • 29.Rühl S, Broz P. Caspase-11 activates a canonical NLRP3 inflammasome by promoting K + efflux. Eur J Immunol. 2015;45:2927–2936. doi: 10.1002/eji.201545772. [DOI] [PubMed] [Google Scholar]
  • 30.Baker PJ, Boucher D, Bierschenk D, Tebartz C, Whitney PG, D’Silva DB, Tanzer MC, Monteleone M, Robertson AAB, Cooper MA, et al. NLRP3 inflammasome activation downstream of cytoplasmic LPS recognition by both caspase-4 and caspase-5. Eur J Immunol. 2015;45:2918–2926. doi: 10.1002/eji.201545655. [DOI] [PubMed] [Google Scholar]
  • 31.Gaidt MM, Ebert TS, Chauhan D, Schmidt T, Schmid-Burgk JL, Rapino F, Robertson AAB, Cooper MA, Graf T, Hornung V. Human Monocytes Engage an Alternative Inflammasome Pathway. Immunity. 2016;44:833–846. doi: 10.1016/j.immuni.2016.01.012. [•• First characterization of the alternative inflammasome by genetic means and phenotypic description.] [DOI] [PubMed] [Google Scholar]
  • 32.Felley LE, Sharma A, Theisen E, James C, Sauer J, Gumperz JE. Human Invariant NKT Cells Induce IL-1 β Secretion by Peripheral Blood Monocytes via a P2X 7 - Independent Pathway. J Immunol. 2016 doi: 10.4049/jimmunol.1600790. [• Description of alternative inflammasome signaling by sensing iNKT cell derived signals.] [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 33.Lemmers B, Salmena L, Bidere N, Su H, Matysiak-Zablocki E, Murakami K, Ohashi PS, Jurisicova A, Lenardo M, Hakem R, et al. Essential role for caspase-8 in toll-like receptors and NFkB signaling. J Biol Chem. 2007;282:7416–7423. doi: 10.1074/jbc.M606721200. [DOI] [PubMed] [Google Scholar]
  • 34.Su H, Bidère N, Zheng L, Cubre A, Sakai K, Dale J, Salmena L, Hakem R, Straus S, Lenardo M. Requirement for caspase-8 in NF-kappaB activation by antigen receptor. Science. 2005;307:1465–1468. doi: 10.1126/science.1104765. [DOI] [PubMed] [Google Scholar]
  • 35.Gurung P, Anand PK, Malireddi RKS, Vande Walle L, Van Opdenbosch N, Dillon CP, Weinlich R, Green DR, Lamkanfi M, Kanneganti T-D. FADD and Caspase-8 Mediate Priming and Activation of the Canonical and Noncanonical Nlrp3 Inflammasomes. J Immunol. 2014;192:1835–1846. doi: 10.4049/jimmunol.1302839. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 36.Allam R, Lawlor KE, Yu EC-W, Mildenhall AL, Moujalled DM, Lewis RS, Ke F, Mason KD, White MJ, Stacey KJ, et al. Mitochondrial apoptosis is dispensable for NLRP3 inflammasome activation but non-apoptotic caspase-8 is required for inflammasome priming. EMBO Rep. 2014;15:1–9. doi: 10.15252/embr.201438463. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 37.Kang S, Fernandes-Alnemri T, Rogers C, Mayes L, Wang Y, Dillon C, Roback L, Kaiser W, Oberst A, Sagara J, et al. Caspase-8 scaffolding function and MLKL regulate NLRP3 inflammasome activation downstream of TLR3. Nat Commun. 2015;6:7515. doi: 10.1038/ncomms8515. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 38.Man SM, Tourlomousis P, Hopkins L, Monie TP, Fitzgerald KA, Bryant CE. Salmonella infection induces recruitment of Caspase-8 to the inflammasome to modulate IL-1beta production. J Immunol. 2013;191:5239–5246. doi: 10.4049/jimmunol.1301581. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 39.Vajjhala PR, Lu A, Brown DL, Pang SW, Sagulenko V, Sester DP, Cridland SO, Hill JM, Schroder K, Stow JL, et al. The inflammasome adaptor ASC induces procaspase-8 death effector domain filaments. J Biol Chem. 2015;290:29217–29230. doi: 10.1074/jbc.M115.687731. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 40.Maelfait J, Vercammen E, Janssens S, Schotte P, Haegman M, Magez S, Beyaert R. Stimulation of Toll-like receptor 3 and 4 induces interleukin-1beta maturation by caspase-8. J Exp Med. 2008;205:1967–73. doi: 10.1084/jem.20071632. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 41.Lawlor KE, Khan N, Mildenhall A, Gerlic M, Croker Ba, D’Cruz Aa, Hall C, Kaur Spall S, Anderton H, Masters SL, et al. RIPK3 promotes cell death and NLRP3 inflammasome activation in the absence of MLKL. Nat Commun. 2015;6:6282. doi: 10.1038/ncomms7282. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 42.Zanoni I, Tan Y, Di Gioia M, Broggi A, Ruan J, Shi J, Donado CA, Shao F, Wu H, Springstead JR, et al. An endogenous caspase-11 ligand elicits interleukin-1 release from living dendritic cells. Science. 2016;352:1232–1236. doi: 10.1126/science.aaf3036. [•• Examines an oxPAPC-induced inflammamasome pathway with similar features compared to alternative inflammasome activation.] [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 43.Wolf AJ, Reyes CN, Liang W, Becker C, Shimada K, Wheeler ML, Cho HC, Popescu NI, Coggeshall KM, Arditi M, et al. Hexokinase Is an Innate Immune Receptor for the Detection of Bacterial Peptidoglycan. Cell. 2016;116:1–13. doi: 10.1016/j.cell.2016.05.076. [•• This paper reports that metabolic perturbations leading to hexokinase translocation into the cytosol trigger inflammasome activation resembling alternative inflammasome activation.] [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 44.Majewski N, Nogueira V, Bhaskar P, Coy PE, Skeen JE, Gottlob K, Chandel NS, Thompson CB, Robey RB, Hay N. Hexokinase-mitochondria interaction mediated by Akt is required to inhibit apoptosis in the presence or absence of Bax and Bak. Mol Cell. 2004;16:819–830. doi: 10.1016/j.molcel.2004.11.014. [DOI] [PubMed] [Google Scholar]
  • 45.Kruidering M, Evan GI. Caspase-8 in apoptosis: the beginning of “the end”? IUBMB Life. 2000;50:85–90. doi: 10.1080/713803693. [DOI] [PubMed] [Google Scholar]
  • 46.Lopez-Castejon G, Brough D. Understanding the mechanism of IL-1β secretion. Cytokine Growth Factor Rev. 2011;22:189–195. doi: 10.1016/j.cytogfr.2011.10.001. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 47.Rubartelli A, Cozzolino F, Talio M, Sitia R. A novel secretory pathway for interleukin-1 beta, a protein lacking a signal sequence. EMBO J. 1990;9:1503–10. doi: 10.1002/j.1460-2075.1990.tb08268.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 48.Zhang M, Sam K, Ge L, Xu K, Schekman R. Translocation of interleukin-1β into a vesicle intermediate in autophagy-mediated secretion. Elife. 2015 doi: 10.7554/eLife.11205. in press. [• Elegant demonstration that unconventional IL-1β secretion requires autophagy-assisted entry into the exocytotic pathway upon Caspase-1 activation in pyroptosis incompetent cells.] [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 49.Chen KW, Groß CJ, Sotomayor F, Stacey KJ, Tschopp J, Sweet MJ, Schroder K. The Neutrophil NLRC4 Inflammasome Selectively Promotes IL-1β Maturation without Pyroptosis during Acute Salmonella Challenge. Cell Rep. 2014;8:570–582. doi: 10.1016/j.celrep.2014.06.028. [DOI] [PubMed] [Google Scholar]
  • 50.Miao E, Rajan J, Aderem A. Caspase-1 induced pyroptotic cell death. Immunol Rev. 2013;243:206–214. doi: 10.1111/j.1600-065X.2011.01044.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 51.Pioli PA, Weaver LK, Schaefer TM, Wright Ja, Wira CR, Guyre PM. Lipopolysaccharide-induced IL-1 beta production by human uterine macrophages up-regulates uterine epithelial cell expression of human beta-defensin 2. J Immunol. 2006;176:6647–6655. doi: 10.4049/jimmunol.176.11.6647. [DOI] [PubMed] [Google Scholar]
  • 52.Wewers MD, Herzyk DJ. Alveolar macrophages differ from blood monocytes in human IL-1 beta release. Quantitation by enzyme-linked immunoassay. J Immunol. 1989;143:1635–1641. [PubMed] [Google Scholar]
  • 53.Yu M, Wang H, Ding A, Golenbock DT, Latz E, Czura CJ, Fenton MJ, Tracey KJ, Yang H. Hmgb1 Signals Through Toll-Like Receptor (Tlr) 4 and Tlr2. Shock. 2006;26:174–179. doi: 10.1097/01.shk.0000225404.51320.82. [DOI] [PubMed] [Google Scholar]
  • 54.Vabulas RM, Ahmad-Nejad P, Da Costa C, Miethke T, Kirschning CJ, Häcker H, Wagner H. Endocytosed HSP60s Use Toll-like Receptor 2 (TLR2) and TLR4 to Activate the Toll/Interleukin-1 Receptor Signaling Pathway in Innate Immune Cells. J Biol Chem. 2001;276:31332–31339. doi: 10.1074/jbc.M103217200. [DOI] [PubMed] [Google Scholar]
  • 55.Jiang D, Liang J, Fan J, Yu S, Chen S, Luo Y, Prestwich GD, Mascarenhas MM, Garg HG, Quinn DA, et al. Regulation of lung injury and repair by Toll-like receptors and hyaluronan. Nat Med. 2005;11:1173–1179. doi: 10.1038/nm1315. [DOI] [PubMed] [Google Scholar]
  • 56.Schaefer L, Babelova A, Kiss E, Hausser HJ, Baliova M, Krzyzankova M, Marsche G, Young MF, Mihalik D, Götte M, et al. The matrix component biglycan is proinflammatory and signals through Toll-like receptors 4 and 2 in macrophages. J Clin Invest. 2005;115:2223–2233. doi: 10.1172/JCI23755. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 57.Johnson GB, Brunn GJ, Kodaira Y, Platt JL. Receptor-mediated monitoring of tissue well-being via detection of soluable heparan sulfate by Toll-like receptor 4. J Immunol. 2002;168:5233–5239. doi: 10.4049/jimmunol.168.10.5233. [DOI] [PubMed] [Google Scholar]
  • 58.Vogl T, Tenbrock K, Ludwig S, Leukert N, Ehrhardt C, van Zoelen MaD, Nacken W, Foell D, van der Poll T, Sorg C, et al. Mrp8 and Mrp14 are endogenous activators of Toll-like receptor 4, promoting lethal, endotoxin-induced shock. Nat Med. 2007;13:1042–1049. doi: 10.1038/nm1638. [DOI] [PubMed] [Google Scholar]
  • 59.Chen GY, Nuñez G. Sterile inflammation: sensing and reacting to damage. Nat Rev Immunol. 2010;10:826–37. doi: 10.1038/nri2873. [DOI] [PMC free article] [PubMed] [Google Scholar]

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