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Review ArticleImmune Homeostasis in Epithelial Cells Evidence and Role ofInflammasome Signaling Reviewed

Paul M Peeters12 Emiel F Wouters1 and Niki L Reynaert1

1Department of RespiratoryMedicine Nutrim School forNutrition Toxicology andMetabolismMaastrichtUniversityMedical Centre6229 HX Maastricht Netherlands2IUF-Leibniz Research Institute for Environmental Medicine Auf rsquom Hennekamp 50 40225 Dusseldorf Germany

Correspondence should be addressed to Paul M Peeters ppeetersmaastrichtuniversitynl

Received 15 January 2015 Accepted 7 July 2015

Academic Editor Kurt Blaser

Copyright copy 2015 Paul M Peeters et alThis is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

The epithelium regulates the interaction between the noxious xenogenous as well as the microbial environment and the immunesystem not only by providing a barrier but also by expressing a number of immunoregulatorymembrane receptors and intracellulardanger sensors and their downstream effectors Amongst these are a number of inflammasome sensor subtypes which have beeninitially characterized in myeloid cells and described to be activated upon assembly into multiprotein complexes by microbial andenvironmental triggers This review compiles a vast amount of literature that supports a pivotal role for inflammasomes in thevarious epithelial barriers of the human body as essential factors maintaining immune signaling and homeostasis

1 Introduction

Inflammation is an extremely complex and fascinatingweapon in mammalian physiology It is the bodyrsquos immediateand carefully orchestrated response to pathogens noxiousstimuli or physical injury In addition responsiveness toplasma- and cell-derived inflammatory mediators reflects amore general role for inflammation in restoring functionalityof the system to basal homeostatic set points The processby which acute inflammation is initiated and develops viamolecular and cellular pathways is well defined [1] In casethe body does not succeed in eliminating or neutralizingthis condition over time a chronic inflammatory state arisesand resets the bodyrsquos reference points and will become mal-adaptiveThis implicates elevated concentrations of cytokinesand chemokines including interleukin- (IL-) 1120573 IL-1120572 IL-6 IL-8 IL-10 IL-18 TNF-120572 and alarmins such as high-mobility group box 1 (HMGB1) These molecules have alsobeen proven to be involved in the progression of chronicinflammatory disorders infections and fibrotic diseases aswell as cancer autoimmune and ageing-associated disorders

It is demonstrated that IL-1 and IL-18 driven inflam-mation through inflammasome activation is initiated by

recognition of endogenous or exogenous danger signals IL-1120573 one of the main classic instigators of inflammation istogether with IL-18 released from the cell upon activationof the inflammasome It has the ability to affect variousbiological properties and has several roles in the proinflam-matory response including activation of the endotheliumand leukocytes Back in 1984 it was predicted that IL-1 wasresponsible for many of the acute responses to infection andinflammation [2] IL-18 a member of the IL-1 cytokine superfamily is recognized as an important regulator of innateand acquired immune responses Its importance is derivedfrom its prominent biological property of inducing interferon(IFN) 120574 IL-18 is expressed at sites of chronic inflammationin autoimmune diseases in a variety of cancers and in thecontext of numerous infectious diseases [3] In addition to IL1and IL18 basic fibroblast growth factor (bFGF) and HMGB1are unconventionally released under caspase-1 activated con-ditions following a number of posttranslationalmodifications[4] Therefore we included bFGF as well as HMGB1 to bereviewed among the factors that could possibly be releasedfrom the epithelial cells upon inflammasome activation ForHMGB1 its earliest functions were described as a nonhistone

Hindawi Publishing CorporationJournal of Immunology ResearchVolume 2015 Article ID 828264 15 pageshttpdxdoiorg1011552015828264

2 Journal of Immunology Research

DNA-binding nuclear protein It facilitates DNA transcrip-tion replication and repair When secreted it has an impor-tant danger signaling and inflammation-promoting activityIt forms highly inflammatory complexes with single stranded(ss) DNA lipopolysaccharide (LPS) IL-1120573 and nucleosomesand interactswith toll-like receptor (TLR) 9 TLR4 IL-1R andTLR2 and the receptor for advanced glycation end-products(RAGE) It has also been reported that HMGB1 can signal viathe well-characterized nuclear factor-120581B pathway to induce arelease of proinflammatory cytokines including TNF-120572 andIL-1120573 bFGF a mitogenic agent is not directly proinflam-matory but can potentiate the recruitment of immune cellsto the site of inflammation This growth factor supports themaintenance of undifferentiated cells and is involved in tissuerepair regeneration and proliferation It is a potent fibrogenicmediator that can be induced by a variety of molecules in dif-ferent cell types such as by silica in lung epithelial cells [5 6]from which it is released in an inflammasome-dependentmode-of-action and potentially linked to fibroblast prolifer-ation [7] Activation of inflammasome-dependent mediatorrelease is typically a very potent reaction and by virtueof the potentially destructive proinflammatory effects ofuncontrolled cytokine and alarmin release their reductionand shutdown mechanisms (eg IL-1 receptor antagonist(IL-1RA) signaling) should therefore be tightly regulated byinnate sensors

To date a number of cytosolic receptors are successfulin the recognition of conserved molecular patterns termedpathogen- or danger-associated molecular patterns (PAMPsor DAMPs) by initiating the formation of an inflammasome[8] These include the nucleotide-binding oligomerizationdomain receptors in short NOD-like receptor (NLR) proteinfamily members NLRP1 NLRP3 NLRP6 NLRP7 NLRP12NLRC4 and NLRC5 as well as the non-NLR pyrin andHIN200 domain-containing (PYHIN) protein family mem-bers absent in melanoma 2 (AIM2) myeloid nuclear dif-ferentiation antigen (MNDA) interferon inducible proteinX (IFIX) and interferon alpha-inducible protein 16 (IFI16)which are all able to oligomerize into a functional inflam-masome [9ndash11] This implies the formation of a multiproteincomplex consisting otherwise also of an adaptor proteinapoptosis-like speck protein containing a CARD (ASC)and IL-1120573 converting enzyme (ICE caspase-1) the enzymeresponsible for maturation of proinflammatory cytokines[12] Upon this assembly caspase-1 undergoes autocatalyticactivation into heterotetramers which further enables thecleavage of its substrates pro-IL-1120573 and pro-IL-18 intomatureIL-1120573 and IL-18 and the unconventional release of bFGFand alarmins by the cell as is illustrated in Figure 1 [13]In addition caspase-1 is associated with the release of IL-1120572 [4] although these mechanisms are poorly understoodInflammasome activation is thus linked to the most impor-tant mediators of inflammation Moreover their release maybe accompanied by pyroptosis an incompletely characterizedproinflammatory mode of cell death [14] Upon activationby a myriad of signaling pathways these inflammasome-dependent mediators are strongly expressed by monocytestissue macrophages and dendritic cells but are also producedby B lymphocytes natural killer (NK) cells and epithelial

cells Activated inflammasomes and subsequently activatedand released cytokines and alarmins play a key and well-controlled role in innate immunity of the lung mucosa andinterstitial microenvironment As mediators of the acutephase of inflammation they are extremely important forthe immune system to react to invading pathogens It ishowever equally important that inflammasomes distinguishpathogenic from nonpathogenic commensals implying thata disturbance in normal danger signaling through the inflam-masome can act as a master switch between tolerance andsensitization in many actively participating tissues [15ndash19]

The activation of inflammasomes in myeloid innateimmune cells as well as their contribution to acute andchronic inflammatory diseases has been characterized pro-foundly over the past 12 years by in vivo and in vitro research[20] Nevertheless we are still far away from understandinghow these molecules actually become activated how theyexert their function and how they can be targeted in therapyAside from the specialized cells of the immune systemthat evolutionary developed to protect organ systems mostforeign pathogens and noxious stimuli are also encounteredby epithelial cells in a barrier lining the organs that are inmostproximal contact with the exterior environment [21] Theseare the tissue specific mucosae of the skin the lung the gutand the urogenital tract as well as nonkeratinizing squamousepithelial cells of the oral mucosa

The epithelium is more and more appreciated to beless passive than what was assumed before and evidenceis mounting that it participates not only in receiving andrelaying inflammatory signals but in functions as an initialsensor of danger and executor of the response as well Wetherefore review here the literature on the presence andfunctionality of inflammasomes in epithelial cells of thevarious organs exposed to the exterior milieu in response toreported insults The aim of this review is to deliver betterunderstanding of inflammatory responses of first-line barrierepithelium in multiple organs and mucosal immunity andencourage further laboratory research to dissect out the roleof epithelial inflammasomes to these processes with moreeffective therapies for the numerous debilitating diseaseswith an acute and chronic inflammatory component as theultimate goal

2 Skin

In addition to its properties as a physical barrier the skinhas many active defense mechanisms Keratinocytes candetect microbial or nonmicrobial danger signals and elicitan immune response prior to the infiltration of myeloid cells[22] Although the action and importance of IL-1120573 IL-18and HMGB1 in inflammatory skin disorders are not com-pletely understood dysregulation of these inflammasome-dependent molecules is an attractive concept that might playa role in many inflammatory abnormalities of the skin In1990 already protein levels of immunoreactive IL-1120573 wereshown to be elevated in psoriatic lesions whereas the amountreleased by normal keratomes or cultured keratinocytes wasundetectable The presence of IL-1120573 was suggested to be

Journal of Immunology Research 3

Non-NLR PYHIN protein family members

AIM2

MNDA

IFIX

IFI16

NOD-like receptor (NLR) protein family members

NLRP1

NLRP3

NLRP6

NLRP7

NLRP12

NLRC4

NLRC5

ASC

Pro-caspase-1

Active caspase-1

Pro-IL-1120573

Pro-IL-18

IL-1120573

IL-18

IL-1120573

IL-18

HMGB1

bFGF

Figure 1 Schematic representation of all intracellular nucleotide-binding oligomerization domain receptors in short NOD-like receptor(NLR) and pyrin and HIN200 domain-containing (PYHIN) inflammasome members which are each able to assemble with the proteasecaspase-1 via the adaptor molecule apoptosis-associated speck-like protein containing a CARD (ASC) when triggered This allows theactivated enzyme to cleave and mature proinflammatory cytokines interleukin- (IL-) 1120573 and IL-18 as well as inducing the unconventionalrelease of basic fibroblast growth factor (bFGF) and high-mobility group box 1 (HMGB1)

due to a novel mechanism of posttranslational processing inthe epidermis [23] This mechanism was identified in 1997as caspase-1 dependent cleavage which could be inducedin human keratinocytes in response to inflammatory andimmunologic stimuli [24]

Later keratinocytes of the nondiseased skin were scarcelystained positive for NLRP1 and NLRP3 [25] and AIM2-like receptor (ALR) inflammasomes [26] These expressionpatterns suggest that multiple inflammasomes are likely toplay a role in the first line of defense against noxious mol-ecules With respect to different functional inflammasomesin the skin human keratinocytes express AIM2 and respondto poly(dAdT) dsDNA with IL-1120573 secretion [26] Recentlythese findings have been supported by detection of activeIL-1120573 and cleaved caspase-1 in human papillomavirus (HPV)infected skin suggesting inflammasome activation by viralDNA [27] Watanabe et al also demonstrated that theNLRP3 inflammasome is present and can be activated in ker-atinocytes [19] as in animalmodels of contact hypersensitivitythis inflammasome was identified as a key regulator of innate

immunity [19] Keratinocytes are obviously also barrier cellsagainst environmental pollutants such as TiO

2and SiO

2 It

was shown that these environmental particles in the nanosizecould induce cleavage of caspase-1 and secretion of IL-1120573 [28]Caspase-1 activity of stratum corneum and serum IL-18 levelwere also increased in patients with Netherton syndromea disease characterized by chronic skin inflammation [29]In a tetanus toxoid-dependent experimental model usingcocultures of monocytes and keratinocytes others further-more observed high levels of IL-1120573 when tetanus toxoid andkeratinocytes were present Indicating that skin epithelialcells are able to secrete caspase-1 and a source for IL-1120573 [30](Figure 2)

In skin injury models relevant to the development ofcancer irradiation with a physiological dose of UVB inducedsecretion of pro-IL-1120572 and of mature and active IL-1120573 andIL-18 in a caspase-1 dependent fashion [31 32] Other studiesof UVB overexposure in sunburned skin demonstrated acti-vated inflammasomes [33] and UV light exposure stimulatedbFGF and HMGB1 release by keratinocytes as well [34 35]

4 Journal of Immunology Research

Chemical compounds∙ Trinitrochlorobenzene∙ ATP

Environmental∙ TiO2

∙ SiO2 (nano)∙ UVB

Microbial∙ Human papilloma virus∙ Mite allergen∙ Poly(dAdT) dsDNA∙ Candida albicans∙ Tetanus toxoid

+

NLRC4

NLRP3

NLRP1

AIM2

ASC

Pro-caspase-1

Active caspase-1

Pro-IL-1120573

Pro-IL-18IL-1120573

IL-18IL-1120573

IL-18

HMGB1

bFGF

Figure 2 Schematic representation of stratified squamous epithelium of the skin and oral mucosa Toxic molecules and environmental andcellular stressors as well as microbial antigens can individually activate one or more inflammasome subtypes leading to caspase-1 activationand the release of IL-1120573 IL-18 bFGF and HMGB1

In an allergic skin disease model mite allergen Dermatoph-agoides pteronyssinus 1 (Der p1 a major allergen of house dustmite) is recognized as a danger signal activated caspase-1and induced release of IL-1120573 and IL-18 from keratinocyteswhich was dependent on the cysteine protease activityMoreover Der p1 stimulated assembly of the inflammasomeby recruiting ASC caspase-1 and NLRP3 to the perinuclearregion [36]

The data reviewed in this section demonstrate that ker-atinocytes are a potent source of cytokines and alarmins uponcontact with a broad spectrum of activators It is clear thatkeratinocytes do not only have a passive role as target cells inthe process of inflammation but also act as stimulators of theinitiation and maintenance of local immune reactions

3 Oral Mucosa

The oral mucosa is exposed to high density and diversityof potential microbial pathogens such as Gram-positive andGram-negative bacteria as well as fungi and others andtherefore has the important function of acting as a physicalbarrier and responding to microbial growth and invasionThe inflammasomes as intracellular immune receptors arethus likely to be important mediators of the inflammatory

response in gingival epithelial cells In a recent study deple-tion of NLRP3 by siRNA abrogated the ability of ATP toinduce IL-1120573 secretion in infected cells [37] ATP is sensed bypurinergic receptors such as P2X ligand-gated ion channel 4(P2X4) Besides numerous reports on the role of P2X7 recep-tors in ATP-mediated inflammasome activation and matureIL-1120573 production inmacrophages in vitro recently in gingivalepithelial cells that were stimulated with extracellular ATP arole for P2X7 dependent-ROS production in the activationof the inflammasome was revealed (Figure 2) [38] Its role invivo has recently been questioned Interestingly in additionto the NLRP3 inflammasome a different inflammasomecontaining NLRC4 appeared to function in the protectionagainst infection with Candida albicans in the mucosal liningof the mouth and intestines rather than in immune cells[39] No further evidence can be found on how immunehomeostasis via inflammasome signaling is maintained inthis environment These studies reveal the epithelial-specificroles of the NLRP3 and NLRC4 inflammasome in innateimmune response of the oral mucosa

4 Gut

In organs where a variety of cell types come in intimate con-tact with commensals and potentially pathogenic microbes

Journal of Immunology Research 5

such as the gut the regulation and maintenance of nor-mal intestinal mucosal barrier function is primordial forthe hostrsquos survival and fitness When cellular integrity andfunctioning of tight junctions between adjacent epithelialcells is disrupted barrier impairment is easily providedresulting in inflammation and the induction of tissue-repairresponsesThe lack of control of this inflammatory conditionis suggested to aggravate in the direction of detrimentalchronic inflammation in the gut Inflammasome-dependentmediators such as IL-1120573 IL-18 and HMGB1 have beenidentified as potent promoters of intestinal pathology whichsuggests that targeting thesemediatorsmay represent a usefultherapeutic approach in inflammatory bowel disease (IBD)[40]

Initially observations suggested that induction of IL-1120573mRNA in enterocytes was causally related to the subsequentinflammatory changes seen in a model of acute experimentalcolitis [41] It was proposed that colon epithelial cells wereprogrammed to provide a set of signals for the activation ofthe mucosal inflammatory response in the earliest phasesafter microbial invasion [42 43] Later a few studies demon-strated that intestinal epithelial cells (IEC) continuouslyexposed to dietary molecules microbial antigens and envi-ronmental influences played a much more active role in thehost immune and inflammatory response via the secretion ofa variety of cytokines limited to not only IL-1120573 but also IL-1120572 and IL-8 [44ndash46]That same year for the first time mouseIECs were proven to be themain producers of IL-18 formerlycalled interferon-gamma-inducing factor [47] under nor-mal physiological conditions suggesting that its constitutiveexpression in IECs may have an important role in the induc-tion of mucosal immunity [48] Two years later IL-18 wasdemonstrated to be localized and increasingly expressed inintestinal mucosal cells of patients with Crohnrsquos disease (CD)[49 50]Within this same period posttranslational activationof IL-18 by caspase-1 cleavage was identified to occur inresponse to viral and bacterial infections [51 52] Specificallya year before inflammasomes were characterized cleavage ofIL-18 in porcine intestinal mucosa by Salmonella choleraesuiswas demonstrated indicating that caspase-1 activation ofIL-18 may be a key step in mucosal immune response tobacterial invasion [53] Expression of IL-18 in human gastricmucosal epithelial cells was also increased by Helicobacterpylori infection or by lactoferrin [54 55] Recently humanIECs showed the ability to release IL-18 upon Salmonellatreatment in a caspase-1 dependent fashion [56] and releaseHMGB1 in their culture medium upon stimulation with LPS[57] and a mixture of TNF-120572 IL-1120573 and IFN-120574 [58]

Microbial activity is required to be constantly monitoredin the epithelial lining of the gut It has become evidentthat a range of inflammasome family members within dif-ferent cell types (eg epithelial and hematopoietic cells)accomplish different but often complementary functionsas watchful guardians eliciting mucosal immune responseswhen activated [59] The most intensively studied inflamma-somes in the gut the NLRP1 NLRP3 NLRP6 and NLRC4inflammasomes have been shown to regulate a number ofcommon intestinal mucosal infections Importantly differententeric infections are sensed by and linked to different

inflammasome functionalities For instance NLRP3 andNLRC4 activation in the intestinal epithelium is essentialfor regulation of permeability and epithelial regenerationthrough sensing of commensal microbes and has been shownto protect against mucosal pathogens [60 61] howeverexcessive inflammasome activationwithin the lamina propriacontributes to severe intestinal inflammation [62] Moreoverwhereas theNLRP6 inflammasome subtype regulated colonicmicrobial ecology and risk for colitis [63] it was also shownto be involved in control of epithelial self-renewal and col-orectal carcinogenesis upon injury [64] Otherwise NLRP6inflammasome-deficient mice have been shown to be unableto clear enteric pathogens from the mucosal surface render-ing themhighly susceptible to persistent infection [65] Addi-tionally recent findings suggest that both hematopoietic- andnonhematopoietic-derived NLRP12 contributed to inflam-mation in an experimental colitis model but the latterdominantly contributed to tumorigenesis Herein NLRP12was profiled as an important add-on in the inflammasomerepertoire and new player in colonic inflammation andtumorigenesis [66] Together these studies reveal intensiveand integrated signaling from multiple inflammasomes toregulate inflammation-induced IBD and colon cancer Inaddition unpublished data report the upregulation of mostinflammasome sensor subtypes (NLRP1 NLRP3 NLRP12NLRC4 AIM2 IFI16 MNDA and PYHIN1) in the colonicmucosa of active IBD patients with the double-stranded (ds)DNA responding PYHIN inflammasome subtypes (AIM2and IFI16) showing the strongest increase These data areaccompanied with enhanced levels of IL-1120573 in primary IECsin culture following dsDNAexposure Immunohistochemicaldata show next to inflammatory cells an epithelial presenceof these inflammasome sensor subunits and some of theireffector molecules (CASP1 and HMGB1) (unpublished data)Together this indicates that a more profound focus on non-NLR signalingmay be justified in IBDThemultiple activatorsof a broad spectrum of inflammasome subtypes implyingcaspase-1 activation and subsequent secretion of specificreadouts in IECs are summarized in Figure 3

Activation of intestinal inflammasomes in different lin-eages of cells regulates physiological reactions and theirhyperactivation or absence can lead to deleterious con-sequences such as inflammation or cancer progression asshown in different models [59] For instance followingtissue damage using the IEC cytotoxic agent dextran sodiumsulphate (DSS) theNLRP3 inflammasome assembles leadingto the production of IL-18 which is then released at themucosal sites [67] Defective NLRP3 inflammasome subtypeactivation was shown to protect against loss of epithelialintegrity and mortality during DSS-induced experimentalcolitis [68] suggesting that genetic and environmental factorsmay activate the NLRP3 inflammasome [69] In additiontheir absence rather than their overproduction could beconsidered deleterious indicating a multifaceted regulatoryrole of NLRP3 in intestinal inflammation Normand et alon the other hand demonstrated that NLRP6-deficient micewere highly susceptible to experimental colitis [64] Furtherit was shown in humans with a leaky intestinal barrier

6 Journal of Immunology Research

Chemical compounds∙ DSS∙ TNBSA

Environmental∙ TiO2

Microbial antigens∙ Bacteria∙ Viruses

+

NLRC4

NLRC5

NLRP3

NLRP1

NLRP6

NLRP12

MNDA

AIM2

ASC

Pro-caspase-1

Active caspase-1Pro-IL-1120573Pro-IL-18

IL-1120573IL-18

IL-1120573IL-18

HMGB1

Mucus producing goblet cell Simple columnar epithelial cell

Basal membrane

Basal membrane

IFI16

IFIX

Figure 3 Schematic representations of simple columnar epithelial cells lining the digestive tract reflecting multiple inflammasomes that aredescribed to be activated by different described agents AIM2 IFI16MNDA and PYHIN1 are subtypes of the non-NLR inflammasome family

(such as seen in IBD patients) that TiO2microparticles

were taken up by IEC and could activate the inflammasomeand induce IL-1120573 and IL-18 secretion in the mucosa ofCrohnrsquos disease patients representing a possible mode ofaggravation of inflammation in susceptible individuals [70]Others have shown that 246-trinitrobenzene sulfonic acid(TNBSA) was unable to induce significant colitis in IL-18deficient mice and that administration of an IL-18 neutral-izing antibody resulted in a dramatic attenuation of mucosalinflammation The proposed function for the NLR and non-NLR inflammasomes is to regulate secretion of IL-18 thatstimulates epithelial cell barrier function and regenerationwhereas in hematopoietic cells inflammasome activationwould have a proinflammatory effect [63 71] This suggeststhat signals produced by the IECsmay play an important rolein inducing the early host inflammatory response to infectionand raises the possibility that interventions that directly targetproduction of inflammatory cytokines by IECs might alterthe course of disease When comparing results of studiesby different groups one should take into considerationthat many of the observed effects may be explained bydefective inflammasome regulation of the composition of themicroflora coupled with differences in native microflora indifferent facilities [59]

Targetingmediator release that is associatedwithmucosalinflammasome activation in the gut could lead to better

understanding of which pathological aspects of inflammationand subsequent increases in permeability contribute to thedevelopment of IBD

5 Lung

Barrier epithelia such as the airway epithelial cells lining therespiratory tract fulfill multiple functions essential for tissuehomeostasis They are because of the immense surface areathat is in intimate contact with the environment a primarytarget of attack by microorganisms and potentially harmfulfactors during every single breath A vicious cycle of exag-gerated responses to chronic stimuli or aberrant responses torather innocent agents may result in chronic inflammationwith permanent structural changes in barrier propertiesincluding smooth muscle hyperplasia airway remodelingand fibrosisThe importance of engagement of pattern recog-nition receptors (PPRs) and their activation is demonstratedvia experimental studies in knockout mice These resultedin evidence suggesting a deleterious role for excessive pro-duction of the inflammasome-dependent proinflammatorycytokines and danger signals IL-1120573 IL-18 and HMGB1 andthe growth factor bFGF which possess multiple pathogenicproperties that could be further enhanced during episodesof disease exacerbations [72ndash76] Because the localizationand the impact of inflammasome sensor activation in airway

Journal of Immunology Research 7

epithelial cells associated with pulmonary inflammation haveyet to be revealed the activation of different inflammasomesin lung epithelium in response to triggers relevant to themainchronic inflammatory diseases asthma COPD pulmonaryfibrosis and pneumoconiosis mainly in in vitro settings issummarized in this section

Historically with respect to evidence of inflammasomeactivation or mediator release from lung epithelium one hasto go back more than a decade prior to the first character-ization of the inflammasome Immunoreactive IL-1120573 wasshown to be released from bronchial epithelial cells exposedto toluene diisocyanate [77] or nitrogen dioxide [78 79]and in tracheal biopsy material from individuals exposedto endotoxin-contaminated grain dust A few years laterin 1996 Hastie et al showed low but significantly higheramounts (2-fold) of IL-1120573 released from bronchial epithe-lial cells from allergic compared to nonallergic individualsfollowing segmental challenge with ragweed [80] Moreoverin experiments with exposure to another allergen Der p1cultured human airway epithelial cells were shown to releaseIL-1120573 [81] In 1998 the IL-1120573 release for the first time waslinked to ICE expression in alveolar epithelial cells uponrespiratory syncytial virus (RSV) infection Interestingly thisstudy showed that this occurred in the absence of apoptosis[48] which could imply that epithelial cell death occurredthrough what is now known as pyroptosis

Different animal models of pulmonary fibrosis havebeen developed to investigate the pathogenic mechanismand potential therapies for idiopathic pulmonary fibrosis(IPF) The most common is the bleomycin model in rodents(mouse rat and hamster) [82] In 2001 caspase-1 mRNAexpression was shown to be elevated in mice treated withbleomycin and bronchiolar and alveolar epithelial cells aswell as myeloid cells showing increased caspase-1 immunore-activity in both nucleus and cytoplasm [83] A decade laterit was suggested that inflammasome signaling in airwayepithelial cells may play an important role in the pathogenesisof diseases like COPD as compounds such as LPS and CpGwere found to induce the releases of IL-1120573 from humanbronchial epithelial cells [84] Next to these environmentaltriggers mechanical stretch was shown that same year toinduce enhanced IL-1120573 levels in the supernatants of alveolarepithelial cells [85] In recent years the panel of media-tors able to activate the inflammasome-dependent caspase-1activity and IL-1120573 release from (primary) lung epithelial cellshas expanded rapidly to include Pseudomonas aeruginosasimvastatin [86] influenza A [19 87 88] RSV [89] andRhinovirus [90] The list of activators seems to be unlimitedThe membrane attack complex of complement apart fromits classical role of lysing cells can also trigger a range ofnonlethal effects on cells including driving inflammationRecent findings demonstrated that sublytic attack by themembrane attack complex of complement leads to caspase-1activation as well as IL-1120573 secretion in primary human lungepithelial cells [91] Another class of inflammasome activatorsin lung epithelial cells includes noxious inhaled particles Apanel of inflammasome-dependent mediators was shown tobe released by bronchial epithelial cells following crystallinesilica exposure [7] and Tran et al demonstrated induction of

IL-1120573 and NLRP3 protein by the proinflammatory stimulusLPS and the combination of IFN-120574 with LPS in primary cellcultures of NHBE cells [92] Hirota and his colleagues char-acterized airway epithelial NLRP3 inflammasome-mediatedimmune responses to urban particulate matter exposureand found significant increases in airway epithelial NLRP3inflammasome-mediated production of IL-1120573 in vitro resultsthat were corroborated in vivo [93]

For interleukin-18 a prolific cytokine involved in manyimmune responses already issued literature research revealedthat its immunoreactivity in airway epithelial cells wasfirst investigated during early stages of host defense withinthe bronchial epithelium of biopsies obtained from controlsubjects and patients with sarcoidosis or asthma [94] LaterWestern blot analysis showed that the 183 kDa mature formof IL-18 appeared in whole cell lysate of Mycobacteriumtuberculosis-stimulated alveolar type II cells whereas bothnonstimulated and Mycobacterium tuberculosis-stimulatedalveolar type II cells contained abundant 24 kDa pro-IL-18 These results indicated that Mycobacterium tuberculosisupregulates IL-18 expression at both transcriptional andposttranscriptional levels [95] implying the involvement ofcaspase-1 enzymatic activity and therefore inflammasomeactivation Piper et al furthermore found that IL-18 wasreleased from Rhinovirus-infected lung epithelia The releasewas not associated with cell death but was dependent oncaspase-1 catalytic activity [90]

The endogenous danger protein HMGB1 was shown tobe released from A549 cells infected with virulent Legionellapneumophila in association with caspase-1 activity [96]HMGB1 levels were furthermore found to be elevated in cellsupernatant from rat alveolar type II cell monolayers thatunderwent scratch wounding [97] Moreover mechanicalstretch significantly increased HMGB1 protein expression inA549 cells [85] These results are important in the contextof injury since epithelial crosstalk to neighboring cells isimportant for normal as well aberrant repair such as inthe case of fibrosis HMGB1 was included in the panelof inflammasome-dependent mediators that were releasedfollowing crystalline silica exposure of bronchial epithelialcells [7]

Epithelial cells express and secrete not only cytokinesand alarmins upon exposure to endogenous or exogenousinflammasome activators but also bFGF The FGFs areinvolved in morphogenesis wound repair inflammationangiogenesis and tumour growth and invasion and requirethe glycosaminoglycan (GAG) side chains of heparin sul-phate proteoglycans for high affinity binding to their specificreceptors [98] Late 20th century bronchial epithelial cellswere shown to secrete bFGF which positively impactedmyofibroblast proliferation in an animal model of asthma Arole for epithelial cells in the expression and release of bFGFfrom heparan sulphate binding sites in bronchial asthma wasdefined a couple of years later [99] Treatment of humanfibroblasts with caspase-1 inhibitors significantly reducedthe amount of secreted bFGF [4] The lung epithelium is amajor source of bFGF as shown by Rhinovirus-induced bFGFrelease in a model that mimics features of airway remodeling

8 Journal of Immunology Research

+

NLRC5

NLRP3

NLRP1

NLRP12

ASC

Pro-caspase-1

Active caspase-1

Pro-IL-1120573Pro-IL-18

IL-1120573IL-18

IL-1120573IL-18

HMGB1bFGF

Mucus producing goblet cell

Tracheobronchial ciliated epithelial cell (TBEC)Basal membrane

TBEC

Chemical compounds∙ Isocyanate∙ NO2

∙ Bleomycin∙ Simvastatin∙ O2

∙ Uric acid crystal

Environmental∙ TiO2

∙ SiO2

∙ MWCNT∙ Particulate matter

Microbial∙ Pseudomonas aeruginosa∙ Influenza A∙ Rhinovirus∙ Mycobacterium tuberculosis∙ Legionella∙ Der p1∙ RSV∙ LPS∙ cPG∙ MAC

Figure 4 Schematic representation of pseudostratified columnar epithelium of the lung indicating a variety of environmental and microbialmolecules that is able to activate the inflammasome with a subsequent release of cytokines alarmins and growth factors

[72] We furthermore showed that crystalline silica exposureof bronchial epithelial cells caused bFGF release [6] whichwas inflammasome- and particle uptake-dependent [7]A pivotal role of surface reactivity of crystalline silica toinflammasome activation was recently demonstrated in cul-tures of epithelial cells with evidence of the inhibitorycapacity of the antioxidant TRX to inflammasome activation[100] Importantly our studies showed that the panel ofsilica-induced NLRP3 inflammasome-dependent mediatorsreleased from airway epithelium leads to fibroblast pro-liferation a characteristic of multiple lung diseases Thesefindings are paralleled by work of Hussain et al in whichit is evidenced that multiwalled carbon nanotubes induce aNLRP3 inflammasome-dependent but TGF-120573 independentprofibrotic response in human bronchial epithelial cells [101]

In contrast there are publications that demonstrate thatlung epithelial cells are not able to secrete IL-1120573 upon expo-sure to differentmicroorganisms and particulates among oth-ers [102 103] This section however summarizes a vast bodyof evidence that lung epithelium participates in early first-line immune defenses via activation of the inflammasomeThe expression of IL-18 IL-1120573 and bFGF as well as HMGB1by these cells demonstrates its participation in the initialresponse to encounters with foreignmoleculesThedescribedmediators may play a prominent role in the cascade of

subsequent steps of the immune response in an autocrine andparacrine as well as chemotactic manner

To date only a subset of inflammasomes has beendescribed in lung epithelial cells so far (Figure 4) NLRP1 wasreported to contribute to the immune response in lungepithelial cells and alveolarmacrophages [25]NLRP3 inflam-masome presence and activation in lung epithelial cellswas demonstrated as well [92 93] Of most caspase-1 acti-vating inflammasomes that have been studiedwell NLRP12 isa uniqueNLR that has been shown to attenuate inflammatorypathways in biochemical assays andmediates the lymph nodehoming of activated skin dendritic cells in contact hyper-sensitivity responses Although its expression was shownin lung cells the overall development of allergic airwaydisease and airway function was not significantly altered byoverall NLRP12 deficiency This suggests that NLRP12 doesnot play a vital role in regulating airway inflammation inthis model [104] Upon Rhinovirus pathogenesis the con-tribution of NLRP3 and NLRC5 inflammasomes and IL-1120573 secretion in Rhinovirus pathogenesis was investigatedand revealed that both inflammasomes act in a cooperativemanner during the assembly by sensing intracellular Ca2+fluxes and triggering IL-1120573 secretion in primary humanbronchial epithelial cells [105] Additionally the importanceof inflammasome signaling in animal models representing

Journal of Immunology Research 9

a cadre of lung diseases such as asthma COPD and acutelung injury as well as fibrosis and pneumoconiosis amongothers has been shown by many groups [106ndash114] and wasrecently reviewed by Brusselle et al [115] However no focuson the epithelium is present in any model

This section evidences that the surface epithelium of theconducting airways can be considered a constitutive pri-mary participant in innate immunity with strong evidencethat epithelial dysfunction is involved in the developmentof inflammatory disorders of the lung and could be aplausible target for therapeutic interventions Often thoughas indicated in the preceding paragraph the importance ofinflammasome activation in the epithelium in animal modelsis not primarily approachedTherefore conditional knockoutmodels or epithelial-specific transgenic animal studies will bea necessity

6 Urogenital Epithelium

With respect to cells lining the urogenital tract evidencesuggests that inflammasomes next to other PPRs haveimportant roles in associated diseases through regulation ofinflammatory and tissue-repair responses to infection andinjury [116] First on the subject of human kidney dis-eases such as Wegenerrsquos granulomatosis and in experimentalmodels of glomerulonephritis glomerular as well as tubularepithelial cells have been shown to synthesize and releaseIL-1120573 constitutively [117ndash119] In a recent study that analyzedthe processing of caspase-1 IL-1120573 and IL-18 after unilateralureteral obstruction (UUO) inmice reflecting chronic kidneydisease it was shown that NLRP3 has a biological function inboth hematopoietic and renal epithelial compartments dur-ing renal injury Additionally in models of ischemic tubularnecrosis and obstruction-induced epithelial-mesenchymaltransition an important role for caspase-1 and IL-18 hasbeen demonstrated under hypoxic conditions and in theabsence of vascular effects [120ndash122] Other cells liningepithelial tracts in contact with the environment conveyinginflammasomes are prostate epithelial cells expressing AIM2with increased caspase-1 activity in an experimental modelof benign prostate hyperplasia (BPH) and human cervicalepithelial cells expressing AIM2 and IFI16 inflammasomesfollowing Chlamydia trachomatis and herpes simplex virus 2respectively [123ndash125] Although the amount of literature oninflammasome activation in these organs is relatively scarceother studies demonstrate a pivotal role of the presenceand activation of various inflammasomes in the epitheliumof urogenital organs exposed to the environment [126ndash128](Figure 5)

7 Conclusion

Epithelial cells form an interface between the body and theenvironmentTherefore they are important guardians for thedetection of danger signals and the consecutive initiation ofan inflammatory response As presented in this review eachorgan and cell type express different sensor subtypes withdiscrepancy in the release of various mediators It should

be emphasized that it is very possible that manifold inflam-masomes are important in multiple epithelial cell types andbecome activated to either overcome detrimental signaling orto cooperate in a constructive fashion combating the diseaseLikewise with respect to relatively lower concentrations ofcytokines released from epithelial cells versus the myeloidcompartment it could be considered that first-line barrierepithelial cells in contact withmany potential danger signalspreferably should not produce high amounts of these verypotent inflammatory cytokines and alarmins as it wouldbe harmful for the microenvironment to have a constantldquohigh-alarm situationrdquo Additionally many more epithelialcells are present in these organs as opposed to for instancemacrophages therefore activated epithelial cells may relayequally large and biologically significant immune signals thatbuild up the important contribution in global inflammasomeactivation at organ level

This review demonstrates that inflammasome activationand subsequent secretion of ldquoalarmingrdquo proteins is notrestricted to macrophages indicating that epithelial cellsshould be considered as highly important cells in innateimmune signaling In future research epithelial-specific con-ditional knockout models and transgenic animal studies willbe a necessary approach to determine this important contri-bution more profoundly

Abbreviations

AEC Alveolar epithelial cellAIM2 Absent in melanoma 2ALR AIM2-like receptorASC Apoptosis-associated speck-like protein

containing a CARDATP Adenosine-51015840-triphosphatebFGF Basic fibroblast growth factorCARD Caspase recruitment domainDAMPs Danger-associated molecular patternsDer p1 Dermatophagoides pteronyssinus 1DNA Deoxyribonucleic aciddsDNA Double-stranded deoxyribonucleic acidDSS Dextran sodium sulphateELISA Enzyme-linked immunosorbent assayHIN-200 200 amino acid hemopoietic IFN-inducible

nuclear proteinsHMGB1 High-mobility group box 1HPV Human papillomavirusIBD Irritable bowel diseaseICE Interleukin-1 converting enzymeIEC Intestinal epithelial cellIFI16 Interferon alpha-inducible protein 16IFIX Interferon inducible protein XIFN InterferonIL-18 Interleukin-18IL-1R Interleukin-1 receptorIL-1120572 Interleukin-1120572IL-1120573 Interleukin-1120573

10 Journal of Immunology Research

Chlamydiatrachomatis Free fatty acids

BSAHerpes simplex virus 2

Cytosolic DNA

IFI16AIM

2

NLRP3

Pro-caspase-1

Active caspase-1

Pro-IL-1120573Pro-IL-18

IL-1120573IL-18

IL-1120573IL-18

ER stress

Basal membrane

ASC

Figure 5 Schematic representations of simple cuboidal epithelial cells lining the urogenital tract in which different inflammasomes havebeen described to be activated by independent instigators triggering the release of inflammasome readouts

LPS LipopolysaccharideLRR Leucine-rich repeat domainMAC Membrane attack complexMNDA Myeloid nuclear differentiation antigenMSU Monosodium urateMTB Mycobacterium tuberculosisMWCNT Multiwalled carbon nanotubesMyD88 Myeloid differentiation primary response

proteinNACHT Nucleotide-binding and oligomerization

domainNADPH Nicotinamide adenine dinucleotide

phosphate-oxidaseNAIP NLR family apoptosis inhibitory proteinNK cells Natural killer cells

NLR Nucleotide-binding domain leucine-richrepeat containing receptors

NLRC4 Nucleotide-binding domain leucine-richrepeat containing receptors with a CARDdomain 4

NLRC5 Nucleotide-binding domain leucine-richrepeat containing receptors with a CARDdomain 5

NLRP1 Nucleotide-binding domain leucine-richrepeat containing receptors with a pyrindomain 1

NLRP3 Nucleotide-binding domain leucine-richrepeat containing receptors with a pyrindomain 3

Journal of Immunology Research 11

NLRP6 Nucleotide-binding domain leucine-richrepeat containing receptors with a pyrindomain 6

NLRP7 Nucleotide-binding domain leucine-richrepeat containing receptors with a pyrindomain 7

NLRP10 Nucleotide-binding domain leucine-richrepeat containing receptors with a pyrindomain 10

NLRP12 Nucleotide-binding domain leucine-richrepeat containing receptors with a pyrindomain 12

NOD Nucleotide-binding oligomerization con-taining domain

P2X7 Purinergic receptor P2X ligand-gated ionchannel 7

PAMPs Pathogen-associated molecular patternsPRR Pattern recognition receptorPYD Pyrin domainPYHIN Pyrin and HIN200 domain-containing

proteinRAGE Receptor for advanced glycation end-pro-

ductRNA Ribonucleic acidROS Reactive oxygen speciesRSV Respiratory syncytial virusSiO2 Silicon dioxide

siRNA Small interfering RNATGF-120573 Transforming growth factor 120573TiO2 Titanium dioxide

TNBSA 246-Trinitrobenzene sulfonic acidTNF-120572 Tumor necrosis factor-120572WCL Whole cell lysate

Gloss

The immune response of the body is determined by a complexinterplay between danger molecules and the host in whichthe recognition of these molecules by myeloid cells has beenlinked with cytosolic sensors such as the inflammasomeOver the past decade there has been tremendous progressin our knowledge of the central roles of NOD-like recep-tor (NLR) and non-NLR inflammasomes in the immuneresponses in many cell types Here we provide evidence thatepithelial cells lining multiple barriers between the innerbody and its possible invaders are equipped with functionalinflammasomes

Conflict of Interests

The authors declare that they have no competing interests

Acknowledgment

This work was performed in the framework of an ERS LongTerm Research Fellowship (LTRF 2013ndash1727) awarded toPMP

References

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[3] J A Gracie S E Robertson and I BMcInnes ldquoInterleukin-18rdquoJournal of Leukocyte Biology vol 73 no 2 pp 213ndash224 2003

[4] M Keller A Ruegg SWerner andH-D Beer ldquoActive caspase-1 is a regulator of unconventional protein secretionrdquo Cell vol132 no 5 pp 818ndash831 2008

[5] J I Herseth V Volden P E Schwarze M Lag andM RefsnesldquoIL-1beta differently involved in IL-8 and FGF-2 release incrystalline silica-treated lung cell co-culturesrdquoParticle and FibreToxicology vol 5 article 16 2008

[6] T N Perkins A Shukla PM Peeters et al ldquoDifferences in geneexpression and cytokine production by crystalline vs amor-phous silica in human lung epithelial cellsrdquo Particle and FibreToxicology vol 9 article 6 2012

[7] P M Peeters T N Perkins E F M Wouters B T Mossmanand N L Reynaert ldquoSilica induces NLRP3 inflammasome acti-vation in human lung epithelial cellsrdquo Particle and Fibre Toxi-cology vol 10 article 3 2013

[8] F Martinon K Burns and J Tschopp ldquoThe Inflammasomea molecular platform triggering activation of inflammatorycaspases and processing of proIL-betardquo Molecular Cell vol 10no 2 pp 417ndash426 2002

[9] S A Schattgen andKA Fitzgerald ldquoThePYHINprotein familyas mediators of host defensesrdquo Immunological Reviews vol 243no 1 pp 109ndash118 2011

[10] P A Keyel ldquoHow is inflammation initiated Individual influ-ences of IL-1 IL-18 and HMGB1rdquo Cytokine vol 69 no 1 pp136ndash145 2014

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12 Journal of Immunology Research

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[25] J A Kummer R Broekhuizen H Everett et al ldquoInflammasomecomponentsNALP 1 and 3 showdistinct but separate expressionprofiles in human tissues suggesting a site-specific role in theinflammatory responserdquo Journal of Histochemistry and Cyto-chemistry vol 55 no 5 pp 443ndash452 2007

[26] V Kopfnagel M Wittmann and T Werfel ldquoHuman keratino-cytes express AIM2 and respond to dsDNA with IL-1betasecretionrdquo Experimental Dermatology vol 20 no 12 pp 1027ndash1029 2011

[27] M Reinholz Y Kawakami S Salzer et al ldquoHPV16 activates theAIM2 inflammasome in keratinocytesrdquoArchives of Dermatolog-ical Research vol 305 no 8 pp 723ndash732 2013

[28] A S Yazdi G Guarda N Riteau et al ldquoNanoparticles activatetheNLRpyrin domain containing 3 (Nlrp3) inflammasome andcause pulmonary inflammation through release of IL-1120572 and IL-1120573rdquoProceedings of theNational Academy of Sciences of theUnitedStates of America vol 107 no 45 pp 19449ndash19454 2010

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[37] O Yilmaz A A Sater L Yao T Koutouzis M Pettengill andD M Ojcius ldquoATP-dependent activation of an inflammasome

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[38] S-C Hung C H Choi N Said-Sadier et al ldquoP2X4 assembleswith P2X7 and pannexin-1 in gingival epithelial cells and mod-ulates ATP-induced reactive oxygen species production andinflammasome activationrdquo PLoS ONE vol 8 no 7 Article IDe70210 2013

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14 Journal of Immunology Research

and induction of the inflammasomerdquo Biochimica et BiophysicaActamdashMolecular Basis of Disease vol 1812 no 9 pp 1104ndash11102011

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[89] K Triantafilou S Kar E Vakakis S Kotecha and M Trianta-filou ldquoHuman respiratory syncytial virus viroporin SH a viralrecognition pathway used by the host to signal inflammasomeactivationrdquoThorax vol 68 no 1 pp 66ndash75 2013

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[96] M Furugen F Higa K Hibiya et al ldquoLegionella pneumophilainfection induces programmed cell death caspase activationand release of high-mobility group box 1 protein in A549alveolar epithelial cells inhibition by methyl prednisolonerdquoRespiratory Research vol 9 article 39 2008

[97] J-F Pittet H Koh X Fang et al ldquoHMGB1 accelerates alveolarepithelial repair via an IL-1120573- and 120572v1205736 integrin-dependentactivation of TGF-1205731rdquo PLoS ONE vol 8 no 5 Article IDe63907 2013

[98] A D Lander and S B Selleck ldquoThe elusive functions of prot-eoglycans in vivo veritasrdquo Journal of Cell Biology vol 148 no2 pp 227ndash232 2000

[99] J K Shute N Solic J ShimizuWMcConnell A E Redingtonand P H Howarth ldquoEpithelial expression and release of FGF-2 from heparan sulphate binding sites in bronchial tissue inasthmardquoThorax vol 59 no 7 pp 557ndash562 2004

[100] P M Peeters I M Eurlings T N Perkins et al ldquoSilica-inducedNLRP3 inflammasome activation in vitro and in rat lungsrdquoParticle and Fibre Toxicology vol 11 no 1 article 58 2014

[101] S Hussain S Sangtian S M Anderson et al ldquoInflammasomeactivation in airway epithelial cells after multi-walled carbonnanotube exposure mediates a profibrotic response in lungfibroblastsrdquo Particle and Fibre Toxicology vol 11 article 28 2014

[102] D D Gillette P A Shah T Cremer et al ldquoAnalysis of humanbronchial epithelial cell proinflammatory response toBurkhold-eria cenocepacia infection inability to secrete IL-1betardquo TheJournal of Biological Chemistry vol 288 no 6 pp 3691ndash36952013

[103] H Unno K Futamura H Morita et al ldquoSilica and double-stranded RNA synergistically induce bronchial epithelial apop-tosis and airway inflammationrdquoAmerican Journal of RespiratoryCell and Molecular Biology vol 51 no 3 pp 344ndash353 2014

[104] I C Allen C M Jania J EWilson et al ldquoAnalysis of NLRP3 inthe development of allergic airway disease in micerdquo Journal ofImmunology vol 188 no 6 pp 2884ndash2893 2012

[105] K Triantafilou S Kar F J M Van Kuppeveld and M Tri-antafilou ldquoRhinovirus-induced calcium flux triggers NLRP3and NLRC5 activation in bronchial cellsrdquo American Journal ofRespiratory Cell and Molecular Biology vol 49 no 6 pp 923ndash934 2013

[106] C Dostert V Petrilli R Van Bruggen C Steele B TMossmanand J Tschopp ldquoInnate immune activation through Nalp3inflammasome sensing of asbestos and silicardquo Science vol 320no 5876 pp 674ndash677 2008

[107] S B Willingham I C Allen D T Bergstralh et al ldquoNLRP3(NALP3 cryopyrin) facilitates in vivo caspase-1 activationnecrosis and HMGB1 release via inflammasome-dependentand -independent pathwaysrdquo Journal of Immunology vol 183no 3 pp 2008ndash2015 2009

[108] PGasse CMary I Guenon et al ldquoIL-1R1MyD88 signaling andthe inflammasome are essential in pulmonary inflammationand fibrosis in micerdquo The Journal of Clinical Investigation vol117 no 12 pp 3786ndash3799 2007

[109] P Gasse N Riteau S Charron et al ldquoUric acid is a danger signalactivating NALP3 inflammasome in lung injury inflammationand fibrosisrdquo American Journal of Respiratory and Critical CareMedicine vol 179 no 10 pp 903ndash913 2009

[110] N S Pauwels K R Bracke L L Dupont et al ldquoRole of IL-1alpha and theNlrp3caspase-1IL-1beta axis in cigarette smoke-induced pulmonary inflammation andCOPDrdquo European Respi-ratory Journal vol 38 no 5 pp 1019ndash1028 2011

[111] S Eltom C S Stevenson J Rastrick et al ldquoP2x7 receptorand caspase 1 activation are central to airway inflammationobserved after exposure to tobacco smokerdquo PLoS ONE vol 6no 9 Article ID e24097 2011

[112] J Fukumoto I Fukumoto P T Parthasarathy et al ldquoNLRP3deletion protects from hyperoxia-induced acute lung injuryrdquoThe American Journal of PhysiologymdashCell Physiology vol 305no 2 pp C182ndashC189 2013

[113] M Ritter K Straubinger S Schmidt et al ldquoFunctional rele-vance of NLRP3 inflammasome-mediated interleukin (IL)-1120573 during acute allergic airway inflammationrdquo Clinical andExperimental Immunology vol 178 no 2 pp 212ndash223 2014

Journal of Immunology Research 15

[114] S Yamagata K Tomita R Sato A Niwa H Higashino and YTohda ldquoInterleukin-18-deficient mice exhibit diminished chro-nic inflammation and airway remodelling in ovalbumin-induced asthma modelrdquo Clinical and Experimental Immunol-ogy vol 154 no 3 pp 295ndash304 2008

[115] G G Brusselle S Provoost K R Bracke A Kuchmiy and MLamkanfi ldquoInflammasomes in respiratory disease from benchto bedsiderdquo Chest vol 145 no 5 pp 1121ndash1133 2014

[116] J C Leemans L Kors H-J Anders and S Florquin ldquoPatternrecognition receptors and the inflammasome in kidney diseaserdquoNature Reviews Nephrology vol 10 no 7 pp 398ndash414 2014

[117] D A S Jenkins D R Wojtacha P Swan S Fleming and A DCumming ldquoIntrarenal localization of interleukin-1 beta mRNAin crescentic glomerulonephritisrdquo Nephrology Dialysis Trans-plantation vol 9 no 9 pp 1228ndash1233 1994

[118] I L Noronha C Kruger K Andrassy E Ritz and RWaldherrldquoIn situ production of TNF-120572 IL-1120573 and IL-2R in ANCA-positive glomerulonephritisrdquo Kidney International vol 43 no3 pp 682ndash692 1993

[119] G H Tesch N Yang H Yu et al ldquoIntrinsic renal cells are themajor source of interleukin-1beta synthesis in normal and dis-eased rat kidneyrdquo Nephrology Dialysis Transplantation vol 12no 6 pp 1109ndash1115 1997

[120] A H Bani-Hani J A Leslie H Asanuma et al ldquoIL-18 neutral-ization ameliorates obstruction-induced epithelial-mesenchy-mal transition and renal fibrosisrdquo Kidney International vol 76no 5 pp 500ndash511 2009

[121] V Y Melnikov S Faubel B Siegmund M Scott Lucia DLjubanovic and C L Edelstein ldquoNeutrophil-independentmechanisms of caspase-1- and IL-18-mediated ischemic acutetubular necrosis in micerdquo Journal of Clinical Investigation vol110 no 8 pp 1083ndash1091 2002

[122] C L Edelstein T S Hoke H Somerset et al ldquoProximal tubulesfrom caspase-1-deficient mice are protected against hypoxia-induced membrane injuryrdquo Nephrology Dialysis Transplanta-tion vol 22 no 4 pp 1052ndash1061 2007

[123] L Ponomareva H Liu X Duan et al ldquoAIM2 an IFN-induciblecytosolic DNA sensor in the development of benign prostatehyperplasia and prostate cancerrdquo Molecular Cancer Researchvol 11 no 10 pp 1193ndash1202 2013

[124] AAAbdul-Sater E KooGHacker andDMOjcius ldquoInflam-masome-dependent caspase-1 activation in cervical epithelialcells stimulates growth of the intracellular pathogen Chlamydiatrachomatisrdquo Journal of Biological Chemistry vol 284 no 39 pp26789ndash26796 2009

[125] K Triantafilou D Eryilmazlar and M Triantafilou ldquoHerpessimplex virus 2-induced activation in vaginal cells involves Toll-like receptors 2 and 9 and DNA sensors DAI and IFI16rdquoAmerican Journal of Obstetrics and Gynecology vol 210 no 2pp 122e1ndash122e10 2014

[126] A Chang K Ko and M R Clark ldquoThe emerging role of theinflammasome in kidney diseasesrdquoCurrent Opinion in Nephrol-ogy and Hypertension vol 23 no 3 pp 204ndash210 2014

[127] L Fang D Xie XWu H CaoW Su and J Yang ldquoInvolvementof endoplasmic reticulum stress in albuminuria inducedinflammasome activation in renal proximal tubular cellsrdquo PLoSONE vol 8 no 8 Article ID e72344 2013

[128] Y Nishi M Satoh H Nagasu et al ldquoSelective estrogen receptormodulation attenuates proteinuria-induced renal tubular dam-age by modulating mitochondrial oxidative statusrdquo KidneyInternational vol 83 no 4 pp 662ndash673 2013

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