REVIEWContribution of the inflammasomes to autoinflammatory diseases and recent mouse models as research tools ☆Michael C.Heymann ⁎,Angela Rösen-Wolff ⁎Department of Pediatrics,University Hospital Carl Gustav Carus,Fetscherstrasse 74,01307Dresden,GermanyReceived 22August 2012;accepted with revision 15January 2013Available online 4February 2013KEYWORDSInnate immune system;Danger signal;Recurrent feverAbstract Inflammasomes are multiprotein complexes that serve as activating platforms for the enzyme caspase-1in response to various danger signals.Active caspase-1can cleave the precursors of the pro-inflammatory cytokines IL-1βand IL-18and thereby activate them.Deregulation of this cascade caused by mutations in genes coding for inflammasomal components and their interaction partners can lead to severe disease.This review summarizes the contribution of deregulated inflammasomes to the field of autoinflammatory syndromes.In addition,it gives insight into currently available mouse models that are used to study and characterize the role of the inflammasome components in the pathophysiology of these diseases.©2013Elsevier Inc.All rights reserved.Contents 1.Introduction .........................................................1762.The concept of “autoinflammation ”............................................1762.1.The cryopyrin associated periodic syndromes (CAPS)...............................1772.2.Other inflammasome components..........................................1782.3.Familial Mediterranean Fever (FMF)........................................1782.4.Pyogenic arthritis,pyoderma gangrenosum,and acne syndrome (PAPA)....................1782.5.Metabolic diseases associated with inflammasomes ................................1793.Animal models ........................................................1793.1.NLRP3..................................... (180)☆The authors are employees of the University Hospital Carl Gustav Carus in Dresden,Germany.Their work is supported by the German Research Foundation (Klinische Forschergruppe KFO 249,project TP1,RO/471-11and TP2,HO 4510/1-1),and the PID-NET project of the Federal Ministry of Education and Research (project A4),and they report no financial conflict of interest.⁎Corresponding authors.Fax:+493514586333.E-mail addresses:michael.heymann@uniklinikum-dresden.de (M.C.Heymann),angela.roesen-wolff@uniklinikum-dresden.de (A.Rösen-Wolff).1521-6616/$-see front matter ©2013Elsevier Inc.All rights reserved./10.1016/j.clim.2013.01.006a v a i l ab l e a t w w w.sc i e n c ed i re c t.c o mC l i n i c a l I m m u n o l o g yw w w.e l s e v i e r.c o m /l o c a t e /y c l i mClinical Immunology (2013)147,175–1843.2.Pyrin..........................................................3.3.(Pro)caspase-1....................................................4.Conclusions......................................................... Conflict of interest statement.................................................. References..............................................................1.IntroductionTen years after their discovery[1],inflammasomes are more than ever believed to play a central role in the recognition of exogenous and endogenous danger signals.In addition to other intracellular pattern-recognition receptors(PRRs),including Nod-like receptors(NLRs)[2],C-type lectin receptors(CLRs) [3,4],and the RIG-like receptors(RLRs),which are capable of sensing RNA[5],these multiprotein complexes contribute to intracellular identification of potentially harmful substances, bacteria,or viruses.Inflammasome activation initiates effec-tive defense mechanisms against potential threads.Inflammasomes typically consist of i)a characteristic scaf-folding protein,ii)the small adapter molecule ASC(apoptosis-associated speck-like protein containing a CARD domain) [6],and iii)the progenitor form of the enzyme caspase-1 (procaspase-1)which is responsible for the activation of pro-inflammatory cytokines such as interleukin-1β(IL-1β)or IL-18[1,7–10].Caspase-1is activated through autoprocessing in response to inflammasome assembly[10].Subsequent secretion of IL-1βand IL-18results in stimulation,proliferation and differentiation of T and B cells which participate in the immunological response to challenging threads[11].To date,four scaffolding proteins that contribute to inflammasome assembly have been described.Thus,four different inflammasomes are capable of sensing a wide range of danger signals.1)The NLRP1(NOD-like receptor family, pyrin domain containing1)inflammasome is activated by muramyl dipeptide(MDP)and lethal toxin from Bacillus anthracis[12,13].2)The AIM2(Absent in melanoma2) inflammasome can sense cytoplasmic dsDNA(double stranded deoxyribonucleic acid)[14],and3)the NLRC4(NLR family CARD[Caspase activation and recruitment domain]domain containing protein4)inflammasome recognizes flagellin [15,16],another bacterial protein.The fourth inflammasome is the NLRP3(NOD-like receptor family,pyrin domain containing3)inflammasome,which is responsible for IL-1βactivation following various stimuli.Inflammasome assembly in general is realized by the binding of ASC to the scaffolding proteins via interaction of the pyrin domain(PYD)of ASC and the PYD of the respective scaffolding protein.Due to the fact that ASC also harbors a caspase-1recruitment domain(CARD)domain,it can bind procasapase-1simultaneously,leading to the assembly of the multiprotein complex.Notably,NLRC4can bind procaspase-1 directly through the interaction of its CARD domain with the CARD domain of procaspase-1[17].In this case,the partici-pation of ASC does not seem to be necessary but may stabilize the inflammasome complex[7].Due to the intriguingly large number of stimuli that can activate the NLRP3inflammasome,it has an exceptional position among the inflammasomes.NLRP3activating stimuli include bacterial proteins,such as MDP[18],pore forming substances including nigericin[19],toxins such as alpha-toxin of Staphylococcus aureus[20],bacterial RNA[21], viruses[22],or fungi[23–25].Also other danger associated molecular patterns(DAMPs)including asbestos[26],alum [27],urate crystals[28],extracellular adenosintriphosphate (ATP)[19],UV-radiation[29],or potassium(K+)-efflux[30] result in NLRP3activation and subsequent IL-1βand IL-18 release.Since the variety of stimuli activating the NLRP3 inflammasome appears to be rather large for one receptor protein,the search for the“real”activator or further molecules that might become activated upstream of NLRP3is an ongoing challenge.Reactive oxygen species(ROS)generated in response to cell stress have been postulated to be activating stimuli for the NLRP3inflammasome[31].However,ROS may also play a role in boosting the expression of several genes,including the genes encoding NLRP3,pro-IL-1βand procaspase-1. Thus,ROS inhibition hypothetically impairs the production of inflammasomal proteins[32].Lysosomal rupture together with cathepsin release into the cytoplasm could also be an alternative upstream event prior to NLRP3activation[27,33].Notably,a constantly growing number of proteins that may be part of inflammasomes have been discovered.Recently, the RIG-I(Retinoic acid-inducible gene-I)inflammasome as an RNA sensor and IFI16(Interferon gamma-inducible protein 16),recognizing nuclear viral DNA,were postulated as new members of the inflammasome family[34–37].2.The concept of“autoinflammation”The concept of“autoinflammation”was first proposed by McDermott and Kastner in1999[38]in a publication elucidating mutations in TNFRSF1A to be responsible for the hereditary inflammatory disease tumor necrosis factor (TNF)-receptor associated periodic syndrome(TRAPS;OMIM #142680)[38].In1997mutations in the MEFV gene had already been identified in patients suffering from Familial Mediterra-nean Fever(FMF;OMIM#249100)[39].In contrast to auto-immune reactions,the inflammatory episodes of these diseases seem to be unprovoked,high-titer autoantibodies are absent and neutrophils and macrophages rather than T or B lymphocytes constitute the involved cell types[38,40,41].A refined definition of autoinflammatory diseases as disorders with“abnormally increased inflammation,mediated predom-inantly by the cells and molecules of the innate immune system,with significant host predisposition”[40]emphasizes the role of the innate immune system in autoinflammation distinguishing it from autoimmunity.In this context,McGonagle and McDermott categorized a continuum of inflammatory diseases with autoinflammatory disorders on the one end and autoimmune disorders on the other end of the spectrum[42]. Monogenic autoinflammatory disorders such as TRAPS or FMF180180180180181176M.C.Heymann,A.Rösen-Wolffare described as “maybe exclusively determined by local tissue-specific factors ”[42]whereas the adaptive immune system mainly seems to determine rare monogenic autoim-mune diseases,such as ALPS (autoimmune lymphoproliferative syndrome),IPEX (immunodysregulation polyendocrinopathy enteropathy X-linked syndrome)or APECED (autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy)[42].Disorders classified in the central range of the continuum are characterized by the influence of both the innate and the adaptive immune system [42].Polygenic autoinflammatory diseases such as Crohn disease or gout,mixed pattern diseases such as psoriasis or Behcet disease and classic polygenic autoimmune diseases such as rheumatoid arthritis or Sjögren syndrome are covered by this continuum [42].In addition to FMF and TRAPS,other autoinflammatory hereditary monogenic diseases have been explored.Due to the fact that activation of inflammasomes is the initial step in the maturation of biologically active forms of the pro-inflammatory cytokines IL-1βand IL-18,it appears obvious that a deregulation of this cascade can cause immune-deregulation and severe autoinflammatory diseases.Accordingly,mutations in several components of inflamma-somes or in inflammasome associated molecules can cause autoinflammatory diseases.Fig.1summarizes the autoin-flammatory syndromes that are associated with inflammasomal components.2.1.The cryopyrin associated periodic syndromes (CAPS)The autoinflammatory diseases caused by mutations in NLRP3,the gene which encodes cryopyrin (=NLRP3),are defined asCAPS [43].Deregulated activation of NLRP3inflammasomes causes secretion of highly enhanced levels of bioactive IL-1β[44,45].This increased production of mature IL-1βis possibly induced by a spontaneous or constitutive assembly of NLRP3molecules which leads to inflammasome activa-tion [46,47].However,the particular signals triggering disease in CAPS patients have not been completely eluci-dated [41].To date,three CAPS subtypes are distinguished.They differ in severity although similar symptoms such as fever,urticarial skin rash or elevated acute-phase response [48]point to the continuum of the clinical characteristics of CAPS [48,49].The less severe manifestation of CAPS is familial cold autoinflammatory syndrome (FCAS;OMIM #120100)[50].The symptoms are triggered by cold and do not only include recurrent febrile episodes but also urticarial rash,arthral-gia,conjunctivitis,and headache.Severe long term damage induced by amyloidosis is very rare [43].Patients with Muckle –Wells syndrome (MWS;OMIM #191900)[50,51]additionally suffer from inflammation of the inner ear which can lead to complete hearing loss.The disease is not mainly triggered by cold.Amyloidosis is more frequent than in FCAS patients and the most fatal and dangerous complication of the disease [43,52].Neonatal-onset multisystem inflammatory disease (NOMID;also CINCA;OMIM #607115)[53]has the most severe impact on the affected individual.Serious inflammation of the central nervous system leads to various impairments including hearing loss and loss of vision both due to progressive damages of the respective nerves.In addition,NOMID is also accompanied by meningitis,bony epiphysial hyperplasia [54],and mental retardation [43,55,56].Figure 1How deregulated inflammasomes lead to enhanced pro-inflammatory signals.After activation,NLRP3interacts with ASC which can subsequently interact with procaspase-1.Pyrin and PSTPIP1do also play a role in inflammasome control.Their contribution is not completely understood yet.After inflammasome assembly,activated caspase-1can cleave pro-IL-1βand pro-IL-18into their active mature variants.Mutated versions of certain proteins lead to different autoinflammatory diseases:NLRP3alterations cause CAPS,PSTPIP1mutations cause PAPA and pyrin alterations lead to FMF.Genetic variants of caspase-1have also been associated with autoinflammatory phenotypes.However,this link needs further studies.ASC:Apoptosis-associated speck-like protein containing a CARD domain;CAPS:Cryopyrin associated periodic syndromes;FMF:Familial Mediterranean Fever;IL-1β:Interleukin-1β;IL-18:Interleukin-18;NLRP3:NOD-like receptor family,pyrin domain containing 3;PAPA:Pyogenic arthritis,pyoderma gangrenosum,and acne syndrome;PSTPIP1:Proline –serine –threonine phosphatase-interacting protein 1.177Contribution of the inflammasomes to autoinflammatory diseases and recent mouse models2.2.Other inflammasome componentsBesides NLRP3,other inflammasome components have been associated with different diseases.One interesting connection between an inflammasome component and autoinflammatory symptoms was recently elucidated by Luksch and colleagues[57].They found several genetic variants of CASP1,the gene encoding procaspase-1,in patients suffering from symptoms similar to autoinflammatory disorders but who did not have any mutation in the respective genes.These genetic alterations of CASP1lead to altered protein structure,expression levels and/or decreased enzy-matic activity resulting in less or abolished IL-1βmaturation. Nevertheless,these reduced levels of IL-1βdid not prevent hyperinflammation[57].It is still open if these CASP1variants cause autoinflammatory conditions.Remarkably,Lamkanfi and colleagues showed a contribution of enzymatically inactive procaspase-1to NF-κB(nuclear factor kappa-light-chain-enhancer of activated B-cells)activation via the RIP2 (receptor interacting protein kinase2)pathway[58,59].This could possibly be a mechanism of how genetic variants of the CASP1gene may contribute to autoinflammatory phenotypes.Genetic variants of AIM2and NLRP1have been linked to diseases,which are not classically proposed as autoinflam-matory.Nonetheless,these associations show the importance of a proper regulation of inflammasomal activity.Inactivation of AIM2,has been linked to colorectal cancer [60]suggesting a tumor suppressor function of the protein. However,a recent publication described AIM2overexpression as putative promoting factor for the development of oral squamous cell carcinoma(OSCC)[61].A relation between mutations in AIM2or altered expression in autoinflammatory diseases is yet uncharacterized.For NLRP1,associations have been made between certain single nucleotide polymorphisms (SNPs)in the NLRP1gene and generalized vitiligo[62,63]and celiac disease[64],both polygenic disorders with yet poorly understood pathophysiology.2.3.Familial Mediterranean Fever(FMF)Mutations in the gene MEFV,which encodes pyrin(also known as marenostrin),are responsible for the most common hereditary autoinflammatory disease FMF[39].Patients suffer from unprovoked febrile episodes and also inflammation of the skin,notably peritonitis,arthritis,synovitis,and serositis [56,65].In most patient families the disease exhibits a recessive trait which means that both alleles of the gene have to be mutated for the clinical manifestation of the syndrome [66].However,about20%of the FMF patients do not have remarkable mutations in the respective gene.This indicates existing mutations which are either more difficult to detect or are present in other genes leading to clinical FMF[65–67].Based on the fact that FMF is described as an autosomal recessive disease[65],it had been assumed that the pyrin protein may serve as a negative regulator of inflammasome activity.Pyrin comprises five domains[68],including a PYD and the B30.2domain,both involved in the putative negative regulatory effect.First,it has been shown that pyrin interacts with the adaptor protein ASC via their PYD domains[69–72].ASC is necessary for activation of at least the AIM2and the NLRP3 inflammasomes and maybe also for activation of the NLRP1 inflammasome in some cases[7].Capturing of ASC by pyrin could be a mechanism how inflammasome activity is con-trolled.This mechanism may be impaired in patients with mutated pyrin protein[73].Second,it has been shown that overexpressed B30.2 domain can interact with caspase-1and inhibit its enzymatic activity[71].Interestingly,most mutations in FMF patients cluster in the B30.2coding region[66].Accordingly,mutated B30.2has reduced binding capacities to caspase-1[71,74]. This could result in less caspase-1control and thereby in increased caspase-1activity.In contrast to the controlling function of pyrin,Yu and colleagues[75]demonstrated an enhanced inflammasome activation by overexpressing the B30.2domain[75].Accord-ingly,a knock-down of pyrin by siRNA resulted in less caspase-1activity[76].Binding ASC or NLRP3via PYD–PYD interaction could point to a supporting role of pyrin in inflammasome activation[77].However,Chae and colleagues created a mouse model in which gain-of-function mutations in pyrin caused severe inflammation depending on ASC but independent of NLRP3[78].This could serve as a hint for an inflammasome-like complex containing pyrin as a backbone but lacking NLRP3[78].Pyrin may promote IL-1βmaturation as a part of a so called pyroptosome,together with caspase-1 and ASC[75].These two opposite functions of pyrin illustrate the still incompletely understood pathogenesis of FMF.However,more studies will be necessary to finally reveal the functions and respective pathways of pyrin.2.4.Pyogenic arthritis,pyoderma gangrenosum, and acne syndrome(PAPA)PAPA(OMIM604416)[79]is another autoinflammatory disease which is indirectly associated with pyrin.Patients suffer from severe inflammation of the skin including pyoderma gangrenosum,skin ulcerations or cystic acne[80].Inflamma-tion of the joints and recurrent febrile episodes,which can occur about1–3days every month,are further characteristics of the syndrome[56].In the case of PAPA,the respective mutations are not located in the pyrin encoding gene MEFV but in the PSTPIP1 gene,which encodes a cytoskeleton organizing protein with the same name,PSTPIP1[81,82].These altered protein molecules exhibit an increased ability to interact with pyrin.A putative contribution of pyrin to inflammation via inflammasome or pyroptosome activation and elevated IL-1βmaturation has been published[75].PSTPIP1could serve as a regulator of pyrin activity.A first possible mechanism to explain hyperinflammation in PAPA patients is enhanced recruitment of pyrin to ASC mediated by PSTPIP1[80,83]. Longer and increased association between mutated PSTPIP1 and pyrin could lead to enhanced secretion of IL-1βand thus to increased inflammation[56,80].Second,PSTPIP1itself could be a regulator of inflammasome or pyroptosome activity.Prolonged interaction with pyrin could cause altered intracellular distribution of PSTPIP1.This would in return lead to deregulated pyroptosome or inflammasome activity[80,83,84].178M.C.Heymann,A.Rösen-WolffThird,concerning a putative role of pyrin as a negative regulator of inflammasome activity,increased binding of pyrin to PSTPIP1may impair the regulatory effect of pyrin on inflammasomes[85].All these possibilities show the necessity of further studies concerning pyrin,PSTPIP1and their contribution to FMF and PAPA pathophysiology.2.5.Metabolic diseases associated with inflammasomesIn the past years,more and more diseases which are not typically considered autoinflammatory syndromes have been associated with a deregulation of inflammasome activity and IL-1βmaturation.Gout is the first example(OMIM#300323).Patients suffer from sudden inflammation of the joints[77].Deposition of monosodium urate(MSU)crystals leads to inflammation of joints and periarticular tissue[28].Jürg Tschopp's group in Lausanne explored the NLRP3inflammasome as key player in the pro-inflammatory signaling that follows the recognition of MSU crystals.Peritoneal macrophages of mice lacking the proteins ASC,procaspase-1or NLRP3,which represent components that are necessary for the formation of the NLRP3-inflammasome,did not secrete mature IL-1βafter incubation with MSU,in contrast to peritoneal macrophages of wild-type mice.Additionally,none of the toll-like receptors (TLRs),which are usually responsible for sensing danger signals at the cell surface,showed significant influence on MSU sensing.Challenging single or TLR2/4double knock-out (KO)mice with MSU did not inhibit inflammation[86].Instead, phagocytosis of the crystals seems to be an important incident for IL-1βactivation after MSU exposure,including the participation of cathepsin B,ROS and the efflux of potassium ions[27,77].Finally,the successful treatment of patients suffering from gout with recombinant IL-1receptor antagonist once more underlined the association of gouty attacks with the inflammasomes[87,88].In addition,type II diabetes(OMIM#125853)has already been linked to IL-1βsince the1980s[89]and recent studies reinforced this link[90,91].Development of type II diabetes is one of the major risks of obesity and exposure to certain molecules which are present at elevated levels in obese patients stimulates inflammasome activation and leads to IL-1βand IL-18mediated inflammation.Those factors include cholesterol crystals[92],free fatty acids[93]and ceramides [94].Patients of type II diabetes show a characteristic insulin resistance and a lack in insulin secretion caused by dysfunc-tional islet beta cells in the pancreas.This in turn leads to increased levels of glucose in the circulating blood[95].Pancreatic cells from type II diabetes patients exhibit amyloid deposits[96].Especially islet amyloid peptide(IAAP) was shown to activate NLRP3and caspase-1.It further co-localized with ASC after inflammasome activation[95]. The subsequently secreted IL-1βled to impaired functionality and viability of beta cells[95].Finally,blockade of IL-1receptor in a clinical study led to a decrease of inflammatory markers in type II diabetes patients and improved the secretory potential of the beta cells[91].Notably,clinical trials of IL-1βblockade in patients with type I diabetes revealed a slowdown in the course of the disease and prevention of an additional decline of beta cell functionality[97,98].In conclusion,the contribution of the inflammasomes to other inflammatory and autoimmune diseases has become obvious.Further research is required to uncover the complex details in signal transduction leading to uncontrolled inflam-mation.Maybe future studies will reveal further diseases in which IL-1βand IL-18play an important pathophysiological role.3.Animal modelsTo further characterize the role of certain proteins in inflam-mation,mouse models appear to be valuable tools.Several manipulations of the murine genome can be performed in order to resemble human disease.1)Disruption of single genes in so-called“knock-out”(KO)mice can be used to imitate nonsense mutations with a loss of protein.2)Knock-in (KI)models harbor artificially introduced genes with or without mutations.Additionally,conditional KO or KI models can provide valuable information about the effect of the respective gene in certain tissues or cell types by using the cre/loxP-system under tissue or cell type specific promoters.In order to introduce a gene,transgenic models or gene-target models could be used.Applying the transgenic technology,the gene may insert at any locus of the genome, whereas the gene-target method ensures the insertion at a specific locus.Introduced gene mutations can result in1)reduced tran-scription of the gene,2)the truncation of protein products and/or3)amino acid exchanges that may affect protein structure or protein function.In order to study the contribution of target genes to the pathophysiology of autoinflammatory disorders,mouse models of different inflammasome components and proteins associat-ed with inflammasome activation have been created(Table1). Table1Contribution of the inflammasomes to autoinflammatory syndromes and selected mouse models. MutatedgeneAffectedproteinAssociatedsyndromeMouse modelsNlrp3NLRP3/cryopyrinFCAS[50]MWS[50,51]NOMID[53]Nlrp3−/−[28]Nlrp3A350V neoR/+[99]Nlrp3L351P neoR/+[99]Nlrp3R258W[100]Nlrp3D301N[101]Casp1(Pro)caspase-1?[57]Casp1−/−Casp11−/−[102]Casp1−/−Casp11tg[103] Mefv Pyrin FMF[39]truncated pyrin[70]KI with mutated humanB30.2domain[78] Pstpip1PSTPIP1PAPA[75,79,81,82]-NLRP3,NOD-like receptor family,pyrin domain containing3;FCAS, Familial Cold Autoinflammatory Syndrome;MWS,Muckle–Wells Syndrome;NOMID,Neonatal-onset multisystem inflammatory disease;FMF,Familial Mediterranean Fever;KI,knock-in;PSTPIP1, Proline–serine–threonine phosphatase-interacting protein1;PAPA, Pyogenic arthritis,pyoderma gangrenosum,and acne syndrome.179Contribution of the inflammasomes to autoinflammatory diseases and recent mouse models3.1.NLRP3NLRP3deficient mice were introduced by Martinon and colleagues in2006[28].The authors documented reduced inflammation in mice deficient of NLRP3when compared to wild-type animals after the administration of MSU crystals.To date,the model has been used in various studies examining whether the inflammation followed by different stimuli was NLRP3dependent[21,22,25,30].Several NLRP3KI models harboring missense mutations are available.The mutations p.A350V[99],p.L351P[99],p.R258W [100],and p.D301N[101]are equivalent to the respective mutations in humans causing CAPS subtypes.Knock-in animals,harboring the mutations p.A350V or p.L351P,develop severe systemic inflammation,causing neonatal or perinatal death[99].The mutation p.A350V led to predominant immigration of neutrophils into different tissues that was accompanied by enhanced levels of various pro-inflammatory cytokines[99].Animals,carrying the mutation p.R258W develop a MWS-like phenotype with skin inflammation,markedly enhanced levels of IL-1β,and a massive systemic expansion of Th17cells[100].Finally,in p.D301N transgenic animals,secretion of IL-1βis massively enhanced,resulting in systemic inflammation. Probably as a result of the inflammation,animals exhibit impaired skeletal growth,which links the mouse model to NOMID[101].3.2.PyrinMost of the mutations in MEFV,the gene that encodes pyrin, are located in the B30.2domain[56,78].In contrast to humans,the murine orthologue lacks this protein domain. Therefore,investigations considering FMF pathogenesis with mouse models turned out to be more complicated.The first animal models analyzed a truncated version of the protein including the complete PYD[70].This protein alteration led to increased caspase-1activation and IL-1βmaturation in homozygous animals.Heterozygous mutations prevented this phenotype and the animals were comparable with wild-type controls[70].Another approach implicated a knock-in strategy of murine pyrin protein fused to a mutated human B30.2 domain as it is present in FMF patients[78].These animals were compared to KO mice deficient in pyrin and several double crossed mice including ASC-KO-and NLRP3-KO-mice. The KI mice harboring the pyrin protein fused to human mutated B30.2variants showed enhanced inflammation and LPS induced IL-1βactivity similar to FMF patients.Moreover, growth retardation,arthritis and dermatitis were also observed in these animals[78].Finally,the inflammatory phenotype was not present in pyrin KO mice and depended on IL-1receptor and ASC expression but not on NLRP3 expression[78].3.3.(Pro)caspase-1The last mouse model that will be discussed in this review deals with the IL-1βconverting enzyme caspase-1.A Casp1KO mouse model has been used in many studies to investigate the role of caspase-1in the immunological response of cells to certain stimuli[102].Unfortunately,the respective Casp1KO mice do not only lack caspaspe-1protein but also caspase-11 [103].Thus,all so far published studies that made use of this mouse model have to be reconsidered carefully.A recent work deals with the contribution of caspase-11 to cell defense against danger signals and pathogens[103]. First,Casp11KO mice were treated with various stimuli to induce an immunological response.Casapase-11was dis-pensable for IL-1βmaturation after stimulation with ATP, flagellin,or dsDNA.In contrast,compared to wild-type animals,Casp11KO mice exhibited a significant reduction in IL-1βsecretion following the exposure to cholera toxinB (CTB),Escherichia coli(E.coli),Citrobacter rodentium,and Vibrio cholerae.Western blot analysis revealed that caspase-1activation failed in Casp11KO mice in response to the latter stimuli[103].For further studies,the scientists introduced the functional Casp11gene isolated from C57BL/6mice into the described caspase-1/caspase-11double KO mice[102,103]to rescue caspase-11expression(=Casp1−/−Casp11tg).In contrast to the double KO mice or Casp11KO mice,the new Casp1−/−Casp11tg animals exhibited enhanced cell death of macrophages and elevated IL-1αsecretion after CTB and E.coli administra-tion.Moreover,the survival of the mice after LPS shock decreased significantly in mice harboring the wild-type transgenic Casp11gene compared to double Casp1−/−Casp11−/−KO mice.In summary,caspase-11seems to play important roles in caspase-1activation and in the response to LPS shock.The protection to LPS induced lethality observed in the classical Casp1/Casp11double KO animals seems to be due to the lack of functional caspase-11and not to the Casp1knock-out,as it had been proposed.The authors postulate a new model, implying caspase-11as an important player in caspase-1 mediated IL-1βand IL-18maturation in response to non-canonical activators.According to these data,the human orthologues caspase-4and caspase-5seem to be more promising targets for medication in sepsis patients than caspase-1[103].4.ConclusionsOver the past decade,the pathophysiology of many recurrent fever syndromes has been linked to increased activity of multiprotein complexes called inflammasomes.However,the molecular mechanisms that result in increased activity of inflammasomes are partially only poorly understood.In addition to spontaneous activation of inflammasomes in autoinflammatory disorders,metabolic diseases can result in inflammasome activation.Thus,further insights into the molecular dynamics of inflammasome activity will lead to a better understanding of multiple diseases,including meta-bolic disorders,and potentially lead to new therapeutics.Undoubtedly,a lot remains to be learned about the complete network of signal cascades ranging from signal recognition to the resulting inflammation.Conflict of interest statementThe authors declare that there are no conflicts of interest.180M.C.Heymann,A.Rösen-Wolff。
