Inflammasome activation and IL-1β and IL-18 processing during infection.pdf

infl ammasome activation and il 1b and il 18 processing during infection frank l van de veerdonk1 2 mihai g netea1 2 charles a dinarello2 3and leo a b joosten1 2 1department of medicine radboud university nijmegen medical center nijmegen the netherlands 2 nijmegen center for infections infl ammation and immunity radboud university nijmegen medical center nijmegen the netherlands 3division of infectious diseases university of colorado denver aurora usa interleukin 1b il 1b andil 18contributetohostdefense againstinfection byaugmentingantimicrobialproperties of phagocytes and initiating th1 and th17 adaptive im mune responses protein complexes called infl amma somes activate intracellular caspase 1 autocatalytically which cleaves the inactive precursors of il 1b and il 18 into bioactive cytokines in this review we discuss the controversies regarding infl ammasome activation and the role of the infl ammasome during infection we highlight alternative mechanisms for processing il 1b and il 18 during infection which involve extracellular cleavage of the inactive cytokines by neutrophil derived serine proteases or proteases released from cytotoxic t cells infl ammasomes as protein platforms responsible for processing cytokines of the il 1 family the control of invading pathogens by innate host defense relies strongly on phagocytosis and killing of microorgan isms by neutrophilic granulocytes monocytes and macro phages 1 2 theactivationofneutrophilsandmacrophages toengulfpathogensandtoreleasetoxicoxygenandnitrogen radicals is dependent on pro infl ammatory cytokines such as il 1b and il 18 produced predominantly by monocytes macrophages 3 4 in contrast to other pro infl ammatory cytokines the inactive precursors of il 1b and il 18 pro il 1b and pro il 18 require cleavage into the bioactive cytokines by an enzymatic process one of the most impor tant enzymes responsible for processing pro il 1b and pro il 18 is the intracellular cysteine protease caspase 1 3 multimericproteinplatformscalledtheinfl ammasomesare important regulators of this process 5 in response to danger signals infl ammasomes are assembled from self oligomerizing scaffold proteins via nacht domain inter actions and these high molecular mass complexes trigger caspase 1 autoactivation 6 this activation of caspase 1 subsequently results in cleavage of il 1b and il 18 in this way two steps control activation of these cytokines fi rst pattern recognition of microorganisms by host cells induces transcription of pro il 1b and pro il 18 second activation of the infl ammasome by danger signals including il 1 itself results in the activation of caspase 1 and cleaving of the pro cytokines into mature bioactive il 1b and il 18 5 in this review we focus on several mechanisms that regulate il 1b and il 18 processing during infections con troversy regarding these mechanisms is discussed with emphasis on infl ammasome dependent and independent pathways activation of the infl ammasome the infl ammasome is classically composed of a nucleotide binding domain leucine rich repeat nlr protein the adaptor molecule apoptosis associated speck like protein containing a card asc domain and caspase 1 four infl ammasome complexes have been described to date figure 1 the most intensely studied is the infl amma some formed by the nlr family member nlrp3 and the adapterproteinasc figure1a thenlrp3infl ammasome can be activated by a large number of stimuli some are of bacterial origin muramyl dipeptide mdp bacterial rna and double stranded rna and others are danger associ ated molecular patterns damps such as uric acid crystals and amyloid b also exogenous compounds such as asbes tos silica or alum adjuvant have been described to activate the nlrp3 infl ammasome 7 11 in addition to the nlrp3 infl ammasome other infl ammasomes described include the nlrp1 infl ammasome activated by bacterial ligands such as mdp or anthrax toxin and the nlrc4 also known as ipaf infl ammasome activated by fl agellins figure 1b andc finally aim2 whichisnotannlrbutamemberof the pyhin family of proteins can form an infl ammasome thatcanbeactivatedbyintracytoplasmicdna figure1d the precise mechanism that initiates activation of the nlrp3 infl ammasome and the subsequent activation of caspase 1 is unclear it is unlikely that direct stimulation of nlrp3 by pathogens or specifi c ligands triggers the infl ammasome because the molecular structures of the compoundsthat activatethe nlrp3 infl ammasomeare very diverse instead three alternative mechanisms have been proposed the fi rst suggests that extracellular atp stimu lates the purogenic p2x7 atp gated ion channel leading to a k effl ux which is then necessary for infl ammasome activation however stimulation of cells with millimolar concentrations of extracellular atp alone does not activate caspase 1 and priming cells with lps is necessary for atp to induce infl ammasome activation 12 lps priming induces nlrp3 expression and this is crucial for nlrp3 activation 13 it has been suggested that k effl ux review corresponding author van de veerdonk f l f veerdonk aig umcn nl 1101471 4906 see front matter 2011 elsevier ltd all rights reserved doi 10 1016 j it 2011 01 003 trends in immunology march 2011 vol 32 no 3 triggers the formation of pores by pannexin 1 allowing the delivery of microbial products into the cytoplasm that will then activate the infl ammasome 14 the second mecha nism proposes that engulfment of damps by phagocytes results in lysosomal damage which is followed by the release of lysosomal compounds such as cathepsin b which trigger nlrp3 activation 15 16 the third mecha nismproposesthatnlrp3 infl ammasome activating ligands also stimulate production of reactive oxygen spe cies which in turn activate nlrp3 and subsequently lead to caspase 1 activation 11 the nlrp1 infl ammasome is the only infl ammasome that has been reconstituted biochemically it has been shown that nlrp1 forms oligomers in the presence of mdp 17 to date however there is no evidence that mdp actually binds nlrp1 interestingly it has been sug gested that nod2 which does bind mdp is able to form a complex with nlrp1 18 the nlr family member nlrc4 ipaf forms an infl ammasome that activates caspase 1 in response to intracellular fl agellin 19 20 nlrc4 also recog nizes proteins in the type iii secretion systems of the gram negative bacteria salmonella typhimurium prgj burkholderia pseudomallei bsak escherichia coli eprj and esci shigella fl exneri mxii and pseudomonas aer uginosa psci 21 nlrc4 recognizes a common motif that is present in these proteins as well as in fl agellin 21 naip5 which is an nlr that can hetero oligomerize with nlrc4 is involved in the fl agellin induced caspase 1 acti vation by legionella pneumophila 22 it is likely that other pattern associated molecular patterns pamps can activate nlrc4 because fl agellin independent nlrc4 acti vating pathways have been described 23 24 and s fl ex neri which does not have fl agellin can activate the nlrc4 infl ammasome 25 moreover a newly described mecha nism involving recognition of bacterial dna by the intra cellular sensor aim2 suggests the existence of a specifi c infl ammasome complex inducing caspase 1 activation up on sensing intracytoplasmic nucleic acids 26 27 differences in infl ammasome activation between cell types despite the progress made in understanding the regula tion of il 1b processing controversy surrounds the capac ity of tlr ligands such as lps to activate caspase 1 and release il 1b the monocyte like leukemia cell line thp 1 does not produce active il 1b in response to lps stimula tion and it was suggested that lps alone does not stimu late il 1b release 7 this however contradicts the results of studies demonstrating that il 1b is released from monocytes after a single stimulation with a tlr ligand 28 29 these inconsistencies were resolved recent ly by a study showing that the synthesis and release of il 1b differs between human monocytes and macrophages 30 monocytes have the ability to activate caspase 1 by autocrine production of atp therefore stimulation with a tlr ligand can immediately lead to the release of active il 1b in monocytes through induction of il 1b transcrip tion without the need for a second signal from the envi ronment to activate caspase 1 figure 2 in contrast macrophages dendritic cells and the monocytic cell line thp 1 require two distinct signals to induce il 1b tran scription and translation and to activate caspase 1 these two signals will subsequently result in il 1b processing and secretion figure 2 30 although caspase 1 is con stitutively active in freshly obtained human blood mono cytes secretion of active il 1b is still dependent on infl ammasome components particularly asc 30 atp is released from endogenous stores in freshly obtained human blood monocytes however the amount of endoge nous atp released from cultured cells is much less than the millimolar concentrations of exogenous atp required for infl ammasome activation in macrophages stimulated td fig trends in immunology nlrp3 inflammasome nlrp1 inflammasomeaim2 inflammasome nlrc4 inflammasome nlrc4 aim2 lrr lrrfiind nacht nacht pyrin pyrin pyrin pyrin pyrinhin lrrnachtcard card card card a b peptidoglycan dna rna candida mdp anthrax toxin caspase 5 caspase 1 caspase 1 caspase 1 caspase 1 flagellin dsdna nlrp3 asc asc asc c d figure 1 the four known inflammasomes a the nlrp3 inflammasome activates caspase 1 by recruiting asc b the nlrp1 inflammasome has a fiind and card domain in addition to its lrr nacht and pyrin domains and can recruit caspase 5 c the nlrc4 inflammasome has a card domain that can directly recruit pro caspase 1 d aim2 consists of a pyrin and dna binding hin domain that forms a complex with asc and caspase 1 asc apoptosis associated speck like protein containing a card fiind function to find domain lrr leucine rich repeat aim2 absent from melanoma 2 mdp muramyl dipeptide reviewtrends in immunologymarch 2011 vol 32 no 3 111 with lps nevertheless atp is essential for the activation of the nlrp3 infl ammasome 31 thus lps alone might induce il 1b secretion in monocytes because endogenous atp activates the p2x7 receptor triggering infl ammasome and caspase 1 activation in contrast macrophages lack the capacity to release signifi cant endogenous atp and require high levels of extracellular atp in addition to lps to activate il 1b secretion the demonstration that monocytes have a constitutive activation of caspase 1 uncouples caspase 1 activation from pamp recognition in human primary monocytes which is likely to represent an adaptation of the monocyte to its environment because circulating monocytes have an important role in the surveillance of an essentially patho gen free environment they must respond promptly to any danger signal by contrast macrophages and dendritic cells are confi ned to a tissue specifi c environment e g alveolar space mucosal surfaces and skin in which they are constantly exposed to microbial stimuli and danger signals if the macrophages or dendritic cells in these tissues are highly sensitive to the presence of microbial ligandsandrespondbyproducingactiveil 1b chronicand deleterious reactions would take place upon each encoun ter thus repeated bouts of infl ammation are likely to be reduced by the requirement of a second signal for the activation of the infl ammasome and release of active il 1b in macrophages and dendritic cells such a secondary signal would be available in the tissue during infection trauma or necrosis when atp levels are elevated and the p2x7 receptor is triggered 32 secondary signals can also be provided by the cathelicidin derived peptide ll37 from infi ltrating neutrophils 33 or the release of bacterial toxins 9 controversy surrounds the role of reactive oxygen spe cies ros in the activation of the nlrp3 infl ammasome as described earlier it has been proposed that all reported nlrp3 infl ammasome ligands share a fi nal common path way that stimulates ros production which activates the infl ammasome 6 but several observations suggest the opposite it is unlikely that ros alone is suffi cient to trigger the nlrp3 infl ammasome because lps alone can trigger ros production in macrophages 34 but is not able to activate the nlrp3 infl ammasome 32 early studies proposinganimportantroleforrosintheactivationofthe infl ammasome included data that infl ammasome activa tion is specifi cally reliant on nadph dependent ros 11 however this concept contrasts with the observation that cells isolated from patients with chronic granulomatous disease cgd which are defi cient in nadph dependent ros production secrete expected or even higher levels of il 1b in a caspase 1 dependent manner upon stimulation of nlrp3 35 36 several other studies are in line with the observation that nadph oxidase is not essential for nlrp3 infl ammasome activation 16 37 furthermore a defi cien cy in superoxide dismutase which leads to elevated ros levels actually inhibits caspase 1 activity 38 one expla nation for the observed inconsistencies in these reports could be the result of using chemical ros inhibitors such as diphenyleneiodonium chloride dpi although dpi is an effective inhibitor of ros generation the compound also exhibits inhibitory effects on il 1b transcription 36 as a result of reduced transcription of il 1b there would be less il 1b secreted in conclusion recent studies lead to the conclusion that nadph dependent ros are unlikely to play a major role in infl ammasome activation although an earlier study suggested that mitochondrial ros does not have a role in the activation of nlrp3 11 it has been proposed recently that excessive ros production by mito chondria can activate the nlrp3 infl ammasome 39 this controversy needs further investigation the role of the infl ammasome in regulating th1 and th17 responses il 1b and il 18 play prominent roles in polarizing t helper responses il 18 is important for the induction of the th1 response which is characterized by the production of ifng 4 ifng activates both neutrophils and macro phages for intracellular killing of bacteria or fungi the importance of ifng in host defense is demonstrated in human subjects with a defi ciency of the il 12 and ifng pathways who have increased risk for severe mycobacte rial and salmonella infections 40 il 1b also regulates ifng mediated responses and potently induces cox 2 leading to large amounts of pge2 that in turn suppress ifng production mice defi cient in il 1b produce more ifng than wild type controls by inhibiting ifng and th1 immunity il 1b contributes to polarization towards a th17 response recent studies have demonstrated that il 1b and its receptor il 1r1 are important for the early differentiation of th17 responses and il 1b can amplify td fig nlrp3 nlrp3 active caspase 1 caspase 1 asc asc tlr tlr il 1 il 1 il 1 il 1 pro il 1 pro il 1 cytoplasm cytoplasm transcription transcription second signal monocyte trends in immunology macrophage figure 2 differential il 1b secretion pathways in monocytes and macrophages caspase 1 is constitutively active in monocytes and these cells release mature il 1b after a single stimulation with a tlr ligand in contrast macrophages need two signals for il 1b secretion one such as a tlr ligand that induces il 1b transcription and a second signal that induces inflammasome activation review trends in immunologymarch 2011 vol 32 no 3 112 th17 responses in the presence of il 23 41 43 in con trast il 18doesnotcausefeveranddoesnotinducecox 2 and hence increases ifng production 44 45 th17 responses have been reported recently to play an important role in host defense during infection in infec tions caused by bacteria and fungi th17 responses mediate protective mucosal host defense th17 cells improve mucosal barrier function during infection by secretion of il 17 and il 22 that in turn stimulate the release of antimicrobial peptides and chemokines for neutrophil recruitment however in viral infections and parasitic infections the role of th17 responses is less clear and il 17 has even been reported to be detri mental for the host th17 responses can inhibit apoptosis of virus infected cells and might contribute to persis tence of the virus 46 persistent infl ammation has also been attributed to il 17 in parasitic infections 47 therefore persistent and chronic induction of th17 responses triggered by pathogens like viruses and para sites that are not suffi ciently cleared can result in immunopathology these data suggest that infl amma some activation leading to active caspase 1 and produc tion of il 1b and il 18 plays an important role in regulating the polarization of t helper responses during infection further studies are needed to characterize the contribution of the infl ammasome to directing protective t cell responses during viral bacterial fungal and para sitic infection infl ammasomes and host defense against infection experimental in vivo infection models reveal the role of the infl ammasome in the host defense against a specifi c path ogen caspase 1 defi cient mice infected with s fl exneri do not develop the acute infl ammation that is characteristic for shigellosis and are more susceptible to infection with s fl exneri 48 supporting an important role for the nlrc4 infl ammasome in shigellosis similarly infection with francisella tularensis a highly infectious gram negative coccobacillus that causes the zoonosis tularemia is much more fulminant in caspase 1 defi cient mice compared to wild type mice 49 there is also evidence for an important role of the nlrp3 infl ammasome in the host defense against infection with listeria monocytogenes caspase 1 defi cient mice that are infected with l monocytogenes have an impaired ability to clear the bacterial burden 50 inter estingly caspase 1 defi cient mice can successfully clear bacterial infection during secondary infection with listeria 50 whichsuggeststhatthenlrp3infl ammaso