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REVIEWThedynamicinterdependenceofamebiasis,innateimmunity,andundernutritionHansP.
Verkerke&WilliamA.
PetriJr.
&ChelseaS.
MarieReceived:17April2012/Accepted:21September2012/Publishedonline:1November2012#Springer-VerlagBerlinHeidelberg2012AbstractEntamoebahistolytica,theprotozoanparasitethatcausesamebicdysentery,greatlycontributestodiseaseburdeninthedevelopingworld.
Effortstoexhaustivelycharacterizethepathogenesisofamebiasishaveincreasedourunderstand-ingofthedynamichost–parasiteinteractionandtheprocessbywhichE.
histolyticatrophozoitestransitionfromgutcom-mensalstoinvadersoftheintestinalepithelium.
Mousemod-elsofdiseasecontinuetobeinstrumentalinthisarea.
Atthesametime,large-scalestudiesinhumanpopulationshaveidentifiedgeneticandenvironmentalfactorsthatinfluencesusceptibilitytoamebiasis.
Nutritionalstatushaslongbeenknowntogloballyinfluenceimmunefunction.
Soitisnotsurprisingthatundernutritionhasemergedasacriticalriskfactor.
AbetterunderstandingofhownutritionalstatusaffectsimmunitytoE.
histolyticawillhavedramaticimplicationsinthedevelopmentofnoveltreatments.
Futureworkshouldcontinuetocharacterizethefascinatinghost–parasitearmsracethatoccursateachstageofinfection.
KeywordsEntamoebahistolytica.
Amebiasis.
Innateimmunity.
UndernutritionIntroductionEntamoebahistolyticaisaprotozoanparasiteendemictomuchofthedevelopingworld.
TheWHOestimatesthat100millionyearlyE.
histolyticainfectionsresultinapproximately100,000deathsfromdysentery,colitis,andextraintestinaldiseasessuchasamebicliverabscess(ALA).
Amebiasisspreadsbyingestionoffoodandwatercontaminatedwithamebiccysts,whichareresistanttothecausticenvironmentofthestomachandpassunimpededintotheintestine.
Excystationandcolonizationoccurinthetermi-nalileum.
E.
histolyticatypicallyestablishesacommensalrelationshipwiththehostandcystsofnonpathogenictroph-ozoitesareexcreted,perpetuatingthelifecycleoftheparasite.
Itisestimatedthat90%ofcasesareasymptomaticwhile10%ofinfectionsprogresstosymptomaticdisease.
Asmallbutsignificantsubpopulationofsymptomaticindividualsdevel-opssevereextraintestinalamebiasis,includingamebicliverabscess[1,2].
IthaslongbeensuspectedbutisnowconfirmedthatundernutritionsignificantlyincreasessusceptibilitytoE.
histolyticainfection.
Undernutritioncontributesto2.
2mil-liondeathsand21%ofdisability-adjustedlifeyearsinchildrenunder5.
Estimatesarethatundernutritionanditsassociateddiseasescontributeto35%ofchilddeathsand11%oftheglobaldiseaseburden[3].
StudiesofamebiasisinhumanpopulationsandmousemodelshaveinformedourknowledgeofhostfactorsthatinfluencesusceptibilitytoE.
histolyticaandparasitefactorsthatmaydirectlyalterviru-lence.
AdaptiveimmunitydevelopsincontinuallyexposedhumanpopulationsandisbelievedtomainlyresultfromproductionofneutralizingantibodiestargetedtotheGal/GalNAcadherencelectin[4].
SecretoryIgAagainstthislectinisassociatedwithprotectioninhumans[5]andhasrecentlybeenshowntocorrelatewithimmunityinexperi-mentallyvaccinatedbaboons[6].
Studiestargetingvaccinedevelopmentinmicesuggestthatprotectiveimmunityismediatedbyaninterferongamma(IFN-γ)-dependentTcellresponseduringinfectionwithE.
histolyticatrophozoites[7,8].
Asvaccinedevelopmentisongoing,thisreviewfocusesontheinnateimmuneresponseduringamebiasis.
WeThisarticleisacontributiontothespecialissueonImmunoparasitology-GuestEditor:MiguelStadeckerH.
P.
Verkerke:W.
A.
PetriJr.
(*):C.
S.
MarieDivisionofInfectiousDiseasesandInternationalHealth,UniversityofVirginiaHealthSystem,MR6buildingRoom1107,29815thSTSW,Charlottesville,VA22903,USAe-mail:wap3g@virginia.
eduSeminImmunopathol(2012)34:771–785DOI10.
1007/s00281-012-0349-1emphasizetheimportanceofenvironmentalfactorsthatalterhostresistancetoamebicdisease,inparticulartheemergingroleofhostnutritionalstatus.
WealsoexplorerecentworkthathasshedlightonbothhostandparasitegeneticfactorsimportantinthepathogenesisofE.
histolyticainfection.
ParasiteKnownamebicvirulencefactorsincludetheGal/GalNAcadherencelectin,cysteineproteases(CPs),arginase,ameba-pores,alcoholdehydrogenase,peroxiredoxin,cyclooxyge-nase2,andlipopeptidophosphoglycan(LPPG)[9,10].
StudiesofE.
histolyticavirulencecontinuetouncovernewfactorsinvolvedinpathogenesisandinteractionwiththehostinnateimmunesystemandmorearelikelytoemergefromfurtheranalysisoftheparasitegenome[11].
Charac-terizedvirulencefactorsofE.
histolyticaareoftendown-regulatedinnonpathogenicisolates[12,13]orabsentinthenonpathogenicamebicspeciesEntamoebadisparandEnt-amoebamoshkovski[14–16]RecentworkinE.
histolyticagenomicssupportsarolefortheparasitegenotypeandgeneticrecombinationintheprogressionofamebiasis.
Alietal.
havefoundthattroph-ozoitesisolatedfromthegutandliverofpatientswithbothintestinalandextraintestinaldiseasearegeneticallydistinctfromparasitesinthestool[17].
Oneexplanationisthatgeneticallydistinctsubpopulationsofvirulenttrophozoitesexistinthegut.
Alternatively,progressionofamebicdiseasemaycorrespondtogeneticreorganizationeventssuchasrecombinationthatpromoteaninvasivephenotype.
Howev-er,thehostandparasiteeffectorsdrivingtheseeventsateachstageofinfectionremainpoorlydefined.
Developingmorepowerfultoolstoperformwithin-patientcomparisonsofamebicvirulenceatdifferentstagesofinfectionwillgreatlyincreaseourunderstandingofthisprocess.
ItalsoremainsunclearwhetherregionalgenomicdifferencesinE.
histolyticastrainsaffectpathogenicity.
Gilchristetal.
compellinglydemonstratedthatexpressionofvirulencegenesisregulatedatthemRNAlevelinthefirstgenome-wideanalysisoftheE.
histolyticatranscriptomein2006.
ThisstudydemonstratedthatinvasivetrophozoitesinmicesignificantlyalteredmRNAexpressionforgeneswithdiverserolesinmetabolismandvirulence[18].
Workonthecalcium-regulatedtranscriptionfactorURE3BPalsosup-portsacrucialrolefortranscriptionalregulationinamebicvirulence.
URE3BPwasoriginallyidentifiedasaregulatoroftheGal/GalNAcadherencelectinandferredoxin.
Laterworkidentifiedacalcium-dependentphospholipid-associatedpeptide,ehC2A,whichseques-tersURE3BPtotheplasmamembraneinhighcalcium[19].
AcomprehensivestudyoftheroleofURE3BPinamebicvirulencehasrevealedthattrophozoitesexpressingadominantpositiveformofURE3BParemorepathogenic,formingdramaticallylargerlesionsintheliversofchallengedgerbils[20].
ThepotentcytotoxiceffectthatE.
histolyticaexertsonhostcellsisevidencethattheparasitepossessesauniquearmamentariumofvirulencefactors.
However,muchworkremainstobedonetodefineparasitefactorsandhowtheyinteractwiththehostateachstageofdisease.
Furtherclarifyingthemechanismsbywhichtrophozoitestransitionfromintestinalcolonizerstoinvasivepathogenshaspoten-tialforfutureanti-parasiticdrugdevelopment.
EnvironmentDiarrhealdiseaseslikeamebiasisareendemictothede-velopingworldwhereanumberofenvironmentalfactorsincludinglimitedaccesstocleanfoodandwater,andexposuretootherenteropathogenscontributetoincreasedriskofinfection[1,3,16,21–23].
Wearenowinthe14thyearoftrackingthemedicalhistoryofacohortofchil-drenlivinginMirpur,anurbanslumofDhaka,Bangla-deshwhereE.
histolyticainfectionisoneofthemajorcausesofdiarrhealillness.
SomeofthemajorfindingsfromthispopulationaresummarizedinTable1.
Apicturehasbeguntoemergeinwhichinfantundernutritionandstuntingpredisposesyoungchildrentoamebiasis,whichinturnexacerbatespreexistingnutritionalimbalancebyalteringgutfunction.
Undernutritionincreasestheriskofdiarrheaanditsas-sociatedmortality[24].
UndernourishedchildrenhavehigherratesofinfectionbyprotozoanparasitesparticularlyE.
histolyticaandCryptosporidiumrelativetowell-nourishedcontrols[25].
ArecentstudyofinfantsfromMirpurinthefirstyearoflifebyMondaletal.
showedthatundernutritionandstuntingat12monthsofageissignifi-cantlyassociatedwithnutritionalstatusatbirth,durationandseverityofdiarrhealepisodes,anddiminishedintestinalbarrierfunction[26].
Whilethisassociationisquiteclear,thecomplexetiologyisnotyetfullyunderstood.
Proteinenergymalnutritionisthemostcommoncauseofhumansecondaryimmunodeficiency,whichisparticularlyproblematicfordiarrhealdiseasesthatfurtherdiminishnu-trientuptakefromthegastrointestinaltract[27–30].
Acutestarvationinitiatesenergy-savingmechanisms,shuntingpowerawayfromimmunefunction.
Chronicnutritionaldeprivationleadstosevereimmunedeficitsexemplifiedbythedramaticthymicatrophyobservedincadaversofunder-nourishedindividuals[31,32].
Complementdeficits,re-ducedIgAproductionandspecificity,alteredgutbarrierfunctionduetorestructuredmicrovilli,impairedphagocyticcapacity,anddiminishedoxidativebursthaveallbeende-scribedasconsequencesofenergydeficitintheimmune772SeminImmunopathol(2012)34:771–785Table1SummaryofstudiesfromMirpuramebiasisbirthcohortAuthorDateStudyMajorfindingsHaqueetal.
2001MeasuredstoolIgAanti-lectinandassociationwithincidenceofEntamoebahistolyticadiarrheaMucousalIgAantibodiesraisedagainsttheamebicGalNAc-lecticareprotectiveagainstE.
histolytica.
Corroboratedin2006Haqueetal.
2002Anti-lecticserumIgGIsolatedtrophozoitesweregeneticallydiverseAnti-lecticfecalIgALackofserumIgGagainsttheGalNAclecticlinkedtoresistancePCRanalysisbyisolatedtrophozoites86%reductioninnewinfectionof12monthsinchildrenwhodevelopedIgAanti-lectinHaqueetal.
2003DiagnostictoolstomeasurecausesofdiarrhealepisodesWell-nourishedchildrenhadfewerepisodesofdiarrhealoverallMostfrequentcausesofdiarrheaincludedenterotoxigenicEscherichiacoli,Shigella,Rotavirus,Giardialamblia,Cryptosporidiumparvum,andE.
histolyticaE.
histolyticacontributedtooverallmorbidityfromdiarrhealillnessDuggaletal.
2004GenotypeleukocyteantigenclassIIallelesChildrenwithDQB1*0601/DRB1*1501haplotypewere10.
1timesmoreresistanttoE.
histolyticaandlesslikelytobesero-positiveforanti-lectinIgGHLAclassII-restrictedimmuneresponsesprotectiveagainstE.
histolyticainfectionTarletonetal.
2006VerbalandnonverbaltestforcognitiveabilityCognitivescorenegativelyassociatedwithstuntinginschool-agechildrenE.
histolyticadiarrheawasnotindependentlyassociatedwithcognitivedeficitswhenthestudywascontrolwithsocialfactorsMondaletal.
2006WeightforageZ-scoreE.
histolytica-associateddiarrhealillnesswasnegativelyassociatedwithgrowthofpreschoolchildrenasmeasuredbyWHOWAZandHAZHeigthforageZ-scoreCryptosporidiumandGiardiadiarrhealepisodesnotassociatedwithgrowthStoolantigendiagnosticsforCryptosporidiumandE.
histolyticaMicroscopicdiagnosicsforGiardiaHaqueetal.
2007IFN-γproductionbyamebicantigenstimulatedPBMCsAbove-averagesecretionofIFN-γbyPBMCsassociatedwithlongersurvivalwithoutE.
histolyticadiarrhealepisodesAdjustedforstunting,theincreasedriskremainedmildlysignificantConcludedthatIFN-γproductionlinkedtonutritionalstatusisprotectiveMondaletal.
2009AnalyzedassociationbetweenE.
histolyticadiarrheaandundaernutritionEnterotoxigenicE.
coli,Cryptosporidium,andE.
histolyticainfectionmorecommoninmalnourishedchildrenMalnutritioncontributestosusceptibilityinasubsetofentericinfectionsPetersonetal.
2010TNF-αproductionbyamebicantigenstimulatedPBMCsHighlevelofTNF-αproductionassociatedwithincreaseriskofthefirstandrecurrentE.
histolyticadiarrhealepisodesMicroarrayanalysisofwholebloodandcolonbiopsysamplesfrompatientsinacuteandconvalescentstagesofamebiasisSymptomaticinfectionwasassociatedwithsignificantlyhigherproductionofTNF-αproteinandmildlyelevatedTNF-αmessagelevelsDuggaletal.
2011MeasuredgeneticvariantsinleptinandtheleptinreceptorandinvestigatedtheirassociationwithamebiasisChildernhomozygousforaSNPencodingasingleaminoacidsubstitutionintheleptinreceptor(Q223R)werefoundtobenearlyfourtimesmorelikelytohaveaninfectioncomparedwiththosehomozygousforthewild-typealleleAdultALApatientswerelesslikelytocarrytheprotectiveglutaminealleleMicecarryingtheprotectivealleleweresignificantlymoreresistanttoexperimentalamebiasisPetersonetal.
2011MicroarrayanalysisofintestinalbiopsiesinacuteandconvalescentamebiasisREG1AandREG1BwerehighlyupregulatedinhumanbiopsiesfrompatientswithacuteversusconvalescentamebiasisRT-qPCRforREG1A/1BResultconfirmedbyRT-qPCRChallengedREG1knockoutmicewithE.
histolyticatrophozoitesREG1knockoutmiceweresignificantlymoresusceptibletoexperimentalamebiasisSeminImmunopathol(2012)34:771–785773system(reviewedin[33]).
Globalsuppressionanddysregu-lationofimmunefunctionassociatedwithacuteandchronicundernutritionmaybethemostimportantindependentpre-dictorofE.
histolyticadiarrheainthefirstyearoflife.
Reciprocally,E.
histolyticadiarrhealepisodesincreasetheriskofbecomingundernourished[23,26,34,35].
Chil-drenwithE.
histolytica-associateddiarrheaarenearlythreetimesmorelikelytobeclinicallyundernourishedandareatdramaticallyincreasedriskofstunting[34].
Childhoodundernutritionanddiarrheaareinextricablylinkedwithprofoundconsequencesfortreatmentout-comesinthedevelopingworld.
Intestinalinflammationresultingfromrepeatedexposuretoentericpathogensleadstomalabsorptionofcriticalnutrients,whilesecond-aryimmunodeficiencyrelatedtocompromisednutritionalstatusmakeschildrenunder5particularlyvulnerabletothedevastatingcycleofundernutrition,disease,andpov-erty.
Tobreakthiscycle,aholisticapproachisneeded.
TreatmentstrategiestotargetE.
histolyticadiarrheainthecontextofanundernourishedpatientmustbedesigned.
Morebroadly,nutritionalsupplementationshouldbeenrichedtoprovideessentialnutrientsaffectedbyrepeat-edexposuretoenteropathogensandsuchsupplementscouldbecomearoutinepartoftreatment.
Finally,suc-cessfulvaccinesmayutilizeadjuvantsthatstimulatein-natefactorsdeficientinundernourishedindividuals.
Whiletherapeuticandnutritionalinterventionsarecritical,humangeneticfactorsthataffectinnateresistancetoE.
histolyticainfectionfurthercomplicatetheinteractionbe-tweennutritionalstatusandparasiticdisease.
HostNaturalimmunityinhumansMountingevidencesuggeststhathostgeneticfactorsinflu-encefrequencyandseverityofE.
histolyticadiarrhealepisodes.
TheHLAclassIIalleleDQB1*0601andthehaplotypeDQB1*0601/DRB1*1501areassociatedwithdrasticallydecreasedratesofE.
histolyticainfection[36].
Morerecently,a12-yearstudyoftheMirpurcohorthasshownthatasingleaminoacidreplacement(Q223R)intheextracellulardomainoftheleptinreceptorencodedbyaSNPisassociatedwithanearlyfourfoldincreaseinsuscep-tibilitytoinfectionbyE.
histolyticaaswellasanoveralldecreaseintimetoinfection.
TheeffectsofthemutationwereconfirmedinapopulationofadultmaleALApatients,wherehomozygosityforthealleleencodingarginineatposition223wasassociatedwithtwofoldincreasedsuscep-tibilitytoextraintestinaldisease.
Theseobservationshigh-lighttheimportanceofleptinsignalingandpointtoaspecificmutationthatincreasessusceptibilitytoE.
histoly-tica,decreasestimetoinfection,andincreasesthelikelihoodofdevelopingextraintestinalamebiasis[37].
Itisclearthatthehostgeneticbackgroundinfluencessusceptibilitytoamebiasis.
However,environmentalfactorscomplicatethepicturebysimultaneouslyaffectingthephe-notype.
Itisimportanttodelineateprimaryresistancedeter-minedbygeneticsandsecondaryresistancedeterminedbynutritionalstatusandotherenvironmentalfactors.
Itiscrit-icaltoidentifymorehostgeneticfactorsthataffectE.
histolyticainfection.
Thesestudieswillnotonlyincreaseourunderstandingofthepathogenesisofamebiasisbutalsoaidintargetingtherapiestoat-riskpopulations.
CytokinesTNF-αTNF-αisacytokinewithdiverserolesinimmunity.
Dependingonthefactorsrecruitedtoligand-boundTNFR1/2,TNF-αcanstimulategrowthanddifferentiationthroughMAPK,survivalandinflammationthroughNFκB,orcelldeaththroughcaspaseactivation[38].
TheamebiclectininducesTNF-αproductionbymacrophages[39]andTable1(continued)AuthorDateStudyMajorfindingsMondaletal.
2012Analysisofassociationbetweenmalnutrition,socioeconomicstate,immunefactorandamebiasisinthefirstyearoflifeChildrenmalnourishedatbirthweresignificantlymoresusceptibletoE.
histolytica,Cryptosporidium,andETECinfectionsMalnutritionin12monthsispredictedbyprolongeddiarrhealepisodes,intestinalbarrierdysfunction,maternaleducation,andfamilyincomeConcludedthatmalnutritionatbirthpredisposeschildrentoamebiasiswhichaltersintestinalbarrierfunctionandinfluencesnutritionalstatusat12monthsHere,wedescribemajorfindingsfromthelong-termprospectivestudyofapopulationlivinginMirpur,anurbanslumofDhaka,Bangladesh.
Childrenwereobservedeveryotherday.
DiarrhealstoolsweretestedforE.
histolyticausingantigendetectionandPCR.
Bloodwasdrawnevery4monthsfordetectionofserumantibodiesandcytokineresponse,andmonthlystoolsampleswereobtainedtomeasuresecretoryimmunoglobulinAresponse.
Anthropometricswereassessedevery4months774SeminImmunopathol(2012)34:771–785TNF-αactsasapotentchemoattractantforE.
histolytica[40];however,theroleofTNF-αinamebiasisisnotentirelyclear.
Invitro,TNF-αenhanceskillingofE.
histolyticatroph-ozoitesbyneutrophilsandmacrophages[41,42].
Invivo,TNF-αseemstoexacerbatedisease.
BlockingTNF-αpro-ductioninthehumanxenographSCIDmousemodelofamebiasisreducesinflammationandintestinaldamage[43].
Thisfindingwasrecentlyextendedtohumanpopula-tionswhereTNF-αproductionbyamebicantigenstimulat-edPBMCscorrelatedwithincreasedsusceptibilitytoE.
histolyticadiarrhea[44].
IthasbeensuggestedthatIFN-γandprostaglandinE2modulateTNF-αproductionduringinfection[45].
Thus,acomplexarrayoffactorslikelydeter-mineswhetherTNF-αhasaprotectiveordestructiveroleduringamebicinfection.
IFN-γAnalysisofcytokineprofilesintheMirpurstudypopulationhasalsoshownthataboveaverageproductionofIFN-γbutnotIL-5byamebicantigenstimulatedPBMCswasassociatedwithfavorableoutcomesanddecreasedincidenceofE.
histo-lyticainfection.
Whencontrolledfornutritionalstatus,theobservationremainedonlymildlysignificantsuggestingthatIFN-γproductionwasdampenedinmalnourishedsubjects[46].
TheimportanceofIFN-γhasbeenrecapitulatedinmurinevaccinationstudies,whichhavedemonstratedthatIFN-γisasignificantcorrelateofprotectionforE.
histolytica[7,8].
IL-10IL-10-deficientmicearemoresusceptibletoamebicinfec-tion[47],andundernutritionmodifiesIL-10production[48]perhapscontributingtoincreasedsusceptibilityofunder-nourishedindividualstoE.
histolyticadiarrhea.
Overall,IL-10islikelytoplayaroleinconnectingnutritionalstatustoamebiasis.
LeptinTheadipocytokineleptinlinksnutritiontoimmunity.
Im-munefunctionsareimpairedandleptinlevelslowinunder-nourishedindividuals[49].
However,theroleofleptininstarvation-inducedimmunedysregulationhasnotbeenfullyelucidated.
Datafromhumanstudiesintotheeffectsofleptindeficiencyonimmunityinmalnourishedchildrenarecontradictory[50,51].
Nonetheless,miceandhumanswithimpairedleptinsignalingexhibitcompromisedim-munefunctionandaremoresusceptibletomanyinfections,includingamebiasis[52,53]Leptinsignalsthroughthelongformofitsreceptorviathereceptor-associatedJanuskinase2(JAK2).
JAK2phosphorylatesintracellulartyrosineresiduesY985,Y1077,andY1138,whichrecruitSHP2,STAT5,andSTAT3,respec-tively.
SHP2positivelyregulatestheERK/c-fospathway[54].
ActivatedSTAT3upregulatesSOCS3,whichservesasbothafeedbackinhibitorofleptinsignalingandarepressorofapo-ptoticpathways.
Leptinreceptor-mediatedSOCS3signalingalsoregulatesanumberofproinflammatorypathways[55]Leptinsignalingplaysacrucialandwell-characterizedroleinanumberofimmunepathwaysthatareimportantforpro-tectionagainstamebiasis.
LeptinenhancesCCLchemokinesecretioninmurinemacrophages[56],whichpromotesche-motaxisofmonocytes.
Leptinalsoactsdirectlytomobilizemonocytepopulationsvialeptin-dependentPI3K/Aktactiva-tion[57].
Inleptinandleptinreceptor-deficientmice,macro-phagesexhibitphenotypicvariationslikediminishedphagocyticcapacityandloweredcytokineproduction[58,59].
LeptinsignalssecretionofproinflammatorycytokinesTNF-αandIL-6[60]andmayregulatethemonocyticoxidativeburst[61].
Stimulationwithleptinupregulatesproductionofinterleukin1receptorantagonist[62]andIFN-γinducibleprotein[63].
Overall,leptincontributestoaTh1proinflammatoryimmuneresponse,whichiscrucialforclearanceoftrophozoitesduringamebiasis.
Leptinre-ceptornullmice(db/db)exhibitaskewedTh2cytokineprofileanddiminishedTcellproliferation[64],whichcor-relatewithincreasedsusceptibilitytoE.
histolytica.
ProinflammatorychemokinesandcytokinesIL-6,IL1-β,andCXCL1areupregulatedinresponsetoleptin[65],andleptinconcentrationsspikeduringgutinflammation[66].
Theprimaryactionofleptininintestinalepithelialcells(IECs)appearstobemediatedthroughSTAT3,atranscrip-tionfactorknowntoregulateepithelialhomeostasisduringinfectionandinflammation[67,68].
PathogenicstrainsofbacteriainmiceactivateSTAT3incolonicepithelialcells,initiatingaprotectiveTh17mucosalimmuneresponse,characterizedbysecretionofIL-17.
TH-17responsestim-ulatesproductionofIL-6andIL-8,potentmediatorsofinflammationandneutrophilchemotaxis,respectively[69].
Leptinstimulatesanti-apoptoticandproliferativepath-waysthatpromotebarrierfunctionandwoundhealinginepitheliallayers[66,69–73].
Leptinsignalingisanti-apoptoticinavarietyofcelltypes[74].
Dendriticcellsfromdb/dbmicearemorepronetoapoptosispotentiallyasaresultofdecreasedPI3K/Akt,STAT3,andIκB-αsignaling[75,76].
Increasedcaspase-3expressionisobservedincolonicepithelialcellsofdb/dbmice[69].
Thepleiotropicroleofleptininimmunefunctionandenergybalancesupportsthehypothesisthatleptiniscriticaltoanappropriateimmuneresponseinentericdiseaseslikeamebiasisandmayexplainmechanisticallythestrongasso-ciationbetweenundernutritionandamebicdisease.
RecentstudiesinhumansandmicehaverevealedtheimportanceofleptinsignalinginprotectionagainstE.
histolytica.
SeminImmunopathol(2012)34:771–785775Guoetal.
haveshownthatob/obanddb/dbmicearemoresusceptibletoamebicinfectionthanarewild-typecontrols.
Moreover,leptinsignalingintheintestinalepithe-liumalonewassufficienttoconferwild-typeresistance.
Thesiteofleptin-mediatedresistancetoE.
histolyticawasde-terminedusingtissuespecificdeletionsoftheleptinreceptorinE.
histolyticaresistantC57B2/6mice.
Deletionoftheleptinreceptorintheintestinalepitheliumwassufficienttorendermicesusceptibletoamebicinfection.
ExpressionofaleptinreceptordeficientinSTAT5signalingalsorescuedthesusceptiblephenotype.
However,complementationwithreceptorsdeficientinSHP-2orSTAT3didnotrestoreresis-tance,implicatingSTAT3andSHP2butnotSTAT5inleptin-mediatedresistancetoamebiasis[52].
Tofurtherunderstandleptinsignalinginhostdefense,wedevelopedanassaytomeasureamebiccytotoxicityinsinglecells.
Physiologicalconcentrationsofleptinwereprotectiveagainstamebiccytotoxicityincellsthatendog-enouslyorexogenouslyexpressedleptinreceptor.
InHEK293Tcellsexogenouslyexpressingleptinreceptorsig-nalingmutants,onlycellsdeficientintheleptin-dependentSTAT3signalingpathwaylostleptin-mediatedresistancetoamebicchallengewhileSHP2andSTAT5mutantsremainedresistant.
TheQ223Rmutationintheleptinreceptoralsoincreasedamebiccytotoxicityinvitro,andcellsexpressingtheQ223formoftheleptinreceptordisplayedsignificantlyhigherlevelsofactivatedSTAT3inresponsetoleptin[77].
Incombination,theseresultsimplythatleptin-dependentSTAT3activationmediatesdirectcellularresistancetoame-biccytotoxicitywhileleptin-activatedSHP-2signalingmaybeimportantincoordinatingtheglobalimmuneresponsetoamebiasis.
Itisimportanttoemphasizethatthismechanismwaselucidatedinmurineandinvitromodelsofamebicdiseaseandhasyettobeextensivelyvalidatedinhumans.
Innatecell-mediatedimmunitytoE.
histolyticaIntestinalimmunityTheintestinalepitheliumisessentialforprotectionagainstamebiasis.
Approximately90%ofE.
histolyticainfectionsarecommensal,suggestingthatthegutishighlyeffectiveatlimitingprogressiontosymptomaticdisease[78].
Thepro-tectiveroleoftheepitheliuminamebiasisismediatedbythemucousallayer,whichpreventsamebafromadheringtoIECs.
IECsandtightgapjunctionsprovideasecondphys-icalbarriertoinvasion.
Asthesebarriersbegintofail,epithelialcellsrecruitandactivateothereffectorsofinnateimmunitythroughchemokineandcytokinerelease.
Resi-dentmicrobesmayalsoaffectE.
histolyticavirulenceandintestinalinflammation.
AsimplifiedmodelofintestinalinvasionbyE.
histolyticatrophozoitesisdepictedinFig.
1.
MucousallayerMucusoffersthefirstlayerofprotectionagainstinvadingtrophozoitesintheintestine.
Gobletcellssecretemucins,whichsolubilizetoformagelatinousbarrierbetweenthelumenandepitheliumformingabarrieragainstentericpathogens.
StudiesbyChadeeetal.
haveshownthatsecret-edcolonicmucinsarecriticalforgastricmucosalrepair[79]andresistancetobacterialcolitis[80].
TheprocessbywhichinvasiveE.
histolyticatrophozoitespenetratetheprotectivemucousbarriertocontactandkillsub-mucousalepithelialcellsiswellunderstood.
TheGal/GalNacadherencelectinofE.
histolyticaadherestoglycosylatedcomponentsofthemucousalmatrix[81].
Next,parasiticcysteineproteasescleavemucinsatsitesoflowglycosylation.
Degradingmucinsandothercomponentsofthemucouslayerallowsamebatobindsub-mucousalcells,inducecelldeath,andpenetratetheepithelium[82].
RecentworkinMUC2knockoutmicehashighlightedthespecificimportanceofmucinsasprotectivemoleculesagainstE.
histolytica.
Bergstrometal.
demonstratedthatMUC2deficientmicechallengedwithE.
histolyticadis-playedanacuteproinflammatoryresponse,characterizedbyincreasedgrosspathology,luminalTNF-αandIFN-γ,andalteredexpressionofgapjunctionproteins[80].
ResultsfromChadeeetal.
recentlyrecapitulatedthisfindingincolonicloopsfromMUC2-deficientmiceandhavegoneontoshowthattrophozoitesalsorequireMUC2toinvadetheepithelium.
E.
histolyticaappearstoinducehigherMUC2expressionandtrophozoitesbecometrappedinthemucouscoatingofthecolonicepitheliumpresumablybe-foreinvading.
E.
histolyticacysteineprotease5(CP5)directlycleavesMUC2.
Expressionofthisproteaselikelyallowstheparasitetoovercometheprotectivemucousbarrier[83].
WhenCP5issilenced,trophozoitesfailtopenetratethecoloniclaminapropriaanddonotinduceproinflammatorycytokinesecre-tioninexvivomodelsofamebiccolonicinvasion[84].
TheseresultssuggestthatdisassociationofE.
histolyticafromthecolonicepitheliumbymucous,preventsadamag-inginflammatoryresponse.
MicrobiomeThemicrobesthatcolonizetheshallowsectionofthemu-cosallayeroftheintestinalepitheliummayimprovebarrierfunctionbyinhibitingadherenceofpathogenicspeciesandcompetingfornutrients.
Entamoebaarenaturallycolonizedbybacteria,whichaffectparasitevirulence[85].
Theaxeni-zationofEntamoebaaltersthesurfaceantigensofthepar-asite[86].
Astrophozoitesfeedprimarilyonresidentgutflora,itisplausiblethatdifferencesinhostmicrobialecol-ogycontributetotheinitiationofavirulentphenotype.
776SeminImmunopathol(2012)34:771–785Already,reductioninlactobacillusspecieshasbeenob-servedinALApatients[87].
Numerousstudieshaveshownthatmicrobialcomposi-tionislinkedtonutritionalstatusanditscompositioncanalternutrientabsorptionandbioavailability[88–91].
Nutri-entdeprivationaltershostsusceptibilitybutmayalsocon-tributetovirulenceoftheparasite.
Severalmicro-andmacronutrientshavebeenshowntoinfluenceE.
histolyticavirulenceincludingcalciumviaregulationofURE3BP[19],glucose[92],cholesterol[93],andiron[94].
Theemergingfieldofmicrobialmetagenomicsislikelytocontinuetorevealanimportantroleforthegutmicro-biomeinthepathogenesisofE.
histolyticaviamodulationoftheimmuneresponseandvirulenceoftheparasiteitself.
AntimicrobialcompoundsSecretedimmunoglobulins,defensins,andcathalecidinsre-inforcemucousalbarrierfunction,activelypreventingpath-ogeniccolonizationbyameba.
Cathelicidins,antimicrobialpeptidesresidentinthegutmucosa,areimportantmediatorsofmammalianinnateimmunityandwhileE.
histolyticatrophozoitesactivatecathelicidinproductioninbothhumanandmouseintestinalepitheliallayers,theyareresistanttotheircytolyticeffect.
AmebiccysteineproteasescleavethecathelicidinsLL-37andCRAMPbutthefragmentsretainbactericidalactivity[95].
Fourregeneratinggene(REG)C-typelectinshavebeencharacterizedinhumans.
Overall,REGsfunctiontoinducecellularproliferationandinhibitapoptosis.
REGIαseemstohaveaphysiologicalroleinthegastricmucosaandREGsarefoundthroughoutthegastrointestinalsystem.
REGIα,REGIβ,andREGIIImRNAsareoverexpressedinthecolonininflammatoryboweldisease,Crohn'sdisease,andulcerativecolitis[96].
Microarrayanalysisofintestinalbi-opsiesfrompatientsinearlyandrecoverystagesofamebi-asishasshownthatREGIαandREGIβarehighlyupregulatedduringacuteinfectionrelativetoconvalescenceandimmunohistochemicalstudiesconfirmthisresult[97].
Thus,REGslikelyhaveafunctionalroleinregenerationoftheintestinalepitheliumafteramebicdisease.
TherecentlydiscoveredantimicrobialfunctionofREGIII[98]suggeststhatREGproteinsmayhavedualfunctionsinintestinalbarrierreinforcementandamebickilling[99].
IntestinalepithelialcellsIECsrecognizemultipleamebicantigensanddirecttheimmuneresponseintheacutephaseofamebiasis.
Itisgenerallythoughtthattheepitheliumdownregulatespatternrecognitionreceptors(PRR)topreventchronicinflamma-tioninresponsetoresidentmicrobialantigens[9].
Interest-ingly,stimulationofIEClayerswiththeadherencelectinofE.
histolyticaincreasesexpressionofTLR2,whichrecog-nizesamebicLPPGandactivatesNFκB-regulatedsecretionofproinflammatorycytokinesfromhumanmonocytes,mac-rophages,anddendriticcells[100].
Invitro,whenstimulat-edwithamebicantigens,IECssecreteTNF-α,IL-6,GROa,andGM-CSF[101].
E.
histolyticaDNAhasbeenshowntoactivateTLR9[100]whichrecognizesunmethylatedCpGdinucleotides[101].
Additionally,CP5bindstoα(V)β(3)integrinonCaco-2coloniccellsviatheRGBmotifandstimulatesanNFκB-mediatedproinflammatoryresponsesviaPI3K/Akt[102,103].
IntheSCIDmouse–humanxeno-graftmodelofALA,amebicchallengeincreasedIL-8andIL-1βsecretionpresumablypromotingacuteneutrophilicinfiltration[104].
IECdeathitselfmayrecruitandstimulateimmunecells.
Ithaslongbeenthoughtthatcontact-Fig.
1Intestinalimmunity.
E.
histolyticatrophozoitesencounteralayeredhostdefenseastheyinvadetheintestine.
Intestinalepithelialcells(IECs)directaninflammatoryresponsetoinvasiveamebabyrecruitingneutrophils,macrophages,andotherimmunecells.
Trophozoitesthatsurvivemayprogresstothecirculatorysystemwheretheyencounterhumoralantibodiesandmustevadecomplement-mediatedlysisSeminImmunopathol(2012)34:771–785777dependentkillingbytrophozoiteswascanonicallyapoptotic.
Amebiccytotoxicityinducesmembraneblebbing,TUNELpositivity,surfaceexposureofphosphatidylserine,andcas-paseactivation.
Furthermore,caspaseinhibitionandoverex-pressionofanti-apoptoticeffectorslikeBcl-2areprotectiveinmousemodelsofdisease[105].
Pyropototicandnecroticdeathareincreasinglyrecognizedasimportantforamebiccytotoxicityashostcellsexperiencearapidlossofmembraneintegritynotconsistentwithcanonicalapoptosis.
Theselatterdeathpathwaysmaycontributemorepotentlytoinflammationintheintestinethroughreleaseofhostcellularcomponents.
AmebicinvasionthroughtheIEClayerandintothelaminapropriamayoccurpassivelythroughsmallgapsresultingfrominflammation[101].
Activepenetrationislikelymediatedbyamebicproteasesthatcleavegapjunc-tions,facilitatingtransepithelialmovement[14].
Interesting-ly,E.
histolyticahasadaptedtodampentheNFκB-mediatedinflammatoryresponseofIECsviainductionofheatshockprotein27(hsp27),whichsuppressesNFκBtranscriptionalregulation[106].
TheseobservationssuggestthattheintestinalepitheliumiscrucialbarriertoinfectionbyE.
histolytica.
PathogenicamebamustbreachthemucousalmatrixtoaccessIECs,whosePRRsrecognizeandrespondtoamebicinvasion.
Attheonsetofacuteamebiccolitis,ulcersformandinflam-matorysignalsfromIECsrecruiteffectorsofinnateimmu-nitytothesiteofdangerwhileinitiatinganadaptiveresponse.
Thedegreeofimmuneactivationintheintestinalepitheliumlikelydeterminestheprogressionandseverityofamebiasisasmuchoftheresultanttissuedamageismediat-edbythehostinflammatoryresponse.
Post-epithelialimmunityNeutrophilsTheprimaryroleofneutrophilsininnateim-munityistodestroyandengulfpathogensinaffectedtis-sues.
NeutrophilextracellularNADPHoxidasegeneratesreactiveoxygenspeciesthataretoxictoinvadingpathogens.
Neutrophilsalsoexpresssurfacereceptorsforopsoninsthatfacilitatephagocytosisofpathogensmarkedbyotherbranchesoftheimmunesystemfordestruction[115].
Neu-trophilsalsoexudeextracellulartraps(NETs)composedofchromatinandserineproteasesthatensnareandneutralizepathogens[116].
AroleforNETsinthepathogenesisofamebiasishasyettobecharacterized.
Trophozoitesthatbreachtheepithelialbarrierelicitarapidinfiltrationofneutrophilstothesiteofinfectioninearlystagesofintestinaldisease[107,108].
IECssecreteIL-8,apotentneutrophilchemoattractant,whenexposedtoE.
histolytica[109].
ActivationandtranslocationmayalsooccurinresponsetoanaphylotoxinsC5aandC3aofthecomplementsystemthoughamebicproteasescleaveandinactivatebotheffectors[110].
Itisgenerallyacceptedthatneutrophilsareresponsibleformuchofthetissuedamageassociatedwithamebiccolitis.
Whileneutrophilsdopossessamebacidalactivity,whichlike-lyaidsinpathogenclearanceforasymptomaticinfections,virulenttrophozoiteshaveadaptedeffectivemechanismsofresistanceandcounterattack.
NeutrophilsincultureamplifythecytopathogenicityofE.
histoltyicatrophozoitesonepithe-lialcells[111]likelyduetoneutrophildegranulationoftoxiccellularcomponents[112].
Leptindeficiencyandgeneticdifferencesinleptinsig-nalingpathwaysmayalsoaffectneutrophilresponsetoamebicinvasion.
Neutrophilsexpressashortformoftheleptinreceptorthatissufficientforleptinsignaling,whichinhibitsapoptoticpathwaysandactsasachemoattractant[54,113],andmaymodulateoxidativecapacityinneutro-philsduringinfection[114].
Thereissomeevidencefrommouseandinvitromodelsofamebiasisthatneutrophilsplayaprotectiveroleearlyininvasion[105,117,118].
NeutrophildepletionintheSCIDmousemodelexacerbatesintestinaldiseaseandliverab-scessformation[105,117,118].
However,theGR-1anti-bodiesusedinthesestudiesmayalsodepletemonocytesandeosinophils.
Inearlystagesofhepaticamebiasis,neutrophilsarethemajoreffectorsoftheacuteinflammatoryresponse,[119]butcanbebothprotective[120]anddestructive[111].
NeutrophilsprimedwithIFN-γ,TNF-α,orLPSdoexhibitamebicidalactivity[112].
However,E.
histolyticahasproventobefarmoreeffectiveatkillingneutrophils,withmorevirulentstrainskillingataratioof1trophozoiteto3,000neutrophils[112].
E.
histolyticatrophozoitesinduceneutrophilapoptosisdirectlybyengagingwithintegrinsandbyactivatingNADPHoxidase[121,122].
Invasiveamebicstrainsareresistanttotherespiratoryburstasanamebicperoxidoredoxinprotectstroph-ozoitesfromreactiveoxygenspecies[121,122].
Neutrophilcorpsesarequicklyphagocytosedbytrophozoites[112].
Figure2summarizessomeofwhatisknownabouttheroleofneutrophilsinhostdefenseagainstE.
histolytica.
MacrophagesMacrophagesplayimportantrolesinthepathogenesisofbothintestinalandhepaticamebiasis.
Macro-phagesareactivatedbyE.
histolyticatrophozoitesandareamebicidalwhenstimulatedwithCSF,IFN-γ,andTNF-α[112].
TheamebicsurfacecomponentLPPGactivatesaclas-sicalinflammatorymacrophageresponseuponTLR2/6andTLR4binding[123].
E.
histolyticaDNAalsopromotesmacrophageactivationbyTLR9binding[104].
Furthermore,macrophagesupregulateTLR2inresponsetotheamebicgalactose-specificlectin[104].
Macrophageamebicidalactiv-ityismediatedbynitricoxidesynthase(NOS)[124].
NOinhibitsimportantamebicvirulencefactorsincludingthecys-teineproteasesandalcoholdehydrogenase2[125].
Invasiveamebaareabletoregulatemacrophageresponsesinanumberofways.
Analogoustotheirinteractionwithneutrophils,trophozoitesinhibittherespiratoryburstof778SeminImmunopathol(2012)34:771–785macrophages.
Withmacrophages,thisfunctiondependsonanamebicarginasethatcompetitivelyconvertsL-arginine,asub-strateofmacrophageNOS,toL-ornithine[126].
Amebaalsomodulatemacrophagecytokinesecretion[127,128].
Produc-tionofTNF-αisblockedbyamebicdegradationofc-fosandTNF-αtranscripts[129].
E.
histolyticaproducesacyclooxy-genase2,theprostaglandinproductofwhichhashost-immunomodulatoryfunction[130,131].
Prostaglandintrig-gersacAMPspikeinmacrophages.
ThissignalactivatesPKAandIL-8secretion[131].
PKAinhibitsexpressionofTh1cytokinesandPKC-mediatedNOsynthesis[130].
Finally,themonocytelocomotioninhibitoryfactor(MLIF)ofE.
his-tolyticahasbeenimplicatedinalteringmacrophagefunction.
MLIFinhibitsNOandproinflammatorycytokineproductionwhilestimulatingthesecretionofIL-10,animportantanti-inflammatoryeffector[132,133].
Macrophagesrepresentasecondarylineofdefenseafterthemassiveinfiltrationofneutrophilsduringacutehepaticamebiasis.
NOSisprotectiveanditsliverspecificdeletionincreasesseverityofALAininfectedmice[124].
IFN-γisalsoimportantformacrophage-mediatedprotectionagainsttrophozoites.
BlockingIFN-γreceptorsincreasessusceptibil-itytoALAinmice[134].
Classicallyactivatedmacrophagesareeffectiveatclearingtrophozoitesandevenalternativeactivationcanpromotewoundrepairandrecruitmentofadap-tiveimmunecells[135].
E.
histolyticasuppressesbothacti-vatedclassesofmacrophages,underminingcell-mediatedimmunityandleadingtoestablishmentofchronicALA[127].
Figure3summarizessomeofwhatisknownabouttheroleofmacrophagesinhostdefenseagainstE.
histolytica.
NaturalkillerandnaturalkillerTcellsNaturalkillercellsdonotrequireprimingtobefunctionallycytotoxicagainstpathogens.
Theircanonicallymyopic"killer"functionhasbeenchallengedinrecentyearsasmorestudieselucidatediverserolesforNKcellsinimmuneregulation[136].
ActivatedNKcellssecreteIFN-γandTNF-α,importantfactorsinhost-immunitytoamebiasis.
NKcellsinfiltratetheliverduringtheacutephaseofexperimentalALAinmousemodels[137].
VirulenttrophozoitesstimulateNKcellcyto-toxicitymoreeffectivelythandonon-pathogenicstrains[138].
E.
histolyticamayregulateNKcellsthroughCP-mediatedcleavageofC5a,ananaphylotoxinknowntoacti-vateNKcellsduringsepsis[139].
NKTcellsexpressmarkersthatdelineateNKcellsbutco-expressαβTCR.
TheydifferfrommostT-cellpopulationsinthattheyrecognizeantigenspresentedbyCD1dinsteadofMHCassociatedantigens.
InvariantNKT(iNKT)cellsareasubsetofNKTcellsthatexpressinvariantα-TCRandproducelargeamountsofpro-andanti-inflammatorycyto-kinesuponactivation.
iNKTcellsareactivatedtoproduceIFN-γbutnotIL-4inaTLR/CD1d-dependentmanner[140].
StudiesinmousemodelsofALAsuggestthatclear-anceoftrophozoitesduringearlystagesofhepaticcoloni-zationisdependentonNKTcellactivity.
MicelackingNKTcellsarehighlysusceptibletoALA[141].
ClearancewasassociatedwithelevatedIFN-γproduction.
Likeneutrophils,NKTcellslikelyhaveacontext-dependenteffectondiseaseprogression.
TheirresponsetoamebicantigensmaycontributetopolarizingtheimmuneresponsetowardtheTh1phenotypethoughttobeprotectiveinamebiasis.
IntermsofALA,aftertheacutephaseofhepaticinfiltration,particularlyresistanttrophozoitessur-viveandexpand.
Atthisstage,iNKTcellsmayincreasetissuedamagebypropagatingdeleteriousinflammatorysig-nalsandstimulatingneutrophilchemotaxis.
Fig.
2NeutrophilsrecruitedtothesiteofE.
histolyticainfectiontargetpathogensbysecretingNADPHoxidase,anenzymethatcatalyzestheproductionofreactiveoxygenspecies.
Interleukins,interferons,andtumornecrosisfactoralphaprimeandrecruitmoreimmunecells.
Theparasiterespondswithperoxiredoxinandcysteineproteases,whichcleaveandneutralizehostfactorstomakewayforlectin-mediatedadherenceandeffectorsofamebiccytotoxicitySeminImmunopathol(2012)34:771–785779ComplementThepathogenesisofextraintestinalamebiasisdependsonevasionofhumoralinnateimmunitybyE.
histolyticaandbothalternativeandclassicalbranchesofthecomplementsystemareinvolved.
Allcomplementpath-waysconvergetoproduceapathogen-associatedC3-convertasethatcleavesC3intotheopsoninC3bandtheinflammatoryanaphylotoxinC3a.
Theproteolyticcascadeculminatesinpathogenlysisviadepositionandpolymeri-zationofpore-formingmembraneattackcomplexes(MACs)onopsonizedinvaders[142].
E.
histolyticacanactivateboththealternativebranchandtheserum-antibody-dependentclassicalbranchofthecom-plementcascade[143,144].
AneutralE.
histolyticacysteineproteasecleavesC3toanactiveisoformofC3b[144].
Introphozoitesthataresusceptibletocomplement,C3b,acleavageproductofamebicCP,successfullyrecruitstermi-naleffectorsofthecascaderesultinginMACformationandparasitelysis[145].
Trophozoitesisolatedfromasymptom-aticcasesarereadilylysedinacomplement-dependentmanner,butpathogenictrophozoitesfrompatientswithamebicliverabscessorinvasivecolitisareresistanttocomplement-mediatedkilling[146].
ThesestudiessuggestthatcomplementisanimportantbarrieragainstinvasionbyE.
histolytica,presumablycontributingtothelowincidenceofextraintestinaldisease.
Furthermore,amebicresistancetocomplement-mediatedkillingrepresentsanadaptationuniquetotrophozoitesthatcauseextraintestinaldisease.
Hostcellspreventtheirowncomplement-mediatedlysisbyexpressingsurfaceCD59.
ExpressionofCD59inhibitsopso-nizationandMACformationonhostcells[147].
TheGal/GalNaclectinofE.
histolyticaisamultisubunitGPI-anchoredcomplexessentialforparasiterecognitionofhostsurfaces.
Bragaetal.
showedthatthislectincontainsaCD59-likeregionthatinhibitsMACformationandprotectstheparasitefromcomplement-mediatedlysis[148].
Inagreementwiththeseresults,globalinhibitionofGPIanchorformationleavespreviouslyresistantE.
histolyticatrophozoitessusceptibletocomplement-mediatedlysis[149].
OtherCD59-likeamebicproteinsmayalsocontributetoresistance.
Venturaetal.
recentlyidentifiedanovel21kDaamebicsurfaceproteinthatreactswithhumananti-CD59[150].
However,itsfunctional-ityasaninhibitorofMACformationanditsmoleculariden-tityhaveyettobeelucidated.
Amebiccysteineproteasesalsoplayaroleincomplementevasion.
ThesurfaceEhCPimplicatedinAPactivationbyC3cleavage[144]alsocleavesandinactivatestheinflammatoryanaphylotoxinsC3aandC5a[110].
ThusCPsappearstoplayopposingrolesinevasionofhostimmunitybyE.
histolytica,ontheonehandactivatingcomplementandontheothersuppressingnumerousinflammatorypathwaysdownstreamFig.
3MacrophagesrecruitedtothesiteofE.
histolyticainfectionareclassicallyactivatedbyinterferongammaandothercytokines.
TheyproduceNOtokillpathogensandsecreteIL-10andIL-8,promotinggutbarrierfunctionandneutrophilrecruitment,respectively.
Thepar-asiticarginasecompetitivelyinhibitsandtheMLIFaltersexpressionofhostnitrousoxidesynthase.
E.
histolyticaCpGDNAactivatesTLR9780SeminImmunopathol(2012)34:771–785ofC3aandC5a.
Athirdmechanismofcomplementresistancemayinvolveenrichmentoflypophosphoglycan(LPG)intheouterleafletoftheE.
hystolyticaplasmamembrane.
WhileLPGisproducedinthepathogenicE.
histolytica,significantlevelsarenotobservedinthenon-pathogenicstrainE.
dispar[151].
LPGprotectsLeishmaniafromcomplement-mediatedlysisbyimpedingthedepositionofproteolyticcomplexesandpreventingMACformation[152].
E.
histolyticatrophozoitesmayemployasimilarstrategy.
ALAisapproximatelyfivetimesmoreprevalentinmenthaninwomen,suggestingasexualdimorphisminsusceptibil-itytoextraintestinalamebiasis[153].
Differencesinthecyto-pathogeniceffectsofcomplementonE.
histolyticabetweenthetwosexeshavebeenimplicated[9].
Nonimmunefemaleserumkillstrophozoitesmoreeffectivelythandoesnonimmunemaleserum[154].
However,itisunlikelythatcomplementissolelyresponsibleforthedifferenceinsusceptibilitytoextraintestinaldisease.
Interestingly,thesexualdimorphismobservedinhumanswascorroboratedinmousestudies[134].
FemaleB6miceweremoreeffectivethanmalesatclearinginvasivetroph-ozoitesfromtheliverinaSCIDmousemodelofALA[141].
ClearancewasassociatedwitharapidNKTcellresponseandhighIFN-γproduction[141].
Malemiceclearedinfectionslesseffectively,IFN-γproductionwaslessrobust,andsignificantIL-4productionwasobserved.
ConclusionsGenerationsofresearchhaveprovidedatremendousbodyofknowledgeabouthostimmunitytoamebiasis.
Recentworkhashighlightedtheimportanceoftheparasitegenomeandtran-scriptomeinprogressiontoavirulentphenotype.
Significantprogresshasalsobeenmadeinvaccineanddrugdevelopmentandwenowhaveabetterunderstandingofenvironmentalfactorsthatcontributetoamebiasis.
ThecomplexityoftheinteractionbetweenE.
histolyticatrophozoitesandhost-innateimmunitycontinuestomakeidentificationoffactorsthatinflu-encesusceptibilitytoinfectionafascinatingchallenge.
Effec-tiveapproacheshaveunitedpowerfultechniquesinpopulationgenetics,immunology,andepidemiologytoprobethefullcomplexityoftherelationship.
Usingworkinhumanpopula-tionstoinformmouseandinvitrostudiesofamebiasishasbeenparticularlyeffective,allowingustofocusonhostandparasitefactorsproventoeffectimmunityinhumans.
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