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Altered macrophage differentiation and immune dysfunction in
tumor development
Antonio Sica1 and Vincenzo Bronte2
1Istituto Clinico Humanitas, Instituto di Ricovero e Cura a
Carattere Scientifico (IRCCS), Rozzano, Italy. 2Istituto Oncologico
Veneto, IRCCS, Padua, Italy.
Tumorsrequireaconstantinfluxofmyelomonocyticcellstosupporttheangiogenesisandstromaremodelingneededfortheirgrowth.Thisismediatedbytumor-derivedfactors,whichcausesustainedmyelopoiesisandtheaccumulationandfunctionaldifferentiationofmyelomonocyticcells,mostofwhicharemacrophages,atthetumorsite.Animportantsideeffectoftheaccumulationandfunctionaldifferentiationofthesecellsisthattheycaninducelymphocytedysfunction.Acompleteunderstandingofthecomplexinterplaybetweenneoplasticandmyelomono-cyticcellsmightoffernoveltargetsfortherapeuticinterventionaimedatdeprivingtumorcellsofimportantgrowthsupportandenhancingtheantitumorimmuneresponse.
Althoughclinicaltrialsevaluatingtheeffectivenessofnovelcancervaccinesindicatethatincancerpatientstheycaninducerobustimmuneresponsesagainsttumorantigens,theclinicalbenefitsofthesevaccineshavebeenlimited(1,2).Thereasonsbehindtheselimitedclinicalresponsesarenotknownbutmightberelated,inpart,totheimmunosuppressiveeffectsoftumors.Immunedysregulationandsuppressionincancerpatientsisacompositeeventinwhichtumor-derivedfactorsconditionnotonlyperiph-eral
immuneniches,
inwhichdysfunctionandevendeathoftumor-specificTcellscanoccur,butalsothebonemarrowandotherhematopoieticorgans(suchasthemousespleen),leadingtoabnormalmyelopoiesisandtheaccumulationofimmunosuppres-sivemyelomonocyticcellsatthetumorsite(3,4).Dysregulationand/orsuppressionoftumor-specificTcellfunction(s)isthere-forelikelytooccurat2separatesites:locally,atthetumor-hostinterface,wherecancercellsdirectlyconditionthetumorstroma;andsystemically,whereanexpandedpoolofimmatureandimmu-nosuppressivemyeloidcellsarefreetocirculateandmediatesup-pressioninthebloodandlymphoidorgans.ThisReviewattemptstoanalyzethemainmyeloidcellpopulationsthatrestrainantitu-morimmuneresponses.
Immunosuppression and cancer: history and
nomenclatureAlthoughapopulationofnotverywelldefinedcellscallednatural
suppressorswasassociatedintheearly1980swithimmunesuppres-sionandtumordevelopment(5),thefirstdescriptionindicatingthatincreasednumbersofmyeloidcellsintumor-bearinghostsmightalterantitumorimmunereactivitywasprovidedbyHansSchreibersgroup(6,7).Inonekeyexperiment,theadministra-tionofaGr-1specificantibodythatrecognizesbothLy6CandLy6Gtoimmunocompetentmicereducedthegrowthofavari-antofaUVlightinducedtumorabletoprogressmoreaggres-sivelythanitsparentaltumorcellline(6).Thisvariantwasknown
toattractmoreleukocytesthantheparentalcellline,apropertyattributedtothereleaseofanoncharacterizedchemotacticfactor,anditsgrowthinvivowasknowntoberestrainedmainlybyCD8+Tcells.Interestingly,eliminationofGr-1+cellsinathymicnudemice(whichlackmostTcells)alsoslowedthegrowthofthisaggressivevariant,suggestingthatGr-1+leukocytesintumor-bearinghostsmightalsopromotetumorgrowthanddevelopment(7).TheeffectofinvivotreatmentwiththisGr-1specificantibodywasoriginallyattributedtotheeliminationofgranulocytes(whichareknowntoexpresshighlevelsofLy6GbutlowlevelsofLy6C),butsuccessivereportsfromseveralgroupsindicatedthattheGr-1specificanti-bodycouldbindandeliminateothercellsintheblood.Gr-1+cellsintumor-bearinghostswere,infact,mostlyCD11b+andcom-prisedbothpolymorphonuclearandmononuclearcells,includingcellsatdifferentstagesofmaturationalongthemyelomonocyticdifferentiationpathway,
thereby
revealingaprofoundaltera-tioninmyelopoiesisduringtumorprogression(4,8)(Figure1).Myelopoiesis,infact,isnotonlyincreasedinthebonemarrowandspleenoftumor-bearingmicebutisalsoaltered,sincethemyelo-monocyticcellscannotproperlydifferentiateintoprofessionalAPCs,suchasDCs(reviewedinref.9).
Heterogeneity of myeloid-derived suppressor
cellsTheheterogeneityoftheCD11b+Gr-1+cellshasgeneratedsomeconfusion,inparticularbecauseofthenomenclatureusedpre-viouslytodefinethem(i.e.,immaturemyeloidcellsormyeloidsuppressorcells).Recently,apanelofleadinginvestigatorsinthefieldagreedtousethecommontermmyeloid-derived
suppressor cells
(MDSCs)(10).TheMDSCdefinitioninvolvesasynthesisofthefunctionalandphenotypicpropertiesofthecells.MDSCscanbedefinedasapopulationofmyelomonocyticcellsnormallylack-ingthemarkersofmaturemyeloidcellsandcommonlyexpressingbothGr-1andCD11binmice,withahighpotentialtosuppressimmuneresponsesinvitroandinvivo.TheexactnatureoftheMDSCpopulationdependsonvariousfactorsdescribedbelow,themostimportantofwhichisprobablythetumortype.Eventhoughnumerousfindingssuggestthatthemonocytic,
ratherthanthegranulocytic,fractionofmouseCD11b+Gr-1+cellsisresponsiblefortheimmunedysfunctionsinducedbythiscellpopulation,bothinvitroandinvivo,inantigen-specificCD8+Tcells
(1113), theuseof thetermmyeloid is
justifiedbytheincompleteunderstandingoftherelationshipbetweenthetwo
Nonstandardabbreviationsused:ARG1,arginase1;CCL2,CCchemokineligand2;CD3,chainoftheCD3componentoftheTCRcomplex;CXCL12,CXCchemokineligand12;CXCR4,CXCchemokinereceptor4;HIF-1,hypoxia-induciblefactor1;IL-4R,IL-4receptor-chain;MDSC,myeloid-derivedsuppressorcell;SHIP,Srchomology2domaincontaininginositol-5-phosphatase;TAM,tumor-associatedmacrophage.
Conflictofinterest:Theauthorshavedeclaredthatnoconflictofinterestexists.
Citationforthisarticle:J. Clin.
Invest.117:11551166(2007).doi:10.1172/JCI31422.
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mainprogenyoftheenhancedmyelopoiesisobservedintumor-bearinghosts(i.e.,granulocytesandmonocytes;Figure1).BothMDSCsandtumor-associatedmacrophages(TAMs)haveaphe-notypesimilartothatofalternativelyactivatedmacrophages(alsoknownasM2macrophages)inthemouse,asdiscussedbelow,andtumor-conditionedgranulocytesmighthavearoleininfluenc-ingthisactivationprocess.Itmustbepointedout,infact,thatinmice,threedifferentneutrophilsubsetshavebeenisolatedthatcanconditionmonocyte/macrophagedifferentiationtowardtheclassicoralternativeactivationpathwaybyreleasingdifferentcytokinesandchemokines(14).Furthermore,humangranulocytesubpopulationsinpatientswithrenalcellcancerhavebeenshowntofunctionasMDSCs(15,16).CD11b+Gr-1+cellsarenormallypresentinthebonemarrowof
healthymiceandaccumulateinthespleenandbloodoftumor-bearingmice(1720).CD11b+Gr-1+cellspresentinsteady-stateconditionsarenotabletoinducesuppressionofantigen-stimulatedTcells,atleastnottothesameextentasthecellsthataccumulateintumor-bearingmice,andrecentdatasupportthepossibility
thatexogenouslyprovidedIL-13mightconferonthemsuppres-siveactivity(21,22).BerzofskyandcolleagueshaveshownthatasubsetofNKTcellsrecognizingtumor-derivedglycolipidspresent-edbytheMHC-likemoleculeCD1releasesIL-13.ThisIL-13canthenactivateCD11b+GR-1+cellstosuppresstumor-specificCTLsthroughaSTAT6pathwayinitiatedbytheIL-4receptor-chain(IL-4R),whichiscommontothereceptorsforIL-4andIL-13(21,22).Thiscircuitisactivatedveryearlyaftertumorimplantationinmice,beforeanyincreaseinthenumberofCD11b+Gr-1+cellsisdetected.Inseveralexperimentalmodels,however,systemicaccu-mulationofCD11b+Gr-1+cells,probablyresultingfrombothdif-ferentiationofprecursorsandrecruitmenttoparticularanatomi-calsites,precedesandisimportantformediatingsuppressionofTcells,notonlyincancerbutalsoduringinfections(Table1).
MDSC suppression of T cell
functionThebiologyandpropertiesofMDSCsintumor-bearinghostshavebeenextensivelydescribedinrecentreviews(4,8,23)andaresum-marizedhereinProperties
of MDSCs.Themechanismsunderlying
Figure 1Current view of TAM and MDSC differentiation. HSCs give
rise to common myeloid precursors (CMPs), which subsequently
originate at least three subsets of cells circulating in
tumor-bearing hosts that can be identified by specific markers:
monocytes (CD11b+Gr-1+F4/80+), granulocytes
(CD11b+Gr-1highF4/80IL-4R), and MDSCs
(CD11b+Gr-1medF4/80low/IL-4R+). Circulating monocytes are recruited
by tumors and differentiate into TAMs, acquiring protumoral
functions. During tumor progression, MDSCs accumulating in blood
and in lymphoid organs such as the spleen may also be recruited to
the tumor microenvironment, where they become F4/80+. This latter
pathway of MDSC-TAM phenotype transition (dashed arrow) was
recently proposed (13, 27). Finally, it has been hypothesized that
immature forms of granulocytes might differentiate into MDSCs or
condition their function and/or further differentiation (red
arrows), as suggested by some studies (14).
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theinhibitoryactivityofMDSCsareprobablyvarious,rangingfromthoserequiringdirectcell-cellcontacttoothersindirectlymediatedbymodificationofthemicroenvironment.MDSCsfresh-lyisolatedfromthespleensoftumor-bearingmicewereoriginallyshowntosuppressthefunctionalactivityofCD8+Tcells,butnotCD4+Tcells,byinterferingwiththeirabilitytosecreteIFN-whenstimulatedwithspecificantigens(19,24).ThiseffectwasthoughttoberelatedtothefactthatMDSCsexpressedMHCclassIbutnotMHCclassIIandwasmediatedbycell-cellcontactandtheproductionofROSsuchashydrogenperoxide(H2O2),triggeredbyMDSCexpressionoftheenzymearginase1(ARG1)(19).TheroleofH2O2asamediatorofTcelldysfunctionseemstocorrelate,atleastinsomestudies,withdecreasedexpressionofthechainoftheCD3componentoftheTCRcomplex(CD3)(25).OtherstudieshaveshownthatcirculatingMDSCshavetobeactivatedbyantigen-experiencedTcellstoexecutetheirsuppressiveprogramandthattheycansuppress,inanMHC-independentfashion,bothantigen-activatedCD4+andCD8+Tcells(11,13,20).Moreover,asubsetofMDSCs(expressingCD11b,Gr-1,CD115,andF4/80)isolatedfromthebonemarrowandspleensoftumor-bearingmicecaninducethedevelopmentofFOXP3+CD4+(FOXP3,fork-headboxp3)TregsinvivobyapathwayrequiringIFN-andIL-10(26).Interestingly,productionofNOwasnotrequiredforMDSC
inductionofTregswhereasNO,releasedbyNOS,hasbeenshowntobeextensivelyinvolvedintheTcelldysfunctioninducedbyMDSCs(Table1),suggestingthatthedifferentbiologicalactivitiesofMDSCsmightbeseparatedatthemolecularlevelandperhapstargetedbydistincttherapeuticapproaches.Someissuesmustbeconsideredwhenanalyzingthepartially
conflictingresultsonthemechanismofMDSC-dependentsup-pressionofTcells.TheinvitroassaysevaluatingtheinhibitorypropertiesofMDSCsarenotstandardized,soindifferentstudiestheymightdifferbothinthetypeofstimuliandsourceofTcells.WhenTcellsarestimulatedinvitrointhepresenceofsupraphysi-ologicnumbersofMDSCs,themechanismsgoverningsuppres-sionmightdifferfromthoseactivatedininvitroassayswheretheratioofMDSCstoTcellsisthesameasfoundinthelymphoidorgansofmice,whereMDSCsarerecruitedinpathologicalsitu-ations.Incontrasttotheinvitroassays,theabilityofMDSCstoinducetumor-specificCD8+Tcellstobecomenonfunctionalinvivohasbeenrepeatedlyconfirmed,althoughmanystudiesarebasedontheuseofeithersmallmoleculesaffectingMDSCinhibi-torypathwaysorantibodiesdepletingGr-1+cells(11,22,2729).Itmustbeemphasizedthattheinterpretationofinvivoexperi-mentswithinhibitorsiscomplicatedbythepossibilitythatthesemoleculesaffectcellsotherthanMDSCs.
Table 1Myeloid celldependent suppression of T cells in mice
Pathology Suppressorcells Phenotype Mousestrain
MechanismofTcellinhibition References isolatedfrom:
Cancer
Colon carcinomas Spleen and tumor CD11b+Gr-1+ BALB/c ARG and NO
dependent (11, 17, (CT26 and C26) 44, 115)Melanoma (B16) Spleen
CD11b+Gr-1+ C57BL/6 NOS dependent (45)Lymphoma (EL-4) Tumor
CD11b+Gr-1+F4/80+ C57BL/6 ARG and NO dependent (13)Colon
adenocarcinoma Tumor F4/80+ C57BL/6 NO and cell-associated form
(116) (MCA-38) of TNF-Mammary carcinoma (4T1) Spleen
CD11b+Gr-1+CD11c+ BALB/c ARG dependent (20)Lewis lung carcinoma
Tumor CD11b+Gr-1F4/80CD80+ C57BL/6 ARG dependent (47)Lewis lung
carcinoma Tumor CD31+ C57BL/6 NO and TGF- dependent (117)T cell
lymphoma (BW-Sp3) Spleen CD11b+Gr-1intLy6GCD115int AKR ARG and NO
independent; (12, 118) partially dependent on PPARFibrosarcoma (C3)
Spleen CD11b+Gr-1+ C57BL/6 ARG and H2O2 (19)Transformed fibroblasts
Spleen CD11b+Gr-1+ BALB/c NKT cells, IL-13, STAT6, TGF-; (21, 22)
(1512RM) NOS independent; ARG not tested
Infection
Candida albicans Blood, spleen CD11b+Gr-1+CD80+ BALB/c IFN-/NO
and CD80 (119) polymorphonuclear cellsTrypanosoma cruzi Spleen
CD11b+Gr-1+ C57BL/6 and Sv129 IFN-/NOS (120)Schistosoma mansoni
Spleen CD11b+Gr-1+CD16+ BALB/c and B10.D2 Unidentified soluble
factor (121) (not IL-4, IL-10, or TGF-)Taenia crassiceps Peritoneum
CD11b+Gr-1+ BALB/c 12/15-Lipoxygenase, NO and ARG
(122)Porphyromonas Spleen, BM but CD11b+Gr-1+ BALB/c IFN- (52)
gingivalis not lymph nodesSchistosome Peritoneum CD11b+Gr-1+F4/80+
BALB/c and C57BL/6 IFN-/NO; partly IL-10 dependent (123)
oligosaccharide (Lacto-N-neotetraose)Cruzipain antigen Spleen
CD11b+Gr-1+ BALB/c Not investigated; ARG and NOS (124) from T.
cruzi (extramedullary activity increased in macrophages
hematopoiesis)
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General properties of MDSCs and their relationship with M2
macrophagesThesuppressiveprogramofMDSCscanbetriggeredbytheirinter-actionwithantigen-activatedCD8+Tcellsbothinvitroandinvivo,throughanIFN-andcell-contactdependentstepthatmightrequiretheexpressionofCD80andCD11bonthesurfaceoftheMDSCs(11,19,30).Interestingly,simpleinvitrocultureofMDSCsalonecanactivatethisprogram.Thereasonbehindthecommonfindingthatcells
isolatedeitherwithGr-1specificorCD11b-specificantibodiesandculturedinvitro(withorwithoutGM-CSF)becomemacrophage-likecells(i.e.,theygainaCD11b+Gr-1
F4/80+CD80+MHCclassII/lowphenotype)withenhancedimmuno-suppressiveactivity(1113)hasnotbeenfullyinvestigated.TheinhibitorypropertiesofMDSCsareprobablymediatedby
theexpressionofinducibleformsofNOS(i.e.,NOS2)andARG(i.e.,ARG1).BothNOS2andARG1areinvolvedinthemetabolismoftheaminoacidl-Arg(Figure2).NOS2,aheme-containingenzymethatcatalyzesthesynthesisofNOandcitrullinefroml-Arg,isexpressedbyvariouscellsoftheimmunesystem,anditsactiva-tionisconsideredahallmarkofclassicallyactivatedmacrophages(alsoknownasM1macrophages),amacrophagesubsetthatpro-ducesproinflammatorycytokinesandactsastheeffectorcellinthekillingofinvadingpathogens(3133).InM1macrophages,expressionofthegeneencodingNOS2dependsontheactiva-tionoftranscriptionfactors,suchasNF-B,JAK3,andSTAT1aswellasJNK(34),anditcanbetranscriptionallyupregulatedbyproinflammatorycytokines(e.g.,IFNs,IL-1,IL-2,andTNF-),bac-terialLPS,andhypoxia(35,36).Bycontrast,ARG1(alsoknownasliver-typeARGbecauseitisfoundpredominantlyinhepatocytes)isamanganesemetalloenzymethatcatalyzesthehydrolysisofl-Argtol-ornithineandurea(Figure2).However,ARG1isalsoinducedincellsoftheinnateimmunesystembyseveralcytokines
includingTGF-(37),themacrophage-stimulatingprotein(MSP)actingonthereceptorRON(38),GM-CSF(39),andeitherIL-4orIL-13,bothofwhichactivateaSTAT6signalingpathway(40).IncontrasttoNOS2,whoseactivationisconsideredahallmarkofM1macrophages,ARG1activationhasbeenregardedasoneofthemostspecificmarkersofM2macrophages,whichactasimpor-tantmediatorsofallergicresponses,controlparasiticinfections,mediatewoundrepairandfibrosis,andhavebeenfoundintheleukocyteinfiltratesofvarioushumanandmousetumors,wheretheyhavebeensuspectedofpromotingtumorigenesis(31,32),asfurtherdiscussedbelow.DespitethisdistinctexpressionofNOS2andARG1inM1andM2macrophages,respectively,MDSCshavebeenshowntoexpressNOS2and/orARG1,andrecentstudiesindicatethatMDSCshavecharacteristicsofbothM1andM2macrophages.Indeed,werecentlydescribedintumor-bearingmiceapopulationofcirculatingCD11b+Gr-1+inflammatorymonocytesexpressingIL-4RandabletoreleasebothIL-13andIFN-(11),characteristicsthatarecompatiblewithafunctionintermediatebetweenthoseofM1andM2macrophages.TosuppressCD8+Tcells,thesecirculatinginflammatorymonocyteshadtobeacti-vatedbyIFN-producedbyantigen-stimulatedTcells,releasetheirownIFN-andIL-13,andberesponsivetoIL-13byexpress-ingafunctionalIL-13receptor,includingtheIL-4Rsubunit(11).IL-4RisthereforeausefulmarkerfordiscriminatingbetweenpopulationsofimmunosuppressiveMDSCs(IL-4R+)andnon-suppressivegranulocytes(IL-4R),bothofwhichareincreasedinthebloodandspleensoftumor-bearingmice(Figure1).Coopera-tionbetweenIL-13andIFN-ledtosustainedactivationofbothARG1andNOS2inMDSCpopulations,causingdysfunctionalTcellresponses(11).Importantly,CD11b+TAMsalsorequirethesamecombinationofcytokines(IL-13andIFN-)tomediatesup-pressionofCD8+Tcells(11).TheseresultssuggestthatMDSCs
Properties of MDSCs
CoexpressionofthemyeloidcellmarkersCD11bandGr-1mustbeassociatedwiththefunctionalabilitytoinhibitTcellactivation.
Normallyfoundinthebonemarrow(inthespleenofnormalmicetheynormallyaccountforlessthan5%ofnucleatedcells),MDSCscanbeincreasedinnumbersinspleenandbloodunderpathologicalconditions.AnincreaseinMDSCnumbersinlymphnodeshasbeenreportedbysomestudiesunderpathologicalsituations(27,105).
MDSCspresentatthetumorsiteandafractionofcellspresentinthespleenofmicebearingtumorsareCD11b+F4/80+Gr-1;thesecellscanariseinvivoandinvitrofromCD11b+Gr-1+precursorsandretaintheirsuppressiveproperties(12,13,17,28).
Invitroeffects:MDSCsinhibitTcellactivation(CD8+TcellsmorethanCD4+Tcells)inducedbyeitherantigensorpolyclonalstimulithroughanMHC-independentmechanismrequiringcell-cellcontact.
EventhoughdirectantigenpresentationtotheTcellsbyMDSCsisnotrequiredforinvitrosuppression,MDSCscantakeupandcross-presenttumor-associatedantigensinthecontextofMHCclassImoleculesinvivo(27).Inthiscase,selectiveimpairmentoftumor-specificimmunityhasbeenshown,indicatingthatMHC-dependentresponsesmightberelevantinvivo.
HumanMDSCequivalentsarenotentirelyknown,butgranulocytesubpopulationsmightbeinvolvedinmediatingsomehumanMDSCinhibitoryactivities(15,16).
VEGF,GM-CSF,IL-3,M-CSF,andIL-6havebeenshowntobeinvolvedinthealterationofnormalmyelopoiesisandrecruitmentofMDSCstoperipheralorgansunderpathologicalsituations.Cytokinesmightberelevantforenhancedmyelopoiesis,mobiliza-tionofMDSCs,andconditioningthematurationofthesecells(4,9).
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andTAMsrespondwithanM2macrophageorientedprogramtoclassicsignalsdrivingmacrophageactivation(dependentonTh1cytokines)andreconcileconflictingdataattributingaprevalenceofeitherIFN-,NOS2,andSTAT1orIL-4/IL-13,ARG,andSTAT6axesinthesuppressionoftheimmuneresponseintumor-bearinghosts(Table1,Figure2;alsodiscussedfurtherbelow).Manyquestions,however,stillawaitanswers.It
isnotclear,
forexample,whetheralltheMDSCprecursorsinapopulationrespondsimilarly(andsynchronously)toTcellmediatedacti-vationorwhetherMDSCpopulationsareheterogeneous,withsomecellsprogrammedtoactivateanM1phenotypeandotherstoactivateanM2phenotype.Alternatively,someplasticitymightexist,i.e.,MDSCsmightbeabletooscillatebetweenM1andM2phenotypes,dependinguponthestimulationtheyreceive.More-over,withrespecttothestatusofpolarization,somedifferenceshavebeen
reportedbetweenmiceandhumans.For
example,ARG1isexpressedinmouse,butnothuman,M2macrophages(41).Inhumans,ARG1isconstitutivelyexpressedbygranulocytes
(42),andARG1-expressinggranulocyteshavebeenreportedtoinducebothdecreasedCD3expressionandattenuatedactivationinTcellsfromrenalcellcarcinomapatients(15).Thesediscrepanciesbetweenhumansandmicemightreflectour
incompleteunderstandingof
thehighlydynamicprocessofmyeloiddifferentia-tionincancer,andonlytheidentificationofthemoleculesreleasedbytumorsandthetranscrip-tionfactorsactivatedinhematopoieticprecur-sorscanaddresstheseissues.WearecurrentlyevaluatingthepossibilityofgeneratingMDSCsfrombonemarrowprecursorsusingdefinedinvitroculturesystemsinanattempttoaddresssomeoftheseissues.
l-Arg metabolism as the mechanism of MDSC
immunosuppressionIncreasedl-Argmetabolism,eitherinmyeloidcellsinfiltratingthetumorstromaorintumor
cells,canimpairantigenresponsivenessofTcells,bothatthetumor-hostinterfaceandsystemically(23,29,43).Immuneregulationbyl-Argmetabolismisnotantigen-specific,buttobesusceptibletotheinhibitoryactivityoftheARG-andNOS-dependentl-Argmetabo-lismpathways,aTcellmustbeactivatedthroughitsTCR.Activa-tionthroughtheTCRpromotesTcellcycling,andmanyoftheinhibitoryeffectsofl-Argmetabolizingenzymesrequireactivelyproliferatingcells.NOS2andARG1canfunctionseparatelyorsynergisticallytoalterTcellfunction;activationofeitherenzymealoneinanAPCinhibitsitsabilitytoinduceTcellproliferationbyinterferingwithintracellularTcellsignaltransductionpath-wayswhereasinductionofbothenzymesgenerateshighlyreactiveoxygenandnitrogenspecies,suchasH2O2andperoxynitrites,thatmightinducesignalingdefectsinproximalimmunecellsandforceantigen-activatedTcellstoundergoapoptosis(Figure2andref.23).Therelativelevelsofexpressionofthe2enzymesseemtoberelatedtothestimulusdrivingMDSCaccumulation(Table1).Inthecaseoftumor-inducedMDSCs,themainfactorsdetermining
Figure 2Inhibitory effects of MDSC l-Arg metabolism on
antigen-activated T cells. l-Arg enters MDSCs through a cationic
amino acid transporter (CAT-2B) and is mainly metabolized by the
inducible forms of NOS and ARG (i.e., NOS2 and ARG1, respec-tively)
although the contribution of other isoforms cannot be ruled out.
Depending on the balance between these enzymes, depletion of
extracellular l-Arg concentration, NO release, and enhanced
production of reactive oxygen and nitrogen species (for example, O2
and H2O2, and ONOO, respec-tively) can ensue. T cells that are
activated in the MDSC-conditioned environment stop proliferating
and eventually die by apoptosis through pathways involving
activation of general control nondere-pressible 2 (GCN2) and
soluble guanylate cyclase (sGC); tyrosine nitration and S-cysteine
nitrosylation of various proteins; loss of CD3; and interference
with the IL-2R signaling pathway (reviewed in ref. 23). cEBP-,
CCAAT enhancerbinding protein ; MSP, macrophage-stimulating
protein.
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whichl-Argmetabolizingenzymeisexpressedatthehighestlevelareasfollows:tumorhistology,anatomicalsitefromwhichtheMDSCsareisolated(spleen,blood,ortumor),geneticbackgroundofmouse(whichprobablydictatestheTh1vs.Th2orientationoftheimmuneresponse),andtypeofstimulatorysignaldeliveredtotheactivatingTcells(Table1andrefs.4446).Interestingly,asdiscussedabove,activationofARG1canlead
tolossofcellsurfaceexpressionofCD3inantigen-activatedTcellsbyconsumptionofl-Argandactivationof
theaminoaciddeficiencysensorgeneralcontrolnonderepressible2(GCN2)(47,48),asensorthatisalsotriggeredbyanotheraminoacidmetabolizingenzymecausingimmunesuppression,indoleamine2,3-dioxygenase
(49,50).The lossofCD3
seemstobemoreimportantforinhibitionofCD4+TcellfunctionthanofCD8+Tcellfunction(51).Indeed,splenicMDSCswereshowntoinducetheCD3chaindownregulationinantigen-stimulatedCD4+butnotCD8+Tcells(51).Moreover,CD3lossmightnotberelat-edexclusivelytotumorMDSCs,sinceMDSCsexpandedduringchronic
inflammation inducedby infectionwithPorphyromo-nas
gingivaliscanalsoinduceitsdownregulation(52).IthasbeenproposedthatthefunctionalroleofMDSCsistolimitchronicstimulationoftheimmuneresponseandpreventunmitigatedTcellactivation,whichcanbedangerous(53).DownregulationofCD3expressionandtheunresponsivenessofTcellsthatensuescontributetotheinflammatoryresponsebeingattenuated;i.e.,thereleaseofproinflammatorycytokinesandothermediatorsthatmightbedetrimentaltothebodywhenproducedinexcessorforaprolongedperiodisattenuated.LossofCD3Tcellsisnottheonlymechanismbywhichheight-
enedl-ArgmetabolismmediatesTcellsuppression.Forexample,CD8+tumor-infiltratinglymphocytes(TILs)presentinindividualswithprostatecancerareinhibitedbyapathwaydependentontheintratumoralactivationofARG2andNOS2(expressedbythecan-cercells),buttheseTILsdonotshowalteredexpressionofCD3
orotherprofounddefectsintheTCRsignalingpathway(54).WethereforethinkthatitisprobablethatCD3downregulationisalateeventintumorprogression,associatedwithadeeperaltera-tioninhostmyelopoiesis.
Origin and molecular basis of TAM
functionsTAMsarethesecondwell-describedpopulationofmyeloidcellsthathavebeenshowntoexertanegativeeffectonantitumorimmuneresponses.TherelationshipbetweenTAMsandMDSCsisnotcompletelydefined,butdatadiscussedbelowsuggestTAMsmight,inpart,bederivedfromorrelatedtoMDSCs(Figure1andProperties
of TAMs).Fordecades,solidtumorshavebeenknowntobestrongly
infiltratedbyinflammatoryleukocytes,andaccumulatingevi-dencehasclearlydemonstrated,invariousmouseandhumanmalignancies,includingcolon,breast,lung,andprostatecan-cer(32,5557),astrictcorrelationbetweenincreasednumbersand/ordensityofmacrophagesandpoorprognosis.Basedonthis,boththerecruitmentandactivationofTAMsareregardedaspivotaltotumorprogression,andTAMsareputativetargetsfortherapeuticintervention.AsoriginallydescribedbyAlbertoMantovaniandcolleaguesin
theearly1980s(57),circulatingmonocytes(Figure1)arerecruit-edtothetumor,wheretheydifferentiateintoTAMs,byatumor-derivedchemotacticfactor,originallyidentifiedasCCchemokineligand2(CCL2;alsoknownasMCP-1)(32).Followingthisobser-vation,otherchemokinesabletorecruitmonocytesweredetect-edinneoplastictissuesasproductsofeitherthetumorcellsorhoststromalelements(55).Inadditiontorecruitingmonocytes,thesemoleculesplayanimportantroleintumorprogressionbydirectlystimulatingneoplasticgrowth,promotinginflamma-tion,andinducingangiogenesis(58).Evidencesupportingapiv-otalroleforchemokines,inadditiontoCCL2,intherecruitmentofmonocytestoneoplastictissuesincludesadirectcorrelation
Properties of TAMs
TAMsarederivedfromcirculatingmonocytesthatarerecruitedtotumorsbychemotacticfactorssuchasCCL2,VEGF,andM-CSF(32,106).
TAMspreferentiallylocalizeinhypoxicareasoftumors(64,107).
M2macrophagepolarization:TAMsexpresshighlevelsofM2macrophagemarkers(IL-10,TGF-,ARG1,andthemannoserecep-tor)andlowlevelsofmediatorsofM1macrophagemediatedinflammation(IL-12,TNF-,andIL-6)(32,75,81).
TAMsexhibitdefectiveNF-BactivityandfunctionalIRF-3/STAT1pathwayactivityinresponsetoTLR4ligands(75).
TAMsexhibitthefollowingprotumoralfunctions:
(a)Inductionofangiogenesisthroughexpressionoftissuefactors,VEGF,CCL2,FGF2,CXCL8,CXCL1,andCXCL2
(32,85,108,109)
(b)Productionofgrowthfactors(e.g.,PDGF,EGF,andVEGF)(85,110,111)
(c)InductionofmatrixremodelingthroughtheproductionofTGF-,CCL2,andMMPssuchasMMP9(32,112)
(d)Immunesuppression,throughtheproductionofimmunosuppressivecytokines(e.g.,IL-10andTGF-)(32,75,76)
andtherecruitmentofTregsthroughthesecretionofCCL22(113)
(e)SkewingofadaptiveimmunitytoaTh2-typeimmuneresponsethroughtheproductionofCCL17(32),CCL18(114),
andCCL22(113).
IncreasednumbersofTAMscorrelatewithvesseldensityandpoorprognosis(56).
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betweenchemokineproductionandmonocyteinfiltrationinmouseandhumantumors(32).Moleculesotherthanchemokinescanalsopromotemono-
cyterecruitment.Inparticular,tumor-derivedcytokinessuchasVEGFandM-CSFpromotemonocyterecruitmentaswellasmacrophagesurvivalandproliferation,andtheirexpressioncor-relateswithtumorgrowth(59).Someofthesefactors,expressedinthetumormicroenvironment,alsoinhibitthedifferentiationofmonocytesintoDCsbyactivatingSTAT3-dependentsignaling(9),therebyimpairingtheinductionofDC-inducedantigen-spe-cificimmuneresponses(60).SeverallinesofevidencesuggestthatsomecirculatingMDSCs
reachthetumorsiteandbecomepartofthetumorstroma,indi-catingthat,inadditiontoperipheralmonocytes,CD11b+Gr-1+MDSCsmightalsobeprecursorsofF4/80+TAMs.Indeed,ithasbeenshownthatGr-1+cellsisolatedfromthespleensoftumor-bearingmicecanreachthetumorandbecomeF4/80+TAMschar-acterizedbyincreasedSTAT1phosphorylationandconstitutiveexpressionofARG1andNOS2(13,27)(Figure1).Intumor-bear-inghosts,increasedbioavailabilityofVEGFandreleaseofsolubleKITligandinthebonemarrowarepromotedthroughthehighexpressionofMMP9bysplenicCD11b+Gr+cells,whichindirectlypromotetumorvascularizationandregulatethemobilizationofmoreCD11b+Gr+cells.TheseCD11b+Gr-1+cellswerealsofoundtodirectlyincorporateintothetumorendothelium(61),wheretheycontributetotumorgrowthandvascularizationbyproduc-ingMMP9anddifferentiatingintoendothelialcells.Moreover,theconceptofashareddifferentiationpathwaybetweencirculatingMDSCsandTAMs(Figure1)isreinforcedbythecommonmolec-ularpathways(activatedbyIFN-andIL-13)necessaryfortheirimmunosuppressiveactivity,aspreviouslydescribed(11).TAMspreferentiallylocalizetopoorlyvascularizedregionsof
tumors(62,63).Thisenvironmentpromotesthemetabolicadap-tationofTAMstohypoxiathroughtheactivationofhypoxia-induciblefactor1(HIF-1)andHIF-2(63).WerecentlyhaveshownthatHIF-1activatedinTAMsbyhypoxiainfluencestheposi-tioningandfunctionoftumorcells,stromalcells,andTAMsbyselectivelyupregulatingtheirexpressionofCXCchemokinerecep-tor4(CXCR4)(64).Moreover,HIF-1activationcanhavearoleintheinductionoftheCXCR4ligand,CXCchemokineligand12(CXCL12)(65),achemokineinvolvedincancermetastasis(66).Together,thesedatasuggestthatoxygenavailabilityhasaroleinguidingthemicroanatomicallocalizationandfunctionofTAMs.Moreover,hypoxiacanalsohaveimportantconsequencesonl-ArgmetabolisminTAMsandtherebyonthesuppressionofadaptiveimmunity,sinceitcaninduceNOS2andARGexpression(inthiscasewithacertainvariabilityintermsofARG1andARG2)invari-ouscelltypes(6769).InadditiontoHIF-1,analysisofthemolecularbasisofthe
TAMphenotypehasidentifiedNF-BasthemasterregulatorofTAMtranscriptionalprograms,andsomeevidencesuggeststhatmodulationofNF-Bactivityinthesecellsisanimportantmecha-nismbywhichtheirprotumoralfunctionscanbecontrolled(32).Although
in inf lammatory leukocytes, in particular
macrophages,NF-Bisanessentialtranscriptionfactorguid-ingtheinflammatoryresponse,thisfactorisalsorecognizedasamajoreffectorofcancercellproliferationandsurvival(70).Incan-cer,NF-Binducesmoreaggressivetumorphenotypesbypromot-ingcellstogrowindependentlyofgrowthsignals;byincreasingtheirinsensitivitytogrowthinhibition;byincreasingtheirresis-
tancetoapoptoticsignals;byimmortalizingthecells;byenhanc-ingangiogenesis;andbyenhancingtissueinvasionandmetastasis(71).TheconstitutiveNF-Bactivationoftenobservedintumorcellsmightbepromotedbyeithersignalsfromthemicroenviron-ment,includingcytokines,hypoxia,andROS,orbygeneticaltera-tions(71).Inparticular,proinflammatorycytokines(e.g.,IL-1andTNF-)expressedbydifferentsubsetsoftumor-infiltratingleuko-cytes(72)canactivateNF-Bincancercellsandcontributetotheirproliferationandsurvival(71).Strikingly,theproliferativeroleofTNF-wasrecentlyconfirmedinprimaryandinvitroestablishedhumanrenalcarcinomacells(73).Thepeculiarabilityoftumorstopromoteleukocyterecruitmentlargelyreliesontheirconstitutiveexpressionofthegenesthatencodeinflammatorychemokines,whoseexpressioniscontrolledbyNF-B(74).Thesedataunder-pinthecentralroleofNF-BinthefunctionalcrosstalkbetweentumorsandtheimmunesystemandsuggestacausalrelationshipbetweenNF-Bmediatedinflammationandtumorigenesis(70).DifferencesareemergingabouttheeffectsofNF-Bincancer
cellsandTAMs.Incontrastwithcancercells,infact,TAMsfromadvancedtumorsshowdefectiveNF-Bactivationinresponsetodifferentproinflammatorysignals(55,75,76).ThisdefectiveNF-BactivationinTAMscorrelateswithimpairedexpressionofNF-Bdependentinflammatoryfunctions(e.g.,theexpressionofcytotoxicmediatorssuchasTNF-,IL-1,andIL-12)(32).Theseobservationsareinapparentcontrastwithaprotumorfunctionofinflammatoryreactionsobservedinmodelsofspontaneousorchemicallyinducedcarcinogenesis(77,78).Althoughintheselattermodels,NF-Binhibitionresultedintumorgrowthdelay(77,78),intumorsatamoreadvancedstageofprogression,atherapeuticeffectwasachievedthroughthereactivationofNF-Bdependentinflammationinthemyeloidcellcompartment(75,79,80).Thisdiscrepancymightreflectadynamicchangeinthetumormicro-environmentduringthetransitionfromearlyneoplasticeventstoadvancedtumorstages,whichwouldresultinprogressivemodula-tionoftheNF-BactivityexpressedbyinfiltratinginflammatorycellsandprogressiveconversionoftheTAMsfromanM1toanM2macrophagephenotype.Importantly,restorationofNF-Bactiv-ityinTAMsfromadvancedtumorsresultsinincreasedexpressionofinflammatorycytokines(e.g.,TNF-)andisassociatedwithadelayintumorgrowth(75).Sofar,NF-Bpathwayshavebeencharacterized,inpart,inTAMs,andsimilarstudiesshouldberep-licatedinMDSCs.
TAMs mediate an M2 macrophageoriented persistent
inflammationCharacterizationofthetranscriptomeofTAMsisolatedfromamousefibrosarcomaconfirmedthatthesecellsmainlyhaveanM2macrophagephenotypebutalsoexpressIFN-inducibleche-mokines(acharacteristicofM1macrophages)
(81).Asimilarmixtureofgeneprofiles(mostlyanM2profilewithM1traits)wasalsorecentlyfoundinmouseMDSCs(11).ThemainlyM2macrophagelikephenotypeofTAMsisassociatedwiththemhav-ingprotumoralfunction.Evidenceforthiscomesfromanumberofstudies.First,pharmacologicalskewingofTAMpolarizationfromanM2macrophagelikephenotypetoafullM1macro-phagephenotypesustainsantitumorimmunity(79,82).Indeed,acombinationofCpGoligodeoxynucleotidesandanIL-10recep-torspecificantibodyswitchedTAMsfromanM2toanM1mac-rophagelikephenotypeandtriggeredaninnateresponsethatwasabletodebulklargetumorswithin16hours(82).Second,
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recentresultssuggestthatSRChomology2domaincontaininginositol-5-phosphatase(SHIP)functionsinvivotorepressskew-ingtoanM2macrophagelikephenotype.PeritonealandalveolarmacrophagesisolatedfromShip/miceconstitutivelyexpresshighlevelsofARG1andshowimpairedLPS-inducedNOproduction.Consistentwiththis,transplantedtumorsgrowmorerapidlyinShip/micethaninwild-typemice(83,84).Third,aDNAvac-cineagainsttheM2macrophageassociatedmoleculelegumain,whichishighlyexpressedbyTAMs,inducedarobustCD8+Tcell
responseagainstTAMs,reducingtheirdensityintumortissuesandleadingtothesuppressionofangiogenesis,tumorgrowth,andmetastasis(85).Finally,wehaverecentlydemonstratedthatTAMsarecharacterizedbynuclearlocalizationoftheinhibitoryp50NF-Bhomodimer,aphenotypeassociatedwithtumorprogres-sionandalackofM1macrophagelikefunction(75).Interest-ingly,
theM2macrophageinducingsignalsPGE2,
IL-10,andTGF-wereshowntopromoteincreasednuclearlocalizationofthep50NF-Bhomodimer(75).Moreover,micelackingexpres-
Figure 3Molecular pathways of macrophage polarization and their
role in tumor progression. The major pathways of macrophage
polarization and cur-rent evidence linking their activation with
either tumor progression (+) or regression () are outlined. The
overall view suggests that M2 macro-phagepolarizing signals (such
as IL-10, IL-4, and IL-13) are mainly associated with tumor
progression. Contrasting evidence associates M1
macrophagepolarizing pathways (such as IFN- and TLR ligation) with
either tumor progression or regression. The crosstalk between the
M1 and M2 macrophagepolarizing pathways, which results in
reciprocal modulation, are also indicated. As shown, IL-10mediated
induction of the p50 NF-B homodimer interferes with NF-B activation
and M1 macrophageinduced inflammation. The balance between
activation of M1 macrophageassociated STAT1 and M2
macrophageassociated STAT3 and STAT6 finely regulates macrophage
polarization and activity. A predominance of NF-B and STAT1
activation results in M1 macrophage polarization, which promotes
cytotoxic and inflammatory functions. In contrast, a predominance
of STAT3 and STAT6 activation results in M2 macrophage
polarization, which is associated with immune suppression and tumor
progression. As discussed in the text, IL-23 might also contribute
to the polarization decision as it activates different STATs,
includ-ing STAT1 and STAT3, in TAMs, but direct evidence is
missing. CC, colorectal carcinoma; HCC, hepatocarcinoma; Fibr,
fibrosarcoma; Mel, melanoma; BC, breast carcinoma; SCC, squamous
cell carcinoma; Bl. Carc, bladder carcinoma.
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sionofp50alsolackexpressionoftheM2macrophagepolarizingcytokinesIL-4,IL-5,andIL-13(86),andintumor-bearingmicelackingexpressionofp50,TAMsexpresscytokinescharacteristicofM1macrophages,andsplenocytesproduceTh1cytokines,bothofwhichareassociatedwithadelayintumorgrowth(75).AllthesefindingstogethersuggestthatM2macrophagelike
inflammationfuelscancerprogressionandleadtothesugges-tionthatNF-BinhibitioninTAMsisassociatedwithM2macro-phagelikeinflammatoryfunctions.Itisprobablethat,althoughfullactivationofNF-Binmacrophagesresidentinpreneoplas-ticsitesmightexacerbatelocalM1macrophagelikeinflamma-tionandfavortumorigenesis(77,78,87),tumorgrowthresultsinprogressiveinhibitionofNF-Bininfiltratingleukocytes,asobservedinbothmyeloid(75,88)andlymphoid(89)cellsfromindividualswithtumors,andintheprogressiveskewingtoM2macrophagelikeinflammation.Ifso,thetherapeuticefficacyofstrategiestargetingNF-Bforthetreatmentofcancersmightbedeterminedbyboththetumorstageandpolarizationstatusoftheinfiltratingleukocytes.
STATs in TAM and MDSC
functionAcentralroleinthepolarizationofmyeloidcellfunctionsaswellasintumorprogressionandthealteredimmuneresponsetocan-cerisemergingforselectedmembersoftheSTATfamilyoftran-scriptionfactors.Inparticular,STAT1,STAT3,andSTAT6havebeenshowntohaveamajorroleintransmittingpolarizingsig-nalstothenucleus(90)andtohavedistinctrolesinmacrophagepolarization(Figure3).STAT1isactivatedinresponsetoM1mac-rophagepolarizingsignals(e.g.,IFN-andLPS)whereasSTAT3andSTAT6areselectivelyactivatedbyM2macrophagepolarizingcytokines(e.g.,IL-10,IL-4,andIL-13)(91).ActivationofspecificSTATs,centralinducersofmacrophagepolarizationprograms,isexpectedtoparalleleithertheantitumoralorprotumoralroleofM1andM2macrophagemediatedinflammation,respectively.OriginalevidenceindicatesthatSTAT1activationisessential
forimmunesurveillanceagainsttumors(92).Inparticular,micedeficientforeithertheIFN-receptor(signalingthroughwhichactivatesSTAT1;ref.93)orSTAT1displayedenhancedresistancetotheinductionoftumorsbymethylcholanthrene(94).Overtheyears,theSTAT1-mediatedantitumoraleffecthasbeenconfirmedinpreclinicaltumormodels(95,96).However,recentreportsargueagainstthissimpleviewandsuggestthattheIFN-/STAT1pathwaymighthaveaprotumoralrole,atleastincertaintumors.Forexam-ple,STAT1wasrecentlydescribedasresponsibleforTAM-mediatedsuppressiveactivityandtumorprogression,anditwasshownthatTAMsisolatedfromSTAT1-deficientmicefailedtosuppressTcellresponses(13).Inaddition,inamousesquamouscellcarcinoma,STAT1deficiencyenhancedIL-12mediatedtumorregressionbyaTcelldependentmechanism(97).InagreementwiththeroleofSTAT1asthecentralmediatorofthebiologicalactivitiesofIFN-,administrationofneutralizingantibodiesspecificforIFN-inhib-itedtumorgrowthinIL-12treatedStat1+/+mice(97).Morerecent-ly,ithasalsobeenshownthatactivationoftheCD8+Tcellsuppres-siveactivityoftumor-inducedMDSCsrequirestheactionofIFN-,thoughincombinationwithIL-13(11).Inlinewiththispicture,micelackingSOCS1,whicharecharacterizedbyhyperactivationofSTAT1,displayspontaneousdevelopmentofcolorectalcarcino-mas(98),supportingtheideathatpersistentactivationofSTAT1-dependentsignalingmightbeassociatedwithtumorprogression.Interestingly,molecularanalysisofthetranscriptomeofTAMs
showedthatthesecellsexpresshighlevelsofIFN-induciblechemo-kinesandSTAT1activity(81).Together,theseresultssuggestthat,alongwithapredominantexpressionofM2macrophagepolarizedfunctionsinTAMsandMDSCs,theparallelactivationofSTAT1inthesecellsmightenhanceimmunedysfunctions,furtherfavoringtumorprogression.ThiscontrastingevidenceontheinfluenceofSTAT1mightbeexplainedbydifferencesamongthetumormodelsinvestigated,thestateoftumorprogression,andthenumberandtypeofinfiltratingleukocytes.STAT3andSTAT6activationareassociatedwithM2macrophage
polarization(32,91).IthasbeenshownthatSTAT3isconstitu-tivelyactivatedintumorcells(99)andindiversetumor-infiltrat-ingimmunecells,includingTAMs(80),leadingtoinhibitionofproinflammatorycytokineandchemokineproductionandtothereleaseoffactorsthatsuppressDCmaturation.AblatingSTAT3inhematopoieticcellstriggersanintrinsicimmunesurveillancesystemthatinhibitstumorgrowthandmetastasisandisassociatedwithenhancedfunctionalactivityofDCs,Tcells,NKcells,andneutro-phils(80).STAT3/JAK2activationinmyeloidcellsbytumor-derivedfactorscanleadtotheaccumulationofCD11b+Gr-1+MDSCs,pre-ventingtheirdifferentiationintomatureDCs,whereasinterferingwithSTAT3signalingreversestheseinhibitoryeffects(100,101).TAMsfromStat6/tumor-bearingmicedisplayanM1macrophagephenotype,withlowlevelsofexpressionofARG1andhighlevelsofexpressionofNOS2,whichpromotestumorcelldeaththroughthecytotoxicactivityofthehighlevelsofNOthatareproduced.Asaresult,thesemicerejectedspontaneousmammarycarcinomasinanimmunesystemdependentmanner(20,102).Therefore,althoughcurrentliteraturestronglysuggestsacrucialroleforpolar-izedinflammationincancerprogression,additionalstudiesshouldclarifywhetheraccumulatingandcontrastingevidencemightbeascribedtospecificmicroenvironmentalconditionsorrelatedtotumortypeand/orstageofdisease.TherecentobservationthatthecytokineIL-23,amemberof
the IL-12cytokine family, isexpressed
inhumanandmousetumorshasunveiledanotherpotentialplayerinTAM-depen-dentimmunosuppression.Inmousetumormodels,expressionofthemRNAencodingtheIL-23p19subunitwasincreasedinCD11b+andCD11c+cells(probablyTAMsandDCs)presentintumorstroma.SimilarlytoIL-12,IL-23promotes
inflamma-toryresponses,buttheneteffectofthecytokineisdeleteriousforantitumorimmunity.IL-23,infact,promotesupregulationofMMP9andincreasestumorangiogenesisbutreducesCD8+Tcellinfiltration(103).Importantly,geneticdeletionstudiesandantibody-mediatedneutralizationofIL-23havedemonstratedadirectnegativeeffectofthecytokineontumorimmunesurveil-lance(103).Furthermore,IL-23stimulationcanactivateSTAT1,STAT3,STAT4,andSTAT5andleadtoenhancedproductionofIL-6(104);itthereforemighthaveanimportantroleininfluenc-ingtheTAMtranscriptomeandfunction.
For the future: therapeutic
perspectivesMDSCsandTAMsprobablyrepresentacontinuumofauniquemyeloidcelldifferentiationprograminducedbytumor-derivedfactorstosupportanincessant
influxofcellsthataidtumorinvasionofnearbytissues,stromaremodeling,andcellprolifera-tionandthatinhibittheinnateandadaptiveantitumorimmuneresponse.Targetingthisdynamicprocessmightofferinterestingperspectivesfornewtherapiesforthetreatmentofcancer(4,79).Inapplyingnovelapproachestorelievingtheimmunosuppression
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inducedbyMDSCsandTAMs,oneaspectmustbeconsidered:therelativecontributionofMDSCsandTAMstotheoverallimpair-mentofantitumorTcellresponseshasnotbeenclearlyestimated.ItisprobablethatinhibitionofCD8+TcellantitumorimmunitybyMDSCsandTAMsintumor-bearinghostsmightoccurindif-ferentplaces,primarilythetumorsiteandthedraininglymphnodesbutalsodistantsitesoftheimmunesystem.MDSCsandTAMsmightalsoaffectdifferentlythesubsetsofcirculatingCD8+Tcellsinrelationtothespreadofmalignanttumorsindifferentpatients.WethinkthatcombiningprotocolsthatinterferewithMDSC-and/orTAM-mediatedimmunesuppressionwitheithercancervaccination(activeimmunotherapy)ortheadoptivetrans-ferofexvivoexpandedtumor-infiltratingTcells(passiveimmu-notherapy)mightprovidetherapeuticbenefitforthetreatment
ofcancer.However,thebenefitofsuchcombinationapproachesislikelytodifferineverypatientaccordingtothestateofimpair-mentoftheantitumorimmuneresponse.
AcknowledgmentsWethankAlbertoMantovaniforhiscriticalreadingandconstantsuggestions.ThisworkhasbeensupportedbygrantsfromtheItal-ianMinistryofHealth,theItalianFoundationforMultipleSclero-sis(FISM),theItalianAssociationforCancerResearch(AIRC),andtheEuropeanCommunity.
Addresscorrespondenceto:VincenzoBronte,IstitutoOncologicoVeneto,ViaGattamelata64,35128Padua,Italy.Phone:39-049-8215897;Fax:39-049-8072854;E-mail:[email protected].
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