9.黏膜免疫 研究生

黏膜免疫

MucosalImmune

2021/10/101黏膜免疫系统（mucosalimmunesystem）：

直接接触病原体的解剖部位并能够分泌粘液的上皮细胞所覆盖，构成黏膜免疫系统。

广泛分布于呼吸道、胃肠道、泌尿生殖道黏膜下及一些外分泌腺体（唾液腺、泪腺、乳腺）处的淋巴组织。

执行局部特异性免疫功能的主要场所。2021/10/102Themucosalimmunesystem.Thetissuesofthemucosalimmunesystemarethelymphoidorgansassociatedwiththeintestine,respiratorytract,andurogenitaltract,aswellastheoralcavityandpharynxandtheglandsassociatedwiththesetissues,suchasthesalivaryglandsandlachrymalglands.Thelactatingbreastisalsopartofthemucosalimmunesystem.黏膜免疫系统亦称黏膜相关淋巴组织（mucosa-associatedlymphoidtissue,MALT）。2021/10/103黏膜免疫系统与免疫应答黏膜免疫系统中的固有免疫应答黏膜免疫系统中的适应性免疫应答黏膜免疫中的免疫耐受和免疫调节黏膜免疫与疾病主要内容2021/10/104小肠黏膜免疫系统的各种细胞成分和器官化的淋巴组织肠腔固有层肠系膜粘膜上皮DCM细胞小肠上皮细胞上皮内淋巴细胞(IEL)绒毛共生菌粘液浆细胞隐窝杯状细胞IgA巨噬细胞肠腺嗜酸细胞B细胞T细胞抗菌肽DC输出淋巴管肥大细胞分立的淋巴滤泡派氏集合淋巴结滤泡T细胞肠系膜淋巴结Cross-sectionaldiagramofthemucousmembraneliningtheintestineshowinganoduleoflymphoidfolliclesthatconstitutesaPeyer’spatchinthesubmucosa.Theintestinallaminapropriacontainslooseclustersoflymphoidcellsanddiffusefollicles.黏膜免疫系统与免疫应答（结构和应答特点）2021/10/105黏膜免疫系统的特点

解剖特征

－粘膜上皮和淋巴组织间因相互作用而联系紧密。

－由散在的淋巴组织和器官化的结构（如派氏集合淋巴结、

分立的淋巴滤泡和扁桃体）共同组成。

－启用抗原摄取机制，如出现派氏集合淋巴结和M细胞。

效应机制

－在无感染发生的情况下拥有大量活化的T细胞和记忆细胞。

－存在非特异性激活的“天然”效应性T细胞和记忆性T细胞。

－大量启用分泌型IgA抗体。

－涉及各种共生微生物菌丛。

免疫调节

－可主动下调针对食物和其它共生性抗原的强势免疫应答。

－可激活抑制性巨噬细胞及诱导耐受的树突状细胞。*黏膜覆盖面大；肠腔中充满各种微生物；防御病原体的入侵，维持对共生菌的耐受。2021/10/106一、组成肠相关淋巴组织的固有免疫细胞二、肠道粘膜相关的固有免疫应答三、上皮内淋巴细胞杀伤入侵病毒和修复损伤组织黏膜免疫中的固有免疫应答2021/10/107Mucosallymphoidtissueinthehumanintestine.2021/10/1082021/10/109Thelaminapropriaandepitheliumoftheintestinalmucosaarediscretelymphoidcompartments.组成肠相关淋巴组织的固有免疫细胞2021/10/1010

lntraepitheliallymphocytesexpressCCR9andtheintegrinE:7,whichbindstoE-cadherinonepithelialcells.TheyaremostlyCD8Tcells,someofwhichexpresstheconventional:formofCD8andotherstheCD8:homodimer.

CD8Tcellspredominateintheepithelium,whereasCD8Tcellspredominateinthelaminapropria.2021/10/1011ThelaminapropriacontainslgA-producingplasmacells,lymphocytes,effectorTcells,dendriticcells,macrophages,andmastcells.Tcellsinthelaminapropriaofthesmallintestineexpresstheintegrin4:7andthechemokinereceptorCCR9,whichattractsthemintothetissuefromthebloodstream.2021/10/1012黏膜淋巴细胞与上皮细胞相互作用：黏膜淋巴细胞：位于黏膜上皮细胞间和上皮细胞基底层一侧的T淋巴细胞，包括固有类T淋巴细胞和NK细胞。分布部位特殊、功能发挥受控于上皮细胞表面各类MHC分子与其受体分子间的相互作用。除了经典的MHCI类和II类分子，上皮细胞还表达范围广泛的各种非经典MHC分子，包括TL、HLA-E、MIC-A/-B、MR1和进化上与之高度同源的CD1d分子，激活多种黏膜淋巴细胞。2021/10/1013TLMHCIIMHCITLMIC-MIC-HLA-ECD1d/MR1

A/-BA/-B

-GalCelCD8CD4CD8

CD8

NKG2DCD94/TCRTCR

TCRTCRTCRNKG2ATCRTCRTCRTCRNKNKTMAITCD8CD4TCD8TCD8TGF-IFN-,IL-4Pf,Gz,FasLIL-6,KGFTNF-,Pf,Gz,IFN-,IL-4?TNF-IFN-FasL

调节Th1/Th2毒性炎症,修复，炎症毒性Th1/Th2？功能毒性抗炎功能？BCRB受体上皮细胞配体粘膜淋巴细胞分泌因子功能

TL:非经典MHCI类分子胸腺白血病抗原；MIC:MHCI类链相关分子；CD1d/a-GalCel:分化抗原CD1d分子提呈的半乳糖神经酰胺；MR1:MHC相关I类分子；MAIT:MR1限制性粘膜相关恒定链T细胞;Pf:穿孔素；Gz:颗粒酶。KGF:角朊细胞生长因子。2021/10/1014物理免疫屏障功能上皮细胞分泌的粘液防止微生物接近上皮细胞上皮细胞产生的防御素具有抗菌活性上皮细胞表达的TLR和NLR显示双重免疫功能固有层中的DC和巨噬细胞具有炎症反应抑制作用和免疫调节作用肠道黏膜相关的固有免疫应答2021/10/1015免疫屏障功能2021/10/1016肠腔固有层肠系膜粘膜上皮DCM细胞小肠上皮细胞上皮内淋巴细胞(IEL)绒毛共生菌粘液浆细胞隐窝杯状细胞IgA巨噬细胞肠腺嗜酸细胞B细胞T细胞抗菌肽DC输出淋巴管肥大细胞分立的淋巴滤泡派氏集合淋巴结滤泡T细胞肠系膜淋巴结分泌的粘液防止微生物接近2021/10/1017肠腔固有层肠系膜粘膜上皮DCM细胞小肠上皮细胞上皮内淋巴细胞(IEL)绒毛共生菌粘液浆细胞隐窝杯状细胞IgA巨噬细胞肠腺嗜酸细胞B细胞T细胞抗菌肽DC输出淋巴管肥大细胞分立的淋巴滤泡派氏集合淋巴结滤泡T细胞肠系膜淋巴结产生的防御素具有抗菌活性含有大量嗜酸颗粒分颗粒含有防御素、溶菌酶对肠道微生物有杀灭功能2021/10/1018Epithelialcellshaveacrucialroleininnatedefenseagainstpathogens.TLRsarepresentinintracellularvesiclesoronthebasolateralorapicalsurfacesofepithelialcells,wheretheyrecognizedifferentcomponentsofinvadingbacteria.NOD1andNOD2pattern-recognitionreceptorsarefoundinthecytoplasmandrecognizecellwallpeptidesfrombacteria.BothTLRsandNODsactivatetheNFBpathwayleadingtothegenerationofpro-inflammatoryresponsesbyepithelialcells.TheseincludetheproductionofchemokinessuchasCXCL8,CXCL1(GROa),CCL1,andCCL2,whichattractneutrophilsandmacrophages,andCCL20and-defensin,whichattractimmaturedendriticcellsinadditiontopossessingantimicrobialproperties.ThecytokinesIL-1andIL-6arealsoproducedandactivatemacrophagesandothercomponentsoftheacuteinflammatoryresponse表达的TLR和NLR显示双重免疫功能2021/10/1019Commensalbacteriacanpreventinflammatoryresponsesintheintestine.Thepro-inflammatorytranscriptionfactorNFBpathwayisactivatedinepithelialcellsviatheligationofTLRsbypathogens(firsttwopanels).Commensalbacteriahavebeenfoundtoinhibitthispathwayandthuspreventinflammation.OnewayisbyactivationofthenuclearreceptorPPAR,leadingtotheexportofNFBfromthenucleus(thirdpanel).AnotherisbyblockingthedegradationoftheinhibitorIBandthusretainingNFBinthecytoplasm(fourthpanel).2021/10/1020小肠上皮细胞及固有层中DC上PRR的表达和功能可降低针对肠腔共生微生物的炎症反应。能识别细菌鞭毛的PRR(NLR:表达于胞质中；TLR：表达于小肠上皮细胞基底膜一侧)，对共生微生物的炎症反应只有当微生物进入或穿越上皮细胞后才能产生。识别LPS的TLR4在小肠上皮细胞及固有层DC上的表达皆下调。能下调TLR信号转导的胞内调节蛋白（在固有层的DC中）可优势表达。TLRTLR4DCNLR

细菌PAMP小肠上皮层固有层DC具有炎症反应抑制作用2021/10/1021IEL参与构筑黏膜防御屏障IEL对病原体的杀伤功能IEL的维稳和保护功能的功能IEL杀伤入侵病毒和修复损伤组织2021/10/10222021/10/1023病毒食物抗原（谷朊肽）TCRMHCI类分子MIC-A/-BNKG2DLTCD8CD8FasFasLGz(颗粒酶)Pf(穿孔素)

a型和b型粘膜上皮细胞间淋巴细胞（IEL）的主要功能A.a型IEL。左：病毒感染粘膜上皮细胞；中：受感染细胞通过MHCI类分子向CD8IEL展示病毒抗原肽，激活IEL；右：激活的IEL行使典型的CTL功能，通过分泌Pf和Gz，以及通过Fas/FasL途径杀伤病毒感染的上皮细胞；B.b型IEL。左：发生应急改变（感染、损伤、接触毒性肽）的上皮细胞表达非经典MHC分子MIC-A、MIC-B和胸腺白血病抗原(LT)；中：IEL表达NKG2D和CD8分子，分别与MIC-A/-B以及LT结合，IEL被激活；右：激活的IEL杀伤受到应急损伤的上皮细胞，机制同上。上皮细胞CD8IELABa型IELb型IEL2021/10/1024一、黏膜免疫系统器官化的淋巴组织二、参与适应性黏膜免疫应答的免疫细胞三、黏膜免疫中的抗体应答四、黏膜免疫中T细胞介导的应答黏膜免疫中的适应性免疫应答2021/10/1025黏膜免疫系统器官化的淋巴组织派氏集合淋巴结与M细胞散在性淋巴滤泡肠系膜淋巴结2021/10/1026派氏集合淋巴结（Peyer’s小结）2021/10/1027肠系膜淋巴结（肿大）肠系膜淋巴结2021/10/1028黏膜免疫系统含有大量效应淋巴细胞黏膜免疫系统中独特的树突状细胞黏膜固有层中T细胞的致敏和归巢参与适应性黏膜免疫应答的免疫细胞2021/10/1029肠道中T细胞依赖的IgA抗体类别转换机制派氏集合淋巴结圆丘状隆起部位的DC获取由M细胞提交的肠腔抗原并迁移至滤泡区近旁后，将抗原提呈给初始CD4T细胞并使之激活和分化成Th。Th与借助其BCR识别了抗原的B细胞发生相互作用，通过T-B间CD40L-CD40的结合，B细胞分化成产生IgA的浆细胞。该过程受DC产生的一氧化氮及TGF-的促进。由此产生的浆细胞经由血循环再归槽至固有层，所分泌的高亲和力IgA抗体，经过上皮细胞胞吞转换进入肠腔，与当初致敏的肠腔抗原结合。CD40LCD40DC初始T激活的ThNOB浆母细胞TGF-经过血流派氏集合淋巴结淋巴滤泡固有层肠腔基底膜上皮细胞肠腔抗原IgAIgA分泌IgA浆细胞DCB7CD28M细胞2021/10/1030Captureofantigensfromtheintestinallumenbymononuclearcellsinthelaminapropria.Firstpanel:solubleantigenssuchasfoodproteinsmightbetransporteddirectlyacrossorbetweenenterocytes,ortheremightbeMcellsinthesurfaceepitheliumoutsidePeyer'spatches.Secondpanel:enterocytescancaptureandinternalizeantigen:antibodycomplexesbymeansoftheFeRnontheirsurfaceandtransportthemacrosstheepitheliumbytranscytosis.Atthebasalfaceoftheepithelium,laminapropriadendriticcellsexpressingFeRnandotherFereceptorspickupandinternalizethecomplexes.Thirdpanel:anenterocyteinfectedwithanintracellularpathogenundergoesapoptosisanditsremainsarephagocytosedbythedendriticcell.Fourthpanel:mononuclearcellshavebeenseenextendingprocessesbetweenthecellsoftheepitheliumwithoutdisturbingitsintegrity.Thecellprocesscouldpickupandinternalizeantigenfromthegutlumenandthenretract.Themicrographshowsmononuclearcells,whichmaybedendriticcellsormacrophages,(stainedgreenwithafluorescenttagontheCD11cmolecule)inthelaminapropriaofavillusofmousesmallintestine.Theepitheliumisnotstainedandappearsblack,butitsluminal(outer)surfaceisshownbythewhiteline.Acellprocesshassqueezedbetweentwoepithelialcellsanditstipispresentinthelumenoftheintestine.Magnificationx200.MicrographfromNiess,J.H.,eta/.:Science2005,307:254-258.Captureofantigensfromtheintestinallumen2021/10/1031参见图9-7黏膜免疫系统中独特的树突状细胞2021/10/1032CCR9CCL2547DCCCR947TSLPRCT肠系膜淋巴结微静脉MadCAM-1E-钙粘素E7固有层肠系膜M细胞固有层上皮细胞TT上皮细胞小肠淋巴细胞的激活和归巢肠系膜淋巴结和派氏集合淋巴结中的DC，在胸腺基质淋巴生成素(TSLP)和其它因子的作用下表达视黄醇脱氢酶(RALDH)，后者将维生素A转化成视黄酸(RC)。RC诱导，已被抗原活化的效应T细胞（及B细胞）表达趋化因子受体CCR9和整合素47，并进入血循环。2021/10/1033CCR9CCL2547DCCCR947TSLPRCT肠系膜淋巴结微静脉MadCAM-1E-钙粘素E7固有层肠系膜M细胞固有层上皮细胞TT上皮细胞小肠淋巴细胞的激活和归巢分布在黏膜固有层中的后毛细血管微静脉的内皮细胞表达MadCAM-1(47配体)，使CCR9+47+T细胞停留于该处并穿越微静脉到达固有层,并变更其表型为CCR9+E7+T。固有层上皮细胞表达CCL25(CCR9配体)和E-钙粘素（E7配体），使效应性淋巴细胞选择性地归巢和停于黏膜固有层。2021/10/10342021/10/1035Molecularcontrolofintestine-specifichomingoflymphocytes.Leftpanel:TandBlymphocytesprimedbyantigeninthePeyer'spatchesormesentericlymphnodesarriveaseffectorlymphocytesinthebloodstreamsupplyingtheintestinalwall).Thelymphocytesexpresstheintegrin4:7,whichbindsspecificallytoMAdCAM-1expressedselectivelyontheendotheliumofbloodvesselsinmucosaltissues.Thisprovidestheadhesionsignalneededfortheemigrationofcellsintothelaminapropria.Rightpanel:ifprimedinthesmallintestine,theeffectorlymphocytesalsoexpressthechemokinereceptorCCR9,whichallowsthemtorespondtoCCL25(greencircles)producedbyepithelialcellsofthesmallintestine;thisenhancesselectiverecruitment.EffectorlymphocytesthathavebeenprimedinthelargeintestinedonotexpressCCR9butdoexpressCCR10.ThismayrespondtoCCL28(bluecircles)producedbycolonepithelialcellstofulfillasimilarfunction.Lymphocytesthatwillentertheepitheliallayerstopexpressingthe4:7integrinandinsteadexpresstheE:7integrin.ThereceptorforthisisE-cadherinontheepithelialcells.Theseinteractionsmayhelpkeeplymphocytesintheepitheliumoncetheyhaveenteredit.2021/10/1036分泌型IgA的特征影响分泌型IgA抗体类别转换的因素分泌型IgA的转运分泌型IgA的意义分泌型IgM可以代偿有缺陷的IgA

黏膜免疫中的抗体应答2021/10/1037参见图9-62021/10/10382021/10/1039分泌型IgA的特征影响分泌型IgA抗体类别转换的因素分泌型IgA的转运分泌型IgA的意义分泌型IgM可以代偿有缺陷的IgA

黏膜免疫中的抗体应答2021/10/1040黏膜DC与炎症反应Th17与黏膜免疫屏障肠道蠕虫感染与Th2型免疫应答黏膜免疫中T细胞介导的应答2021/10/10412021/10/1042对肠道蠕虫感染的保护性应答和病理性应答大部分肠道蠕虫即可启动CD4T细胞介导的保护性应答也可诱导病理性应答。其中Th2相关应答有利于清除寄生虫，属保护性应答；当DC接触抗原时产生IL-12，则产生Th1型应答。通常两种应答并存，若Th2介导的保护性应答不能处于优势地位，Th1型应答将使感染持续，并造成小肠的慢性病理性损伤。置换被寄生的上皮细胞，用粘液阻止寄生虫粘附，促其脱落。肥大细胞产生TNF,组胺等介质，募集炎症细胞,重塑粘膜。产生能固定补体的抗体IgG2a。活化的巨噬细胞以其产物引起组织损伤，诱导组织重塑。IgE激活肥大细胞，介导ADCC。嗜酸粒细胞通过主要碱性蛋白

(MBP)杀伤寄生虫。激活巨噬细胞招募和激活嗜酸粒细胞驱动

B

淋巴细胞产生IgE招募肥大细胞，抗蠕虫驱动B淋巴细胞产生IgG2aIL-13IL-5IL-4IL-9IL-3IFN-诱导上皮细胞修复,分泌粘液.................................................................Th-2Th-1DC

初始CD4T保护效应宿主损伤2021/10/1043一、黏膜DC与免疫耐受二、正常肠道的大量共生菌不引发有害的免疫反应三、黏膜耐受的诱导黏膜免疫中的免疫耐受和免疫调节2021/10/1044黏膜DC---CD103+DCiTreg2021/10/1045正常肠道的大量共生菌不引发有害的免疫反应肠道共生菌对维持人体的健康发挥着重要的作用。能够促进食物如纤维素的代谢；能分解毒素；能产生重要的辅助因子如维生素K1和短链脂肪酸。通过竞争空间和营养成份可以抑制病原体在肠道的繁殖和入侵共生菌能够直接作用于黏膜上皮细胞，对维持黏膜的屏障功能有重要作用。共生菌和其产物对于免疫系统的发展和功能有重要作用。

2021/10/1046Localresponsestocommensals.Severallocalprocessesensurepeacefulcoexistencebetweenthemicrobiotaandthehost,allowingthecommensalorganismstoberecognizedbytheimmunesystemwithoutinducinginflammationoranimmuneresponsethatwouldeliminatethem.CommensalbacteriainthelumengainaccesstotheimmunesystemviaMcellsinPeyer'spatchesandisolatedfollicles(leftpanel).UptakeandpresentationofthesenoninvasiveorganismsbyrestingdendriticcellsgenerateslgA-switchedBcellsthatlocalizeinthelaminapropriaaslgA-producingplasmacells(rightpanel).ThesecretorylgAthatisproducedlimitstheaccessofcommensalstotheepitheliumandhelpspreventtheirpenetration.Thisisassistedfurtherbythepresenceofthicklayersofmucus,whichalsocontainmucinglycoproteinsthathaveantibacterialproperties.Inaddition,stimulationofpattern-recognitionreceptorsonepithelialcellsandlocalleukocytesinducestheproductionofantimicrobialpeptidessuchasdefensins.2021/10/1047A.健康小鼠饲以卵清蛋白，7天后，用同一抗原加佐剂作皮下免疫，14天后测定小鼠针对卵清蛋白的血清抗体和T细胞应答水平。B.同一品系的对照小鼠，喂饲无关蛋白，而7天后注射的卵清蛋白属首次免疫，诱导出的典型保护性免疫针对卵清蛋白。C.B组小鼠以无关蛋白喂饲后若注射同一无关蛋白，同样可诱导针对该无关蛋白的粘膜耐受。特异应答

＋＋＋－7d7d14d14dAB特异应答黏膜耐受保护性免疫

－7d14dC特异应答

黏膜

耐受黏膜耐受的诱导2021/10/1048一、病原体感染与宿主免疫反应之间的消长

决定了感染的结局二、针对共生菌的免疫应答与肠道疾病三、肠道中与免疫应答相关的一些临床疾病黏膜免疫与疾病2021/10/1049Mucosalinfectionsareoneofthebiggesthealthproblemsworldwide.Mostofthepathogensthatcausethedeathsoflargenumbersofpeoplearethoseofmucosalsurfacesorenterthebodythroughtheseroutes.Respiratoryinfectionsarecausedbynumerousbacteria(suchasStreptococcuspneumoniaeandHaemophilusinfluenzae,whichcausepneumonia,andBordetellapertussis,thecauseofwhoopingcough)andviruses(suchasinfluenzaandrespiratorysyncytialvirus).Diarrhealdiseasesarecausedbybothbacteria(suchasthecholerabacteriumCholeravibrio)andviruses(suchasrotaviruses).Thehumanimmunodeficiencyvirus(HIV)thatcausesAIDSentersthroughthemucosaoftheurogenitaltractorissecretedintobreastmilkandispassedfrommothertochildinthisway.ThebacteriumMycobacteriumtuberculosis,whichcausestuberculosis,alsoentersthroughtherespiratorytract.Measlesmanifestsitselfasasystemicdisease,butitoriginallyentersviatheoral/respiratoryroute.HepatitisBisalsoasexuallytransmittedvirus.Finally,parasiticwormsinhabitingtheintestinecausechronicdebilitatingdiseaseandprematuredeath.Mostofthesedeaths,especiallythosefromacuterespiratoryanddiarrhealdiseases,occurinchildrenunder5yearsoldinthedevelopingworld,andtherearestillnoeffectivevaccinesagainstmanyofthesepathogens.Numbersshownarethemostrecentestimatedfiguresavailable(TheGlobalBurdenofDisease:2004Update.WorldHealthOrganization,2008).*Doesnotincludedeathsfromlivercancerorcirrhosisresultingfromchronicinfection.2021/10/1050InfectionbyClostridiumdifficile.Treatmentwithantibioticscausesmassivedeathofthecommensalbacteriathatnormallycolonizethecolon.Thisallowspathogenicbacteriatoproliferateandtooccupyanecologicalnichethatisnormallyoccupiedbyharmlesscommensalbacteria.Clostridiumdifficileisanexampleofapathogenproducingtoxinsthatcancauseseverebloodydiarrheainpatientstreatedwithantibiotics.病原体感染与宿主免疫反应之间的消长决定了感染的结局2021/10/1051Shigellaflexneri,acauseofbacterialdysentery,infectsintestinalepithelialcells,triggeringactivationoftheNFBpathway.ShigellaflexneribindstoMcellsandistranslocatedbeneaththegutepithelium(firstpanel).Thebacteriainfectintestinalepithelialcellsfromtheirbasalsurfaceandarereleasedintothecytoplasm(secondpanel).MuramyltripeptidescontainingdiaminopimelicacidinthecellwallsoftheshigellaebindtoandoligomerizetheproteinNOD1.OligomerizedNOD1bindstheserine/threoninekinaseRIPK2,whichtriggersactivationoftheNFBpathway,leadingtothetranscriptionofgenesforchemokinesandcytokines(thirdpanel).ActivatedepithelialcellsreleasethechemokineCXCL8,whichactsasaneutrophilchemoattractant(fourthpanel).IB,inhibitorofNFB;IK,IBkinase.2021/10/1052针对共生菌的免疫应答与肠道疾病2021/10/1053Mucosaldendriticcellsregulatetheinductionoftoleranceandimmunityintheintestine.Undernormalconditions(leftpanels),dendriticcellsarepresentinthemucosaunderlyingtheepitheliumandcanacquireantigensfromfoodsorcommensalorganisms.Theytaketheseantigenstothedrainingmesentericlymphnode,wheretheypresentthemtonaiveCD4Tcells.Thereis,however,constitutiveproductionbyepithelialcellsandmesenchymalcellsofmoleculessuchasTGF-,thymicstromallymphopoietin(TSLP),andprostaglandinE2(PGE2),whichmaintainthelocaldendriticcellsinaquiescentstatewithlowlevelsofco-stimulatorymolecules,sothatwhentheypresentantigentonaiveCD4Tcells,anti-inflammatoryorregulatoryTcellsaregenerated.Theserecirculatebacktotheintestinalwallandmaintaintolerancetotheharmlessantigens.Invasionbypathogensoramassiveinfluxofcommensalbacteria(rightpanels)overcomesthes