TargetingtheGutMicrobiomeasaTreatmentforPrimar.pdf

Targeting the Gut Microbiome as a Treatment for PrimarySclerosing Cholangitis:A Conceptional FrameworkAyesha Shah,MBBS,FRACP1,2,3,Graeme A.Macdonald,MBBS,FRACP,PhD,FAASLD1,2,3,Mark Morrison,PhD1,2,3,4andGerald Holtmann,MD,PhD,MBA,FRACP,FRCP,FAHMS1,2,3Primarysclerosingcholangitis(PSC)isarare,immune-mediated,chroniccholestaticliverdiseaseassociatedwithauniquephenotype of inflammatory bowel disease thatfrequentlymanifests as pancolitis with right-sided predominance.Availabledata suggest a bidirectional interplay of the gut-liver axis with critical roles for the gastrointestinal microbiome andcirculating bile acids(BAs)in the pathophysiology of PSC.BAs shape the gut microbiome,whereas gut microbes have thepotentialtoalterBAs,andthereareemergingdatathatalterationsofBAsandthemicrobiomearenotsimplyaconsequencebutthecauseofPSC.ClusteringofPSCinfamiliesmaysuggestthatPSCoccursingeneticallysusceptibleindividuals.Afterexposuretoanenvironmentaltrigger(e.g.,microbialbyproductsorBAs),anaberrantorexaggeratedcholangiocyte-inducedimmune cascade occurs,ultimately leading to bile duct damage and progressive fibrosis.The pathophysiology can beconceptualizedasatriadof(1)gutdysbiosis,(2)alteredBAmetabolism,and(3)immune-mediatedbiliaryinjury.Immuneactivation seems to be central to the disease process,but immunosuppression does not improve clinical outcomes or alterthe natural history of PSC.Currently,orthoptic liver transplantation is the only established life-saving treatment,whereasantimicrobial therapy or fecal transplantation is an emerging therapeutic option for PSC.The beneficial effects of thesemicrobiome-basedtherapiesarelikelymediatedbyashiftofthegutmicrobiomewithfavorableeffectsonBAmetabolism.Inthefuture,personalizedapproacheswillallowtobettertargettheinterdependencebetweenmicrobiome,immunefunction,and BA metabolism and potentially cure patients with PSC.Am J Gastroenterol 2020;115:814822.https:/doi.org/10.14309/ajg.0000000000000604INTRODUCTIONDefinitions and clinical presentationPrimary sclerosing cholangitis(PSC)is an uncommon chroniccholestatic liver disease characterized by inflammation and fi-brosis of intra-and extrahepatic bile ducts,leading to the for-mation of multifocal bile duct strictures and progressive fibrotictransformation of bile ducts(1).Hepatic fibrosis is promoted bythese biliary changes,ultimately leading to cirrhosis and liverfailure(2).There is a strong association between PSC and in-flammatory bowel disease(IBD),particularly ulcerative colitis(UC).Between 2.5%and 7.5%of individuals with IBD willeventually develop PSC.Conversely,between 60%and 70%ofpatients with PSC have IBD(3).Furthermore,the phenotype ofIBD occurring in PSC is unique and differs from the typicalphenotype seen in patients with UC or Crohns disease(CD)(4).Patients with PSC-IBD typically have mild intestinal disease ac-tivity and an increased incidence of extensive colitis typicallypancolitis,with a right-sided predominance,rectal sparing,andbackwash ileitis.DiagnosisIntheclinicalsetting,adiagnosisofPSC(1)ismadeinpatientswitha cholestatic biochemical profile when cholangiography(e.g.,magnetic resonance cholangiography or endoscopic retrogradecholangiography)shows characteristic bile duct changes withmultifocal strictures and segmental dilatations,and secondarycauses of sclerosing cholangitis have been excluded(5).EpidemiologyPSC occurs more commonly in men than in women(2:1).Themean age at the time of diagnosis is approximately 40 years(6).Using a random-effects model,a systematic review and meta-analysis concluded that the pooled incidence rate for PSC was0.77(0.451.09)per 100,000 person-years(7).Morbidity and mortalityPSC is often progressive,leading to fibrosis,cirrhosis,and end-stageliverdisease.Intheabsenceofanypharmacologicaltherapy,orthotopic liver transplantation(OLT)represents the only cu-rativeoption(8,9).ThemedianlifeexpectancyafteradiagnosisofPSC is 13.221.3 years without liver transplantation(6).There isa 3-fold mortality rate increase(hazard ratio 2.92,95%CI2.163.94)anda2-foldincreaseinriskofanymalignancy(hazardratio2.23,95%CI0.886.11)inpatientswithPSCcomparedwiththe general population;(10).However,cancer-related death farexceeds death caused by end-stage liver disease and other non-malignant complications of the disease(11).1Faculty of Medicine,The University of Queensland,Brisbane,QLD,Australia;2Translational Research Institute,Brisbane,QLD,Australia;3Department ofGastroenterology and Hepatology,Princess Alexandra Hospital,Brisbane,QLD,Australia;4Diamantina Institute,The University of Queensland,Brisbane,QLD,Australia.Correspondence:Gerald J.Holtmann,MD,PhD,MBA,FRACP,FRCP,FAHMS.E-mail:g.holtmannuq.edu.au.Received July 19,2019;accepted February 26,2020;published online March 25,2020The American Journal ofGASTROENTEROLOGYVOLUME 115|JUNE REVIEW ARTICLE814REVIEW ARTICLEPATHOPHYSIOLOGYThe pathophysiology of PSC is multifactorial,with genetic andenvironmental factors implicated.GeneticsIt is now believed that PSC occurs in genetically susceptibleindividuals(12)after exposure to an environmental trigger(13).This exposure initiates a series of events involving complexinteractions between the innate and adaptive immune systems,ultimately leading to gut-derived activated lymphocytes migrateto the liver,cholangiocyte damage,and progressive fibrosis(8).The obvious association between PSC and IBD points towardapossibleroleofautoimmunity(2).Indeed,characteristichumanleukocyteantigenhaplotypeassociationshavebeenrecognizedinPSC(2),and large-scale genome-wide association studies haveidentified genotypic associations in cohorts with PSC(14,15).More than 70%of patients with PSC also have IBD,and at least 2common gene loci have been identified in genome-wide associ-ation studies for PSC and UC(14).Overall,the most plausibleexplanationforthepathogenesisofPSCisimmunogenicprimingin a genetically predisposed individual,leading to the character-istic phenotype,that is likely further influenced by other host orenvironmental factor(8).Host/patient immune responseSeveral important observations,coupled with the strong associ-ation between certain human leukocyte antigen haplotypes andfrequency of concurrent extrahepatic autoimmune disorders,support the concept that PSC is an immune-mediated phenom-enon.The possible role of liver-gut crosstalk in PSC and UCpathogenesis may be related to the enterohepatic circulationof lymphocytes(16)referred to as the gut lymphocyte hominghypothesis.This theory postulates that a subset activated T cellsfrom the gut appear home in on the liver,then initiate and inflictdamage(17).Gastrointestinal microbiotaAs more than 60%of patients with PSC have coexistent IBD,thepathogenesis of PSC seems to be related to the manifestation oftheinflammatorychangesofthebowelthatseemtomirrorUC.Itis now widely accepted that the microbiome plays a key role forthe manifestation and progression of IBD(18).Similarly,there isnow accumulating evidence that alterations of the gastrointesti-nal microbiome are central to the pathogenesis of PSC(19).Several clinical trials(Table 1)provide evidence that the stoolmicrobiomeandthemicrobiomeadherenttothecolonicmucosaaredistinctinpatientswithPSCcomparedwithpatientswithIBDwithout PSC and non-IBD controls.Thethusfarlargeststudyonthemicrobiome(20)included137patients with PSC andfound differences inthemicrobiota profilesbetweenpatientswithPSCandUC.Despitetheobserveddysbiosis,itwasnotpossibletodifferentiatePSCandUCbasedonmicrobialtaxonomic characteristics.The taxonomic differences betweenpatients with PSC and UC may have pathophysiological sig-nificances(21).Microbes can contributeeither in isolation orcollectively via specific metabolic products and/or interferencewiththemucosalimmunesystemtowardthepathophysiologyofPSC(orIBD).Several studies(20,2227)have shownthatpatientswithPSChaveanoverrepresentationofVeillonellaspp.Thistaxon(andothersenrichedinpatientswithPSC-IBD)canproduceamineoxidases.Amine oxidase such as vascular adhesion protein-1(VAP-1)facilitatesadhesion ofgut-tropic lymphocytes to the liverendothelium(28)and potentially contributes to the manifestationof PSC-IBD via aberrant lymphocyte tracking between the boweland liver(29).The alteration in the relative abundance of Veillo-nellainthegutmicrobiomeisnotspecificforPSCandispotentiallyamarkerofadvancedliverdisease(30,31).Inaddition,Vieria-Silvaetal.(32)haveveryobservedthatanincreaseinEnterococcusspp.ispositivelyassociatedwithserumalkalinephosphatase(ALP)levelsand bile duct obstruction.They also reported that increases inVeillonella and Fusobacterium spp.are both capable of driving theproinflammatory burden associated with PSC and PSC-IBD butare coexclusive,with members of the latter genus favored inpatientswithCDorPSC-CD.Lemoinneetal.(25),reportedfungaldysbiosis in patients with PSC.They found a decrease in the pro-portion of the Saccharomycetaceae family,including the speciesSaccharomyces cerevisiae,known to have anti-inflammatoryproperties and shown to be decreased in patients with active IBD(33).All these findings suggest that the gut microbiome might beinvolved in the pathogenesis of PSC(and associated IBD)whilepatientswithPSCandIBDhaveamicrobiomethatisdistinctfrompatients with IBD alone.Thus far,no specific microbial signatureunique to PSC has been identified based on relative abundancesalone,and more studies with comprehensive functional andquantitative characterization of the microbiome are warranted.Interplay between gastrointestinal microbiome,BA metabolism,and immune function in PSCThe gastrointestinal microbiome and metabolism of BAs.Thehuman gut microbiota consists of thousands of different bacterialspecies and other microorganisms with defined functions.Thismicrobial ecosystem maintains homeostasis through a tight bal-ance between the mucosal immune system,cell-to-cell signaling,and subsequent release of antimicrobial peptides to controlneighboring bacterial clades(34).Commensal microbial metabo-lism produces essential vitamins and short-chain fatty acids(SCFAs)and modifies a variety of endogenous and exogenoussources of complex molecules(e.g.,BAs and xenobiotics)that cancontributetothegutmetabolome(35,36).TheSCFAs,particularlybutyrate,regulate innate and adaptive immune cell generation,trafficking,andfunctionandhaveanti-inflammatoryeffects.Thus,SCFAs play an active role in modulating the mucosal immunesystem to establish a tolerant phenotype against beneficial com-mensalmicrobiota(37).However,thebiodiversityofgutmicrobesimplicatedincoordinatingBAmetabolismandtheirconversion(s)from primary to secondary BAs is narrow and perhaps better de-finedfromageneticandbiochemicalcontextthanotheraspectsofmicrobial metabolism in the human gut(38).Hepatocytes synthesize primary BAs,cholic acid(CA),andchenodeoxycholic acid from cholesterol,which are conjugatedwith taurine or glycine(making them more water soluble)forsecretionintobile.PrimaryBAsaresecretedintothebileductsandare stored in the gallbladder during the interdigestive phase(39).After a meal,cholecystokinin(CCK)released from the pancreasstimulates gallbladder contraction to release BAs into the duode-num.A small proportion of bile secreted into the duodenum rea-ches the terminal ileum and colon(40),where 95%of BAs areabsorbed by active transport and transported via the portalvein to the liver where they are taken up by hepatocytes and re-excreted.This process in known as enterohepatic circulation.Microbes in the ileum and colon transform primary BAs(CA andCopyright 2020 The Author(s).Published by Wolters Kluwer Health,Inc.on behalf of The American College of GastroenterologyThe American Journal ofGASTROENTEROLOGYREVIEW ARTICLEGut Microbiome as a Treatment for PSC815chenodeoxycholic acid)into hydrophobic secondary BAs(lith-ocholic acid and deoxycholic acid)by wide array of bacterialtransformation including deconjugation catalyzed by the bile salthydrolase,dehydroxylation by 7-alpha dehydroxylase,oxidation/reduction,and epimerization(41).Some of these secondary BAsare reabsorbed during colonic transit and are also circulated inportalbloodtotheliver.Theenterohepaticcirculationofthehighlyhydrophobic and toxic secondary BAs and increased concen-trations in the liver have been linked to inflammation,cholestasis,gallstone formation(42),and carcinogenesis(43).Because of thecolonicabsorptionofBAs,onlyasmallproportionofBAs(0.5g/d)is excreted in the feces.These are replaced by de novo synthesis ofBAs in the liver.This could explain the similarity of fecal BAprofiles in patients with IBD and PSC,IBD alone,or healthycontrols(44).There is emerging evidence that luminal bile acids affect thegastrointestinal microbiome.BAs have antimicrobial activity bydamaging the bacterial cell membrane and thus inhibiting bacteriaoutgrowth(45).Reduced BA concentrations are associated withbacterial overgrowth(46,47).Indeed,a recent systematic review andmeta-analysis(48)suggested that advanced chronic liver disease isassociated with bacterial overgrowth.Gut bacteria metabolize BAsand thus influence BA composition and BA hydrophobicity(49).These BAs also shape the gut microbiota communities,whereas in-testinal microbes alter BAs.Thus,there is a bidirectional interactionbetween the intestinalmicrobiotaand BAs(50).Thus,there isa linkbetweengastrointestinalmicrobesandhepaticBAmetabolisms.Thisgut-to-liver axis may play a critical role in the regulation of enter-ohepatic circulation of BAs,BA pool size,and BA composition.Alterations of the BA homeostasis and excessive intrahepatic accu-mulation of(potentially toxic)BAs and/or their metabolites arethought to play a pivotal role in mediating the hepatic injury ofcholestatic diseases(51).BA metabolism and mucosal immune function.The regulatoryfunctionsofBAsaremainlytheresultofactivationofintracellularligand-activated nuclear receptors,such as the farnesoid X re-ceptor(FXR,NR1H4)and cell surface G proteincoupledreceptors,specifically the G proteincoupled BA receptor(TGR5orGPBAR1)(52).Bothreceptorsarehighlyrepresentedincellsofinnate immunity such as intestinal and liver macrophages,den-dritic cells,and natural killer T cells.Thus,both FXR and TGR5are critical regulators of BA metabolism,BA circulation,andintestinalimmunefunctionandcontributetothemaintenanceofa tolerogenic phenotype in enterohepatic tissues(52).Bile acids also directly shape mucosal immune function viainteractions with both dedicated BA receptors(FXR and GPBAR1)and nonspecific BA sensors(pregnane X receptor,vitamin D re-ceptor,and constitutive androstane receptor).In the context of in-nate immunity(5355),BAs suppress NF-kBdependent signalingpathways(56)and inhibit NLRP3-dependent inflammasomeTable 1.Studies of the gut microbiota in PSCNo.StudyYearMaterialPSC,nDiversitySpecific changes in the gut microbiome inPSC,using 16S rRNA gene sequencing1Rossen et al.(92)2015Ileocecal mucosa12ReducedClostridiales2Kevans et al.(26)2016Colon mucosa31No changeNo consistent indicator found3Torres et al.(93)2016Terminal ileum,right a