review article endotoxin-induced tryptophan degradation

Review ArticleEndotoxin-Induced Tryptophan Degradation alongthe Kynurenine Pathway The Role of Indolamine23-Dioxygenase and Aryl Hydrocarbon Receptor-MediatedImmunosuppressive Effects in Endotoxin Toleranceand Cancer and Its Implications for Immunoparalysis

Elisa Wirthgen and Andreas Hoeflich

Institute of Genome Biology Leibniz Institute for Farm Animal Biology Germany

Correspondence should be addressed to Elisa Wirthgen wirthgenfbn-dummerstorfde

Received 28 August 2015 Revised 28 October 2015 Accepted 6 December 2015

Academic Editor Mario Herrera-Marschitz

Copyright copy 2015 E Wirthgen and A HoeflichThis is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in anymedium provided the originalwork is properly cited

The degradation of tryptophan (TRP) along the kynurenine pathway plays a crucial role as a neuro- and immunomodulatorymechanism in response to inflammatory stimuli such as lipopolysaccharides (LPS) In endotoxemia or sepsis an enhancedactivation of the rate-limiting enzyme indoleamine 23-dioxygenase (IDO) is associated with a higher mortality risk It is assumedthat IDO induced immunosuppressive effects provoke the development of a protracted compensatory hypoinflammatory phaseup to a complete paralysis of the immune system which is characterized by an endotoxin tolerance However the role ofIDO activation in the development of life-threatening immunoparalysis is still poorly understood Recent reports described theimpact of inflammatory IDO activation and aryl hydrocarbon receptor- (AhR-) mediated pathways on the development of LPStolerance and immune escape of cancer cells These immunosuppressive mechanisms offer new insights for a better understandingof the development of cellular dysfunctions in immunoparalysis This review provides a comprehensive update of significantbiological functions of TRPmetabolites along the kynurenine pathway and the complex regulation of LPS-induced IDO activationIn addition the review focuses on the role of IDO-AhR-mediated immunosuppressive pathways in endotoxin tolerance andcarcinogenesis revealing the significance of enhanced IDO activity for the establishment of life-threatening immunoparalysis insepsis

1 Introduction

Each day animal organisms are exposed to a multitude ofimmunological stressors and pathogens and they react withappropriate immune responses For a successful host defensea balance between pro- and anti-inflammatory parts of theimmune response is essential A loss of this balance leadseither to an overreaction or to a suppression of immuneresponse both of which could be life-threatening situationsWithin this regulation of immune response the degradationof tryptophan (TRP) along the kynurenine pathway plays acrucial roleThis pathway is amajor link between the immuneand nervous systems [1] In the context of immune response

indoleamine 23-dioxygenase 1 (IDO1) is the rate-limitingenzyme for the degradation of tryptophan (TRP) alongthe kynurenine pathway to biologically active metabolitessuch as kynurenine (KYN) kynurenic acid (KYNA) orquinolinic acid (QUIN) which have been shown to takepart in diverse physiological and pathological processes [2]IDO1 is activated by proinflammatory cytokines as a part ofinnate immunity and has different physiological functionswith a highly cell type specific pattern of inducibility Theimmunosuppressive function of IDO1 activation was shownfor the first time in murine placenta [3] The high expressionof IDO1 in the placenta leads to a local suppression of mater-nal immune response and protects the fetus from rejection

Hindawi Publishing CorporationJournal of Amino AcidsVolume 2015 Article ID 973548 13 pageshttpdxdoiorg1011552015973548

2 Journal of Amino Acids

by its mother Even in malignancies it has been shownthat an increased IDO1 activity promotes the developmentof an immune tolerance protecting the tumor against theimmune response [4] In patients with sepsis an increasedIDO1 activity provokes systemic immunosuppression and isassociatedwith an increased risk formortality when it is usedas a prognostic indicator for probability of survival [5 6]In addition to the immunological consequences the mod-ulation of TRP metabolism influences the nervous systemand behavior The activation of the kynurenine pathway inthe brain depletion of TRP and generation of neurologicallyactive metabolites are associated with diseases includingschizophrenia [7] Alzheimerrsquos disease [8] and depression[9 10]

2 Tryptophan Metabolism as a Link betweenNutrition and Immune and Nervous System

TRP as an essential amino acid is not synthesized by theanimal organismTherefore dietary TRP is transported fromthe digestive tract through the portal vein to the liver whereit is used for the synthesis of proteins [11] In numerous celltypes approximately 95 of the dietary TRP is metabolizedvia the kynurenine pathway [2 12] For a long time it wasthought that TRP is only completely oxidized to NAD+(coenzyme) or to ATP (energy source) in liver cells but ithas been shown that this occurs also in extrahepatic cells(eg in astrocytes) [13] In many cell types the metabolites ofthe kynurenine pathway mediate mainly anti-inflammatoryeffects [14] whereby they act regulatory in response toproinflammatory stimuli In cells of the nervous systemKYNA anthranilic acid (AA) 3-hydroxyanthranilic acid (3-HA) or QUIN modulate neurological functions KYNA actsas an antagonist of the glutamate receptor and is thereforedescribed as a neuroprotective metabolite whereas QUINan agonist of the glutamate receptor mediates neurotoxiceffects [15] In addition to its importance in the kynureninepathway the degradation of TRP to serotonin (5-HT) andto melatonin in cells of the pineal gland has importantphysiological functions 5-HT acts both as a hormone andas a neurotransmitter Melatonin acts as a neurohormoneand is associated with the development of circadian rhythmand the sleep-wake cycle [16] Because the expression ofenzymes along the kynurenine pathway may differ substan-tially between specific cell types a crude classification of fourcompartments wasmade [11]The first compartment includescells of somatic tissues such as the lungs and intestineswhere TRP is mainly degraded to KYN In addition to theimmunosuppressive effects of KYN it has been shown thatKYN acts as an endothelium-derived relaxing factor that isable to decrease blood pressure in hypertensive rats [17] Thesecond compartment contains cells of the immune systemsuch as dendritic cells or macrophages which express manyenzymes of the kynurenine pathway exclusive of enzymesfor the synthesis of the coenzyme nicotinamide adeninedinucleotide (NAD+) which is required for redox reactionsin cells Hepatic cells represent the third compartment andexpress all enzymes required for the total oxidation of TRP

In the fourth compartment including cells of the centralnervous system TRP is specifically degraded to KYNA butother neurologically active metabolites such as quinolinicacid are also generated Additionally KYNA has been foundto be generated by macrophages [18] Increased concentra-tions of KYNA and xanthurenic acid (3-Hydroxy KYNAXA) were detected in the plasma of patients with type 2diabetes presumably due to chronic stress or the low-gradeinflammation that are prominent risk factors for diabetes [19]Thermochemical and kinetic data show that KYNA and XAare the best free-radical scavengers from the eight tested TRPmetabolites [20] suggesting that the production is a regula-tory mechanism to attenuate damage by the inflammation-induced production of reactive oxygen species AA and 3-HA can act as antioxidants in certain chemical environmentsbut in the presence of iron(II) 3-HA exhibits prooxidantactivity [21] Patients suffering from neurological disorders asHuntingtonrsquos disease or brain injury often showed decreasedlevels of 3-HA combined with increased levels of AA [22]However the biological importance of the 3-HA to AA ratioas either neurotoxic or neuroprotective mechanism is stilldiscussed [22 23]

One characteristic of TRP metabolism is that the rate-limiting step of the catalysis fromTRP toKYN is generated byboth the hepatic enzyme tryptophan 23-dioxygenase (TDO)and the ubiquitous expressed enzyme IDO1 [24 25] TDO isessential for homeostasis of TRP concentrations in organismsand has a lower affinity to TRP than IDO1 Its expression isactivated mainly by increased plasma TRP concentrationsbut can also be activated by glucocorticoids and glucagon[26] In contrast to TDO IDO1 expression is specificallyinduced by inflammatory stimuli such as the cytokinesTNF-120572 or IFN-120574 IDO has a high affinity to L-tryptophanbut will also catalyze the oxidation of other substrates suchas serotonin (5-HT) or melatonin [27] It has been assumedthat TDO is only expressed in the liver however studiesin mice indicate that TDO is also expressed in the brain[28 29] In addition to IDO1 and TDO IDO2 is describedas the third rate-limiting enzyme of the kynurenine pathway[30] IDO1 and IDO2 share a significant identity at the aminoacid level but are not structurally related to TDO IDO2 ispredominantly expressed in the kidneys followed by theepididymis testis and liver [31] The expression in distinctcell types as well as the response to stimuli differs suggestingthat IDO1 and IDO2 are not functionally redundant [31]IDO2 has even been found to be expressed in human tumorcells but the physiologically significant degradation of TRPto KYN is catalyzed by IDO1 [32] At present the biologicalfunction of IDO2 is not well established Recent studiesindicate that IDO2 is a critical mediator in the production ofautoantibodies [33] and in IDO1-mediatedT cell regulation inthe context of inflammation [34] The existence of three rate-limiting enzymes differing in structure substrate specificityand activation supports the significance of the kynureninepathway for both nutritional and immunoregulatory func-tions An overview of the TRP degradation pathways andtheir biological significance is presented in Figure 1

Journal of Amino Acids 3

Tryptophan

N-formyl kynurenine

Aminocarboxymuconatesemialdehyde

Picolinic acid

NAD+

Glutaryl-CoA

5-Hydroxytryptophan

N-acetyl-5-methoxy-

IDO

IDO

IDO

FOR

KAT

KMO

KYNU

HADO

HAAO

TPH

AANAT

HIOMT

AADC

Kynurenine pathway

Biological functions

IFN-120574

TNF-120572IL-1

Cortisol

TRP

IL-2

KYNU

AMO

Anthranilic acid 5

TDO IDO

Serotonin67

Melatonin56

kynuramine67

Kynurenine14

Kynurenic acid13

IFN-120574

IFN-120574

IFN-120574

3-Hydroxykynurenine124

3-Hydroxyanthranilic acid1 4 5

Quinolinic acid 24

CO2 + ATP

1Immunosuppressive2Neurotoxic3Neuroprotective4Generation of free radicals

5Antioxidative6Acting as hormone7Modulation of behavior

Figure 1 Main pathways of TRP degradation including enzymes and their potential stimuli (modified after [1 4 16 35]) Superscriptednumbers indicate described biological effects of TRP metabolites Black arrows mark enzymatic reactions and dashed arrows include morethan one catalytic reaction step Enzymes FOR= formamidase IDO= indolamine 23-dioxygenase TDO= tryptophan 23-dioxygenase TPH= tryptophan hydroxylase KAT = kynurenine aminotransferase KMO = kynurenine 3-monooxygenase KYNU = kynureninase HADO =3-hydroxyanthranilic acid dioxygenase HAAO = 3-hydroxyanthranilic acid oxidase AMO = anthranilate 3-monooxygenase and AADC =aromatic L-amino acid decarboxylase AANAT = N-acetyltransferase HIOMT = hydroxy-O-methyltransferase


3 Regulation of LPS-InducedExpression of IDO1

The ubiquitously expressed intracellular enzyme IDO is anoxidoreductase and catalyzes the degradation of L-TRP toN-formylkynurenine which under physiologic conditionsis rapidly converted to KYN [36] Thereby L-TRP andKYN are transported bidirectionally by the L-amino acidtransporter (LAT) to the IDO expressing cell or to the bloodplasma respectively [36] The activation of IDO gene (Ido)expression is mainly induced by proinflammatory cytokinesbut a constitutive expression of IDO has also been described[27 37 38] indicating different physiological functions ofIDO LPS act as potential ligands for various cell types suchas peripheral mononuclear cells (PBMCs) endothelial cellscells of smooth vascular muscles granulocytes or thrombo-cytes [39] The activation of macrophages or monocytes byLPS initiates the release of bioactive lipids reactive oxygenspecies and cytokines including IFN-120574 TNF-120572 interleukin-1 (IL-1) IL-6 IL-8 or IL-10 which induce synergisticallyantagonistically or independently acting signaling pathways[40] The activation of immune cells by LPS is mainlymediated by toll-like receptor (TLR) 4 signaling but alsoby TLR-2 [41] Therefore LPS interact with the soluble LPSbinding protein (LPB) The LPSLPB complex binds to themembrane protein cluster of differentiation 14 (CD14) whichinteracts with TLR-4 and activates nuclear factor ldquokappa-light-chain-enhancerrdquo of activated B-cells (NF-120581B) associatedsignaling pathways [42 43] The transcription factor NF-120581B controls more than 150 target genes which are essentialfor the expression of cytokines immune receptors antigen-presenting molecules proteins of acute phase and regulatoryproteins such as transcription factors or enzymes [44] Inaddition to NF-120581B associated IDO activation IFN-120574 initiatesthe transcription of IDO through the pathway of a signaltransducer and activator of transcription 1 (STAT1) In humancancer cells it has been shown that IDO sustains its ownexpression via an autocrine AhR-IL-6-STAT3 signaling loopleading to a constitutive IDO expression in IDO positivetumor cells [45] Ido genes have been identified in variousspecies of mammals [46] In mice and humans Ido is locatedon chromosome eight and includes ten exons with approx-imately 15 kbp of DNA [44] The human Ido gene containsmultiple promoter elements and the cDNA encodes a full-length protein containing 403 amino acids with a molecularweight of 45 kDa [27] In addition a truncated IDO1 variantcomposed of 316 amino acids has been described [30]The expression and activity of the IDO enzyme is complexand is regulated on transcriptional posttranscriptional andposttranslational levels On the transcriptional level IFN-120574-induced IDO expression is controlled by thewidely expressedadaptor protein bridging integrator 1 (BIN1) or the enteralfatty acid sodium butyrate (NaB) A reduced expression oractivity of BIN1 found in various types of cancers enablesan increased IDO activity which promotes the appearanceof immunotolerance [47ndash49] In vitro studies of carcinomacells demonstrated that NaB inhibits IFN-120574-induced IDOexpression by reducing the phosphorylation and nucleartranslocation of STAT1 [50]The IDOenzyme is amonomeric

protein containing a prosthetic heme group The catalyticactivity of IDO is posttranscriptionally regulated by theintracellular redox status indicating the existence of an IDO-specific reducing system similar to that of erythrocytes [25 5152]The oxidized enzyme (IDO-Fe3+) represents the catalyticinactive ferrous form whereas the reduced ferric form ofthe enzyme (IDO-Fe2+) is catalytically active and enablesthe binding of TRP and oxygen [53] Furthermore there isevidence for NaB-dependent ubiquitination of IDO as thepreparation for proteasomal degradation [54] In the contextof inflammatory immune response the release of IFN-120574induces the generation of nitric-oxide (NO) by induciblenitric-oxide synthase (iNOS) [55] NO is able to inhibit IDOactivity in a reversible manner by building IDO2+NO-TRPcomplex and preventing oxidation of TRP [56] FurthermoreNO induces the degradation of IDO via mediating thebinding of suppressor of cytokine signaling 3 (SOCS3) ata specific phosphotyrosine [57 58] An overview of theregulation mechanisms in response to inflammatory stimulisuch as LPS is given by Figure 2

4 The Kynurenine Pathway inEndotoxemia and Sepsis

Dependent on dose and application endotoxins includingLPS induce the activation of inflammatory pathways whichcan lead to serious systemic reactions such as septic shock[59] In animal models the in vivo application of LPS isoften used to simulate inflammatory conditions includingsystemic inflammatory response syndrome (SIRS) bacterialinfection [60] or systemic inflammation [61] Although theLPS-mediated activation of proinflammatory cytokines isessential for an adequate host defense an overreaction ofthe proinflammatory immune response provokes serioustissue damage leading to life-threatening situations includingseptic shock [62] In addition to the release of IL-10 [63]the activation of IDO is one potent immunosuppressiveand regulatory mechanism of innate immunity It has beenshown that IDO expression in dendritic cells or macrophagessuppresses T cells by depriving TRP [49 64] Addition-ally it has been shown that KYN inhibits T cell-mediatedinflammation by suppressing T cell proliferation [65] Studiesof human malignancies have found that IDO is highlyexpressed in many malignant tumor cells leading to aneffective immune escape by inhibiting the T cell response[47] Furthermore the generation of immunomodulatingmetabolites of the kynurenine pathway induced the arrest ofT cells and the arrest or death of natural killer cells leadingto the development of tolerogenic antigen-presenting cells(APCs) migrating to lymph nodes andmediating suppressiveeffects [14] In humans it has been shown that plasmaconcentrations of KYN are correlated with sepsis severitydemonstrating increased IDO activity in patients with septicshock [6] The ratio of KYN to TRP is a sensitive marker forIDO activation and a significant increase can be used as aprognostic indicator formortality in sepsis or trauma patients[6 66] Interestingly KYN is associated with the degreeof hypotension in experimental human endotoxemia [67]


R

TLR-4

TNFR

STAT1

NF-120581B

IDO

LPS

IDO-Fe3+IDO-Fe -TRP-O2+

2

Immune cellLPS

TLR-4

TNF-120572

TNF-120572IFN-120574

IFN-120574

IFN-120574

mRNA

NaB

IDOTranslation

mRNA

IDO

Nucleus

Cytoplasm

SOCS3

Ubiquitination

Bin1

Inactive Active

TRP TRPTRP

TRP

Blood plasma

TRP KYN

TRP

TRP

KYN

Cofactor

NO

KYN

LAT

KYN

DepletionAccumulation

KYN

Inhibition

KYNKYN

Immunomodulation

IDO expressingcell

KYN

Proteosomaldegradation

Activation

1A 1BAlternate

407 aa

316 aa

IDO

NaB

eminus

5998400 exons

Figure 2 Schematic illustration of an inflammatory IDO activation by LPS and inflammatory cytokines and possible regulatory mechanismson transcriptional posttranscriptional and posttranslational level

indicating that there are functions of IDO activation otherthan in immunosuppression This is supported by studies ofIDO knockout mice and mice treated with the IDO inhibitor1-methyltryptophan After IDO knockout or inhibition themice showed decreased levels of proinflammatory TNF-120572IL-6 and IL-12 but increased levels of the anti-inflammatorycytokine IL-10 which prevented them fromdying from septicshock 48 h after the LPS challenge [68] Furthermore in thisstudy the balance of IL-12 to IL-10 had an impact on IDOprotein expression whereby IL-10 has suppressive effects on

IDO protein expression if IL-10 is much higher than IL-12 Itis possible that the effects of IDO induced increased hypoten-sion or a disturbance of the balance between pro- and anti-inflammatory cytokine response These are risk factors formortality in early sepsis independent of immunosuppressiveeffects Studies of patients with septic shock and acute kidneyinjury demonstrated that a failed reduction of KYNA afterhemofiltration treatment may predict fatal outcomes [69] Ithas been shown that KYNA in vitro inhibited the secretionof TNF-120572 by LPS treated mononuclear cells [70 71] This


was confirmed by studies of mice indicating that the in vivoapplication of KYNA provokes a reduced release of TNF-120572 inresponse to an ex vivo LPS challenge [71] Furthermore in anin vitro vascular flow model KYNA triggered the firm arrestof monocytes to both fibronectin and intercellular adhesionmolecule 1 (ICAM-1) via 1205731 integrin- and 1205732 integrin-mediated mechanisms respectively [72] Notably elevatedplasma levels of KYNA have been reported in patients withinflammatory bowel diseases but tryptophan and metabo-lites such as 3-hydroxykynurenine 3-hydroxyanthranilicacid and xanthurenic acid were unchanged in all patients[73] In vitro studies have also provided that KYNA actsas a ligand for the G protein-coupled receptor GPR35that is highly expressed in human peripheral monocytesand in cells of the gastrointestinal tract [70] However theinvolvement of GPR35 in gastrointestinal disorders and itsimmunosuppressive effects remain unclear

5 IDO-AhR-Mediated ImmunosuppressiveEffects on Endotoxin Tolerance and Cancer

Endotoxin tolerance in vivo is characterized by a reducedinducibility of inflammatory cytokines and the HPA axisand a reduced development of illness symptoms includingfever or weight loss in response to a repeated endotoxinchallenge [74 75] In the context of endotoxin-induced sepsisthe hyperinflammatory immune response is followed by acompensatory hypoinflammatory phase up to a completeparalysis of the immune system [76] which is character-ized by an endotoxin tolerance In this phase the properimmune response failed leading to secondary infections (egventilator-associated pneumonia) [77] In addition to sepsisendotoxin tolerance has been reported in several diseasesincluding trauma surgery or pancreatitis Recent studieson IDO knockout mice indicated that IDO1 plays a crucialrole in the generation of LPS-induced endotoxin toleranceand the combined effects of AhR IDO1 and transforminggrowth factor-120573 (TGF-120573) are required [78] Notably L-kynurenine alone failed to restore tolerance in the absenceof IDO1 which indicates that nonenzymatic functions suchas intracellular signaling are involved in the development ofa regulatory phenotype in splenic dendritic cells It has alsobeen found that IDO1 specific phosphorylation reprogramsgene expression leading to TGF-120573 production in responseto TLR signaling [79] Interestingly the phosphorylation ofIDO1 in response to LPS is dependent on the upregulationof AhR [78] which triggers the activity of Src kinase [80]resulting in phosphorylation of the target proteins [81] Theimportance of AhR activation and TGF-120573 for the inducementof counterregulatory mechanisms such as LPS toleranceis supported by previous studies on mice indicating thatthe generation of regulatory T cells (Tregs) is dependent onAhR and TGF-120573 [82 83] Notably the suppressive activityof human T cells treated with TGF-1120573 plus TCDD as AhRligand is greater than that of T cells treated only with TCDD[83] The increase of Tregs is deleterious in sepsis patientsand associated with decreased proliferation of effector T cells[84] Participation of AhR in the transcriptional regulation of

LPS-induced gene expression is supported by further studiesshowing multiple AhR associated pathways [85ndash87] It hasbeen shown that AHR-deficient mice are more sensitive toendotoxin shock than wild type mice indicating the crucialrole of AhR in modulating the TLR-4-induced inflammatoryimmune response [88] It is possible that the lack of AhRin these animals prevents the successful establishment ormaintenance of endotoxin tolerance as a counterregulatorymechanism in response to an endotoxin challenge AhRis a ligand-dependent cytosolic transcription factor that isable to translocate to the cell nucleus after ligand binding[86] The tryptophan metabolites L-kynurenine [78] 6-formylindolcarbazole (FICZ a photoproduct of TRP) [89]and KYNA [90] are described as natural AhR ligands medi-ating immunosuppressive functions To induce transcriptionof AhR target genes in the nucleus AhR partners withproteins such as AhR nuclear translocator (ARNT) or NF-120581B subunit RelB Studies on human cancer cells have shownthat KYN activates the AhR-ARNT associated transcriptionof IL-6 which induced autocrine activation of IDO1 viaSTAT3 This AhR-IL-6-STAT3 loop is associated with a poorprognosis in lung cancer [45] supporting the idea that IDO-mediated immunosuppression enables the immune escape oftumor cells Notably in trauma patients with increased IDOactivity nonsurvivors had higher concentrations of IL-6 aftertrauma than survivors [91] It is possible that the autocrineconstitutive activation of IDO is associated with immunedysfunctions and poor survival The partnership of AhR andRelB has been shown to induce expression of IDO1 andIDO2 and to promote maturation of DCs promoting localtolerance at mucosal sites such as the lungs or gut [86] Theprominence of AhR in immunosuppression and endotoxintolerance may be a link between similar mechanisms foundin inflammation and carcinogenesis Potential connectionsbetween inflammatory IDO activation and AhR-mediatedimmunosuppressive pathways are shown in Figure 3

However to ensure a successful defense against patho-gens LPS tolerance is not a global downregulation of signal-ing proteins but is rather a mechanism of host defense thatremains active [74] In endotoxin-tolerant mice increasednumbers of Kupffer cells were detected leading to increasedphagocytosis of bacteria [92 93] Furthermore endotoxin-tolerant mice have an increased number of neutrophilsindicating the increased recruitment of cells from bonemarrow [92] or reduced apoptosis [94] In pigeons areduction of leukocytes and eosinophil granulocytes concurswith an increase in lymphocytes [95] Even the compositionof lymphocytes was changed in tolerant animals whereas thepercentage of cytotoxic T cells increased but the percentageof T-helper cells remained unalteredThis would suggest thatendotoxin tolerance helps to control inflammatory-mediatedeffects of bacterial infection It was found that endotoxin-tolerant state protects mice against immunopathology inGram-negative and Gram-positive infections [74] Further-more in these studies endotoxin tolerance does not preventthe early events in protective TLR-2 signaling but prohibitsthe later onset of hostrsquos control over local or systemic disease


TGF-120573

Cytoplasm

IDO

IDO

p

IDOTDOSTAT1

TDO

Inflammation

IDO

TRP

SrcAhR

IL-6

IL-6

STAT3

KYN

SrcAhR

KYN

KYN

AhR

KYN

KYN

AhR

KYN

ARN

T

IL-6

JAK

IDO

Nucleus

pIDO

Src

IL-6

IL-6

KYN

Inactive kinase

Active kinase

NF-120581B

Figure 3 Schematic illustration of possible interaction between inflammatory IDO activation andAhR-mediated effects IDO1 plays a crucialrole in the generation of LPS-induced endotoxin tolerance whereas combined effects of AhR IDO1 and TGF-120573 are required L-kynureninealone failed to restore tolerance in the absence of IDO1 indicating nonenzymatic functions as intracellular signaling It was found that IDO1specific phosphorylation by AhR triggered kinase activity induces reprograming of gene expression leading to TGF-120573 in response to TLRsignaling Studies on human cancer cells showed that IDO induced KYN activates the AhR-ARNT associated transcription of IL-6 whichpromotes autocrine activation of IDO1 via STAT3 supporting the theory that IDO-mediated immunosuppression enables effectively immuneescape of tumor cells

6 From Immunosuppression toImmunoparalysis The Significance ofEnhanced IDO Activation

The mechanisms that cause the development of immunopa-ralysis are poorly understood However it has been shownthat the concept of endotoxin tolerance serves as an exper-imental analogue to the clinical entity of immunoparalysis[96] Microarray studies of human PBMCs showed that theexpression profile of sepsis patients is strongly associatedwith the signature for endotoxin tolerance Furthermorein early stage of sepsis the risk for the development ofa confirmed sepsis or organ dysfunction was predicted bythis endotoxin tolerance signature [97] The development ofimmunoparalysis is strongly associated with a poor outcomein patients because the homeostasis between pro- and anti-inflammatory immune response is not restored within afew days [96] Therefore the protracted immunosuppres-sive phase prohibits the adequate control of the primaryinfection or of secondary hospital-acquired infections withopportunistic pathogens [98] The role of IDO activation

in the development of life-threatening immunoparalysis isstill poorly understood A recent report which examinedthe AhR-mediated impact of IDO activation on immuno-suppressive functions such as LPS tolerance and immuneescape of cancer cells offers new insights for a better under-standing of the development of cellular dysfunctions inimmunoparalysis It has been shown that enhanced IDOactivity is associatedwith the severity of sepsis or septic shock[6 66 91 99] The data indicate that patients with a pooroutcome have enhanced IDO activity in the early stage ofsepsis leading to increased levels of KYN and KYNA andprovoking the expression of TGF-120573 and IL-6 and the develop-ment of endotoxin tolerance Increased plasma levels of TGF-120573 and IL-6 were found in early sepsis (day 1) of nonsurvivorscompared to survivors [100] Notably in the ongoing sepsisof nonsurvivors (day 7) IL-6 did not significantly decreasein plasma enabling the autocrine activation of IDO via theAhR-IL-6-STAT3 loop as described in human cells [45] Thetheory of enhanced IDO activity and immunosuppressionin ongoing sepsis is supported by data from patients withsevere sepsis and septic shock There nonsurvivors had


Ongoing sepisEarly sepsis

KYN KYNA TGF-120573 IL-6endotoxin tolerance signature

Poor prognosis

Good prognosis

Enhanced IDO activityFunctional IDO activity

IDOactivation

LPS

Immunoparalysis

Homeostasis

Indicators forenhanced IDO

activityKYN IL-6 Tregs

Figure 4 Enhanced IDO activity and its indicators as potential risk factors for the development of immunoparalysis and poor prognosis insepsis Patients with poor outcome have enhanced IDO activity in early state of sepsis leading to increased levels of KYN and KYNA thatprovokes expression of TGF-120573 IL-6 and development of endotoxin tolerance In ongoing sepsis IL-6 does not decrease significantly in plasmaof nonsurvivors enabling the autocrine activation of IDOviaAhR-IL-6-STAT3 loopThe enhanced IDOactivitymediates immunosuppressiveeffects as increased generation of Tregs in ongoing sepsis provoking the establishment of immunoparalysis

elevated KYNTRP ratios compared to survivors even on day7 of sepsis indicating enhanced IDO activity [6] Notablyin patients with sepsis-associated immunosuppression thetreatment with granulocyte-macrophage colony-stimulatingfactor (GM-CSF) reduced IDO activity and downstreammetabolites significantly [101] GM-CSF is known to restorethe immunocompetence of monocytes in sepsis and mayshorten the time of both intrahospital and intensive care unitstay [102] These data support that enhanced IDO activity isone of several risk factors in poor prognosis of sepsis Anillustration of risk factors detected in early and ongoing sepsisthat indicates significance of enhanced IDO activity for pooroutcome is given in Figure 4

At present there are no data regarding the IDO geneor protein expression from early to ongoing sepsis IDOactivity is often measured indirectly by the KYNTRP ratiobut TRP is also depleted by TDO and IDO2 Studies on pigshave provided LPS-inducible IDO1 protein expression in theplasma 3 h and 6 h after the LPS challenge comparable withan increase of the metabolites KYN and KYNA [61] Even inliver and lung tissue IDO1 protein expression was detectable6 h after LPS One day after the LPS challenge in both plasmaand tissue IDO1 protein expression was not detectable inLPS-challenged pigs in accordance with the normalization ofpro- and anti-inflammatory cytokines and TRP metabolitesA second LPS challenge 24 h after the first challenge didnot induce cytokine production indicating LPS tolerance

However IDO1 activation was induced neither in vivo norex vivo [103] suggesting that constitutive IDO1 activity is notrequired for maintenance of the tolerant state It is possiblethat the initial phase of IDO activation is critical for a goodor poor outcome after septic shock

7 Conclusion

The IDO-mediated degradation of TRP along the kynureninepathway plays a significant role as counterregulatory mecha-nismwithin the inflammatory immune response Endotoxin-induced IDO activation results in the depletion of TRPand the generation of biological active TRP metabolitessuch as KYN and KYNA which have important immuno-suppressive properties Furthermore IDO-mediated effectsare required for the establishment of LPS tolerance asanti-inflammatory mechanism permitting an adequate hostdefense However an imbalance in the cytokine activationand kynurenine cascade may result in severe immune dys-functions as immunoparalysis Studies of patients with sepsisfound that an enhanced IDO activity was associated withan increasedmortality risk However molecular mechanismsremained poorly understood Recent findings of IDO-AhR-mediated immunosuppressive effects in endotoxin toleranceand cancer offer new insights for the better understandingof molecular mechanisms in sepsis In endotoxin tolerancethe AhR activation by KYN mediates the phosphorylation of


IDO inducing reprograming of gene expression and leadingto TGF-120573 production in response to TLR signaling In cancercells a constitutive IDO activation was associated with poorprognosis This is provoked by immunosuppressive effects ofIDO expressing tumor cells on the local environment leadingto an effective immune escape of the cancer cells NotablyAhR also mediates the constitutive IDO expression in cancercells Thereby KYN activates AhR-mediated transcription ofIL-6 leading to an autocrine activation of a constitutive IDOexpression It has been shown that increased concentrationsof KYN or KYNA both ligands of AhR and increasedconcentrations of TGF-120573 and IL-6 are risk factors for estab-lishment of an immunoparalysis and for a poor prognosisof patients in early sepsis instead of recovery In ongoingsepsis there are indices that prolonged IDO activity supportsthe maintenance of a protracted immunosuppressive phaseThis might be due to the generation of regulatory T cellsvia TGF-120573 mediated pathways and an increased productionof immunosuppressive metabolites such as KYN or KYNAIncreased concentrations of IL-6 found in ongoing sepsis ofpatients with poor prognosis may promote the constitutiveIDO activation leading to maintenance of a life-threateningimmunosuppressive phase in sepsis The significance of IDOinduced AhR-mediated effects on sepsis has still to beinvestigated However the molecular mechanisms of IDOinduced immunosuppression found in endotoxin toleranceand carcinogenesis can offer new insights on critical path-ways in sepsis-associated immunoparalysis and enable newapproaches for therapeutic interventions

Abbreviations

3-HA 3-Hydroxyanthranilic acid5-HT Serotonin (5-hydroxytryptamine)AA Anthranilic acidAhR Aryl hydrocarbon receptorAPC Antigen-presenting cellARNT Aryl hydrocarbon receptor nuclear

translocatorBIN1 Bridging integrator 1CD Cluster of differentiationGM-CSF Granulocyte-macrophage colony-stimulating

factorGPR35 G protein-coupled receptorHPA Hypothalamic-pituitary-adrenalIDO Indolamine 23-dioxygenaseIFN InterferonIL InterleukiniNOS Inducible nitric-oxide synthaseKYN KynurenineKYNA Kynurenic acidLBP LPS binding proteinLPS LipopolysaccharidesNaB Sodium butyrateNAD+ Coenzyme nicotinamide adenine

dinucleotideNF-120581B Nuclear factor ldquokappa-light-chain-enhancerrdquo

of activated B-cellsNO Nitric-oxide

PBMC Peripheral mononuclear cellQUIN Quinolinic acidRelB Subunit of the NF-120581B complexSIRS Systemic inflammatory response syndromeSOCS3 Suppressor of cytokine signaling 3STAT Signal transducer and activator of transcrip-

tionTCDD 2378-Tetrachlorodibenzo-p-dioxinTDO Tryptophan 23-dioxygenaseTLR Toll-like receptorTNF Tumor necrosis factorTNFR TNF receptorTGF Transforming growth factorTreg Regulatory T cellTRP TryptophanXA Xanthurenic acid (3-hydroxykynurenic acid)

Conflict of Interests

Elisa Wirthgen and Andreas Hoeflich are related to LigandisGbR This relation has no effect on the content of the paper

Acknowledgments

The authors thank Christian Manteuffel and the staff ofSignal Transduction Unit from Leibniz Institute for FarmAnimal Biology for helpful comments and suggestions Fur-thermore the authors thank Winfried Otten Ellen KanitzMargret Tuchscherer Ulrike Gimsa and Christine Schuttfor introducing in the topics tryptophan metabolism andneuroimmunoendocrinology

References

[1] Y Mandi and L Vecsei ldquoThe kynurenine system and immuno-regulationrdquo Journal of Neural Transmission vol 119 no 2 pp197ndash209 2012

[2] O Takikawa ldquoBiochemical and medical aspects of the indole-amine 23-dioxygenase-initiated l-tryptophan metabolismrdquoBiochemical and Biophysical Research Communications vol 338no 1 pp 12ndash19 2005

[3] D H Munn M Zhou J T Attwood et al ldquoPrevention ofallogeneic fetal rejection by tryptophan catabolismrdquo Sciencevol 281 no 5380 pp 1191ndash1193 1998

[4] X Liu R C Newton S M Friedman and P A ScherleldquoIndoleamine 23-dioxygenase an emerging target for anti-cancer therapyrdquo Current Cancer Drug Targets vol 9 no 8 pp938ndash952 2009

[5] R Huttunen J Syrjanen J Aittoniemi et al ldquoHigh activity ofindoleamine 2 3 dioxygenase enzyme predicts disease severityand case fatality in bacteremic patientsrdquo Shock vol 33 no 2 pp149ndash154 2010

[6] P Tattevin DMonnier O Tribut et al ldquoEnhanced indoleamine23-dioxygenase activity in patients with severe sepsis and septicshockrdquo Journal of Infectious Diseases vol 201 no 6 pp 956ndash966 2010

[7] S Erhardt L Schwieler L Nilsson K Linderholm and GEngberg ldquoThe kynurenic acid hypothesis of schizophreniardquoPhysiology amp Behavior vol 92 no 1-2 pp 203ndash209 2007


[8] G J Guillemin K R Williams D G Smith G A SmytheJ Croitoru-Lamoury and B J Brew ldquoQuinolinic acid in thepathogenesis of Alzheimerrsquos diseaserdquo in Developments in Tryp-tophan and Serotonin Metabolism pp 167ndash176 Springer BerlinGermany 2003

[9] J C OrsquoConnor M A Lawson C Andre et al ldquoLipopolysac-charide-induced depressive-like behavior is mediated by indol-eamine 23-dioxygenase activation in micerdquoMolecular Psychia-try vol 14 no 5 pp 511ndash522 2009

[10] R Dantzer J C OrsquoConnor M A Lawson and K W Kel-ley ldquoInflammation-associated depression from serotonin tokynureninerdquo Psychoneuroendocrinology vol 36 no 3 pp 426ndash436 2011

[11] J R Moffett and M A Namboodiri ldquoTryptophan and theimmune responserdquo Immunology and Cell Biology vol 81 no 4pp 247ndash265 2003

[12] J Peters ldquoTryptophan nutrition and metabolism an overviewrdquoin Kynurenine and Serotonin Pathways pp 345ndash358 Springer1991

[13] R S Grant H Naif M Espinosa and V Kapoor ldquoIDOinduction in IFN-120574 activated astroglia a role in improving cellviability during oxidative stressrdquo Redox Report vol 5 no 2-3pp 101ndash104 2000

[14] A Gonzalez N Varo E Alegre A Diaz and I MeleroldquoImmunosuppression routed via the kynurenine pathway abiochemical and pathophysiologic approachrdquoAdvances in Clin-ical Chemistry vol 45 pp 155ndash197 2008

[15] J P Ruddick A K Evans D J Nutt S L Lightman G AW Rook and C A Lowry ldquoTryptophan metabolism in thecentral nervous system medical implicationsrdquo Expert Reviewsin Molecular Medicine vol 8 no 20 pp 1ndash27 2006

[16] T W Stone and L G Darlington ldquoEndogenous kynurenines astargets for drug discovery and developmentrdquo Nature ReviewsDrug Discovery vol 1 no 8 pp 609ndash620 2002

[17] Y Wang H Liu G McKenzie et al ldquoKynurenine is anendothelium-derived relaxing factor produced during inflam-mationrdquo Nature Medicine vol 16 no 3 pp 279ndash285 2010

[18] FMoroni A CozziM Sili andGMannaioni ldquoKynurenic acida metabolite with multiple actions and multiple targets in brainand peripheryrdquo Journal of Neural Transmission vol 119 no 2pp 133ndash139 2012

[19] G F Oxenkrug ldquoIncreased plasma levels of xanthurenic andkynurenic acids in type 2 diabetesrdquoMolecular Neurobiology vol52 no 2 pp 805ndash810 2015

[20] A Perez-Gonzalez J R Alvarez-Idaboy and A Galano ldquoFree-radical scavenging by tryptophan and its metabolites throughelectron transfer based processesrdquo Journal of Molecular Model-ing vol 21 article 213 2015

[21] V Chobot F Hadacek W Weckwerth and L KubicovaldquoIron chelation and redox chemistry of anthranilic acid and3-hydroxyanthranilic acid a comparison of two structurallyrelated kynurenine pathway metabolites to obtain improvedinsights into their potential role in neurological disease devel-opmentrdquo Journal of Organometallic Chemistry vol 782 pp 103ndash110 2015

[22] L G Darlington C M Forrest G M Mackay et al ldquoOnthe biological importance of the 3-hydroxyanthranilic acidanthranilic acid ratiordquo International Journal of TryptophanResearch vol 3 no 1 pp 51ndash59 2010

[23] D Krause H-S Suh L Tarassishin et al ldquoThe tryptophanmetabolite 3-hydroxyanthranilic acid plays anti-inflammatory

and neuroprotective roles during inflammation role ofhemeoxygenase-1rdquoThe American Journal of Pathology vol 179no 3 pp 1360ndash1372 2011

[24] M W Taylor and G Feng ldquoRelationship between interferon-120574indoleamine 2 3-dioxygenase and tryptophan catabolismrdquoTheFASEB Journal vol 5 no 11 pp 2516ndash2522 1991

[25] S R Thomas and R Stocker ldquoRedox reactions relatedto indoleamine 23-dioxygenase and tryptophan metabolismalong the kynurenine pathwayrdquo Redox Report vol 4 no 5 pp199ndash220 1999

[26] N Le FlocrsquohWOtten andEMerlot ldquoTryptophanmetabolismfrom nutrition to potential therapeutic applicationsrdquo AminoAcids vol 41 no 5 pp 1195ndash1205 2011

[27] N J C King and S R Thomas ldquoMolecules in focus indol-eamine 23-dioxygenaserdquoThe International Journal of Biochem-istry and Cell Biology vol 39 no 12 pp 2167ndash2172 2007

[28] K Ohira H Hagihara K Toyama et al ldquoExpression of tryp-tophan 23-dioxygenase in mature granule cells of the adultmouse dentate gyrusrdquo Molecular Brain vol 3 no 1 article 262010

[29] M Kanai T Nakamura and H Funakoshi ldquoIdentificationand characterization of novel variants of the tryptophan 2 3-dioxygenase gene differential regulation in the mouse nervoussystem during developmentrdquoNeuroscience Research vol 64 no1 pp 111ndash117 2009

[30] H J Ball H J Yuasa C J D Austin S Weiser and N HHunt ldquoIndoleamine 2 3-dioxygenase-2 a new enzyme in thekynurenine pathwayrdquoThe International Journal of Biochemistryand Cell Biology vol 41 no 3 pp 467ndash471 2009

[31] H J Ball A Sanchez-Perez SWeiser et al ldquoCharacterization ofan indoleamine 2 3-dioxygenase-like protein found in humansand micerdquo Gene vol 396 no 1 pp 203ndash213 2007

[32] S Lob A Konigsrainer D Zieker et al ldquoIDO1 and IDO2 areexpressed in human tumors levo- but not dextro-1-methyl tryp-tophan inhibits tryptophan catabolismrdquo Cancer ImmunologyImmunotherapy vol 58 no 1 pp 153ndash157 2009

[33] L M F Merlo E Pigott J B DuHadaway et al ldquoIDO2 is acritical mediator of autoantibody production and inflammatorypathogenesis in a mouse model of autoimmune arthritisrdquo TheJournal of Immunology vol 192 no 5 pp 2082ndash2090 2014

[34] R Metz C Smith J B DuHadaway et al ldquoIDO2 is critical forIDO1-mediated T-cell regulation and exerts a non-redundantfunction in inflammationrdquo International Immunology vol 26no 7 pp 357ndash367 2014

[35] Z Bohar J Toldi F Fulop and L Vecsei ldquoChanging the faceof kynurenines and neurotoxicity therapeutic considerationsrdquoInternational Journal of Molecular Sciences vol 16 no 5 pp9772ndash9793 2015

[36] S Lancellotti L Novarese and R De Cristofaro ldquoBiochemicalproperties of indoleamine 23-dioxygenase from structure tooptimized design of inhibitorsrdquo Current Medicinal Chemistryvol 18 no 15 pp 2205ndash2214 2011

[37] R Yoshida T Nukiwa Y Watanabe M Fujiwara F Hirataand O Hayaishi ldquoRegulation of indoleamine 23-dioxygenaseactivity in the small intestine and the epididymis of micerdquoArchives of Biochemistry and Biophysics vol 203 no 1 pp 343ndash351 1980

[38] R Yoshida J Imanishi T Oku T Kishida and O HayaishildquoInduction of pulmonary indoleamine 23-dioxygenase byinterferonrdquo Proceedings of the National Academy of Sciences ofthe United States of America vol 78 no 1 pp 129ndash132 1981


[39] E T Reitschel T Kirikae FU Shade et al ldquoBacterial endotoxinmolecular relationships of structure to activity and functionrdquoThe FASEB Journal vol 8 no 2 pp 217ndash225 1994

[40] S N Vogel and M M Hogan Role of Cytokines in Endotoxin-Mediated Host Responses Oxford University Press New YorkNY USA 1990

[41] R P Darveau T-T T Pham K Lemley et al ldquoPorphyromonasgingivalis lipopolysaccharide contains multiple lipid A speciesthat functionally interact with both toll-like receptors 2 and 4rdquoInfection and Immunity vol 72 no 9 pp 5041ndash5051 2004

[42] S M Dauphinee and A Karsan ldquoLipopolysaccharide signalingin endothelial cellsrdquo Laboratory Investigation vol 86 no 1 pp9ndash22 2006

[43] D J Kahler and A L Mellor ldquoT cell regulatory plasmacytoiddendritic cells expressing indoleamine 23 dioxygenaserdquo inDen-dritic Cells vol 188 ofHandbook of Experimental Pharmacologypp 165ndash196 Springer Berlin Germany 2009

[44] H L Pahl ldquoActivators and target genes of RelNF-120581B transcrip-tion factorsrdquo Oncogene vol 18 no 49 pp 6853ndash6866 1999

[45] U M Litzenburger C A Opitz F Sahm et al ldquoConstitutiveIDO expression in human cancer is sustained by an autocrinesignaling loop involving IL-6 STAT3 and theAHRrdquoOncotargetvol 5 no 4 pp 1038ndash1051 2014

[46] H J Yuasa M Takubo A Takahashi T Hasegawa H Nomaand T Suzuki ldquoEvolution of vertebrate indoleamine 23-dioxygenasesrdquo Journal of Molecular Evolution vol 65 no 6 pp705ndash714 2007

[47] C Uyttenhove L Pilotte I Theate et al ldquoEvidence for atumoral immune resistance mechanism based on tryptophandegradation by indoleamine 23-dioxygenaserdquoNatureMedicinevol 9 no 10 pp 1269ndash1274 2003

[48] A L Mellor and D H Munn ldquoIDO expression by dendriticcells tolerance and tryptophan catabolismrdquo Nature ReviewsImmunology vol 4 no 10 pp 762ndash774 2004

[49] D H Munn and A L Mellor ldquoIDO and tolerance to tumorsrdquoTrends in Molecular Medicine vol 10 no 1 pp 15ndash18 2004

[50] Y-W He H-S Wang J Zeng et al ldquoSodium butyrate inhibitsinterferon-gamma induced indoleamine 23-dioxygenaseexpression via STAT1 in nasopharyngeal carcinoma cellsrdquo LifeSciences vol 93 no 15 pp 509ndash515 2013

[51] Y Iwamoto I S M Lee R Kido and M Tsubaki ldquoTryptophan23-dioxygenase in Saccharomyces cerevisiaerdquoCanadian Journalof Microbiology vol 41 no 1 pp 19ndash26 1995

[52] N D Papadopoulou M Mewies K J McLean et al ldquoRedoxand spectroscopic properties of human indoleamine 2 3-dioxygenase and aHis303Ala variant implications for catalysisrdquoBiochemistry vol 44 no 43 pp 14318ndash14328 2005

[53] T Taniguchi M Sono F Hirata et al ldquoIndoleamine 23-dioxygenase Kinetic studies on the binding of superoxide anionand molecular oxygen to enzymerdquo The Journal of BiologicalChemistry vol 254 no 9 pp 3288ndash3294 1979

[54] G-M Jiang Y-W He R Fang et al ldquoSodium butyrate down-regulation of indoleamine 2 3-dioxygenase at the transcrip-tional and post-transcriptional levelsrdquoThe International Journalof Biochemistry amp Cell Biology vol 42 no 11 pp 1840ndash18462010

[55] L Lu C A Bonham F G Chambers et al ldquoInduction of nitricoxide synthase in mouse dendritic cells by IFN-120574 endotoxinand interaction with allogeneic T cells nitric oxide productionis associated with dendritic cell apoptosisrdquo The Journal ofImmunology vol 157 no 8 pp 3577ndash3586 1996

[56] S R Thomas A C Terentis H Cai et al ldquoPost-translationalregulation of human indoleamine 2 3-dioxygenase activity bynitric oxiderdquo Journal of Biological Chemistry vol 282 no 33 pp23778ndash23787 2007

[57] COrabonaM T Pallotta C Volpi et al ldquoSOCS3 drives protea-somal degradation of indoleamine 23-dioxygenase (IDO) andantagonizes IDO-dependent tolerogenesisrdquo Proceedings of theNational Academy of Sciences of the United States of Americavol 105 no 52 pp 20828ndash20833 2008

[58] M T Pallotta C Orabona C Volpi U Grohmann P Puccettiand F Fallarino ldquoProteasomal degradation of indoleamine 2 3-dioxygenase in CD8+ dendritic cells is mediated by suppressorof cytokine signaling 3 (SOCS3)rdquo International Journal ofTryptophan Research vol 3 pp 91ndash97 2010

[59] J G Wagner and R A Roth ldquoNeutrophil migration duringendotoxemiardquo Journal of Leukocyte Biology vol 66 no 1 pp10ndash24 1999

[60] R C Bone R A Balk F B Cerra et al ldquoDefinitions forsepsis and organ failure and guidelines for the use of innovativetherapies in sepsis The ACCPSCCM Consensus ConferenceCommittee American College of Chest PhysiciansSociety ofCritical Care Medicinerdquo Chest Journal vol 101 no 6 pp 1644ndash1655 1992

[61] E Wirthgen M Tuchscherer W Otten et al ldquoActivation ofindoleamine 2 3-dioxygenase by LPS in a porcine modelrdquoInnate Immunity vol 20 no 1 pp 30ndash39 2014

[62] C Galanos andM A Freudenberg ldquoMechanisms of endotoxinshock and endotoxin hypersensitivityrdquo Immunobiology vol 187no 3 pp 346ndash356 1993

[63] K Asadullah W Sterry and H D Volk ldquoInterleukin-10therapymdashreview of a new approachrdquo Pharmacological Reviewsvol 55 no 2 pp 241ndash269 2003

[64] P Hwu M X Du R Lapointe M Do M W Taylor and HA Young ldquoIndoleamine 23-dioxygenase production by humandendritic cells results in the inhibition of T cell proliferationrdquoJournal of Immunology vol 164 no 7 pp 3596ndash3599 2000

[65] P Terness T M Bauer L Rose et al ldquoInhibition of allo-geneic T cell proliferation by indoleamine 23-dioxygenasendashexpressing dendritic cells mediation of suppression by trypto-phan metabolitesrdquo The Journal of Experimental Medicine vol196 no 4 pp 447ndash457 2002

[66] D Changsirivathanathamrong Y Wang D Rajbhandari et alldquoTryptophan metabolism to kynurenine is a potential novelcontributor to hypotension in human sepsisrdquo Critical CareMedicine vol 39 no 12 pp 2678ndash2683 2011

[67] J-S Padberg M VanMeurs J T Kielstein et al ldquoIndoleamine-23-dioxygenase activity in experimental human endotoxemiardquoExperimental amp Translational Stroke Medicine vol 4 no 1article 24 2012

[68] I D Jung M-G Lee J H Chang et al ldquoBlockade of indol-eamine 2 3-dioxygenase protects mice against lipopolysac-charide-induced endotoxin shockrdquoThe Journal of Immunologyvol 182 no 5 pp 3146ndash3154 2009

[69] W Dabrowski T Kocki J Pilat J Parada-Turska and M L NG Malbrain ldquoChanges in plasma kynurenic acid concentrationin septic shock patients undergoing continuous veno-venoushaemofiltrationrdquo Inflammation vol 37 no 1 pp 223ndash234 2014

[70] J Wang N Simonavicius X Wu et al ldquoKynurenic acid as aligand for orphan G protein-coupled receptor GPR35rdquo Journalof Biological Chemistry vol 281 no 31 pp 22021ndash22028 2006

[71] C Kiank J-P Zeden S Drude et al ldquoPsychological stress-induced IDO1-dependent tryptophan catabolism implications


on immunosuppression in mice and humansrdquo PLoS ONE vol5 no 7 Article ID e11825 2010

[72] M C Barth N Ahluwalia T J T Anderson et al ldquoKynurenicacid triggers firm arrest of leukocytes to vascular endotheliumunder flow conditionsrdquoThe Journal of Biological Chemistry vol284 no 29 pp 19189ndash19195 2009

[73] C M Forrest S R Gould L G Darlington and T W StoneldquoLevels of purine kynurenine and lipid peroxidation productsin patients with inflammatory bowel diseaserdquo Advances inExperimental Medicine and Biology vol 527 pp 395ndash400 2003

[74] M D Lehner and T Hartung ldquoEndotoxin tolerancemdashmechanisms and beneficial effects in bacterial infectionrdquo inReviews of Physiology Biochemistry and Pharmacology vol 144of Reviews of Physiology Biochemistry and Pharmacology pp95ndash141 Springer Berlin Germany 2002

[75] J Li D F Li J J Xing Z B Cheng and C H Lai ldquoEffects of120573-glucan extracted from Saccharomyces cerevisiae on growthperformance and immunological and somatotropic responsesof pigs challenged with Escherichia coli lipopolysacchariderdquoJournal of Animal Science vol 84 no 9 pp 2374ndash2381 2006

[76] M FOsuchowski KWelch J Siddiqui andDG Remick ldquoCir-culating cytokineinhibitor profiles reshape the understandingof the SIRSCARS continuum in sepsis and predict mortalityrdquoJournal of Immunology vol 177 no 3 pp 1967ndash1974 2006

[77] J S Boomer J M Green and R S Hotchkiss ldquoThe chang-ing immune system in sepsis is individualized immuno-modulatory therapy the answerrdquoVirulence vol 5 no 1 pp 45ndash56 2014

[78] A Bessede M Gargaro M T Pallotta et al ldquoAryl hydrocarbonreceptor control of a disease tolerance defence pathwayrdquoNaturevol 511 no 7508 pp 184ndash190 2014

[79] A De Luca C Montagnoli T Zelante et al ldquoFunctional yetbalanced reactivity to Candida albicans requires TRIF MyD88and IDO-dependent inhibition of Rorcrdquo Journal of Immunologyvol 179 no 9 pp 5999ndash6008 2007

[80] B Dong W Cheng W Li et al ldquoFRET analysis of proteintyrosine kinase c-Src activation mediated via aryl hydrocarbonreceptorrdquoBiochimica et BiophysicaActa (BBA) General Subjectsvol 1810 no 4 pp 427ndash431 2011

[81] M Backlund and M Ingelman-Sundberg ldquoRegulation of arylhydrocarbon receptor signal transduction by protein tyrosinekinasesrdquo Cellular Signalling vol 17 no 1 pp 39ndash48 2005

[82] J D Mezrich J H Fechner X Zhang B P Johnson WJ Burlingham and C A Bradfield ldquoAn interaction betweenkynurenine and the aryl hydrocarbon receptor can generateregulatory T cellsrdquo The Journal of Immunology vol 185 no 6pp 3190ndash3198 2010

[83] R Gandhi D Kumar E J Burns et al ldquoActivation of the arylhydrocarbon receptor induces human type 1 regulatory T cell-like and Foxp3+ regulatory T cellsrdquo Nature Immunology vol 11no 9 pp 846ndash853 2010

[84] F Venet C-S Chung H Kherouf et al ldquoIncreased circulatingregulatory T cells (CD4+CD25+CD127minus) contribute to lympho-cyte anergy in septic shock patientsrdquo Intensive Care Medicinevol 35 no 4 pp 678ndash686 2009

[85] L Stejskalova Z Dvorak and P Pavek ldquoEndogenous andexogenous ligands of aryl hydrocarbon receptor current stateof artrdquoCurrent DrugMetabolism vol 12 no 2 pp 198ndash212 2011

[86] R Thomas ldquoRelB and the aryl hydrocarbon receptor dendriticcell tolerance at the epithelial interfacerdquo Immunology and CellBiology vol 91 no 9 pp 543ndash544 2013

[87] F J Quintana ldquoThe aryl hydrocarbon receptor a molecu-lar pathway for the environmental control of the immuneresponserdquo Immunology vol 138 no 3 pp 183ndash189 2013

[88] H Sekine JMimuraMOshima et al ldquoHypersensitivity of arylhydrocarbon receptor-deficient mice to lipopolysaccharide-induced septic shockrdquo Molecular and Cellular Biology vol 29no 24 pp 6391ndash6400 2009

[89] R P Bunaciu and A Yen ldquo6-Formylindolo (32-b)carbazole(FICZ) enhances retinoic acid (RA)-induced differentiation ofHL-60 myeloblastic leukemia cellsrdquo Molecular Cancer vol 12article 39 2013

[90] B C DiNatale I A Murray J C Schroeder et al ldquoKynurenicacid is a potent endogenous aryl hydrocarbon receptor ligandthat synergistically induces interleukin-6 in the presence ofinflammatory signalingrdquo Toxicological Sciences vol 115 no 1pp 89ndash97 2010

[91] M Ploder A Spittler K Kurz et al ldquoAccelerated trypto-phan degradation predicts poor survival in trauma and sepsispatientsrdquo International Journal of Tryptophan Research vol 3pp 61ndash67 2010

[92] M D Lehner J Ittner D S Bundschuh N van Rooijen AWendel and T Hartung ldquoImproved innate immunity of endo-toxin-tolerant mice increases resistance to Salmonella entericaserovar typhimurium infection despite attenuated cytokineresponserdquo Infection and Immunity vol 69 no 1 pp 463ndash4712001

[93] A P Bautista and J J Spitzer ldquoAcute endotoxin tolerancedownregulates superoxide anion release by the perfused liverand isolated hepatic nonparenchymal cellsrdquoHepatology vol 21no 3 pp 855ndash862 1995

[94] C Yamamoto S-I Yoshida H Taniguchi M H Qin HMiyamoto and Y Mizuguchi ldquoLipopolysaccharide and granu-locyte colony-stimulating factor delay neutrophil apoptosis andingestion by guinea pig macrophagesrdquo Infection amp Immunityvol 61 no 5 pp 1972ndash1979 1993

[95] K Dudek and D Bednarek ldquoCellular immune response ofpigeons in the conditions of endotoxin fever and pyrogenictolerancerdquo Polish Journal of Veterinary Sciences vol 14 no 1 pp127ndash133 2011

[96] W J Frazier and M W Hall ldquoImmunoparalysis and adverseoutcomes from critical illnessrdquo Pediatric Clinics of North Amer-ica vol 55 no 3 pp 647ndash668 2008

[97] O M Pena D G Hancock N H Lyle et al ldquoAn endotoxintolerance signature predicts sepsis and organ dysfunction atinitial clinical presentationrdquo EBioMedicine vol 1 no 1 pp 64ndash71 2014

[98] R S Hotchkiss G Monneret and D Payen ldquoSepsis-in-duced immunosuppression from cellular dysfunctions toimmunotherapyrdquo Nature Reviews Immunology vol 13 no 12pp 862ndash874 2013

[99] J-P Zeden G Fusch B Holtfreter et al ldquoExcessive tryptophancatabolism along the kynurenine pathway precedes ongoingsepsis in critically ill patientsrdquoAnaesthesia amp Intensive Care vol38 no 2 pp 307ndash316 2010

[100] H-P Wu C-C Shih C-Y Lin C-C Hua and D-Y ChuangldquoSerial increase of IL-12 response and human leukocyte antigen-DR expression in severe sepsis survivorsrdquo Critical Care vol 15no 5 article R224 2011

[101] J C Schefold J-P Zeden R Pschowski et al ldquoTreatment withgranulocyte-macrophage colony-stimulating factor is associ-ated with reduced indoleamine 23-dioxygenase activity and


kynurenine pathway catabolites in patients with severe sepsisand septic shockrdquo Scandinavian Journal of Infectious Diseasesvol 42 no 3 pp 164ndash171 2010

[102] C Meisel J C Schefold R Pschowski et al ldquoGranulocyte-macrophage colony-stimulating factor to reverse sepsis-associated immunosuppression a double-blind randomizedplacebo-controlled multicenter trialrdquo American Journal ofRespiratory and Critical Care Medicine vol 180 no 7 pp640ndash648 2009

[103] D-B E Wirthgen LPS-induzierte aktivierung des enzymsindolamin 2 3-dioxygenase beim schwein physiologische kon-sequenzen und ihre blockade durch 1-methyltryptophan [PhDthesis] Universitat Rostock Rostock Germany 2012

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Microbiology


by its mother Even in malignancies it has been shownthat an increased IDO1 activity promotes the developmentof an immune tolerance protecting the tumor against theimmune response [4] In patients with sepsis an increasedIDO1 activity provokes systemic immunosuppression and isassociatedwith an increased risk formortality when it is usedas a prognostic indicator for probability of survival [5 6]In addition to the immunological consequences the mod-ulation of TRP metabolism influences the nervous systemand behavior The activation of the kynurenine pathway inthe brain depletion of TRP and generation of neurologicallyactive metabolites are associated with diseases includingschizophrenia [7] Alzheimerrsquos disease [8] and depression[9 10]

2 Tryptophan Metabolism as a Link betweenNutrition and Immune and Nervous System

TRP as an essential amino acid is not synthesized by theanimal organismTherefore dietary TRP is transported fromthe digestive tract through the portal vein to the liver whereit is used for the synthesis of proteins [11] In numerous celltypes approximately 95 of the dietary TRP is metabolizedvia the kynurenine pathway [2 12] For a long time it wasthought that TRP is only completely oxidized to NAD+(coenzyme) or to ATP (energy source) in liver cells but ithas been shown that this occurs also in extrahepatic cells(eg in astrocytes) [13] In many cell types the metabolites ofthe kynurenine pathway mediate mainly anti-inflammatoryeffects [14] whereby they act regulatory in response toproinflammatory stimuli In cells of the nervous systemKYNA anthranilic acid (AA) 3-hydroxyanthranilic acid (3-HA) or QUIN modulate neurological functions KYNA actsas an antagonist of the glutamate receptor and is thereforedescribed as a neuroprotective metabolite whereas QUINan agonist of the glutamate receptor mediates neurotoxiceffects [15] In addition to its importance in the kynureninepathway the degradation of TRP to serotonin (5-HT) andto melatonin in cells of the pineal gland has importantphysiological functions 5-HT acts both as a hormone andas a neurotransmitter Melatonin acts as a neurohormoneand is associated with the development of circadian rhythmand the sleep-wake cycle [16] Because the expression ofenzymes along the kynurenine pathway may differ substan-tially between specific cell types a crude classification of fourcompartments wasmade [11]The first compartment includescells of somatic tissues such as the lungs and intestineswhere TRP is mainly degraded to KYN In addition to theimmunosuppressive effects of KYN it has been shown thatKYN acts as an endothelium-derived relaxing factor that isable to decrease blood pressure in hypertensive rats [17] Thesecond compartment contains cells of the immune systemsuch as dendritic cells or macrophages which express manyenzymes of the kynurenine pathway exclusive of enzymesfor the synthesis of the coenzyme nicotinamide adeninedinucleotide (NAD+) which is required for redox reactionsin cells Hepatic cells represent the third compartment andexpress all enzymes required for the total oxidation of TRP

In the fourth compartment including cells of the centralnervous system TRP is specifically degraded to KYNA butother neurologically active metabolites such as quinolinicacid are also generated Additionally KYNA has been foundto be generated by macrophages [18] Increased concentra-tions of KYNA and xanthurenic acid (3-Hydroxy KYNAXA) were detected in the plasma of patients with type 2diabetes presumably due to chronic stress or the low-gradeinflammation that are prominent risk factors for diabetes [19]Thermochemical and kinetic data show that KYNA and XAare the best free-radical scavengers from the eight tested TRPmetabolites [20] suggesting that the production is a regula-tory mechanism to attenuate damage by the inflammation-induced production of reactive oxygen species AA and 3-HA can act as antioxidants in certain chemical environmentsbut in the presence of iron(II) 3-HA exhibits prooxidantactivity [21] Patients suffering from neurological disorders asHuntingtonrsquos disease or brain injury often showed decreasedlevels of 3-HA combined with increased levels of AA [22]However the biological importance of the 3-HA to AA ratioas either neurotoxic or neuroprotective mechanism is stilldiscussed [22 23]

One characteristic of TRP metabolism is that the rate-limiting step of the catalysis fromTRP toKYN is generated byboth the hepatic enzyme tryptophan 23-dioxygenase (TDO)and the ubiquitous expressed enzyme IDO1 [24 25] TDO isessential for homeostasis of TRP concentrations in organismsand has a lower affinity to TRP than IDO1 Its expression isactivated mainly by increased plasma TRP concentrationsbut can also be activated by glucocorticoids and glucagon[26] In contrast to TDO IDO1 expression is specificallyinduced by inflammatory stimuli such as the cytokinesTNF-120572 or IFN-120574 IDO has a high affinity to L-tryptophanbut will also catalyze the oxidation of other substrates suchas serotonin (5-HT) or melatonin [27] It has been assumedthat TDO is only expressed in the liver however studiesin mice indicate that TDO is also expressed in the brain[28 29] In addition to IDO1 and TDO IDO2 is describedas the third rate-limiting enzyme of the kynurenine pathway[30] IDO1 and IDO2 share a significant identity at the aminoacid level but are not structurally related to TDO IDO2 ispredominantly expressed in the kidneys followed by theepididymis testis and liver [31] The expression in distinctcell types as well as the response to stimuli differs suggestingthat IDO1 and IDO2 are not functionally redundant [31]IDO2 has even been found to be expressed in human tumorcells but the physiologically significant degradation of TRPto KYN is catalyzed by IDO1 [32] At present the biologicalfunction of IDO2 is not well established Recent studiesindicate that IDO2 is a critical mediator in the production ofautoantibodies [33] and in IDO1-mediatedT cell regulation inthe context of inflammation [34] The existence of three rate-limiting enzymes differing in structure substrate specificityand activation supports the significance of the kynureninepathway for both nutritional and immunoregulatory func-tions An overview of the TRP degradation pathways andtheir biological significance is presented in Figure 1


Tryptophan

N-formyl kynurenine


Picolinic acid

NAD+

Glutaryl-CoA

5-Hydroxytryptophan

N-acetyl-5-methoxy-

IDO

IDO

IDO

FOR

KAT

KMO

KYNU

HADO

HAAO

TPH

AANAT

HIOMT

AADC

Kynurenine pathway


IFN-120574

TNF-120572IL-1

Cortisol

TRP

IL-2

KYNU

AMO

Anthranilic acid 5

TDO IDO

Serotonin67

Melatonin56

kynuramine67

Kynurenine14

Kynurenic acid13

IFN-120574

IFN-120574

IFN-120574



Quinolinic acid 24

CO2 + ATP











R

TLR-4

TNFR

STAT1

NF-120581B

IDO

LPS


2

Immune cellLPS

TLR-4

TNF-120572

TNF-120572IFN-120574

IFN-120574

IFN-120574

mRNA

NaB

IDOTranslation

mRNA

IDO

Nucleus

Cytoplasm

SOCS3

Ubiquitination

Bin1

Inactive Active

TRP TRPTRP

TRP

Blood plasma

TRP KYN

TRP

TRP

KYN

Cofactor

NO

KYN

LAT

KYN


KYN

Inhibition

KYNKYN

Immunomodulation

IDO expressingcell

KYN


Activation

1A 1BAlternate

407 aa

316 aa

IDO

NaB

eminus

5998400 exons











TGF-120573

Cytoplasm

IDO

IDO

p

IDOTDOSTAT1

TDO

Inflammation

IDO

TRP

SrcAhR

IL-6

IL-6

STAT3

KYN

SrcAhR

KYN

KYN

AhR

KYN

KYN

AhR

KYN

ARN

T

IL-6

JAK

IDO

Nucleus

pIDO

Src

IL-6

IL-6

KYN

Inactive kinase

Active kinase

NF-120581B








Poor prognosis

Good prognosis


IDOactivation

LPS

Immunoparalysis

Homeostasis







7 Conclusion




Abbreviations










Acknowledgments


References






















































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology


Tryptophan

N-formyl kynurenine


Picolinic acid

NAD+

Glutaryl-CoA

5-Hydroxytryptophan

N-acetyl-5-methoxy-

IDO

IDO

IDO

FOR

KAT

KMO

KYNU

HADO

HAAO

TPH

AANAT

HIOMT

AADC

Kynurenine pathway


IFN-120574

TNF-120572IL-1

Cortisol

TRP

IL-2

KYNU

AMO

Anthranilic acid 5

TDO IDO

Serotonin67

Melatonin56

kynuramine67

Kynurenine14

Kynurenic acid13

IFN-120574

IFN-120574

IFN-120574



Quinolinic acid 24

CO2 + ATP











R

TLR-4

TNFR

STAT1

NF-120581B

IDO

LPS


2

Immune cellLPS

TLR-4

TNF-120572

TNF-120572IFN-120574

IFN-120574

IFN-120574

mRNA

NaB

IDOTranslation

mRNA

IDO

Nucleus

Cytoplasm

SOCS3

Ubiquitination

Bin1

Inactive Active

TRP TRPTRP

TRP

Blood plasma

TRP KYN

TRP

TRP

KYN

Cofactor

NO

KYN

LAT

KYN


KYN

Inhibition

KYNKYN

Immunomodulation

IDO expressingcell

KYN


Activation

1A 1BAlternate

407 aa

316 aa

IDO

NaB

eminus

5998400 exons











TGF-120573

Cytoplasm

IDO

IDO

p

IDOTDOSTAT1

TDO

Inflammation

IDO

TRP

SrcAhR

IL-6

IL-6

STAT3

KYN

SrcAhR

KYN

KYN

AhR

KYN

KYN

AhR

KYN

ARN

T

IL-6

JAK

IDO

Nucleus

pIDO

Src

IL-6

IL-6

KYN

Inactive kinase

Active kinase

NF-120581B








Poor prognosis

Good prognosis


IDOactivation

LPS

Immunoparalysis

Homeostasis







7 Conclusion




Abbreviations










Acknowledgments


References






















































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology








R

TLR-4

TNFR

STAT1

NF-120581B

IDO

LPS


2

Immune cellLPS

TLR-4

TNF-120572

TNF-120572IFN-120574

IFN-120574

IFN-120574

mRNA

NaB

IDOTranslation

mRNA

IDO

Nucleus

Cytoplasm

SOCS3

Ubiquitination

Bin1

Inactive Active

TRP TRPTRP

TRP

Blood plasma

TRP KYN

TRP

TRP

KYN

Cofactor

NO

KYN

LAT

KYN


KYN

Inhibition

KYNKYN

Immunomodulation

IDO expressingcell

KYN


Activation

1A 1BAlternate

407 aa

316 aa

IDO

NaB

eminus

5998400 exons











TGF-120573

Cytoplasm

IDO

IDO

p

IDOTDOSTAT1

TDO

Inflammation

IDO

TRP

SrcAhR

IL-6

IL-6

STAT3

KYN

SrcAhR

KYN

KYN

AhR

KYN

KYN

AhR

KYN

ARN

T

IL-6

JAK

IDO

Nucleus

pIDO

Src

IL-6

IL-6

KYN

Inactive kinase

Active kinase

NF-120581B








Poor prognosis

Good prognosis


IDOactivation

LPS

Immunoparalysis

Homeostasis







7 Conclusion




Abbreviations










Acknowledgments


References






















































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology








TGF-120573

Cytoplasm

IDO

IDO

p

IDOTDOSTAT1

TDO

Inflammation

IDO

TRP

SrcAhR

IL-6

IL-6

STAT3

KYN

SrcAhR

KYN

KYN

AhR

KYN

KYN

AhR

KYN

ARN

T

IL-6

JAK

IDO

Nucleus

pIDO

Src

IL-6

IL-6

KYN

Inactive kinase

Active kinase

NF-120581B








Poor prognosis

Good prognosis


IDOactivation

LPS

Immunoparalysis

Homeostasis







7 Conclusion




Abbreviations










Acknowledgments


References






















































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology




Poor prognosis

Good prognosis


IDOactivation

LPS

Immunoparalysis

Homeostasis







7 Conclusion




Abbreviations










Acknowledgments


References






















































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology



Abbreviations










Acknowledgments


References






















































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology















































































































Volume 2014

Zoology






















Advances in

Virolog y



Volume 2014




Enzyme Research



Microbiology

review article endotoxin-induced tryptophan degradation

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