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1.  Local Expression of Indoleamine 2,3 Dioxygenase in Syngeneic Fibroblasts Significantly Prolongs Survival of an Engineered Three-Dimensional Islet Allograft 
Diabetes  2010;59(9):2219-2227.
OBJECTIVE
The requirement of systemic immunosuppression after islet transplantation is of significant concern and a major drawback to clinical islet transplantation. Here, we introduce a novel composite three-dimensional islet graft equipped with a local immunosuppressive system that prevents islet allograft rejection without systemic antirejection agents. In this composite graft, expression of indoleamine 2,3 dioxygenase (IDO), a tryptophan-degrading enzyme, in syngeneic fibroblasts provides a low-tryptophan microenvironment within which T-cells cannot proliferate and infiltrate islets.
RESEARCH DESIGN AND METHODS
Composite three-dimensional islet grafts were engineered by embedding allogeneic mouse islets and adenoviral-transduced IDO–expressing syngeneic fibroblasts within collagen gel matrix. These grafts were then transplanted into renal subcapsular space of streptozotocin diabetic immunocompetent mice. The viability, function, and criteria for graft take were then determined in the graft recipient mice.
RESULTS
IDO-expressing grafts survived significantly longer than controls (41.2 ± 1.64 vs. 12.9 ± 0.73 days; P < 0.001) without administration of systemic immunesuppressive agents. Local expression of IDO suppressed effector T-cells at the graft site, induced a Th2 immune response shift, generated an anti-inflammatory cytokine profile, delayed alloantibody production, and increased number of regulatory T-cells in draining lymph nodes, which resulted in antigen-specific impairment of T-cell priming.
CONCLUSIONS
Local IDO expression prevents cellular and humoral alloimmune responses against islets and significantly prolongs islet allograft survival without systemic antirejection treatments. This promising finding proves the potent local immunosuppressive activity of IDO in islet allografts and sets the stage for development of a long-lasting nonrejectable islet allograft using stable IDO induction in bystander fibroblasts.
doi:10.2337/db09-1560
PMCID: PMC2927944  PMID: 20522587
2.  Genotyping and expression analysis of IDO2 in human pancreatic cancer: a novel, active target 
Background
We recently discovered that the enzyme indoleamine 2,3-dioxygenase (IDO) is overexpressed in primary pancreatic ductal adenocarcinomas (PDA) and in lymph node metastases (J Am Coll Surg. 5:849-54: 2008). IDO2 is a recently discovered relative of IDO that has unique signaling properties (Cancer Res. 67:7082-7087: 2007). Notably, the IDO2 gene has two functional polymorphisms commonly found in human populations that abolish its enzymatic activity (R235W and Y359STOP). Both IDO and IDO2 repress the immune system and we hypothesize that expression of these enzymes in PDA may help cancer cells evade immune detection.
Methods
Based on evidence that the IDO2 may be a preferential target of D-1-methyl-tryptophan (1-MT), a clinical lead inhibitor of IDO currently being evaluated in Phase I trials, we sequenced IDO2 in 36 resected PDAs and evaluated its expression in relation to the two known genetic polymorphisms.
Results
In our patient cohort, we found that 58% (21/36) of the cases were heterozygous for the R235W polymorphism; 28% (10/36) were homozygous wild-type; and only 14% (5/36) were homozygous for the functionally inactive polymorphism. Interestingly, IDO2 had a homozygous wild-type configuration in two pancreatic cancer cell lines whereas one cell line (MiaPaCa2 cells) was homozygous for the R235W polymorphism. As for the Y359STOP polymorphism (seen in the cell line Hs766T), we found that 27% (10/36) of the cases were heterozygous, 62% (22/36) were homozygous wild-type, and only 11% (4/36) were homozygous for this functionally inactive allele. Ruling out the possibility of compound polymorphic variants, we estimated 75% of our resected patient cohort had an active IDO2 enzyme with a conservative estimate that 58% of the patients had at least one functional allele. In immunohistochemical analyses, we found that IDO2 was equally overexpressed in pancreatic cancer tissue from each genetially polymorphic subgroup. We also detected IDO2 protein expression in the genetically distinct pancreatic cancer cell lines after exposure with IFN-γ, establishing that even functionally polymorphic IDO2 sequences can generate IDO2 protein.
Conclusions
These are the first data to report IDO2 expression in PDA and indicate that IDO2 genetic polymorphisms do not negate IFN-γ-inducible protein expression. IDO2 genotyping and expression analysis of our PDA patient tissue bank and cell lines show that IDO2 is active and expressed in a majority of PDA patients. Taken together, these data strongly suggest that the clinical lead compound D-1-MT acting through IDO2 might be useful in treatment of PDA, either alone or in combination with other anti-tumor modalities.
doi:10.1016/j.jamcollsurg.2008.12.018
PMCID: PMC3176891  PMID: 19476837
3.  Aging-associated increase in indoleamine 2,3-dioxygenase (IDO) activity appears to be unrelated to the transcription of the IDO1 or IDO2 genes in peripheral blood mononuclear cells 
Background
Old age is associated with increased levels of circulating pro-inflammatory cytokines, a phenomenon termed inflamm-aging. Elevated levels of pro-inflammatory cytokines have been associated with several age-associated diseases and with a shortened lifespan. Indoleamine 2,3-dioxygenase (IDO) has immunomodulatory properties and its activity is elevated in inflammation, autoimmune disorders and malignancies. We have previously shown that IDO activity is increased in nonagenarians compared to young individuals and that high IDO activity is associated with mortality at old age.
Findings
In this study our aim was to assess whether this difference in IDO activity in the plasma was due to the differential expression of either the IDO1 or IDO2 gene in peripheral blood mononuclear cells. Our results show that IDO1 and IDO2 are not differently expressed in nonagenarians compared to controls and that the expression of IDO genes is not associated with the level of IDO activity in the plasma.
Conclusion
The level of IDO activity in the plasma is not regulated through the expression of IDO1 or IDO2 in the peripheral blood mononuclear cells.
doi:10.1186/1742-4933-8-9
PMCID: PMC3198939  PMID: 21989355
4.  Performance-enhanced mesenchymal stem cells via intracellular delivery of steroids 
Scientific Reports  2014;4:4645.
Inadequate immunomodulatory potency of mesenchymal stem cells (MSC) may limit their therapeutic efficacy. We report glucocorticoid steroids augment MSC expression and activity of indoleamine-2,3-dioxygenase (IDO), a primary mediator of MSC immunomodulatory function. This effect depends on signaling through the glucocorticoid receptor and is mediated through up-regulation of FOXO3. Treatment of MSCs with glucocorticoids, budesonide or dexamethasone, enhanced IDO expression following IFN-γ stimulation in multiple donors and was able to restore IDO expression in over-passaged MSCs. As IDO enhancement was most notable when cells were continuously exposed to budesonide, we engineered MSC with budesonide loaded PLGA microparticles. MSC efficiently internalized budesonide microparticles and exhibited 4-fold enhanced IDO activity compared to budesonide preconditioned and naïve MSC, resulting in a 2-fold improvement in suppression of stimulated peripheral blood mononuclear cells in an IDO-dependent manner. Thus, the augmentation of MSC immune modulation may abrogate challenges associated with inadequate potency and enhance their therapeutic efficacy.
doi:10.1038/srep04645
PMCID: PMC3982175  PMID: 24717973
5.  Uveal Melanoma Expression of Indoleamine 2, 3 Deoxygenase: Establishment of an Immune Privileged Environment by Tryptophan Depletion 
Experimental eye research  2007;85(5):617-625.
The enzyme indoleamine 2, 3-dioxygenase (IDO) catalyzes degradation of tryptophan, an essential amino acid required for lymphocyte activation and proliferation. Many tumors express IDO which implied that it acts as a mechanism to evade T cell-mediated immune attack, and also to establish an immunosuppressive tumor microenvironment. The purpose of this study was to determine whether primary and metastatic uveal melanoma expressed the IDO gene and whether uveal melanoma cells could deplete tryptophan. In situ expression of IDO in primary uveal melanoma from tumor bearing eyes and metastatic uveal melanoma liver tissues was determined by immunohistostaining with IDO-specific antibody. Reverse transcription PCR was used to assess IDO gene transcription by primary and metastatic uveal melanoma cell lines. IDO protein expression was determined by Western blot of uveal melanoma cell protein lysate. IDO catalytic activity was assessed by measuring the presence of kynurenine, a product generated by tryptophan degradation, in uveal melanoma culture supernatants.
Primary uveal melanoma from tumor-bearing eyes and metastatic uveal melanoma from the liver did not express IDO in situ. IDO was not constitutively expressed in either primary or metastatic uveal melanoma cell lines. However, stimulation of primary and metastatic uveal melanoma cell cultures with interferon-gamma (IFN-γ) universally upregulated both IDO gene and protein expression. Culture supernatants from IFN-γ treated primary and metastatic uveal melanoma cell cultures contained elevated levels of kynurenine. Addition of the IDO inhibitor 1-methyl DL-tryptophan significantly diminished kynurenine levels in IFN-γ treated uveal melanoma cell cultures. The results from this study suggest that IFN-γ inducible IDO upregulation by primary and metastatic uveal melanoma may generate a local immune privileged microenvironment to promote escape from T cell-mediated immune surveillance.
doi:10.1016/j.exer.2007.07.014
PMCID: PMC2695208  PMID: 17870068
6.  Indoleamine 2,3-dioxygenase (IDO) induced by Leishmania infection of human dendritic cells 
Parasite immunology  2012;34(10):464-472.
Dendritic cells (DC) play a pivotal role in regulating immunity, establishing immunologically privileged tissue micro-environments and maintaining homeostasis. It is becoming increasingly clear that one key mechanism that mediates many DC functions is production of the immunomodulatory enzyme indoleamine 2,3-dioxygenase (IDO). For pathogens that cause chronic infection, exploitation of host DCs is a solution to establish and persist within a host. Leishmania parasites cause a range of clinical manifestations, all involving chronic infection, and are proficient at avoiding immune responses. We demonstrate here that infection of human myeloid-derived DC with L. major and L. donovani induces IDO expression using a mechanism that involves autocrine or paracrine stimulation with a DC secreted factor. Leishmania-induced IDO suppresses allogeneic and tetanus toxoid-specific lymphocyte proliferation; an inhibition that is reversed with the IDO inhibitor, 1-Methyl Tryptophan (1-MT). Furthermore, IDO expression by human DC does not require live Leishmania infection, as parasite lysates also up-regulate IDO mRNA production. Our data suggest that one mechanism Leishmania parasites utilize to circumvent immune clearance may be to promote the induction of IDO among host DC within the infection micro-environment.
doi:10.1111/j.1365-3024.2012.01380.x
PMCID: PMC3572781  PMID: 22803643
Leishmania; Indoleamine 2; 3-dioxygenase; dendritic cell
7.  The cystine/glutamate antiporter regulates indoleamine 2,3-dioxygenase protein levels and enzymatic activity in human dendritic cells 
Indoleamine 2,3-dioxygenase (IDO) is the rate-limiting enzyme in the tryptophan-catabolizing pathway and a key regulator of peripheral immune tolerance. As the suppressive effects of IDO are predominantly mediated by dendritic cells (DCs) and IDO-competent DCs promote long-term immunologic tolerance, a detailed understanding of how IDO expression and activity is regulated in these cells is central to the rational design of therapies to induce robust immune tolerance. We previously reported that the cystine/glutamate antiporter modulates the functional expression of IDO in human monocyte-derived DCs. Specifically, we showed that blocking antiporter uptake of cystine significantly increased both IDO mRNA and IDO enzymatic activity and that this correlated with impaired DC presentation of exogenous antigen to T cells via MHC class II and the cross-presentation pathway. The antiporter regulates intracellular and extracellular redox by transporting cystine into the cell in exchange for glutamate. Intracellular cystine is reduced to cysteine to support biosynthesis of the major cellular antioxidant glutathione and cysteine is exported from the cell where it functions as an extracellular antioxidant. Here we show that antiporter control of IDO expression in DCs is reversible, independent of interferon-γ, regulated by redox, and requires active protein synthesis. These findings highlight a role for antiporter regulation of cellular redox as a critical control point for modulating IDO expression and activity in DCs. Thus, systemic disease and aging, processes that perturb redox homeostasis, may adversely affect immunity by promoting the generation of IDO-competent DCs.
PMCID: PMC3520491  PMID: 23243629
Indoleamine 2,3-dioxygenase; human monocyte-derived dendritic cells; tryptophan; kynurenine; cystine/glutamate antiporter; glutathione; cystine; cysteine; peripheral immune tolerance; redox
8.  The cystine/glutamate antiporter regulates indoleamine 2,3-dioxygenase protein levels and enzymatic activity in human dendritic cells 
Indoleamine 2,3-dioxygenase (IDO) is the rate-limiting enzyme in the tryptophan-catabolizing pathway and a key regulator of peripheral immune tolerance. As the suppressive effects of IDO are predominantly mediated by dendritic cells (DCs) and IDO-competent DCs promote long-term immunologic tolerance, a detailed understanding of how IDO expression and activity is regulated in these cells is central to the rational design of therapies to induce robust immune tolerance. We previously reported that the cystine/glutamate antiporter modulates the functional expression of IDO in human monocyte-derived DCs. Specifically, we showed that blocking antiporter uptake of cystine significantly increased both IDO mRNA and IDO enzymatic activity and that this correlated with impaired DC presentation of exogenous antigen to T cells via MHC class II and the cross-presentation pathway. The antiporter regulates intracellular and extracellular redox by transporting cystine into the cell in exchange for glutamate. Intracellular cystine is reduced to cysteine to support biosynthesis of the major cellular antioxidant glutathione and cysteine is exported from the cell where it functions as an extracellular antioxidant. Here we show that antiporter control of IDO expression in DCs is reversible, independent of interferon-γ, regulated by redox, and requires active protein synthesis. These findings highlight a role for antiporter regulation of cellular redox as a critical control point for modulating IDO expression and activity in DCs. Thus, systemic disease and aging, processes that perturb redox homeostasis, may adversely affect immunity by promoting the generation of IDO-competent DCs.
PMCID: PMC3520491  PMID: 23243629
Indoleamine 2; 3-dioxygenase; human monocyte-derived dendritic cells; tryptophan; kynurenine; cystine/glutamate antiporter; glutathione; cystine; cysteine; peripheral immune tolerance; redox
9.  Inhibition of Allogeneic T Cell Proliferation by Indoleamine 2,3-Dioxygenase–expressing Dendritic Cells 
Indoleamine 2,3-dioxygenase (IDO), an enzyme involved in the catabolism of tryptophan, is expressed in certain cells and tissues, particularly in antigen-presenting cells of lymphoid organs and in the placenta. It was shown that IDO prevents rejection of the fetus during pregnancy, probably by inhibiting alloreactive T cells, and it was suggested that IDO-expression in antigen-presenting cells may control autoreactive immune responses. Degradation of tryptophan, an essential amino acid required for cell proliferation, was reported to be the mechanism of IDO-induced T cell suppression. Because we wanted to study the action of IDO-expressing dendritic cells (DCs) on allogeneic T cells, the human IDO gene was inserted into an adenoviral vector and expressed in DCs. Transgenic DCs decreased the concentration of tryptophan, increased the concentration of kynurenine, the main tryptophan metabolite, and suppressed allogeneic T cell proliferation in vitro. Kynurenine, 3-hydroxykynurenine, and 3-hydroxyanthranilic acid, but no other IDO-induced tryptophan metabolites, suppressed the T cell response, the suppressive effects being additive. T cells, once stopped in their proliferation, could not be restimulated. Inhibition of proliferation was likely due to T cell death because suppressive tryptophan catabolites exerted a cytotoxic action on CD3+ cells. This action preferentially affected activated T cells and increased gradually with exposure time. In addition to T cells, B and natural killer (NK) cells were also killed, whereas DCs were not affected. Our findings shed light on suppressive mechanisms mediated by DCs and provide an explanation for important biological processes in which IDO activity apparently is increased, such as protection of the fetus from rejection during pregnancy and possibly T cell death in HIV-infected patients.
doi:10.1084/jem.20020052
PMCID: PMC2196057  PMID: 12186837
immunosuppression; immunoregulation; immunological tolerance; tryptophan; kynurenine
10.  Identification of a Variable Number of Tandem Repeats Polymorphism and Characterization of LEF-1 Response Elements in the Promoter of the IDO1 Gene 
PLoS ONE  2011;6(9):e25470.
Background
Indoleamine 2,3-dioxygenase (IDO) catalyzes the first and rate-limiting step of the kynurenine pathway that is an important component of immunomodulatory and neuromodulatory processes. The IDO1 gene is highly inducible by IFN-γ and TNF-α through interaction with cis-acting regulatory elements of the promoter region. Accordingly, functional polymorphisms in the IDO1 promoter could partly explain the interindividual variability in IDO expression that has been previously documented.
Methodology/Principal Findings
A PCR-sequencing strategy, applied to DNA samples from healthy Caucasians, allowed us to identify a VNTR polymorphism in the IDO1 promoter, which correlates significantly with serum tryptophan concentration, controlled partially by IDO activity, in female subjects, but not in males. Although this VNTR does not appear to affect basal or cytokine-induced promoter activity in gene reporter assays, it contains novel cis-acting elements. Three putative LEF-1 binding sites, one being located within the VNTR repeat motif, were predicted in silico and confirmed by chromatin immunoprecipitation. Overexpression of LEF-1 in luciferase assays confirmed an interaction between LEF-1 and the predicted transcription factor binding sites, and modification of the LEF-1 core sequence within the VNTR repeat motif, by site-directed mutagenesis, resulted in an increase in promoter activity.
Conclusions/Significance
The identification of a VNTR in the IDO1 promoter revealed a cis-acting element interacting with the most downstream factor of the Wnt signaling pathway, suggesting novel mechanisms of regulation of IDO1 expression. These data offer new insights, and suggest further studies, into the role of IDO in various pathological conditions, particularly in cancer where IDO and the Wnt pathway are strongly dysregulated.
doi:10.1371/journal.pone.0025470
PMCID: PMC3181322  PMID: 21980470
11.  Exosomes from IDO+ DC are therapeutic in CIA and DTH disease models 
Arthritis and rheumatism  2009;60(2):380-389.
Objective
We have demonstrated previously that dendritic cells (DC), modified with immunosuppressive cytokines, and exosomes derived from the DC can suppress the onset of murine CIA and reduce the severity of established arthritis. Indoleamine 2,3-dioxygenase (IDO) is a tryptophan degrading enzyme important for immune regulation and tolerance maintenance. DC expressing functional IDO can inhibit T cells by either depleting them of essential tryptophan and/or by producing toxic metabolites, as well as by generating regulatory T cells. In this study, we examined the immunosuppressive effects of bone marrow derived DC, genetically modified to express IDO, and IDO+-DC-derived exosomes.
Methods
Bone marrow derived DC were adenovirally transduced with IDO or CTLA4-Ig (an inducer of IDO), and the resulting DC and exosomes were tested for their immunosuppressive ability in the collagen-induced arthritis and delayed type hypersensitivity murine models.
Results
We demonstrate that both DC and exosomes derived from DC overexpressing IDO are anti-inflammatory in collagen-induced arthritis and delayed type hypersensitivity murine models. The suppressive effects were partially dependent on B7 costimulatory molecules. In addition, gene transfer of CTLA4-Ig to DC resulted in induction of IDO in the DC and exosomes able to reduce inflammation in an IDO-dependent manner.
Conclusion
These results demonstrate that both IDO expressing DC and DC-derived exosomes are immunosuppressive and anti-inflammatory, and are able to reverse established arthritis. Therefore, exosomes from IDO+ DC may represent a novel therapy for rheumatoid arthritis.
doi:10.1002/art.24229
PMCID: PMC3491653  PMID: 19180475
Dendritic cells; Exosomes; IDO; Arthritis; Inflammatory disease
12.  Indoleamine 2,3-dioxygenase is a novel prognostic indicator for endometrial cancer 
British Journal of Cancer  2006;95(11):1555-1561.
Indoleamine 2,3-dioxygenase (IDO) is a tryptophan-catabolising enzyme inducing immune tolerance. The present study aimed to investigate IDO expression and its prognostic significance in endometrial cancer. Indoleamine 2,3-dioxygenase expression in endometrial cancer tissues (n=80) was immunohistochemically scored as four groups (IDO−, 1+, 2+, and 3+). The high IDO expression (IDO2+ or 3+) in tumour cells was found in 37 (46.3%) of the 80 cases, and was positively correlated with surgical stage, myometrial invasion, lymph-vascular space involvement, and lymph node metastasis, but not with the histological grade. Patients with high IDO expression had significantly impaired overall survival and progression-free survival (PFS) (P=0.002 and P=0.001, respectively) compared to patients with no or weak expression of IDO (IDO− or 1+). The 5-year PFS for IDO−/1+, 2+, and 3+ were 97.7, 72.9, and 36.4%, respectively. Even in patients with early-stage disease (International Federation of Gynecology and Obstetrics I/II, n=64), the PFS for IDO2+/3+ was significantly poor (P=0.001) compared to that for IDO−/1+. On multivariate analysis, IDO expression was an independent prognostic factor for PFS (P=0.020). These results indicated that the high IDO expression was involved in the progression of endometrial cancer and correlated with the impaired clinical outcome, suggesting that IDO is a novel and reliable prognostic indicator for endometrial cancer.
doi:10.1038/sj.bjc.6603477
PMCID: PMC2360726  PMID: 17117179
indoleamine 2,3-dioxygenase (IDO); endometrial cancer; prognostic factor; progression-free survival (PFS)
13.  Inhibition of  T Cell Proliferation by Macrophage Tryptophan Catabolism  
The Journal of Experimental Medicine  1999;189(9):1363-1372.
We have recently shown that expression of the enzyme indoleamine 2,3-dioxygenase (IDO) during murine pregnancy is required to prevent rejection of the allogeneic fetus by maternal T cells. In addition to their role in pregnancy, IDO-expressing cells are widely distributed in primary and secondary lymphoid organs. Here we show that monocytes that have differentiated under the influence of macrophage colony-stimulating factor acquire the ability to suppress T cell proliferation in vitro via rapid and selective degradation of tryptophan by IDO. IDO was induced in macrophages by a synergistic combination of the T cell–derived signals IFN-γ and CD40-ligand. Inhibition of IDO with the 1-methyl analogue of tryptophan prevented macrophage-mediated suppression. Purified T cells activated under tryptophan-deficient conditions were able to synthesize protein, enter the cell cycle, and progress normally through the initial stages of G1, including upregulation of IL-2 receptor and synthesis of IL-2. However, in the absence of tryptophan, cell cycle progression halted at a mid-G1 arrest point. Restoration of tryptophan to arrested cells was not sufficient to allow further cell cycle progression nor was costimulation via CD28. T cells could exit the arrested state only if a second round of T cell receptor signaling was provided in the presence of tryptophan. These data reveal a novel mechanism by which antigen-presenting cells can regulate T cell activation via tryptophan catabolism. We speculate that expression of IDO by certain antigen presenting cells in vivo allows them to suppress unwanted T cell responses.
PMCID: PMC2193062  PMID: 10224276
macrophage; indoleamine 2,3-dioxygenase; T cells; tryptophan; macrophage colony-stimulating factor
14.  Immuno-Regulatory Function of Indoleamine 2,3 Dioxygenase through Modulation of Innate Immune Responses 
PLoS ONE  2013;8(8):e71044.
Successful long-term treatment of type-1 diabetes mainly relies on replacement of β-cells via islet transplantation. Donor shortage is one of the main obstacles preventing transplantation from becoming the treatment of choice. Although animal organs could be an alternative source for transplantation, common immunosuppressive treatments demonstrate low efficacy in preventing xenorejection. Immunoprotective effects of indoleamine 2,3-dioxygenase (IDO) on T-cell mediated allorejection has been extensively studied. Our studies revealed that IDO expression by fibroblasts, induced apoptosis in T-cells while not affecting non-immune cell survival/function. Since macrophages play a pivotal role in xenograft rejection, herein we investigated the effect of IDO-induced tryptophan deficiency/kynurenine accumulation on macrophage function/survival. Moreover, we evaluated the local immunosuppressive effect of IDO on islet-xenograft protection. Our results indicated that IDO expression by bystander fibroblasts significantly reduced the viability of primary macrophages via apoptosis induction. Treatment of peritoneal macrophages by IDO-expressing fibroblast conditioned medium significantly reduced their proinflammatory activity through inhibition of iNOS expression. To determine whether IDO-induced tryptophan starvation or kynurenine accumulation is responsible for macrophage apoptosis and inhibition of their proinflammatory activity, Raw264.7 cell viability and proinflammatory responses were evaluated in tryptophan deficient medium or in the presence of kynurenine. Tryptophan deficiency, but not kynurenine accumulation, reduced Raw264.7 cell viability and suppressed their proinflammatory activity. Next a three-dimensional islet-xenograft was engineered by embedding rat islets within either control or IDO–expressing fibroblast-populated collagen matrix. Islets morphology and immune cell infiltration were then studied in the xenografts transplanted into the C57BL/6 mouse renal sub-capsular space. Local IDO significantly decreased the number of infiltrating macrophages (11±1.47 vs. 70.5±7.57 cells/HPF), T-cells (8.75±1.03 vs. 75.75±5.72 cells/HPF) and iNOS expression in IDO-expressing xenografts versus controls. Islet morphology remained intact in IDO-expressing grafts and islets were strongly stained for insulin/glucagon compared to control. These findings support the immunosuppressive role of IDO on macrophage-mediated xeno-rejection.
doi:10.1371/journal.pone.0071044
PMCID: PMC3733714  PMID: 23940687
15.  IDO in human gut Graft-versus-Host Disease 
While rodent graft-versus-host disease (GVHD) models have suggested that indoleamine 2, 3-dioxygenase (IDO) is a critical regulator of gastrointestinal GVHD parallel human studies on IDO expression have not been reported. IDO expression was assessed in 20 patients who underwent duodenal biopsy. IDO was upregulated in epithelial cells. In situ analyses reveal that macrophages and dendritic cells stain positive for IDO, but that most of the IDO+ cells were a novel population of CD3+CD4+IDO+ cells. The proportion of CD4+IDO+ T-cells was significantly higher in patients with moderate GVHD. In situ regulatory T-cell and Th17 numbers correlated with overall severity. Although needing confirmatory results from larger sample sets, these data are consistent with the hypothesis that IDO is involved in regulating gastrointestinal GVHD.
doi:10.1016/j.bbmt.2011.08.002
PMCID: PMC3565565  PMID: 21835147
Graft-versus-host disease; regulatory T-cells; Th17; IDO
16.  Astrocyte Indoleamine 2,3-Dioxygenase Is Induced by the TLR3 Ligand Poly(I:C): Mechanism of Induction and Role in Antiviral Response▿  
Journal of Virology  2007;81(18):9838-9850.
Indoleamine 2,3-dioxygenase (IDO) is the first and rate-limiting enzyme in the kynurenine pathway of tryptophan catabolism and has been implicated in neurotoxicity and suppression of the antiviral T-cell response in HIV encephalitis (HIVE). Here we show that the Toll-like receptor 3 (TLR3) ligand poly(I:C) (PIC) induces the expression of IDO in human astrocytes. PIC was less potent than gamma interferon (IFN-γ) but more potent than IFN-β in inducing IDO. PIC induction of IDO was mediated in part by IFN-β but not IFN-γ, and both NF-κB and interferon regulatory factor 3 (IRF3) were required. PIC also upregulated TLR3, thereby augmenting the primary (IFN-β) and secondary (IDO and viperin) response genes upon subsequent stimulation with PIC. In HIVE, the transcripts for TLR3, IFN-β, IDO, and viperin were increased and IDO immunoreactivity was detected in reactive astrocytes as well as macrophages and microglia. PIC caused suppression of intracellular replication of human immunodeficiency virus pseudotyped with vesicular stomatitis virus G protein and human cytomegalovirus in a manner dependent on IRF3 and IDO. The involvement of IDO was demonstrated by partial but significant reversal of the PIC-mediated antiviral effect by IDO RNA interference and/or tryptophan supplementation. Importantly, the cytokine interleukin-1 abolished IFN-γ-induced IDO enzyme activity in a nitric oxide-dependent manner without suppressing protein expression. Our results demonstrate that IDO is an innate antiviral protein induced by double-stranded RNA and suggest a therapeutic utility for PIC in human viral infections. They also show that IDO activity can be dissociated from protein expression, indicating that the local central nervous system cytokine and nitric oxide environment determines IDO function.
doi:10.1128/JVI.00792-07
PMCID: PMC2045380  PMID: 17626075
17.  Regulation of colonic epithelial cell turnover by IDO contributes to the innate susceptibility of SCID mice to Trichuris muris infection 
Parasite Immunology  2011;33(4):244-249.
Tryptophan catabolism via the kynurenine pathway is dependent on the enzyme Indoleamine 2,3-dioxygenase (IDO). Expression of IDO is upregulated in a number of inflammatory settings such as wounding and infection, and the resulting local tryptophan depletion may inhibit the replication of intracellular pathogens. Indo gene expression is upregulated in the gut during chronic infection with the mouse whipworm Trichuris muris. We demonstrate an increase in the rate of colonic epithelial cell turnover after inhibition of IDO in T.muris-infected SCID mice, leading to a significant expulsion of parasite burden. We identify the goblet cell as a novel source of IDO and present data revealing a new role for IDO in the regulation of epithelial cell turnover post-infectious challenge.
doi:10.1111/j.1365-3024.2010.01272.x
PMCID: PMC3084993  PMID: 21392042
indoleamine 2,3-dioxygenase (IDO); inflammation; large intestine; Trichuris muris
18.  Leishmania major Attenuates Host Immunity by Stimulating Local Indoleamine 2,3-Dioxygenase Expression 
The Journal of Infectious Diseases  2011;203(5):715-725.
Inflammation stimulates immunity but can create immune privilege in some settings. Here, we show that cutaneous Leishmania major infection stimulated expression of the immune regulatory enzyme indoleamine 2,3 dioxygenase (IDO) in local lymph nodes. Induced IDO attenuated the T cell stimulatory functions of dendritic cells and suppressed local T cell responses to exogenous and nominal parasite antigens. IDO ablation reduced local inflammation and parasite burdens, as did pharmacologic inhibition of IDO in mice with established infections. IDO ablation also enhanced local expression of proinflammatory cytokines and induced some CD4+ T cells to express interleukin (IL) 17. These findings showed that IDO induced by L. major infection attenuated innate and adaptive immune responses. Thus, IDO acts as a molecular switch regulating host responses, and IDO inhibitor drugs are a potential new approach to enhance host immunity to established leishmania infections.
doi:10.1093/infdis/jiq095
PMCID: PMC3072725  PMID: 21282196
19.  Myeloid differentiation primary response protein 88 blockade upregulates indoleamine 2,3-dioxygenase expression in rheumatoid synovial fibroblasts 
Experimental & Molecular Medicine  2011;43(8):446-454.
Indoleamine 2,3-dioxygenase (IDO) is a key negative regulator of immune responses and has been implicated in tumor tolerance, autoimmune disease and asthma. IDO was detected in the joint synovial tissue in the inflammatory microenvironment of rheumatoid arthritis (RA), but IDO expression in joint synovial tissue is not sufficient to overcome the inflamed synovial environment. This study aimed to unravel the mechanisms involving the failure to activate tolerogenic IDO in the inflamed joint. We demonstrate that both poly (I:C) and lipopolysaccharide (LPS) induce expression of IDO in synovial fibroblasts. However, inflammatory cytokines such as IL-17, TNF-α, IL-12, IL-23 and IL-16 did not induce IDO expression. Poly (I:C) appeared to induce higher IDO expression than did LPS. Surprisingly, toll-like receptor (TLR)4-mediated IDO expression was upregulated after depletion of myeloid differentiation primary response protein 88 (MyD88) in synovial fibroblasts using small interfering RNA (siRNA). IDO, TLR3 and TLR4 were highly expressed in synovial tissue of RA patients compared with that of osteoarthritis patients. In addition, RA patients with severe disease activity had higher levels of expression of IDO, TLR3 and TLR4 in the synovium than patients with mild disease activity. These data suggest that upregulation of IDO expression in synovial fibroblasts involves TLR3 and TLR4 activation by microbial constituents. We showed that the mechanisms responsible for IDO regulation primarily involve MyD88 signaling in synovial fibroblasts, as demonstrated by siRNA-mediated knockdown of MyD88.
doi:10.3858/emm.2011.43.8.050
PMCID: PMC3174378  PMID: 21654189
indoleamine-pyrrole 2,3,-dioxygenase; myeloid differentiation factor 88; rheumatoid arthritis; TICAM1 protein, human; toll-like receptors
20.  Inhibition of indoleamine 2,3-dioxygenase-mediated tryptophan catabolism accelerates collagen-induced arthritis in mice 
Indoleamine 2,3-dioxygenase (IDO) is one of the initial and rate-limiting enzymes involved in the catabolism of the essential amino acid tryptophan. In cultured cells, the induction of IDO leads to depletion of tryptophan and tryptophan starvation. Recent studies suggest that modulation of tryptophan concentration via IDO plays a fundamental role in innate immune responses. Induction of IDO by interferon-γ in macrophages and dendritic cells results in tryptophan depletion and suppresses the immune-mediated activation of fibroblasts and T, B, and natural killer cells. To assess the role of IDO in collagen-induced arthritis (CIA), a model of rheumatoid arthritis characterized by a primarily Th1-like immune response, activity of IDO was inhibited by 1-methyl-tryptophan (1-MT) in vivo. The results showed significantly increased incidence and severity of CIA in mice treated with 1-MT. Activity of IDO, as determined by measuring the levels of kynurenine/tryptophan ratio in the sera, was increased in the acute phase of arthritis and was higher in collagen-immunized mice that did not develop arthritis. Treatment with 1-MT resulted in an enhanced cellular and humoral immune response and a more dominant polarization to Th1 in mice with arthritis compared with vehicle-treated arthritic mice. The results demonstrated that development of CIA was associated with increased IDO activity and enhanced tryptophan catabolism in mice. Blocking IDO with 1-MT aggravated the severity of arthritis and enhanced the immune responses. These findings suggest that IDO may play an important and novel role in the negative feedback of CIA and possibly in the pathogenesis of rheumatoid arthritis.
doi:10.1186/ar2205
PMCID: PMC2206348  PMID: 17511858
21.  Induction of TDO2 and IDO2 in Liver by High-Fat Feeding in Mice: Discrepancies with Human Obesity 
Low-grade and chronic inflammation is elicited in white adipose tissue in human obesity. The presence of inflammatory molecules leads to an increased tryptophan catabolism through the induction of indoleamine-2,3-dioxygenase-1 (IDO1). In order to characterize the mechanisms underlying this dysregulation, we have studied 2 mouse models of obesity. Unexpectedly, we did not detect any IDO1 expression in obese or lean mice adipose tissue. In a previous study, we did not find any significant difference in the liver for IDO2 and tryptophan-2,3-dioxygenase (TDO2) gene expression between normal weight and obese patients. IDO2 and TDO2 expression was increased in the liver of high-fat fed mice, but not in ob/ob mice, and was strongly correlated with hydroxysteroid-(11-beta) dehydrogenase-1 (HSD11B1) expression, an enzyme that generates active cortisol within tissues. In conclusion, despite a dysregulation of tryptophan metabolism, obese mice display discrepancies with human obesity metabolism, rendering them inappropriate for further investigations in this animal model.
doi:10.4137/IJTR.S11717
PMCID: PMC3729279
tryptophan 2; 3-dioxygenase; indoleamine 2; 3-dioxygenase 2; obesity; high fat diet
22.  Exploitation of the IDO Pathway in the Therapy of Rheumatoid Arthritis 
Indoleamine 2,3-dioxygenase (IDO) is the first and rate-limiting step along the kynurenine pathway and is thought to play a key role in immune homeostasis through depletion of tryptophan and accumulation of kynurenines. In this review we summarize recent research into the possibility of harnessing the IDO pathway for the therapy of rheumatoid arthritis. Inhibition of IDO activity, or knockout of the gene encoding IDO, was shown to cause an increase in the severity of collagen-induced arthritis, an animal model of rheumatoid arthritis. The increased severity of disease was associated with elevated numbers of pathogenic Th1 and Th17 cells in the joints and draining lymph nodes. In another study, analysis of the kinetics of expression of downstream kynurenine pathway enzymes during the course of arthritis revealed a potential role for tryptophan metabolites in resolution of arthritis. Furthermore, the therapeutic administration of L-kynurenine or [3,4-dimethoxycinnamonyl]-anthranilic acid (a synthetic derivative of 3-hydroxy-anthranilic acid) significantly reduced both clinical and histological progression of experimental arthritis. These findings raise the possibility of exploiting the IDO pathway for the therapy of autoimmune disease.
doi:10.4137/IJTR.S11737
PMCID: PMC3729277  PMID: 23922504
rheumatoid arthritis; indoleamine 2; 3-dioxygenase; animal models
23.  Mechanism Underlying Defective Interferon Gamma-Induced IDO Expression in Non-obese Diabetic Mouse Fibroblasts 
PLoS ONE  2012;7(5):e37747.
Indoleamine 2,3-dioxygenase (IDO) can locally suppress T cell-mediated immune responses. It has been shown that defective self-tolerance in early prediabetic female non-obese diabetic (NOD) mice can be attributed to the impaired interferon-gamma (IFN-γ)- induced IDO expression in dendritic cells of these animals. As IFN-γ can induce IDO in both dendritic cells and fibroblasts, we asked the question of whether there exists a similar defect in IFN-γ-induced IDO expression in NOD mice dermal fibroblasts. To this end, we examined the effect of IFN-γ on expression of IDO and its enzymatic activity in NOD dermal fibroblasts. The results showed that fibroblasts from either prediabetic (8 wks of age) female or male, and diabetic female or male (12 and 24 wks of age respectively) NOD mice failed to express IDO in response to IFN-γ treatment. To find underlying mechanisms, we scrutinized the IFN- γ signaling pathway and investigated expression of other IFN-γ-modulated factors including major histocompatibility complex class I (MHC-I) and type I collagen (COL-I). The findings revealed a defect of signal transducer and activator of transcription 1 (STAT1) phosphorylation in NOD cells relative to that of controls. Furthermore, we found an increase in MHC-I and suppression of COL-I expression in fibroblasts from both NOD and control mice following IFN-γ treatment; indicating that the impaired response to IFN-γ in NOD fibroblasts is specific to IDO gene. Finally, we showed that an IFN-γ-independent IDO expression pathway i.e. lipopolysaccharide (LPS)-mediated-c-Jun kinase is operative in NOD mice fibroblast. In conclusion, the findings of this study for the first time indicate that IFN-γ fails to induce IDO expression in NOD dermal fibroblasts; this may partially be due to defective STAT1 phosphorylation in IFN-γ-induced-IDO signaling pathway.
doi:10.1371/journal.pone.0037747
PMCID: PMC3360620  PMID: 22662207
24.  Serum Analysis of Tryptophan Catabolism Pathway: Correlation with Crohn’s Disease Activity 
Inflammatory Bowel Diseases  2011;18(7):1214-1220.
BACKGROUND
Indoleamine 2,3 dioxygenase 1 (IDO1) is a tryptophan catabolizing enzyme with immunotolerance promoting functions. We sought to determine if increased gut expression of IDO1 in Crohn’s disease (CD) would result in detectable changes in serum levels of tryptophan and the initial IDO1 pathway catabolite, kynurenine.
METHODS
Individuals were prospectively enrolled through the Washington University Digestive Diseases Research Center. Montreal classification was used for disease phenotyping. Disease severity was categorized by physician’s global assessment. Serum tryptophan and kynurenine were measured by high pressure liquid chromatography. IDO1 immunohistochemical staining was performed on formalin-fixed tissue blocks.
RESULTS
25 CD patients and 11 controls were enrolled. 8 CD patients had serum collected at two different time points and levels of disease activity. Strong IDO1 expression exists in both the lamina propria and epithelium during active CD compared to controls. Suppressed serum tryptophan levels and an elevated kynurenine/tryptophan (K/T) ratio were found in individuals with active CD as compared to those in remission or the control population. K/T ratios correlated positively with disease activity as well as with C-reactive protein and erythrocyte sedimentation rate. In the subgroup of CD patients with two serum measurements, tryptophan levels elevated while kynurenine levels and the K/T ratio lowered as the disease activity lessened.
CONCLUSIONS
IDO1 expression in Crohn’s disease is associated with lower serum tryptophan and an elevated K/T ratio. These levels may serve a reasonable objective marker of gut mucosal immune activation and surrogate for Crohn’s Disease activity.
doi:10.1002/ibd.21849
PMCID: PMC3235239  PMID: 21823214
Indoleamine 2,3 dioxygenase; tryptophan; Crohn; Biomarker
25.  Plasmacytoid dendritic cells from mouse tumor-draining lymph nodes directly activate mature Tregs via indoleamine 2,3-dioxygenase 
The Journal of Clinical Investigation  2007;117(9):2570-2582.
A small population of plasmacytoid DCs (pDCs) in mouse tumor-draining LNs can express the immunoregulatory enzyme indoleamine 2,3-dioxygenase (IDO). We show that these IDO+ pDCs directly activate resting CD4+CD25+Foxp3+ Tregs for potent suppressor activity. In vivo, Tregs isolated from tumor-draining LNs were constitutively activated and suppressed antigen-specific T cells immediately ex vivo. In vitro, IDO+ pDCs from tumor-draining LNs rapidly activated resting Tregs from non–tumor-bearing hosts without the need for mitogen or exogenous anti-CD3 crosslinking. Treg activation by IDO+ pDCs was MHC restricted, required an intact amino acid–responsive GCN2 pathway in the Tregs, and was prevented by CTLA4 blockade. Tregs activated by IDO markedly upregulated programmed cell death 1 ligand 1 (PD-L1) and PD-L2 expression on target DCs, and the ability of Tregs to suppress target T cell proliferation was abrogated by antibodies against the programmed cell death 1/PD-L (PD-1/PD-L) pathway. In contrast, Tregs activated by anti-CD3 crosslinking did not cause upregulation of PD-Ls, and suppression by these cells was unaffected by blocking the PD-1/PD-L pathway. Tregs isolated from tumor-draining LNs in vivo showed potent PD-1/PD-L–mediated suppression, which was selectively lost when tumors were grown in IDO-deficient hosts. We hypothesize that IDO+ pDCs create a profoundly suppressive microenvironment within tumor-draining LNs via constitutive activation of Tregs.
doi:10.1172/JCI31911
PMCID: PMC1940240  PMID: 17710230

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