Indoleamine 2,3-dioxygenase expression in human inflammatory bowel disease

Evaluating the role of kynurenine/tryptophan ratio as an indicator of disease activity in Indian patients with inflammatory bowel disease. A case-control study

BACKGROUND

Tryptophan (T), an essential amino acid, is primarily metabolized (∼90%) to kynurenine (K) by indoleamine 2,3-dioxygenase 1 (IDO1) mainly in intestinal cells. In inflammatory bowel disease (IBD), there is an increase in IDO1 activity which would increase Kynurenine levels and Kynurenine/Tryptophan (K/T) ratio. We hypothesize that alteration in K/T may be an indicator of disease severity in IBD.

METHODS

55 healthy controls (HC), 55 Ulcerative colitis (UC) (35 active and 20 remission) and 30 Crohn's disease (CD) (20 active and 10 remission) were enrolled from November 2020 to March 2023. Plasma Kyn & Trp were simultaneously estimated using ultra-high-pressure liquid chromatography (UPLC). K/T ratio was correlated with disease activity and fecal calprotectin. In 25 patients follow-up samples were also collected with change in disease activity.

RESULTS

Median K/T ratio was significantly higher in patients with active disease as compared to those in remission and HC (p < .0001). A cut-off of ≤41 distinguished remission/healthy controls with a sensitivity of 92.73%, specificity of 76.36%, and an AUC of 0.9 (95% CI: 0.83-0.95, p < .001). The K/T ratio correlated with FC levels at a diagnostic cut-off of 250 µg/g. A significant reduction in K/T ratio with disease activity was noted in 80% of follow-up patients.

CONCLUSION

The K/T ratio with a cut-off of 41, correlated with the disease activity in 82% of patients, suggesting that the K/T ratio alters remarkably with disease activity in IBD patients. These findings can be further assessed for disease marker in a larger cohort of IBD patients.

Evaluation of TNF-α and IFN-γ primed conditioned medium of mesenchymal stem cell in acetic acid-induced mouse model of acute colitis

IBD, an autoimmune-inflammatory disorder that affects people who are genetically prone to inflammation. There is a lot of interest in MSC-CM therapy, especially when primed with TNF-α + IFN-γ. Throughout the study, data were collected on the percentage of apoptotic cells, gene expression of ZO-1, Foxp3, GATA3, IDO-1, Muc2, T-bet, Notch1, TNFR2, and ROR-γt, colon weight and length, histopathological analysis, and DAI. TNF-α and IL-10 levels were assessed in addition to the NO level. The results suggest that primed MSC-CM improved DAI, mucosal deterioration, intestinal inflammation and NO concentration. The amount of TNF-α was decreased, but IL-10 and the colon's percentage of apoptotic cells was increased. The mRNA expression of ZO-1, Foxp3, GATA3, IDO-1, and Muc2 genes increased greatly in the treatment groups, while the expression of T-bet, Notch1, TNFR2, and ROR-γt genes has decreased. These studies suggest that primed MSC-CM may combine with common treatments to improve responsiveness.

Impaired Kynurenine Pathway in Inflammatory Bowel Disease

Background/Objectives: Inflammatory bowel diseases primarily encompass Crohn's disease and ulcerative colitis. Insufficient levels of tryptophan cause an imbalance in the gut microbiota, leading to inflammation in the gastrointestinal tract. The main catabolic pathway of tryptophan is the kynurenine pathway. Our study aims to evaluate serum tryptophan, the kynurenine pathway, and oxidative stress parameters, including total oxidant status and total antioxidant capacity, in patients with Crohn's disease and ulcerative colitis. Methods: The study included 80 follow-up patients in remission diagnosed with Crohn's disease and ulcerative colitis who attended the Gastroenterology Outpatient Clinic, as well as 78 healthy controls. Serum tryptophan, kynurenine, 3-hydroxykynurenine, 3-hydroxyanthranilic acid, and kynurenic acid levels were measured with liquid chromatography and tandem mass spectrometry (LC-MS/MS). All statistical analysis was performed using R version 4.2.1. Statistical Language. Results: Serum tryptophan, 3-hydroxyanthranilic acid, and total antioxidant capacity were lower in patients with ulcerative colitis and Crohn's disease compared to those in the control group. The serum total oxidant status in the control group was significantly lower than in patients with Crohn's disease and ulcerative colitis. Conclusions: The results of our research indicate that tryptophan and kynurenine pathway metabolites could potentially contribute to the pathogenesis of inflammatory bowel diseases.

Association between metabolites in tryptophan-kynurenine pathway and inflammatory bowel disease: a two-sample Mendelian randomization

Previous observational studies have suggested an association between tryptophan (TRP)-kynurenine (KYN) pathway and inflammatory bowel disease (IBD). However, whether there is a causal relationship among them remains unclear. Therefore, a two-sample Mendelian randomization (MR) study was conducted to explore the potential causal effects of crucial metabolites in TRP-KYN pathway on IBD and its subtypes. Using summary data from genome-wide association studies, a two-sample MR was employed to evaluate the genetic associations between TRP and KYN as exposures and IBD as an outcome. The inverse variance weighted method was used as the primary MR analysis, with MR-Egger, weighted mode, simple mode, and weighted median methods as complementary analyses. The odds ratios (OR) and 95% confidence intervals (CI) were determined for TRP-IBD (OR 0.739, 95% CI [0.697; 0.783]), TRP-UC (OR 0.875, 95% CI [0.814; 0.942]), TRP-CD (OR 0.685, 95% CI [0.613; 0.765]), KYN-IBD (OR 4.406, 95% CI [2.247; 8.641]), KYN-UC (OR 2.578, 95% CI [1.368; 4.858], and KYN-CD (OR 13.516, 95% CI [4.919; 37.134]). Collectively, the MR analysis demonstrated a significant protective association between TRP and IBD, whereas KYN was identified as a risk factor for IBD.

Microbial-Derived Tryptophan Metabolites and Their Role in Neurological Disease: Anthranilic Acid and Anthranilic Acid Derivatives

The gut microbiome provides the host access to otherwise indigestible nutrients, which are often further metabolized by the microbiome into bioactive components. The gut microbiome can also shift the balance of host-produced compounds, which may alter host health. One precursor to bioactive metabolites is the essential aromatic amino acid tryptophan. Tryptophan is mostly shunted into the kynurenine pathway but is also the primary metabolite for serotonin production and the bacterial indole pathway. Balance between tryptophan-derived bioactive metabolites is crucial for neurological homeostasis and metabolic imbalance can trigger or exacerbate neurological diseases. Alzheimer's, depression, and schizophrenia have been linked to diverging levels of tryptophan-derived anthranilic, kynurenic, and quinolinic acid. Anthranilic acid from collective microbiome metabolism plays a complex but important role in systemic host health. Although anthranilic acid and its metabolic products are of great importance for host-microbe interaction in neurological health, literature examining the mechanistic relationships between microbial production, host regulation, and neurological diseases is scarce and at times conflicting. This narrative review provides an overview of the current understanding of anthranilic acid's role in neurological health and disease, with particular focus on the contribution of the gut microbiome, the gut-brain axis, and the involvement of the three major tryptophan pathways.

Modulation of Indoleamine 2,3-Dioxygenase 1 During Inflammatory Bowel Disease Activity in Humans and Mice

Background and Aims

Indoleamine 2,3 dioxygenase-1 (IDO1), a key enzyme in tryptophan metabolism, is strongly up-regulated both in human inflammatory bowel disease (IBD) and animal models of colitis, however its role in the pathogenesis is still controversial. In this study, we investigated IDO1 expression and activity in a mouse model of DSS-induced chronic colitis as well as in colon biopsies and sera from IBD patients.

Methods

Chronic colitis was induced in mice through the oral administration of dextran sodium sulfate (DSS), and IDO1 activity was induced by i.p. treatment with N-acetyl serotonin (NAS). IDO1 expression and catalytic activity (measured as Kyn/Trp ratio) was evaluated in sera and tissue samples collected from mice and 93 IBD patients under immunotherapy with Vedolizumab (VDZ) or Ustekinumab (UST).

Results

Strong up-regulation of IDO1 was found in colons of mice with acute colitis, which follows disease activity. Enhanced IDO1 activity by NAS treatment protects the intestinal mucosa during the recovery phase of chronic colitis. In IBD patients, IDO1 expression and activity correlate with the severity of mucosal inflammation with inflamed regions showing higher IDO1 expression compared to non-inflamed regions within the same patient. Endoscopic response to VDZ/UST treatment is associated with decreased expression of IDO1.

Conclusions

This is the first study demonstrating immunomodulatory activity of IDO1 in a chronic mouse model of DSS-induced colitis. As its expression and catalytic activity correlate with the grade of mucosal inflammation and treatment response, IDO1 could represent a promising biomarker for disease severity and treatment monitoring in IBD.

Cassane diterpenoid ameliorates dextran sulfate sodium-induced experimental colitis by regulating gut microbiota and suppressing tryptophan metabolism

Ulcerative colitis (UC) is one form of inflammatory bowel disease (IBD), characterized by chronic relapsing intestinal inflammation. As increasing morbidity of UC and deficiency of conventional therapies, there is an urgent need for attractive treatment. Cassane diterpenoids, the characteristic chemical constituents of Caesalpinia genus plants, have been studied extensively owing to various and prominent biological activities. This study attempted to investigate the bioactivity of caesaldekarin e (CA), a cassane diterpenoid isolated from C. bonduc in our previous work, on dextran sulfate sodium (DSS)-induced experimental colitis and clarify the function mechanism. The results indicated that CA ameliorated mice colitis by relieving disease symptoms, suppressing inflammatory infiltration and maintaining intestinal barrier integrity. Furthermore, 16S rRNA gene sequencing analysis indicated that CA could improve the gut microbiota imbalance disrupted by DSS and especially restored abundance of Lactobacillus. In addition, untargeted metabolomics analysis suggested that CA regulated metabolism and particularly the tryptophan metabolism by inhibiting the upregulation of indoleamine 2,3-dioxygenase 1 (IDO-1). It also been proved in IFN-γ induced RAW264.7 cells. Overall, this study suggests that CA exhibits anti-UC effect through restoring gut microbiota and regulating tryptophan metabolism and has the potential to be a treatment option for UC.

Tryptophan metabolites modulate inflammatory bowel disease and colorectal cancer by affecting immune system

Tryptophan is an essential amino acid, going through three different metabolic pathways in the intestines. Indole pathway in the gut microbiota, serotonin system in the enterochromaffin cells and kynurenine pathway in the immune cells and intestinal lining are the three arms of tryptophan metabolism in the intestines. Clinical, in vivo and in vitro studies showed that each one of these arms has a significant impact on IBD. This review explains how different metabolites of tryptophan are involved in the pathophysiology of IBD and colorectal cancer, as a major complication of IBD. Indole metabolites alleviate colitis and protect against colorectal cancer while serotonin arm follows a more complicated and receptor-specific pattern. Indole metabolites and kynurenine interact with aryl hydrocarbon receptor (AHR) to induce T regulatory cells differentiation, confine Th17 and Th1 response and produce anti-inflammatory mediators. Kynurenine decreases tumor-infiltrating CD8+ cells and mediates tumor cells immune evasion. Serotonin system also increases colorectal cancer cells proliferation and metastasis while, indole metabolites can profoundly decrease colorectal cancer growth. Targeted therapy for tryptophan metabolites may improve the management of IBD and colorectal cancer, e.g. supplementation of indole metabolites such as indole-3-carbinol (I3C), inhibition of kynurenine monooxygenase (KMO) and selective stimulation or inhibition of specific serotonergic receptors can mitigate colitis. Furthermore, it will be explained how indole metabolites supplementation, inhibition of indoleamine 2,3-dioxygenase 1 (IDO1), KMO and serotonin receptors can protect against colorectal cancer. Additionally, extensive molecular interactions between tryptophan metabolites and intracellular signaling pathways will be thoroughly discussed.

Impact of Microbial Metabolites on Microbiota-Gut-Brain Axis in Inflammatory Bowel Disease

The complex bidirectional communication system existing between the gastrointestinal tract and the brain initially termed the "gut-brain axis" and renamed the "microbiota-gut-brain axis", considering the pivotal role of gut microbiota in sustaining local and systemic homeostasis, has a fundamental role in the pathogenesis of Inflammatory Bowel Disease (IBD). The integration of signals deriving from the host neuronal, immune, and endocrine systems with signals deriving from the microbiota may influence the development of the local inflammatory injury and impacts also more distal brain regions, underlying the psychophysiological vulnerability of IBD patients. Mood disorders and increased response to stress are frequently associated with IBD and may affect the disease recurrence and severity, thus requiring an appropriate therapeutic approach in addition to conventional anti-inflammatory treatments. This review highlights the more recent evidence suggesting that alterations of the microbiota-gut-brain bidirectional communication axis may concur to IBD pathogenesis and sustain the development of both local and CNS symptoms. The participation of the main microbial-derived metabolites, also defined as "postbiotics", such as bile acids, short-chain fatty acids, and tryptophan metabolites in the development of IBD-associated gut and brain dysfunction will be discussed. The last section covers a critical evaluation of the main clinical evidence pointing to the microbiome-based therapeutic approaches for the treatment of IBD-related gastrointestinal and neuropsychiatric symptoms.

Tryptophan Metabolism, Regulatory T Cells, and Inflammatory Bowel Disease: A Mini Review

Inflammatory bowel disease (IBD) is a chronic inflammatory disorder of the gastrointestinal tract resulting from the homeostasis imbalance of intestinal microenvironment, immune dysfunction, environmental and genetic factors, and so on. This disease is associated with multiple immune cells including regulatory T cells (Tregs). Tregs are a subset of T cells regulating the function of various immune cells to induce immune tolerance and maintain intestinal immune homeostasis. Tregs are correlated with the initiation and progression of IBD; therefore, strategies that affect the differentiation and function of Tregs may be promising for the prevention of IBD-associated pathology. It is worth noting that tryptophan (Trp) metabolism is effective in inducing the differentiation of Tregs through microbiota-mediated degradation and kynurenine pathway (KP), which is important for maintaining the function of Tregs. Interestingly, patients with IBD show Trp metabolism disorder in the pathological process, including changes in the concentrations of Trp and its metabolites and alteration in the activities of related catalytic enzymes. Thus, manipulation of Treg differentiation through Trp metabolism may provide a potential target for prevention of IBD. The purpose of this review is to highlight the relationship between Trp metabolism and Treg differentiation and the role of this interaction in the pathogenesis of IBD.

Increased expression of kynurenine aminotransferases mRNA in lymphocytes of patients with inflammatory bowel disease

BACKGROUND

Complex interaction of genetic defects with environmental factors seems to play a substantial role in the pathogenesis of inflammatory bowel disease (IBD). Accumulating data implicate a potential role of disturbed tryptophan metabolism in IBD. Kynurenic acid (KYNA), a derivative of tryptophan (TRP) along the kynurenine (KYN) pathway, displays cytoprotective and immunomodulating properties, whereas 3-OH-KYN is a cytotoxic compound, generating free radicals.

METHODS

The expression of lymphocytic mRNA encoding enzymes synthesizing KYNA (KAT I-III) and serum levels of TRP and its metabolites were evaluated in 55 patients with IBD, during remission or relapse [27 patients with ulcerative colitis (UC) and 28 patients with Crohn's disease (CD)] and in 50 control individuals.

RESULTS

The increased expression of KAT1 and KAT3 mRNA characterized the entire cohorts of patients with UC and CD, as well as relapse-remission subsets. Expression of KAT2 mRNA was enhanced in patients with UC and in patients with CD in remission. In the entire cohorts of UC or CD, TRP levels were lower, whereas KYN, KYNA and 3-OH-KYN were not altered. When analysed in subsets of patients with UC and CD (active phase-remission), KYNA level was significantly lower during remission than relapse, yet not versus control. Functionally, in the whole groups of patients with UC or CD, the TRP/KYN ratio has been lower than control, whereas KYN/KYNA and KYNA/3-OH-KYN ratios were not altered. The ratio KYN/3-OH-KYN increased approximately two-fold among all patients with CD; furthermore, patients with CD with relapse, manifested a significantly higher KYNA/3-OH-KYN ratio than patients in remission.

CONCLUSION

The presented data indicate that IBD is associated with an enhanced expression of genes encoding KYNA biosynthetic enzymes in lymphocytes; however, additional mechanisms appear to influence KYNA levels. Higher metabolic conversion of serum TRP in IBD seems to be followed by the functional shift of KYN pathway towards the arm producing KYNA during exacerbation. We propose that KYNA, possibly via interaction with aryl hydrocarbon receptor or G-protein-coupled orphan receptor 35, may serve as a counter-regulatory mechanism, decreasing cytotoxicity and inflammation in IBD. Further longitudinal studies evaluating the individual dynamics of TRP and KYN pathway in patients with IBD, as well as the nature of precise mechanisms regulating KYNA synthesis, should be helpful in better understanding the processes underlying the observed changes.

Indoleamine-pyrrole 2,3-dioxygenase-1 (IDO-1) mRNA is over-expressed in the duodenal mucosa and is negatively correlated with serum tryptophan concentrations in dogs with protein-losing enteropathy

INTRODUCTION

Dogs with protein-losing enteropathy (PLE) have decreased serum tryptophan concentrations, which may contribute to disease pathogenesis. Indoleamine-pyrrole 2,3-dioxygenase-1 (IDO-1) expression is associated with low serum tryptophan concentrations and is increased in the gastrointestinal tract of humans with inflammatory bowel disease (IBD). Therefore, the objective of our study was to determine if the mRNA expression of IDO-1 is increased in the duodenal mucosa of dogs with PLE as compared to dogs with chronic enteropathy (CE) and healthy dogs, and whether this expression is correlated with changes in serum tryptophan concentration.

METHODS

Our study was a retrospective study using archived paraffin-embedded duodenal biopsy specimens from 8 healthy Beagle dogs from the Iowa State University Canine Service Colony and 18 and 6 client-owned dogs diagnosed with CE and PLE, respectively at the Bristol Veterinary School. A novel RNA in situ hybridization (ISH) technology, RNAscope, was used to identify IDO-1 mRNA mucosal expression in duodenal tissues. An IDO-1 specific probe was hybridized onto 10 duodenal biopsy sections from each dog whereby RNAscope signal (mRNA expression) was quantified by a single operator using light microscopy.

RESULTS

Dogs with PLE had significantly higher mRNA expression of IDO-1 in the duodenal mucosa compared to healthy dogs (mucosal percentage IDO-1 positive: P = 0.0093, (mean ± S.D) control: 19.36 ± 7.08, PLE: 34.12 ± 5.98, average fold difference: 1.76 and mucosal IDO-1 H-score: P = 0.0356, (mean ± S.D) control: 45.26 ± 19.33, PLE: 84.37 ± 19.86, average fold difference: 1.86). The duodenal mucosal mRNA expression of IDO-1 was negatively correlated with serum tryptophan concentrations in dogs with PLE (mucosal IDO-1 H-score: Spearman's rank correlation coefficient = -0.94, P = 0.0048).

CONCLUSIONS

In conclusion, our study suggests that decreased serum tryptophan concentrations in dogs with PLE is associated with increased intestinal IDO-1 expression. Further studies are needed to determine potential inflammatory pathways responsible for increased expression of IDO-1 in the intestinal tract of dogs with PLE.

Butyrate Produced by Commensal Bacteria Down-Regulates Indolamine 2,3-Dioxygenase 1 (IDO-1) Expression via a Dual Mechanism in Human Intestinal Epithelial Cells

Commensal bacteria are crucial for the development and maintenance of a healthy immune system therefore contributing to the global well-being of their host. A wide variety of metabolites produced by commensal bacteria are influencing host health but the characterization of the multiple molecular mechanisms involved in host-microbiota interactions is still only partially unraveled. The intestinal epithelial cells (IECs) take a central part in the host-microbiota dialogue by inducing the first microbial-derived immune signals. Amongst the numerous effector molecules modulating the immune responses produced by IECs, indoleamine 2,3-dioxygenase-1 (IDO-1) is essential for gut homeostasis. IDO-1 expression is dependent on the microbiota and despites its central role, how the commensal bacteria impacts its expression is still unclear. Therefore, we investigated the impact of individual cultivable commensal bacteria on IDO-1 transcriptional expression and found that the short chain fatty acid (SCFA) butyrate was the main metabolite controlling IDO-1 expression in human primary IECs and IEC cell-lines. This butyrate-driven effect was independent of the G-protein coupled receptors GPR41, GPR43, and GPR109a and of the transcription factors SP1, AP1, and PPARγ for which binding sites were reported in the IDO-1 promoter. We demonstrated for the first time that butyrate represses IDO-1 expression by two distinct mechanisms. Firstly, butyrate decreases STAT1 expression leading to the inhibition of the IFNγ-dependent and phosphoSTAT1-driven transcription of IDO-1. In addition, we described a second mechanism by which butyrate impairs IDO-1 transcription in a STAT1-independent manner that could be attributed to its histone deacetylase (HDAC) inhibitor property. In conclusion, our results showed that IDO-1 expression is down-regulated by butyrate via a dual mechanism: the reduction of STAT1 level and the HDAC inhibitor property of SCFAs.

Role of indoleamine 2,3-dioxygenase in pathology of the gastrointestinal tract

Indoleamine 2,3-dioxygenase (IDO) has the most important role in modulation of tryptophan-dependent effects in the gastrointestinal tract, including modulation of intestinal immune response. An increased IDO activity maintains immune tolerance and attenuates ongoing inflammation but allows immune escape and uncontrolled growth of gastrointestinal tumors. Accordingly, IDO represents a novel therapeutic target for the treatment of inflammatory and malignant diseases of the gastrointestinal tract. In this review article, we summarize current knowledge about molecular and cellular mechanisms that are involved in IDO-dependent effects. We provide a brief outline of experimental and clinical studies that increased our understanding of how enhanced IDO activity: controls host-microbiota interactions in the gut; regulates detrimental immune response in inflammatory disorders of the gastrointestinal system; and allows immune escape and uncontrolled growth of gastrointestinal tumors. Additionally, we present future perspectives regarding modulation of IDO activity in the gut as possible new therapeutic approaches for the treatment of inflammatory and malignant diseases of the gastrointestinal system.

Redox Properties of Tryptophan Metabolism and the Concept of Tryptophan Use in Pregnancy

During pregnancy, tryptophan (Trp) is required for several purposes, and Trp metabolism varies over time in the mother and fetus. Increased oxidative stress (OS) with high metabolic, energy and oxygen demands during normal pregnancy or in pregnancy-associated disorders has been reported. Taking the antioxidant properties of Trp and its metabolites into consideration, we made four hypotheses. First, the use of Trp and its metabolites is optional based on their antioxidant properties during pregnancy. Second, dynamic Trp metabolism is an accommodation mechanism in response to OS. Third, regulation of Trp metabolism could be used to control/attenuate OS according to variations in Trp metabolism during pregnancy. Fourth, OS-mediated injury could be alleviated by regulation of Trp metabolism in pregnancy-associated disorders. Future studies in normal/abnormal pregnancies and in associated disorders should include measurements of free Trp, total Trp, Trp metabolites, and activities of Trp-degrading enzymes in plasma. Abnormal pregnancies and some associated disorders may be associated with disordered Trp metabolism related to OS. Mounting evidence suggests that the investigation of the use of Trp and its metabolites in pregnancy will be meanful.

Depressive disorder and gastrointestinal dysfunction after myocardial infarct are associated with abnormal tryptophan-5-hydroxytryptamine metabolism in rats

In this study, we investigated the relationship between tryptophan-5-hydroxytryptamine metabolism, depressive disorder, and gastrointestinal dysfunction in rats after myocardial infarction. Our goal was to elucidate the physiopathologic bases of somatic/psychiatric depression symptoms after myocardial infarction. A myocardial infarction model was established by permanent occlusion of the left anterior descending coronary artery. Depression-like behavior was evaluated using the sucrose preference test, open field test, and forced swim test. Gastric retention and intestinal transit were detected using the carbon powder labeling method. Immunohistochemical staining was used to detect indoleamine 2,3-dioxygenase expression in the hippocampus and ileum. High-performance liquid chromatography with fluorescence and ultraviolet detection determined the levels of 5-hydroxytryptamine, its precursor tryptophan, and its metabolite 5-hydroxyindoleacetic acid in the hippocampus, distal ileum, and peripheral blood. All data were analyzed using one-way analyses of variance. Three weeks after arterial occlusion, rats in the model group began to exhibit depression-like symptoms. For example, the rate of sucrose consumption was reduced, the total and central distance traveled in the open field test were reduced, and immobility time was increased, while swimming, struggling and latency to immobility were decreased in the forced swim test. Moreover, the gastric retention rate and gastrointestinal transit rate were increased in the model group. Expression of indoleamine 2,3-dioxygenase was increased in the hippocampus and ileum, whereas 5-hydroxytryptamine metabolism was decreased, resulting in lower 5-hydroxytryptamine and 5-hydroxyindoleacetic acid levels in the hippocampus and higher levels in the ileum. Depressive disorder and gastrointestinal dysfunction after myocardial infarction involve abnormal tryptophan-5-hydroxytryptamine metabolism, which may explain the somatic, cognitive, and psychiatric symptoms of depression commonly observed after myocardial infarction. Peripheral 5-hydroxytryptamine is an important substance in the gut-brain axis, and its abnormal metabolism is a critical finding after myocardial infarct.

Inhibition of indoleamine 2,3-dioxygenase 1 expression alters immune response in colon tumor microenvironment in mice

Indoleamine 2,3-dioxygenase (IDO), an enzyme that degrades the essential amino acid l-tryptophan along the kynurenine pathway, exerts immunomodulatory effects in a number of diseases. IDO expression is increased in tumor tissue and in draining lymph nodes; this increase is thought to play a role in tumor evasion by suppressing the immune response. A competitive inhibitor of IDO is currently being tested in clinical trials for the treatment of relapsed or refractory solid tumors, but the efficacy of IDO inhibition in colorectal tumors remains to be fully elucidated. In this study, we investigated the effect of IDO deficiency on colon tumorigenesis in mice by genetic deletion and pharmacological inhibition. Ido1-deficient^(−/−) mice were crossed with Apc^Min/+ mice or were administered azoxymethane with or without dextran sodium sulfate. Ido1 deficiency did not lead to significant differences in the size and number of colon tumors. Similarly, the pharmacological inhibition of IDO using 1-methyltryptophan (1-mT) also resulted in no significant differences in tumor size and number in Apc^Min/+ mice. However, Ido1 deficiency altered the immune response in the tumor microenvironment, showing a significant increase in mRNA expression of pro-inflammatory cytokines and a significant decrease in the number of Foxp3-positive regulatory T cells in the colon tumors of Ido1^(−/−) mice. Importantly, 1-mT treatment also significantly altered cytokine expression in the colon tumor tissues. These results suggest that IDO inhibition alone cannot sufficiently suppress colon cancer development in mice despite its immunomodulatory activity in the tumor microenvironment.

Role of indoleamine 2,3-dioxygenase in health and disease

IDO1 (indoleamine 2,3-dioxygenase 1) is a member of a unique class of mammalian haem dioxygenases that catalyse the oxidative catabolism of the least-abundant essential amino acid, L-Trp (L-tryptophan), along the kynurenine pathway. Significant increases in knowledge have been recently gained with respect to understanding the fundamental biochemistry of IDO1 including its catalytic reaction mechanism, the scope of enzyme reactions it catalyses, the biochemical mechanisms controlling IDO1 expression and enzyme activity, and the discovery of enzyme inhibitors. Major advances in understanding the roles of IDO1 in physiology and disease have also been realised. IDO1 is recognised as a prominent immune regulatory enzyme capable of modulating immune cell activation status and phenotype via several molecular mechanisms including enzyme-dependent deprivation of L-Trp and its conversion into the aryl hydrocarbon receptor ligand kynurenine and other bioactive kynurenine pathway metabolites, or non-enzymatic cell signalling actions involving tyrosine phosphorylation of IDO1. Through these different modes of biochemical signalling, IDO1 regulates certain physiological functions (e.g. pregnancy) and modulates the pathogenesis and severity of diverse conditions including chronic inflammation, infectious disease, allergic and autoimmune disorders, transplantation, neuropathology and cancer. In the present review, we detail the current understanding of IDO1’s catalytic actions and the biochemical mechanisms regulating IDO1 expression and activity. We also discuss the biological functions of IDO1 with a focus on the enzyme's immune-modulatory function, its medical implications in diverse pathological settings and its utility as a therapeutic target.

IDO expressing dendritic cells suppress allograft rejection of small bowel transplantation in mice by expansion of Foxp3+ regulatory T cells

BACKGROUND

Indoleamine 2,3-dioxygenase (IDO), the enzyme that catalyzes the first and rate-limiting step of tryptophan catabolism, suppresses T-cell responses by tryptophan depletion and accumulation of kynurenine metabolites. IDO prevents allograft rejection in various transplantations.

METHODS

Dendritic cells (DC) highly expressing IDO (IDO(+) DC) were cultured through transduction of adenovirus vectors carrying the IDO sequence. IDO(+) DC were incubated with CD4(+) CD25(-) T cells to detect T cell proliferation. The effects of IDO(+) DC and 3-Hydroxyanthranilic acid (3-HAA) were verified in an allogeneic murine small bowel transplantation (SBT) model. Foxp3(+) Treg cells of recipient mice were detected by flow cytometry and cytokines in plasma were determined by ELISA.

RESULTS

IDO(+) DC effectively suppressed proliferation of CD4(+) CD25(-) T cells in vitro, and this effect could be enhanced by adding 3-HAA. In the SBT transplantation model, both 3-HAA (P < 0.05) and IDO(+) DC (P < 0.01) prolonged the survival time of transplanted mice. Mice treated with IDO(+) DC achieved longer mean survival time than 3-HAA administrated mice (11.5d vs. 18.5d). Grafts from IDO(+) DC, 3-HAA and combination treatment group showed reduced inflammation and minimal architectural distortion. IFN-γ production was significantly inhibited by IDO(+) DC and 3-HAA (P<0.05). The expression of IL-2 was slightly lower with 3-HAA or IDO(+) DC treatment. However, IL-10 was higher in 3-HAA, IDO(+) DC and combination treatment groups, while TGF-β was elevated in all non-control groups.

CONCLUSIONS

IDO(+) DC plus 3-HAA has an immunoprotective role and represents a potential strategy to suppress acute rejection and prolong survival of grafts in SBT.

Collective unconscious: how gut microbes shape human behavior

The human gut harbors a dynamic and complex microbial ecosystem, consisting of approximately 1 kg of bacteria in the average adult, approximately the weight of the human brain. The evolutionary formation of a complex gut microbiota in mammals has played an important role in enabling brain development and perhaps sophisticated social interaction. Genes within the human gut microbiota, termed the microbiome, significantly outnumber human genes in the body, and are capable of producing a myriad of neuroactive compounds. Gut microbes are part of the unconscious system regulating behavior. Recent investigations indicate that these microbes majorly impact on cognitive function and fundamental behavior patterns, such as social interaction and stress management. In the absence of microbes, underlying neurochemistry is profoundly altered. Studies of gut microbes may play an important role in advancing understanding of disorders of cognitive functioning and social interaction, such as autism.

A novel gas chromatography mass spectrometry-based serum diagnostic and assessment approach to ulcerative colitis

BACKGROUND & AIMS

To improve the clinical course of ulcerative colitis (UC), more accurate serum diagnostic and assessment methods are required. We used serum metabolomics to develop diagnostic and assessment methods for UC.

METHODS

Sera from UC patients, Crohn's disease (CD) patients, and healthy volunteers (HV) were collected at multiple institutions. The UC and HV were randomly allocated to the training or validation set, and their serum metabolites were analyzed by gas chromatography mass spectrometry (GC/MS). Using the training set, diagnostic and assessment models for UC were established by multiple logistic regression analysis. Then, the models were assessed using the validation set. Additionally, to establish a diagnostic model for discriminating UC from CD, the CD patients' data were used.

RESULTS

The diagnostic model for discriminating UC from HV demonstrated an AUC of 0.988, 93.33% sensitivity, and 95.00% specificity in the training set and 95.00% sensitivity and 98.33% specificity in the validation set. Another model for discriminating UC from CD exhibited an AUC of 0.965, 85.00% sensitivity, and 97.44% specificity in the training set and 83.33% sensitivity in the validation set. The model for assessing UC showed an AUC of 0.967, 84.62% sensitivity, and 88.23% specificity in the training set and 84.62% sensitivity, 91.18% specificity, and a significant correlation with the clinical activity index (rs=0.7371, P<0.0001) in the validation set.

CONCLUSIONS

Our models demonstrated high performance and might lead to the development of a novel treatment selection method based on UC condition.

Indoleamine 2,3-dioxygenase: expressing cells in inflammatory bowel disease-a cross-sectional study

Aim. To characterise and enumerate IDO⁺ cells, Tregs, and T cell subsets in patients with ulcerative colitis (UC) and Crohn's disease (CD) with regard to their clinical activity. Methods. Ten active UC (aUC), 10 inactive UC (iUC), 6 aCD, and 8 iCD patients and 10 healthy individuals were included in the study. Circulating Foxp3-, IDO-, IL-17A-, IL-4-, IFN-γ-, and IL-10-expressing CD4⁺ T cells were quantitated by flow cytometry. Interleukin-17-expressing cells, CD25⁺/Foxp3⁺ Tregs, and CD123⁺/IDO⁺ plasmacytoid dendritic cells were evaluated in intestinal biopsies from 10 aUC, 6 aCD, and 10 noninflamed tissues. Results. All CD4⁺ T subsets were increased in aIBD patients compared with healthy donors. Meanwhile, frequency of CD8α⁺/CD16⁺/IDO⁺, CD8α⁺/CD56⁺/IDO⁺, CD8α⁺/CD80⁺/IDO⁺, CD8α⁺/CD123⁺/IDO⁺ large granular nonlymphoid cells, and CCR6⁺/CD123⁺/IDO⁺ plasmacytoid dendritic cells was higher in aIBD patients versus healthy donors or iIBD patients. Tissue IL-17A⁺ cells were present in higher amounts in aIBD versus noninflamed controls. IDO- and Foxp3-expressing cells were increased in aUC versus aCD patients and noninflamed tissues. Conclusions. The findings represent an original work in Mexican Mestizo patients with IBD. It shows that Tregs and IDO-expressing cells are increased with regard to disease activity. These cells could significantly shape inflammatory bowel disease pathophysiology, severity, and tolerance loss.

IDO1 plays an immunosuppressive role in 2,4,6-trinitrobenzene sulfate-induced colitis in mice

IDO, an enzyme that degrades the essential amino acid L-tryptophan to N-formylkynurenine, is known to exert immunomodulatory effects in a number of diseases and disorders. IDO expression is increased in tumors, where it is thought to be involved in tumor evasion by suppressing the immune response. A competitive inhibitor of IDO is currently being tested in clinical trials for relapsed or refractory solid tumors; however, there remains a concern that attenuation of the immunosuppressive function of IDO might exacerbate inflammatory responses. In this study, we investigated the role of IDO in 2,4,6-trinitrobenzene sulfate (TNBS)-induced colitis in mice by gene deletion and pharmacological inhibition. TNBS treatment induced significantly more severe colitis in Ido1 gene-deficient (Ido1⁻/⁻) mice than in Ido1 wild-type (Ido1⁺/⁺) mice, indicating a role for IDO1 in suppression of acute colitis. Consistent with this, the expression of Ido1 was increased in the colonic interstitial tissues of TNBS-treated Ido1⁺/⁺ mice. Furthermore, transplantation of Ido1⁺/⁺ bone marrow cells into Ido1⁻/⁻ mice reduced the pathological damage associated with colitis, altered the expression of cytokines, including IFN-γ, TNF-α, and IL-10, and increased the number of CD4⁺ Foxp3⁺ regulatory T cells in the colon. Pharmacological inhibition of IDO enzymatic activity by oral administration of 1-methyltryptophan (1-methyl-L-tryptophan or 1-methyl-D-tryptophan) significantly increased the severity of TNBS-induced colitis in mice, demonstrating that both stereoisomers can promote colitis. Collectively, our data indicate that IDO1 plays an important immunoregulatory role in the colon.

Indoleamine 2,3 dioxygenase in intestinal disease

PURPOSE OF REVIEW

Several gastrointestinal diseases including the inflammatory bowel diseases (IBDs) and malignancy are associated with elevated expression of indoleamine 2,3 dioxygenase-1 (IDO1). IDO1 initiates tryptophan catabolism along a pathway that generates several bioactive kynurenine-based metabolites. Promotion of T-cell-mediated tolerance and antimicrobial effects are among the variety of functions attributed to IDO1 activity. Recent advances addressing the diverse implications of gut-associated IDO1 expression are herein reviewed.

RECENT FINDINGS

In active IBD, IDO1 is highly expressed both in the cells of the lamina propria and epithelium. Experimental models demonstrate that IDO1 promotes gut immune homeostasis by limiting inflammatory responses and protecting the epithelium. In human colon cancer, high expression of IDO1 by the neoplastic epithelium correlates with poor prognosis. The serum kynurenine : tryptophan ratio is elevated in both active Crohn's disease and in colon cancer, suggesting this measurement may prove useful as a disease biomarker. IDO1 inhibitors have moved to clinical trials providing new hope as immunotherapy for advanced malignancy.

SUMMARY

IDO1 activity significantly shapes gastrointestinal disease pathophysiology and severity. Measures of IDO1 activity may be useful as a disease biomarker. Manipulation of IDO1 activity has great potential as a treatment for both inflammatory and malignancy associated gastrointestinal disease.

The circadian clock circuitry and the AHR signaling pathway in physiology and pathology

Life forms populating the Earth must face environmental challenges to assure individual and species survival. The strategies predisposed to maintain organismal homeostasis and grant selective advantage rely on anticipatory phenomena facing periodic modifications, and compensatory phenomena facing unpredictable changes. Biological processes bringing about these responses are respectively driven by the circadian timing system, a complex of biological oscillators entrained to the environmental light/dark cycle, and by regulatory and metabolic networks that precisely direct the body's adjustments to variations of external conditions and internal milieu. A critical role in organismal homeostatic functions is played by the aryl hydrocarbon receptor (AHR) complex, which senses environmental and endogenous compounds, influences metabolic responses controlling phase I/II gene expression, and modulates vital phenomena such as development, inflammation and adaptive immunity. A physiological cross-talk between circadian and AHR signaling pathways has been evidenced. The alteration of AHR signaling pathway deriving from genetic damage with polymorphisms or mutations, or produced by exogenous or endogenous AHR activation, and chronodisruption caused by mismatch between the body's internal clock and geophysical time/social schedules, are capable of triggering pathological mechanisms involved in metabolic, immune-related and neoplastic diseases. On the other hand, the molecular components of the circadian clock circuitry and AHR signaling pathway may represent useful tools for preventive interventions and valuable targets of therapeutic approaches.