Low efficiency IDO2 enzymes are conserved in lower vertebrates, whereas higher efficiency IDO1 enzymes are dispensable

Neuroimmune modulation by tryptophan derivatives in neurological and inflammatory disorders

The nervous and immune systems are highly developed, and each performs specialized physiological functions. However, they work together, and their dysfunction is associated with various diseases. Specialized molecules, such as neurotransmitters, cytokines, and more general metabolites, are essential for the appropriate regulation of both systems. Tryptophan, an essential amino acid, is converted into functional molecules such as serotonin and kynurenine, both of which play important roles in the nervous and immune systems. The role of kynurenine metabolites in neurodegenerative and psychiatric diseases has recently received particular attention. Recently, we found that hyperactivity of the kynurenine pathway is a critical risk factor for septic shock. In this review, we first outline neuroimmune interactions and tryptophan derivatives and then summarized the changes in tryptophan metabolism in neurological disorders. Finally, we discuss the potential of tryptophan derivatives as therapeutic targets for neuroimmune disorders.

Dietary Tryptophan Plays a Role as an Anti-Inflammatory Agent in European Seabass (Dicentrarchus labrax) Juveniles during Chronic Inflammation

Where teleost fish are concerned, studies in tryptophan immunomodulation generally point to immunosuppressive properties, thus presenting a potential anti-inflammatory dietary strategy. The goal of the present work was to evaluate the effects of tryptophan dietary supplementation on immune and neuroendocrine responses of the European seabass, Dicentrarchus labrax, undergoing chronic inflammation. Juvenile European seabass were intraperitoneally injected with either Freund's Incomplete Adjuvant (FIA, inflamed group) or a saline solution (control group). Within each group, fish were fed a control (CTRL) and a CTRL-based diet supplemented with tryptophan (0.3% DM basis; TRP) for 4 weeks. Different tissues were sampled every week for the assessment of immune-related parameters. When TRP was provided to FIA-injected fish, mcsfr gene expression increased from 1 to 2 weeks and remained high until the end of the experiment. The same fish showed a concurrent increase in peripheral monocyte counts. Moreover, il34 expression at 1 week post-FIA injection was higher in TRP-fed than in CTRL-fed fish. After one week, molecular patterns of anti-inflammatory processes seemed to be favoured by TRP (mcsfr, gr1, il34 and tgfβ). Altogether, the results show that the feeding period seems to be critical where tryptophan supplementation is concerned since at later inflammatory stages-and longer feeding periods-fish fed TRP displayed a molecular profile similar to that of the CTRL group. In contrast, shorter administration periods might accelerate immune regulatory pathways.

Unusual Evolution of Cephalopod Tryptophan Indole-Lyases

Tryptophan indole-lyase (TIL), a pyridoxal-5-phosphate-dependent enzyme, catalyzes the hydrolysis of L-tryptophan (L-Trp) to indole and ammonium pyruvate. TIL is widely distributed among bacteria and bacterial TILs consist of a D2-symmetric homotetramer. On the other hand, TIL genes are also present in several metazoans. Cephalopods have two TILs, TILα and TILβ, which are believed to be derived from a gene duplication that occurred before octopus and squid diverged. However, both TILα and TILβ individually contain disruptive amino acid substitutions for TIL activity, and neither was active when expressed alone. When TILα and TILβ were coexpressed, however, they formed a heterotetramer that exhibited low TIL activity. The loss of TIL activity of the heterotetramer following site-directed mutagenesis strongly suggests that the active heterotetramer contains the TILα/TILβ heterodimer. Metazoan TILs generally have lower kcat values for L-Trp than those of bacterial TILs, but such low TIL activity may be rather suitable for metazoan physiology, where L-Trp is in high demand. Therefore, reduced activity may have been a less likely target for purifying selection in the evolution of cephalopod TILs. Meanwhile, the unusual evolution of cephalopod TILs may indicate the difficulty of post-gene duplication evolution of enzymes with catalytic sites contributed by multiple subunits, such as TIL.

Identification and characterization of tryptophan-kynurenine pathway-related genes involving lamprey (Lampetra japonica) innate immunity

The tryptophan-kynurenine (TRP-KYN) pathway is involved in several biological functions, including immunosuppression, inflammatory response, and tumor suppression. Six TRP-KYN pathway-related genes, tryptophan 2,3-dioxygenase (TDO), indoleamine 2,3-dioxygenase 2 (IDO2), aminoadipate aminotransferase (AADAT), glutamate oxaloacetate transaminase 2 (GOT2), kynurenine monooxygenase (KMO), and kynureninase (KYNU) have been identified and cloned from the jawless vertebrate lamprey (Lampetra japonica) to gain insights into their evolution and characterization. Expression distribution showed that the key gene Lj-TDO was highly expressed in the oral gland. Real-time quantitative PCR showed that TRP-KYN pathway-related genes were significantly overexpressed after multi-stimulation. RNA interference showed that Lj-IDO2 knockdown regulated the expression of inflammatory factors. In conclusion, our study successfully clarified the ancestral features and functions of the TRP-KYN pathway, while providing valuable insights into the involvement of this pathway in the immune responses of a jawless vertebrate.

Indoleamine 2,3-dioxygenase 2 immunohistochemical expression in medullary thyroid carcinoma: implications in prognosis and immunomodulatory effects

BACKGROUND

The linkage between IDO2 expression and cancer progression is still unclear, particularly in medullary thyroid carcinoma (MTC). Our purpose is to unveil the potential correlations between IDO2 status, clinical-pathological parameters, patients' prognosis, and the possible immunomodulatory functions in MTC.

METHODS

Immunohistochemical expression levels of IDO2 were evaluated in the resected MTC surgical specimens and corresponding lymph nodes. CD4 + T cell infiltration was also evaluated by immunohistochemical analysis in the MTC tissues. The association of the IDO2 expression level with clinicopathologic characteristics, overall survival (OS)/recurrence-free survival (RFS), and CD4 + T cell infiltration were retrospectively investigated.

RESULTS

High expression of IDO2 is closely associated with more aggressive clinicopathological features, such as multifocality, ETE, a higher pT stage and especially a higher pN stage. Moreover, a significant difference in RFS was observed between the IDO2-high and IDO2-low groups. IDO2 expression of lymph node tissues was significantly related to the metastasis status. Furthermore, we found that IDO2 expression is negatively correlated with CD4 + T cell infiltrations in MTC tissues.

CONCLUSION

The expression level of IDO2 is associated with aggressive characteristics and is predictive of poor prognosis in patients with MTC. Also, an interesting observation is that IDO2 involvement in MTC showed a moderate sexual dimorphism, of which female patients tend to be more affected by IDO2 status. Moreover, our results showed the potential immunomodulatory functions of IDO2. The close relationship between IDO2 and CD4 + T cell infiltration in the MTC microenvironment, together with its potential prognostic implications, makes it possible for IDO2 to serve as an alternative drug target in cancer immunotherapy and as a new prognostic tool.

Species Differences in Tryptophan Metabolism and Disposition

Major species differences in tryptophan (Trp) metabolism and disposition exist with important physiological, functional and toxicity implications. Unlike mammalian and other species in which plasma Trp exists largely bound to albumin, teleosts and other aquatic species possess little or no albumin, such that Trp entry into their tissues is not hampered, neither is that of environmental chemicals and toxins, hence the need for strict measures to safeguard their aquatic environments. In species sensitive to toxicity of excess Trp, hepatic Trp 2,3-dioxygenase (TDO) lacks the free apoenzyme and its glucocorticoid induction mechanism. These species, which are largely herbivorous, however, dispose of Trp more rapidly and their TDO is activated by smaller doses of Trp than Trp-tolerant species. In general, sensitive species may possess a higher indoleamine 2,3-dioxygenase (IDO) activity which equips them to resist immune insults up to a point. Of the enzymes of the kynurenine pathway beyond TDO and IDO, 2-amino-3-carboxymuconic acid-6-semialdehyde decarboxylase (ACMSD) determines the extent of progress of the pathway towards NAD⁺ synthesis and its activity varies across species, with the domestic cat (Felis catus) being the leading species possessing the highest activity, hence its inability to utilise Trp for NAD⁺ synthesis. The paucity of current knowledge of Trp metabolism and disposition in wild carnivores, invertebrates and many other animal species described here underscores the need for further studies of the physiology of these species and its interaction with Trp metabolism.

Indoleamine 2,3-dioxygenase 1 (IDO1): an up-to-date overview of an eclectic immunoregulatory enzyme

Indoleamine 2,3-dioxygenase 1 (IDO1) catalyzes the initial rate-limiting step in the degradation of the essential amino acid tryptophan along the kynurenine pathway. When discovered more than 50 years ago, IDO1 was thought to be an effector molecule capable of mediating a survival strategy based on the deprivation of bacteria and tumor cells of the essential amino acid tryptophan. Since 1998, when tryptophan catabolism was discovered to be crucially involved in the maintenance of maternal T-cell tolerance, IDO1 has become the focus of several laboratories around the world. Indeed, IDO1 is now considered as an authentic immune regulator not only in pregnancy, but also in autoimmune diseases, chronic inflammation, and tumor immunity. However, in the last years, a bulk of new information-including structural, biological, and functional evidence-on IDO1 has come to light. For instance, we now know that IDO1 has a peculiar conformational plasticity and, in addition to a complex and highly regulated catalytic activity, is capable of performing a nonenzymic function that reprograms the expression profile of immune cells toward a highly immunoregulatory phenotype. With this state-of-the-art review, we aimed at gathering the most recent information obtained for this eclectic protein as well as at highlighting the major unresolved questions.

BJ, Silvio B, Fabio T. Identification and characterization of the kynurenine pathway in the pond snail Lymnaea stagnalis

Dysregulation of the kynurenine pathway (KP) is implicated in many human diseases and disorders, from immunological, metabolic, neurodegenerative, and neuropsychiatric conditions to cancer, and represents an appealing target for new therapeutic approaches. In this intricate scenario, invertebrates, like Lymnaea stagnalis (LS), provide a flexible tool to unravel the complexity of the KP. Starting from the available LS genome and transcriptome, we identified putative transcripts of all KP enzymes containing an ORF; each predicted protein possessed a high degree of sequence conservation to known orthologues of other invertebrate and vertebrate model organisms. Sequences were confirmed by qualitative PCR and sequencing. At the same time, the qRT-PCR analysis revealed that Lym IDO-like, Lym TDO-like, Lym AFMID-like, Lym KMO-like, Lym AADAT-like, Lym KYAT I/III-like, Lym KYNU-like, Lym HAAO-like, and Lym ACMSD-like showed widespread tissue expression. Then, tryptophan, kynurenine, kynurenic acid, anthranilic acid, 3-hydroxy-kynurenine, xanthurenic acid, picolinic acid, and quinolinic acid were identified in the hemolymph of LS by UHPLC-Q exactive mass spectrometer. Our study provides the most thorough characterization to date of the KP in an invertebrate model, supporting the value of LS for future functional studies of this pathway at the cellular, synaptic, and behavioral levels.

The Role of Tryptophan Metabolites in Neuropsychiatric Disorders

In recent decades, neuropsychiatric disorders such as major depressive disorder, schizophrenia, bipolar, etc., have become a global health concern, causing various detrimental influences on patients. Tryptophan is an important amino acid that plays an indisputable role in several physiological processes, including neuronal function and immunity. Tryptophan’s metabolism process in the human body occurs using different pathways, including the kynurenine and serotonin pathways. Furthermore, other biologically active components, such as serotonin, melatonin, and niacin, are by-products of Tryptophan pathways. Current evidence suggests that a functional imbalance in the synthesis of Tryptophan metabolites causes the appearance of pathophysiologic mechanisms that leads to various neuropsychiatric diseases. This review summarizes the pharmacological influences of tryptophan and its metabolites on the development of neuropsychiatric disorders. In addition, tryptophan and its metabolites quantification following the neurotransmitters precursor are highlighted. Eventually, the efficiency of various biomarkers such as inflammatory, protein, electrophysiological, genetic, and proteomic biomarkers in the diagnosis/treatment of neuropsychiatric disorders was discussed to understand the biomarker application in the detection/treatment of various diseases.

Unbalanced IDO1/IDO2 Endothelial Expression and Skewed Keynurenine Pathway in the Pathogenesis of COVID-19 and Post-COVID-19 Pneumonia

Despite intense investigation, the pathogenesis of COVID-19 and the newly defined long COVID-19 syndrome are not fully understood. Increasing evidence has been provided of metabolic alterations characterizing this group of disorders, with particular relevance of an activated tryptophan/kynurenine pathway as described in this review. Recent histological studies have documented that, in COVID-19 patients, indoleamine 2,3-dioxygenase (IDO) enzymes are differentially expressed in the pulmonary blood vessels, i.e., IDO1 prevails in early/mild pneumonia and in lung tissues from patients suffering from long COVID-19, whereas IDO2 is predominant in severe/fatal cases. We hypothesize that IDO1 is necessary for a correct control of the vascular tone of pulmonary vessels, and its deficiency in COVID-19 might be related to the syndrome’s evolution toward vascular dysfunction. The complexity of this scenario is discussed in light of possible therapeutic manipulations of the tryptophan/kynurenine pathway in COVID-19 and post-acute COVID-19 syndromes.

Regulatory T Cell Modulation by Lactobacillus rhamnosus Improves Feather Damage in Chickens

It is currently unclear whether potential probiotics such as lactic acid bacteria could affect behavioral problems in birds. To this end, we assessed whether a supplementation of Lactobacillus rhamnosus JB-1 can reduce stress-induced severe feather pecking (SFP), feather damage and fearfulness in adult birds kept for egg laying. In parallel, we assessed SFP genotypic and phenotypic-related immune responses and aromatic amino acid status linked to neurotransmitter production. Social stress aggravated plumage damage, while L. rhamnosus treatment improved the birds' feather cover in non-stressed birds, but did not impact fearfulness. Our data demonstrate the significant impact of L. rhamnosus supplementation on the immune system. L. rhamnosus supplementation induced immunosuppressive regulatory T cells and cytotoxic T cells in both the cecal tonsils and the spleen. Birds exhibiting the SFP phenotype possessed lower levels of cecal tonsils regulatory T cells, splenic T helper cells and a lower TRP:(PHE+TYR). Together, these results suggest that bacteria may have beneficial effects on the avian immune response and may be useful therapeutic adjuncts to counteract SFP and plumage damage, thus increasing animal health and welfare.

Inhibitory effect of ascorbate on tryptophan 2,3-dioxygenase

Tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO) catalyze the same reaction, oxidative cleavage of L-tryptophan (L-Trp) to N-formyl-kynurenine. In both enzymes, the ferric (FeIII) form is inactive, and ascorbate (Asc) is frequently used as a reductant in in vitro assays to activate the enzymes by reducing the heme iron. Recently, it has been reported that Asc activates IDO2 by acting as a reductant, however, it is also a competitive inhibitor of the enzyme. Here, the effect of Asc on human TDO (hTDO) is investigated. Similar to its interaction with IDO2, Asc acts as both a reductant and a competitive inhibitor of hTDO in the absence of catalase, and its inhibitory effect was enhanced by the addition of H2O2. Interestingly, however, no inhibitory effect of Asc was observed in the presence of catalase. TDO is known to be activated by H2O2 and a ferryl-oxo (FeIV=O) intermediate (Compound II) is generated during the activation process. The observation that Asc acts as a competitive inhibitor of hTDO only in the absence of catalase can be explained by assuming that the target of Asc is Compound II. Asc seems to compete with L-Trp in an unusual manner.

Kynurenine to tryptophan ratio as a biomarker of acute stress in fish

The aim of this study was to determine the kynurenine (KYN) to tryptophan (TRP) ratio (KTR) in fish tissue to assess its usefulness as a biomarker of acute stress. Laboratory held rainbow trout (Oncorhynchus mykiss) were subjected to an acute stressor and KYN, TRP and cortisol were measured in liver and brain tissues at 4- and 48-h post-stress. The analytical method used to determine our analytes was based on lyophilization, and liquid-solid extraction followed by isotope dilution high-performance liquid chromatography positive ion electrospray tandem mass spectrometry. The [KYN]/[TRP] ratio (KTR) was greater in fish liver and brain in the 48-h post-stress exposure group (n = 8) relative to controls (n = 8, p < 0.05); a similar increase was not observed in fish in the 4-h post-stress exposure group. Hepatic and brain cortisol levels were also elevated in fish from both stress-induced groups relative to their respective controls implying that cortisol responded more quickly to the stressful stimulus than KYN and TRP. Our results suggest that the KTR is a promising acute stress diagnostic biomarker in fish. Efforts are ongoing to assess whether the KTR can be used as a biomarker for chronic stress in fish exposed to aquatic contaminants and other environmental stressors and if similar assessments can be made on tissues collected via non-lethal approaches.

L. rhamnosus improves the immune response and tryptophan catabolism in laying hen pullets

In mammals, early-life probiotic supplementation is a promising tool for preventing unfavourable, gut microbiome-related behavioural, immunological, and aromatic amino acid alterations later in life. In laying hens, feather-pecking behaviour is proposed to be a consequence of gut-brain axis dysregulation. Lactobacillus rhamnosus decreases stress-induced severe feather pecking in adult hens, but whether its effect in pullets is more robust is unknown. Consequently, we investigated whether early-life, oral supplementation with a single Lactobacillus rhamnosus strain can prevent stress-induced feather-pecking behaviour in chickens. To this end, we monitored both the short- and long-term effects of the probiotic supplement on behaviour and related physiological parameters. We hypothesized that L. rhamnosus would reduce pecking behaviour by modulating the biological pathways associated with this detrimental behaviour, namely aromatic amino acid turnover linked to neurotransmitter production and stress-related immune responses. We report that stress decreased the proportion of cytotoxic T cells in the tonsils (P = 0.047). Counteracting this T cell depression, birds receiving the L. rhamnosus supplementation significantly increased all T lymphocyte subset proportions (P < 0.05). Both phenotypic and genotypic feather peckers had lower plasma tryptophan concentrations compared to their non-pecking counterparts. The probiotic supplement caused a short-term increase in plasma tryptophan (P < 0.001) and the TRP:(PHE + TYR) ratio (P < 0.001). The administration of stressors did not significantly increase feather pecking in pullets, an observation consistent with the age-dependent onset of pecking behaviour. Despite minimal changes to behaviour, our data demonstrate the impact of L. rhamnosus supplementation on the immune system and the turnover of the serotonin precursor tryptophan. Our findings indicate that L. rhamnosus exerts a transient, beneficial effect on the immune response and tryptophan catabolism in pullets.

Ingestion of Lactobacillus rhamnosus modulates chronic stress-induced feather pecking in chickens

Feather pecking (FP) is a stress-induced neuropsychological disorder of birds. Intestinal dysbiosis and inflammation are common traits of these disorders. FP is, therefore, proposed to be a behavioral consequence of dysregulated communication between the gut and the brain. Probiotic bacteria are known to favorably modulate the gut microbiome and hence the neurochemical and immune components of the gut-brain axis. Consequently, probiotic supplementation represents a promising new therapeutic to mitigate widespread FP in domestic chickens. We monitored FP, gut microbiota composition, immune markers, and amino acids related to the production of neurochemicals in chickens supplemented with Lactobacillus rhamnosus or a placebo. Data demonstrate that, when stressed, the incidence of FP increased significantly; however, L. rhamnosus prevented this increase. L. rhamnosus supplementation showed a strong immunological effect by increasing the regulatory T cell population of the spleen and the cecal tonsils, in addition to limiting cecal microbiota dysbiosis. Despite minimal changes in aromatic amino acid levels, data suggest that catecholaminergic circuits may be an interesting target for further studies. Overall, our findings provide the first data supporting the use of a single-strain probiotic to reduce stress-induced FP in chickens and promise to improve domestic birds' welfare.

Tryptophan and Its Metabolites in Lung Cancer: Basic Functions and Clinical Significance

Lung cancer is the most lethal malignancy worldwide. Recently, it has been recognized that metabolic reprogramming is a complex and multifaceted factor, contributing to the process of lung cancer. Tryptophan (Try) is an essential amino acid, and Try and its metabolites can regulate the progression of lung cancer. Here, we review the pleiotropic functions of the Try metabolic pathway, its metabolites, and key enzymes in the pathogenic process of lung cancer, including modulating the tumor environment, promoting immune suppression, and drug resistance. We summarize the recent advance in therapeutic drugs targeting the Try metabolism and kynurenine pathway and their clinical trials.

Methylene blue and ascorbate interfere with the accurate determination of the kinetic properties of IDO2

Indoleamine 2,3-dioxygenases (IDOs) catalyze the oxidative cleavage of L-tryptophan (Trp) to N-formylkynurenine. Two IDOs, IDO1 and IDO2, are present in vertebrates. IDO1 is a high-affinity Trp-degrading enzyme involved in several physiological processes. By comparison, IDO2 generally has been reported to have low affinity (high K_m -value) for Trp, and the enzyme's in vivo function remains unclear. Using IDOs from different species, we show that compared with ferrous-oxy (Fe²⁺ -O₂ ) IDO1, Fe²⁺ -O₂ IDO2 is substantially more stable and engages in multiple turnovers of the reaction in the absence of a reductant. Without reductant, Fe²⁺ -O₂ IDO2 showed K_m -values in the range of 80-356 μM, that is, values substantially lower than reported previously and close to the physiological concentrations of Trp. Methylene blue and ascorbate (Asc), used commonly as the reducing system for IDO activity determination, significantly affected the enzymatic activity of IDO2: In combination, the two reductants increased the apparent K_m - and k_cat -values 8- to 117-fold and 2-fold, respectively. Asc alone both activated and inhibited IDO2 by acting as a source of electrons and as a weak competitive inhibitor, respectively. In addition, ferric (Fe³⁺ ) IDO1 and IDO2 exhibited weak dioxygenase activity, similar to tryptophan 2,3-dioxygenase. Our results shed new light in the enzymatic activity of IDO2, and they support the view that this isoform of IDO also participates in the metabolism of Trp in vivo.

The emerging roles of IDO2 in cancer and its potential as a therapeutic target

During the past decades, tryptophan metabolism disorder was discovered to play a vital and complex role in the development of cancer. Indoleamine 2,3-dioxygenase 2 (IDO2) is one of the initial and rate-limiting enzymes of the kynurenine pathway of tryptophan catabolism. Increasing evidence indicates that IDO2 is upregulated in some tumors and plays a role in the development of cancer. In spite of the growing body of research, few reviews focused on the role of IDO2 in cancer. Here, we review the emerging knowledge on the roles of IDO2 in cancer and its potential as a therapeutic target. Firstly, the main biological features and regulatory mechanisms are reviewed, after which we focus on the expression and roles of IDO2 in cancer. Finally, we discuss the potential of IDO2 as a therapeutic target for cancer treatment.

Indoleamine and tryptophan 2,3-dioxygenases as important future therapeutic targets

Conversion of tryptophan to N-formylkynurenine is the first and rate-limiting step of the tryptophan metabolic pathway (i.e., the kynurenine pathway). This conversion is catalyzed by three enzyme isoforms: indoleamine 2,3-dioxygenase 1 (IDO1), indoleamine 2,3-dioxygenase 2 (IDO2), and tryptophan 2,3-dioxygenase (TDO). As this pathway generates numerous metabolites that are involved in various pathological conditions, IDOs and TDO represent important targets for therapeutic intervention. This pathway has especially drawn attention due to its importance in tumor resistance. Over the last decade, a large number of IDO and TDO inhibitors have been developed, many of which have entered clinical trials. Here, detailed structural comparisons of these three enzymes (with emphasis on their active sites), their involvement in cellular signaling, and their role(s) in pathological conditions are discussed. Furthermore, the most important recent inhibitors described in papers and patents and involved in clinical trials are reviewed, with a focus on both selective and multiple inhibitors. A short overview of the biochemical and cellular assays used for inhibitory potency evaluation is also presented. This review summarizes recent advances on IDO and TDO as potential drug targets, and provides the key features and perspectives for further research and development of potent inhibitors of the kynurenine pathway.

Indoleamine 2,3-Dioxygenase 2 Immunohistochemical Expression in Resected Human Non-small Cell Lung Cancer: A Potential New Prognostic Tool

Indoleamine 2,3-dioxygenase 2 (IDO2) is an analog of the tryptophan degrading and immunomodulating enzyme indoleamine 2,3-dioxygenase 1 (IDO1). Although the role of IDO1 is largely understood, the function of IDO2 is not yet well-elucidated. IDO2 overexpression was documented in some human tumors, but the linkage between IDO2 expression and cancer progression is still unclear, in particular in non-small cell lung cancer (NSCLC). Immunohistochemical expression and cellular localization of IDO2 was evaluated on 191 formalin-fixed and paraffin-embedded resected NSCLC. Correlations between IDO2 expression, clinical-pathological data, tumor-infiltrating lymphocytes (TILs), immunosuppressive tumor molecules (IDO1 and programmed cell death ligand-1 – PD-L1 –) and patients' prognosis were evaluated. IDO2 high expression is strictly related to high PD-L1 level among squamous cell carcinomas group (p = 0.012), to either intratumoral or mixed localization of TILs (p < 0.001) and to adenocarcinoma histotype (p < 0.001). Furthermore, a significant correlation between IDO2 high expression and poor non-small cell lung cancer prognosis was detected (p = 0.011). The current study reaches interesting knowledge about IDO2 in non-small cell lung cancer. The close relationship between IDO2 expression, PD-L1 increased levels, TILs localization and NSCLC poor prognosis, assumed IDO2 as a potential prognostic biomarker to be exploited for optimizing innovative combined therapies with immune checkpoint inhibitors.

Switch of NAD Salvage to de novo Biosynthesis Sustains SIRT1-RelB-Dependent Inflammatory Tolerance

A typical inflammatory response sequentially progresses from pro-inflammatory, immune suppressive to inflammatory repairing phases. Although the physiological inflammatory response resolves in time, severe acute inflammation usually sustains immune tolerance and leads to high mortality, yet the underlying mechanism is not completely understood. Here, using the leukemia-derived THP-1 human monocytes, healthy and septic human peripheral blood mononuclear cells (PBMC), we report that endotoxin dose-dependent switch of nicotinamide adenine dinucleotide (NAD) biosynthesis pathways sustain immune tolerant status. Low dose endotoxin triggered nicotinamide phosphoribosyltransferase (NAMPT)-dependent NAD salvage activity to adapt pro-inflammation. In contrast, high dose endotoxin drove a shift of NAD synthesis pathway from early NAMPT-dependent NAD salvage to late indoleamine 2,3-dioxygenase-1 (IDO1)-dependent NAD de novo biosynthesis, leading to persistent immune suppression. This is resulted from the IDO1-dependent expansion of nuclear NAD pool and nuclear NAD-dependent prolongation of sirtuin1 (SIRT1)-directed epigenetics of immune tolerance. Inhibition of IDO1 activity predominantly decreased nuclear NAD level, which promoted sequential dissociations of immunosuppressive SIRT1 and RelB from the promoter of pro-inflammatory TNF-α gene and broke endotoxin tolerance. Thus, NAMPT-NAD-SIRT1 axis adapts pro-inflammation, but IDO1-NAD-SIRT1-RelB axis sustains endotoxin tolerance during acute inflammatory response. Remarkably, in contrast to the prevention of sepsis death of animal model by IDO1 inhibition before sepsis initiation, we demonstrated that the combination therapy of IDO1 inhibition by 1-methyl-D-tryptophan (1-MT) and tryptophan supplementation rather than 1-MT administration alone after sepsis onset rescued sepsis animals, highlighting the translational significance of tryptophan restoration in IDO1 targeting therapy of severe inflammatory diseases like sepsis.

Danshensu, a novel indoleamine 2,3-dioxygenase1 inhibitor, exerts anti-hepatic fibrosis effects via inhibition of JAK2-STAT3 signaling

BACKGROUND

Indoleamine 2,3-dioxygenase 1 (IDO1), an important intracellular rate-limiting enzyme in the development of Hepatic fibrosis (HF), and has been proposed as a hallmark of HF. Danshensu (DSS) is a major bioactive component that isolated from a edible traditional Chinese medicinal herb Salviae Miltiorrhizae Radix et Rhizoma (Danshen), while, the anti-HF mode and mechanism of action of DSS have not been fully elucidated.

METHODS

Carbon tetrachloride (CCl4)-induced rat HF model and TGF-β1-induced hepatic stellate cell (HSC) model were employed to assess the in vivo and in vitro anti-HF effects of DSS. HSC-T6 cells stably expressing IDO1, a constitutively active IDO1 mutant, was used to determine the role of JAK2-STAT3 signaling in the DSS's anti-HF effects.

RESULTS

We found that intragastric administration of DSS potently reduced fibrosis, inhibited IDO1 expression and STAT3 activity both in vitro and in vivo. Using molecular docking and molecular dynamics analysis, DSS was identified as a novel IDO1 inhibitor. Mechanistic studies indicated that DSS inhibited JAK2-STAT3 signaling, it reduced IDO1 expression, STAT3 phosphorylation and STAT3 nuclear localization. More importantly, overexpression of IDO1 diminished DSS's anti-HF effects.

CONCLUSION

Our findings provide a pharmacological justification for the clinical use of DSS in treating HF, and suggest that DSS has the potential to be developed as a modern alternative and/or complimentary agent for HF treatment and prevention.

A comprehensive comparison of the metazoan tryptophan degrading enzymes

Tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO) have an independent origin; however, they have distinctly evolved to catalyze the same reaction. In general, TDO is a single-copy gene in each metazoan species, and TDO enzymes demonstrate similar enzyme activity regardless of their biological origin. In contrast, multiple IDO paralogues are observed in many species, and they display various enzymatic properties. Similar to vertebrate IDO2, invertebrate IDOs generally show low affinity/catalytic efficiency for L-Trp. Meanwhile, two IDO isoforms from scallop (IDO-I and -III) and sponge IDOs show high L-Trp catalytic activity, which is comparable to vertebrate IDO1. Site-directed mutagenesis experiments have revealed that primarily two residues, Tyr located at the 2nd residue on the F-helix (F2^nd) and His located at the 9th residue on the G-helix (G9^th), are crucial for the high affinity/catalytic efficiency of these 'high performance' invertebrate IDOs. Conversely, those two amino acid substitutions (F2^nd/Tyr and G9^th/His) resulted in high affinity and catalytic activity in other molluscan 'low performance' IDOs. In human IDO1, G9^th is Ser¹⁶⁷, whereas the counterpart residue of G9^th in human TDO is His⁷⁶. Previous studies have shown that Ser¹⁶⁷ could not be substituted by His because the human IDO1 Ser¹⁶⁷His variant showed significantly low catalytic activity. However, this may be specific for human IDO1 because G9^th/His was demonstrated to be very effective in increasing the L-Trp affinity even in vertebrate IDOs. Therefore, these findings indicate that the active sites of TDO and IDO are more similar to each other than previously expected.

Genetic Polymorphisms Affecting IDO1 or IDO2 Activity Differently Associate With Aspergillosis in Humans

Aspergillus is the causative agent of human diseases ranging from asthma to invasive infection. Genetic and environmental factors are crucial in regulating the interaction between the host and Aspergillus. The role played by the enzyme indoleamine 2,3-dioxygenase 1 (IDO1), which catalyzes the first and rate-limiting step of tryptophan catabolism along the kynurenine pathway, is increasingly being recognized, but whether and how genetic variation of IDO1 influences the risk of aspergillosis in susceptible patients is incompletely understood. In addition, whether the closely related protein IDO2 plays a similar role remains unexplored. In the present study, we performed genetic association studies in two different cohorts of susceptible patients [cystic fibrosis (CF) patients and recipients of hematopoietic stem cell transplantation (HSCT)], and identified IDO1 polymorphisms that associate with the risk of infection in both cohorts. By using human bronchial epithelial cells and PBMC from CF and HSCT patients, respectively, we could show that the IDO1 polymorphisms appeared to down-modulate IDO1 expression and function in response to IFNγ or Aspergillus conidia, and to associate with an increased inflammatory response. In contrast, IDO2 polymorphisms, including variants known to profoundly affect protein expression and function, were differently associated with the risk of aspergillosis in the two cohorts of patients as no association was found in CF patients as opposed to recipients of HSCT. By resorting to a murine model of bone marrow transplantation, we could show that the absence of IDO2 more severely affected fungal burden and lung pathology upon infection with Aspergillus as compared to IDO1, and this effect appeared to be linked to a deficit in the antifungal effector phagocytic activity. Thus, our study confirms and extends the role of IDO1 in the response to Aspergillus, and shed light on the possible involvement of IDO2 in specific clinical settings.

Tryptophan Metabolic Pathways and Brain Serotonergic Activity: A Comparative Review

The essential amino acid L-tryptophan (Trp) is the precursor of the monoaminergic neurotransmitter serotonin (5-hydroxytryptamine, 5-HT). Numerous studies have shown that elevated dietary Trp has a suppressive effect on aggressive behavior and post-stress plasma cortisol concentrations in vertebrates, including teleosts. These effects are believed to be mediated by the brain serotonergic system, even though all mechanisms involved are not well understood. The rate of 5-HT biosynthesis is limited by Trp availability, but only in neurons of the hindbrain raphe area predominantly expressing the isoform TPH2 of the enzyme tryptophan hydroxylase (TPH). In the periphery as well as in brain areas expressing TPH1, 5-HT synthesis is probably not restricted by Trp availability. Moreover, there are factors affecting Trp influx to the brain. Among those are acute stress, which, in contrast to long-term stress, may result in an increase in brain Trp availability. The mechanisms behind this stress induced increase in brain Trp concentration are not fully understood but sympathetic activation is likely to play an important role. Studies in mammals show that only a minor fraction of Trp is utilized for 5-HT synthesis whereas a larger fraction of the Trp pool enters the kynurenic pathway. The first stage of this pathway is catalyzed by the hepatic enzyme tryptophan 2,3-dioxygenase (TDO) and the extrahepatic enzyme indoleamine 2,3-dioxygenase (IDO), enzymes that are induced by glucocorticoids and pro-inflammatory cytokines, respectively. Thus, chronic stress and infections can shunt available Trp toward the kynurenic pathway and thereby lower 5-HT synthesis. In accordance with this, dietary fatty acids affecting the pro-inflammatory cytokines has been suggested to affect metabolic fate of Trp. While TDO seems to be conserved by evolution in the vertebrate linage, earlier studies suggested that IDO was only present mammals. However, recent phylogenic studies show that IDO paralogues are present within the whole vertebrate linage, however, their involvement in the immune and stress reaction in teleost fishes remains to be investigated. In this review we summarize the results from previous studies on the effects of dietary Trp supplementation on behavior and neuroendocrinology, focusing on possible mechanisms involved in mediating these effects.

Lipopolysaccharide shock reveals the immune function of indoleamine 2,3-dioxygenase 2 through the regulation of IL-6/stat3 signalling

Indoleamine 2,3-dioxygenase 2 (Ido2) is a recently identified catalytic enzyme in the tryptophan-kynurenine pathway that is expressed primarily in monocytes and dendritic cells. To elucidate the biological role of Ido2 in immune function, we introduced lipopolysaccharide (LPS) endotoxin shock to Ido2 knockout (Ido2 KO) mice, which led to higher mortality than that in the wild type (WT) mice. LPS-treated Ido2 KO mice had increased production of inflammatory cytokines (including interleukin-6; IL-6) in serum and signal transducer and activator of transcription 3 (stat3) phosphorylation in the spleen. Moreover, the peritoneal macrophages of LPS-treated Ido2 KO mice produced more cytokines than did the WT mice. By contrast, the overexpression of Ido2 in the murine macrophage cell line (RAW) suppressed cytokine production and decreased stat3 expression. Finally, RAW cells overexpressing Ido2 did not alter nuclear factor κB (NF-κB) or stat1 expression, but IL-6 and stat3 expression decreased relative to the control cell line. These results reveal that Ido2 modulates IL-6/stat3 signalling and is induced by LPS, providing novel options for the treatment of immune disorders.

Upregulation of tryptophanyl-tRNA synthethase adapts human cancer cells to nutritional stress caused by tryptophan degradation

Tryptophan (Trp) metabolism is an important target in immuno-oncology as it represents a powerful immunosuppressive mechanism hijacked by tumors for protection against immune destruction. However, it remains unclear how tumor cells can proliferate while degrading the essential amino acid Trp. Trp is incorporated into proteins after it is attached to its tRNA by tryptophanyl-tRNA synthestases. As the tryptophanyl-tRNA synthestases compete for Trp with the Trp-catabolizing enzymes, the balance between these enzymes will determine whether Trp is used for protein synthesis or is degraded. In human cancers expression of the Trp-degrading enzymes indoleamine-2,3-dioxygenase-1 (IDO1) and tryptophan-2,3-dioxygenase (TDO2) was positively associated with the expression of the tryptophanyl-tRNA synthestase WARS. One mechanism underlying the association between IDO1 and WARS identified in this study is their joint induction by IFNγ released from tumor-infiltrating T cells. Moreover, we show here that IDO1- and TDO2-mediated Trp deprivation upregulates WARS expression by activating the general control non-derepressible-2 (GCN2) kinase, leading to phosphorylation of the eukaryotic translation initiation factor 2α (eIF2α) and induction of activating transcription factor 4 (ATF4). Trp deprivation induced cytoplasmic WARS expression but did not increase nuclear or extracellular WARS levels. GCN2 protected the cells against the effects of Trp starvation and enabled them to quickly make use of Trp for proliferation once it was replenished. Computational modeling of Trp metabolism revealed that Trp deficiency shifted Trp flux towards WARS and protein synthesis. Our data therefore suggest that the upregulation of WARS via IFNγ and/or GCN2-peIF2α-ATF4 signaling protects Trp-degrading cancer cells from excessive intracellular Trp depletion.

Genetic alterations affecting the genes encoding the enzymes of the kynurenine pathway and their association with human diseases

Tryptophan is metabolized primarily via the kynurenine pathway (KP), which involves several enzymes, including indoleamine 2,3-dioxygenase, tryptophan 2,3 dioxygenase (TDO), kynurenine aminotransferases (KATs), kynurenine monooxygenase (KMO) etc. The majority of metabolites are neuroactive: some of them, such as kynurenic acid, show neuroprotective effects, while others contribute to free radical production, leading to neurodegeneration. Imbalance of the pathway is assumed to contribute to the development of several neurodegenerative diseases, psychiatric disorders, migraine and multiple sclerosis. Our aim was to summarize published data on genetic alterations of enzymes involved in the KP leading to disturbances of the pathway that can be related to different diseases. To achieve this, a PubMed literature search was performed for publications on genetic alterations of the KP enzymes upto April 2017. Several genetic alterations of the KP have been identified and have been proposed to be associated with diseases. Here we must emphasize that despite the large number of recognized genetic alterations, the number of firmly established causal relations with specific diseases is still small. The realization of this by those interested in the field is very important and finding such connections should be a major focus of related research. Polymorphisms of the genes encoding the enzymes of the KP have been associated with autism, multiple sclerosis and schizophrenia, and were shown to affect the immune response of patients with bacterial meningitis, just to mention a few. To our knowledge, this is the first comprehensive review of the genetic alterations of the KP enzymes. We believe that the identification of genetic alterations underlying diseases has great value regarding both treatment and diagnostics in precision medicine, as this work can promote the understanding of pathological mechanisms, and might facilitate medicinal chemistry approaches to substitute missing components or correct the disturbed metabolite balance of KP.

A single amino acid residue regulates the substrate affinity and specificity of indoleamine 2,3-dioxygenase

Indoleamine 2,3-dioxygenase (IDO) is a heme-containing enzyme that catalyses the oxidative cleavage of L-Trp. The ciliate Blepharisma stoltei has four IDO genes (IDO-I, -II, -III and -IV), which seem to have evolved via two sequential gene duplication events. Each IDO enzyme has a distinct enzymatic property, where IDO-III has a high affinity for L-Trp, whereas the affinity of the other three isoforms for L-Trp is low. IDO-I also exhibits a significant catalytic activity with another indole compound: 5-hydroxy-l-tryptophan (5-HTP). IDO-I is considered to be an enzyme that is involved in the biosynthesis of the 5-HTP-derived mating pheromone, gamone 2. By analysing a series of chimeric enzymes based on extant and predicted ancestral enzymes, we identified Asn131 in IDO-I and Glu132 in IDO-III as the key residues responsible for their high affinity for each specific substrate. These two residues were aligned in an identical position as the substrate-determining residue (SDR). Thus, the substrate affinity and specificity are regulated mostly by a single amino acid residue in the Blepharisma IDO-I and IDO-III enzymes.

Investigation of the Tissue Distribution and Physiological Roles of Indoleamine 2,3-Dioxygenase-2

Indoleamine 2,3-dioxygenase-2 (IDO2) is 1 of the 3 enzymes that can catalyze the first step in the kynurenine pathway of tryptophan metabolism. Of the 2 other enzymes, tryptophan 2,3-dioxygenase is highly expressed in the liver and has a role in tryptophan homeostasis, whereas indoleamine 2,3-dioxygenase-1 (IDO1) expression is induced by inflammatory stimuli. Indoleamine 2,3-dioxygenase-2 is reportedly expressed comparatively narrow, including in liver, kidney, brain, and in certain immune cell types, and it does not appear to contribute significantly to systemic tryptophan catabolism under normal physiological conditions. Here, we report the identification of an alternative splicing pattern, including the use of an alternative first exon, that is conserved in the mouse Ido1 and Ido2 genes. These findings prompted us to assess IDO2 protein expression and enzymatic activity in tissues. Our analysis, undertaken in Ido2^+/+ and Ido2^−/− mice using immunohistochemistry and measurement of tryptophan and kynurenine levels, suggested an even more restricted pattern of tissue expression than previously reported. We found IDO2 protein to be expressed in the liver with a perinuclear/nuclear, rather than cytoplasmic, distribution. Consistent with earlier reports, we found Ido2 ^−/− mice to be phenotypically similar to their Ido2^+/+ counterparts regarding levels of tryptophan and kynurenine in the plasma and liver. Our findings suggest a specialized function or regulatory role for IDO2 associated with its particular subcellular localization.

Differential expression of novel metabolic and immunological biomarkers in oysters challenged with a virulent strain of OsHV-1

Early lifestages of the Pacific oyster (Crassostrea gigas) are highly susceptible to infection by OsHV-1 μVar, but little information exists regarding metabolic or pathophysiological responses of larval hosts. Using a metabolomics approach, we identified a range of metabolic and immunological responses in oyster larvae exposed to OsHV-1 μVar; some of which have not previously been reported in molluscs. Multivariate analyses of entire metabolite profiles were able to separate infected from non-infected larvae. Correlation analysis revealed the presence of major perturbations in the underlying biochemical networks and secondary pathway analysis of functionally-related metabolites identified a number of prospective pathways differentially regulated in virus-exposed larvae. These results provide new insights into the pathogenic mechanisms of OsHV-1 infection in oyster larvae, which may be applied to develop disease mitigation strategies and/or as new phenotypic information for selective breeding programmes aiming to enhance viral resistance.

Synthesis and evaluation of oxindoles as promising inhibitors of the immunosuppressive enzyme indoleamine 2,3-dioxygenase 1

Indoleamine 2,3-dioxygenase 1 (IDO1) is considered as an important therapeutic target for the treatment of cancer, chronic infections and other diseases that are associated with immune suppression. Recent developments in understanding the catalytic mechanism of the IDO1 enzyme revealed that conversion of l-tryptophan (l-Trp) to N-formylkynurenine proceeded through an epoxide intermediate state. Accordingly, we synthesized a series of 3-substituted oxindoles from l-Trp, tryptamine and isatin. Compounds with C3-substituted oxindole moieties showed moderate inhibitory activity against the purified human IDO1 enzyme. Their optimization led to the identification of potent compounds, 6, 22, 23 and 25 (IC₅₀ = 0.19 to 0.62 μM), which are competitive inhibitors of IDO1 with respect to l-Trp. These potent compounds also showed IDO1 inhibition potencies in the low-micromolar range (IC₅₀ = 0.33-0.49 μM) in MDA-MB-231 cells. The cytotoxicity of these potent compounds was trivial in different model cancer (MDA-MB-231, A549 and HeLa) cells and macrophage (J774A.1) cells. Stronger selectivity for the IDO1 enzyme (124 to 210-fold) over the tryptophan 2,3-dioxygenase (TDO) enzyme was also observed for these compounds. These results suggest that the oxindole moiety of the compounds could mimic the epoxide intermediate state of l-Trp. Therefore, the structural simplicity and low-micromolar inhibition potencies of these 3-substituted oxindoles make them quite attractive for further investigation of IDO1 function and immunotherapeutic applications.

Effect of water-immersion restraint stress on tryptophan catabolism through the kynurenine pathway in rat tissues

The aim of this study was to clarify the effect of water-immersion restraint stress (WIRS) on tryptophan (Trp) catabolism through the kynurenine (Kyn) pathway in rat tissues. The tissues of rats subjected to 6 h of WIRS (+WIRS) had increased tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO) activities and increased TDO and IDO1 (one of two IDO isozymes in mammals) mRNA expression levels, with decreased Trp and increased Kyn contents in the liver. +WIRS rats had unchanged TDO and IDO activities in the kidney, decreased TDO activity and unchanged IDO activity in the brain, and unchanged IDO activity in the lung and spleen, with increased Kyn content in all of these tissues. Pretreatment of stressed rats with RU486, a glucocorticoid antagonist, attenuated the increased TOD activity, but not the increased IDO activity, with partial recoveries of the decreased Trp and increased Kyn contents in the liver. These results indicate that WIRS enhances hepatic Trp catabolism by inducing both IDO1 and TDO in rats.

IDO2: A Pathogenic Mediator of Inflammatory Autoimmunity

Indoleamine 2,3-dioxygenase 2 (IDO2), a homolog of the better-studied tryptophan-catabolizing enzyme IDO1, is an immunomodulatory molecule with potential effects on various diseases including cancer and autoimmunity. Here, we review what is known about the direct connections between IDO2 and immune function, particularly in relationship to autoimmune inflammatory disorders such as rheumatoid arthritis and lupus. Accumulating evidence indicates that IDO2 acts as a pro-inflammatory mediator of autoimmunity, with a functional phenotype distinct from IDO1. IDO2 is expressed in antigen-presenting cells, including B cells and dendritic cells, but affects inflammatory responses in the autoimmune context specifically by acting in B cells to modulate T cell help in multiple model systems. Given that expression of IDO2 can lead to exacerbation of inflammatory responses, IDO2 should be considered a potential therapeutic target for autoimmune disorders.

Sea lampreys elicit strong transcriptomic responses in the lake trout liver during parasitism

BACKGROUND

The sea lamprey (Petromyzon marinus) is a jawless vertebrate that parasitizes fish as an adult and, with overfishing, was responsible for the decline in lake trout (Salvelinus namaycush) populations in the Great Lakes. While laboratory studies have looked at the rates of wounding on various fish hosts, there have been few investigations on the physiological effects of lamprey wounding on the host. In the current study, two morphotypes of lake trout, leans and siscowets, were parasitized in the laboratory by sea lampreys and the liver transcriptomes of parasitized and nonparasitized fish were analyzed by RNA-seq (DESeq2 and edgeR) to determine which genes and gene pathways (Ingenuity Pathway Analysis) were altered by lamprey parasitism.

RESULTS

Overall, genes encoding molecules involved in catalytic (e.g., enzymatic) and binding activities (factors and regulators) predominated the regulated gene lists. In siscowets, the top upregulated gene was growth arrest and DNA-damage-inducible protein and for leans it was interleukin-18-binding protein. In leans, the most significantly downregulated gene was UDP-glucuronosyltransferase 2A2 - DESeq2 or phosphotriesterase related - edgeR. For siscowets, the top downregulated gene was C-C motif chemokine 19 - DESeq2 or GTP-binding protein Rhes - edgeR. Gene pathways associated with inflammatory-related responses or factors (cytokines, chemokines, oxidative stress, apoptosis) were regulated following parasitism in both morphotypes. However, pathways related to energy metabolism (glycolysis, gluconeogenesis, lipolysis, lipogenesis) were also regulated. These pathways or the intensity or direction (up/downregulation) of regulation were different between leans and siscowets. Finally, one of the most significantly downregulated pathways in both leans and siscowets was the kynurenine (tryptophan degradation) pathway.

CONCLUSIONS

The results indicate a strong transcriptional response in the lake trout to lamprey parasitism that entails genes involved in the regulation of inflammation and cellular damage. Responses to energy utilization as well as hydromineral balance also occurred indicating an adjustment in the host to energy demands and osmotic imbalances during parasitism. Given the role of the kynurenine pathway in promoting immunotolerance in mammals, the downregulation observed in this pathway during parasitism may signify an attempt by the host to inhibit any feedback suppression of the immune response to the lamprey.

IDO2 Modulates T Cell-Dependent Autoimmune Responses through a B Cell-Intrinsic Mechanism

Mechanistic insight into how adaptive immune responses are modified along the self-nonself continuum may offer more effective opportunities to treat autoimmune disease, cancer, and other sterile inflammatory disorders. Recent genetic studies in the KRN mouse model of rheumatoid arthritis demonstrate that the immunomodulatory molecule IDO2 modifies responses to self-antigens; however, the mechanisms involved are obscure. In this study, we show that IDO2 exerts a critical function in B cells to support the generation of autoimmunity. In experiments with IDO2-deficient mice, adoptive transplant experiments demonstrated that IDO2 expression in B cells was both necessary and sufficient to support robust arthritis development. IDO2 function in B cells was contingent on a cognate, Ag-specific interaction to exert its immunomodulatory effects on arthritis development. We confirmed a similar requirement in an established model of contact hypersensitivity, in which IDO2-expressing B cells are required for a robust inflammatory response. Mechanistic investigations showed that IDO2-deficient B cells lacked the ability to upregulate the costimulatory marker CD40, suggesting IDO2 acts at the T-B cell interface to modulate the potency of T cell help needed to promote autoantibody production. Overall, our findings revealed that IDO2 expression by B cells modulates autoimmune responses by supporting the cross talk between autoreactive T and B cells.