Effects of 1-methyltryptophan stereoisomers on IDO2 enzyme activity and IDO2-mediated arrest of human T cell proliferation

Prolonged indoleamine 2,3-dioxygenase-2 activity and associated cellular stress in post-acute sequelae of SARS-CoV-2 infection

BACKGROUND

Post-acute sequela of SARS-CoV-2 infection (PASC) encompass fatigue, post-exertional malaise and cognitive problems. The abundant expression of the tryptophan-catabolizing enzyme indoleamine 2,3-dioxygenase-2 (IDO2) in fatal/severe COVID-19, led us to determine, in an exploratory observational study, whether IDO2 is expressed and active in PASC, and may correlate with pathophysiology.

METHODS

Plasma or serum, and peripheral blood mononuclear cells (PBMC) were obtained from well-characterized PASC patients and SARS-CoV-2-infected individuals without PASC. We assessed tryptophan and its degradation products by UPLC-MS/MS. IDO2 activity, its potential consequences, and the involvement of the aryl hydrocarbon receptor (AHR) in IDO2 expression were determined in PBMC from another PASC cohort by immunohistochemistry (IHC) for IDO2, IDO1, AHR, kynurenine metabolites, autophagy, and apoptosis. These PBMC were also analyzed by metabolomics and for mitochondrial functioning by respirometry. IHC was also performed on autopsy brain material from two PASC patients.

FINDINGS

IDO2 is expressed and active in PBMC from PASC patients, as well as in brain tissue, long after SARS-CoV-2 infection. This is paralleled by autophagy, and in blood cells by reduced mitochondrial functioning, reduced intracellular levels of amino acids and Krebs cycle-related compounds. IDO2 expression and activity is triggered by SARS-CoV-2-infection, but the severity of SARS-CoV-2-induced pathology appears related to the generated specific kynurenine metabolites. Ex vivo, IDO2 expression and autophagy can be halted by an AHR antagonist.

INTERPRETATION

SARS-CoV-2 infection triggers long-lasting IDO2 expression, which can be halted by an AHR antagonist. The specific kynurenine catabolites may relate to SARS-CoV-2-induced symptoms and pathology.

FUNDING

None.

The tryptophan-metabolizing enzyme indoleamine 2,3-dioxygenase 1 regulates polycystic kidney disease progression

Autosomal dominant polycystic kidney disease (ADPKD), the most common monogenic nephropathy, is characterized by phenotypic variability that exceeds genic effects. Dysregulated metabolism and immune cell function are key disease modifiers. The tryptophan metabolites, kynurenines, produced through indoleamine 2,3-dioxygenase 1 (IDO1), are known immunomodulators. Here, we study the role of tryptophan metabolism in PKD using an orthologous disease model (C57BL/6J Pkd1RC/RC). We found elevated kynurenine and IDO1 levels in Pkd1RC/RC kidneys versus wild type. Further, IDO1 levels were increased in ADPKD cell lines. Genetic Ido1 loss in Pkd1RC/RC animals resulted in reduced PKD severity, as measured by cystic index and percentage kidney weight normalized to body weight. Consistent with an immunomodulatory role of kynurenines, Pkd1RC/RC;Ido1-/- mice presented with significant changes in the cystic immune microenvironment (CME) versus controls. Kidney macrophage numbers decreased and CD8+ T cell numbers increased, both known PKD modulators. Also, pharmacological IDO1 inhibition in Pkd1RC/RC mice and kidney-specific Pkd2-knockout mice with rapidly progressive PKD resulted in less severe PKD versus controls, with changes in the CME similar to those in the genetic model. Our data suggest that tryptophan metabolism is dysregulated in ADPKD and that its inhibition results in changes to the CME and slows disease progression, making IDO1 a therapeutic target for ADPKD.

The exploitation of enzyme-based cancer immunotherapy

Cancer immunotherapy utilizes the immune system and its wide-ranging components to deliver anti-tumor responses. In immune escape mechanisms, tumor microenvironment-associated soluble factors and cell surface-bound molecules are mainly accountable for the dysfunctional activity of tumor-specific CD8⁺ T cells, natural killer (NK) cells, tumor associated macrophages (TAMs) and stromal cells. The myeloid-derived suppressor cells (MDSCs) and Foxp3⁺ regulatory T cells (Tregs), are also key tumor-promoting immune cells. These potent immunosuppressive networks avert tumor rejection at various stages, affecting immunotherapies' outcomes. Numerous clinical trials have elucidated that disruption of immunosuppression could be achieved via checkpoint inhibitors. Another approach utilizes enzymes that can restore the body's potential to counter cancer by triggering the immune system inhibited by the tumor microenvironment. These immunotherapeutic enzymes can catalyze an immunostimulatory signal and modulate the tumor microenvironment via effector molecules. Herein, we have discussed the immuno-metabolic roles of various enzymes like ATP-dephosphorylating ectoenzymes, inducible Nitric Oxide Synthase, phenylamine, tryptophan, and arginine catabolizing enzymes in cancer immunotherapy. Understanding the detailed molecular mechanisms of the enzymes involved in modulating the tumor microenvironment may help find new opportunities for cancer therapeutics.

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.

Discovery of the First Selective IDO2 Inhibitor As Novel Immunotherapeutic Avenues for Rheumatoid Arthritis

Indoleamine 2,3-dioxygenase 2 (IDO2), a closely related homologue of well-studied immunomodulatory enzyme IDO1, has been identified as a pathogenic mediator of inflammatory autoimmunity in preclinical models. Therapeutic targeting IDO2 in autoimmune diseases has been challenging due to the lack of small-molecule IDO2 inhibitors. Here, based on our previously developed IDO1/IDO2 dual inhibitor, guided by the homology model of the IDO2 structure, we discovered compound 22, the most potent inhibitor targeting IDO2 with good in vitro inhibitory activity (IDO2 IC₅₀ = 112 nM). Notably, treatment with 22 alleviated disease severity and reduced inflammatory cytokines in both the collagen-induced arthritis (CIA) mice model and adjuvant arthritis (AA) rat model. Our study offered for the first time a selective small-molecule IDO2 inhibitor 22 with IC₅₀ at the nanomolar level, which may be used not only as a candidate compound for the treatment of autoimmune diseases but also as a tool compound for further IDO2-related mechanistic study.

Fighting in a wasteland: deleterious metabolites and antitumor immunity

As cancers progress, they produce a local environment that acts to redirect, paralyze, exhaust, or otherwise evade immune detection and destruction. The tumor microenvironment (TME) has long been characterized as a metabolic desert, depleted of essential nutrients such as glucose, oxygen, and amino acids, that starves infiltrating immune cells and renders them dysfunctional. While not incorrect, this perspective is only half the picture. The TME is not a metabolic vacuum, only consuming essential nutrients and never producing by-products. Rather, the by-products of depleted nutrients, "toxic" metabolites in the TME such as lactic acid, kynurenine, ROS, and adenosine, play an important role in shaping immune cell function and cannot be overlooked in cancer immunotherapy. Moreover, while the metabolic landscape is distinct, it is not unique, as these toxic metabolites are encountered in non-tumor tissues, where they evolutionarily shape immune cells and their response. In this Review, we discuss how depletion of essential nutrients and production of toxic metabolites shape the immune response within the TME and how toxic metabolites can be targeted to improve current cancer immunotherapies.

Tryptophan Metabolism in Atherosclerosis and Diabetes

The essential amino acid tryptophan (Trp) undergoes catabolism through several pathways, producing biologically active metabolites that significantly impact physiological processes. The metabolic pathway responsible for the majority of Trp catabolism is the kynurenine synthesis pathway (KP). Serotonin and melatonin are among the most essential Trp pathways degradation products. It has emerged that a strong relationship exists between alterations in Trp metabolism and the onset and progression of atherosclerosis and diabetes. Atherosclerosis is a chronic inflammatory disease of the small and medium arteries wall caused by maladaptive local immune responses, which underpins several cardiovascular diseases (CVD). Systemic low-grade immune-mediated inflammation is implicated in atherosclerosis where pro-inflammatory cytokines, such as interferon-γ (IFN-γ), play a significant role. IFN-γ upregulates the enzyme indoleamine 2,3-dioxygenase (IDO), decreasing serum levels of the Trp and increasing metabolite levels of kynurenine. Increased IDO expression and activity could accelerate the atherosclerosis process. Therefore, activated IDO inhibition could offer possible treatment options regarding atherosclerosis management. Diabetes is a chronic metabolic disease characterized by hyperglycemia that, over time, leads to severe damage to the heart, blood vessels, eyes, kidneys, and peripheral nerves. Trp serum levels and lower activity of IDO were higher in future type 2 diabetes (T2DM) patients. This article reviews recent findings on the link between mammalian Trp metabolism and its role in atherosclerosis and diabetes and outlines the intervention strategies.

Tumor Cell IDO Enhances Immune Suppression and Decreases Survival Independent of Tryptophan Metabolism in Glioblastoma

PURPOSE

Glioblastoma (GBM) is an incurable primary brain tumor that has not benefited from immunotherapy to date. More than 90% of GBM expresses the tryptophan (Trp) metabolic enzyme, indoleamine 2,3-dioxygenase 1 (IDO). This observation supported the historical hypothesis that IDO suppresses the antitumor immune response solely through a mechanism that requires intratumoral Trp depletion. However, recent findings led us to investigate the alternative hypothesis that IDO suppresses the anti-GBM immune response independent of its association with Trp metabolism.

EXPERIMENTAL DESIGN

IDO-deficient GBM cell lines reconstituted with IDO wild-type or IDO enzyme-null cDNA were created and validated in vitro and in vivo. Microarray analysis was conducted to search for genes that IDO regulates, followed by the analysis of human GBM cell lines, patient GBM and plasma, and The Cancer Genome Atlas (TCGA) database. Ex vivo cell coculture assays, syngeneic and humanized mouse GBM models, were used to test the alternative hypothesis.

RESULTS

Nonenzymic tumor cell IDO activity decreased the survival of experimental animals and increased the expression of complement factor H (CFH) and its isoform, factor H like protein 1 (FHL-1) in human GBM. Tumor cell IDO increased CFH and FHL-1 expression independent of Trp metabolism. Increased intratumoral CFH and FHL-1 levels were associated with poorer survival among patients with glioma. Similar to IDO effects, GBM cell FHL-1 expression increased intratumoral regulatory T cells (Treg) and myeloid-derived suppressor cells while it decreased overall survival in mice with GBM.

CONCLUSIONS

Our study reveals a nonmetabolic IDO-mediated enhancement of CFH expression and provides a new therapeutic target for patients with GBM.

IDO1 Expression in Ovarian Cancer Induces PD-1 in T Cells via Aryl Hydrocarbon Receptor Activation

The immunoregulatory enzyme, indoleamine 2,3-dioxygenase (IDO1) and the PD-1/PD-L1 axis are potent mechanisms that impede effective anti-tumor immunity in ovarian cancer. However, whether the IDO pathway regulates PD-1 expression in T cells is currently unknown. Here we show that tumoral IDO1 expression led to profound changes in tryptophan, nicotinate/nicotinamide, and purine metabolic pathways in the ovarian tumor microenvironment, and to an increased frequency of PD-1+CD8+ tumor infiltrating T cells. We determined that activation of the aryl hydrocarbon receptor (AHR) by kynurenine induced PD-1 expression, and this effect was significantly abrogated by the AHR antagonist CH223191. Mechanistically, kynurenine alters chromatin accessibility in regulatory regions of T cell inhibitory receptors, allowing AHR to bind to consensus XRE motifs in the promoter region of PD-1. These results enable the design of strategies to target the IDO1 and AHR pathways for enhancing anti-tumor immunity in ovarian cancer.

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.

IDO inhibits a tryptophan sufficiency signal that stimulates mTOR: A novel IDO effector pathway targeted by D-1-methyl-tryptophan

Tryptophan catabolism by indoleamine 2,3-dioxygenase (IDO) alters inflammation and favors T-cell tolerance in cancer, but the underlying molecular mechanisms remain poorly understood. The integrated stress response kinase GCN2, a sensor of uncharged tRNA that is activated by amino acid deprivation, is recognized as an important effector of the IDO pathway. However, in a mouse model of inflammatory carcinogenesis, ablation of Gcn2 did not promote resistance against tumor development like the absence of IDO does, implying the existence of additional cancer-relevant pathways that operate downstream of IDO. Addressing this gap in knowledge, we report that the IDO-mediated catabolism of tryptophan also inhibits the immunoregulatory kinases mTOR and PKC-Θ, along with the induction of autophagy. These effects were relieved specifically by tryptophan but also by the experimental agent 1-methyl-D-tryptophan (D-1MT, also known as NLG8189), the latter of which reversed the inhibitory signals generated by IDO with higher potency. Taken together, our results implicate mTOR and PKC-Θ in IDO-mediated immunosuppressive signaling, and they provide timely insights into the unique mechanism of action of D-1MT as compared with traditional biochemical inhibitors of IDO. These findings are important translationally, because they suggest broader clinical uses for D-1MT against cancers that overexpress any tryptophan catabolic enzyme (IDO, IDO2 or TDO). Moreover, they define mTOR and PKC-Θ as candidate pharmacodynamic markers for D-1MT responses in patients recruited to ongoing phase IB/II cancer trials, addressing a current clinical need.

Development of Therapeutic Vaccines for Ovarian Cancer

Ovarian cancer remains the deadliest of all gynecologic malignancies. Our expanding knowledge of ovarian cancer immunology has allowed the development of therapies that generate systemic anti-tumor immune responses. Current immunotherapeutic strategies include immune checkpoint blockade, cellular therapies, and cancer vaccines. Vaccine-based therapies are designed to induce both adaptive and innate immune responses directed against ovarian cancer associated antigens. Tumor-specific effector cells, in particular cytotoxic T cells, are activated to recognize and eliminate ovarian cancer cells. Vaccines for ovarian cancer have been studied in various clinical trials over the last three decades. Despite evidence of vaccine-induced humoral and cellular immune responses, the majority of vaccines have not shown significant anti-tumor efficacy. Recently, improved vaccine development using dendritic cells or synthetic platforms for antigen presentation have shown promising clinical benefits in patients with ovarian cancer. In this review, we provide an overview of therapeutic vaccine development in ovarian cancer, discuss proposed mechanisms of action, and summarize the current clinical experience.

Differential Roles of IDO1 and IDO2 in T and B Cell Inflammatory Immune Responses

Indoleamine-2,3-dioxygenase (IDO)1 and IDO2 are two closely related tryptophan catabolizing enzymes encoded by linked genes. The IDO pathway is also immunomodulatory, with IDO1 well-characterized as a mediator of tumor immune evasion. Due to its homology with IDO1, IDO2 has been proposed to have a similar immunoregulatory function. Indeed, IDO2, like IDO1, is necessary for the differentiation of regulatory T cells in vitro. However, compared to IDO1, in vivo studies demonstrated a contrasting role for IDO2, with experiments in preclinical models of autoimmune arthritis establishing a proinflammatory role for IDO2 in mediating B and T cell activation driving autoimmune disease. Given their potentially opposing roles in inflammatory responses, interpretation of results obtained using IDO1 or IDO2 single knockout mice could be complicated by the expression of the other enzyme. Here we use IDO1 and IDO2 single and double knockout (dko) mice to define the differential roles of IDO1 and IDO2 in B cell-mediated immune responses. Autoreactive T and B cell responses and severity of joint inflammation were decreased in IDO2 ko, but not IDO1 ko arthritic mice. Dko mice had a reduction in the number of autoantibody secreting cells and severity of arthritis: however, percentages of differentiated T cells and their associated cytokines were not reduced compared to IDO1 ko or wild-type mice. These data suggest that autoreactive B cell responses are mediated by IDO2, while autoreactive T cell responses are indirectly affected by IDO1 expression in the IDO2 ko mice. IDO2 also influenced antibody responses in models of influenza infection and immunization with T cell-independent type II antigens. Taken together, these studies provide evidence for the contrasting roles IDO1 and IDO2 play in immune responses, with IDO1 mediating T cell suppressive effects and IDO2 working directly in B cells as a proinflammatory mediator of B cell responses.

Silencing IDO2 in dendritic cells: A novel strategy to strengthen cancer immunotherapy in a murine lung cancer model

While dendritic cell (DC)‑based immunotherapy has achieved satisfactory results in animal models, its effects were not satisfactory as initially expected in clinical applications, despite the safety and varying degrees of effectiveness in various types of cancer. Improving the efficacy of the DC‑based vaccine is essential for cancer immunotherapy. The present study aimed to investigate methods with which to amplify and enhance the antitumor immune response of a DC‑based tumor vaccine by silencing the expression of indoleamine 2,3‑dioxygenase 2 (IDO2), a tryptophan rate‑limiting metabolic enzyme in DCs. In vitro experiments revealed that the silencing of IDO2 in DCs did not affect the differentiation of DCs, whereas it increased their expression of costimulatory molecules following stimulation with tumor necrosis factor (TNF)‑α and tumor lysate from Lewis lung cancer (LLC) cells. In a mixed co‑culture system, the IDO2‑silenced DCs promoted the proliferation of T‑cells and reduced the induction of regulatory T‑cells (Tregs). Further in vivo experiments revealed that the silencing of IDO2 in DCs markedly suppressed the growth of tumor cells. Moreover, treatment with the IDO2‑silenced DC‑based cancer vaccine enhanced cytotoxic T lymphocyte activity, whereas it decreased T‑cell apoptosis and the percentage of CD4+CD25+Foxp3+ Tregs. On the whole, the present study provides evidence that the silencing of the tryptophan rate‑limiting metabolic enzyme, IDO2, has the potential to enhance the efficacy of DC‑based cancer immunotherapy.

Both IDO1 and TDO contribute to the malignancy of gliomas via the Kyn-AhR-AQP4 signaling pathway

Indoleamine 2,3-dioxygenase 1 (IDO1), indoleamine 2,3-dioxygenase 2 (IDO2), and tryptophan 2,3-dioxygenase (TDO) initiate the first step of the kynurenine pathway (KP), leading to the transformation of L-tryptophan (Trp) into L-kynurenine (Kyn) and other downstream metabolites. Kyn is known as an endogenous ligand of the aryl hydrocarbon receptor (AhR). Activation of AhR through TDO-derived Kyn is a novel mechanism to support tumor growth in gliomas. However, the role of IDO1 and IDO2 in this mechanism is still unknown. Herein, by using clinical samples, we found that the expression and activity of IDO1 and/or TDO (IDO1/TDO) rather than IDO2 were positively correlated with the pathologic grades of gliomas. The expression of IDO1/TDO rather than IDO2 was positively correlated with the Ki67 index and overall survival. The expression of IDO1/TDO was positively correlated with the expression of aquaporin 4 (AQP4), implying the potential involvement of IDO1/TDO in glioma cell motility. Mechanistically, we found that IDO1/TDO accounted for the release of Kyn, which activated AhR to promote cell motility via the Kyn-AhR-AQP4 signaling pathway in U87MG glioma cells. RY103, an IDO1/TDO dual inhibitor, could block the IDO1/TDO-Kyn-AhR-AQP4 signaling pathway and exert anti-glioma effects in GL261 orthotopic glioma mice. Together, our results showed that the IDO1/TDO-Kyn-AhR-AQP4 signaling pathway is a new mechanism underlying the malignancy of gliomas, and suggest that both IDO1 and TDO might be valuable therapeutic targets for gliomas.

The therapeutic potential of targeting tryptophan catabolism in cancer

Based on its effects on both tumour cell intrinsic malignant properties as well as anti-tumour immune responses, tryptophan catabolism has emerged as an important metabolic regulator of cancer progression. Three enzymes, indoleamine-2,3-dioxygenase 1 and 2 (IDO1/2) and tryptophan-2,3-dioxygenase (TDO2), catalyse the first step of the degradation of the essential amino acid tryptophan (Trp) to kynurenine (Kyn). The notion of inhibiting IDO1 using small-molecule inhibitors elicited high hopes of a positive impact in the field of immuno-oncology, by restoring anti-tumour immune responses and synergising with other immunotherapies such as immune checkpoint inhibition. However, clinical trials with IDO1 inhibitors have yielded disappointing results, hence raising many questions. This review will discuss strategies to target Trp-degrading enzymes and possible down-stream consequences of their inhibition. We aim to provide comprehensive background information on Trp catabolic enzymes as targets in immuno-oncology and their current state of development. Details of the clinical trials with IDO1 inhibitors, including patient stratification, possible effects of the inhibitors themselves, effects of pre-treatments and the therapies the inhibitors were combined with, are discussed and mechanisms proposed that might have compensated for IDO1 inhibition. Finally, alternative approaches are suggested to circumvent these problems.

N-Benzyl/Aryl Substituted Tryptanthrin as Dual Inhibitors of Indoleamine 2,3-Dioxygenase and Tryptophan 2,3-Dioxygenase

Indoleamine 2,3-dioxygenase 1 (IDO1), which catalyzes the initial and rate-limiting step of the kynurenine pathway of tryptophan catabolism, has emerged as a key target in cancer immunotherapy because of its role in enabling cancers to evade the immune system. Tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase 2 (IDO2) catalyze the same reaction and play a potential role in cancer immunotherapy. Starting from our previously discovered tryptanthrin IDO1 inhibitor scaffold, we synthesized novel N-benzyl/aryl substituted tryptanthrin derivatives and evaluated their inhibitory efficacy on IDO1, TDO, and IDO2. Most compounds showed similar high inhibitory activities on both IDO1 and TDO, which were significantly superior over that of IDO2 with magnitude difference. We showed that N-benzyl/aryl substituted tryptanthrin directly interacted with IDO1, TDO, and IDO2, significantly augmented the proliferation of T cells in vitro, blocked the kynurenine pathway, and suppressed tumor growth when administered to LLC and H22 tumor-bearing mice.

Toxoplasma Effector TgIST Targets Host IDO1 to Antagonize the IFN-γ-Induced Anti-parasitic Response in Human Cells

Toxoplasma gondii is an important human and animal pathogen that causes life-threatening toxoplasmosis. Interferon-γ (IFN-γ) is critical for anti-T. gondii cell-autonomous immunity in both humans and mice. To proliferate efficiently within the hosts, virulent strains of T. gondii can suppress IFN-γ-dependent immunity. During parasite infection, it is well-characterized that various virulence effectors are secreted to transcriptionally or post-translationally target IFN-γ-inducible GTPases, which are essential for anti-parasite responses in mice. However, the role of IFN-γ-inducible GTPases in anti-T. gondii responses in human cells is controversial since they are non-functional or absent in humans. Instead, IFN-γ-induced tryptophan degradation by indole-2,3-dioxygenase (IDO) is important for the anti-T. gondii human response. To date, the T. gondii virulent mechanism targeting IDO in human cells remains elusive. Here we show that although humans possess two IDO isozymes, IDO1 and IDO2, human cells of various origins require IDO1 but not IDO2 for IFN-γ-induced cell-autonomous immunity to T. gondii. T. gondii secretes an effector TgIST to inhibit IDO1 mRNA expression. Taken together, the data suggests that T. gondii possesses virulence programs operated by TgIST to antagonize IFN-γ-induced IDO1-mediated anti-parasite cell-autonomous immunity in human cells.

Indoximod: An Immunometabolic Adjuvant That Empowers T Cell Activity in Cancer

Exploding interest in immunometabolism as a source of new cancer therapeutics has been driven in large part by studies of tryptophan catabolism mediated by IDO/TDO enzymes. A chief focus in the field is IDO1, a pro-inflammatory modifier that is widely overexpressed in cancers where it blunts immunosurveillance and enables neovascularization and metastasis. The simple racemic compound 1-methyl-D,L-tryptophan (1MT) is an extensively used probe of IDO/TDO pathways that exerts a variety of complex inhibitory effects. The L isomer of 1MT is a weak substrate for IDO1 and is ascribed the weak inhibitory activity of the racemate on the enzyme. In contrast, the D isomer neither binds nor inhibits the purified IDO1 enzyme. However, clinical development focused on D-1MT (now termed indoximod) due to preclinical cues of its greater anticancer activity and its distinct mechanisms of action. In contrast to direct enzymatic inhibitors of IDO1, indoximod acts downstream of IDO1 to stimulate mTORC1, a convergent effector signaling molecule for all IDO/TDO enzymes, thus possibly lowering risks of drug resistance by IDO1 bypass. In this review, we survey the unique biological and mechanistic features of indoximod as an IDO/TDO pathway inhibitor, including recent clinical findings of its ability to safely enhance various types of cancer therapy, including chemotherapy, chemo-radiotherapy, vaccines, and immune checkpoint therapy. We also review the potential advantages indoximod offers compared to selective IDO1-specific blockade, which preclinical studies and the clinical study ECHO-301 suggest may be bypassed readily by tumors. Indoximod lies at a leading edge of broad-spectrum immunometabolic agents that may act to improve responses to many anticancer modalities, in a manner analogous to vaccine adjuvants that act to boost immunity in settings of infectious disease.

The enhanced antitumour response of pimozide combined with the IDO inhibitor L‑MT in melanoma

Melanoma is one of the most fatal and therapy-resistant types of cancer; therefore, identifying novel therapeutic candidates to improve patient survival is an ongoing effort. Previous studies have revealed that pimozide is not sufficient to treat melanoma; therefore, enhancing the treatment is necessary. Indoleamine 2, 3‑dioxygenase (IDO) is an immunosuppressive, intracellular rate-limiting enzyme, which contributes to immune tolerance in various tumours, including melanoma, and inhibition of IDO may be considered a novel therapeutic strategy when combined with pimozide. The present study aimed to assess the antitumour activities of pimozide in vitro, and to investigate the effects of pimozide combined with L‑methyl-tryptophan (L‑MT) in vivo. For in vitro analyses, the B16 melanoma cell line was used. Cell cytotoxicity assay, cell viability assay, wound‑healing assay and western blotting were conducted to analyse the effects of pimozide on B16 cells. Furthermore, B16 cell-bearing mice were established as the animal model. Haematoxylin and eosin staining, immunohistochemistry, terminal deoxynucleotidyl transferase dUTP nick end-labelling staining, western blotting and flow cytometry were performed to determine the effects of monotherapy and pimozide and L‑MT cotreatment on melanoma. The results demonstrated that pimozide exhibited potent antitumour activity via the regulation of proliferation, apoptosis and migration. Furthermore, the antitumour effects of pimozide were enhanced when combined with L‑MT, not only via regulation of proliferation, apoptosis and migration, but also via immune modulation. Notably, pimozide may regulate tumour immunity through inhibiting the activities of signal transducer and activator of transcription (Stat)3 and Stat5. In conclusion, the present study proposed the use of pimozide in combination with the IDO inhibitor, L‑MT, as a potential novel therapeutic strategy for the treatment of melanoma.

Gene silencing of indoleamine 2,3-dioxygenase 2 in melanoma cells induces apoptosis through the suppression of NAD+ and inhibits in vivo tumor growth

Indoleamine 2,3-dioxygenase 2 (IDO2) is a newly discovered enzyme that catalyzes the initial and rate-limiting step in the degradation of tryptophan. As a homologous protein of IDO1, IDO2 plays an inhibitory role in T cell proliferation, and it is essential for regulatory T cell (Treg) generation in healthy conditions. Little is known about the immune-independent functions of IDO2 relevant to its specific contributions to physiology and pathophysiology in cancer cells. The purpose of this study was to assess the impact of IDO2 gene silencing as a way to inhibit B16-BL6 cancer cells in a murine model. Here, for the first time, we show that knockdown of IDO2 using small interfering RNA (siRNA) inhibits cancer cell proliferation, arrests cell cycle in G1, induces greater cell apoptosis, and reduces cell migration in vitro. Knockdown of IDO2 decreased the generation of nicotinamide adenine dinucleotide (NAD+) while increasing the generation of reactive oxygen species (ROS). We further demonstrate that cell apoptosis, induced by IDO2 downregulation, can be weakened by addition of exogenous NAD+, suggesting a novel mechanism by which IDO2 promotes tumor growth through its metabolite product NAD+. In addition to in vitro findings, we also demonstrate that IDO2 silencing in tumor cells using short hairpin RNA (shRNA) delayed tumor formation and arrested tumor growth in vivo. In conclusion, this study demonstrates a new non-immune-associated mechanism of IDO2 in vitro and IDO2 expression in B16-BL6 cells contributes to cancer development and progression. Our research provides evidence of a novel target for gene silencing that has the potential to enhance cancer therapy.

Immunotherapy in ovarian cancer

Immunological destruction of tumors is a multistep, coordinated process that can be modulated or targeted at several critical points to elicit tumor rejection. These steps in the cancer immunity cycle include: (i) generation of sufficient numbers of effector T cells with high avidity recognition of tumor antigens in vivo; (ii) trafficking and infiltration into the tumor; (iii) overcoming inhibitory networks in the tumor microenvironment; (iv) direct recognition of tumor antigens and generation of an effector anti-tumor response; and (v) persistence of the anti-tumor T cells. In an effort to understand whether the immune system plays a role in controlling ovarian cancer, our group and others demonstrated that the presence of tumor infiltrating lymphocytes (TILs) is associated with improved clinical outcome in ovarian cancer patients. Recently, we hypothesized that the quality of infiltrating T cells could also be a critical determinant of outcome in ovarian cancer patients. In the past decade, several immune-based interventions have gained regulatory approval in many solid tumors and hematologic malignancies. These interventions include immune checkpoint blockade, cancer vaccines, and adoptive cell therapy. There are currently no approved immune therapies for ovarian cancer. Immunotherapy in ovarian cancer will have to consider the immune suppressive networks within the ovarian tumor microenvironment; therefore, a major direction is to develop biomarkers that would predict responsiveness to different types of immunotherapies, and allow for treatment selection based on the results. Moreover, such biomarkers would allow rational combination of immunotherapies, while minimizing toxicities. In this review, the current understanding of the host immune response in ovarian cancer patients will be briefly reviewed, progress in immune therapies, and future directions for exploiting immune based strategies for long lasting durable cure.

Major Developments in the Design of Inhibitors along the Kynurenine Pathway

Disrupted kynurenine pathway (KP) metabolism has been implicated in the progression of neurodegenerative disease, psychiatric disorders and cancer. Modulation of enzyme activity along this pathway may therefore offer potential new therapeutic strategies for these conditions. Considering their prominent positions in the KP, the enzymes indoleamine 2,3-dioxygenase, kynurenine 3-monooxygenase and kynurenine aminotransferase, appear the most attractive targets. Already, increasing interest in this pathway has led to the identification of a number of potent and selective enzyme inhibitors with promising pre-clinical data and the elucidation of several enzyme crystal structures provides scope to rationalize the molecular mechanisms of inhibitor activity. The field seems poised to yield one or more inhibitors that should find clinical utility.

Tryptophan Catabolism and Cancer Immunotherapy Targeting IDO Mediated Immune Suppression

Over the last decade, tryptophan catabolism has been firmly established as a powerful mechanism of innate and adaptive immune tolerance. The catabolism of tryptophan is a central pathway maintaining homeostasis by preventing autoimmunity or immunopathology that would result from uncontrolled and overreacting immune responses. This is driven by the key and rate-limiting enzymes indoleamine-2,3-dioxygenase 1 (IDO1) and tryptophan-2,3-dioxygenase 2 (TDO), resulting in local depletion of tryptophan, while tryptophan catabolites accumulate, including kynurenine and its derivatives, depending on the presence of downstream enzymes in the kynurenine pathway. These metabolic modifications result in a local microenvironment that is profoundly immunosuppressive, as a result of various mechanisms whose respective role remains incompletely characterized. Drugs targeting this pathway, specifically IDO1, are already in clinical trials with the aim at reverting cancer-induced immunosuppression. Recent studies have demonstrated favorable pharmacokinetics profiles for first-generation (Indoximod NLG8189) and second-generation IDO1 inhibitors (INCB024360 and NLG919). Targeting tryptophan catabolism in combination with additional methods of therapy may improve efficacy of cancer immunotherapy. These methods include, but are not limited to vaccination, adoptive cellular therapy, checkpoint inhibitor blockade, and cyclooxygenase-2 (COX2) inhibition. Over the last decade, there has been a considerable increase in our understanding of the regulation and downstream mediators of tryptophan metabolism. This detailed understanding will expand opportunities to interfere with the pathway therapeutically on multiple levels. The object of this chapter is to highlight current and past key findings that implicate tryptophan catabolism as an important mediator of cancer immunity and discuss the development of multiple therapeutic targets.

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.

Trial Watch-Small molecules targeting the immunological tumor microenvironment for cancer therapy

Progressing malignancies establish robust immunosuppressive networks that operate both systemically and locally. In particular, as tumors escape immunosurveillance, they recruit increasing amounts of myeloid and lymphoid cells that exert pronounced immunosuppressive effects. These cells not only prevent the natural recognition of growing neoplasms by the immune system, but also inhibit anticancer immune responses elicited by chemo-, radio- and immuno therapeutic interventions. Throughout the past decade, multiple strategies have been devised to counteract the accumulation or activation of tumor-infiltrating immunosuppressive cells for therapeutic purposes. Here, we review recent preclinical and clinical advances on the use of small molecules that target the immunological tumor microenvironment for cancer therapy. These agents include inhibitors of indoleamine 2,3-dioxigenase 1 (IDO1), prostaglandin E2, and specific cytokine receptors, as well as modulators of intratumoral purinergic signaling and arginine metabolism.

Indolamine 2,3-dioxygenase expression by monocytes and dendritic cell populations in hepatitis C patients

Dendritic cells (DCs) play an important role in the induction of the primary immune response to infection. DCs may express the tryptophan-catabolizing enzyme indolamine2,3-dioxygenase (IDO), which is an inducer of immune tolerance. Because there is evidence that chronic hepatitis C virus (HCV) infection leads to functional impairment of certain DC populations, we analysed IDO expression in DCs and monocytes from chronically infected and recovered HCV patients. The IDO1 and -2 expression was increased significantly in the monocytes of chronic HCV patients but, interestingly, not in those from recovered patients. The myeloid DCs from chronically infected HCV patients also showed enhanced IDO1 expression, while no change in either IDO1 or -2 was found for plasmacytoid DCs. Up-regulation of IDO1 gene expression was confirmed by the presence of enhanced kynurenine/tryptophan ratios in the plasma from chronic HCV patients. Increased IDO1 and -2 expression was also observed in monocytes from healthy donors infected with an adapted mutant of the HCV JFH-1 strain ex vivo, confirming a direct effect of HCV infection. These changes in IDO expression could be prevented by treatment with the IDO inhibitor 1-methyl tryptophan (1-mT). Furthermore, maturation of monocyte-derived DCs from chronically infected HCV patients, as well as well as monocyte-derived DCs infected ex vivo with HCV, was impaired, but this was reversed by 1-mT treatment. This suggests that IDO inhibitors may be used to treat chronic HCV patients in vivo, in conjunction with current therapies, or to activate DCs from patients ex vivo, such that they can be administered back as a DC-based therapeutic vaccine.

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.

Interferon Lambda Upregulates IDO1 Expression in Respiratory Epithelial Cells After Influenza Virus Infection

Influenza infection causes an increase in indoleamine 2, 3-dioxygenase (IDO) activity in the lung parenchyma. IDO catabolizes tryptophan into kynurenine, leading to immune dampening. Multiple cell types express IDO, and while IFN-γ upregulates IDO in dendritic cells and macrophages, it is unclear how IDO is affected in respiratory epithelial cells during influenza infection. In this study, the role of IFN-λ in IDO regulation was investigated after influenza infection of respiratory epithelial cells. IDO1 expression increased concurrently with IFN-λ expression. In differentiated NHBE cells, the IDO metabolite was released basolaterally. Recombinant IFN-λ upregulated IDO1 activity, and silencing of IFN-λ decreased IDO1 expression during influenza infection. During IFN-λ stimulation, most differentiated cell types are able to express IDO but during influenza infection, IDO is primarily expressed in uninfected cells. These studies show a role for IDO in the host response to influenza infection, and they provide insights into novel approaches for enhancing vaccine responses and therapeutic approaches.

The role of IDO in brain tumor immunotherapy

Malignant glioma comprises the majority of primary brain tumors. Coincidently, most of those malignancies express an inducible tryptophan catabolic enzyme, indoleamine 2,3 dioxygenase 1 (IDO1). While IDO1 is not normally expressed at appreciable levels in the adult central nervous system, it's rapidly induced and/or upregulated upon inflammatory stimulus. The primary function of IDO1 is associated with conversion of the essential amino acid, tryptophan, into downstream catabolites known as kynurenines. The depletion of tryptophan and/or accumulation of kynurenine has been shown to induce T cell deactivation, apoptosis and/or the induction of immunosuppressive programming via the expression of FoxP3. This understanding has informed immunotherapeutic design for the strategic development of targeted molecular therapeutics that inhibit IDO1 activity. Here, we review the current knowledge of IDO1 in brain tumors, pre-clinical studies targeting this enzymatic pathway, alternative tryptophan catabolic mediators that compensate for IDO1 loss and/or inhibition, as well as proposed clinical strategies and questions that are critical to address for increasing future immunotherapeutic effectiveness in patients with incurable brain cancer.

Trial watch: IDO inhibitors in cancer therapy

Indoleamine 2,3-dioxigenase 1 (IDO1) is the main enzyme that catalyzes the first, rate-limiting step of the so-called "kynurenine pathway", i.e., the metabolic cascade that converts the essential amino acid L-tryptophan (Trp) into L-kynurenine (Kyn). IDO1, which is expressed constitutively by some tissues and in an inducible manner by specific subsets of antigen-presenting cells, has been shown to play a role in the establishment and maintenance of peripheral tolerance. At least in part, this reflects the capacity of IDO1 to restrict the microenvironmental availability of Trp and to favor the accumulation of Kyn and some of its derivatives. Also, several neoplastic lesions express IDO1, providing them with a means to evade anticancer immunosurveillance. This consideration has driven the development of several IDO1 inhibitors, some of which (including 1-methyltryptophan) have nowadays entered clinical evaluation. In animal tumor models, the inhibition of IDO1 by chemical or genetic interventions is indeed associated with the (re)activation of therapeutically relevant anticancer immune responses. This said, several immunotherapeutic regimens exert robust clinical activity in spite of their ability to promote the expression of IDO1. Moreover, 1-methyltryptophan has recently been shown to exert IDO1-independent immunostimulatory effects. Here, we summarize the preclinical and clinical studies testing the antineoplastic activity of IDO1-targeting interventions.

IDO2 in Immunomodulation and Autoimmune Disease

IDO2 is a relative of IDO1 implicated in tryptophan catabolism and immune modulation but its specific contributions to normal physiology and pathophysiology are not known. Evolutionary genetic studies suggest that IDO2 has a unique function ancestral to IDO1. In mice, IDO2 gene deletion does not appreciably affect embryonic development or hematopoiesis, but it leads to defects in allergic or autoimmune responses and in the ability of IDO1 to influence the generation of T regulatory cells. Gene expression studies indicate that IDO2 is a basally and more narrowly expressed gene than IDO1 and that IDO2 is uniquely regulated by AhR, which serves as a physiological receptor for the tryptophan catabolite kynurenine. In the established KRN transgenic mouse model of rheumatoid arthritis, where IDO1 gene deletion has no effect, IDO2 deletion selectively blunts responses to autoantigen but has no effect on responses to neoantigen challenge. In human populations, natural variations in IDO2 gene sequence that attenuate enzymatic activity have been reported to influence brain cancer control and adaptive immune responses to the IDO2 protein itself, consistent with the concept that IDO2 is involved in shaping immune tolerance in human beings. Biochemical and pharmacological studies provide further evidence of differences in IDO2 enzymology and function relative to IDO1. We suggest that IDO2 may act in a distinct manner from IDO1 as a set-point for tolerance to "altered-self" antigens along the self-non-self continuum where immune challenges from cancer and autoimmunity may arise.

Tryptophan-catabolizing enzymes - party of three

Indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO) are tryptophan-degrading enzymes that have independently evolved to catalyze the first step in tryptophan catabolism via the kynurenine pathway (KP). The depletion of tryptophan and formation of KP metabolites modulates the activity of the mammalian immune, reproductive, and central nervous systems. IDO and TDO enzymes can have overlapping or distinct functions depending on their expression patterns. The expression of TDO and IDO enzymes in mammals differs not only by tissue/cellular localization but also by their induction by distinct stimuli. To add to the complexity, these genes also have undergone duplications in some organisms leading to multiple isoforms of IDO or TDO. For example, many vertebrates, including all mammals, have acquired two IDO genes via gene duplication, although the IDO1-like gene has been lost in some lower vertebrate lineages. Gene duplications can allow the homologs to diverge and acquire different properties to the original gene. There is evidence for IDO enzymes having differing enzymatic characteristics, signaling properties, and biological functions. This review analyzes the evolutionary convergence of IDO and TDO enzymes as tryptophan-catabolizing enzymes and the divergent evolution of IDO homologs to generate an enzyme family with diverse characteristics not possessed by TDO enzymes, with an emphasis on the immune system.

Indoleamine 2,3-dioxygenase pathways of pathogenic inflammation and immune escape in cancer

Genetic and pharmacological studies of indoleamine 2,3-dioxygenase (IDO) have established this tryptophan catabolic enzyme as a central driver of malignant development and progression. IDO acts in tumor, stromal and immune cells to support pathogenic inflammatory processes that engender immune tolerance to tumor antigens. The multifaceted effects of IDO activation in cancer include the suppression of T and NK cells, the generation and activation of T regulatory cells and myeloid-derived suppressor cells, and the promotion of tumor angiogenesis. Mechanistic investigations have defined the aryl hydrocarbon receptor, the master metabolic regulator mTORC1 and the stress kinase Gcn2 as key effector signaling elements for IDO, which also exerts a non-catalytic role in TGF-β signaling. Small-molecule inhibitors of IDO exhibit anticancer activity and cooperate with immunotherapy, radiotherapy or chemotherapy to trigger rapid regression of aggressive tumors otherwise resistant to treatment. Notably, the dramatic antitumor activity of certain targeted therapeutics such as imatinib (Gleevec) in gastrointestinal stromal tumors has been traced in part to IDO downregulation. Further, antitumor responses to immune checkpoint inhibitors can be heightened safely by a clinical lead inhibitor of the IDO pathway that relieves IDO-mediated suppression of mTORC1 in T cells. In this personal perspective on IDO as a nodal mediator of pathogenic inflammation and immune escape in cancer, we provide a conceptual foundation for the clinical development of IDO inhibitors as a novel class of immunomodulators with broad application in the treatment of advanced human cancer.

IDO2 is a critical mediator of autoantibody production and inflammatory pathogenesis in a mouse model of autoimmune arthritis

Rheumatoid arthritis and other autoimmune disorders are associated with altered activity of the immunomodulatory enzyme IDO. However, the precise contributions of IDO function to autoimmunity remain unclear. In this article, we examine the effect of two different IDO enzymes, IDO1 and IDO2, on the development of autoimmune arthritis in the KRN preclinical model of rheumatoid arthritis. We find that IDO2, not IDO1, is critical for arthritis development, providing direct evidence of separate in vivo functions for IDO1 and IDO2. Mice null for Ido2 display decreased joint inflammation relative to wild-type mice owing to a reduction in pathogenic autoantibodies and Ab-secreting cells. Notably, IDO2 appears to specifically mediate autoreactive responses, but not normal B cell responses, as total serum Ig levels are not altered and IDO2 knockout mice are able to mount productive Ab responses to model Ags in vitro and in vivo. Reciprocal adoptive transfer studies confirm that autoantibody production and arthritis are modulated by IDO2 expression in a cell type extrinsic to the T cell. Taken together, our results, provide important insights into IDO2 function by defining its pathogenic contributions to autoantibody-mediated autoimmunity.

IDO2 is critical for IDO1-mediated T-cell regulation and exerts a non-redundant function in inflammation

IDO2 is implicated in tryptophan catabolism and immunity but its physiological functions are not well established. Here we report the characterization of mice genetically deficient in IDO2, which develop normally but exhibit defects in IDO-mediated T-cell regulation and inflammatory responses. Construction of this strain was prompted in part by our discovery that IDO2 function is attenuated in macrophages from Ido1 (-/-) mice due to altered message splicing, generating a functional mosaic with implications for interpreting findings in Ido1 (-/-) mice. No apparent defects were observed in Ido2 (-/-) mice in embryonic development or hematopoietic differentiation, with wild-type profiles documented for kynurenine in blood serum and for immune cells in spleen, lymph nodes, peritoneum, thymus and bone marrow of naive mice. In contrast, upon immune stimulation we determined that IDO1-dependent T regulatory cell generation was defective in Ido2 (-/-) mice, supporting Ido1-Ido2 genetic interaction and establishing a functional role for Ido2 in immune modulation. Pathophysiologically, both Ido1 (-/-) and Ido2 (-/-) mice displayed reduced skin contact hypersensitivity responses, but mechanistic distinctions were apparent, with only Ido2 deficiency associated with a suppression of immune regulatory cytokines that included GM-CSF, G-CSF, IFN-γ, TNF-α, IL-6 and MCP-1/CCL2. Different contributions to inflammation were likewise indicated by the finding that Ido2 (-/-) mice did not phenocopy Ido1 (-/-) mice in the reduced susceptibility of the latter to inflammatory skin cancer. Taken together, our results offer an initial glimpse into immune modulation by IDO2, revealing its genetic interaction with IDO1 and distinguishing its non-redundant contributions to inflammation.

Indoleamine 2,3-dioxygenase activity contributes to local immune suppression in the skin expressing human papillomavirus oncoprotein e7

Chronic infection of anogenital epithelium with human papillomavirus (HPV) promotes development of cancer. Many pathogens evoke immunosuppressive mechanisms to enable persistent infection. We have previously shown that grafted skin expressing HPV16 E7 oncoprotein from a keratin-14 promoter (K14E7) is not rejected by a syngeneic, immunocompetent host. In this study we show that indoleamine 2,3-dioxygenase (IDO) 1, an IFN-γ-inducible immunoregulatory molecule, is more highly expressed by langerin(-ve) dermal dendritic cells (DCs) from K14E7 skin than nontransgenic control skin. Furthermore, inhibiting IDO activity using 1-methyl-dl-tryptophan (1-D/L-MT) promotes K14E7 skin graft rejection. Increased IDO1 expression and activity in K14E7 skin requires IFN-γ and invariant natural killer T (iNKT) cells, both of which have been shown to negatively regulate T-cell effector function and suppress K14E7 graft rejection. Furthermore, DCs from K14E7 skin express higher levels of IFN-γ receptor (IFN-γR) than DCs from control skin. K14E7 transgenic skin recruits significantly higher numbers of DCs, independent of IFN-γ and IFN-γR expression. Consistent with these observations in a murine model, we found higher expression of IDO1 and IFN-γ but not IDO2 in the cervical epithelium of patients with HPV-associated cervical intraepithelial neoplasia (CIN) 2/3. Our data support a hypothesis that induction of IDO1 in HPV-infected skin contributes to evasion of host immunity.

Indoleamine 2,3-dioxygenase 2 (IDO2) and the kynurenine pathway: characteristics and potential roles in health and disease

The kynurenine pathway is the major route for the oxidative degradation of the amino acid tryptophan. Activity of the pathway is involved in several disease conditions, both in the periphery and the central nervous system, including cancer, inflammatory disorders, neurological conditions, psychiatric disorders and neurodegenerative diseases. Three enzymes are now known to catalyze the first and rate-limiting step in the catabolism of tryptophan along this pathway: tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO, subsequently named IDO1), both of which have been extensively studied, and a third enzyme, indoleamine 2,3-dioxygenase 2 (IDO2), a relative newcomer to the kynurenine pathway field. The adjuvant chemotherapeutic agent, 1-methyl-D-tryptophan, was intially suggested to target IDO2, implying involvement of IDO2 in tumorigenesis. Subsequently this compound has been suggested to have alternative actions and the physiological and pathophysiological roles of IDO2 are unclear. Targeted genetic interventions and selective inhibitors provide approaches for investigating the biology of IDO2. This review focuses on the current knowledge of IDO2 biology and discusses tools that will assist in further characterizing the enzymes of the kynurenine pathway.

Induction of TDO2 and IDO2 in Liver by High-Fat Feeding in Mice: Discrepancies with Human Obesity

Low-grade and chronic inflammation is elicited in white adipose tissue in human obesity. The presence of inflammatory molecules leads to an increased tryptophan catabolism through the induction of indoleamine-2,3-dioxygenase-1 (IDO1). In order to characterize the mechanisms underlying this dysregulation, we have studied 2 mouse models of obesity. Unexpectedly, we did not detect any IDO1 expression in obese or lean mice adipose tissue. In a previous study, we did not find any significant difference in the liver for IDO2 and tryptophan-2,3-dioxygenase (TDO2) gene expression between normal weight and obese patients. IDO2 and TDO2 expression was increased in the liver of high-fat fed mice, but not in ob/ob mice, and was strongly correlated with hydroxysteroid-(11-beta) dehydrogenase-1 (HSD11B1) expression, an enzyme that generates active cortisol within tissues. In conclusion, despite a dysregulation of tryptophan metabolism, obese mice display discrepancies with human obesity metabolism, rendering them inappropriate for further investigations in this animal model.

Intracerebroventricular administration of lipopolysaccharide induces indoleamine-2,3-dioxygenase-dependent depression-like behaviors

BACKGROUND

Activation of the tryptophan degrading enzyme indoleamine-2,3-dioxygenase 1 (IDO1) is associated with the development of behavioral signs of depression. Systemic immune challenge induces IDO1 in both the periphery and the brain, leading to increased circulating and brain concentrations of kynurenines. However, whether IDO1 activity within the brain is necessary for the manifestation of depression-like behavior of mice following a central immune challenge remains to be elucidated.

METHODS

We investigated the role of brain IDO1 in mediating depression-like behavior of mice in response to intracerebroventricular injection of saline or lipopolysaccharide (LPS, 10 ng).

RESULTS

LPS increased the duration of immobility in the tail suspension test and decreased preference for a sucrose solution. These effects were associated with an activation of central but not peripheral IDO1, as LPS increased brain kynurenine but had no effect on plasma concentrations of kynurenine. Interestingly, genetic deletion or pharmacological inhibition of IDO1, using 1-methyl-tryptophan, abrogated the reduction in sucrose preference induced by intracerebroventricular LPS. 1-Methyl-tryptophan also blocked the LPS-induced increase in duration of immobility during the tail suspension test.

CONCLUSIONS

These data indicate that activation of brain IDO1 is sufficient to induce depression-like behaviors of mice in response to central LPS.

Indoleamine 2,3-dioxygenase is a critical resistance mechanism in antitumor T cell immunotherapy targeting CTLA-4

Indoleamine 2,3-dioxygenase suppresses infiltration and accumulation of tumor-reactive T cells in the context of anti–CTLA-4 immunotherapy and attenuates the anti-tumor efficacy.

The cytotoxic T lymphocyte antigen-4 (CTLA-4)–blocking antibody ipilimumab results in durable responses in metastatic melanoma, though therapeutic benefit has been limited to a fraction of patients. This calls for identification of resistance mechanisms and development of combinatorial strategies. Here, we examine the inhibitory role of indoleamine 2,3-dioxygenase (IDO) on the antitumor efficacy of CTLA-4 blockade. In IDO knockout mice treated with anti–CTLA-4 antibody, we demonstrate a striking delay in B16 melanoma tumor growth and increased overall survival when compared with wild-type mice. This was also observed with antibodies targeting PD-1–PD-L1 and GITR. To highlight the therapeutic relevance of these findings, we show that CTLA-4 blockade strongly synergizes with IDO inhibitors to mediate rejection of both IDO-expressing and nonexpressing poorly immunogenic tumors, emphasizing the importance of the inhibitory role of both tumor- and host-derived IDO. This effect was T cell dependent, leading to enhanced infiltration of tumor-specific effector T cells and a marked increase in the effector-to-regulatory T cell ratios in the tumors. Overall, these data demonstrate the immunosuppressive role of IDO in the context of immunotherapies targeting immune checkpoints and provide a strong incentive to clinically explore combination therapies using IDO inhibitors irrespective of IDO expression by the tumor cells.

Targeting Tregs in Malignant Brain Cancer: Overcoming IDO

One of the hallmark features of glioblastoma multiforme (GBM), the most common adult primary brain tumor with a very dismal prognosis, is the accumulation of CD4⁺CD25⁺Foxp3⁺ regulatory T cells (Tregs). Regulatory T cells (Tregs) segregate into two primary categories: thymus-derived natural Tregs (nTregs) that develop from the interaction between immature T cells and thymic epithelial stromal cells, and inducible Tregs (iTregs) that arise from the conversion of CD4⁺FoxP3⁻ T cells into FoxP3 expressing cells. Normally, these Treg subsets complement one another’s actions by maintaining tolerance of self-antigens, thereby suppressing autoimmunity, while also enabling effective immune responses toward non-self-antigens, thus promoting infectious protection. However, Tregs have also been shown to be associated with the promotion of pathological outcomes, including cancer. In the setting of GBM, nTregs appear to be primary players that contribute to immunotherapeutic failure, ultimately leading to tumor progression. Several attempts have been made to therapeutically target these cells with variable levels of success. The blood brain barrier-crossing chemotherapeutics, temozolomide, and cyclophosphamide (CTX), vaccination against the Treg transcriptional regulator, FoxP3, as well as mAbs against Treg-associated cell surface molecules CD25, CTLA-4, and GITR are all different therapeutic approaches under investigation. Contributing to the poor success of past approaches is the expression of indoleamine 2,3-dioxygenase 1 (IDO), a tryptophan catabolizing enzyme overexpressed in GBM, and critically involved in regulating tumor-infiltrating Treg levels. Herein, we review the current literature on Tregs in brain cancer, providing a detailed phenotype, causative mechanisms involved in their pathogenesis, and strategies that have been used to target this population, therapeutically.