The Two Sides of Indoleamine 2,3-Dioxygenase 2 (IDO2)

Indoleamine 2,3-dioxygenase 1 (IDO1) and IDO2 originated from gene duplication before vertebrate divergence. While IDO1 has a well-defined role in immune regulation, the biological role of IDO2 remains unclear. Discovered in 2007, IDO2 is located near the IDO1 gene. Because of their high sequence similarity, IDO2 was initially thought to be a tryptophan (Trp)-degrading enzyme like IDO1. Differently from what expected, IDO2 displays extremely low catalytic activity toward Trp. Nevertheless, many studies, often contradictory, have tried to demonstrate that IDO2 modulates immune responses by catabolizing Trp into kynurenine, an unconvincing hypothesis linked to an incomplete understanding of IDO2's activity. In this study, we review IDO2's functional role beyond Trp metabolism. IDO2's evolutionary persistence across species, despite being almost inactive as an enzyme, suggests it has some relevant biological importance. IDO2 expression in human normal cells is poor, but significant in various cancers, with two prevalent SNPs. Overall, the comparison of IDO2 to IDO1 as a Trp-degrading enzyme may have led to misunderstandings about IDO2's true physiological and pathological roles. New insights suggest that IDO2 might function more as a signaling molecule, particularly in cancer contexts, and further studies could reveal its potential as a target for cancer therapy.

Endothelial Indoleamine-2,3-Dioxygenase-1 is not Critically Involved in Regulating Antitumor Immunity in the Central Nervous System

The vascular niche of malignant gliomas is a key compartment that shapes the immunosuppressive brain tumor microenvironment (TME). The blood-brain-barrier (BBB) consisting of specialized endothelial cells (ECs) and perivascular cells forms a tight anatomical and functional barrier critically controlling transmigration and effector function of immune cells. During neuroinflammation and tumor progression, the metabolism of the essential amino acid tryptophan (Trp) to metabolites such as kynurenine has long been identified as an important metabolic pathway suppressing immune responses. Previous studies have demonstrated that indoleamine-2,3-dioxygenase-1 (IDO1), a key rate-limiting enzyme in tryptophan catabolism, is expressed within the TME of high-grade gliomas. Here, we investigate the role of endothelial IDO1 (eIDO1) expression for brain tumor immunity. Single-cell RNA sequencing data revealed that in human glioma tissue, IDO1 is predominantly expressed by activated ECs showing a JAK/STAT signaling pathway-related CXCL11⁺ gene expression signature. In a syngeneic experimental glioma model, eIDO1 is induced by low-dose tumor irradiation. However, cell type-specific ablation of eIDO1 in experimental gliomas did not alter frequency and phenotype of tumor-infiltrating T cells nor tumor growth. Taken together these data argue against a dominant role of eIDO1 for brain tumor immunity.

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.

The CXCL11-CXCR3A axis influences the infiltration of CD274 and IDO1 in oral squamous cell carcinoma

BACKGROUND

The CXCL9/10/11-CXCR3 axis plays pivotal roles in the recruitment of immune cells and the formation of cancer-specific immunity in various cancers. High expression of immune checkpoints, which could be regulated by cytokines, is closely related to the establishment of immune escape in tumor microenvironment. Therefore, the study was tried to provide insights into the influence of the CXCL9/10/11-CXCR3 axis on immune checkpoints in oral squamous cell carcinoma (OSCC) and oral potentially malignant disorders (OPMDs), especially oral leukoplakia (OLK).

METHODS

The mRNA levels of CXCL9/10/11 and CXCR3 were analyzed in TCGA, GEO and Oncomine and verified in OLK and OSCC. The specimens were used to analysis the relationship between CXCL9/10/11 and CXCR3 variants. The correlation between CXCL9/10/11 and immune checkpoint/ligand in head and neck squamous cell carcinoma was analyzed in TIMER and confirmed in samples. Small interference transfection of CXCL11 in SCC25 cells was used to evaluate the function of CXCL11 on CD274/IDO1 expression.

RESULTS

CXCL9/10/11 had increase expression trends from normal tissues to OSCC. The proportion of CXCR3A (one variant of CXCR3) was significantly increased in OSCC comparing with normal tissues, while other variants-CXCR3B and CXCR3alt-did not. CXCL9/10/11 was positively correlated with CXCR3A and immune checkpoints/ligand (IDO1, LAG3, and CD274) in OLK and OSCC. CXCL11-knockdown SCC25 cells could directly inhibit the intracellular expression of CD274 and IDO1.

CONCLUSION

The upregulated CXCL9/10/11-CXCR3A axis may interact with immune checkpoints/their ligands in OLK and OSCC. Furthermore, CXCL11 may affect the expression of CD274 and IDO1 in an autocrine mode in OSCC.

Kynurenine produced by indoleamine 2,3-dioxygenase 2 exacerbates acute liver injury by carbon tetrachloride in mice

Kynurenine (Kyn) plays an important role as an immune check-point molecule and regulates various immune responses through its aryl hydrocarbon receptor (Ahr). Kyn is synthesized by indoleamine 2,3-dioxygenase (Ido) and tryptophan 2,3-dioxygenase (Tdo). Ido contributes approximately 90% of tryptophan catabolism. Although Kyn is increased in various liver disorders, the roles of Kyn in liver injury are complicated because Ido1, Ido2, and Tdo are activated in different cell types. In this study, the roles of Ido2 in carbon tetrachloride (CCl₄; 1 ml/kg, i.p.)-induced acute liver injury were examined using Ido2 knockout mice and Ido2 inhibitor. After CCl₄ treatment, the ratio of Kyn to tryptophan and levels of Kyn in the liver were increased, accompanied by activation of Ahr-mediated signaling, as revealed by increased nuclear Ahr and Cyp1a1 mRNA. Knockout of Ido2 (Ido2^-/-) and treatment with Ido2 inhibitor 1-methyl-D-tryptophan (D-1MT; 100 mg/kg, i.p.) attenuated CCl₄-induced liver injury, with decreased induction of Ahr-mediated signaling. Administration of D-Kyn (100 mg/kg, i.p.) to Ido2^-/- mice canceled the effect of Ido2 deficiency and exacerbated acute liver damage by CCl₄ treatment. In addition, liver fibrosis induced by repeated CCl₄ administration was suppressed in Ido2^-/- mice. In conclusion, the action of Ido2 and Kyn in the liver may prevent severe hepatocellular damage and liver fibrosis.

Inhibition of indoleamine 2,3-dioxygenase attenuates endotoxin-mediated tolerance in granulosa cells through kynurenine pathway

Ovarian granulosa cells (GCs) have been shown to have innate immune capabilities, which modulate their native endocrine functions through toll-like receptors (TLRs). We have recently shown that GCs exposed to lipopolysaccharide (LPS; 1.0 µg/mL) transiently regulate proinflammatory cytokine expression (interleukin 1β [IL-1β], IL-6, and tumor necrosis factor α) through chromatin remodeling. In the present study, we have demonstrated that GCs become tolerant to LPS on repeated exposure of LPS. To understand the mechanism of this endotoxin tolerance (ET) phenomenon in buffalo GCs, we have further studied the genome-wide transcriptomic analyses in buffalo GCs (unpublished data) and identified indoleamine 2,3-dioxygenase 1 (IDO1) gene, known to be involved in tryptophan catabolism, was found to be highly upregulated in endotoxin-tolerant GCs. Real-time gene expression analyses also showed similar results. Further analyses of tryptophan and tryptophan metabolite, kynurenine, showed that tryptophan was found to be depleted with the accumulation of kynurenine in the endotoxin-tolerant GCs. The effect of IDO1 induced ET was reversed when cells were pretreated with IDO1 inhibitor (1-methyl tryptophan, 1 mM). To the best of our knowledge, this is the first report describing the role of IDO1 gene in ET in GCs mimicked by repeated endotoxin exposure in vitro. In summary, the present study convincingly demonstrated that the tryptophan catabolism, through the kynurenine pathway, plays a crucial role as an immunomodulatory mechanism of ET in GCs. The finding could be exploited in developing potential therapeutics to treat impaired GCs function due to the ET underlying prolonged uterine or systemic infection leads to accumulation of endotoxin in follicular fluid.

Unraveling the ECM-Immune Cell Crosstalk in Skin Diseases

The extracellular matrix (ECM) is a complex network of proteins and proteoglycans secreted by keratinocytes, fibroblasts and immune cells. The function of the skin ECM has expanded from being a scaffold that provides structural integrity, to a more dynamic entity that is constantly remodeled to maintain tissue homeostasis. The ECM functions as ligands for cell surface receptors such as integrins, dystroglycans, and toll-like receptors (TLRs) and regulate cellular signaling and immune cell dynamics. The ECM also acts as a sink for growth factors and cytokines, providing critical cues during epithelial morphogenesis. Dysregulation in the organization and deposition of ECMs lead to a plethora of pathophysiological conditions that are exacerbated by aberrant ECM-immune cell interactions. In this review, we focus on the interplay between ECM and immune cells in the context of skin diseases and also discuss state of the art therapies that target the key molecular players involved.

Higher CXCL16 exodomain is associated with aggressive ovarian cancer and promotes the disease by CXCR6 activation and MMP modulation

Ovarian cancer (OvCa) is the leading cause of death from gynecological malignancies. Five-year survival rate of OvCa ranges from 30–92%, depending on the spread of disease at diagnosis. Role of chemokines is well appreciated in cancer, including OvCa. However, their precise role is understudied. Here, we show clinical and biological significance of CXCR6-CXCL16 and ADAM10 in OvCa. Expression of CXCR6 and N-terminal CXCL16 was significantly higher in serous carcinoma tissues compared to endometrioid. OvCa cells (SKOV-3 and OVCAR-3) also showed higher expression of CXCR6 than normal ovarian epithelial cells (IOSE-7576) while CXCL16 was higher in SKOV-3 than IOSE-7576. Furthermore, N-terminal CXCL16 was higher in conditioned media of OvCa cells than IOSE-7576. Compared to OVCAR-3, SKOV-3 cells, which had higher CXCL16, expressed significantly higher transcripts of ADAM10, a protease that cleaves CXCL16. OVCAR-3 cells showed higher CXCR6 specific migration whereas SKOV-3 cells showed more invasion. Difference in invasive potential of these cells was due to modulation of different MMPs after CXCL16 stimulation. Higher CXCR6 expression in serous papillary carcinoma tissues suggests its association with aggressive OvCa. Increased migration-invasion towards CXCL16 implies its role in metastatic spread. Therefore, CXCR6-CXCL16 axis could be used to differentiate between aggressive versus non-aggressive disease and as a target for better prognosis.

The Role of CXCR3 and Its Chemokine Ligands in Skin Disease and Cancer

Chemokines and their receptors play an important role in the recruitment, activation and differentiation of immune cells. The chemokine receptor, CXCR3, and its ligands, CXCL9, CXCL10, and CXCL11 are key immune chemoattractants during interferon-induced inflammatory responses. Inflammation of the skin resulting from infections or autoimmune disease drives expression of CXCL9/10/11 and the subsequent recruitment of effector, CXCR3⁺ T cells from the circulation. The relative contributions of the different CXCR3 chemokines and the three variant isoforms of CXCR3 (CXCR3A, CXCR3B, CXCR3alt) to the inflammatory process in human skin requires further investigation. In skin cancers, the CXCR3 receptor can play a dual role whereby expression on tumor cells can lead to cancer metastasis to systemic sites while receptor expression on immune cells can frequently promote anti-tumor immune responses. This review will discuss the biology of CXCR3 and its associated ligands with particular emphasis on the skin during inflammation and carcinogenesis.

Altered tryptophan metabolism in human meningioma

Meningiomas are the neoplasms that arise from the arachnoid cells of the meninges. It was reported that cancer cells escape from immune system through the metabolism of an aromatic essential amino acid tryptophan (TRP) via Kynurenine (KYN) pathway. However, the role of TRP metabolites such as, 5-Hydroxy tryptophan (5-HTP), 5-Hydroxy tryptamine (5-HT), N-acetyl serotonin (NAS), Melatonin (MEL), KYN, N-acetyl tryptamine, 5-Hydroxy indole acetic acid (5-HIAA) and 5-Methoxy indole acetic acid is not yet evaluated in human meningioma. Therefore, in the current study we have evaluated the levels of TRP and its metabolites in the progression of human meningioma using tumor biopsy samples and autopsy control meninges with Reverse Phase-HPLC. We here report that TRP metabolism favors towards KYN pathway in human meningioma and it could be due to increased indoleamine 2,3-dioxygenase 2 levels as we found its m-RNA levels to be up regulated in human meningioma. We observed significant increase in KYN and 5HIAA levels and significant decrease in TRP, 5-HTP, 5-HT, NAS and MEL levels in meningioma compared to control meninges. Since TRP metabolites regulate inducible nitric oxide synthase (INOS) gene expression and thereby nitric oxide (NO) production, we have also evaluated the INOS and NO levels. The INOS and NO levels were up regulated in human meningioma. The present data corroborates with existing data on TRP metabolism in tumor progression and may serve to target TRP metabolism as a therapeutic intervention.

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.

The role of placental tryptophan catabolism

This review discusses the mechanisms and consequences of degradation of tryptophan (Trp) in the placenta, focusing mainly on the role of indoleamine 2,3-dioxygenase-1 (IDO1), one of three enzymes catalyzing the first step of the kynurenine pathway of Trp degradation. IDO1 has been implicated in regulation of feto-maternal tolerance in the mouse. Local depletion of Trp and/or the presence of metabolites of the kynurenine pathway mediate immunoregulation and exert antimicrobial functions. In addition to the decidual glandular epithelium, IDO1 is localized in the vascular endothelium of the villous chorion and also in the endothelium of spiral arteries of the decidua. Possible consequences of IDO1-mediated catabolism of Trp in the endothelium encompass antimicrobial activity and immunosuppression, as well as relaxation of the placental vasotonus, thereby contributing to placental perfusion and growth of both placenta and fetus. It remains to be evaluated whether other enzymes mediating Trp oxidation, such as indoleamine 2,3-dioxygenase-2, Trp 2,3-dioxygenase, and Trp hydroxylase-1 are of relevance to the biology of the placenta.

Kynurenines with neuroactive and redox properties: relevance to aging and brain diseases

The kynurenine pathway (KP) is the main route of tryptophan degradation whose final product is NAD(+). The metabolism of tryptophan can be altered in ageing and with neurodegenerative process, leading to decreased biosynthesis of nicotinamide. This fact is very relevant considering that tryptophan is the major source of body stores of the nicotinamide-containing NAD(+) coenzymes, which is involved in almost all the bioenergetic and biosynthetic metabolism. Recently, it has been proposed that endogenous tryptophan and its metabolites can interact and/or produce reactive oxygen species in tissues and cells. This subject is of great importance due to the fact that oxidative stress, alterations in KP metabolites, energetic deficit, cell death, and inflammatory events may converge each other to enter into a feedback cycle where each one depends on the other to exert synergistic actions among them. It is worth mentioning that all these factors have been described in aging and in neurodegenerative processes; however, has so far no one established any direct link between alterations in KP and these factors. In this review, we describe each kynurenine remarking their redox properties, their effects in experimental models, their alterations in the aging process.

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.