Association of indoleamine 2,3 dioxygenase, brain derived neurotrophic factor and cellular senescence in type 2 diabetes mellitus with depression: a clinical approach

BACKGROUND AND AIM

Type 2 diabetes mellitus (T2DM) and depression are often linked. Several studies have reported the role of molecular markers either in diabetes or depression. The present study aimed at molecular level profiling of Indoleamine-2,3-dioxygenase (IDO), brain-derived neurotrophic factor (BDNF) and cellular senescence in patients with type 2 diabetes with and without depression compared to individuals with healthy controls.

METHODS

A total of 120 individuals diagnosed with T2DM were enlisted for the study, with a subset of participants with and without exhibiting depression. The gene expression analysis was done using quantitative real-time PCR.

RESULTS

Indoleamine 2,3 dioxygenase (p < 0.001) and senescence genes (p < 0.001) were significantly upregulated, while brain derived neurotrophic factor (p < 0.01) was significantly downregulated in T2DM patients comorbid with and without depression when compared to healthy controls.

CONCLUSION

Indoleamine 2,3 dioxygenase, Brain derived neurotrophic factor and cellular senescence may play a role in the progression of the disease. The aforementioned discoveries offer significant contributions to our understanding of the molecular mechanisms that underlie T2DM with depression, potentially aiding in the advancement of prediction and diagnostic methods for this particular ailment.

The plasma kynurenine-to-tryptophan ratio as a biomarker of tuberculosis disease in people living with HIV on antiretroviral therapy: an exploratory nested case-control study

BACKGROUND

Non-sputum-based tests are needed to predict or diagnose tuberculosis (TB) disease in people living with HIV (PWH). The enzyme indoleamine 2, 3-dioxygenase-1 (IDO1) is expressed in tuberculoid granuloma and catabolizes tryptophan (Trp) to kynurenine (Kyn). IDO1 activity compromises innate and adaptive immune responses, promoting mycobacterial survival. The plasma Kyn-to-Trp (K/T) ratio is a potential TB diagnostic and/or predictive biomarker in PWH on long-term antiretroviral therapy (ART).

METHODS

We compared plasma K/T ratios in samples from PWH, who were followed up prospectively and developed TB disease after ART initiation. Controls were matched for age and duration of ART. Kyn and Trp were measured at 3 timepoints; at TB diagnosis, 6 months before TB diagnosis and 6 months after TB diagnosis, using ultra performance liquid chromatography combined with mass spectrometry.

RESULTS

The K/T ratios were higher for patients with TB disease at time of diagnosis (median, 0.086; IQR, 0.069-0.123) compared to controls (0.055; IQR 0.045-0.064; p = 0.006), but not before or after TB diagnosis. K/T ratios significantly declined after successful TB treatment, but increased upon treatment failure. The K/T ratios showed a parabolic correlation with CD4 cell counts in participants with TB (p = 0.005), but there was no correlation in controls.

CONCLUSIONS

The plasma K/T ratio helped identify TB disease and may serve as an adjunctive biomarker for for monitoring TB treatment in PWH. Validation studies to ascertain these findings and evaluate the optimum cut-off for diagnosis of TB disease in PWH should be undertaken in well-designed prospective cohorts.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT00411983.

Dysregulated placental expression of kynurenine pathway enzymes is associated with inflammation and depression in pregnancy

BACKGROUND

Perinatal depression (including antenatal-, postnatal-, and depression that spans both timepoints) is a prevalent disorder with high morbidity that affects both mother and child. Even though the full biological blueprints of perinatal depression remain incomplete, multiple studies indicate that, at least for antenatal depression, the disorder has an inflammatory component likely linked to a dysregulation of the enzymatic kynurenine pathway. The production of neuroactive metabolites in this pathway, including quinolinic acid (QUIN), is upregulated in the placenta due to the multiple immunological roles of the metabolites during pregnancy. Since neuroactive metabolites produced by the pathway also may affect mood by directly affecting glutamate neurotransmission, we sought to investigate whether the placental expression of kynurenine pathway enzymes controlling QUIN production was associated with both peripheral inflammation and depressive symptoms during pregnancy.

METHODS

68 placentas obtained at birth were analyzed using qPCR to determine the expression of kynurenine pathway enzymes. Cytokines and metabolites were quantified in plasma using high-sensitivity electroluminescence and ultra-performance liquid chromatography, respectively. Maternal depressive symptoms were assessed using the Edinburgh Postnatal Depression Scale (EPDS) throughout pregnancy and the post-partum. Associations between these factors were assessed using robust linear regression with ranked enzymes.

RESULTS

Low placental quinolinate phosphoribosyl transferase (QPRT), the enzyme responsible for degrading QUIN, was associated with higher IL-6 and higher QUIN/kynurenic acid ratios at the 3rd trimester. Moreover, women with severe depressive symptoms in the 3rd trimester had significantly lower placental expression of both QPRT and 2-amino-3-carboxymuconate-6-semialdehyde decarboxylase (ACMSD); impaired activity of these two enzymes leads to QUIN accumulation.

CONCLUSION

Overall, our data support that a compromised placental environment, featuring low expression of critical kynurenine pathway enzymes is associated with increased levels of plasma cytokines and the dysregulated kynurenine metabolite pattern observed in depressed women during pregnancy.

Indoximod-based chemo-immunotherapy for pediatric brain tumors: A first-in-children phase I trial

BACKGROUND

Recurrent brain tumors are the leading cause of cancer death in children. Indoleamine 2,3-dioxygenase (IDO) is a targetable metabolic checkpoint that, in preclinical models, inhibits anti-tumor immunity following chemotherapy.

METHODS

We conducted a phase I trial (NCT02502708) of the oral IDO-pathway inhibitor indoximod in children with recurrent brain tumors or newly diagnosed diffuse intrinsic pontine glioma (DIPG). Separate dose-finding arms were performed for indoximod in combination with oral temozolomide (200 mg/m2/day x 5 days in 28-day cycles), or with palliative conformal radiation. Blood samples were collected at baseline and monthly for single-cell RNA-sequencing with paired single-cell T cell receptor sequencing.

RESULTS

Eighty-one patients were treated with indoximod-based combination therapy. Median follow-up was 52 months (range 39-77 months). Maximum tolerated dose was not reached, and the pediatric dose of indoximod was determined as 19.2 mg/kg/dose, twice daily. Median overall survival was 13.3 months (n = 68, range 0.2-62.7) for all patients with recurrent disease and 14.4 months (n = 13, range 4.7-29.7) for DIPG. The subset of n = 26 patients who showed evidence of objective response (even a partial or mixed response) had over 3-fold longer median OS (25.2 months, range 5.4-61.9, p = 0.006) compared to n = 37 nonresponders (7.3 months, range 0.2-62.7). Four patients remain free of active disease longer than 36 months. Single-cell sequencing confirmed emergence of new circulating CD8 T cell clonotypes with late effector phenotype.

CONCLUSIONS

Indoximod was well tolerated and could be safely combined with chemotherapy and radiation. Encouraging preliminary evidence of efficacy supports advancing to Phase II/III trials for pediatric brain tumors.

Association between altered tryptophan metabolism, plasma aryl hydrocarbon receptor agonists, and inflammatory Chagas disease

Introduction

Chagas disease causes a cardiac illness characterized by immunoinflammatory reactions leading to myocardial fibrosis and remodeling. The development of Chronic Chagas Cardiomyopathy (CCC) in some patients while others remain asymptomatic is not fully understood, but dysregulated inflammatory responses are implicated. The Aryl hydrocarbon receptor (AhR) plays a crucial role in regulating inflammation. Certain tryptophan (Trp) metabolites have been identified as AhR ligands with regulatory functions.

Methods results and discussion

We investigated AhR expression, agonist response, ligand production, and AhR-dependent responses, such as IDO activation and regulatory T (Treg) cells induction, in two T. cruzi-infected mouse strains (B6 and Balb/c) showing different polymorphisms in AhR. Furthermore, we assessed the metabolic profile of Trp catabolites and AhR agonistic activity levels in plasma samples from patients with chronic Chagas disease (CCD) and healthy donors (HD) using a luciferase reporter assay and liquid chromatography-mass spectrophotometry (LC-MS) analysis. T. cruzi-infected B6 mice showed impaired AhR-dependent responses compared to Balb/c mice, including reduced IDO activity, kynurenine levels, Treg cell induction, CYP1A1 up-regulation, and AhR expression following agonist activation. Additionally, B6 mice exhibited no detectable AhR agonist activity in plasma and displayed lower CYP1A1 up-regulation and AhR expression upon agonist activation. Similarly, CCC patients had decreased AhR agonistic activity in plasma compared to HD patients and exhibited dysregulation in Trp metabolic pathways, resulting in altered plasma metabolite profiles. Notably, patients with severe CCC specifically showed increased N-acetylserotonin levels in their plasma. The methods and findings presented here contribute to a better understanding of CCC development mechanisms and may identify potential specific biomarkers for T. cruzi infection and the severity of associated heart disease. These insights could be valuable in designing new therapeutic strategies. Ultimately, this research aims to establish the AhR agonistic activity and Trp metabolic profile in plasma as an innovative, non-invasive predictor of prognosis for chronic Chagas disease.

Angucyclinones with IDO and TDO inhibitory activities isolated from the actinomycetes Umezawaea beigongshangensis

Four previously undescribed angucyclinones umezawaones A-D (1-4) were isolated from the liquid cultures of Umezawaea beigongshangensis. Their structures were determined by spectroscopic analyses, single crystal X-ray diffraction, quantum chemical 13C NMR and electronic circular dichroism calculations. All compounds displayed strong inhibitory activities against indoleamine 2,3-dioxygenase and tryptophan-2,3-dioxygenase in enzymatic assay, especially compound 2.

Extracellular vesicles derived from nasopharyngeal carcinoma induce the emergence of mature regulatory dendritic cells using a galectin-9 dependent mechanism

Nasopharyngeal carcinoma-derived small extracellular vesicles (NPCSEVs) have an immunosuppressive impact on the tumour microenvironment. In this study, we investigated their influence on the generation of tolerogenic dendritic cells and the potential involvement of the galectin-9 (Gal9) they carry in this process. We analysed the phenotype and immunosuppressive properties of NPCSEVs and explored the ability of DCs exposed to NPCSEVs (NPCSEV-DCs) to regulate T cell proliferation. To assess their impact at the pathophysiological level, we performed real-time fluorescent chemoattraction assays. Finally, we analysed phenotype and immunosuppressive functions of NPCSEV-DCs using a proprietary anti-Gal9 neutralising antibody to assess the role of Gal9 in this effect. We described that NPCSEV-DCs were able to inhibit T cell proliferation despite their mature phenotype. These mature regulatory DCs (mregDCs) have a specific oxidative metabolism and secrete high levels of IL-4. Chemoattraction assays revealed that NPCSEVs could preferentially recruit NPCSEV-DCs. Finally, and very interestingly, the reduction of the immunosuppressive function of NPCSEV-DCs using an anti-Gal9 antibody clearly suggested an important role for vesicular Gal9 in the induction of mregDCs. These results revealed for the first time that NPCSEVs promote the emergence of mregDCs using a galectin-9 dependent mechanism and open new perspectives for antitumour immunotherapy targeting NPCSEVs.

Generation and quality control of mature monocyte-derived dendritic cells for immunotherapy

Dendritic cell vaccination is a form of active immunotherapy that aims to exploit the crucial role of DC in the initiation of T-cell responses. Numerous vaccination trials have been conducted targeting various tumor entities, including glioblastoma, the most frequent and aggressive malignant brain tumor in adults. They have demonstrated feasibility and safety and suggest improved survival, associated with induction of anti-tumoral immunity. Here, we describe in detail a large-scale 2-step protocol for successive GMP-compliant generation of immature and mature dendritic cells, yielding a highly homogenous population of CD83+ mature DC expressing CD40, CD80, CD86 and HLA-DR at high density, lacking activity of the immunosuppressive enzyme indoleamine-2,3-dioxygenase, migrating towards the chemokine CCL19 and showing highly potent T-cell stimulatory activity. Loaded with autologous tumor lysate, these cells are currently being evaluated in a phase II controlled randomized clinical trial (GlioVax) in glioblastoma patients.

Involvement of indoleamine 2, 3-dioxygenase (IDO) and brain-derived neurotrophic factor (BDNF) in the neuroprotective mechanisms of ferulic acid against depressive-like behaviour

OBJECTIVE

Ferulic acid (FA) is a common food ingredient that is abundantly present in various routinely consumed food and beverages. Like many cinnamic acid derivatives, FA produces wide-ranging effects in a dose-dependent manner and various studies link FA consumption with reduced risk of depressive disorders. The aim of this study was to exploit the neuroprotective mechanisms of FA including indoleamine 2,3-dioxygenase (IDO), brain-derived neurotrophic factor (BDNF), and other pro-inflammatory cytokines by employing lipopolysaccharide (LPS)-induced depressive-like behaviour model.

METHODS

C57BL/6J male mice were divided into 4 groups consisting of saline (SAL), LPS, FA and Imipramine (IMI). Animals were pretreated orally with FA (10 mg/kg) and IMI (10 mg/kg) for 21 days once daily and all groups except SAL were challenged with LPS (0.83 mg/kg) intraperitoneally on day 21.

RESULTS

LPS administration produced a biphasic change in the behaviour of the animals where the animals lost a significant weight and express high immobility time at 24 h. Proinflammatory cytokines including, TNF-α, IL-6, IL-1β, and IFN-γ were significantly increased along with increased lipid peroxidation and reduced BDNF. Furthermore, the increased kynurenine to tryptophan ratio was indicative of elevated IDO activity.

CONCLUSION

The results of this study emphasise that low dose of FA is effective in attenuating depressive-like behaviour by modulating IDO, BDNF and reducing neuroinflammation.

Inhalant Cannabidiol Inhibits Glioblastoma Progression Through Regulation of Tumor Microenvironment

Introduction: Glioblastoma (GBM) is the most common invasive brain tumor composed of diverse cell types with poor prognosis. The highly complex tumor microenvironment (TME) and its interaction with tumor cells play important roles in the development, progression, and durability of GBM. Angiogenic and immune factors are two major components of TME of GBM; their interplay is a major determinant of tumor vascularization, immune profile, as well as immune unresponsiveness of GBM. Given the ineffectiveness of current standard therapies (surgery, radiotherapy, and concomitant chemotherapy) in managing patients with GBM, it is necessary to develop new ways of treating these lethal brain tumors. Targeting TME, altering tumor ecosystem may be a viable therapeutic strategy with beneficial effects for patients in their fight against GBM. Materials and Methods: Given the potential therapeutic effects of cannabidiol (CBD) in a wide spectrum of diseases, including malignancies, we tested, for the first time, whether inhalant CBD can inhibit GBM tumor growth using a well-established orthotopic murine model. Optical imaging, histology, immunohistochemistry, and flow cytometry were employed to describe the outcomes such as tumor progression, cancer cell signaling pathways, and the TME. Results: Our findings showed that inhalation of CBD was able to not only limit the tumor growth but also to alter the dynamics of TME by repressing P-selectin, apelin, and interleukin (IL)-8, as well as blocking a key immune checkpoint-indoleamine 2,3-dioxygenase (IDO). In addition, CBD enhanced the cluster of differentiation (CD) 103 expression, indicating improved antigen presentation, promoted CD8 immune responses, and reduced innate Lymphoid Cells within the tumor. Conclusion: Overall, our novel findings support the possible therapeutic role of inhaled CBD as an effective, relatively safe, and easy to administer treatment adjunct for GBM with significant impacts on the cellular and molecular signaling of TME, warranting further research.

IDO blockade negatively regulates the CTLA-4 signaling in breast cancer cells

Cancer is classified into metabolic and/or genetic disorders; notably, the tryptophan catabolism pathway is vital in different cancer types. Here, we focused on the interaction and molecular connection between the cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) receptor and indoleamine-2,3-dioxygenase (IDO) enzyme. To test the impact of the selected immunotherapies on breast cancer cell migration and cell survival, we used in vitro assays. Also, we test the impact of anti-CTLA-4 antibody on the IDO-positive cells. The results of cell migration and clonogenic assays showed that anti-CTLA-4 antibody reduces cancer cell migration and clonogenic abilities of murine breast cancer cells. In addition, the result of flow cytometry showed that the anti-CTLA-4 antibody did not change the percentage of IDO-positive cancer cells. Notably, administrating an IDO blocker, 1-Methyl-DL-tryptophan (1MT), reduces the efficiency of the antiCTLA-4 antibody. The enzymatic blocking of the IDO reduces the efficiency of the anti-CTLA-4 antibody on cell migration and clonogenic abilities suggesting that there is an inhibitory interaction at the molecular level between functions of CTLA-4 and IDO. It is unclear via which mechanism(s) IDO interacts with CTLA-4 signaling and also why blocking IDO makes disruption in CTLA-4 signaling in cancer cells. Indeed, evaluating the role of IDO in CTLA-4 signaling in cancer cells may assist in clarifying a poor response to CTLA-4 immunotherapies by some patients. Hence, further investigation of the molecular interaction between CTLA-4 and IDO might help to improve the efficiency of CTLA-4 immunotherapy.

PD-1, PD-L1, IDO, CD70 and microsatellite instability as potential targets to prevent immune evasion in sarcomas

Background: Soft tissue and bone sarcomas are rare entities, hence, standardized therapeutic strategies are difficult to assess. Materials & methods: Immunohistochemistry was performed on 68 sarcoma samples to assess the expression of PD-1, PD-L1, IDO and CD70 in different tumor compartments and molecular analysis was performed to assess microsatellite instability status. Results: PD-1/PD-L1, IDO and CD70 pathways are at play in the immune evasion of sarcomas in general. Soft tissue sarcomas more often show an inflamed phenotype compared with bone sarcomas. Specific histologic sarcoma types show high expression levels of different markers. Finally, this is the first presentation of a microsatellite instability-high Kaposi sarcoma. Discussion/conclusion: Immune evasion occurs in sarcomas. Specific histologic types might benefit from immunotherapy, for which further investigation is needed.

IDO-Kynurenine pathway mediates NLRP3 inflammasome activation-induced postoperative cognitive impairment in aged mice

AIM

Postoperative cognitive dysfunction (POCD) is a common postoperative complication, especially in elderly patients. It extends hospital stay, increases the mortality rate and are heavy burdens to the family and society. Accumulating research has indicated that overactivation of pyrin domain-containing protein 3 (NLRP3) inflammasomes is related to POCD andplays a critical role in activating pro-inflammatory cytokines. According to existing studies, indoleamine 2,3-dioxygenase (IDO) is potently up-regulated by inflammatory factors, tryptophan in brain is mainly catalyzed by IDO to kynurenine (KYN), KYN metabolism may contribute to the development of depressive disorder and memory deficits. Hence, this study elucidated whether IDO-Kynurenine pathway mediates NLRP3 inflammasome activation-induced postoperative cognitive impairment in aged mice.

MATERIAL AND METHODS

POCD model was established in aged C57BL/6J mice by exploratory laparotomy under isoflurane anesthesia. Learning and memory were determined using Morris water maze.

RESULTS

The data showed that IDO and kynurenine aminotransferase-II (KAT-II) mRNA in hippocampus was up-regulated, and NLRP3, caspase recruitment domain (ASC), interleukin-1b (IL-1b) and IDO overexpressed, KYN levels increased after anesthesia and surgery. NLRP3 inflammasome inhibitor (MCC950) reversed NLRP3, ASC, IL-1b and IDO overexpression, and the elevation of KYN levels. To clarify the role of IDO-Kynurenine pathway in postoperative cognitive impairment, IDO inhibitor (1-methyl-Ltryptophan 1-MT) reduced the elevation of KYN and kynurenic acid (KYNA) levels, reduction of tryptophan (TRP), as well as improved learning and memory abilities. Finally, KAT-II inhibitor (PF-04859989) reduced brain KYNA levels and restored the cognitive impairment.

CONCLUSION

These results reveal that IDO-Kynurenine pathway mediates NLRP3 inflammasome activation-induced postoperative cognitive impairment.

Metabolomics profiling reveals differences in proliferation between tumorigenic and non-tumorigenic Madin-Darby canine kidney (MDCK) cells

Background

Madin-Darby canine kidney (MDCK) cells are a cellular matrix in the production of influenza vaccines. The proliferation rate of MDCK cells is one of the critical factors that determine the vaccine production cycle. It is yet to be determined if there is a correlation between cell proliferation and alterations in metabolic levels. This study aimed to explore the metabolic differences between MDCK cells with varying proliferative capabilities through the use of both untargeted and targeted metabolomics.

Methods

To investigate the metabolic discrepancies between adherent cell groups (MDCK-M60 and MDCK-CL23) and suspension cell groups (MDCK-XF04 and MDCK-XF06), untargeted and targeted metabolomics were used. Utilizing RT-qPCR analysis, the mRNA expressions of key metabolites enzymes were identified.

Results

An untargeted metabolomics study demonstrated the presence of 81 metabolites between MDCK-M60 and MDCK-CL23 cells, which were mainly affected by six pathways. An analysis of MDCK-XF04 and MDCK-XF06 cells revealed a total of 113 potential metabolites, the majority of which were impacted by ten pathways. Targeted metabolomics revealed a decrease in the levels of choline, tryptophan, and tyrosine in MDCK-CL23 cells, which was in accordance with the results of untargeted metabolomics. Additionally, MDCK-XF06 cells experienced a decrease in 5'-methylthioadenosine and tryptophan, while S-adenosylhomocysteine, kynurenine, 11Z-eicosenoic acid, 3-phosphoglycerate, glucose 6-phosphate, and phosphoenolpyruvic acid concentrations were increased. The mRNA levels of MAT1A, MAT2B, IDO1, and IDO2 in the two cell groups were all increased, suggesting that S-adenosylmethionine and tryptophan may have a significant role in cell metabolism.

Conclusions

This research examines the effect of metabolite fluctuations on cell proliferation, thus offering a potential way to improve the rate of MDCK cell growth.

Immune Responses in Lung Granulomas during Mtb/HIV Co-Infection: Implications for Pathogenesis and Therapy

HIV and TB are the cause of significant worldwide mortality and pose a grave danger to the global public health. TB is the leading cause of death in HIV-infected persons, with one in four deaths attributable to TB. While the majority of healthy individuals infected with M. tuberculosis (Mtb) are able to control the infection, co-infection with HIV increases the risk of TB infection progressing to TB disease by over 20-fold. While antiretroviral therapy (ART), the cornerstone of HIV care, decreases the incidence of TB in HIV-uninfected people, this remains 4- to 7-fold higher after ART in HIV-co-infected individuals in TB-endemic settings, regardless of the duration of therapy. Thus, the immune control of Mtb infection in Mtb/HIV-co-infected individuals is not fully restored by ART. We do not fully understand the reasons why Mtb/HIV-co-infected individuals maintain a high susceptibility to the reactivation of LTBI, despite an effective viral control by ART. A deep understanding of the molecular mechanisms that govern HIV-induced reactivation of TB is essential to develop improved treatments and vaccines for the Mtb/HIV-co-infected population. We discuss potential strategies for the mitigation of the observed chronic immune activation in combination with both anti-TB and anti-retroviral approaches.

Aryl hydrocarbon receptor-kynurenine axis promotes oncogenic activity in BCP-ALL

B-cell precursor acute lymphoblastic leukemia (BCP-ALL), the most common childhood cancer, originates from lymphoid precursor cells in bone marrow committed to the B-cell lineage. Environmental factors and genetic abnormalities disturb the normal maturation of these precursor cells, promoting the formation of leukemia cells and suppressing normal hematopoiesis. The underlying mechanisms of progression are unclear, but BCP-ALL incidence seems to be increasing in parallel with the adoption of modern lifestyles. This study hypothesized that air pollution and haze are risk factors for BCP-ALL progression. The current study revealed that indeno(1,2,3-cd)pyrene (IP), a major component of polycyclic aromatic hydrocarbons (PAHs) in air, promotes oncogenic activities (proliferation, transformation, and disease relapse) in vitro and in vivo. Mechanistically, IP treatment activated the aryl hydrocarbon receptor (AHR)-indoleamine-2,3-dioxygenase (IDOs) axis, thereby enhancing tryptophan metabolism and kynurenine (KYN) level and consequent promoting the KYN-AHR feedback loop. IP treatment decreased the time to disease relapse and increased the BCP-ALL cell count in an orthotopic xenograft mouse model. Additionally, in 50 clinical BCP-ALL samples, AHR and IDO were co-expressed in a disease-specific manner at mRNA and protein levels, while their mRNA levels showed a significant correlation with disease-free survival duration. These results indicated that PAH/IP exposure promotes BCP-ALL disease progression.

Aberrant tryptophan metabolism leads to unfavorable outcomes in lenalidomide-treated myeloma patients

Indoleamine 2,3-dioxygenase 1 (IDO), an enzyme that metabolizes tryptophan (Trp) to kynurenine (Kyn), is an important microenvironmental factor suppressing antitumor immunity. Here, we investigated the clinical impact of aberrant Trp metabolism in patients with multiple myeloma (MM) treated with lenalidomide (Len) and evaluated its effects on T cell immunity ex vivo. Kyn and Trp concentrations were quantified in sera from 72 patients with relapsed or refractory MM prior to the initiation of therapy with Len plus dexamethasone (Ld). Associations of the Kyn/Trp ratio with progression-free survival (PFS) and overall survival (OS) were analyzed. The expressions of IDO in tumor and stromal cells were evaluated during co-culture, and the effects of culture medium containing low Trp and high Kyn concentrations on T cells in the presence of Len were investigated. Patients with high serum Kyn/Trp ratios (≥46.0, n = 22) had significantly shorter PFS and OS than those with low ratios (4.9 vs. 12.6 months, and 15.5 vs. 45.7 months, respectively). MM cells promoted IDO expression in stromal cells during co-culture in both a direct contact and an indirect manner. Incubation in medium with a high Kyn/Trp ratio significantly inhibited T cell cytokine production and upregulated the expression of inhibitory immune receptors. These effects were sustained even in the presence of Len. In conclusion, a high serum Kyn/Trp ratio is associated with poor prognosis in patients with MM. We propose that aberrant Trp metabolism reduces anti-tumor immunity and the efficacy of Len therapy.

Melanoma Cells Inhibit iNKT Cell Functions via PGE2 and IDO1

Invariant natural killer T (iNKT) cells are a distinct group of immune cells known for their immunoregulatory and cytotoxic activities, which are crucial in immune surveillance against tumors. They have been extensively investigated as a potential target for adoptive cell immunotherapy. Despite the initial promise of iNKT cell-based immunotherapy as a treatment for melanoma patients, its effective utilization has unfortunately yielded inconsistent outcomes. The primary cause of this failure is the immunosuppressive tumor microenvironment (TME). In this study, we specifically directed our attention towards melanoma cells, as their roles within the TME remain partially understood and require further elucidation. Methods: We conducted co-culture experiments involving melanoma cell lines and iNKT cells. Results: We demonstrated that melanoma cell lines had a significant impact on the proliferation and functions of iNKT cells. Our findings revealed that co-culture with melanoma cell lines led to a significant impairment in the expression of the NKG2D receptor and cytolytic granules in iNKT cells. Moreover, we observed a strong impairment of their cytotoxic capability induced by the presence of melanoma cells. Furthermore, through the use of selective inhibitors targeting IDO1 and COX-2, we successfully demonstrated that the melanoma cell line's ability to impair iNKT cell activation and functions was attributed to the up-regulation of IDO1 expression and PGE2 production.

Modulation of T cells by tryptophan metabolites in the kynurenine pathway

Lymphocytes maturing in the thymus (T cells) are key factors in adaptive immunity and the regulation of inflammation. The kynurenine pathway of tryptophan metabolism includes several enzymes and compounds that can modulate T cell function, but manipulating these pharmacologically has not achieved the expected therapeutic activity for the treatment of autoimmune disorders and cancer. With increasing knowledge of other pathways interacting with kynurenines, the expansion of screening methods, and the application of virtual techniques to understanding enzyme structures and mechanisms, details of interactions between kynurenines and other pathways are being revealed. This review surveys some of these alternative approaches to influence T cell function indirectly via the kynurenine pathway and summarizes the most recent work on the development of compounds acting directly on the kynurenine pathway.

Beneficial or detrimental activity of regulatory T cells, indoleamine 2,3-dioxygenase, and heme oxygenase-1 in the lungs is influenced by the level of virulence of Mycobacterium tuberculosis strain infection

Tuberculosis (TB) caused by the complex Mycobacterium tuberculosis (Mtb) is the main cause of death by a single bacterial agent. Last year, TB was the second leading infectious killer after SARS-CoV-2. Nevertheless, many biological and immunological aspects of TB are not completely elucidated, such as the complex process of immunoregulation mediated by regulatory T cells (Treg cells) and the enzymes indoleamine 2,3-dioxygenase (IDO) and heme oxygenase 1 (HO-1). In this study, the contribution of these immunoregulatory factors was compared in mice infected with Mtb strains with different levels of virulence. First Balb/c mice were infected by intratracheal route, with a high dose of mild virulence reference strain H37Rv or with a highly virulent clinical isolate (strain 5186). In the lungs of infected mice, the kinetics of Treg cells during the infection were determined by cytofluorometry and the expression of IDO and HO-1 by RT-PCR and immunohistochemistry. Then, the contribution of immune-regulation mediated by Treg cells, IDO and HO-1, was evaluated by treating infected animals with specific cytotoxic monoclonal antibodies for Treg cells depletion anti-CD25 (PC61 clone) or by blocking IDO and HO-1 activity using specific inhibitors (1-methyl-D,L-tryptophan or zinc protoporphyrin-IX, respectively). Mice infected with the mild virulent strain showed a progressive increment of Treg cells, showing this highest number at the beginning of the late phase of the infection (28 days), the same trend was observed in the expression of both enzymes being macrophages the cells that showed the highest immunostaining. Animals infected with the highly virulent strain showed lower survival (34 days) and higher amounts of Treg cells, as well as higher expression of IDO and HO-1 one week before. In comparison with non-treated animals, mice infected with strain H37Rv with depletion of Treg cells or treated with the enzymes blockers during late infection showed a significant decrease of bacilli loads, higher expression of IFN-g and lower IL-4 but with a similar extension of inflammatory lung consolidation determined by automated morphometry. In contrast, the depletion of Treg cells in infected mice with the highly virulent strain 5186 produced diffuse alveolar damage that was similar to severe acute viral pneumonia, lesser survival and increase of bacillary loads, while blocking of both IDO and HO-1 produced high bacillary loads and extensive pneumonia with necrosis. Thus, it seems that Treg cells, IDO and HO-1 activities are detrimental during late pulmonary TB induced by mild virulence Mtb, probably because these factors decrease immune protection mediated by the Th1 response. In contrast, Treg cells, IDO and HO-1 are beneficial when the infection is produced by a highly virulent strain, by regulation of excessive inflammation that produced alveolar damage, pulmonary necrosis, acute respiratory insufficiency, and rapid death.

Indoleamine-2,3 dioxygenase: a fate-changer of the tumor microenvironment

Indoleamine-2,3 dioxygenase is a rate-limiting enzyme in the tryptophan catabolism in kynurenine pathways that has an immunosuppressive effect and supports cancer cells to evade the immune system in different cancer types. Diverse cytokines and pathways upregulate the production of indoleamine-2,3 dioxygenase enzymes in the tumor microenvironment and cause more production and activity of this enzyme. Ultimately, this situation results in anti-tumor immune suppression which is in favor of tumor growth. Several inhibitors such as 1-methyl-tryptophan have been introduced for indoleamine-2,3 dioxygenase enzyme and some of them are widely utilized in pre-clinical and clinical trials. Importantly at the molecular level, indoleamine-2,3 dioxygenase is positioned in a series of intricate signaling and molecular networks. Here, the main objective is to provide a focused view of indoleamine-2,3 dioxygenase enhancer pathways and propose further studies to cover the gap in available information on the function of indoleamine-2,3 dioxygenase enzyme in the tumor microenvironment.

Regulation of IDO2 by the Aryl Hydrocarbon Receptor (AhR) in Breast Cancer

Indoleamine 2,3-dioxygenase 2 (IDO2) is a tryptophan-catabolizing enzyme and a homolog of IDO1 with a distinct expression pattern compared with IDO1. In dendritic cells (DCs), IDO activity and the resulting changes in tryptophan level regulate T-cell differentiation and promote immune tolerance. Recent studies indicate that IDO2 exerts an additional, non-enzymatic function and pro-inflammatory activity, which may play an important role in diseases such as autoimmunity and cancer. Here, we investigated the impact of aryl hydrocarbon receptor (AhR) activation by endogenous compounds and environmental pollutants on the expression of IDO2. Treatment with AhR ligands induced IDO2 in MCF-7 wildtype cells but not in CRISPR-cas9 AhR-knockout MCF-7 cells. Promoter analysis with IDO2 reporter constructs revealed that the AhR-dependent induction of IDO2 involves a short-tandem repeat containing four core sequences of a xenobiotic response element (XRE) upstream of the start site of the human ido2 gene. The analysis of breast cancer datasets revealed that IDO2 expression increased in breast cancer compared with normal samples. Our findings suggest that the AhR-mediated expression of IDO2 in breast cancer could contribute to a pro-tumorigenic microenvironment in breast cancer.

Elevated Kynurenine Levels in Patients with Primary Sjögren's Syndrome

OBJECTIVE

We aimed to investigate the plasma levels of tryptophan (Trp) and its metabolites in patients with primary Sjögren's syndrome (pSS).

METHODS

The study included 34 pSS patients and 42 healthy individuals, and serum Trp and kynurenine (Kyn) concentrations were measured by liquid chromatography with tandem mass spectrometry. Trp degradation was predicted using the ratio of Kyn and Trp concentrations (Kyn/Trp).

RESULTS

In our study, the mean serum Trp concentration was found to be considerably lower in the pSS group than in the control group (P = .001). The levels of Kyn (P = .019) and the Kyn/Trp ratio (P < .001) were significantly higher in the pSS group than in the control group. The Kyn/Trp ratio was negatively correlated with C-reactive protein (r = -0.369, P = .032).

CONCLUSION

We found that Kyn pathway metabolism was altered in patients with pSS. This suggests that Trp metabolism may be closely linked to the disease pathogenesis of pSS.

Fatigue in Patients on Chronic Hemodialysis: The Role of Indoleamine 2,3-Dioxygenase (IDO) Activity, Interleukin-6, and Muscularity

Fatigue is a frequent symptom in hemodialysis (HD), and the indolamine-2,3-dioxygenase (IDO) metabolic trap has been hypothesized in the pathogenesis of fatigue. The association between IDO activity according to fatigue and its relationship with muscle mass and function in HD patients was verified. Chronic HD patients were considered, and fatigue was assessed. The plasma kynurenines and tryptophan ratio (Kyn/Trp), as surrogate of IDO activity, and interleukin (IL)-6 were measured. Muscularity was assessed by BIA and muscle strength by hand-grip dynamometer. 50 HD patients were enrolled, and fatigue was present in 24% of the cohort. Patients with fatigue showed higher Kyn/Trp (p = 0.005), were older (p = 0.007), and IL-6 levels resulted higher than in non-fatigue patients (p < 0.001). HD patients with fatigue showed lower intracellular water (surrogate of muscle mass) (p < 0.001), as well as lower hand grip strength (p = 0.02). The Kyn/Trp ratio positively correlated with IL-6 and ECW/ICW (p = 0.004 and p = 0.014). By logistic regression analysis, higher ICW/h² was associated with lower odds of fatigue (OR, 0.10; 95% CI, 0.01 to 0.73). In conclusion, our cohort fatigue was associated with a higher Kyn/Trp ratio, indicating a modulation of IDO activity. The Kyn/Trp ratio correlated with IL-6, suggesting a potential role of IDO and inflammation in inducing fatigue and changes in muscularity.

Canagliflozin attenuates chronic unpredictable mild stress induced neuroinflammation via modulating AMPK/mTOR autophagic signaling

Although vast progress has been made to understand the pathogenesis of depression, existing antidepressant remedies, with several adverse effects, are not fully adequate. Interestingly, new emerging theories implicating an altered HPA-axis, tryptophan metabolism, neuroinflammation and altered gut integrity were proposed to further identify novel therapeutic targets. Along these lines, canagliflozin (CAN), a novel antidiabetic medication with anti-inflammatory and neuroprotective activity may present an effective treatment for depression; nevertheless, no studies have explored its effect on depressive disorder yet. To this end, this study aimed to investigate the possible antidepressant activity of CAN in CUMS and the mechanisms underlying its action on the gut-brain inflammation axis as well as the alteration in the TRY/KYN pathway in addition to its role in modulating the autophagic signaling cascade. Interestingly, CAN successfully attenuated the CUMS-induced elevations in despair and anhedonic behaviors as well as the elevated serum CORT. Furthermore, it enhanced gut integrity via hampering the CUMS-induced colonic inflammation and amending colonic tight junction proteins. The enhanced gut integrity was further corroborated by a notable anti-inflammatory and neuroprotective activity manifested via the observed mitigation of immune cell activation in addition to IDO hippocampal protein content and promotion of the autophagy cascade. Our findings postulate the possible anti-inflammatory and neuroprotective effects of CAN and the implication of TRY/KYN and AMPK/mTOR signaling pathways in the CUMS-induced MDD. Hence, this study shed light to the promising role of CAN in the augmentation of the current antidepressant treatments.

Electroacupuncture attenuates LPS-induced depression-like behavior through kynurenine pathway

Background

A growing body of evidence suggests that inflammation and changes in glutamate neurotransmission are two pathophysiological mechanisms underlying depression. Electroacupuncture (EA) is a common therapeutic tool for the treatment of depression. However, the potential antidepressant mechanism of EA remains obscure. The change of the kynurenine pathway (KP) is the research priority of antidepressant mechanisms. This study will investigate the role of EA on lipopolysaccharide (LPS)-induced depression-like behavior and explore its possible mechanism of action.

Methods

Lipopolysaccharide was used to induce depression-like behavior, and EA was given at Hegu (L14) and Taichong (LR3) acupoints in C57BL/6J mice. Depression-like behaviors were measured by behavioral tests, including tail suspension test (TST), sucrose preference test (SPT), force swim test (FST), and open field test (OFT). The levels of inflammatory cytokines IL-1β, IL-6, and TNF-α, and KP enzyme IDO1 were measured by qPCR and enzyme-linked immunosorbent assay (ELISA), while high-performance liquid chromatography (HPLC) was performed to detect the content of prefrontal cortex and hippocampal as well as serum glutamate, tryptophan (TRP), kynurenic (KYN), and quinolinic acid (QA).

Results

The results showed that (1) as evidenced by increased spontaneous locomotor activities, decreased immobility duration, and a stronger preference for sucrose in the sucrose preference test, EA reversed LPS-challenged depressive-like behavior. (2) EA at L14 and LR3 decreased the levels of inflammatory cytokines, inhibited IDO1, and regulated KP metabolisms, as well as lowered the concentration of glutamate. (3) EA may exert anti-depression effects by acting on the kynurenine pathway.

Conclusion

This study evaluated the effects of EA on depression-like behaviors induced by lipopolysaccharide (LPS) and its regulation of inflammation and the glutamatergic system. Our results suggest that EA can ameliorate depression-like behaviors, lower the level of inflammation, and reduce the release of glutamate, possibly through the regulation of the kynurenine pathway in the brain.

Effect of apigenin on tryptophan metabolic key enzymes expression in lipopolysaccharide-induced microglial cells and its mechanism

[Aims] Flavonoid apigenin (API) has a wide range of biological functions, particularly anti-inflammation. Indoleamine 2,3-dioxygenase (IDO) and 2-Amino-3-carboxymuconate-6-semialdehyde decarboxylase (ACMSD) are important tryptophan metabolic enzymes that play pivotal roles in the production of toxic metabolite quinolinic acid. However, the relationship between inflammation and ACMSD remains unclear. The present study investigated the relationship between inflammation and tryptophan metabolic key enzymes. Similarly, the anti-inflammatory effect of API on important tryptophan metabolic enzymes was examined in lipopolysaccharide (LPS)-treated microglial cells. [Main methods] MG6 cells were exposed to LPS with or without API treatment for 24-48 h. IDO and ACMSD mRNA expression and production of inflammatory mediators were analyzed. Activation of inflammatory signaling pathways, such as mitogen-activated protein kinase (MAPK) and nuclear factor-κB (NF-κB), was also examined to study the mechanism of API in the inflammatory state. [Key findings] LPS suppressed ACMSD expression and enhanced IDO expression. However, API elevated ACMSD mRNA expression and suppressed IDO mRNA expression in LPS-treated MG6 cells. Furthermore, API suppressed interleukin-6 and nitric oxide production, whereas overproduction of inflammatory mediators enhanced IDO expression and assisted tryptophan degradation. API also inhibited activation of extracellular signal-regulated kinase (Erk) and jun N-terminal kinase (JNK) MAPK, and degradation of IκBα. [Significance] These results indicate alteration of ACMSD expression under inflammatory conditions. Moreover, API recovers expression of tryptophan metabolic key enzymes, which may be mediated by inhibition of proinflammatory mediator production via inactivation of Erk, JNK MAPK, and NF-κB pathways in LPS-stimulated microglial cells.

Cancer Immunotherapy: The Checkpoint between Chronic Colitis and Colorectal Cancer

Inflammatory Bowel Disease (IBD) is a group of diseases that cause intestinal inflammation and lesions because of an abnormal immune response to host gut microflora. Corticosteroids, anti-inflammatories, and antibiotics are often used to reduce non-specific inflammation and relapse rates; however, such treatments are ineffective over time. Patients with chronic colitis are more susceptible to developing colorectal cancer, especially those with a longer duration of colitis. There is often a limit in using chemotherapy due to side effects, leading to reduced efficacy, leaving an urgent need to improve treatments and identify new therapeutic targets. Cancer immunotherapy has made significant advances in recent years and is mainly categorized as cancer vaccines, adoptive cellular immunotherapy, or immune checkpoint blockade therapies. Checkpoint markers are expressed on cancer cells to evade the immune system, and as a result checkpoint inhibitors have transformed cancer treatment in the last 5-10 years. Immune checkpoint inhibitors have produced long-lasting clinical responses in both single and combination therapies. Winnie mice are a viable model of spontaneous chronic colitis with immune responses like human IBD. Determining the expression levels of checkpoint markers in tissues from these mice will provide insights into disease initiation, progression, and cancer. Such information will lead to identification of novel checkpoint markers and the development of treatments with or without immune checkpoint inhibitors or vaccines to slow or stop disease progression.

Impact of the combination of sintilimab and chemotherapy on the tumor and paratumor PD-L1, IDO, TIM-3, FOXP3+ and CD8 expressions in patients with advanced esophageal squamous cell carcinoma

BACKGROUND

Anti-PD-1/PD-L1 therapeutics have been widely used in the clinic in various tumors, including advanced esophageal cancer, showing remarkable treatment efficacy. Factors determining the response to anti-PD-1/PD-L1 therapeutics are numerous, including the tumor microenvironment, such as CD8+ T cells and expression of PD-1/PD-L1. Our study aimed to explore the effect of chemoimmunotherapy on the expression of CD8+ T cells, TIM-3, and FOXP3+ in tumor, paratumor tissues, and the expression of PD-L1, IDO, in tumor, paratumor tissues, and lymph nodes, and analyze the correlation among these markers.

METHODS

A total of 18 patients were allocated into two treatment groups: a treatment group and a concurrent control group. A total of 38 tissue samples, 114 slides (tumor, paratumor, and lymph node) were collected in the treatment group, and 37 tissue samples, 111 slides (tumor, paratumor, and lymph node) were collected in the concurrent control group.

RESULTS

The expression of PD-L1, CD8+, FOXP3+, TIM-3, and IDO in tumors, paratumor tissues, but not lymph nodes, was significantly affected by chemoimmunotherapy. Compared with patients without chemoimmunotherapy, the expression of CD8+ T cells, IDO, and PD-L1 was significantly decreased in tumor and paratumor tissues after chemoimmunotherapy, while FOXP3+ expression was significantly decreased only in tumor tissues, and TIM-3 expression was significantly decreased only in paratumor tissues. Moreover, the correlation between these markers was also completely altered after chemoimmunotherapy. In addition, N staging was associated with high expression of CD8 in advanced esophageal squamous cell carcinoma in the concurrent control group.

CONCLUSION

This study provides new insight into the effects of CI treatment on isolated CD8+ T cell infiltration, PD-L1, IDO, FOXP3+ and TIM-3 expression as well as their cross-talk in different tissues enabling a better understanding of the impact of CI treatment on the immune microenvironment.

A phthalocyanine-based photosensitizer for effectively combating triple negative breast cancer with enhanced photodynamic anticancer activity and immune response

Although photodynamic therapy (PDT) has attracted great interest, the photosensitizers in clinical had weak inhibition on metastasis and invasion of cancers. Additionally the immune response induced by PDT was insufficient to eradicate cancer. Herein, indoximod, an inhibitor of indoleamine 2,3-dioxygenase (IDO), is introduced to concatenate with zinc phthalocyanines (ZnPc) for effectively overcoming above inadequacy. Due to indoximod moiety, photosensitizer 1-MT-Pc can obtain enhanced intracellular uptake and high reactive oxygen species (ROS) generation. More impressively, 1-MT-Pc can achieve remarkable photocytotoxicity towards TNBC cells and negligible damage to normal cells. Meanwhile, 1-MT-Pc effectively inhibits metastasis and invasion of TNBC cells. Importantly, 1-MT-Pc exhibit elevated inhibitory effect on 4T1 tumor by enhanced PDT and immunotherapy.

Interleukin-6 and indoleamine-2,3-dioxygenase as potential adjuvant targets for Papillomavirus-related tumors immunotherapy

High-risk Human papillomavirus (HPV) infections represent an important public health issue. Nearly all cervical malignancies are associated with HPV, and a range of other female and male cancers, such as anogenital and oropharyngeal. Aiming to treat HPV-related tumors, our group developed vaccines based on the genetic fusion of the HSV-1 glycoprotein D (gD) with the HPV-16 E7 oncoprotein (gDE7 vaccines). Despite the promising antitumor results reached by gDE7 vaccines in mice, combined therapies may increase the therapeutic effects by improving antitumor responses and halting immune suppressive mechanisms elicited by tumor cells. Considering cancer immunosuppressive mechanisms, indoleamine-2,3-dioxygenase (IDO) enzyme and interleukin-6 (IL-6) stand out in HPV-related tumors. Since IL-6 sustained the constitutive IDO expression, here we evaluated the therapeutic outcomes achieved by the combination of active immunotherapy based on a gDE7 protein-based vaccine with adjuvant treatments involving blocking IDO, either by use of IDO inhibitors or IL-6 knockout mice. C57BL/6 wild-type (WT) and transgenic IL-6-/- mice were engrafted with HPV16-E6/E7-expressing TC-1 cells and treated with 1-methyl-tryptophan isoforms (D-1MT and DL-1MT), capable to inhibit IDO. In vitro, the 1MT isoforms reduced IL-6 gene expression and IL-6 secretion in TC-1 cells. In vivo, the multi-targeted treatment improved the antitumor efficacy of the gDE7-based protein vaccine. Although the gDE7 immunization achieves partial tumor mass control in combination with D-1MT or DL-1MT in WT mice or when administered in IL-6-/- mice, the combination of gDE7 and 1MT in IL-6-/- mice further enhanced the antitumor effects, reaching total tumor rejection. The outcome of the combined therapy was associated with an increased frequency of activated dendritic cells and decreased frequencies of intratumoral polymorphonuclear myeloid-derived suppressor cells and T regulatory cells. In conclusion, the present study demonstrated that IL-6 and IDO negatively contribute to the activation of immune cells, particularly dendritic cells, reducing gDE7 vaccine-induced protective immune responses and, therefore, opening perspectives for the use of combined strategies based on inhibition of IL-6 and IDO as immunometabolic adjuvants for immunotherapies against HPV-related tumors.

Islet allografts expressing a PD-L1 and IDO fusion protein evade immune rejection and reverse preexisting diabetes in immunocompetent mice without systemic immunosuppression

Allogeneic islet transplantation is a promising experimental therapy for poorly controlled diabetes. Despite pharmacological immunosuppression, long-term islet engraftment remains elusive. Here, we designed a synthetic fusion transgene coupling PD-L1 and indoleamine dioxygenase [hereafter PIDO] whose constitutive expression prevents immune destruction of genetically engineered islet allograft transplanted in immunocompetent mice. PIDO expressing murine islets maintain robust dynamic insulin secretion in vitro and when transplanted in allogeneic hyperglycemic murine recipients reverse pre-existing streptozotocin-induced and autoimmune diabetes in the absence of pharmacological immunosuppression for more than 50 and 8 weeks, respectively, and is dependent on host CD4 competence. Additionally, PIDO expression in allografts preserves endocrine functional viability of islets and promotes a localized tolerogenic milieu characterized by the suppression of host CD8 T cell and phagocyte recruitment and accumulation of FOXP3⁺ Tregs. Furthermore, in the canine model of xenogeneic islet transplantation, muscle implanted PIDO-expressing porcine islets displayed physiological glucose-responsive insulin secretion competency in euglycemic recipient for up to 20 weeks. In conclusion, the PIDO transgenic technology enables host CD4⁺ T cell-modulated immune evasiveness and long-term functional viability of islet allo- and xenografts in immune-competent recipients without the need for pharmacological immune suppression and would allow for improved outcomes for tissue transplantation.

Indoleamine 2,3-dioxygenase (IDO)-activity in Severe Psychiatric Disorders: A Systemic Review

BACKGROUND

Indoleamine 2,3-dioxygenase (IDO) activity is induced by cellular immune activation and therefore associated with inflammatory diseases, among others psychiatric disorders. This review aims to elucidate IDO activity reflected by kynurenine (KYN) to tryptophan (TRP) ratio in severe mental disorders.

METHODS

A systematic literature search in MEDLINE and EMBASE was conducted targeting clinical trials in English language measuring KYN/TRP in individuals with a diagnosis of depression, bipolar disorder, or schizophrenia.

RESULTS

Five out of 15 studies found higher levels of KYN/TRP in depression compared to a control group while the same amount found no difference. Moreover, three studies showed lower levels. In bipolar disorder, four out of six, and in psychotic disorders, three out of four trials found higher levels in patients compared to controls. There are only two studies comparing KYN/TRP in major depression and bipolar disorder, showing conflicting results. Eight studies focused on associations between KYN/TRP and clinical parameters, whereas two studies found positive correlations between KYN/TRP and severity of depressive symptoms. In contrast, four studies did not show an association. IDO activity during specific psychiatric treatment was analyzed by eight studies.

CONCLUSION

In summary, this review demonstrates an inconsistency in the findings of studies investigating KYN/TRP in severe mental disorders. Although there are hints that inflammation associated with TRP catabolism towards the KYN pathway via elevated IDO activity seems likely, no conclusive statements can be drawn. Presumably, the consideration of influencing factors such as inflammatory processes, metabolic activities and psychological/neuropsychiatric symptoms are pivotal for a deeper understanding of the underlying mechanisms.

Tumor microenvironment antigens

The identification and characterization of tumor antigens are central objectives in developing anti-cancer immunotherapy. Traditionally, tumor-associated antigens (TAAs) are considered relatively restricted to tumor cells (i.e., overexpressed proteins in tumor cells), whereas tumor-specific antigens (TSAs) are considered unique to tumor cells. Recent studies have focused on identifying patient-specific neoantigens, which might be highly immunogenic because they are not expressed in normal tissues. The opposite strategy has emerged with the discovery of anti-regulatory T cells (anti-Tregs) that recognize and attack many cell types in the tumor microenvironment, such as regulatory immune cells, in addition to tumor cells. The term proposed in this review is "tumor microenvironment antigens" (TMAs) to describe the antigens that draw this attack. As therapeutic targets, TMAs offer several advantages that differentiate them from more traditional tumor antigens. Targeting TMAs leads not only to a direct attack on tumor cells but also to modulation of the tumor microenvironment, rendering it immunocompetent and tumor-hostile. Of note, in contrast to TAAs and TSAs, TMAs also are expressed in non-transformed cells with consistent human leukocyte antigen (HLA) expression. Inflammation often induces HLA expression in malignant cells, so that targeting TMAs could additionally affect tumors with no or very low levels of surface HLA expression. This review defines the characteristics, differences, and advantages of TMAs compared with traditional tumor antigens and discusses the use of these antigens in immune modulatory vaccines as an attractive approach to immunotherapy. Different TMAs are expressed by different cells and could be combined in anti-cancer immunotherapies to attack tumor cells directly and modulate local immune cells to create a tumor-hostile microenvironment and inhibit tumor angiogenesis. Immune modulatory vaccines offer an approach for combinatorial therapy with additional immunotherapy including checkpoint blockade, cellular therapy, or traditional cancer vaccines. These combinations would increase the number of patients who can benefit from such therapeutic measures, which all have optimal efficiency in inflamed tumors.

IDO1 is highly expressed in macrophages of patients in advanced tumour stages of oral squamous cell carcinoma

PURPOSE

Strategies for Indolamine-2,3-dioxygenase 1 (IDO1) inhibition in cancer immunotherapy once produced encouraging results, but failed in clinical trials. Recent evidence indicates that immune cells in the tumour microenvironment, especially macrophages, contribute to immune dysregulation and therefore might play a critical role in drug resistance.

METHODS

In this study, we investigated the significance of IDO1 expressing immune cells in primary tumours and corresponding lymph node metastases (LNMs) in oral squamous cell carcinoma (OSCC) by immunohistochemistry. The link between IDO1 and macrophages was investigated by flow cytometry in tumour tissue, healthy adjacent tissue and peripheral blood mononuclear cells (PBMCs). IDO1 activity (measured as Kynurenine/Tryptophan ratio) was assessed by ELISAs.

RESULTS

High IDO1 expression in tumour-infiltrating immune cells was significantly correlated with advanced stages [Spearman's rank correlation (SRC), p = 0.027] and reduced progression-free survival (multivariate Cox regression, p = 0.034). IDO1 was significantly higher expressed in PBMCs of patients in advanced stages than in healthy controls (ANOVA, p < 0.05) and IDO1⁺ macrophages were more abundant in intratumoural areas than peritumoural (t test, p < 0.001). IDO1 expression in PBMCs was significantly correlated with IDO1 activity in serum (SRC, p < 0.05). IDO1 activity was significantly higher in patients with LNMs (t test, p < 0.01).

CONCLUSION

All in all, IDO1 expressing immune cells, especially macrophages, are more abundant in advanced stages of OSCC and are associated with reduced progression-free survival. Further investigations are needed to explore their role in local and systemic immune response. The IDO1 activity might be a suitable biomarker of metastasis in OSCC patients.

Mitochondrial interaction of fibrosis-protective 5-methoxy tryptophan enhances collagen uptake by macrophages

5-methoxy tryptophan (5-MTP) is an anti-fibrotic metabolite made by fibroblasts and epithelial cells, present in a micromolar concentrations in human blood, and is associated with the progression of fibrotic kidney disease, but the mechanism is unclear. Here, we show by microscopy and functional assays that 5-MTP influences mitochondria in human peripheral blood monocyte-derived macrophages. As a result, the mitochondrial membranes are more rigid, more branched, and are protected against oxidation. The macrophages also change their metabolism by reducing mitochondrial import of acyl-carnitines, intermediates of fatty acid metabolism, driving glucose import. Moreover, 5-MTP increases the endocytosis of collagen by macrophages, and experiments with inhibition of glucose uptake showed that this is a direct result of their altered metabolism. However, 5-MTP does not affect the macrophages following pathogenic stimulation, due to 5-MTP degradation by induced expression of indole-amine oxygenase-1 (IDO-1). Thus, 5-MTP is a fibrosis-protective metabolite that, in absence of pathogenic stimulation, promotes collagen uptake by anti-inflammatory macrophages by altering the physicochemical properties of their mitochondrial membranes.

Quantification of Linrodostat and its metabolites: Overcoming bioanalytical challenges in support of a discovery Indoleamine 2,3 dioxygenase program

BMS-986205 (Linrodostat) is a small molecule inhibitor of Indoleamine 2, 3 dioxygenase (IDO) that is currently being evaluated in clinical trials for the oral treatment of advanced cancer. Initially, there were concerns regarding possible toxicity following administration, since BMS-986205 undergoes metabolism to form 4-chloroaniline. However, it was later determined that the downstream metabolites of 4-chloroaniline might be a greater concern. To evaluate the potential toxicity of these metabolites, a sensitive LC-MS/MS analytical method was needed to quantify both the parent compound and multiple metabolites. This presented a challenge since the method required the analysis of multiple analytes while still retaining the analytical sensitivity required to support studies. By utilizing a multi-function analytical method, we were able to quantify the necessary analytes using a complex LC-MS/MS-based method including the application of both negative and positive electrospray ionization.

The tryptophan metabolism, kynurenine pathway and oxidative stress - Implications for glioma pathobiology

The kynurenine pathway receives increasing attention due to its involvement in central nervous system pathologies, i.a. neurodegenerative and psychiatric disorders, but also due to the contribution to the pathomechanism of neoplasms, including brain tumors.The present review focuses on kynurenine pathway activity in gliomas, brain tumors of glial origin. The upregulation of kynurenine pathway enzyme, indoleamine 2,3-dioxygenase (IDO), resulting in a decreased level of tryptophan and augmented kynurenine synthesis (increased (KYN/Trp ratio) are the most recognised hallmark of malignant transformation, characterised with immunomodulatory adaptations, providing an escape from defence mechanisms of the host, growth-beneficial milieu and resistance to some therapeutics. The review addresses, however, the oxidative/nitrosative stress-associated mechanisms of tryptophan catabolism, mainly the kynurenine pathway activity, linking them with glioma pathobiology.

Xenotransplantation of cryopreserved human clumps of mesenchymal stem cells/extracellular matrix complexes pretreated with IFN-γ induces rat calvarial bone regeneration

Introduction

Three-dimensional (3D) clumps of mesenchymal stem cells (MSCs)/extracellular matrix (ECM) complexes, composed with cells and self-produced intact ECM, can be grafted into defect areas without artificial scaffold to induce successful bone regeneration. Moreover, C-MSCs pretreated with IFN-γ (C-MSCsγ) increased the immunomodulatory enzyme indoleamine 2,3-dioxygenase (IDO) expression and thereby inhibited T cell activity. Xenotransplantation of human C-MSCsγ suppressed host T cell immune rejection and induced bone regeneration in mice. Besides, we have also reported that C-MSCs retain the 3D structure and bone regenerative property even after cryopreservation. To develop the "off-the-shelf" cell preparation for bone regenerative therapy that is promptly provided when needed, we investigated whether C-MSCsγ can retain the immunosuppressive and osteogenic properties after cryopreservation.

Methods

Confluent human MSCs that had formed on the cellular sheet were scratched using a micropipette tip and then torn off. The sheet was rolled to make a round clump of cells. The round cell clumps were incubated with a growth medium for 3 days, and then C-MSCs were obtained. To generate C-MSCsγ, after 2 days' culture, C-MSCs were stimulated with 50 ng/ml of IFN-γ. Both C-MSCs and C-MSCsγ were cryopreserved for 2 days and then thawed to obtain Cryo-C-MSCs and Cryo-C-MSCsγ, respectively. The biological properties of those cell clumps were assessed in vitro. In addition, to test whether human Cryo-C-MSCsγ attenuates immune rejection to induce bone regeneration, a xenograft study using a rat calvarial defect was performed.

Results

Both IFN-γ pretreatment and cryopreservation process did not affect the 3D structure and cell viability in all human cell clumps. Interestingly, Cryo-C-MSCsγ showed significantly increased IDO mRNA expression equivalent to C-MSCsγ. More importantly, xenotransplantation of human C-MSCsγ and Cryo-C-MSCsγ induced rat calvarial bone regeneration by suppressing rat T cells infiltration and the grafted human cells reduction in the grafted area. Finally, there were no human donor cells in the newly formed bone, implying that the bone reconstruction by C-MSCsγ and Cryo-C-MSCsγ can be due to indirect host osteogenesis.

Conclusion

These findings implied that Cryo-C-MSCsγ can be a promising bone regenerative allograft therapy that can be certainly and promptly supplied on demand.

Extracellular adenosine triphosphate induces IDO and IFNγ expression of human periodontal ligament cells through P2 X7 receptor signaling

BACKGROUND

Mechanical stimuli induce the release of adenosine triphosphate into the extracellular environment by human periodontal ligament cells (hPDLCs). Extracellular adenosine triphosphate (eATP) plays the role in both inflammation and osteogenic differentiation. eATP involves in immunosuppressive action by increasing immunosuppressive molecules IDO and IFNγ expression on immune cells. However, the role of eATP on the immunomodulation of hPDLCs remains unclear. This study aimed to examine the effects of eATP on the IDO and IFNγ expression of hPDLCs and the participation of purinergic P₂ receptors in this phenomenon.

METHODS

hPDLCs were treated with eATP. The mRNA and protein expression of indoleamine-pyrrole 2,3-dioxygenase (IDO) and interferon-gamma (IFNγ) were determined. The role of the purinergic P₂ receptor was determined using calcium chelator (EGTA) and PKC inhibitor (PKCi). Chemical inhibitors (KN62 and BBG), small interfering RNA (siRNA), and P₂ X₇ receptor agonist (BzATP) were used to confirm the involvement of P₂ X₇ receptors on IDO and IFNγ induction by hPDLCs.

RESULTS

eATP significantly enhanced mRNA expression of IDO and IFNγ. Moreover, eATP increased kynurenine which is the active metabolite of tryptophan breakdown catalyzed by the IDO enzyme and significantly induced IFNγ protein expression. EGTA and PKCi reduced eATP-induced IDO and IFNγ expressions by hPDLCs, confirming the role of calcium signaling. Chemical P₂ X₇ inhibitors (KN62 and BBG) and siRNA targeting the P₂ X₇ receptor significantly inhibited the eATP-induced IDO and IFNγ production. Correspondingly, BzATP markedly increased IDO and IFNγ expression.

CONCLUSION

eATP induced immunosuppressive function of hPDLCs by promoting IDO and IFNγ production via P₂ X₇ receptor signaling. eATP may become a promising target for periodontal regeneration by modulating immune response and further triggering tissue healing.

Mesenchymal Stem Cells Alleviate Inflammatory Bowel Disease Via Tr1 Cells

Mesenchymal stem cells (MSCs) have been used to achieve exciting therapeutic outcomes in many animal studies and clinical trials for various autoimmune diseases, including inflammatory bowel disease (IBD). Type 1 regulatory T (Tr1) cells are the main source of interleukin (IL) 10 in the intestine. Whether Tr1 cells are involved during MSC-mediated IBD treatment is unclear. We treated a murine model of 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced colitis with human umbilical cord-derived MSCs (hUCMSCs) and found that the disease severity was alleviated significantly in a dose-dependent manner. hUCMSCs increased the proportion of Tr1 cells and decreased that of T helper (Th)-1 and Th17 cells in the spleen and mesenteric lymph nodes in different stages of colitis. We found that the upregulation of Tr1 cells by hUCMSCs was abrogated after blocking indoleamine-2,3-dioxygenase (IDO), and IDO knockdown in hUCMSCs reversed the increase in Tr1 cell proportions caused by hUCMSCs in colitis. Moreover, hUCMSCs inhibited apoptosis and promoted the proliferation of Tr1 cells. Our results suggest that Tr1 cells play an important role in the amelioration of IBD by MSCs, and they are the target population for the alleviation of IBD by MSCs, providing meaningful references for the study of therapeutic mechanisms of MSCs in other inflammatory diseases.

ETC Model: How One Small Dialysis Organization Is Navigating Uncharted Policy Waters

The ETC model proposes to increase access to home dialysis and transplant for patients with ESRD. Implementation of this model is happening while many dialysis organizations are still suffering the far-reaching effects of the coronavirus disease 2019 (COVID-19) pandemic. In addition, the model has the potential to negatively affect small and independent dialysis organizations disproportionately. It incentivizes home dialysis over transplant and promotes development of new home dialysis programs, rewards achievement over improvement, and places an excessive burden on small and independent dialysis organizations. Advantages of the program include the focus on self-care as an acceptable alternative to home dialysis for some patients and the potential for some organizations to make improvements in care with increased reimbursements. The authors hope that the Centers for Medicare and Medicaid Services will address many of these concerns in updated rulemaking and guidance.

Tryptophan, Kynurenine and Kynurenic Acid Concentrations in Milk and Serum of Dairy Cows with Prototheca Mastitis

The aim of this work was to investigate serum and milk levels of tryptophan (TRP), kynurenine (KYN), and kynurenic acid (KYNA), as well as the activity of indoleamine 2,3-dioxygenase (IDO) in cows with mastitis due to Prototheca algae. The study was prompted by previous research showing a link between the KYN pathway of TRP metabolism and bovine mastitis of bacterial etiology. The study was carried out over a 2-year period (2018-2019) and included quarter milk and serum samples collected from six dairy herds in Poland. The samples were obtained from healthy cows and cows with Prototheca mastitis of either clinical and subclinical manifestation, as determined upon direct measurement of the somatic cell count or indirectly by performing a California Mastitis Test on suspected quarters. Both TRP and KYN concentrations were significantly lower in milk of mastitic cows compared to healthy animals (0.8 vs. 8.72 µM, p = 0.001; 0.07 vs. 0.32 µM, p = 0.001, respectively). The difference in TRP and KYN concentrations in the sera of the two animal groups was much less pronounced (25.55 vs. 27.57 µM, 3.03 vs. 3.56 nM, respectively). The concentration of KYNA was almost at the same level in milk (1.73 vs. 1.70 nM) and in serum (80.47 vs. 75.48 nM) of both mastitic and healthy cows. The data showed that the level of TRP and its metabolites in serum was conspicuously higher compared to milk in all cows under the study. The activity of IDO was significantly higher in milk of cows with Prototheca mastitis compared to healthy animals (71.4 vs. 40.86, p < 0.05), while in serum it was pretty much the same (135.94 vs. 124.98, p > 0.05). The IDO activity differed significantly between serum and milk both for mastitic (135.94 vs. 71.4, p < 0.05) and healthy cows (124.98 vs. 40.86, p < 0.001). In conclusion, low values of TRP and KYN concentrations or elevated IDO activity in milk samples might be used as markers of mastitis due to infectious causes, including Prototheca spp.

Indoleamine 2, 3-Dioxygenase Promotes Aryl Hydrocarbon Receptor-Dependent Differentiation Of Regulatory B Cells in Lung Cancer

Regulatory B cells (Breg) are IL-10 producing subsets of B cells that contribute to immunosuppression in the tumor microenvironment (TME). Breg are elevated in patients with lung cancer; however, the mechanisms underlying Breg development and their function in lung cancer have not been adequately elucidated. Herein, we report a novel role for Indoleamine 2, 3- dioxygenase (IDO), a metabolic enzyme that degrades tryptophan (Trp) and the Trp metabolite L-kynurenine (L-Kyn) in the regulation of Breg differentiation in the lung TME. Using a syngeneic mouse model of lung cancer, we report that Breg frequencies significantly increased during tumor progression in the lung TME and secondary lymphoid organs, while Breg were reduced in tumor-bearing IDO deficient mice (IDO^-/-). Trp metabolite L-Kyn promoted Breg differentiation in-vitro in an aryl hydrocarbon receptor (AhR), toll-like receptor-4-myeloid differentiation primary response 88, (TLR4-MyD88) dependent manner. Importantly, using mouse models with conditional deletion of IDO in myeloid-lineage cells, we identified a significant role for immunosuppressive myeloid-derived suppressor cell (MDSC)-associated IDO in modulating in-vivo and ex-vivo differentiation of Breg. Our studies thus identify Trp metabolism as a therapeutic target to modulate regulatory B cell function during lung cancer progression.

p-Coumaric acid regulates macrophage polarization in myocardial ischemia/reperfusion by promoting the expression of indoleamine 2, 3-dioxygenase

Macrophage infiltration is a hallmark pathological change observed in early stage myocardial ischemia/reperfusion (MI/R) injury and one of the main causes of myocardial damage. Here, we investigated the effects of p-Coumaric acid (p-CA) on macrophage polarization following MI/R injury and its mechanisms. In vitro, p-CA decreases the expression of LPS/IFN-γ-induced M1 macrophage markers (TNF-α, IL-6, iNOS and CCL2) and increases IL-4-induced M2 macrophage markers (IL-10, CD206, Arg1 and Mrc) in mouse bone marrow-derived macrophages (BMDMs). Additionally, p-CA elevated indoleamine 2, 3-dioxygenase (IDO) protein expression levels, M2 macrophage polarization and M2 macrophage markers through IL-4. In contrast, repression of IDO attenuated p-CA functions regulating BMDMs through IL-4. In vivo, IDO expression was downregulated in mouse hearts subjected to MI/R injury. Treatment of p-CA increased IDO expression in the hearts of MI/R mice. Functionally, p-CA decreases M1 macrophage markers, the number of M1 macrophages and inflammation around heart tissue following MI/R injury. Importantly, p-CA reduces cardiomyocyte apoptosis caused by MI/R. Altogether, our study identified that p-CA modulates macrophage polarization by promoting IDO expression and that p-CA attenuates macrophage-mediated inflammation following MI/R by promoting M2 macrophage polarization through IDO.

Indoleamine 2,3-Dioxygenase Cannot Inhibit Chlamydia trachomatis Growth in HL-60 Human Neutrophil Granulocytes

Aims

Neutrophil granulocytes are the major cells involved in Chlamydia trachomatis (C. trachomatis)-mediated inflammation and histopathology. A key protein in human intracellular antichlamydial defense is the tryptophan-degrading enzyme indoleamine 2,3-dioxygenase (IDO) which limits the growth of the tryptophan auxotroph Chlamydia. Despite its importance, the role of IDO in the intracellular defense against Chlamydia in neutrophils is not well characterized.

Methods

Global gene expression screen was used to evaluate the effect of C. trachomatis serovar D infection on the transcriptome of human neutrophil granulocytes. Tryptophan metabolite concentrations in the Chlamydia-infected and/or interferon-gamma (IFNG)-treated neutrophils were measured by ultra-high-performance liquid chromatography–tandem mass spectrometry (UHPLC–MS/MS).

Results

Our results indicate that the C. trachomatis infection had a major impact on neutrophil gene expression, inducing 1,295 genes and repressing 1,510 genes. A bioinformatics analysis revealed that important factors involved in the induction of neutrophil gene expression were the interferon-related transcription factors such as IRF1-5, IRF7-9, STAT2, ICSB, and ISGF3. One of the upregulated genes was ido1, a known infection- and interferon-induced host gene. The tryptophan-degrading activity of IDO1 was not induced significantly by Chlamydia infection alone, but the addition of IFNG greatly increased its activity. Despite the significant IDO activity in IFNG-treated cells, C. trachomatis growth was not affected by IFNG. This result was in contrast to what we observed in HeLa human cervical epithelial cells, where the IFNG-mediated inhibition of C. trachomatis growth was significant and the IFNG-induced IDO activity correlated with growth inhibition.

Conclusions

IDO activity was not able to inhibit chlamydial growth in human neutrophils. Whether the IDO activity was not high enough for inhibition or other chlamydial growth-promoting host mechanisms were induced in the infected and interferon-treated neutrophils needs to be further investigated.

Content of tryptophan and kynurenines in serum and milk of dairy cows with mastitis caused by Streptococcus spp

The aim of the study was to investigate serum and milk concentrations of tryptophan (TRP), kynurenine (KYN) and kynurenic acid (KYNA), and activity of indoleamine 2,3-dioxygenase (IDO) in cows suffering from mastitis caused by Streptococcus spp. The blood and milk samples were collected from Holstein-Friesian cows farmed in the Lublin region of Poland. It was found that TRP was lower in cows with mastitis both in serum and milk. KYN was lower in serum but not in milk. KYNA was not significantly altered in diseased cows both in serum and milk. The activity of IDO calculated as KYN to TRP ratio was unchanged in serum but was markedly elevated in milk of cows with mastitis. Our findings may have important implications for diagnosis of mastitis in cows because an increase of activity of IDO and reduction of TRP in milk might be a valuable early marker predicting the occurrence of the disease.

Combination of IDO1high and CCL19low expression in the tumor tissue reduces survival in HPV positive cervical cancer

The over expression of Indoleamine 2, 3-Dioxygenase (IDO1), an immune checkpoint inhibitor, is well known in cervical cancer. However, its association with chemokine signals promoting cellular infiltration in the cervical tumor microenvironment, is unknown. In the current study, we evaluated the expression and enzymatic activity of IDO1. We also profiled the expression of chemokine ligand-receptors- CCR4-CCL22, CXCR3-CXCL10, CXCR4-CXCL12, and CCR7-CCL19 using immunohistochemistry (IHC), and studied their association with IDO1, statistically. After getting an informed consent, punch biopsy samples were obtained from 105 patients diagnosed with cervical cancer. HPV typing by Sanger sequencing, realtime PCR for quantifying IDO1 mRNA expression, HPLC for determining the K/T ratio and IHC for all the above chemokine receptor-ligand pairs along with IDO1 were performed. We found a significant increase in the expression of IDO1 and K/T levels in early and locally advanced stages when compared to Stage IV disease. Among the chemokine ligand -receptor pairs profiled, we found that high CCL19 marker expression was a good prognostic indicator of patients' disease-free (p = 0.013) and overall survival (p = 0.043). Although we could not identify IDO1 as an independent prognostic factor, we found that high levels of IDO1 expression may further reduce survival outcomes in patients with low CCL19 expression. This could be vital for designing immuno therapeutic interventions targeting IDO1.

Emerging Roles on Immunological Effect of Indoleamine 2,3-Dioxygenase in Liver Injuries

Indoleamine 2,3-dioxygenase (IDO) is one of the initial rate-limiting enzymes of the kynurenine pathway (KP), which causes immune suppression and induction of T cell anergy. It is associated with the imbalance of immune homeostasis in numerous diseases including cancer, chronic viral infection, allergy, and autoimmune diseases. Recently, IDO has extended its role to liver field. In this review, we summarize the dysregulation and potentials of IDO in the emerging field of liver injuries, as well as current challenges for IDO targets. In particular, we discuss unexpected conclusions against previous work published. IDO is induced by pro-inflammatory cytokines in liver dysfunction and exerts an immunosuppressive effect, whereas the improvement of liver injury may require consideration of multiple factors besides IDO.