Cell-autonomous control of interferon type I expression by indoleamine 2,3-dioxygenase in regulatory CD19+ dendritic cells

Involvement of tumor immune microenvironment metabolic reprogramming in colorectal cancer progression, immune escape, and response to immunotherapy

Metabolic reprogramming is a k`ey hallmark of tumors, developed in response to hypoxia and nutrient deficiency during tumor progression. In both cancer and immune cells, there is a metabolic shift from oxidative phosphorylation (OXPHOS) to aerobic glycolysis, also known as the Warburg effect, which then leads to lactate acidification, increased lipid synthesis, and glutaminolysis. This reprogramming facilitates tumor immune evasion and, within the tumor microenvironment (TME), cancer and immune cells collaborate to create a suppressive tumor immune microenvironment (TIME). The growing interest in the metabolic reprogramming of the TME, particularly its significance in colorectal cancer (CRC)-one of the most prevalent cancers-has prompted us to explore this topic. CRC exhibits abnormal glycolysis, glutaminolysis, and increased lipid synthesis. Acidosis in CRC cells hampers the activity of anti-tumor immune cells and inhibits the phagocytosis of tumor-associated macrophages (TAMs), while nutrient deficiency promotes the development of regulatory T cells (Tregs) and M2-like macrophages. In CRC cells, activation of G-protein coupled receptor 81 (GPR81) signaling leads to overexpression of programmed death-ligand 1 (PD-L1) and reduces the antigen presentation capability of dendritic cells. Moreover, the genetic and epigenetic cell phenotype, along with the microbiota, significantly influence CRC metabolic reprogramming. Activating RAS mutations and overexpression of epidermal growth factor receptor (EGFR) occur in approximately 50% and 80% of patients, respectively, stimulating glycolysis and increasing levels of hypoxia-inducible factor 1 alpha (HIF-1α) and MYC proteins. Certain bacteria produce short-chain fatty acids (SCFAs), which activate CD8+ cells and genes involved in antigen processing and presentation, while other mechanisms support pro-tumor activities. The use of immune checkpoint inhibitors (ICIs) in selected CRC patients has shown promise, and the combination of these with drugs that inhibit aerobic glycolysis is currently being intensively researched to enhance the efficacy of immunotherapy.

An immuno-lipidomic signature revealed by metabolomic and machine-learning approaches in labial salivary gland to diagnose primary Sjögren's syndrome

Introduction

Assessing labial salivary gland exocrinopathy is a cornerstone in primary Sjögren's syndrome. Currently this relies on the histopathologic diagnosis of focal lymphocytic sialadenitis and computing a focus score by counting lym=phocyte foci. However, those lesions represent advanced stages of primary Sjögren's syndrome, although earlier recognition of primary Sjögren's syndrome and its effective treatment could prevent irreversible damage to labial salivary gland. This study aimed at finding early biomarkers of primary Sjögren's syndrome in labial salivary gland combining metabolomics and machine-learning approaches.

Methods

We used a standardized targeted metabolomic approach involving high performance liquid chromatography coupled with mass spectrometry among newly diagnosed primary Sjögren's syndrome (n=40) and non- primary Sjögren's syndrome sicca (n=40) participants in a prospective cohort. A metabolic signature predictive of primary Sjögren's syndrome status was explored using linear (logistic regression with elastic-net regularization) and non-linear (random forests) machine learning architectures, after splitting the data set into training, validation, and test sets.

Results

Among 126 metabolites accurately measured, we identified a discriminant signature composed of six metabolites with robust performances (ROC-AUC = 0.86) for predicting primary Sjögren's syndrome status. This signature included the well-known immune-metabolite kynurenine and five phospholipids (LysoPC C28:0; PCaa C26:0; PCaaC30:2; PCae C30:1, and PCaeC30:2). It was split into two main components: the first including the phospholipids was related to the intensity of lymphocytic infiltrates in salivary glands, while the second represented by kynurenine was independently associated with the presence of anti-SSA antibodies in participant serum.

Conclusion

Our results reveal an immuno-lipidomic signature in labial salivary gland that accurately distinguishes early primary Sjögren's syndrome from other causes of sicca symptoms.

Immunometabolic reprogramming, another cancer hallmark

Molecular carcinogenesis is a multistep process that involves acquired abnormalities in key biological processes. The complexity of cancer pathogenesis is best illustrated in the six hallmarks of the cancer: (1) the development of self-sufficient growth signals, (2) the emergence of clones that are resistant to apoptosis, (3) resistance to the antigrowth signals, (4) neo-angiogenesis, (5) the invasion of normal tissue or spread to the distant organs, and (6) limitless replicative potential. It also appears that non-resolving inflammation leads to the dysregulation of immune cell metabolism and subsequent cancer progression. The present article delineates immunometabolic reprogramming as a critical hallmark of cancer by linking chronic inflammation and immunosuppression to cancer growth and metastasis. We propose that targeting tumor immunometabolic reprogramming will lead to the design of novel immunotherapeutic approaches to cancer.

HIF: a master regulator of nutrient availability and metabolic cross-talk in the tumor microenvironment

A role for hypoxia-inducible factors (HIFs) in hypoxia-dependent regulation of tumor cell metabolism has been thoroughly investigated and covered in reviews. However, there is limited information available regarding HIF-dependent regulation of nutrient fates in tumor and stromal cells. Tumor and stromal cells may generate nutrients necessary for function (metabolic symbiosis) or deplete nutrients resulting in possible competition between tumor cells and immune cells, a result of altered nutrient fates. HIF and nutrients in the tumor microenvironment (TME) affect stromal and immune cell metabolism in addition to intrinsic tumor cell metabolism. HIF-dependent metabolic regulation will inevitably result in the accumulation or depletion of essential metabolites in the TME. In response, various cell types in the TME will respond to these hypoxia-dependent alterations by activating HIF-dependent transcription to alter nutrient import, export, and utilization. In recent years, the concept of metabolic competition has been proposed for critical substrates, including glucose, lactate, glutamine, arginine, and tryptophan. In this review, we discuss how HIF-mediated mechanisms control nutrient sensing and availability in the TME, the competition for nutrients, and the metabolic cross-talk between tumor and stromal cells.

Assessment of interferon gamma and indoleamine 2,3-dioxygenase 1 analysis during respiratory syncytial virus infection in infants in Italy: an observational case-control study

OBJECTIVES

The aim of this study was to measure interferon gamma (IFN-γ) and indoleamine 2,3-dioxygenase 1 (IDO1) values in the White blood cells of infants during respiratory tract infections and to compare these with healthy age-matched controls.

DESIGN

This was a prospective, observational case-control study conducted in 2019-2020.

SETTING

The study took place at Regina Margherita Children's Hospital, Turin, Italy.

PARTICIPANTS

The study comprised 63 infants, including 26 patients hospitalised for bronchiolitis due to a respiratory syncytial virus (RSV) infection and 37 age-matched controls. The inclusion criteria included a positive RSV test for an infant with bronchiolitis.

METHODS

We collected peripheral blood and measured the relative quantification of messenger RNA (mRNA) expression of IFN-γ and IDO1 with TaqMan real-time PCR amplification. The data were collected on the first day of admission.

RESULTS

The mean age of the 26 patients with RSV bronchiolitis (53.8% female) was 85 (9-346) days when they were admitted to the hospital. Their mean gestational age at birth was 38 weeks and their mean birth weight was 3100 (2780-3730) g. The expression of IFN-γ was significantly reduced in patients with bronchiolitis RSV compared with healthy controls (p=0.0132). However, there was no significant difference between the two groups when the IDO1 mRNA expression values in their WCC were measured (p=0.0642).

CONCLUSION

Our findings did not clarify whether IDO1 expression was related to the early stage of the disease or to the young age of the infants. The data provide evidence that IFN-γ was significantly reduced in infants with bronchiolitis due to RSV, compared with age-matched healthy controls, but the IDO1 was not different. New investigations that focus on subjects infected with RSV at different stages of infancy would help to clarify whether IDO1 expression can be related to age.

Targeting Tryptophan Catabolism in Cancer Immunotherapy Era: Challenges and Perspectives

Metabolism of tryptophan (Trp), an essential amino acid, represent a major metabolic pathway that both promotes tumor cell intrinsic malignant properties as well as restricts antitumour immunity, thus emerging as a drug development target for cancer immunotherapy. Three cytosolic enzymes, namely indoleamine 2,3-dioxygenase 1 (IDO1), IDO2 and tryptophan 2,3-dioxygenase (TDO2), catalyzes the first-rate limiting step of the degradation of Trp to kynurenine (Kyn) and modulates immunity toward immunosuppression mainly through the aryl hydrocarbon receptor (AhR) activation in numerous types of cancer. By restoring antitumor immune responses and synergizing with other immunotherapies, the encouraging preclinical data of IDO1 inhibitors has dramatically failed to translate into clinical success when combined with immune checkpoints inhibitors, reigniting the debate of combinatorial approach. In this review, we i) provide comprehensive evidences on immunomodulatory role of the Trp catabolism metabolites that highlight this pathway as relevant target in immuno-oncology, ii)ii) discuss underwhelming results from clinical trials investigating efficacy of IDO1 inhibitors and underlying mechanisms that might have contributed to this failure, and finally, iii) discuss the current state-of-art surrounding alternative approaches of innovative antitumor immunotherapies that target molecules of Trp catabolism as well as challenges and perspectives in the era of immunotherapy.

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

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

Blockade of IDO-Kynurenine-AhR Axis Ameliorated Colitis-Associated Colon Cancer via Inhibiting Immune Tolerance

BACKGROUND & AIMS

Chronic inflammation in colon section is associated with an increased risk of colorectal cancer (CRC). Proinflammatory cytokines were produced in a tumor microenvironment and correlated with poor clinical outcome. Tumor-infiltrating T cells were reported to be greatly involved in the development of colon cancer. In this study, we demonstrated that kynurenine (Kyn), a metabolite catalyzed by indoleamine 2,3-dioxygenase (IDO), was required for IDO-mediated T cell function, and adaptive immunity indeed played a critical role in CRC.

METHODS

Supernatant of colon cancer cells was used to culture activated T cells and mice spleen lymphocytes, and the IDO1-Kyn-aryl hydrocarbon (AhR) receptor axis was determined in vitro. In vivo, an azoxymethane (AOM)/dextran sodium sulfate (DSS)-induced CRC model was established in IDO-/-, Rag1-/-, and wild-type mice, and tumor-associated T lymphocyte infiltration and Kyn/AhR signaling pathway changes were measured in each group.

RESULTS

Kyn promoted AhR nuclear translocation increased the transcription of Foxp3, a marker of regulatory T cells (Tregs), through improving the interaction between AhR and Foxp3 promoter. Additionally, compared WT mice, IDO-/- mice treated with AOM/DSS exhibited fewer and smaller tumor burdens in the colon, with less Treg and more CD8+ T cells infiltration, while Kyn administration abolished this regulation. Rag1-/- mice were more sensitive to AOM/DSS-induced colitis-associated colon cancer (CRC) compared with the wild-type mice, suggesting that T cell-mediated adaptive immunity indeed played a critical role in CRC.

CONCLUSIONS

We demonstrated that inhibition of IDO diminished Kyn/AhR-mediated Treg differentiation and could be an effective strategy for the prevention and treatment of inflammation-related colon cancer.

miRNAs as novel immunoregulators in cancer

The immune system is a well-known vital regulator of tumor growth, and one of the main hallmarks of cancer is evading the immune system. Immune system deregulation can lead to immune surveillance evasion, sustained cancer growth, proliferation, and metastasis. Tumor-mediated disruption of the immune system is accomplished by different mechanisms that involve extensive crosstalk with the immediate microenvironment, which includes endothelial cells, immune cells, and stromal cells, to create a favorable tumor niche that facilitates the development of cancer. The essential role of non-coding RNAs such as microRNAs (miRNAs) in the mechanism of cancer cell immune evasion has been highlighted in recent studies. miRNAs are small non-coding RNAs that regulate a wide range of post-transcriptional gene expression in a cell. Recent studies have focused on the function that miRNAs play in controlling the expression of target proteins linked to immune modulation. Studies show that miRNAs modulate the immune response in cancers by regulating the expression of different immune-modulatory molecules associated with immune effector cells, such as macrophages, dendritic cells, B-cells, and natural killer cells, as well as those present in tumor cells and the tumor microenvironment. This review explores the relationship between miRNAs, their altered patterns of expression in tumors, immune modulation, and the functional control of a wide range of immune cells, thereby offering detailed insights on the crosstalk of tumor-immune cells and their use as prognostic markers or therapeutic agents.

Discovery and Preclinical Evaluation of BMS-986242, a Potent, Selective Inhibitor of Indoleamine-2,3-dioxygenase 1

Indoleamine 2,3-dioxygenase 1 (IDO1) is a heme-containing dioxygenase enzyme implicated in cancer immune response. This account details the discovery of BMS-986242, a novel IDO1 inhibitor designed for the treatment of a variety of cancers including metastatic melanoma and renal cell carcinoma. Given the substantial interest around this target for cancer immunotherapy, we sought to identify a structurally differentiated clinical candidate that performs comparably to linrodostat (BMS-986205) in terms of both in vitro potency and in vivo pharmacodynamic effect in a mouse xenograft model. On the basis of its preclinical profile, BMS-986242 was selected as a candidate for clinical development.

MicroRNAs and Their Targetomes in Tumor-Immune Communication

The development of cancer is a complex and dynamically regulated multiple-step process that involves many changes in gene expression. Over the last decade, microRNAs (miRNAs), a class of short regulatory non-coding RNAs, have emerged as key molecular effectors and regulators of tumorigenesis. While aberrant expression of miRNAs or dysregulated miRNA-mediated gene regulation in tumor cells have been shown to be capable of directly promoting or inhibiting tumorigenesis, considering the well-reported role of the immune system in cancer, tumor-derived miRNAs could also impact tumor growth through regulating anti-tumor immune responses. Here, we discuss howmiRNAs can function as central mediators that influence the crosstalk between cancer and the immune system. Moreover, we also review the current progress in the development of novel experimental approaches for miRNA target identification that will facilitate our understanding of miRNA-mediated gene regulation in not only human malignancies, but also in other genetic disorders.

Resident plasmacytoid dendritic cells patrol vessels in the naïve limbus and conjunctiva

Plasmacytoid dendritic cells (pDCs) constitute a unique population of bone marrow-derived cells that play a pivotal role in linking innate and adaptive immune responses. While peripheral tissues are typically devoid of pDCs during steady state, few tissues do host resident pDCs. In the current study, we aim to assess presence and distribution of pDCs in naïve murine limbus and bulbar conjunctiva. Immunofluorescence staining followed by confocal microscopy revealed that the naïve bulbar conjunctiva of wild-type mice hosts CD45+ CD11clow PDCA-1+ pDCs. Flow cytometry confirmed the presence of resident pDCs in the bulbar conjunctiva through multiple additional markers, and showed that they express maturation markers, the T cell co-inhibitory molecules PD-L1 and B7-H3, and minor to negligible levels of T cell co-stimulatory molecules CD40, CD86, and ICAM-1. Epi-fluorescent microscopy of DPE-GFP×RAG1-/- transgenic mice with GFP-tagged pDCs indicated lower density of pDCs in the bulbar conjunctiva compared to the limbus. Further, intravital multiphoton microscopy revealed that resident pDCs accompany the limbal vessels and patrol the intravascular space. In vitro multiphoton microscopy showed that pDCs are attracted to human umbilical vein endothelial cells and interact with them during tube formation. In conclusion, our study shows that the limbus and bulbar conjunctiva are endowed with resident pDCs during steady state, which express maturation and classic T cell co-inhibitory molecules, engulf limbal vessels, and patrol intravascular spaces.

Discovery of Highly Potent Benzimidazole Derivatives as Indoleamine 2,3-Dioxygenase-1 (IDO1) Inhibitors: From Structure-Based Virtual Screening to in Vivo Pharmacodynamic Activity

In this study, a successful medicinal chemistry campaign that exploited virtual, biophysical, and biological investigations led to the identification of a novel class of IDO1 inhibitors based on a benzimidazole substructure. This family of compounds is endowed with an extensive bonding network in the protein active site, including the interaction with pocket C, a region not commonly exploited by previously reported IDO1 inhibitors. The tight packing of selected compounds within the enzyme contributes to the strong binding interaction with IDO1, to the inhibitory potency at the low nanomolar level in several tumoral settings, and to the selectivity toward IDO1 over TDO and CYPs. Notably, a significant reduction of L-Kyn levels in plasma, together with a potent effect on abrogating immunosuppressive properties of MDSC-like cells isolated from patients affected by pancreatic ductal adenocarcinoma, was observed, pointing to this class of molecules as a valuable template for boosting the antitumor immune system.

The therapeutic effect of dendritic cells expressing indoleamine 2,3-dioxygenase (IDO) on an IgA nephropathy mouse model

OBJECTIVE

IgA nephropathy (IgAN) is one of the most common glomerulonephritis in the world, especially in Asian population. IgAN usually progresses slowly, but it is still an important cause of chronic renal failure. IgAN is characterized by abnormal increase of IgA1 level and deposition in mesangium. At present, there is no specific treatment.

MATERIALS AND METHODS

Previous reports have shown that DC cells expressing immunosuppressive factors can significantly reduce the symptoms of arthritis in arthritis models. Indoleamine 2,3-dioxygenase (IDO) is an important tryptophan degrading enzyme and an important factor regulating immunotolerance. DC expressing functional IDO can inhibit effector T cells by consuming essential tryptophan and/or producing toxic metabolites and promoting the differentiation of Treg cells, which exhibits immunosuppressive effect. In this study, we constructed a IgAN mouse model. The mature DC cells overexpressing IDO were induced in vitro and transfused back to IgAN mice to observe their effects on inflammation and renal injury.

RESULTS

The results showed that overexpression of IDO did not affect the maturation of DC cells. The proportion of CD3 + CD4 + and CD3 + CD8 + cells decreased significantly and the proportion of CD4 + CD25 + Foxp3 + cells increased significantly in kidney tissue of IgAN mice after the reinfusion of IDO-expressing DC. The contents of IL-2, IL-4, IL-6, and IL-17A in kidney tissue of IgAN mice also decreased significantly, the damage of kidney tissue was alleviated, ACR was decreased, collagen fibre content in kidney tissue was decreased, and IgA deposition in glomerular mesangium was decreased in IgAN mice.

CONCLUSIONS

It has the potential to treat IgAN by upregulating the expression of IDO in DC cells by genetic engineering and reinfusion into vivo.

Current Paradigms of Tolerogenic Dendritic Cells and Clinical Implications for Systemic Lupus Erythematosus

Tolerogenic dendritic cells (tolDCs) are central players in the initiation and maintenance of immune tolerance and subsequent prevention of autoimmunity. Recent advances in treatment of autoimmune diseases including systemic lupus erythematosus (SLE) have focused on inducing specific tolerance to avoid long-term use of immunosuppressive drugs. Therefore, DC-targeted therapies to either suppress DC immunogenicity or to promote DC tolerogenicity are of high interest. This review describes details of the typical characteristics of in vivo and ex vivo tolDC, which will help to select a protocol that can generate tolDC with high functional quality for clinical treatment of autoimmune disease in individual patients. In addition, we discuss the recent studies uncovering metabolic pathways and their interrelation intertwined with DC tolerogenicity. This review also highlights the clinical implications of tolDC-based therapy for SLE treatment, examines the current clinical therapeutics in patients with SLE, which can generate tolDC in vivo, and further discusses on possibility and limitation on each strategy. This synthesis provides new perspectives on development of novel therapeutic approaches for SLE and other autoimmune diseases.

Limited Effect of Indolamine 2,3-Dioxygenase Expression and Enzymatic Activity on Lupus-Like Disease in B6.Nba2 Mice

B6.Nba2 mice spontaneously develop a lupus-like disease characterized by elevated levels of serum anti-nuclear autoantibody (ANA) immune complexes and constitutive type I interferon (IFNα) production. During disease progression, both plasmacytoid dendritic cells (pDCs) and antibody secreting plasma cells accumulate in spleens of B6.Nba2 mice. Indoleamine 2,3-dioxygenase (IDO) has been suggested to play a role in several autoimmune diseases including in the MRL/lpr model of mouse lupus-like disease; however, it remains unknown if IDO is involved in disease development and/or progression in other spontaneous models. We show here that IDO1 protein and total IDO enzymatic activity are significantly elevated in lupus-prone B6.Nba2 mice relative to B6 controls. IDO1 expression was restricted to PCs and SignR1⁺ macrophages in both strains, while significantly increased in B6.Nba2-derived SiglecH⁺ (SigH⁺) pDCs. Despite this unique expression pattern, neither pharmacologic inhibition of total IDO nor IDO1 gene ablation altered serum autoantibody levels, splenic immune cell activation pattern, or renal inflammation in B6.Nba2 mice. Interestingly, IDO pharmacologic inhibition, but not IDO1 deficiency, resulted in diminished complement factor C'3 fixation to kidney glomeruli, suggesting a possible therapeutic benefit of IDO inhibition in SLE patients with renal involvement.

Immunomodulation by food: impact on gut immunity and immune cell function

Recent studies have revealed that various food components affect the immune response. These components act on various immune cells, and their effects are mediated through the intestinal immune system and, in some cases, the intestinal microbiota. In this review, we describe the immunomodulating effects of various food components, including probiotics, prebiotics, polysaccharides, vitamins, minerals, fatty acids, peptides, amino acids and polyphenols. Some of these components enhance immune responses, leading to host defense against infection, whereas others inhibit immune responses, thus suppressing allergy and inflammation.

Interferons: Reprogramming the Metabolic Network against Viral Infection

Viruses exploit the host and induce drastic metabolic changes to ensure an optimal environment for replication and the production of viral progenies. In response, the host has developed diverse countermeasures to sense and limit these alterations to combat viral infection. One such host mechanism is through interferon signaling. Interferons are cytokines that enhances the transcription of hundreds of interferon-stimulated genes (ISGs) whose products are key players in the innate immune response to viral infection. In addition to their direct targeting of viral components, interferons and ISGs exert profound effects on cellular metabolism. Recent studies have started to illuminate on the specific role of interferon in rewiring cellular metabolism to activate immune cells and limit viral infection. This review reflects on our current understanding of the complex networking that occurs between the virus and host at the interface of cellular metabolism, with a focus on the ISGs in particular, cholesterol-25-hydroxylase (CH25H), spermidine/spermine acetyltransferase 1 (SAT1), indoleamine-2,3-dioxygenase (IDO1) and sterile alpha motif and histidine/aspartic acid domain-containing protein 1 (SAMHD1), which were recently discovered to modulate specific metabolic events and consequently deter viral infection.

Antiviral Effect of IDO in Mouse Fibroblast Cells During Influenza Virus Infection

While upregulation of 2,3-dioxygenase (IDO) accompanied by degradation of tryptophan along the kynurenine pathway have been reported to exert antimicrobial effects against a wide range of infectious agents, its role in the replication of influenza A virus remains uncertain. We performed experiments using influenza A/WSN/33 virus infection of mouse fibroblast cell-line (NIH-3T3) to study the effects of IDO on viral replication. Influenza infection resulted in prominent elevations of transcripts encoding IDO, interferon (IFN)-β, and segment 8 of the virus in NIH-3T3 cells. Introduction of siRNA targeted against IDO followed by infection resulted in further increased levels of viral RNA without altering IFN-β expression. Inhibition of IDO during the infection also resulted in reduction of virus-driven upregulation of 3-hydroxyanthranilate 3,4-dioxygenase (HAAO), but not kynurenine 3-monooxygenase (KMO), which are enzymes downstream in the kynurenine pathway. Thus, induction of IDO appears to contribute to limiting replication of the WSN/33 strain of influenza A virus in murine NIH-3T3 cells.

Chemo-Immunotherapy: Role of Indoleamine 2,3-Dioxygenase in Defining Immunogenic Versus Tolerogenic Cell Death in the Tumor Microenvironment

In certain settings, chemotherapy can trigger an immunogenic form of tumor cell death. More often, however, tumor cell death after chemotherapy is not immunogenic, and may be actively tolerizing. However, even in these settings the dying tumor cells may be much more immunogenic than previously recognized, if key suppressive immune checkpoints such as indoleamine 2,3-dioxygenase (IDO) can be blocked. This is an important question, because a robust immune response to dying tumor cells could potentially augment the efficacy of conventional chemotherapy, or enhance the strength and duration of response to other immunologic therapies. Recent findings using preclinical models of self-tolerance and autoimmunity suggest that IDO and related downstream pathways may play a fundamental role in the decision between tolerance versus immune activation in response to dying cells. Thus, in the period of tumor cell death following chemotherapy or immunotherapy, the presence of IDO may help dictate the choice between dominant immunosuppression versus inflammation, antigen cross-presentation, and epitope spreading. The IDO pathway thus differs from other checkpoint-blockade strategies, in that it affects early immune responses, at the level of inflammation, activation of antigen-presenting cells, and initial cross-presentation of tumor antigens. This "up-stream" position may make IDO a potent target for therapeutic inhibition.

Tryptophan Catabolism and Cancer Immunotherapy Targeting IDO Mediated Immune Suppression

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

IDO-orchestrated crosstalk between pDCs and Tregs inhibits autoimmunity

Plasmacytoid dendritic cells (pDCs) have been shown to both mediate and prevent autoimmunity, and the regulation of their immunogenic versus tolerogenic functions remains incompletely understood. Here we demonstrate that, compared to other cells, pDCs are the major expressors of Indoleamine-2,3-dioxygenase (IDO) in steady-state lymph nodes (LNs). IDO expression by LN pDCs was closely dependent on MHCII-mediated, antigen-dependent, interactions with Treg. We further established that IDO production by pDCs was necessary to confer suppressive function to Tregs. During EAE development, IDO expression by pDCs was required for the generation of Tregs capable of dampening the priming of encephalitogenic T cell and disease severity. Thus, we describe a novel crosstalk between pDCs and Tregs: Tregs shape tolerogenic functions of pDCs prior to inflammation, such that pDCs in turn, promote Treg suppressive functions during autoimmunity.

Prevention and treatment of cancers by immune modulating nutrients

Epidemiological and laboratory data support the protective effects of bioactive nutrients in our diets for various diseases. Along with various factors, such as genetic history, alcohol, smoking, exercise, and dietary choices play a vital role in affecting an individual's immune responses toward a transforming cell, by either preventing or accelerating a neoplastic transformation. Ample evidence suggests that dietary nutrients control the inflammatory and protumorigenic responses in immune cells. Immunoprevention is usually associated with the modulation of immune responses that help in resolving the inflammation, thus improving clinical outcome. Various metabolic pathway-related nutrients, including glutamine, arginine, vitamins, minerals, and long-chain fatty acids, are important components of immunonutrient mixes. Epidemiological studies related to these substances have reported different results, with no or minimal effects. However, several studies suggest that these nutrients may have immune-modulating effects that may lower cancer risk. Preclinical studies submit that most of these components may provide beneficial effects. The present review discusses the available data, the immune-modulating functions of these nutrients, and how these substances could be used to study immune modulation in a neoplastic environment. Further research will help to determine whether the mechanistic signaling pathways in immune cells altered by nutrients can be exploited for cancer prevention and treatment.

IDO in the Tumor Microenvironment: Inflammation, Counter-Regulation, and Tolerance

Indoleamine 2,3-dioxygenase (IDO) has immunoregulatory roles associated with tryptophan metabolism. These include counter-regulation (controlling inflammation) and acquired tolerance in T cells. Recent findings reveal that IDO can be triggered by innate responses during tumorigenesis, and also by attempted T cell activation, either spontaneous or due to immunotherapy. Here we review the current understanding of mechanisms by which IDO participates in the control of inflammation and in peripheral tolerance. Focusing on the tumor microenvironment, we examine the role of IDO in response to apoptotic cells and the impact of IDO on Treg cell function. We discuss how the counter-regulatory and tolerogenic functions of IDO can be targeted for cancer immunotherapy and present an overview of the current clinical progress in this area.

Stress-activated Dendritic Cells (DC) Induce Dual Interleukin (IL)-15- and IL1β-mediated Pathways, Which May Elicit CD4+ Memory T Cells and Interferon (IFN)-stimulated Genes

The prevailing evidence suggests that immunological memory does not require antigenic re-stimulation but is maintained by low level tonic stimulation. We examined the hypothesis that stress agents contribute to tonic cellular activation and maintain immunological memory. Stimulation of monocyte-derived dendritic cells (DC) with stress agents elicits reactive oxygen species and HSP70. NFκB is activated, which up-regulates membrane-associated (ma) IL-15, caspase-1 and IL-1β. Co-culture of stress-treated DC with mononuclear cells activates IL-15 and IL-1β receptors on CD4(+) T cells, eliciting CD40L, proliferation, and up-regulation of CD45RO(+) memory T cells. The transcription factors Tbet(high) and RORγt are up-regulated, whereas FoxP3 is down-regulated, resulting in enhanced Th1 and Th17 expression and the corresponding cytokines. The interaction between maIL-15 expressed by DC and IL-15R on CD4(+) T cells results in one pathway and the corresponding cells expressing IL-1β and IL1βR as a second pathway. Importantly, inhibition studies with IL-15 antibodies and IL-1βR inhibitor suggest that both pathways may be required for optimum CD4(+) CD45RO(+) memory T cell expression. Type 1 IFN expression in splenic CD11c DC of stress-treated mice demonstrated a significant increase of IFN-α in CD11c CD317(+) and CD8α(+) DC. Analysis of RNA in human CD4(+) memory T cells showed up-regulation of type 1 IFN-stimulated genes and inhibition with histone methyltransferase inhibitor. We suggest the paradigm that stress-induced tonic stimulation might be responsible for the robust persistence of the immune response in vaccination and that epigenetic changes are involved in maintaining CD4(+) T cell memory.

B-Cells induce regulatory T cells through TGF-β/IDO production in A CTLA-4 dependent manner

A number of studies have suggested that B cell mediated-regulation contributes to the establishment of immunological tolerance. However, the precise mechanisms by which regulatory B cells establish and maintain tolerance in humans remain to be determined. The objective of the current study is to understand the cellular and molecular bases of B-cell regulatory functions in humans. To describe the mechanisms regulating the functional plasticity of regulatory B cells, we used an in vitro co-culture model based on autologous mixed lymphocyte cultures involving freshly isolated B and T cells. The results show that activated B cells regulate T cell proliferation through producing transforming growth factor (TGF)-β and indoleamine 2,3-dioxygenase (IDO). The production of TGF-β and IDO leads to the induction of not only "natural" regulatory T cells but also of TGF-β-producing CD4(+) T cells and IL-10-producing regulatory T cells. Furthermore, we evidenced for the first time that CTLA-4 induces B-cells to produce IDO and to become effective induced regulatory B cells (iBregs). This study emphasizes a novel regulatory axis and open news insights in how to manage regulatory B cell functions in autoimmunity.

Peripheral regulatory cells immunophenotyping in primary Sjögren's syndrome: a cross-sectional study

Introduction

IL-10--producing B cells, Foxp3-expressing T cells (Tregs) and the IDO-expressing dendritic cells (pDC) are able to modulate inflammatory processes, to induce immunological tolerance and, in turn, to inhibit the pathogenesis of autoimmune disease.

The aim of the study was to characterize and to enumerate peripheral IL-10--producing B cells, Tregs and pDCregs in primary Sjögren's Syndrome (pSS) patients in regard of their clinical and serologic activity.

Methods

Fifty pSS patients and 25 healthy individuals were included in the study. CD19⁺--expressing peripheral B lymphocytes were purified by positive selection. CD19⁺/CD24^hi/CD38^hi/IL-10--producing B cells, CD4⁺/CD25^hi/Foxp3⁺ and CD8⁺/CD28^-/Foxp3⁺ Tregs, as well as CCR6⁺/CD123⁺/IDO⁺ DCs, were quantitated by flow cytometry.

Results

Immature/transitional circulating IgA⁺ IL-10--producing B cells had higher levels in pSS patients versus control group, whereas CD19⁺/CD38^hi/IgG⁺/IL-10⁺ cells had lower percentage versus control. Indeed CD19⁺/CD24^hi/CD38^hi/CD5⁺/IL-10⁺, CD19⁺/CD24^hi/CD38^hi/CD10⁺/IL-10⁺, CD19⁺/CD24^hi/CD38^hi/CD20⁺/IL-10⁺, CD19⁺/CD24^hi/CD38^hi/CD27^-/IL-10⁺, and CD19⁺/CD24^hi/CD38^hi/CXCR7⁺/IL-10⁺ cells had higher frequency in clinical inactive pSS patients when compared with control group. Remarkably, only percentages of CD19⁺/CD24^hi/CD38^hi/CD10⁺/IL-10⁺ and CD19⁺/CD24^hi/CD38^hi/CD27^-/IL-10⁺ subsets were increased in pSS serologic inactive versus control group (P < 0.05). The percentage of IDO-expressing pDC cells was higher in pSS patients regardless of their clinical or serologic activity. There were no statistically significant differences in the percentage of CD4⁺/CD25^hi/Foxp3⁺ Tregs between patient groups versus controls. Nonetheless, a decrease in the frequency of CD8⁺/CD28^-/Foxp3⁺ Tregs was found in inactive pSS patients versus controls (P < 0.05).

Conclusions

The findings of this exploratory study show that clinical inactive pSS patients have an increased frequency of IL-10--producing B cells and IDO-expressing pDC cells.

Tolerogenic splenic IDO (+) dendritic cells from the mice treated with induced-Treg cells suppress collagen-induced arthritis

TGF-β-induced regulatory T cells (iTregs) retain Foxp3 expression and immune-suppressive activity in collagen-induced arthritis (CIA). However, the mechanisms whereby transferred iTregs suppress immune responses, particularly the interplay between iTregs and dendritic cells (DCs) in vivo, remain incompletely understood. In this study, we found that after treatment with iTregs, splenic CD11c⁺DCs, termed “DC_iTreg,” expressed tolerogenic phenotypes, secreted high levels of IL-10, TGF-β, and IDO, and showed potent immunosuppressive activity in vitro. After reinfusion with DC_iTreg, marked antiarthritic activity improved clinical scores and histological end-points were observed. The serological levels of inflammatory cytokines and anti-CII antibodies were low and TGF-β production was high in the DC_iTreg-treated group. DC_iTreg also induced new iTregs in vivo. Moreover, the inhibitory activity of DC_iTreg on CIA was lost following pretreatment with the inhibitor of indoleamine 2,3-dioxygenase (IDO). Collectively, these findings suggest that transferred iTregs could induce tolerogenic characteristics in splenic DCs and these cells could effectively dampen CIA in an IDO-dependent manner. Thus, the potential therapeutic effects of iTregs in CIA are likely maintained through the generation of tolerogenic DCs in vivo.

Type I Interferon at the Interface of Antiviral Immunity and Immune Regulation: The Curious Case of HIV-1

Type I interferon (IFN-I) play a critical role in the innate immune response against viral infections. They actively participate in antiviral immunity by inducing molecular mechanisms of viral restriction and by limiting the spread of the infection, but they also orchestrate the initial phases of the adaptive immune response and influence the quality of T cell immunity. During infection with the human immunodeficiency virus type 1 (HIV-1), the production of and response to IFN-I may be severely altered by the lymphotropic nature of the virus. In this review I consider the different aspects of virus sensing, IFN-I production, signalling, and effects on target cells, with a particular focus on the alterations observed following HIV-1 infection.

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

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

The role of indoleamine 2,3-dioxygenase in LP-BPM5 murine retroviral disease progression

Background

Indoleamine 2,3-dioxygenase (IDO) is an immunomodulatory intracellular enzyme involved in tryptophan degradation. IDO is induced during cancer and microbial infections by cytokines, ligation of co-stimulatory molecules and/or activation of pattern recognition receptors, ultimately leading to modulation of the immune response. LP-BM5 murine retroviral infection induces murine AIDS (MAIDS), which is characterized by profound and broad immunosuppression of T- and B-cell responses. Our lab has previously described multiple mechanisms regulating the development of immunodeficiency of LP-BM5-induced disease, including Programmed Death 1 (PD-1), IL-10, and T-regulatory (Treg) cells. Immunosuppressive roles of IDO have been demonstrated in other retroviral models, suggesting a possible role for IDO during LP-BM5-induced retroviral disease progression and/or development of viral load.

Methods

Mice deficient in IDO (B6.IDO−/−) and wildtype C57BL/6 (B6) mice were infected with LP-BM5 murine retrovirus. MAIDS and LP-BM5 viral load were assessed at termination.

Results

As expected, IDO was un-inducible in B6.IDO−/− during LP-BM5 infection. B6.IDO−/− mice infected with LP-BM5 retrovirus succumbed to MAIDS as indicated by splenomegaly, serum hyper IgG2a and IgM, decreased responsiveness to B- and T-cell mitogens, conversion of a proportion of CD4⁺ T cells from Thy1.2⁺ to Thy1.2^-, and increased percentages of CD11b⁺Gr-1⁺ cells. LP-BM5 infected B6.IDO−/− mice also demonstrated the development of roughly equivalent disease kinetics as compared to infected B6 mice. Splenic viral loads of B6 and B6.IDO−/− mice were also equivalent after infection as measured by LP-BM5-specific Def Gag and Eco Gag viral mRNA, determined by qRT-PCR.

Conclusions

Collectively, these results demonstrate IDO neither plays an essential role, nor is required, in LP-BM5-induced disease progression or LP-BM5 viral load.

Plasmacytoid dendritic cells and immunotherapy in multiple sclerosis

Plasmacytoid dendritic cells (pDCs) are specialized APCs implicated in the pathogenesis of many human diseases. Compared with other peripheral blood mononuclear cells, pDCs express a high level of TLR9, which recognizes viral DNA at the initial phase of viral infection. Upon stimulation, these cells produce large amounts of type I interferon and other proinflammatory cytokines and are able to prime T lymphocytes. Thus, pDCs regulate innate and adaptive immune responses. This article reviews select aspects of pDC biology relevant to the disease pathogenesis and immunotherapy in multiple sclerosis. Many unresolved questions remain in this area, promising important future discoveries in pDC research.

Remarkable role of indoleamine 2,3-dioxygenase and tryptophan metabolites in infectious diseases: potential role in macrophage-mediated inflammatory diseases

Indoleamine 2,3-dioxygenase 1 (IDO1), the L-tryptophan-degrading enzyme, plays a key role in the immunomodulatory effects on several types of immune cells. Originally known for its regulatory function during pregnancy and chronic inflammation in tumorigenesis, the activity of IDO1 seems to modify the inflammatory state of infectious diseases. The pathophysiologic activity of L-tryptophan metabolites, kynurenines, is well recognized. Therefore, an understanding of the regulation of IDO1 and the subsequent biochemical reactions is essential for the design of therapeutic strategies in certain immune diseases. In this paper, current knowledge about the role of IDO1 and its metabolites during various infectious diseases is presented. Particularly, the regulation of type I interferons (IFNs) production via IDO1 in virus infection is discussed. This paper offers insights into new therapeutic strategies in the modulation of viral infection and several immune-related disorders.

Indoleamine 2,3-dioxygenase and dendritic cell tolerogenicity

This article summarizes the molecular and cellular mechanisms that regulate the activity of indoleamine 2,3-dioxygenase (IDO), a potent immune-suppressive enzyme, in dendritic cells (DCs). Specific attention is given to differential up-regulation of IDO in distinct DC subsets, its function in immune homeostasis/autoimmunity, infection and cancer; and the associated immunological outcomes. The review will conclude with a discussion of the poorly defined mechanisms that mediate the long-term maintenance of IDO-expression in response to inflammatory stimuli and how selective modulation of IDO activity may be used in the treatment of disease.

Amino acid catabolism: a pivotal regulator of innate and adaptive immunity

Enhanced amino acid catabolism is a common response to inflammation, but the immunologic significance of altered amino acid consumption remains unclear. The finding that tryptophan catabolism helped maintain fetal tolerance during pregnancy provided novel insights into the significance of amino acid metabolism in controlling immunity. Recent advances in identifying molecular pathways that enhance amino acid catabolism and downstream mechanisms that affect immune cells in response to inflammatory cues support the notion that amino acid catabolism regulates innate and adaptive immune cells in pathologic settings. Cells expressing enzymes that degrade amino acids modulate antigen-presenting cell and lymphocyte functions and reveal critical roles for amino acid- and catabolite-sensing pathways in controlling gene expression, functions, and survival of immune cells. Basal amino acid catabolism may contribute to immune homeostasis that prevents autoimmunity, whereas elevated amino acid catalytic activity may reinforce immune suppression to promote tumorigenesis and persistence of some pathogens that cause chronic infections. For these reasons, there is considerable interest in generating novel drugs that inhibit or induce amino acid consumption and target downstream molecular pathways that control immunity. In this review, we summarize recent developments and highlight novel concepts and key outstanding questions in this active research field.

Integrated stress response-altered pro-inflammatory signals in mucosal immune-related cells

Various cells are associated with the integrated stress response (ISR) that leads to translation arrest via phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2. Pathogenic insults or nutritional imbalance in the mucosal tissues including the intestinal, airway, and genitourinary epithelia can cause ISRs, which have been linked to different mucosal inflammatory responses and subsequent systemic diseases. In particular, translational arrest caused by the early recognition of luminal microbes as well as nutritional status allows the human body to mount appropriate responses and maintain homeostasis both at the cellular and systemic levels. However, an over- or reduced ISR can create pathogenic conditions such as inflammation and carcinogenesis. This present review explores the association between eIF2α kinase-linked pathways and mucosal or systemic pro-inflammatory signals activated by xenobiotic insults (such as ones caused by microbes or nutritional abnormalities). Understanding ISR-modulated cellular alterations will provide progressive insights into approaches for treating human mucosal inflammatory and metabolic disorders.

Peripheral regulatory cells immunophenotyping in kidney transplant recipients with different clinical profiles: a cross-sectional study

Regulatory Foxp3-expressing T cells (Tregs), IL-10-producing B cells (Bregs), and IDO-expressing dendritic cells (DCregs) downregulate inflammatory processes and induces peripheral tolerance. These subpopulations also might participate in maintaining allograft immunological quiescence in kidney transplant recipients (KTRs) with an excellent long-term graft function under immunosuppression (ELTGF). The aim of the study was to characterize and to enumerate peripheral Tregs, Bregs, and DCregs in KTR with an ELTGF for more than 5 years after transplant. Fourteen KTR with an ELTGF, 9 KTR with chronic graft dysfunction (CGD), and 12 healthy donors (HDs) were included in the study. CD19⁺-expressing peripheral B lymphocytes were purified by positive selection. IL-10-producing B cells, CD4⁺/CD25^hi, and CD8⁺/CD28⁻ Tregs, as well as CCR6⁺/CD123⁺/IDO⁺ DCs, were quantitated by flow cytometry. IL-10-producing Bregs (immature/transitional, but not CD19⁺/CD38^hi/CD24^hi/CD27⁺B10 cells), CCR6⁺/CD123⁺/IDO⁺ DCs, and Tregs from ELTGF patients had similar or higher percentages versus HD (P < 0.05). By contrast, number of Tregs, DCregs, and Bregs except for CD27⁺B10 cells from CGD patients had lower levels versus HD and ELTGF patients (P < 0.05). The findings of this exploratory study might suggest that in ELTGF patients, peripheral tolerance mechanisms could be directly involved in the maintenance of a quiescent immunologic state and graft function stability.

Indoleamine 2,3 dioxygenase and metabolic control of immune responses

Sustained access to nutrients is a fundamental biological need, especially for proliferating cells, and controlling nutrient supply is an ancient strategy to regulate cellular responses to stimuli. By catabolizing the essential amino acid TRP, cells expressing the enzyme indoleamine 2,3 dioxygenase (IDO) can mediate potent local effects on innate and adaptive immune responses to inflammatory insults. Here, we discuss recent progress in elucidating how IDO activity promotes local metabolic changes that impact cellular and systemic responses to inflammatory and immunological signals. These recent developments identify potential new targets for therapy in a range of clinical settings, including cancer, chronic infections, autoimmune and allergic syndromes, and transplantation.

Indoleamine 2,3-dioxygenase expression and functional activity in dendritic cells exposed to cholera toxin

Indoleamine 2,3-dioxygenase (IDO), a tryptophan-metabolizing enzyme expressed by dendritic cells (DC), has the potential to inhibit T cell responses and to promote tolerance. In contrast, cholera toxin (CT), the enterotoxin produced by Vibrio cholerae, promotes T cell responses, partly through its ability to induce DC maturation and promote antigen presentation. We hypothesized that the adjuvant activity of CT is associated with a lack of induction of IDO in DC. To test this hypothesis, monocyte-derived DC were pulsed with CT, and the IDO mRNA expression, IDO functional activity and cytokine production were measured as well as the ability of DC to induce T cell responses in vitro. Cholera toxin exposure induced enhanced levels of IDO mRNA in DC but no functional IDO protein activity. Cholera toxin pulsing however primed DC for CD40L-induced IDO protein activity. CD40L stimulation of CT-pulsed DC induced a modest IL-12p40 production, but not IL-12p70 or IL-23 secretion. Furthermore, CT-pulsed DC induced strong allogeneic and autologous T cell responses in vitro, which were not affected by the IDO-specific inhibitor 1-methyl tryptophan. Our results show that CT per se does not induce the expression of functional IDO protein, although it primes DC for CD40L-mediated IDO production and IL-12p40 secretion. Furthermore, CT-treated DC were equally powerful in their T cell stimulatory capacity as cytokine-matured DC.

A comparative study of stress-mediated immunological functions with the adjuvanticity of alum

The efficacy of a vaccine is generally dependent on an adjuvant, which enhances the immune functions and alum has been widely used in human immunization. Alum activates the intracellular stress sensors inflammasomes, but whether these are responsible for the adjuvanticity is controversial. The objectives of this investigation were to examine the hypothesis that alum-mediated adjuvanticity is a function of stress and conversely that stress agents will elicit adjuvanticity. The investigation was carried out in BALB/c mice by SC immunization with ovalbumin (OVA) mixed with alum. This elicited inflammasomes, with significant activation of caspase 1, production of IL-1β, and adjuvanticity, demonstrated by enhancing OVA-specific serum IgG antibodies, CD4(+) T cells, and proliferation. The novel finding that alum induced HSP70 suggests that stress is involved in the mechanism of adjuvanticity. This was confirmed by inhibition studies with PES (phenylethynesulfonamide), which disrupts inducible HSP70 function, and inhibited both inflammasomes and the adjuvant function. Parallel studies were pursued with an oxidative agent (sodium arsenite), K-releasing agent (Gramicidin) and a metal ionophore (dithiocarbamate). All 3 stress agents induced HSP70, inflammasomes, and the adjuvant functions. Furthermore, up-regulation of membrane associated IL-15 on DC and CD40L on T cells in the animals treated with alum or the stress agents mediate the interactions between splenic CD11c DC and CD4(+) or CD8(+) T cells. The results suggest that the three stress agents elicit HSP70, a hallmark of stress, as well as inflammasomes and adjuvanticity, commensurate with those of alum, which may provide an alternative strategy in developing novel adjuvants.

Indoleamine 2,3-dioxygenase-expressing peripheral cells in rheumatoid arthritis and systemic lupus erythematosus: a cross-sectional study

BACKGROUND

Indoleamine 2,3-dioxygenase (IDO) is a tryptophan-degrading enzyme which suppresses T lymphocyte activity and induces Foxp3+ CD4+ regulatory T cells (Tregs) polarisation. The aim of this study was to evaluate the expression of IDO in freshly isolated peripheral cells as well as to enumerate Tregs and Th17 subpopulation in rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) patients.

MATERIALS AND METHODS

The percentage of IDO-expressing cells as well as Tregs and Th17 was evaluated in 14 active RA- (aRA), 13 inactive RA- (iRA), 7 aSLE-, 12 iSLE-treated patients and 11 healthy donors (controls). Intracellular IDO was analysed by flow cytometry in CD14+, CD8α+, CD16+ and CD123+ cell subpopulations. Tregs and Th17 were assessed by intracellular of Foxp3 and IL17A detection in CD4+ CD14- cells. A total of 50,000 events were recorded for each sample.

RESULTS

The amounts of CD14+/CD16-/IDO+, CD14-/CD16+/IDO+ and CD14+/CD16+/IDO+-expressing peripheral cells were slightly lower in inactive vs. active disease in RA and SLE patients. Notwithstanding, only inactive patients had statistically significant lower percentages when compared to controls. aRA and iRA showed a statistically significant decrease in CD8α+/CD123+/IDO+ vs. controls. Meanwhile, only iSLE patients had lower CD8α+/CD123+/IDO+ cells vs. aSLE patients and controls. Th17 subset was present in higher amounts in aRA and iRA patients vs. controls. Tregs showed an increase in aRA patients vs. controls.

CONCLUSIONS

A decreased percentage of IDO-expressing peripheral cells were determined in iRA and iSLE compared to controls. It could play a critical role in tolerance loss in these diseases.

Physiologic control of IDO competence in splenic dendritic cells

Dendritic cells (DCs) competent to express the regulatory enzyme IDO in mice are a small but distinctive subset of DCs. Previously, we reported that a high-dose systemic CpG treatment to ligate TLR9 in vivo induced functional IDO exclusively in splenic CD19(+) DCs, which stimulated resting Foxp3-lineage regulatory T cells (Tregs) to rapidly acquire potent suppressor activity. In this paper, we show that IDO was induced in spleen and peripheral lymph nodes after CpG treatment in a dose-dependent manner. Induced IDO suppressed local T cell responses to exogenous Ags and inhibited proinflammatory cytokine expression in response to TLR9 ligation. IDO induction did not occur in T cell-deficient mice or in mice with defective B7 or programmed death (PD)-1 costimulatory pathways. Consistent with these findings, CTLA4 or PD-1/PD-ligand costimulatory blockade abrogated IDO induction and prevented Treg activation via IDO following high-dose CpG treatment. Consequently, CD4(+)CD25(+) T cells uniformly expressed IL-17 shortly after TLR9 ligation. These data support the hypothesis that constitutive interactions from activated T cells or Tregs and IDO-competent DCs via concomitant CTLA4→B7 and PD-1→PD-ligand signals maintain the default potential to regulate T cell responsiveness via IDO. Acute disruption of these nonredundant interactions abrogated regulation via IDO, providing novel perspectives on the proinflammatory effects of costimulatory blockade therapies. Moreover, interactions between IDO-competent DCs and activated T cells in lymphoid tissues may attenuate proinflammatory responses to adjuvants such as TLR ligands.

IDO induces expression of a novel tryptophan transporter in mouse and human tumor cells

IDO is the rate-limiting enzyme in the kynurenine pathway, catabolizing tryptophan to kynurenine. Tryptophan depletion by IDO-expressing tumors is a common mechanism of immune evasion inducing regulatory T cells and inhibiting effector T cells. Because mammalian cells cannot synthesize tryptophan, it remains unclear how IDO(+) tumor cells overcome the detrimental effects of local tryptophan depletion. We demonstrate that IDO(+) tumor cells express a novel amino acid transporter, which accounts for ∼50% of the tryptophan uptake. The induced transporter is biochemically distinguished from the constitutively expressed tryptophan transporter System L by increased resistance to inhibitors of System L, resistance to inhibition by high concentrations of most amino acids tested, and high substrate specificity for tryptophan. Under conditions of low extracellular tryptophan, expression of this novel transporter significantly increases tryptophan entry into IDO(+) tumors relative to tryptophan uptake through the low-affinity System L alone, and further decreases tryptophan levels in the microenvironment. Targeting this additional tryptophan transporter could be a way of pharmacological inhibition of IDO-mediated tumor escape. These findings highlight the ability of IDO-expressing tumor cells to thrive in a tryptophan-depleted microenvironment by expressing a novel, highly tryptophan-specific transporter, which is resistant to inhibition by most other amino acids. The additional transporter allows tumor cells to strike the ideal balance between supply of tryptophan essential for their own proliferation and survival, and depleting the extracellular milieu of tryptophan to inhibit T cell proliferation.

Targeting the immunoregulatory indoleamine 2,3 dioxygenase pathway in immunotherapy

Natural immune tolerance is a formidable barrier to successful immunotherapy to treat established cancers and chronic infections. Conversely, creating robust immune tolerance via immunotherapy is the major goal in treating autoimmune and allergic diseases, and enhancing survival of transplanted organs and tissues. In this review, we focus on a natural mechanism that creates local T-cell tolerance in many clinically relevant settings of chronic inflammation involving expression of the cytosolic enzyme indoleamine 2,3-dioxygenase (IDO) by specialized subsets of dendritic cells. IDO-expressing dendritic cells suppress antigen-specific T-cell responses directly, and induce bystander suppression by activating regulatory T cells. Thus, manipulating IDO is a promising strategy to treat a range of chronic inflammatory diseases.

Tolerogenic plasmacytoid DC

Plasmacytoid DC (pDC) are type-I IFN-producing cells known for their capacity to promote anti-viral innate and adaptive immune responses. Despite their potent anti-viral function, when compared with conventional DC, pDC exhibit poor immunostimulatory ability and their interaction with T cells often favors the generation of Treg. pDC are activated primarily in response to ssRNA and ssDNA through TLR7 and TLR9, respectively, but also through TLR-independent mechanisms. Non-lymphoid tissue pDC, such as those residing in the airways, gut, and liver, play a significant role in regulating mucosal immunity and are critical for the development of tolerance to inhaled or ingested antigens. Herein we discuss properties that define tolerogenic pDC and how their unique characteristics translate into an ability to regulate immunity and promote the development of tolerance. We cover the importance of pDC during intrathymic Treg development and the maintenance of peripheral tolerance, as well as their regulatory role in transplantation, autoimmunity, and cancer. We highlight recent findings regarding danger-associated molecular pattern and PAMP signaling in the regulation of pDC function, and how the ability of pDC to promote tolerance translates into the potential clinical applications of these cells as therapeutic targets to regulate immune reactivity.