Indoleamine 2,3-dioxygenase production by human dendritic cells results in the inhibition of T cell proliferation

Human mesenchymal stromal cell-mediated immunoregulation: mechanisms of action and clinical applications

Mesenchymal stromal cells (MSCs) are multipotent cells found in connective tissues that can differentiate into bone, cartilage, and adipose tissue. Interestingly, they can regulate immune responses in a paracrine way and allogeneic MSCs do not elicit immune response. These properties have encouraged a number of clinical trials in a broad range of regenerative therapies. Although these trials were first focused on their differentiation properties, in the last years, the immunosuppressive features have gained most of the attention. In this review, we will summarize the up-to-date knowledge about the immunosuppressive mechanisms of MSCs in vivo and in vitro and the most promising approaches in clinical investigation.

HIV-1 Tat protein induces the production of IDO in human monocyte derived-dendritic cells through a direct mechanism: effect on T cells proliferation

During HIV-1 infection, an increase of indoleamine 2,3 dioxygenase (IDO) expression, and dendritic cells (DC) dysfunction were often associated with AIDS disease progression. In this work, we investigated the effect of HIV-1 Tat protein on the expression of IDO, in MoDCs. We show that Tat induces IDO protein expression and activity in a dose dependent manner by acting at the cell membrane. Using Tat-mutants, we show that the N-Terminal domain, Tat 1–45, but not the central region, Tat 30–72, is sufficient to induce the expression of active IDO. Tat protein is also able to induce several cytokines in MoDCs, including IFN-γ, a strong inducer of IDO. In order to understand whether IDO is induced directly by Tat protein or indirectly following IFN-γ production, complementary experiments were performed and showed that: i) at the kinetic level, Tat induced IDO expression before the production of IFN-γ ii) treatment of MoDCs with Tat-conditioned medium was unable to stimulate IDO expression, iii) coculture of MoDCs in a transwell cell system did not allow IDO expression in MoDCs not previously treated by Tat, iv) direct contact between Tat-treated and untreated MoDCs was not sufficient to induce IDO expression in a Tat-independent manner, and v) treatment of MoDCs in the presence of IFN-γ pathway inhibitors, Jak I and Ly294002, inhibited IFN-γ-induced IDO but had no effect on Tat-induced IDO. At the functional level, our data showed that treatment of MoDCs with Tat led to the inhibition of their capacity to stimulate T cell proliferation. This impairement was totally abolished when the stimulation was performed in the presence of 1MT, an inhibitor of IDO activity, arguing for the implication of the kynurenine pathway. By inducing IDO, Tat protein may be considered, as a viral pathogenic factor, in the dysregulation of the DC functions during HIV-1 infection.

The kynurenine pathway in stem cell biology

The kynurenine pathway (KP) is the main catabolic pathway of the essential amino acid tryptophan. The KP has been identified to play a critical role in regulating immune responses in a variety of experimental settings. It is also known to be involved in several neuroinflammatory diseases including Huntington's disease, amyotrophic lateral sclerosis, and Alzheimer's disease. This review considers the current understanding of the role of the KP in stem cell biology. Both of these fundamental areas of cell biology have independently been the focus of a burgeoning research interest in recent years. A systematic review of how the two interact has not yet been conducted. Several inflammatory and infectious diseases in which the KP has been implicated include those for which stem cell therapies are being actively explored at a clinical level. Therefore, it is highly relevant to consider the evidence showing that the KP influences stem cell biology and impacts the functional behavior of progenitor cells.

Type III TGF-β receptor downregulation generates an immunotolerant tumor microenvironment

Cancers subvert the host immune system to facilitate disease progression. These evolved immunosuppressive mechanisms are also implicated in circumventing immunotherapeutic strategies. Emerging data indicate that local tumor-associated DC populations exhibit tolerogenic features by promoting Treg development; however, the mechanisms by which tumors manipulate DC and Treg function in the tumor microenvironment remain unclear. Type III TGF-β receptor (TGFBR3) and its shed extracellular domain (sTGFBR3) regulate TGF-β signaling and maintain epithelial homeostasis, with loss of TGFBR3 expression promoting progression early in breast cancer development. Using murine models of breast cancer and melanoma, we elucidated a tumor immunoevasion mechanism whereby loss of tumor-expressed TGFBR3/sTGFBR3 enhanced TGF-β signaling within locoregional DC populations and upregulated both the immunoregulatory enzyme indoleamine 2,3-dioxygenase (IDO) in plasmacytoid DCs and the CCL22 chemokine in myeloid DCs. Alterations in these DC populations mediated Treg infiltration and the suppression of antitumor immunity. Our findings provide mechanistic support for using TGF-β inhibitors to enhance the efficacy of tumor immunotherapy, indicate that sTGFBR3 levels could serve as a predictive immunotherapy biomarker, and expand the mechanisms by which TGFBR3 suppresses cancer progression to include effects on the tumor immune microenvironment.

Suppression of acute graft-versus-host response by TCDD is independent of the CTLA-4-IFN-γ-IDO pathway

Activation of the aryl hydrocarbon receptor (AhR) by its prototypic ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), induces potent suppression of an acute graft-versus-host (GVH) response and prevents GVH disease (GVHD). Suppression is associated with development of a regulatory population of donor CD4(+) CD25(+)T-cells that express high levels of cytotoxic T-lymphocyte antigen 4 (CTLA-4). However, a direct link between these AhR-induced Tregs (AhR-Tregs) and suppression of GVHD remains to be shown. CTLA-4 is a negative regulator of T-cell responses and is associated with the induction of tolerogenic dendritic cells (DCs) that produce indoleamine 2,3-dioxygenase (IDO). We hypothesized that AhR-Tregs mediate suppression via their enhanced expression of CTLA-4, which, in turn, induces IFN-γ and IDO in host DCs. Subsequent depletion of tryptophan by IDO leads to termination of the donor T-cell response prior to development of effector CTL. Here, we show that despite increased expression of Ifng, Irf3, Irf7, Ido1, and Ido2 in the lymph nodes of TCDD-treated host mice, inhibition of IDO enzyme activity by 1-methyl-tryptophan was unable to relieve TCDD-mediated suppression of the GVH response. Furthermore, treatment with an anti-CTLA-4 antibody that blocks CTLA-4 signaling was also unable to alleviate TCDD-mediated suppression. Alternatively, we investigated the possibility that donor-derived AhR-Tregs produce IFN-γ to suppress effector CTL development. However, suppression of GVHD by TCDD was not affected by the use of Ifng-deficient donor cells. Together, these results indicate that neither overexpression of CTLA-4 nor production of IFN-γ by AhR-Tregs plays a major role in the manifestation of their immunosuppressive function in vivo.

Systemic tryptophan and kynurenine catabolite levels relate to severity of rhinovirus-induced asthma exacerbation: a prospective study with a parallel-group design

BACKGROUND

Patients with allergic asthma have exacerbations which are frequently caused by rhinovirus infection. The antiviral tryptophan-catabolising enzyme indoleamine 2,3-dioxygenase (IDO) is induced by interferon-γ and suppressed by Th2 mediators interleukin (IL)-4 and IL-13. We hypothesised that local IDO activity after viral airway infection is lower in patients with allergic asthma than in healthy controls.

OBJECTIVE

To determine whether IDO activity differs between patients with allergic asthma and healthy individuals before and after rhinovirus infection.

METHODS

Healthy individuals and patients with allergic asthma were experimentally infected with low-dose (10 TCID50) rhinovirus 16. Blood, bronchoalveolar lavage fluid and exhaled breath condensate (for mass spectrometry by UPLC-MS/MS) were obtained before and after rhinovirus challenge.

RESULTS

IDO activity was not induced by rhinovirus infection in either group, despite increases in cold scores. However, baseline pulmonary IDO activity was lower in patients with allergic asthma than in healthy individuals. In contrast, systemic tryptophan and its catabolites were markedly higher in patients with allergic asthma. Moreover, systemic quinolinic acid and tryptophan were associated with eosinophil cationic protein (r=0.43 and r=0.78, respectively) and eosinophils (r=0.38 and r=0.58, respectively) in bronchoalveolar lavage fluid and peak asthma symptom scores after rhinovirus challenge (r=0.53 and r=0.64, respectively).

CONCLUSIONS

Rhinovirus infection by itself induces no IDO activity, but the reduced pulmonary IDO activity in patients with allergic asthma at baseline may underlie a reduced control of viral infections. Notably, the enhanced systemic catabolism of tryptophan in patients with allergic asthma was strongly related to the outcome of rhinovirus challenge in asthma and may serve as a prognostic factor.

The niacin required for optimum growth can be synthesized from L-tryptophan in growing mice lacking tryptophan-2,3-dioxygenase

In mammals, nicotinamide (Nam) is biosynthesized from l-tryptophan (l-Trp). The enzymes involved in the initial step of the l-Trp→Nam pathway are l-Trp-2,3-dioxygenase (TDO) and indoleamine-2,3-dioxygenase (IDO). We aimed to determine whether tdo-knockout (tdo(-/-)) mice fed a diet without preformed niacin can synthesize enough Nam to sustain optimum growth. Wild-type (WT) and tdo(-/-) mice were fed a chemically defined 20% casein diet with or without preformed niacin (30 mg nicotinic acid/kg) for 28 d. Body weight, food intake, and liver NAD concentrations did not differ among the groups. In the groups of mice fed the niacin-free diet, urinary concentrations of the upstream metabolites kynurenine (320% increase, P < 0.0001), kynurenic acid (270% increase, P < 0.0001), xanthurenic acid (770% increase, P < 0.0001), and 3-hydroxyanthranilic acid (3-HA; 450% increase, P < 0.0001) were higher in the tdo(-/-) mice than in the WT mice, while urinary concentrations of the downstream metabolite quinolinic acid (QA; 50% less, P = 0.0010) and the sum of Nam and its catabolites (10% less, P < 0.0001) were lower in the tdo(-/-) mice than in the WT mice. These findings show that the kynurenine formed in extrahepatic tissues by IDO and subsequent enzymes can be metabolized up to 3-HA, but not into QA. However, the tdo(-/-) mice sustained optimum growth even when fed the niacin-free diet for 1 mo, suggesting they can synthesize the minimum necessary amount of Nam from l-Trp, because the liver can import blood kynurenine formed in extrahepatic tissues and metabolize it into Nam via NAD and the resulting Nam is then distributed back into extrahepatic tissues.

CD8(+) T Cells Are Required For Glatiramer Acetate Therapy in Autoimmune Demyelinating Disease

The exact mechanism of glatiramer acetate (GA, Copaxone®), an FDA-approved immunomodulatory therapy for multiple sclerosis (MS), remains unclear after decades of research. Previously, we have shown that GA therapy of MS induces CD8⁺ T cell responses that can potentially suppress pathogenic CD4⁺ T cell responses. Using a murine model of MS, experimental autoimmune encephalomyelitis (EAE), we now demonstrate that CD8⁺ T cells are necessary in mediating the therapeutic effects of GA. Further, adoptive transfer of GA-induced CD8⁺ T cells resulted in amelioration of EAE, establishing a role as a viable immunotherapy in demyelinating disease. Generation of these cells required indoleamine-2,3-dioxygenase (IDO), while suppressive function depended on non-classical MHC class I, IFN-γ, and perforin expression. GA-induced regulatory myeloid cells, previously shown to activate CD4⁺ regulatory T cells in an antigen-independent manner, required CD8⁺ T cells for disease suppression in vivo. These studies demonstrate an essential role for CD8⁺ T cells in GA therapy and identify their potential as an adoptive immunotherapeutic agent.

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

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

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

Immunosuppressive mechanisms of regulatory dendritic cells in cancer

Three major functional subsets of dendritic cells (DCs) have been described in the tumor microenvironment in patients with cancer and tumor-bearing animals: (i) conventional DCs with intact antigen-presenting capabilities, (ii) functionally defective DCs with decreased motility and low ability to uptake, process and present antigens or produce cytokines and (iii) regulatory DCs with high capacity to suppress T cell proliferation, induce differentiation of regulatory T cells or support immune tolerance. Phenotypic characteristics of regulatory DCs (regDCs), as well as the mechanisms of T cell inhibition, vary in different experimental conditions and environments, suggesting high level of their plasticity and probably different origin. Although new data demonstrate that regDCs may play an important role at early stages of tumor development, functional differences and clinical significance of emergence of different myeloid regulatory cells (MDSCs, regDCs, M2 macrophages, N2 neutrophils, mast cells) in cancer remain to be determined.

The aryl hydrocarbon receptor: a novel target for immunomodulation in organ transplantation

The aryl hydrocarbon receptor (AHR), which has been central to studies in toxicology for years as the receptor for the toxicant dioxin, is rapidly gaining interest in immunology based on its ability to influence T-cell differentiation. Multiple studies have documented that binding of this receptor with certain ligands favors T-cell differentiation toward regulatory T cells, and paradoxically, binding of this same receptor with different ligands enhances Th17 effector cell differentiation. This finding has been confirmed in both in vitro and in vivo models, where different ligands are able to either ameliorate or conversely aggravate autoimmunity in experimental autoimmune encephalomyelitis. The AHR has both an endogenous role that is important in development and normal physiology and an exogenous role as a receptor for manmade toxicants, with their binding leading to transcription of cytochrome P450 enzymes that metabolize these same ligands. Based on recent reports that will be summarized in this overview, we will consider the role that the AHR might play as a sensor to the outside environment, leading to alteration of the acquired immune system that might have relevance in transplantation or other medical conditions. In addition to describing the data in normal physiology and T-cell differentiation, we will present examples of the importance of this receptor in preclinical models of disease and highlight specific ligands that target the AHR and will have efficacy in treating transplant rejection and in tolerance protocols.

Targeting Tregs in Malignant Brain Cancer: Overcoming IDO

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

Differences in gastric carcinoma microenvironment stratify according to EBV infection intensity: implications for possible immune adjuvant therapy

Epstein-Barr virus (EBV) is associated with roughly 10% of gastric carcinomas worldwide (EBVaGC). Although previous investigations provide a strong link between EBV and gastric carcinomas, these studies were performed using selected EBV gene probes. Using a cohort of gastric carcinoma RNA-seq data sets from The Cancer Genome Atlas (TCGA), we performed a quantitative and global assessment of EBV gene expression in gastric carcinomas and assessed EBV associated cellular pathway alterations. EBV transcripts were detected in 17% of samples but these samples varied significantly in EBV coverage depth. In four samples with the highest EBV coverage (hiEBVaGC – high EBV associated gastric carcinoma), transcripts from the BamHI A region comprised the majority of EBV reads. Expression of LMP2, and to a lesser extent, LMP1 were also observed as was evidence of abortive lytic replication. Analysis of cellular gene expression indicated significant immune cell infiltration and a predominant IFNG response in samples expressing high levels of EBV transcripts relative to samples expressing low or no EBV transcripts. Despite the apparent immune cell infiltration, high levels of the cytotoxic T-cell (CTL) and natural killer (NK) cell inhibitor, IDO1, was observed in the hiEBVaGCs samples suggesting an active tolerance inducing pathway in this subgroup. These results were confirmed in a separate cohort of 21 Vietnamese gastric carcinoma samples using qRT-PCR and on tissue samples using in situ hybridization and immunohistochemistry. Lastly, a panel of tumor suppressors and candidate oncogenes were expressed at lower levels in hiEBVaGC versus EBV-low and EBV-negative gastric cancers suggesting the direct regulation of tumor pathways by EBV.

Epstein-Barr virus (EBV) is detected in roughly 10% of gastric carcinoma (GC) cases worldwide. Despite a strong link between EBV and gastric carcinoma, the contribution of EBV to the tumor environment in EBV associated gastric carcinoma is unclear. We performed a global assessment of EBV and host cell gene expression in gastric carcinoma tumors from 71 patients to link EBV genes (and expression intensities) to cell and microenvironmental changes. In addition to the finding that EBV is associated with down-regulated tumor regulatory genes, this study revealed that samples with high levels of EBV gene expression (hiEBVaGCs) displayed elevated immune cell infiltration with high interferon-gamma (IFNG) expression compared to samples with low or no EBV gene expression. Despite this evidence of increased immune posturing, hiEBVaGC samples also showed elevated expression of the potent immune cell inhibitor, IDO1. This finding may partly explain the persistence of these virus associated tumors in the face of local immune cell concentration. Importantly, the small molecule IDO inhibitor, 1MT (1-methyl Tryptophan), has been shown to reverse the tolerance inducing effects of IDO1 in other tumors. We propose that stratification of gastric carcinomas into EBV-negative, EBV-low and EBV-high may provide indicator value for the use of IDO1 inhibitors as adjuvant therapies against hiEBVaGCs.

Neem leaf glycoprotein overcomes indoleamine 2,3 dioxygenase mediated tolerance in dendritic cells by attenuating hyperactive regulatory T cells in cervical cancer stage IIIB patients

Tolerogenic dendritic cells (DCs) are a subset of DCs characterized by abundant indoleamine 2,3 dioxygenase (IDO) expressions. IDO may be co-operatively induced in DCs by regulatory T (Tregs) cells and various DC maturation agents. Tregs are markedly amplified in the physiological system of cancer patients, inducing over tolerance in DCs that leads to the hyper accumulation of immunosuppressive IDO in tumor microenvironment, thereby, hampering anti-tumor immunity. Consequently, a major focus of current immunotherapeutic strategies in cancer is to minimize IDO, which is possible by reducing Tregs and using various IDO inhibitors. Neem leaf glycoprotein (NLGP), a natural and nontoxic immunomodulator, demonstrated several unique immunoregulatory activities. Noteworthy activities of NLGP are to mature DCs and to inhibit Tregs. As Tregs are inducer of IDO in DCs and hyperactive Tregs is a hallmark of cancer, we anticipated that NLGP might abrogate IDO induction in DCs by inhibiting Tregs. Evidences are presented here that in a co-culture of DCs and Tregs isolated from cervical cancer stage IIIB (CaCx-IIIB) patients, NLGP does inhibit IDO induction in DCs by curtailing the over expression of Cytotoxic T-Lymphocyte Antigen 4 (CTLA4) on Tregs and concomitantly induces optimal DC maturation. In contrast, in the presence of LPS as maturation agent the DCs displays a tolerogenic profile. This finding suggests the reduction of tolerogenecity of DCs in CaCx-IIIB patients by reducing the IDO pool using NLGP. Accordingly, this study sheds more light on the diverse immunomodulatory repertoire of NLGP.

Silencing of indoleamine 2,3-dioxygenase enhances dendritic cell immunogenicity and antitumour immunity in cancer patients

Dendritic cells (DCs) are being explored as a therapeutic vaccine for cancers. However, their immunogenic potential is limited by the presence of immunosuppressive factors. Among these factors is the tryptophan-degrading enzyme indoleamine 2,3-dioxygenase (IDO). In this study, we have investigated the safety, immunogenicity and clinical response of IDO-silenced DC vaccine in four patients with gynecological cancers. DCs were transfected with IDO small interfering RNA and mRNA encoding human telomerase reverse transcriptase (hTERT) or survivin, two universal tumour antigens. Silencing of IDO in DCs did not affect the expression of the co-stimulatory molecules CD80 and CD86, but enhanced the expression of the CCR7 and CD40 molecules. IDO-silenced DCs showed superior potency to activate allogeneic T cells compared to their IDO-positive counterparts. The immunisation with this novel DC cancer vaccine was well tolerated and all patients developed delayed-type hypersensitivity skin reaction and specific T-cell response against hTERT and survivin tumour antigens. Perhaps most importantly, the immune response seen in the patients was related to objective clinical response. Thus, IDO silencing can enhance the immunogenic function of DCs in vitro and in vivo. Overall, the data provide proof-of-principle that immunisation with IDO-silenced DC vaccine is safe and effective in inducing antitumour immunity.

Inhibition of indoleamine 2,3-dioxygenase enhances the T-cell response to influenza virus infection

Influenza infection induces an increase in the level of indoleamine 2,3-dioxygenase (IDO) activity in the lung parenchyma. IDO is the first and rate-limiting step in the kynurenine pathway where tryptophan is reduced to kynurenine and other metabolites. The depletion of tryptophan, and production of associated metabolites, attenuates the immune response to infection. The impact of IDO on the primary immune response to influenza virus infection was determined using the IDO inhibitor 1-methyl-D,L-tryptophan (1MT). C57BL/6 mice treated with 1MT and infected with A/HKx31 influenza virus had increased numbers of activated and functional CD4⁺ T-cells, influenza-specific CD8⁺ T-cells and effector memory cells in the lung. Inhibition of IDO increased the Th1 response in CD4⁺ T-cells as well as enhanced the Th17 response. These studies show that inhibition of IDO engenders a more robust T-cell response to influenza virus, and suggests an approach for enhancing the immune response to influenza vaccination by facilitating increased influenza-specific T-cell response.

Myeloid-derived suppressor cells and anti-tumor T cells: a complex relationship

Myeloid-Derived Suppressor Cells (MDSC) are immature myeloid cells that are potent inhibitors of immune cell function and which accumulate under conditions of inflammation, especially cancer. MDSC are suggested to promote the growth of cancer by both enhancement of tumor angiogenesis and metastasis and also inhibition of antitumor immune responses. The presence of deficient and/or defective antitumor adaptive and innate immune responses, coincident with accumulation of MDSC in lymphoid organs and tumor parenchyma, supports the notion of a causal relationship. The potent ability of MDSC to inhibit several components and phases of immune response highlights the likelihood that targeting the inhibitory functions of MDSC may maximize the therapeutic potential of antitumor immunotherapy. In order to guide the rational development of immunotherapeutic strategies that incorporate inhibition of MDSC activity and enzymatic functions, thorough understanding of the role of MDSC in antitumor immune responses is required. In this manuscript we review the multifaceted inhibitory functions of MDSC and consider the role of MDSC-induced inhibition of antitumor T cell effector phase. Support for this research is from NIH R01 CA108573.

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.

Interleukin 21-induced granzyme B-expressing B cells infiltrate tumors and regulate T cells

The pathogenic impact of tumor-infiltrating B cells is unresolved at present, however, some studies suggest that they may have immune regulatory potential. Here, we report that the microenvironment of various solid tumors includes B cells that express granzyme B (GrB, GZMB), where these B cells can be found adjacent to interleukin (IL)-21-secreting regulatory T cells (Treg) that contribute to immune tolerance of tumor antigens. Because Tregs and plasmacytoid dendritic cells are known to modulate T-effector cells by a GrB-dependent mechanism, we hypothesized that a similar process may operate to modulate regulatory B cells (Breg). IL-21 induced outgrowth of B cells expressing high levels of GrB, which thereby limited T-cell proliferation by a GrB-dependent degradation of the T-cell receptor ζ-chain. Mechanistic investigations into how IL-21 induced GrB expression in B cells to confer Breg function revealed a CD19(+)CD38(+)CD1d(+)IgM(+)CD147(+) expression signature, along with expression of additional key regulatory molecules including IL-10, CD25, and indoleamine-2,3-dioxygenase. Notably, induction of GrB by IL-21 integrated signals mediated by surface immunoglobulin M (B-cell receptor) and Toll-like receptors, each of which were enhanced with expression of the B-cell marker CD5. Our findings show for the first time that IL-21 induces GrB(+) human Bregs. They also establish the existence of human B cells with a regulatory phenotype in solid tumor infiltrates, where they may contribute to the suppression of antitumor immune responses. Together, these findings may stimulate novel diagnostic and cell therapeutic approaches to better manage human cancer as well as autoimmune and graft-versus-host pathologies.

Kynurenines in the CNS: recent advances and new questions

Various pathologies of the central nervous system (CNS) are accompanied by alterations in tryptophan metabolism. The main metabolic route of tryptophan degradation is the kynurenine pathway; its metabolites are responsible for a broad spectrum of effects, including the endogenous regulation of neuronal excitability and the initiation of immune tolerance. This Review highlights the involvement of the kynurenine system in the pathology of neurodegenerative disorders, pain syndromes and autoimmune diseases through a detailed discussion of its potential implications in Huntington's disease, migraine and multiple sclerosis. The most effective preclinical drug candidates are discussed and attention is paid to currently under-investigated roles of the kynurenine pathway in the CNS, where modulation of kynurenine metabolism might be of therapeutic value.

Natural inhibitors of indoleamine 3,5-dioxygenase induced by interferon-gamma in human neural stem cells

Indoleamine dioxygenase (IDO) is a heme- containing enzyme that catalyzes the oxidation of tryptophan to N-formylkynurenine, kynurenine and the downstream quinolinic acid. Though IDO is physiologically important in maintaining tissue integrity, aberrant IDO expression represses T cell function and promotes regulatory T cells (Treg) in cancer. It additionally exacerbates Alzheimer, depression, Huntington and Parkinson diseases via quinolinic acid. Inhibition of IDO has thus been recently proposed as a strategy for treating cancer and neuronal disorders. In the present study, we have developed a cell-based assay to evaluate the suppressive effect of anti-inflammatory phytochemicals on the enzyme. When stimulated by INF-γ, profound high expressions of IDO-1 mRNA as well as the protein were detected in human neural stem cells (hNSC) and verified by real-time retro-transcribed PCR and western blot analysis, respectively. The protein activity was measured by kynurenine concentration and the assay was validated by dose-responsive inhibition of IDO-1 antagonists including 1-methyltryptaphan, indomethacin and acetylsalicylic acid. Among the tested compounds, apigenin, baicalein, chrysin, and wogonin exhibit a potent repressive activity with IC(50s) comparable to that of indomethacin. The inhibition was further found to be independent of gene expression and protein translation because of the unaltered levels of mRNA and protein expression. Although curcumin displayed a potent inhibitory activity to the enzyme, it appeared to be cytotoxic to hNSCs. Morphological examination of hNSC revealed that baicalein and wogonin at the inhibitory concentrations induced neurite outgrowth. In conclusion, our data shows that certain phytochemicals with 2-phenyl-1-benzopyran-4-one backbone (flavones) attenuate significantly the IDO-1 protein activity without harming hNSCs. The inhibitory activity might have partially contributed to the anti-cancer and neuro-protective property of the compounds.

Serum metabolic profiling study of hepatocellular carcinoma infected with hepatitis B or hepatitis C virus by using liquid chromatography-mass spectrometry

The objective of the present study was to explore the common and specific metabolic alterations of hepatocellular carcinoma (HCC) infected with hepatitis B virus (HBV) or hepatitis C virus (HCV). Serum profiling data revealed that the two HCC groups shared a mainly similar metabolic profile, providing a basis for investigating their common tumor pathogenesis mechanism and early diagnosis biomarkers. Arachidonic acid as a pro-inflammatory precursor increased significantly in the HCC group compared to the cirrhosis and healthy control. And the lysophosphatidylcholines (lysoPCs) with polyunsaturated fatty acid acyl chain with potent anti-inflammatory activity significantly decreased in the HCC and cirrhosis groups compared to those in the healthy control group, which may partly contribute to maintaining chronic inflammation and benefit the initiation and progression of the malignant hepatic tumor. The decreased ratios of polyunsaturated lysoPCs to saturated lysoPCs in HCC groups compared to chronic liver diseases infected with HBV or HCV and healthy control further demonstrated that a malignant liver tumor exerts profound influences independent of virus infection. Especially, serum endocannabinoids anandamide (AEA) and palmitylethanolamide (PEA) were found significantly elevated in HCC groups compared to healthy control, and in HCC with HCV compared to corresponding chronic liver diseases. AEA, PEA, or their combination showed better sensitivity, specificity, and the area under the curve for distinguishing HCC from chronic liver diseases, showing they are potential biomarkers to distinguish the HCC from cirrhosis infected with HCV.

Generation of myeloid-derived suppressor cells using prostaglandin E2

Myeloid-derived suppressor cells (MDSCs) are natural immunosuppressive cells and endogenous inhibitors of the immune system. We describe a simple and clinically compatible method of generating large numbers of MDSCs using the cultures of peripheral blood-isolated monocytes supplemented with prostaglandin E₂ (PGE₂). We observed that PGE₂ induces endogenous cyclooxygenase (COX)2 expression in cultured monocytes, blocking their differentiation into CD1a⁺ dendritic cells (DCs) and inducing the expression of indoleamine 2,3-dioxygenase 1, IL-4Rα, nitric oxide synthase 2 and IL-10 - typical MDSC-associated suppressive factors. The establishment of a positive feedback loop between PGE₂ and COX2, the key regulator of PGE₂ synthesis, is both necessary and sufficient to promote the development of CD1a⁺ DCs to CD14⁺CD33⁺CD34⁺ monocytic MDSCs in granulocyte macrophage colony stimulating factor/IL-4-supplemented monocyte cultures, their stability, production of multiple immunosuppressive mediators and cytotoxic T lymphocyte-suppressive function. In addition to PGE₂, selective E-prostanoid receptor (EP)2- and EP4-agonists, but not EP3/1 agonists, also induce the MDSCs development, suggesting that other activators of the EP2/4- and EP2/4-driven signaling pathway (adenylate cyclase/cAMP/PKA/CREB) may be used to promote the development of suppressive cells. Our observations provide a simple method for generating large numbers of MDSCs for the immunotherapy of autoimmune diseases, chronic inflammatory disorders and transplant rejection.

Silencing IDO in dendritic cells: a novel approach to enhance cancer immunotherapy in a murine breast cancer model

Cancer immunotherapeutic agents (vaccines) in the form of antigen-loaded dendritic cells (DCs) reached an important milestone with the recent approval of Provenge, the first DC vaccine for treatment of prostate cancer. Although this heralds a new era of tumor immunotherapy, it also highlights the compelling need to optimize such DC-based therapies as they are increasingly tested and used to treat human patients. In this study we sought to augment and enhance the antitumor activity of a DC-based vaccine using siRNA to silence expression of immunosuppressive enzyme indoleamine 2,3-dioxygenase (IDO) in DCs. We report here that DCs loaded with tumor antigens, but with siRNA-silenced IDO expression, were introduced into 4T1 breast tumor-bearing mice, the treatment: (i) lengthened the time required for tumor onset, (ii) decreased tumor size compared to tumors grown for equal lengths of time in mice treated with antigen-loaded DCs without IDO silencing and (iii) reduced CD4(+) and CD8(+) T cell apoptosis. Furthermore, immunization with IDO-silenced DCs enhanced tumor antigen-specific T cell proliferation and CTL activity, and decreased numbers of CD4(+) CD25(+) Foxp3(+) T(reg). This study provides evidence to support silencing of immunosuppressive genes (IDO) as an effective strategy to enhance the efficacy of DC-based cancer immunotherapeutic.

Proteome signatures of inflammatory activated primary human peripheral blood mononuclear cells

Proteome profiling is the method of choice to identify marker proteins whose expression may be characteristic for certain diseases. The formation of such marker proteins results from disease-related pathophysiologic processes. In healthy individuals, peripheral blood mononuclear cells (PBMCs) circulate in a quiescent cell state monitoring potential immune-relevant events, but have the competence to respond quickly and efficiently in an inflammatory manner to any invasion of potential pathogens. Activation of these cells is most plausibly accompanied by characteristic proteome alterations. Therefore we investigated untreated and inflammatory activated primary human PBMCs by proteome profiling using a 'top down' 2D-PAGE approach in addition to a 'bottom up' LC-MS/MS-based shotgun approach. Furthermore, we purified primary human T-cells and monocytes and activated them separately. Comparative analysis allowed us to characterize a robust proteome signature including NAMPT and PAI2 which indicates the activation of PBMCs. The T-cell specific inflammation signature included IRF-4, GBP1 and the previously uncharacterized translation product of GBP5; the corresponding monocyte signature included PDCD5, IL1RN and IL1B. The involvement of inflammatory activated PBMCs in certain diseases as well as the responsiveness of these cells to anti-inflammatory drugs may be evaluated by quantification of these marker proteins. This article is part of a Special Issue entitled: Integrated omics.

A cross-talk of decidual stromal cells, trophoblast, and immune cells: a prerequisite for the success of pregnancy

Embryo implantation and formation of a functional placenta are complex processes that require a plethora of regulatory mechanisms involving both mother and embryo cells. Recently, an important role in this complicated cells and factors network was assigned to the decidual stromal cells (DSC) and trophoblast cells. Decidualization includes biochemical changes that trigger DSC to produce a number of factors required for the implantation and induction of immunotolerance in maternal immune system. Immunotolerance is achieved by a cascade of strictly controlled events starting with selective homing of immune cells to the feto-maternal site, regulated proliferation, and predominant differentiation into a regulatory type of immune cells. Furthermore, cytotoxic effector functions are reduced owing to the influence of steroid hormones, factors, cytokines, and inhibitory receptors. Altogether the entire immune system of the mother is switched to tolerogenic functional state which is a prerequisite for the successful maintenance of pregnancy.

SOCS3 in immune regulation of inflammatory bowel disease and inflammatory bowel disease-related cancer

Inflammatory bowel disease (IBD) has unclear pathogenesis and it is related to the increasing risk of developing colorectal cancer (CRC). Recent studies have uncovered the molecular mechanism of intracellular signaling pathways of inflammatory cytokines such as tumor necrosis factor (TNF)-α, interferon (IFN)-γ and interleukin (IL)-6. The major transcription factors including STAT3 have been shown to play a major role in transmitting inflammatory cytokine signals to the nucleus. The suppressors of cytokine signaling (SOCS) 3 protein is the key physiological regulators of cytokine-mediated STAT3 signaling. As such it influences the development of inflammatory and malignant disorders like this associated with IBD. Here we review the complex function of SOCS3 in innate and adaptive immunity, different cell types (macrophages, neutrophils, dendritic cells, B cells, T cells and intestinal epithelial cells) and the role of SOCS3 on the pathogenesis of inflammatory bowel disease (IBD) and IBD-related cancer. Finally, we explore how this knowledge may open novel avenues for the rational treatment of IBD and IBD-related cancer.

Apicomplexan parasite, Eimeria falciformis, co-opts host tryptophan catabolism for life cycle progression in mouse

The obligate intracellular apicomplexan parasites, e.g. Toxoplasma gondii and Plasmodium species, induce an IFNγ-driven induction of host indoleamine 2,3-dioxygenase (IDO), the first and rate-limiting enzyme of tryptophan catabolism in the kynurenine pathway. Induction of IDO1 supposedly depletes cellular levels of tryptophan in host cells, which is proposed to inhibit the in vitro growth of auxotrophic pathogens. In vivo function of IDO during infections, however, is not clear, let alone controversial. We show that Eimeria falciformis, an apicomplexan parasite infecting the mouse caecum, induces IDO1 in the epithelial cells of the organ, and the enzyme expression coincides with the parasite development. The absence or inhibition of IDO1/2 and of two downstream enzymes in infected animals is detrimental to the Eimeria growth. The reduced parasite yield is not due to a lack of an immunosuppressive effect of IDO1 in the parasitized IDO1(-/-) or inhibitor-treated mice because they did not show an accentuated Th1 and IFNγ response. Noticeably, the parasite development is entirely rescued by xanthurenic acid, a by-product of tryptophan catabolism inducing exflagellation in male gametes of Plasmodium in the mosquito mid-gut. Our data demonstrate a conceptual subversion of the host defense (IFNγ, IDO) by an intracellular pathogen for progression of its natural life cycle. Besides, we show utility of E. falciformis, a monoxenous parasite of a well appreciated host, i.e. mouse, to identify in vivo factors underlying the parasite-host interactions.

Combining immunotherapy and targeted therapies in cancer treatment

During the past two decades, the paradigm for cancer treatment has evolved from relatively nonspecific cytotoxic agents to selective, mechanism-based therapeutics. Cancer chemotherapies were initially identified through screens for compounds that killed rapidly dividing cells. These drugs remain the backbone of current treatment, but they are limited by a narrow therapeutic index, significant toxicities and frequently acquired resistance. More recently, an improved understanding of cancer pathogenesis has given rise to new treatment options, including targeted agents and cancer immunotherapy. Targeted approaches aim to inhibit molecular pathways that are crucial for tumour growth and maintenance; whereas, immunotherapy endeavours to stimulate a host immune response that effectuates long-lived tumour destruction. Targeted therapies and cytotoxic agents also modulate immune responses, which raises the possibility that these treatment strategies might be effectively combined with immunotherapy to improve clinical outcomes.

An epigenetic mechanism for high, synergistic expression of indoleamine 2,3-dioxygenase 1 (IDO1) by combined treatment with zebularine and IFN-γ: potential therapeutic use in autoimmune diseases

IDO1 can be induced by interferon gamma (IFN-γ) in multiple cell types. We have earlier described that the DNA methyltransferase inhibitor zebularine also induces IDO1 in several rat cell clones. We now describe a synergistic induction of IDO1 expression by IFN-γ and zebularine. To elucidate the mechanism of the IDO1 induction we have studied the methylation status in the promoter region of the IDO1 gene from both human monocytic THP-1 cells and H1D2 rat colon cancer cells. Interestingly, the IDO1 promoter is hypermethylated and IFN-γ is shown to induce a significant demethylation. The synergism in effect of zebularine and IFN-γ on IDO1 expression is paralleled by a similar synergistic effect on expression of two other IFN-γ-responsive genes, the transcription factors STAT1 and IRF1 with binding sites in the IDO1 promoter region. The demonstrated synergistic activation of IDO1 expression has implications in relation to therapeutic induction of immunosuppression in autoimmunity and chronic inflammation.

Dissecting paracrine effectors for mesenchymal stem cells

There has been increasing interest in the application of mesenchymal stem cells (MSCs) in regenerative medicine in recent years. In this context, the beneficial effects of MSCs have been ascribed mainly to a paracrine action rather than to direct replacement of the injured tissue. Indeed, MSCs produce a great variety of trophic and immunomodulatory factors. In this chapter, we provide an overview of growth factors and chemokines involved in stimulation of cell proliferation, inhibition of apoptosis, enhancement of angiogenesis, and suppression of inflammatory and immune response. In addition, we discuss the emerging role of the extracellular vesicles released from MSCs as possible paracrine mediators.

PGE(2)-driven induction and maintenance of cancer-associated myeloid-derived suppressor cells

Myeloid-derived suppressor cells (MDSCs) are critical mediators of tumor-associated immune suppression, with their numbers and activity strongly increased in most human cancers and animal models. MDSCs suppress anti-tumor immunity through multiple mechanisms, including the manipulation of arginine and tryptophan metabolism by such factors as arginase (Arg), inducible nitric oxide synthase (iNOS/NOS2), and indoleamine-2,3-dioxygenase (IDO). Prostaglandin E(2) (PGE(2)), a mediator of chronic inflammation and tumor progression, has emerged as a key molecule in MDSC biology. PGE(2) promotes MDSC development and their induction by additional factors, directly suppresses T cell immune responses and participates in the induction of other MDSC-associated suppressive factors, including Arg, iNOS and IDO. It further promotes MDSC recruitment to tumor environments through the local induction of CXCL12/SDF-1 and the induction and stabilization of the CXCL12 receptor, CXCR4, on tumor-associated MDSCs. The establishment of a positive feedback loop between PGE(2) and cyclooxygenase 2 (COX-2), the key regulator of PGE(2) synthesis, stabilizes the MDSC phenotype and is required for their suppressive function. The central role of PGE(2) in MDSC biology provides for a feasible target for counteracting MDSC-mediated immune suppression in cancer.

Human periodontal ligament cells facilitate leukocyte recruitment and are influenced in their immunomodulatory function by Th17 cytokine release

The objective of this in vitro study was to examine the immunomodulatory impact of human periodontal ligament (PDL) cells on the nature and magnitude of the leukocyte infiltrate in periodontal inflammation, particularly with regard to Th17 cells. PDL cells were challenged with pro-inflammatory cytokines (IL-1ß, IL-17A, and IFN-γ) and analyzed for the expression of cytokines involved in periodontal immunoinflammatory processes (IL-6, MIP-3 alpha, IL-23A, TGFß1, IDO, and CD274). In order to further investigate a direct involvement of PDL cells in leukocyte function, co-culture experiments were conducted. The expression of the immunomodulatory cytokines studied was significantly increased under pro-inflammatory conditions in PDL cells. Although PDL cells did not stimulate leukocyte proliferation or Th17 differentiation, these cells induced the recruitment of leukocytes. The results of our study suggest that PDL cells might be involved in chronic inflammatory mechanisms in periodontal tissues and thus in the transition to an adaptive immune response in periodontitis.

Fat-1 transgenic cattle as a model to study the function of ω-3 fatty acids

ω-3 polyunsaturated fatty acids have been shown to play an important role in health. Enriched with ω-3 polyunsaturated fatty acids modulate expression of a number of genes with such broad functions as cell proliferation, growth and apoptosis and cell signaling and transduction, these effects, seem to regulate coronary artery disease, hypertension, atherosclerosis, psychiatric disorders and various cancer. In this context, fat-1 transgenic cattle was designed to convert ω-6 to ω-3 fatty acids could form an ideal model to study the effect of ω-3 fatty acids on the above functions. This study focuses on the total genomic difference of gene expression between fat-1 transgenic cattle and wild-type using cDNA microarrays, several genes were found to be overexpressed or suppressed in transgenic cattle relative to wild-type, these discrepancy genes related with lipid metabolism, immunity, inflammation nervous development and fertility.

Role of IFN-γ in the establishment of anterior chamber-associated immune deviation (ACAID)-induced CD8+ T regulatory cells

Introduction of alloantigens into the AC induces a form of immune tolerance known as ACAID, which induces antigen-specific CD8+ Tregs, contributing to ocular immune privilege by down-regulating immune responses. Recent evidence suggests IFN-γ is needed for the suppressive function of CD8+ ACAID Tregs. This study tested the hypothesis that IFN-γ is needed for alloantigen-specific ACAID CD8+ Tregs to execute their suppressive function but is not required for the establishment of ACAID CD8+ Tregs. To address this hypothesis, ACAID was induced by injecting BALB/c spleen cells into the AC of WT C57BL/6 mice, IFN-γ(-/-) C57BL/6 mice, or anti-IFN-γ-treated WT C57BL/6 mice. LAT assays using C57BL/6 APCs as stimulators, CD4+ T cells from C57BL/6 mice previously immunized toward BALB/c alloantigens as effector cells, and IFN-γ-competent, IFN-γ(-/-), or IFN-γR(-/-) CD8+ Tregs were used to evaluate the suppressive function of CD8+ ACAID Tregs in response to IFN-γ. IFN-γ(-/-) mice or mice treated with anti-IFN-γ antibody prior to AC injection of alloantigen failed to develop ACAID. The suppressive function of IFN-γ(-/-) ACAID CD8+ Tregs was restored through the administration of exogenous IFN-γ. This suppressive responsiveness toward IFN-γ was CD8+ Treg-intrinsic, as CD8+ Tregs from IFN-γR(-/-) mice, which were primed in the AC with alloantigens, were not able to suppress alloantigen-specific DTH responses. These results indicate that IFN-γ is not needed for the induction of CD8+ ACAID Tregs but is required for ACAID Tregs to exert the suppression of allospecific DTH responses.

Amelioration of 2,4,6-trinitrobenzene sulfonic acid-induced colitis in mice by immunoregulatory dendritic cells

BACKGROUND

Dendritic cells (DCs) are widely distributed throughout the lymphoid and nonlymphoid tissues, and are important initiators of acquired immunity. They also serve as regulators by inducing self-tolerance. However, it has not been thoroughly clarified whether DCs are somehow involved in the regulation or treatment of inflammatory bowel diseases.

METHODS

We established an ileitis model by transmurally injecting 2,4,6-trinitrobenzene sulfonic acid (TNBS) into the lumen of the ileocolonic junction. The kinetic movement of DCs at the inflammatory sites was analyzed histologically and by flow cytometry, and DCs obtained from the small intestine were analyzed in order to determine the expression of paired immunoglobulin-like receptor-A/B (PIR-A/B) by flow cytometry and quantitative RT-PCR. Furthermore, the regulatory role of DCs was directly determined by a transfer experiment using TNBS-induced colitis model mice.

RESULTS

We observed three DC subsets (PIR-A/B(high), PIR-A/B(med), and PIR-A/B(-) DCs) in the conventional DCs (cDCs) from day 3, and the number of PIR-A/B(med) cDCs increased from the time the inflammatory responses ceased (day 7). PIR-A/B(med) cDCs actually migrated to the inflamed colon, and ameliorated the colitis induced by TNBS when transferred to colitis-induced recipients. The colitis was greatly exacerbated when mice had been treated with the indoleamine-pyrrole 2,3-dioxygenase (IDO) inhibitor 1-methyltryptophan (1-mT) at the time PIR-A/B(med) cDCs were transferred, indicating that the therapeutic ability of PIR-A/B(med) cDCs is partially dependent on IDO.

CONCLUSION

The PIR-A/B(med) cDCs, which increase in number during the final stages of inflammation, can be used to treat colitis via an IDO-dependent mechanism.