Role of IDO in organ transplantation: promises and difficulties

Indoleamine 2,3-Dioxygenase (IDO) Activity: A Perspective Biomarker for Laboratory Determination in Tumor Immunotherapy

Indoleamine 2,3-dioxygenase 1 (IDO1) is a heme enzyme involved in catalyzing the conversion of tryptophan (Trp) into kynurenine (Kyn) at the first rate-limiting step in the kynurenine pathway of L-tryptophan metabolism. It has been found to be involved in several biological functions such as aging, immune microorganism, neurodegenerative and infectious diseases, and cancer. IDO1 plays an important role in immune tolerance by depleting tryptophan in the tumor microenvironment and inhibiting the proliferation of effector T cells, which makes it an important emerging biomarker for cancer immunotherapy. Therefore, the research and development of IDO1 inhibitors are of great importance for tumor therapy. Of interest, IDO activity assays are of great value in the screening and evaluation of inhibitors. Herein, we mainly review the biological functions of IDO1, immune regulation, key signaling molecules in the response pathway, and the development of IDO1 inhibitors in clinical trials. Furthermore, this review provides a comprehensive overview and, in particular, a discussion of currently available IDO activity assays for use in the evaluation of IDO inhibitors in human blood. We believe that the IDO activity is a promising biomarker for the immune escape and laboratory evaluation of tumor immunotherapy.

The role of metabolism on regulatory T cell development and its impact in tumor and transplantation immunity

Regulatory CD4⁺ T (Treg) cells play a key role in the induction of immune tolerance and in the prevention of autoimmune diseases. Treg cells are defined by the expression of transcription factor FOXP3, which ensures proliferation and induction of the suppressor activity of this cell population. In a tumor microenvironment, after transplantation or during autoimmune diseases, Treg cells can respond to various signals from their environment and this property ensures their suppressor function. Recent studies showed that a metabolic signaling pathway of Treg cells are essential in the control of Treg cell proliferation processes. This review presents the latest research highlights on how the influence of extracellular factors (e.g. nutrients, vitamins and metabolites) as well as intracellular metabolic signaling pathways regulate tissue specificity of Treg cells and heterogeneity of this cell population. Understanding the metabolic regulation of Treg cells should provide new insights into immune homeostasis and disorders along with important therapeutic implications for autoimmune diseases, cancer and other immune-system-mediated disorders.

Immune-Related Genes for Predicting Future Kidney Graft Loss: A Study Based on GEO Database

Objective

We aimed to identify feature immune-related genes that correlated with graft rejection and to develop a prognostic model based on immune-related genes in kidney transplantation.

Methods

Gene expression profiles were obtained from the GEO database. The GSE36059 dataset was used as a discovery cohort. Then, differential expression analysis and a machine learning method were performed to select feature immune-related genes. After that, univariate and multivariate Cox regression analyses were used to identify prognosis-related genes. A novel Riskscore model was built based on the results of multivariate regression. The levels of these feature genes were also confirmed in an independent single-cell dataset and other GEO datasets.

Results

15 immune-related genes were expressed differently between non-rejection and rejection kidney allografts. Those differentially expressed immune-related genes (DE-IRGs) were mainly associated with immune-related biological processes and pathways. Subsequently, a 5-immune-gene signature was constructed and showed favorable predictive results in the GSE21374 dataset. Recipients were divided into the high-risk and low-risk groups according to the median value of RiskScore. The GO and KEGG analysis indicated that the differentially expressed genes (DEGs) between high-risk and low-risk groups were mainly involved in inflammatory pathways, chemokine-related pathways, and rejection-related pathways. Immune infiltration analysis demonstrated that RiskScore was potentially related to immune infiltration. Kaplan-Meier survival analysis suggested that recipients in the high-risk group had poor graft survival. AUC values of 1- and 3-year graft survival were 0.804 and 0.793, respectively.

Conclusion

Our data suggest that this immune-related prognostic model had good sensitivity and specificity in predicting the 1- and 3-year kidney graft survival and might act as a useful tool for predicting kidney graft loss.

Tryptophan: From Diet to Cardiovascular Diseases

Cardiovascular disease (CVD) is one of the major causes of mortality worldwide. Inflammation is the underlying common mechanism involved in CVD. It has been recently related to amino acid metabolism, which acts as a critical regulator of innate and adaptive immune responses. Among different metabolites that have emerged as important regulators of immune and inflammatory responses, tryptophan (Trp) metabolites have been shown to play a pivotal role in CVD. Here, we provide an overview of the fundamental aspects of Trp metabolism and the interplay between the dysregulation of the main actors involved in Trp metabolism such as indoleamine 2, 3-dioxygenase 1 (IDO) and CVD, including atherosclerosis and myocardial infarction. IDO has a prominent and complex role. Its activity, impacting on several biological pathways, complicates our understanding of its function, particularly in CVD, where it is still under debate. The discrepancy of the observed IDO effects could be potentially explained by its specific cell and tissue contribution, encouraging further investigations regarding the role of this enzyme. Thus, improving our understanding of the function of Trp as well as its derived metabolites will help to move one step closer towards tailored therapies aiming to treat CVD.

Shikonin Prolongs Allograft Survival via Induction of CD4+FoxP3+ Regulatory T Cells

A transplanted organ is usually rejected without any major immunosuppressive treatment because of vigorous alloimmune responsiveness. However, continuous global immunosuppression may cause severe side effects, including nephrotoxicity, tumors, and infections. Therefore, it is necessary to seek novel immunosuppressive agents, especially natural ingredients that may provide sufficient efficacy in immunosuppression with minimal side effects. Shikonin is a bioactive naphthoquinone pigment, an ingredient originally extracted from the root of Lithospermum erythrorhizon. Previous studies have shown that shikonin regulates immunity and exerts anti-inflammatory effects. In particular, it can ameliorate arthritis in animal models. However, it is unclear whether shikonin inhibits alloimmunity or allograft rejection. In this study and for the first time, we demonstrated that shikonin significantly prolonged the survival of skin allografts in wild-type mice. Shikonin increased the frequencies of CD4⁺Foxp3⁺ regulatory T cells (Tregs) post-transplantation and induced CD4⁺Foxp3⁺ Tregs in vitro as well. Importantly, depleting the Tregs abrogated the extension of skin allograft survival induced by shikonin. It also decreased the frequencies of CD8⁺CD44^highCD62L^low effector T cells and CD11c⁺CD80⁺/CD11c⁺CD86⁺ mature DCs after transplantation. Moreover, we found that shikonin inhibited the proliferation of T cells in vitro and suppressed their mTOR signaling. It also reduced the gene expression of pro-inflammatory cytokines, including IFNγ, IL-6, TNFα, and IL-17A, while increasing the gene expression of anti-inflammatory mediators IL-10, TGF-β1, and indoleamine-2, 3-dioxygenase (IDO) in skin allografts. Further, shikonin downregulated IDO protein expression in skin allografts and DCs in vitro. Taken together, shikonin inhibits allograft rejection via upregulating CD4⁺Foxp3⁺ Tregs. Thus, shikonin is a novel immunosuppressant that could be potentially used in clinical transplantation.

Dendritic Cells Treated with Exogenous Indoleamine 2,3-Dioxygenase Maintain an Immature Phenotype and Suppress Antigen-specific T cell Proliferation

Indoleamine 2,3-dioxygenase (IDO), an intracellular enzyme responsible for catalyzing the rate limiting step of tryptophan catabolism, plays a critical role in immune cell suppression and tolerance. Indoleamine 2,3-dioxygenase-mediated depletion of the essential amino acid tryptophan increases susceptibility of T cells to apoptosis, while kynurenine and its downstream metabolites, such as 3-hydroxyanthranilic acid and quinolinic acid, have a direct cytotoxic effect on conventional effector T cells. Additionally, IDO-expressing antigen presenting cells (APCs) induce proliferation of regulatory T cells. When expressed by an APC, the immunosuppressive effects of IDO may act directly on the APC as well as indirectly upon local T cells. One approach to elicit immune tolerance or reduce inflammation therefore is to promote expression of IDO. However, this approach is constrained by several factors including the potential for deleterious biologic effects of conventional IDO-inducing agents such as interferon gamma (IFNγ), and the potential limitations of constitutive gene transfection. Alternatively, direct action of recombinant IDO enzyme supplied exogenously as a potential therapeutic in the extracellular space has not been investigated previously, and is the focus of this work. Results indicate exogenous recombinant human IDO supplementation influences murine dendritic cell (DC) maturation and ability to suppress antigen specific T cell proliferation. Following treatment, DCs were refractory to maturation by LPS as defined by co-stimulatory molecule expression (CD80 and CD86) and major histocompatibility complex II (MHC-II) expression. Dendritic cells exhibited skewing toward an anti-inflammatory cytokine release profile, with reduced secretion of IL-12p70 and maintained basal level of secreted IL-10. Notably, IDO-treated DCs suppressed proliferation of ovalbumin (OVA) antigen-specific CD4⁺ and CD8⁺ T cells in the presence of cognate antigen presentation in a manner dependent on active enzyme, as introduction of IDO inhibitor 1-methyl-tryptophan, restored T cell proliferation. Defined media experiments indicate a cumulative role for both tryptophan depletion and kynurenine presence, in the suppressive programming of DCs. In sum, we report that exogenously supplied IDO maintains immunoregulatory function on DCs, suggesting that IDO may have potential as a therapeutic protein for suppressive programming with application toward inflammation and tolerance.

Exosomes Derived from IDO1-Overexpressing Rat Bone Marrow Mesenchymal Stem Cells Promote Immunotolerance of Cardiac Allografts

Background:

The immunosuppressive activity of mesenchymal stem cells (MSCs) has been exploited to induce tolerance after organ transplantation. The indoleamine 2,3-dioxygenase (IDO) may have beneficial effects in the immunoregulatory properties of MSCs. It was recently revealed that exosomes derived from MSCs play important roles in mediating the biological functions of MSCs. This study aimed to explore the roles of exosomes derived from MSCs in the induction of immune tolerance.

Methods:

Dendritic cells (DCs) and T-cells were cultured with exosomes derived from rat bone marrow MSCs (BMSCs) overexpressing IDO1 or controls. For the in-vivo study, rats received heart transplants and were treated with exosomes from IDO-BMSCs and heart function was evaluated. Flow cytometry was used to detect expression of cell surface markers. Cytokine levels were detected in culture supernatants or serum samples. Protein and microRNA expressions in exosomes were investigated by chips.

Results:

Exosomes from IDO-BMSCs cultured with DCs and T-cells (1) downregulated CD40, CD86, CD80, MHC-II, CD45RA, CD45RA+CD45RB, OX62, and upregulated CD274 expression, (2) increased the number of regulatory T-cells (Tregs) and decreased the number of CD8+ T-cells, and (3) decreased the levels of pro-inflammatory cytokines, but increased the levels of anti-inflammatory cytokines compared with the other groups. Transplanted rats, which were injected with exosomes from IDO-BMSCs, had reduced allograft-targeting immune responses and improved cardiac allograft function. Exosomes secreted by IDO-BMSCs exhibited significant upregulations of the immunoregulatory protein FHL-1, miR-540-3p, and a downregulation of miR-338-5p.

Conclusion:

Exosomes derived from IDO-BMSCs can be used to promote immunotolerance and prolong the survival of cardiac allografts.

The tryptophan/kynurenine pathway, systemic inflammation, and long-term outcome after kidney transplantation

Tryptophan is metabolized along the kynurenine pathway, initially to kynurenine, and subsequently to cytotoxic 3-hydroxykynurenine. There is increasing interest in this pathway because of its proinflammatory nature, and drugs interfering in it have received increasing attention. We aimed to investigate whether serum and urinary parameters of the tryptophan/kynurenine pathway, and particularly cytotoxic 3-hydroxykynurenine, are associated with systemic inflammation and long-term outcome in renal transplant recipients (RTR). Data were collected in outpatient RTR with a functioning graft for >1 yr. Tryptophan, kynurenine, and 3-hydroxykynurenine in serum and urine were measured using LC-MS/MS. A total of 561 RTR (age: 51 ± 12 yr; 56% male) were included at a median of 6.0 (2.6-11.6) yr posttransplantation. Baseline median serum tryptophan was 40.0 (34.5-46.0) µmol/l, serum kynurenine was 1.8 (1.4-2.2) µmol/l, and serum 3-hydroxykynurenine was 42.2 (31.0-61.7) nmol/l. Serum kynurenine and 3-hydroxykynurenine were strongly associated with parameters of systemic inflammation. During follow-up for 7.0 (6.2-7.5) yr, 51 RTR (9%) developed graft failure and 120 RTR (21%) died. Both serum kynurenine and 3-hydroxykynurenine were independently associated with graft failure [HR 1.72 (1.23-2.41), P = 0.002; and HR 2.03 (1.42-2.90), P < 0.001]. Serum 3-hydroxykynurenine was also independently associated with mortality [HR 1.37 (1.08-1.73), P = 0.01], whereas serum kynurenine was not. Urinary tryptophan/kynurenine pathway parameters were not associated with outcome. Of tryptophan metabolites, serum 3-hydroxykynurenine is cross-sectionally most strongly and consistently associated with systemic inflammation and prospectively with adverse long-term outcome after kidney transplantation. Serum 3-hydroxykynurenine may be an interesting biomarker and target for the evaluation of drugs interfering in the tryptophan/kynurenine pathway.

The expression and prognostic relevance of indoleamine 2,3-dioxygenase in tongue squamous cell carcinoma

Conclusion IDO might be useful for predicting progression of primary tumor stage T2 and T3 in tongue squamous cell carcinoma (TSCC), but does not seem like a specific biomarker for diagnosing TSCC and predicting patient survival. Objectives Indoleamine 2,3-dioxygenase (IDO) is expressed in many cells and it catabolises the essential amino acid tryptophan to kynurenine. IDO acts as an immune modulator through suppression of T-cell immunity and other pathways. In cancer cells, IDO has been proposed to promote tumor progression by enabling malignant cells to escape from the immune system. The aim of this study was to evaluate the association and prognostic relevance of IDO expression in TSCC. Method One hundred and eight retrospective tongue and lymph node specimens were stained immunohistochemically with monoclonal antibody anti-indoleamine 2,3-dioxygenase. The relative abundance of IDO positive epithelial cells, IDO staining intensity, and inflammation were assessed semi-quantitatively with light microscopy. Results IDO was expressed stronger in tongue hyperplasia than in TSCC. However, IDO expression associated with poor survival in the sub-groups with primary tumor stage T2-T4 and in the sub-group with strong inflammation in tumors' invasive front.

Verification of association of elevated serum IDO enzyme activity with acute rejection and low CD4-ATP levels with infection

BACKGROUND

Both acute rejection (AR) and major infection events (MIE) can reduce long-term allograft survival. We assessed the simultaneous efficacy of serum and urine biomarker indoleamine 2,3-dioxygenase (IDO) enzyme activity and peripheral blood CD4-ATP levels for AR and MIE association, respectively.

METHODS

We prospectively tested 217 blood and 167 urine serial samples, collected monthly for 12 months after transplantation from 29 consecutive children receiving a kidney transplant. The indoleamine 2,3-dioxygenase activity was assessed by mass spectrometry assays using the ratio of product L-kynurenine (kyn) to substrate tryptophan (trp). Kyn/trp ratios and blood CD4 T-cell ATP levels were correlated with AR, MIE, or stable group (no events) in the next 30 days.

RESULTS

Using absolute cutoffs and allocating to samples to AR, MIE, or stable group, mean serum kyn/trp ratios were significantly elevated in the group that experienced AR (P=0.0007). Similarly, peripheral blood CD4-ATP levels were significantly lower in the group experiencing MIE (P=0.0351). Urine kyn/trp ratios and blood tacrolimus levels were not different between AR and stable groups. Within-subject analyses, accounting for repeated measures in subjects, also showed that, over time, serum kyn/trp ratios were higher before AR (P=0.031) and blood CD4-ATP levels were lower before MIE (P=0.008).

CONCLUSIONS

These results from our pilot discovery group suggest that a panel of biomarkers together can predict overimmunosuppression or underimmunosuppression. Further independent validation in a multicenter cohort is suggested.

Alternative immunomodulatory strategies for xenotransplantation: CD80/CD86-CTLA4 pathway-modified immature dendritic cells promote xenograft survival

Background

Xenotransplantation is a promising approach to circumventing the current organ shortage. However, T-cell-dependent anti-xenoresponses are a major challenge to successful xenografts. Given the advantages of the use of CTLA4-Ig in the survival of allografts, the purpose of the study was to investigate the therapeutic potential of CTLA4-IgG4 modified immature dendritic cells (imDCs) in the prevention of islets xenograft rejection.

Methods

CTLA4-IgG4 was constructed by the fusion of the extracellular regions of porcine CTLA4 to human the hIgG4 Fc region. The imDCs were induced and cultured from porcine peripheral blood mononuclear cells (PBMC). The CTLA4-IgG4 modified imDCs were delivered via the portal vein to the liver of diabetic mice (insulin-dependent diabetes mellitus) before islet xenografting, and mCTLA4-Ig was administered intravenously after xenotransplantation.

Results

The xenograft survival of mice receiving unmodified imDCs was approximately 30 days. However, following administration of CTLA4-IgG4 modified imDCs before grafting and mCTLA4-Ig after grafting, xenografts survived for more than 100 days. Flow cytometric analysis showed that the CD4⁺CD25⁺Foxp3⁺ Treg population was increased in spleens. The efficacy of donor CTLA4-IgG4 modified imDCs correlated partially with the amplification of Tregs.

Conclusions

These results confirm that selective inhibition of the direct and indirect pathways of T-cell activation by donor CTLA4-IgG4 modified imDCs and receptor CTLA4-Ig is a highly effective strategy to promote survival of xenografts.

Investigation of the kynurenine pathway in Indoleamine 2, 3 dioxygenase deficient mice with inflammatory arthritis

Tryptophan is an essential amino acid involved in the protein synthesis, cognition, and immunity. Oxidative catabolism of tryptophan is executed by the sets of biochemical reactions collectively referred to as the kynurenine pathway. In the immune system, two distinct enzymes, Indoleamne 2,3 dioxygenase 1 (IDO1) and Indoleamine 2, 3 dioxygenase 2 (IDO2) can initiate metabolic flux through the kynurenine pathway. Rheumatoid arthritis is an autoimmune disease driven by the exacerbated immune response towards self antigens and characterized by the chronic inflammatory reaction of the diarthrodial joints. Collagen induced arthritis (CIA) is an animal model of rheumatoid arthritis. Using CIA in wild type (WT) and mice deficient with Indoleamine 2,3 dioxygenase (Ido1KO), it was of interest to test the impact of Ido1 deletion on the concentration of tryptophan and its catabolites as well as on mRNA expression for other genes on the kynurenine pathway. Here, when compared with samples taken from naïve WT animals and those with CIA, it was found that only in the inguinal lymph nodes (iLN) taken from Ido1KO mice with CIA tryptophan concentration was significantly increased. In contrast, mRNA expression for Ido2 was decreased in naïve as well as in the diseased iLN taken from Ido1KO mice. Deletion of Ido1 and reduced mRNA expression for Ido2 neither affected the concentration of the downstream metabolites of tryptophan nor mRNA expression for downstream genes on the kynurenine pathway in iLN. Moreover, the concentration of kynurenine in sera of mice with CIA was significantly decreased in Ido1KO mice with arthritis.

Persistence of gene expression profile in CD200 transgenic skin allografts is associated with graft survival on retransplantation to normal recipients

BACKGROUND

Expression of CD200 increases allograft survival by suppressing inflammation and acquired immunity. Increased allograft survival in transgenic mice overexpressing CD200 (CD200) occurs in association with increased intragraft expression of messenger RNAs (mRNAs) for genes associated with altered T-cell differentiation.

METHODS

We investigated whether donor CD200 BL/6 skin grafts taken from primary control or CD200 recipient BALB/c mice persisted after retransplantation at 14 days to control (nontransgenic) secondary BALB/c recipients, with or without transfer of splenocytes from autologous primary recipients. Splenocytes from primary and secondary recipients were analyzed 14 days after grafting, using in vitro mixed leukocyte cultures (MLCs) incubated with irradiated BL/6 (or third-party C3H/HeJ) stimulator cells and assayed for antigen-specific cytotoxic T lymphocyte. Cytotoxic T lymphocyte responses were correlated with changes in the mRNA gene expression profile observed in the skin tissue harvested from primary or secondary recipients on day 14 after grafting, using real-time polymerase chain reaction to compare quantitative mRNA expression in the graft tissue of primary/secondary recipients.

RESULTS AND CONCLUSIONS

Adoptive transfer of tolerance in MLC to BL/6 grafts was most evident when both skin and splenocytes were transferred from primary BALB/c recipients, although there was an attenuation of MLC responses after graft transfer alone. Adoptive transfer of tolerance occurred concomitant with persistent overexpression of genes encoding Foxp3, transforming growth factor β, interleukin 10, and PD-1 (and PD-L1/PD-L2) in tolerant skin grafts and increased expression of mRNAs for indoleamine 2,3-dioxygenase and the subunits encoding interleukin 35. Infusion of anti-CD4 or anti-transforming growth factor β to secondary recipients on retransplantation abolished increased graft survival, suggesting the importance of each to the final outcome.

Gene therapy with adenovirus-delivered indoleamine 2,3-dioxygenase improves renal function and morphology following allogeneic kidney transplantation in rat

BACKGROUND

Indoleamine 2,3-dioxygenase (IDO), the rate-limiting enzyme in the tryptophan catabolism, has recently emerged as an important immunosuppressive enzyme involved in the regulation of both physiologic (maternal tolerance), as well as pathologic (neoplasia, autoimmune diseases, asthma) processes. Accumulating evidence points to a role for IDO in suppressing T-cell responses, thereby promoting tolerance. In the present study, we investigate the effects of adenovirus-mediated gene therapy with IDO on the acute rejection of the transplanted kidneys.

METHODS

The experiments were performed in a rat Fisher to Lewis acute renal rejection model. RGD modified adenovirus carrying IDO gene (RGD-AdTIDO, n = 9) or RGD modified adenovirus carrying green fluorescent protein gene (RGD-AdTL, n = 8) were injected into the renal artery of the donor kidney before transplantation. A group receiving saline (n = 8) served as control. Rats were sacrificed after 7 days.

RESULTS

Successful gene delivery was confirmed with real-time polymerase chain reaction and immunohistochemistry. RGD-AdTIDO significantly decreased elevated plasma creatinine (93.7 ± 18.9 µmol/l) compared to the RGD-AdTL (248.2 ± 43.6 µmol/l) and saline (228.3 ± 46.4 µmol/l) treated rats. Moreover, RGD-AdTIDO therapy diminished the infiltration of CD8+ T cells and macrophages into the graft and reduced renal interstitial pre-fibrosis. Also, it limited the up-regulation of kidney injury molecule-1, interleukin (IL)-2, IL-17 and transforming growth factor-β mRNA expression, and increased foxp3 mRNA expression compared to controls.

CONCLUSIONS

RGD-AdTIDO therapy improves renal function and morphology in a clinically relevant model of acute rejection.

Targeting the tumor microenvironment by immunotherapy: part 2

Cancer therapy was traditionally centered on the neoplastic cells. This included mainly surgery, radiation, and chemotherapy, in some cases hormone therapy and to a lesser extent immunotherapy – all traditionally targeted to the highly proliferating mutated tumor cells. In view of our present understanding of the powerfull influence of the tumor microenvironment (TME) on cancer behavior and response – and lack of response – to treatment, this previously ignored constituent of cancer now has to be considered as an important, even indispensable target for therapy. The TME may be targeted both to its immune and to its nonimmune components. The various immune evasion elements of the TME should be targeted as well.

Influence of immunosuppressive agents on tryptophan degradation and neopterin production in human peripheral blood mononuclear cells

The anti-proliferative and immunomodulatory enzyme indoleamine 2,3-dioxygenase (IDO) degrades the essential amino acid tryptophan via the kynurenine pathway. IDO is stimulated during cellular immune responses preferentially by Th1-type cytokine interferon-γ (IFN-γ). IDO activity is estimated by calculating the kynurenine to tryptophan ratio (Kyn/Trp). In human monocyte-derived macrophages and dendritic cells, GTP-cyclohydrolase I is induced in parallel to IDO and produces neopterin. This study investigated the effects of common immunosuppressants on freshly isolated human peripheral blood mononuclear cells (PBMC) in vitro. PBMC were incubated with compounds for 30 min and then either left unstimulated or stimulated with mitogen phytohaemagglutinin (PHA). Concentrations of tryptophan, kynurenine and neopterin were measured in supernatants after 48 h. Kyn/Trp, neopterin and IFN-γ concentrations were significantly higher in PHA-stimulated vs. unstimulated PBMC. Tacrolimus (FK506), cyclosporine A (CsA), sirolimus and methylprednisolone dose-dependently inhibited tryptophan degradation and neopterin production. FK506, CsA and sirolimus showed significant inhibition at concentrations as low as 0.1 μg/ml, whereas prednisolone and methylprednisolone required higher doses to suppress tryptophan degradation. Mycophenolate-mofetil suppressed neopterin formation more efficiently than Kyn/Trp. All tested drugs also strongly decreased mitogen-induced IFN-γ concentrations. Overall the investigated immunosuppressants are effective to inhibit IDO activity and neopterin production in a similar and dose-dependent manner, however with some differences in IC50s when comparing individual compounds. The corresponding changes of IFN-γ concentrations are in line with its role as a trigger of both biochemical changes.

Immune biomarker panel monitoring utilizing IDO enzyme activity and CD4 ATP levels: prediction of acute rejection vs. viral replication events

Infections have become as important an event as acute rejection posttransplant for long-term allograft survival. Less invasive biomarkers tested so far predict risk for one event or the other, not both. We prospectively tested blood and urine monthly for 12 months posttransplant from children receiving a kidney transplant. The IDO enzyme pathway was assessed by MS assays using the ratio of product l-kyn to substrate trp. Kyn/trp ratios and blood CD4 T-cell ATP levels were correlated with acute rejection or major infection events or stable group (no events) in the next 30 days. The 25 subjects experienced six discrete episodes of acute rejection in five subjects and 16 discrete events of major infection in 14 subjects (seven BK viruria, six cytomegaloviremia, one EB and cytomegaloviremia, and two transplant pyelonephritis). Mean serum kyn/trp ratios were significantly elevated in the group that experienced acute rejection (p = 0.02). Within-subject analyses revealed that over time, urine kyn/trp ratios showed an increase (p = 0.01) and blood CD4-ATP levels showed a decrease (p = 0.007) prior to a major infection event. These pilot results suggest that a panel of biomarkers together can predict over- or under-immunosuppression, but need independent validation.

Indoleamine 2,3-dioxygenase: is it an immune suppressor?

This article covers what is currently known about the role of the enzyme indoleamine 2,3-dioxygenase (IDO) in cancer-related immunosuppression and the clinical research on IDO inhibitors. A PUBMED search was performed using the terms IDO, indoleamine 2,3-dioxygenase, 1-MT. IDO is an inducible enzyme that catalyzes the rate-limiting first step in tryptophan catabolism. This enzyme is overexpressed in response to IFNgamma in a variety of different malignancies. IDO causes immunosuppression through breakdown of tryptophan in the tumor microenvironment and tumor-draining lymph nodes. The depletion of tryptophan and toxic catabolites renders effector T cells inactive and dendritic cells immunosuppressive. Preclinical data suggest that IDO inhibition can delay tumor growth, enhance dendritic cell vaccines, and synergize with chemotherapy through immune-mediated mechanisms. The lead IDO inhibitor, d-1-methyl-tryptophan (d-1-MT), was selected for phase I trials and seems to have immune modulating activity. Subsequently, another isoform of IDO, IDO2, was discovered and found to be the target of d-1-MT. Multiple single-nucleotide polymorphisms in IDO2 affecting its catalytic activity may serve as a pharmacogenetic predictive biomarker for d-1-MT. The IDO pathway is an important mechanism of tumor-related immunosuppression and blocking it could improve cancer immunotherapy outcomes. Clinical development of d-1-MT and other IDO inhibitors as systemic immunomodulators to be combined with other immune modulators, vaccines, and chemotherapy are ongoing.

Indoleamine 2,3-dioxygenase mediates the antiviral effect of gamma interferon against hepatitis B virus in human hepatocyte-derived cells

Alpha interferon (IFN-α) is an approved medication for chronic hepatitis B. Gamma interferon (IFN-γ) is a key mediator of host innate and adaptive antiviral immunity against hepatitis B virus (HBV) infection in vivo. In an effort to elucidate the antiviral mechanism of these cytokines, 37 IFN-stimulated genes (ISGs), which are highly inducible in hepatocytes, were tested for their ability to inhibit HBV replication upon overexpression in human hepatoma cells. One ISG candidate, indoleamine 2,3-dioxygenase (IDO), an IFN-γ-induced enzyme catalyzing tryptophan degradation, efficiently reduced the level of intracellular HBV DNA without altering the steady-state level of viral RNA. Furthermore, expression of an enzymatically inactive IDO mutant did not inhibit HBV replication, and tryptophan supplementation in culture completely restored HBV replication in IDO-expressing cells, indicating that the antiviral effect elicited by IDO is mediated by tryptophan deprivation. Interestingly, IDO-mediated tryptophan deprivation preferentially inhibited viral protein translation and genome replication but did not significantly alter global cellular protein synthesis. Finally, tryptophan supplementation was able to completely restore HBV replication in IFN-γ- but not IFN-α-treated cells, which strongly argues that IDO is the primary mediator of IFN-γ-elicited antiviral response against HBV in human hepatocyte-derived cells.

Indoleamine 2,3-dioxygenase in human hematopoietic stem cell transplantation

In recent years tryptophan metabolism and its rate limiting enzyme indoleamine 2,3-dioxygenase (IDO) have attracted increasing attention for their potential to modulate immune responses including the regulation of transplantation tolerance. The focus of this review is to discuss some features of IDO activity which particularly relate to hematopoietic stem cell transplantation (HSCT). HSCT invariably involves the establishment of some degree of a donor-derived immune system in the recipient. Thus, the outstanding feature of tolerance in HSCT is that in this type of transplantation it is not rejection, which causes the most severe problems to HSCT recipients, but the reverse, graft-versus-host (GvH) directed immune responses. We will discuss the peculiar role of IDO activity and accelerated tryptophan metabolism at the interface between immune activation and immune suppression and delineate from theoretical and experimental evidence the potential significance of IDO in mediating tolerance in HSCT. Finally, we will examine therapeutic options for exploitation of IDO activity in the generation of allo-antigen-specific tolerance, i.e. avoiding allo-reactivity while maintaining immunocompetence, in HSCT.

How tolerogenic dendritic cells induce regulatory T cells

Since their discovery by Steinman and Cohn in 1973, dendritic cells (DCs) have become increasingly recognized for their crucial role as regulators of innate and adaptive immunity. DCs are exquisitely adept at acquiring, processing, and presenting antigens to T cells. They also adjust the context (and hence the outcome) of antigen presentation in response to a plethora of environmental inputs that signal the occurrence of pathogens or tissue damage. Such signals generally boost DC maturation, which promotes their migration from peripheral tissues into and within secondary lymphoid organs and their capacity to induce and regulate effector T cell responses. Conversely, more recent observations indicate that DCs are also crucial to ensure immunological peace. Indeed, DCs constantly present innocuous self- and nonself-antigens in a fashion that promotes tolerance, at least in part, through the control of regulatory T cells (Tregs). Tregs are specialized T cells that exert their immunosuppressive function through a variety of mechanisms affecting both DCs and effector cells. Here, we review recent advances in our understanding of the relationship between tolerogenic DCs and Tregs.