Normal development and function of dendritic cells in mice lacking IDO-1 expression

IDO1 scavenges reactive oxygen species in myeloid-derived suppressor cells to prevent graft-versus-host disease

This study reveals that the tryptophan-degrading reaction catalyzed by indoleamine 2,3-dioxygenase 1 (IDO1) is linked to reactive oxygen species (ROS) scavenging in Gr-1⁺CD11b⁺ myeloid cells. The IDO1-mediated ROS scavenging promotes myeloid-derived suppressor cell characteristics in Gr-1⁺CD11b⁺ cells, suppressing their differentiation into proinflammatory neutrophils. These results could explain the increased lethality in graft-versus-host disease as well as the enhanced proinflammatory and reduced regulatory T cell responses after transplantation of IDO1-deficient bone marrow cells. Our findings provide a mechanistic insight into the immune-modulatory roles of IDO1.

Tryptophan-catabolizing enzyme indoleamine 2,3-dioxygenase 1 (IDO1) also has an immunological function to suppress T cell activation in inflammatory circumstances, including graft-versus-host disease (GVHD), a fatal complication after allogeneic bone marrow transplantation (allo-BMT). Although the mononuclear cell expression of IDO1 has been associated with improved outcomes in GVHD, the underlying mechanisms remain unclear. Herein, we used IDO-deficient (Ido1^−/−) BMT to understand why myeloid IDO limits the severity of GVHD. Hosts with Ido1^−/− BM exhibited increased lethality, with enhanced proinflammatory and reduced regulatory T cell responses compared with wild type (WT) allo-BMT controls. Despite the comparable expression of the myeloid-derived suppressor cell (MDSC) mediators, arginase-1, inducible nitric oxide synthase, and interleukin 10, Ido1^−/− Gr-1⁺CD11b⁺ cells from allo-BMT or in vitro BM culture showed compromised immune-suppressive functions and were skewed toward the Ly6C^lowLy6G^hi subset, compared with the WT counterparts. Importantly, Ido1^−/−Gr-1⁺CD11b⁺ cells exhibited elevated levels of reactive oxygen species (ROS) and neutrophil numbers. These characteristics were rescued by human IDO1 with intact heme-binding and catalytic activities and were recapitulated by the treatment of WT cells with the IDO1 inhibitor L1-methyl tryptophan. ROS scavenging by N-acetylcysteine reverted the Ido1^−/−Gr-1⁺CD11b⁺ composition and function to an MDSC state, as well as improved the survival of GVHD hosts with Ido1^−/− BM. In summary, myeloid-derived IDO1 enhances GVHD survival by regulating ROS levels and limiting the ability of Gr-1⁺CD11b⁺ MDSCs to differentiate into proinflammatory neutrophils. Our findings provide a mechanistic insight into the immune-regulatory roles of the metabolic enzyme IDO1.

MiR-669b-3p regulates CD4+ T cell function by down-regulating indoleamine-2, 3-dioxygenase

OBJECTIVE

Acute rejection is a major cause of morbidity and mortality after solid organ transplantation. Therefore, optimizing treatment strategies and improving curative effect is urgent and necessary. Reliable biomarkers for acute rejection and the underlying molecular mechanisms remain to be determined.

METHODS

In this study, we established a mouse-to-mouse cardiac transplantation model and identified miR-669b-3p as a potential biomarker of acute rejection using a microRNA polymerase chain reaction (PCR)-based chip assay.

RESULTS

Further analyses showed that miR-669b-3p negatively regulated indoleamine-2,3-dioxygenase (IDO), a rate-limiting enzyme of tryptophan catabolism inhibiting T cell function. Using mixed lymphocyte reaction assay, we showed that miR-669b-3p increased proliferation stimulation index and inhibited apoptosis in CD4⁺ T cells. Moreover, miR-669b-3p regulated the expression of inflammatory cytokines such as tumor necrosis factor alpha (TNF-α) and Interleukin 10 (IL-10) and contributed to cytokine shift towards a Th2-dominant response.

CONCLUSION

Our results advance the current understanding of the immune regulatory function of miRNA and shed light on the role of miR-669b-3p in CD4⁺ T cells, suggesting that miR-669b-3p is a potential target for acute allograft rejection.

Prognostic value of tumour-infiltrating CD8+ lymphocytes in rectal cancer after neoadjuvant chemoradiation: is indoleamine-2,3-dioxygenase (IDO1) a friend or foe?

The prognostic value of the local immune phenotype in patients with colorectal cancer has been extensively studied. Neoadjuvant radiotherapy and/or chemotherapy may potentially influence these immune responses. In this study, we examined the prognostic role of indoleamine-2,3-Dioxygenase (IDO1) and infiltrating cytotoxic T lymphocytes (CD8+) in locally advanced rectal carcinomas after neoadjuvant treatment. Expression of IDO1 and CD8 was evaluated by immunohistochemistry in 106 archival tumour tissue samples from patients following neoadjuvant chemoradiation and radical resection. The average infiltration of IDO1+ and CD8+ cells was calculated along the tumour invasive front, in the tumour centre and within the neoplastic cells and expressed as total scores. Of the tumour specimens evaluable for immunohistochemistry, 100% showed CD8+ lymphocyte infiltration and 93.4% stained positive for IDO1. Total IDO1 score positively correlated with total CD8 score for all three subsites (p = 0.002, Kendall-tau-b 0.357). A high total CD8 score was positively correlated with lower ypUICC-stages (p = 0.047) and lower ypT-categories (p = 0.032). Total IDO1 expression showed a clear trend towards a lower risk of recurrence (p = 0.078). A high total IDO1 score was an independent prognostic marker for prolonged disease-free survival (HR 0.38, p = 0.046) and a high total CD8 score for favourable overall survival (HR 0.16, p = 0.029). Analysis of the local CD8 and IDO1 expression profile may be a helpful tool in predicting prognosis for patients with locally advanced rectal cancer following neoadjuvant chemoradiation.

Sodium valproate modulates immune response by alternative activation of monocyte-derived macrophages in systemic lupus erythematosus

The anti-inflammatory role of macrophages in apoptotic cells (ACs) clearance is involved in Systemic Lupus Erythematosus (SLE) pathogenesis. The efferocytic capability of macrophages is altered by M1/M2 polarization. Histone deacetylase inhibitors (HDACi) are proposed to enhance the expansion of M2 macrophages. Sodium valproate (VPA) is an HDACi with different anti-inflammatory properties. Here, we aimed to investigate the effects of HDACi by VPA on the polarization of monocyte-derived macrophages (MDMs) and regulating the expression of anti-inflammatory cytokines in SLE. We studied the ex vivo alterations of MDMs among 15 newly diagnosed SLE patients and 10 normal subjects followed by ACs and VPA treatments. M1/M2 polarization was assessed by expression of CD86/CD163, IL1-β, IDO-1, and MRC-1 among treated and non-treated MDMs. We also evaluated the production of IL-10, IL-12, TGF-β1, and TNF-α cytokines in the cell culture supernatants. CD163 was overexpressed upon VPA treatment, while CD86 showed no significant change. IL1-β and IDO-1 genes were significantly downregulated, and the mRNA expression of MRC-1 was increased among VPA-treated MDMs of SLE patients. The anti-inflammatory cytokines (IL-10 and TGF-β1) were overproduced while TNF-α level was decreased in response to VPA. The population of classically activated macrophages was more prevalent among SLE patients and efferocytosis was defected. VPA could successfully enhance the anti-inflammatory immune response through alternative activation of MDMs in SLE patients.

Dendritic cells transfected with indoleamine 2,3-dioxygenase gene suppressed acute rejection of cardiac allograft

BACKGROUND

Immunomodulation by indoleamine 2,3-dioxygenase (IDO) has been documented in many studies yet its underlying mechanisms remain undefined, especially in solid organ transplantation. Recent research demonstrated that the active expression of IDO in dendritic cells (DCs) regulates immune reaction. This study assessed whether DCs transfected with IDO gene inhibit T cells responses and suppress cardiac allograft rejection.

METHODS

Adenovirus vector containing IDO gene was transfected into DCs to obtain IDO-positive DCs (IDO(+) DCs). To evaluate the effect of IDO(+) DCs on T cells in vitro, CD4(+) T cell proliferation and apoptosis was assessed in mixed lymphocyte reactions and measured by flow cytometry, respectively. IDO(+) DCs from C57BL/6 mice were injected into BALB/c recipients before heterotopic cardiac transplantation.

RESULTS

Supernatant fluids from cultures of IDO(+) DCs had decreased tryptophan and increased kynurenine levels, reflecting IDO activity. IDO(+) DCs suppressed CD4(+) T cell responses in vitro, as reflected by decreased proliferation and increased apoptosis. In the transplant model, IDO(+) DCs prolonged survival and alleviated rejection of cardiac allograft in recipients injected with IDO(+) DCs. In vivo, IDO(+) DCs also significantly impaired CD4(+) T cell responses promoting increased apoptosis and a Th2-dominant cytokine shift.

CONCLUSIONS

IDO overexpression in DCs suppressed T cells alloresponses in vitro, and IDO(+) DCs attenuated acute allograft rejection in vivo. Regulation of tryptophan catabolism by means of IDO overexpression in DCs may be a useful approach in cardiac transplantation and immunological tolerance.

Role of indoleamine 2,3-dioxygenase in health and disease

IDO1 (indoleamine 2,3-dioxygenase 1) is a member of a unique class of mammalian haem dioxygenases that catalyse the oxidative catabolism of the least-abundant essential amino acid, L-Trp (L-tryptophan), along the kynurenine pathway. Significant increases in knowledge have been recently gained with respect to understanding the fundamental biochemistry of IDO1 including its catalytic reaction mechanism, the scope of enzyme reactions it catalyses, the biochemical mechanisms controlling IDO1 expression and enzyme activity, and the discovery of enzyme inhibitors. Major advances in understanding the roles of IDO1 in physiology and disease have also been realised. IDO1 is recognised as a prominent immune regulatory enzyme capable of modulating immune cell activation status and phenotype via several molecular mechanisms including enzyme-dependent deprivation of L-Trp and its conversion into the aryl hydrocarbon receptor ligand kynurenine and other bioactive kynurenine pathway metabolites, or non-enzymatic cell signalling actions involving tyrosine phosphorylation of IDO1. Through these different modes of biochemical signalling, IDO1 regulates certain physiological functions (e.g. pregnancy) and modulates the pathogenesis and severity of diverse conditions including chronic inflammation, infectious disease, allergic and autoimmune disorders, transplantation, neuropathology and cancer. In the present review, we detail the current understanding of IDO1’s catalytic actions and the biochemical mechanisms regulating IDO1 expression and activity. We also discuss the biological functions of IDO1 with a focus on the enzyme's immune-modulatory function, its medical implications in diverse pathological settings and its utility as a therapeutic target.

Tryptophan-degrading enzymes in tumoral immune resistance

Tryptophan is required for T lymphocyte effector functions. Its degradation is one of the mechanisms selected by tumors to resist immune destruction. Two enzymes, tryptophan-2,3-dioxygenase and indoleamine 2,3-dioxygenase 1, control tryptophan degradation through the kynurenine pathway. A third protein, indoleamine 2,3-dioxygenase 2, was identified more recently. All three enzymes were reported to be expressed in tumors, and are candidate targets for pharmacological inhibition aimed at restoring effective anti-tumoral immunity. In this review, we compare these three enzymes in terms of structure, activity, regulation, and expression in healthy and cancerous tissues, in order to appreciate their relevance to tumoral immune resistance.

Extensive profiling of the expression of the indoleamine 2,3-dioxygenase 1 protein in normal and tumoral human tissues

Tryptophan catabolism by indoleamine 2,3-dioxygenase 1 (IDO1) plays a key role in tumoral resistance to immune rejection. In humans, constitutive expression of IDO1 has been observed in several tumor types. However, a comprehensive analysis of its expression in normal and tumor tissues is still required to anticipate the risks and potential benefits of IDO1 inhibitors. Using a newly validated monoclonal antibody to human IDO1, we performed an extensive immunohistochemical analysis of IDO1 expression in normal and tumor tissues. In normal tissues, IDO1 was expressed by endothelial cells in the placenta and lung and by epithelial cells in the female genital tract. In lymphoid tissues, IDO1 was expressed in mature dendritic cells with a phenotype (CD83(+), DC-LAMP(+), langerin(-), CD123(-), CD163(-)) distinct from plasmacytoid dendritic cells. Importantly, IDO1-expressing dendritic cells were not enriched in tumor-draining lymph nodes, in contrast with previously reported findings. IDO1-expressing cells were observed in a large fraction (505/866, 58%) of human tumors. They comprised tumor cells, endothelial cells, and stromal cells in proportions that varied depending on the tumor type. Tumors showing the highest proportions of IDO1-immunolabeled samples were carcinomas of the endometrium and cervix, followed by kidney, lung, and colon. This hierarchy of IDO1 expression was confirmed by gene expression data mined from The Cancer Genome Atlas database. Expression of IDO1 may be used to select tumors likely to benefit from targeted therapy with IDO1 inhibitors.

Key role of group v secreted phospholipase A2 in Th2 cytokine and dendritic cell-driven airway hyperresponsiveness and remodeling

Background

Previous work has shown that disruption of the gene for group X secreted phospholipase A₂ (sPLA₂-X) markedly diminishes airway hyperresponsiveness and remodeling in a mouse asthma model. With the large number of additional sPLA₂s in the mammalian genome, the involvement of other sPLA₂s in the asthma model is possible – in particular, the group V sPLA₂ (sPLA₂-V) that like sPLA₂-X is highly active at hydrolyzing membranes of mammalian cells.

Methodology and Principal Findings

The allergen-driven asthma phenotype was significantly reduced in sPLA₂-V-deficient mice but to a lesser extent than observed previously in sPLA₂-X-deficient mice. The most striking difference observed between the sPLA₂-V and sPLA₂-X knockouts was the significant impairment of the primary immune response to the allergen ovalbumin (OVA) in the sPLA₂-V^−/− mice. The impairment in eicosanoid generation and dendritic cell activation in sPLA2-V^−/− mice diminishes Th2 cytokine responses in the airways.

Conclusions

This paper illustrates the diverse roles of sPLA₂s in the immunopathogenesis of the asthma phenotype and directs attention to developing specific inhibitors of sPLA₂-V as a potential new therapy to treat asthma and other allergic disorders.

Induction of indoleamine 2,3-dioxygenase expression via heme oxygenase-1-dependant pathway during murine dendritic cell maturation

Heme oxygenase (HO)-1 is expressed in a variety of conditions involved in the regulation of immune responses. In this study, we examined the role of HO-1 in dendritic cell (DC) maturation and expression of indoleamine 2,3-dioxygenase (IDO), a key enzyme that catalyzes the initial, rate-limiting step in tryptophan degradation. IDO deficiency led to diminished phenotypic and functional maturation of DCs in vitro and in vivo. IDO expression and DC maturation was abrogated by the HO inhibitor zinc protoporphrin, but increased by hemin, a potent inducer of HO-1. Moreover, LPS-induced HO-1 expression was mediated by an NF-kappaB-dependent pathway. Our findings provide additional insight into the immunological functions of IDO and HO-1, and suggest possible therapeutic adjuvants for the treatment of DC-related acute and chronic diseases.

Role of IDO in organ transplantation: promises and difficulties

Induction of donor-antigen-specific immunological tolerance still remains the "holy grail" in organ transplantation. Recently, Indoleamine-2,3 Dioxygenase (IDO)--a tryptophan degrading enzyme--has been shown to be implicated in one of nature's most impressive examples of tolerance, which is maternal acceptance of the semi-allogeneic foetus. Although many experimental findings propose IDO as a key player in induction and maintenance of peripheral tolerance, scepticism exists as to whether IDO represents a promising therapeutic target with clinical relevance. In this review article we will discuss the role of IDO in transplantation and take a critical look at IDO-based therapeutic strategies.

Manipulation of indoleamine 2,3-dioxygenase (IDO) for clinical transplantation: promises and challenges

BACKGROUND

Since the discovery that indoleamine 2,3-dioxygenase (IDO) is a modulator for maintenance of fetomaternal immuno-privilege state, it has been implicated in tumour tolerance, autoimmune diseases and asthma. IDO is an IFN-gamma-inducible, intracellular enzyme that catalyzes the initial and rate-limiting step in the degradation of tryptophan. It has been suggested that IDO can regulate the immune system either through deprivation of tryptophan that is essential for T cell proliferation or via cytotoxic effects of kynurenine pathway metabolites on T cell survival.

METHODS

The sources of information used were obtained through Pubmed/Medline.

RESULTS/CONCLUSION

While IDO emerges as a regulator of immunity, its role in controlling allo-response is unfolding. IDO can control T cell responses to allo-antigens and induce generation of allo-specific regulatory T cells. Exploiting IDO as a modulator of transplant rejection, many groups have manipulated its activity to prolong allograft survival in transplantation models. Despite the initial promise, its application to clinical transplantation may be limited. We therefore examine the potentials and limitations associated with clinical translation of IDO into a therapeutic.