Inhibition of indoleamine 2,3-dioxygenase-mediated tryptophan catabolism accelerates collagen-induced arthritis in mice

Differences in IDO1+ dendritic cells and soluble CTLA-4 are associated with differential clinical responses to methotrexate treatment in rheumatoid arthritis

Objective

Antigen-presenting dendritic cells (DCs) and monocytes play an essential role in rheumatoid arthritis (RA) pathogenesis, however, their tolerogenic potential remains unclear. Herein, the tolerogenic profiles of DCs are characterized in treatment-naïve RA patients to determine their role to inflammatory arthritis management.

Methods

Thirty-six treatment-naïve RA patients were enrolled, of which 62% were non-responders to methotrexate (MTX) monotherapy based on disease activity score (DAS) after 6-months of therapy. DC and monocyte subset frequencies, activation (CD40, CD86, CD209 expression), and tolerogenic profile (intracellular indoleamine-2,3-dioxygenase [IDO1] and cytotoxic T lymphocyte antigen 4 [CTLA-4] expression) were examined in the baseline peripheral blood by multicolor flow-cytometry. Soluble CTLA-4 (sCTLA-4) levels in plasma were measured.

Results

DC subsets were decreased in RA compared to healthy controls (HC), and the frequency of conventional DCs (cDC) inversely correlated with inflammatory markers and improvement in disease activity. CD141+ cDC1s were the major IDO1-expressing cells. IDO1+cDC1s were reduced in RA patients compared to HC. The baseline frequency of IDO1+cDC1s inversely correlated with improvement in disease activity. CTLA-4 expression in CD1c+ cDC2s and monocytes was lower in RA patients compared to HC. Moreover, MTX-responders had a significantly lower frequency of IDO1+cDC1 cells and higher level of sCTLA-4 in the plasma compared to MTX non-responders. There was a strong predictive association of low IDO1+cDC1 cells, low sCTLA-4 and non-response to MTX.

Conclusions

Our findings reveal altered DC and monocytes immunophenotypes that are associated with RA pathology and treatment response. The frequencies of tolerogenic IDO1+cDC1s and the low level of sCTLA-4 are strongly associated with MTX non-responsiveness and therapeutic outcome. These results suggest that investigation of the association IDO1+cDC1 and sCTLA-4 with response to treatment may be more generalizable to other autoimmune diseases.

Maladaptive T-Cell Metabolic Fitness in Autoimmune Diseases

Immune surveillance and adaptive immune responses, involving continuously circulating and tissue-resident T-lymphocytes, provide host defense against infectious agents and possible malignant transformation while avoiding autoimmune tissue damage. Activation, migration, and deployment of T-cells to affected tissue sites are crucial for mounting an adaptive immune response. An effective adaptive immune defense depends on the ability of T-cells to dynamically reprogram their metabolic requirements in response to environmental cues. Inability of the T-cells to adapt to specific metabolic demands may skew cells to become either hyporesponsive (creating immunocompromised conditions) or hyperactive (causing autoimmune tissue destruction). Here, we review maladaptive T-cell metabolic fitness that can cause autoimmune diseases and discuss how T-cell metabolic programs can potentially be modulated to achieve therapeutic benefits.

Hypomethylation-induced regulatory programs in T cells unveiled by transcriptomic analyses

Regulatory T cells (Tregs) are essential mediators of tolerance mitigating aberrant immune responses. While naturally occurring Treg (nTreg) development and function are directed by epigenetic events, induced Treg (iTreg) identity and mechanisms of action remain elusive. Mirroring the epigenetic circuits of nTregs, we and others have used hypomethylation agents (HAs) to ex vivo convert T cells into iTregs (HA-iTregs) and further showed that the suppressive properties of the HA-iTregs are predominantly confined in an emergent population, which de novo expresses the immunomodulatory molecule HLA-G, consequently providing a surface marker for isolation of the suppressive HA-iTreg compartment (G+ cells). We isolated the HA-induced G+ cells and their G- counterparts and employed high-throughput RNA-sequencing (RNA-seq) analyses to uncover the G+-specific transcriptomic changes guiding T cells toward a regulatory trajectory upon their exposure to HA. We found a distinct transcriptional upregulation of G+ cells accompanied by enrichment of immune-response-related pathways. Although single-cell RNA-seq profiling revealed regulatory G+ cells to have molecular features akin to nTregs, when assessed in conjunction with the comparative transcriptomic analysis and profiling of secreted cytokines against the non-suppressive G- cells, FOXP3 and other T-helper signatures appear to play a minor role in their suppressive phenotype. We found an ectopic expression of IDO-1 and CCL17/22 in G+ cells, denoting that in vitro exposure of T cells to HA may well unlock myeloid suppressor genes. This report provides transcriptional data shaping the molecular identity of a highly purified and potent HA-iTreg population and hints toward ectopic myeloid-specific molecular mechanisms mediating HA-iTreg function.

Intra-articular delivery of an indoleamine 2,3-dioxygenase galectin-3 fusion protein for osteoarthritis treatment in male Lewis rats

OBJECTIVE

Osteoarthritis (OA) is driven by low-grade inflammation, and controlling local inflammation may offer symptomatic relief. Here, we developed an indoleamine 2,3-dioxygenase and galectin-3 fusion protein (IDO-Gal3), where IDO increases the production of local anti-inflammatory metabolites and Gal3 binds carbohydrates to extend IDO's joint residence time. In this study, we evaluated IDO-Gal3's ability to alter OA-associated inflammation and pain-related behaviors in a rat model of established knee OA.

METHODS

Joint residence was first evaluated with an analog Gal3 fusion protein (NanoLuc™ and Gal3, NL-Gal3) that produces luminescence from furimazine. OA was induced in male Lewis rats via a medial collateral ligament and medial meniscus transection (MCLT + MMT). At 8 weeks, NL or NL-Gal3 were injected intra-articularly (n = 8 per group), and bioluminescence was tracked for 4 weeks. Next, IDO-Gal3s's ability to modulate OA pain and inflammation was assessed. Again, OA was induced via MCLT + MMT in male Lewis rats, with IDO-Gal3 or saline injected into OA-affected knees at 8 weeks post-surgery (n = 7 per group). Gait and tactile sensitivity were then assessed weekly. At 12 weeks, intra-articular levels of IL6, CCL2, and CTXII were assessed.

RESULTS

The Gal3 fusion increased joint residence in OA and contralateral knees (p < 0.0001). In OA-affected animals, both saline and IDO-Gal3 improved tactile sensitivity (p = 0.008), but IDO-Gal3 also increased walking velocities (p ≤ 0.033) and improved vertical ground reaction forces (p ≤ 0.04). Finally, IDO-Gal3 decreased intra-articular IL6 levels within the OA-affected joint (p = 0.0025).

CONCLUSION

Intra-articular IDO-Gal3 delivery provided long-term modulation of joint inflammation and pain-related behaviors in rats with established OA.

The impact of exosomes derived from distinct sources on rheumatoid arthritis

Rheumatoid arthritis (RA) is an autoimmune disease that can induce joint deformities and functional impairment, significantly impacting the overall well-being of individuals. Exosomes, which are cellularly secreted vesicles, possess favorable biological traits such as biocompatibility, stability, and minimal toxicity. Additionally, they contain nucleic acids, lipids, proteins, amino acids, and metabolites, serving as mediators in cellular communication and information exchange. Recent studies have demonstrated the association between exosomes and the pathogenesis of RA. Exosomes derived from mesenchymal stem cells, dendritic cells, and neutrophils exert influence on the biological functions of immune cells and joint cells, however, the precise mechanism remains largely unclarified. This comprehensive review systematically analyzes and summarizes the biological characteristics and functionalities of exosomes derived from diverse cellular sources, thus establishing a scientific foundation for the utilization of exosomes as diagnostic targets and therapeutic modalities in the context of RA.

Modulation of T cells by tryptophan metabolites in the kynurenine pathway

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

Intra-Articular Delivery of an Indoleamine 2,3-Dioxygenase Galectin-3 Fusion Protein for Osteoarthritis Treatment in Male Lewis Rats

Objective : Controlling joint inflammation can improve osteoarthritis (OA) symptoms; however, current treatments often fail to provide long-term effects. We have developed an indoleamine 2,3-dioxygenase and galectin-3 fusion protein (IDO-Gal3). IDO converts tryptophan to kynurenines, directing the local environment toward an anti-inflammatory state; Gal3 binds carbohydrates and extends IDO's joint residence time. In this study, we evaluated IDO-Gal3's ability to alter OA-associated inflammation and pain-related behaviors in a rat model of established knee OA. Methods : Joint residence was first evaluated with an analog Gal3 fusion protein (NanoLuc™ and Gal3, NL-Gal3) that produces luminescence from furimazine. OA was induced in male Lewis rats via a medial collateral ligament and medial meniscus transection (MCLT+MMT). At 8 weeks, NL or NL-Gal3 were injected intra-articularly (n=8 per group), and bioluminescence was tracked for 4 weeks. Next, IDO-Gal3's ability to modulate OA pain and inflammation was assessed. Again, OA was induced via MCLT+MMT in male Lewis rats, with IDO-Gal3 or saline injected into OA-affected knees at 8 weeks post-surgery (n=7 per group). Gait and tactile sensitivity were then assessed weekly. At 12 weeks, intra-articular levels of IL6, CCL2, and CTXII were assessed. Results : The Gal3 fusion increased joint residence in OA and contralateral knees (p<0.0001). In OA-affected animals, IDO-Gal3 improved tactile sensitivity (p=0.002), increased walking velocities (p≤0.033), and improved vertical ground reaction forces (p≤0.04). Finally, IDO-Gal3 decreased intra-articular IL6 levels within the OA-affected joint (p=0.0025). Conclusion : Intra-articular IDO-Gal3 delivery provided long-term modulation of joint inflammation and pain-related behaviors in rats with established OA.

Elevated Kynurenine Levels in Patients with Primary Sjögren's Syndrome

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Effectiveness of Soluble CTLA-4-Fc in the Inhibition of Bone Marrow T-Cell Activation in Context of Indoleamine 2.3-Dioxygenase (IDO) and CD4+Foxp3+ Treg Induction

Background

Rheumatoid arthritis (RA) is a chronic autoimmune disease with systemic inflammation finally resulting in damaged joints. One of the RA development models suggests bone marrow (BM) as a place of inflammation development further leading to disease progression. We aimed to investigate the potential of CTLA-4-Fc molecule in inducing tolerogenic milieu in BM measured as indoleamine 2,3-dioxygenase (IDO) expression, CD4⁺Foxp3⁺ Treg induction, and T cell activation control. The expression of IDO-pathway genes was also examined in monocytes to estimate the tolerogenic potential in the periphery.

Methods

Bone marrow mononuclear cells (BMMC) were stimulated by pro-inflammatory cytokines and CTLA-4-Fc. Next IDO expression, CD4⁺CD69⁺ and CD4⁺Foxp3⁺ percentage were estimated by PCR and FACS staining, respectively. Enzymatic activity of IDO was confirmed by HPLC in BM plasma and blood plasma. Genes expressed in IDO-pathway were analyzed by NGS in peripheral monocytes isolated from RA patients and healthy controls.

Results

We found that CTLA-4-Fc and IFN-γ stimulation results in IDO production by BMMC. CTLA-4-Fc induced tryptophan catabolism can inhibit mitogen-induced CD4⁺ T cells activation without influencing CD8⁺ cells, but did not control CD25 nor Foxp3 expression in BM cells. Significantly higher expression of selected IDO-pathway genes was detected on peripheral monocytes isolated from RA as compared to healthy controls.

Conclusion

This study sheds light on some immunosuppression aspects present or induced in BM. The potential of IDO-mediated pathways were confirmed in the periphery, what may represent the promising candidates for therapeutic strategies in RA.

Impact of IDO1 and IDO2 on the B Cell Immune Response

Indoleamine-2,3-dioxygenase (IDO)1 and IDO2 are closely related tryptophan catabolizing enzymes that have immunomodulatory properties. Although initially studied as modifiers of T cell activity, emerging evidence suggests IDO1 and IDO2 also have important roles as modulators of B cell function. In this context, IDO1 and IDO2 appear to play opposite roles, with IDO1 inhibiting and IDO2 driving inflammatory B cell responses. In this mini review, we discuss the evidence for IDO1 and IDO2 modulation of B cell function, focusing on the effect of these enzymes on autoimmunity, allergic responses, protective immunity, and response to pathogens. We summarize strategies to target IDO1 and/or IDO2 as potential therapeutics for inflammatory autoimmune disease and highlight outstanding questions and areas that require future study.

A Review of Metabolomic Profiling in Rheumatoid Arthritis: Bringing New Insights in Disease Pathogenesis, Treatment and Comorbidities

Metabolomic analysis provides a wealth of information that can be predictive of distinctive phenotypes of pathogenic processes and has been applied to better understand disease development. Rheumatoid arthritis (RA) is an autoimmune disease with the establishment of chronic synovial inflammation that affects joints and peripheral tissues such as skeletal muscle and bone. There is a lack of useful disease biomarkers to track disease activity, drug response and follow-up in RA. In this review, we describe potential metabolic biomarkers that might be helpful in the study of RA pathogenesis, drug response and risk of comorbidities. TMAO (choline and trimethylamine oxide) and TCA (tricarboxylic acid) cycle products have been suggested to modulate metabolic profiles during the early stages of RA and are present systemically, which is a relevant characteristic for biomarkers. Moreover, the analysis of lipids such as cholesterol, FFAs and PUFAs may provide important information before disease onset to predict disease activity and treatment response. Regarding therapeutics, TNF inhibitors may increase the levels of tryptophan, valine, lysine, creatinine and alanine, whereas JAK/STAT inhibitors may modulate exclusively fatty acids. These observations indicate that different disease modifying antirheumatic drugs have specific metabolic profiles and can reveal differences between responders and non-responders. In terms of comorbidities, physical impairment represented by higher fatigue scores and muscle wasting has been associated with an increase in urea cycle, FFAs, tocopherols and BCAAs. In conclusion, synovial fluid, blood and urine samples from RA patients seem to provide critical information about the metabolic profile related to drug response, disease activity and comorbidities.

Targeting immune checkpoints in gynecologic cancer: updates & perspectives for pathologists

Checkpoint inhibitor-based immunotherapy is increasingly used in the treatment of gynecologic cancers, and most often targets the PD-1/PD-L1 axis. Pathologists should be familiar with the biomarkers required to determine candidacy for these treatments based on existing FDA approvals, including mismatch repair protein immunohistochemistry, microsatellite instability testing, tumor mutation burden testing, and PD-L1 immunohistochemistry. This review summarizes the rationale behind these treatments and their associated biomarkers and delivers guidance on how to utilize and readout these tests. It also introduces additional biomarkers which may provide information regarding immunotherapeutic vulnerability in the future such as neoantigen load; POLE mutation status; and immunohistochemical expression of immunosuppressive checkpoints like LAG-3, TIM-3, TIGIT, and VISTA; immune-activating checkpoints such as CD27, CD40, CD134, and CD137; enzymes such as IDO-1 and adenosine-related compounds; and MHC class I.

The Immunomodulatory Enzyme IDO2 Mediates Autoimmune Arthritis through a Nonenzymatic Mechanism

IDO2 is one of two closely related tryptophan catabolizing enzymes induced under inflammatory conditions. In contrast to the immunoregulatory role defined for IDO1 in cancer models, IDO2 has a proinflammatory function in models of autoimmunity and contact hypersensitivity. In humans, two common single-nucleotide polymorphisms have been identified that severely impair IDO2 enzymatic function, such that <25% of individuals express IDO2 with full catalytic potential. This, together with IDO2's relatively weak enzymatic activity, suggests that IDO2 may have a role outside of its function in tryptophan catabolism. To determine whether the enzymatic activity of IDO2 is required for its proinflammatory function, we used newly generated catalytically inactive IDO2 knock-in mice together with established models of contact hypersensitivity and autoimmune arthritis. Contact hypersensitivity was attenuated in catalytically inactive IDO2 knock-in mice. In contrast, induction of autoimmune arthritis was unaffected by the absence of IDO2 enzymatic activity. In pursuing this nonenzymatic IDO2 function, we identified GAPDH, Runx1, RANbp10, and Mgea5 as IDO2-binding proteins that do not interact with IDO1, implicating them as potential mediators of IDO2-specific function. Taken together, our findings identify a novel function for IDO2, independent of its tryptophan catabolizing activity, and suggest that this nonenzymatic function could involve multiple signaling pathways. These data show that the enzymatic activity of IDO2 is required only for some inflammatory immune responses and provide, to our knowledge, the first evidence of a nonenzymatic role for IDO2 in mediating autoimmune disease.

Indoleamine 2, 3 Dioxygenase 1 Impairs Chondrogenic Differentiation of Mesenchymal Stem Cells in the Joint of Osteoarthritis Mice Model

Osteoarthritis (OA) is a serious joint inflammation that leads to cartilage degeneration and joint dysfunction. Mesenchymal stem cells (MSCs) are used as a cell-based therapy that showed promising results in promoting cartilage repair. However, recent studies and clinical trials explored unsatisfied outcomes because of slow chondrogenic differentiation and increased calcification without clear reasons. Here, we report that the overexpression of indoleamine 2,3 dioxygenase 1 (IDO1) in the synovial fluid of OA patients impairs chondrogenic differentiation of MSCs in the joint of the OA mice model. The effect of MSCs mixed with IDO1 inhibitor on the cartilage regeneration was tested compared to MSCs mixed with IDO1 in the OA animal model. Further, the mechanism exploring the effect of IDO1 on chondrogenic differentiation was investigated. Subsequently, miRNA transcriptome sequencing was performed for MSCs cocultured with IDO1, and then TargetScan was used to verify the target of miR-122-5p in the SF-MSCs. Interestingly, we found that MSCs mixed with IDO1 inhibitor showed a significant performance to promote cartilage regeneration in the OA animal model, while MSCs mixed with IDO1 failed to stimulate cartilage regeneration. Importantly, the overexpression of IDO1 showed significant inhibition to Sox9 and Collagen type II (COL2A1) through activating the expression of β-catenin, since inhibiting of IDO1 significantly promoted chondrogenic signaling of MSCs (Sox9, COL2A1, Aggrecan). Further, miRNA transcriptome sequencing of SF-MSCs that treated with IDO1 showed significant downregulation of miR-122-5p which perfectly targets Wnt1. The expression of Wnt1 was noticed high when IDO1 was overexpressed. In summary, our results suggest that IDO1 overexpression in the synovial fluid of OA patients impairs chondrogenic differentiation of MSCs and cartilage regeneration through downregulation of miR-122-5p that activates the Wnt1/β-catenin pathway.

Metabolites as drivers and targets in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a chronic autoimmune disease characterized by neovascularization, immune cell infiltration, and synovial hyperplasia, which leads to degradation of articular cartilage and bone, and subsequent functional disability. Dysregulated angiogenesis, synovial hypoxia, and immune cell infiltration result in a ‘bioenergetic crisis’ in the inflamed joint which further exacerbates synovial invasiveness. Several studies have examined this vicious cycle between metabolism, immunity, and inflammation and the role metabolites play in these interactions. To add to this complexity, the inflamed synovium is a multicellular tissue with many cellular subsets having different metabolic requirements. Metabolites can shape the inflammatory phenotype of immune cell subsets during disease and act as central signalling hubs. In the RA joint, the increased energy demand of stromal and immune cells leads to the accumulation of metabolites such as lactate, citrate, and succinate as well as adipocytokines which can regulate downstream signalling pathways. Transcription factors such as HIF1ɑ and mTOR can act as metabolic sensors to activate synovial cells and drive pro-inflammatory effector function, thus perpetuating chronic inflammation further. These metabolic intermediates may be potential therapeutic targets and so understanding the complex interplay between metabolites and synovial cells in RA may allow for identification of novel therapeutic strategies but also may provide significant insight into the underlying mechanisms of disease pathogenesis.

Enhancing extracellular vesicles for therapeutic treatment of arthritic joints

Extracellular vesicles are small membrane-derived packages of information that are released from virtually all cell types. These nano-packages contain regulatory material including proteins, lipids, mRNA and microRNA and are a key mechanism of paracellular communication within a given microenvironment. Encompassed with a lipid bilayer, these organelles have been attributed numerous roles in regulating both physiological and pathological functions. Herein, we describe the role of EVs in the context of Rheumatoid and Osteoarthritis and explore how they could be harnessed to treat inflammatory and degenerative joint conditions. These structures offer a promising therapeutic strategy for treating musculoskeletal diseases due to their bioactive content, stability, small size and intrinsic ability to enter the avascular cartilage, a notoriously challenging tissue to target. We also discuss how EVs can be manipulated to load therapeutic cargo or present additional targeting moieties to enhance their beneficial actions and tissue regenerative properties.

Development of a novel formulation method to prepare liposomal Epacadostat

BACKGROUND

One of the important metabolic pathways in cancer progression is tryptophan catabolism by the indoleamin-2,3-dioxygenase (IDO) enzyme, which suppresses the immune system and induces tolerance. Inhibition of IDO1 is an important therapeutic goal for immunotherapy in many cancers such as metastatic melanoma. Epacadostat (EPA) is a very strong inhibitor of IDO1, and its clinical studies are being performed in a higher clinical phase than other inhibitors. In this study, we have developed a new liposomal EPA formulation to reduce the dose, side effects, and treatment costs.

METHODS

Liposomes containing EPA were formulated using a novel remote loading method. Their morphology, particle size, surface charge, total phospholipid content, and drug loading were evaluated. Validation method studies to assay of EPA were carried out according to ICHQ2B guidelines. For in-vivo study, B16F10 melanoma bearing C57BL/6 mice were treated with the free or liposomal forms of EPA, and then monitored for tumor size and survival rate.

RESULTS

A validated method for EPA determination in liposomal form using UV-visible spectrophotometry was developed which was a precise, accurate and robust method. The particle size, zeta potential, and encapsulation efficacy of liposomes was 128.1 ± 1.1 nm, -16.5 ± 1 mV, and 64.9 ± 3.5, respectively. The half maximal inhibitory concentration (IC50) of liposomal EPA was 64 ng/ml that was lower than free EPA (128 ng/ml). In-vivo results also showed that tumor growth was slower in mice receiving liposomal EPA than in the group receiving free EPA.

CONCLUSION

A new method was developed to load EPA into liposomes. Moreover, the use of the nanoliposomal EPA showed more efficacy than EPA in inhibiting the tumor growth in melanoma model. Therefore, it might be used in further clinical studies as a good candidate for immunotherapy alone or in combination with other treatments.

Evidence that a deviation in the kynurenine pathway aggravates atherosclerotic disease in humans

BACKGROUND

The metabolism of tryptophan (Trp) along the kynurenine pathway has been shown to carry strong immunoregulatory properties. Several experimental studies indicate that this pathway is a major regulator of vascular inflammation and influences atherogenesis. Knowledge of the role of this pathway in human atherosclerosis remains incomplete.

OBJECTIVES

In this study, we performed a multiplatform analysis of tissue samples, in vitro and in vivo functional assays to elucidate the potential role of the kynurenine pathway in human atherosclerosis.

METHODS AND RESULTS

Comparison of transcriptomic data from carotid plaques and control arteries revealed an upregulation of enzymes within the quinolinic branch of the kynurenine pathway in the disease state, whilst the branch leading to the formation of kynurenic acid (KynA) was downregulated. Further analyses indicated that local inflammatory responses are closely tied to the deviation of the kynurenine pathway in the vascular wall. Analysis of cerebrovascular symptomatic and asymptomatic carotid stenosis data showed that the downregulation of KynA branch enzymes and reduced KynA production were associated with an increased probability of patients to undergo surgery due to an unstable disease. In vitro, we showed that KynA-mediated signalling through aryl hydrocarbon receptor (AhR) is a major regulator of human macrophage activation. Using a mouse model of peritoneal inflammation, we showed that KynA inhibits leukocyte recruitment.

CONCLUSIONS

We have found that a deviation in the kynurenine pathway is associated with an increased probability of developing symptomatic unstable atherosclerotic disease. Our study suggests that KynA-mediated signalling through AhR is an important mechanism involved in the regulation of vascular inflammation.

Metabolic Reprogramming of Fibroblasts as Therapeutic Target in Rheumatoid Arthritis and Cancer: Deciphering Key Mechanisms Using Computational Systems Biology Approaches

Fibroblasts, the most abundant cells in the connective tissue, are key modulators of the extracellular matrix (ECM) composition. These spindle-shaped cells are capable of synthesizing various extracellular matrix proteins and collagen. They also provide the structural framework (stroma) for tissues and play a pivotal role in the wound healing process. While they are maintainers of the ECM turnover and regulate several physiological processes, they can also undergo transformations responding to certain stimuli and display aggressive phenotypes that contribute to disease pathophysiology. In this review, we focus on the metabolic pathways of glucose and highlight metabolic reprogramming as a critical event that contributes to the transition of fibroblasts from quiescent to activated and aggressive cells. We also cover the emerging evidence that allows us to draw parallels between fibroblasts in autoimmune disorders and more specifically in rheumatoid arthritis and cancer. We link the metabolic changes of fibroblasts to the toxic environment created by the disease condition and discuss how targeting of metabolic reprogramming could be employed in the treatment of such diseases. Lastly, we discuss Systems Biology approaches, and more specifically, computational modeling, as a means to elucidate pathogenetic mechanisms and accelerate the identification of novel therapeutic targets.

Amino Acid Metabolism in Rheumatoid Arthritis: Friend or Foe?

In mammals, amino acid metabolism has evolved to act as a critical regulator of innate and adaptive immune responses. Rheumatoid arthritis (RA) is the most common form of inflammatory arthropathy sustained by autoimmune responses. We examine here the current knowledge of tryptophan and arginine metabolisms and the main immunoregulatory pathways in amino acid catabolism, in both RA patients and experimental models of arthritis. We found that l-tryptophan (Trp) metabolism and, in particular, the kynurenine pathway would exert protective effects in all experimental models and in some, but not all, RA patients, possibly due to single nucleotide polymorphisms in the gene coding for indoleamine 2,3-dioxygenase 1 (IDO1; the enzyme catalyzing the rate-limiting step of the kynurenine pathway). The function, i.e., either protective or pathogenetic, of the l-arginine (Arg) metabolism in RA was less clear. In fact, although immunoregulatory arginase 1 (ARG1) was highly induced at the synovial level in RA patients, its true functional role is still unknown, possibly because of few available preclinical data. Therefore, our analysis would indicate that amino acid metabolism represents a fruitful area of research for new drug targets for a more effective and safe therapy of RA and that further studies are demanding to pursue such an important objective.

Hypoxia decreases the T helper cell-suppressive capacity of synovial fibroblasts by downregulating IDO1-mediated tryptophan metabolism

OBJECTIVE

The development of RA is linked to local infiltration of immune cells and to changes in the phenotype of synovial fibroblasts. Synovial fibroblasts possess the capacity to suppress T cell responses through indoleamine 2, 3-dioxygenase 1 (IDO1)-mediated tryptophan metabolism. However, synovial fibroblasts from RA patients are restricted in this immune-modulatory function. Moreover, hypoxic conditions are detected within synovial tissues of RA patients, with oxygen tensions of only 3.2% O2. This study aims at investigating the effects of hypoxia on the interaction between T cells and synovial fibroblasts, particularly on the T cell-suppressive capacities of synovial fibroblasts.

METHODS

Synovial fibroblasts were cultured with Th cells under normoxic and hypoxic conditions (3% O2). Th cell proliferation was detected by flow cytometry. Tryptophan and kynurenine amounts were measured by HPLC. IDO1 expression and signal transducer and activator of transcription 1 (STAT1) phosphorylation were quantified by real-time PCR or western blot, and cytokine secretion by ELISA.

RESULTS

Hypoxic conditions strongly diminished the Th cell-suppressive capacities of both OA synovial fibroblasts and RA synovial fibroblasts. Accordingly, IDO1 mRNA and protein expression, STAT1 phosphorylation and tryptophan metabolism were greatly reduced in OA synovial fibroblasts by hypoxia. MMP-3, IL-6, IL-10 and IFNγ secretion were significantly decreased under hypoxia in synovial fibroblast-Th cell co-cultures, while IL-17A levels were elevated. Supplementation with IFNγ, a well-known inducer of IDO1 expression, could rescue neither IDO1 expression nor Th cell suppression under hypoxic conditions.

CONCLUSION

Hypoxia strongly affected the crosstalk between synovial fibroblasts and Th cells. By reducing the efficiency of synovial fibroblasts to restrict Th cell proliferation and by increasing the expression of IL-17A, hypoxia might have implications on the pathophysiology of RA.

B-Cell-Targeted 3DNA Nanotherapy Against Indoleamine 2,3-Dioxygenase 2 (IDO2) Ameliorates Autoimmune Arthritis in a Preclinical Model

The tryptophan catabolizing enzyme indoleamine 2,3-dioxygenase 2 (IDO2) has been identified as an immunomodulatory agent promoting autoimmunity in preclinical models. As such, finding ways to target the expression of IDO2 in B cells promises a new avenue for therapy for debilitating autoimmune disorders such as rheumatoid arthritis. IDO2, like many drivers of disease, is an intracellular protein expressed in a range of cells, and thus therapeutic inhibition of IDO2 requires a mechanism for targeting this intracellular protein in specific cell types. DNA nanostructures are a promising novel way of delivering small molecule drugs, antibodies, or siRNAs to the cytoplasm of a cell. These soluble, branched structures can carry cell-specific targeting moieties along with their therapeutic deliverable. Here, we examined a 3DNA nanocarrier specifically targeted to B cells with an anti-CD19 antibody. We find that this 3DNA is successfully delivered to and internalized in B cells. To test whether these nanostructures can deliver an efficacious therapeutic dose to alter autoimmune responses, a modified anti-IDO2 siRNA was attached to B-cell-directed 3DNA nanocarriers and tested in an established preclinical model of autoimmune arthritis, KRN.g7. The anti-IDO2 3DNA formulation ameliorates arthritis in this system, delaying the onset of joint swelling and reducing total arthritis severity. As such, a 3DNA nanocarrier system shows promise for delivery of targeted, specific, low-dose therapy for autoimmune disease.

Differential Roles of IDO1 and IDO2 in T and B Cell Inflammatory Immune Responses

Indoleamine-2,3-dioxygenase (IDO)1 and IDO2 are two closely related tryptophan catabolizing enzymes encoded by linked genes. The IDO pathway is also immunomodulatory, with IDO1 well-characterized as a mediator of tumor immune evasion. Due to its homology with IDO1, IDO2 has been proposed to have a similar immunoregulatory function. Indeed, IDO2, like IDO1, is necessary for the differentiation of regulatory T cells in vitro. However, compared to IDO1, in vivo studies demonstrated a contrasting role for IDO2, with experiments in preclinical models of autoimmune arthritis establishing a proinflammatory role for IDO2 in mediating B and T cell activation driving autoimmune disease. Given their potentially opposing roles in inflammatory responses, interpretation of results obtained using IDO1 or IDO2 single knockout mice could be complicated by the expression of the other enzyme. Here we use IDO1 and IDO2 single and double knockout (dko) mice to define the differential roles of IDO1 and IDO2 in B cell-mediated immune responses. Autoreactive T and B cell responses and severity of joint inflammation were decreased in IDO2 ko, but not IDO1 ko arthritic mice. Dko mice had a reduction in the number of autoantibody secreting cells and severity of arthritis: however, percentages of differentiated T cells and their associated cytokines were not reduced compared to IDO1 ko or wild-type mice. These data suggest that autoreactive B cell responses are mediated by IDO2, while autoreactive T cell responses are indirectly affected by IDO1 expression in the IDO2 ko mice. IDO2 also influenced antibody responses in models of influenza infection and immunization with T cell-independent type II antigens. Taken together, these studies provide evidence for the contrasting roles IDO1 and IDO2 play in immune responses, with IDO1 mediating T cell suppressive effects and IDO2 working directly in B cells as a proinflammatory mediator of B cell responses.

The role of indoleamine 2,3 dioxygenase 1 in the osteoarthritis

Osteoarthritis (OA) is a chronic degenerative joint disease and a leading cause of disability. It involves articular cartilage destruction and a whole joint inflammation. In spite of OA pathogenesis is still unclear, new studies on the OA pathophysiological aetiology and immunomodulation therapy continuously achieve significant advances with new concepts. Here, we focus on the indoleamine-2,3-dioxygenase1 (IDO1) activity in the osteoarthritis (OA), which is one of the noticeable enzymes in the synovial fluid of arthritis patients. It was recognized as an essential mediator of autoreactive B and T cell responses in rheumatoid arthritis (RA) and an interesting therapeutic target against RA. However, the role IDO1 plays in the OA pathogenesis hasn't been discussed. The new OA experimental analysis evidenced IDO1 overexpression in the synovial fluid of OA patients, and recent studies reported that IDO1 metabolites were found higher in the OA synovial fluid than RA and spondyloarthropathies (SpA) patients. Moreover, the positive relation of IDO1 metabolites with OA pain and joint stiffness has been confirmed. Thus, the IDO1 plays a pivotal role in the pathogenesis of OA. In this review, the role IDO1 plays in the OA pathogenesis has been deeply discussed. It could be a promising target in the immunotherapy of OA disease.

Indoleamine 2, 3-dioxygenase 1enhanceshepatocytes ferroptosis in acute immune hepatitis associated with excess nitrative stress

Ferroptosis is a recently recognized form of regulated cell death that is characterized by lipid peroxidation. However, the molecular mechanisms of ferroptosis in acute immune hepatitis (AIH) are largely unknown. In this study, we investigated the classical ferroptotic events in the livers of mice with concanavalin A (ConA) to induce AIH. The dramatically upregulated gene indoleamine 2, 3-dioxygenase 1 (IDO1) was identified with AIH, and its role in generation of ferroptosis and reactive nitrogen species (RNS) was assessed both in vitro and in vivo by genetic deletion or pharmacologic inhibition of IDO1. We observed that ferroptosis contributed to the ConA-induced hepatic damage, which was confirmed by the therapeutical effects of ferroptosis inhibitor (ferrostatin-1). Noteworthy, upregulation of hepatic IDO1 and nitrative stress in ConA-induced hepatic damage were also remarkably inhibited by the ferroptosis abolishment. Additionally, IDO1 deficiency contributed to ferroptosis resistance by activating solute carrier family 7 member 11 (SLC7A11; also known as xCT) expression, accompanied with the reductions of murine liver lesions and RNS. Meanwhile, IDO inhibitor 1-methyl tryptophan alleviated murine liver damage with the reduction of inducible nitric oxide synthase and 3-nitrotyrosine expression. Consistent with the results in vivo, hepatocytes-specific knockdown of IDO1 led to ferroptosis resistance upon exposure to ferroptosis-inducing compound (Erastin) in vitro, whereas IDO1 overexpression aggravated the classical ferroptotic events, and the RNS stress. Overall, these results revealed a novel molecular mechanism of ferroptosis with the key feature of nitrative stress in ConA-induced liver injury, and also identified IDO1-dependent ferroptosis as a potential target for the treatment of AIH.

Circulating Pro- and Anti-Inflammatory Metabolites and Its Potential Role in Rheumatoid Arthritis Pathogenesis

Rheumatoid arthritis (RA) is a chronic systemic autoimmune disease that affects synovial joints, leading to inflammation, joint destruction, loss of function, and disability. Although recent pharmaceutical advances have improved the treatment of RA, patients often inquire about dietary interventions to improve RA symptoms, as they perceive pain and/or swelling after the consumption or avoidance of certain foods. There is evidence that some foods have pro- or anti-inflammatory effects mediated by diet-related metabolites. In addition, recent literature has shown a link between diet-related metabolites and microbiome changes, since the gut microbiome is involved in the metabolism of some dietary ingredients. But diet and the gut microbiome are not the only factors linked to circulating pro- and anti-inflammatory metabolites. Other factors including smoking, associated comorbidities, and therapeutic drugs might also modify the circulating metabolomic profile and play a role in RA pathogenesis. This article summarizes what is known about circulating pro- and anti-inflammatory metabolites in RA. It also emphasizes factors that might be involved in their circulating concentrations and diet-related metabolites with a beneficial effect in RA.

Modeling Tryptophan/Indoleamine 2,3-Dioxygenase with Heme Superoxide Mimics: Is Ferryl the Key Intermediate?

Tryptophan oxidation in biology has been recently implicated in a vast array of paramount pathogenic conditions in humans, including multiple sclerosis, rheumatoid arthritis, type-I diabetes, and cancer. This 2,3-dioxygenative cleavage of the indole ring of tryptophan with dioxygen is mediated by two heme enzymes, tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO), during its conversion to N-formylkynurenine in the first and rate-limiting step of kynurenine pathway. Despite the pivotal significance of this enzymatic transformation, a vivid viewpoint of the precise mechanistic events is far from complete. A heme superoxide adduct is thought to be the active oxidant in both TDO and IDO, which, following O-O bond cleavage, presumably generates a key ferryl (Fe^IV=O) reaction intermediate. This study, for the first time in model chemistry, demonstrates the potential of synthetic heme superoxide adducts to mimic the bioinorganic chemistry of indole dioxygenation by TDO and IDO, challenging the widely accepted categorization of these metal adducts as weak oxidants. Herein, an electronically divergent series of ferric heme superoxo oxidants mediates the facile conversion of an array of indole substrates into their corresponding 2,3-dioxygenated products, while shedding light on an unequivocally occurring, putative ferryl intermediate. The oxygenated indole products have been isolated in ∼31% yield, and characterized by LC-MS, ¹H and ¹³C NMR, and FT-IR methodologies, as well as by ¹⁸O_2(g) labeling experiments. Distinctly, the most electron-deficient superoxo adduct is observed to react the fastest, specifically with the most electron-rich indole substrate, underscoring the cruciality of electrophilicity of the heme superoxide moiety in facilitating the initial indole activation step. Comprehensive understanding of such mechanistic subtleties will benefit future attempts in the rational design of salient therapeutic agents, including next generation anticancer drug targets with amplified effectivity.

Potent Activation of Indoleamine 2,3-Dioxygenase by Polysulfides

Indoleamine 2,3-dioxygenase (IDO1) is a heme enzyme that catalyzes the oxygenation of the indole ring of tryptophan to afford N-formylkynurenine. This activity significantly suppresses the immune response, mediating inflammation and autoimmune reactions. These consequential effects are regulated through redox changes in the heme cofactor of IDO1, which autoxidizes to the inactive ferric state during turnover. This change in redox status increases the lability of the heme cofactor leading to further suppression of activity. The cell can thus regulate IDO1 activity through the supply of heme and reducing agents. We show here that polysulfides bind to inactive ferric IDO1 and reduce it to the oxygen-binding ferrous state, thus activating IDO1 to maximal turnover even at low, physiologically significant concentrations. The on-rate for hydrogen disulfide binding to ferric IDO1 was found to be >10⁶ M^-1 s^-1 at pH 7 using stopped-flow spectrometry. Fe K-edge XANES and EPR spectroscopy indicated initial formation of a low-spin ferric sulfur-bound species followed by reduction to the ferrous state. The μM affinity of polysulfides for IDO1 implicates these polysulfides as important signaling factors in immune regulation through the kynurenine pathway. Tryptophan significantly enhanced the relatively lower-affinity binding of hydrogen sulfide to IDO1, inspiring the use of the small molecule 3-mercaptoindole (3MI), which selectively binds to and activates ferric IDO1. 3MI sustains turnover by catalytically transferring reducing equivalents from glutathione to IDO1, representing a novel strategy of upregulating innate immunosuppression for treatment of autoimmune disorders. Reactive sulfur species are thus likely unrecognized immune-mediators with potential as therapeutic agents through these interactions with IDO1.

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

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

Fibroblast-Like Synoviocytes Glucose Metabolism as a Therapeutic Target in Rheumatoid Arthritis

Metabolomic studies show that rheumatoid arthritis (RA) is associated with metabolic disruption that may be therapeutically targetable. Among them, glucose metabolism and glycolytic intermediaries seem to have an important role in fibroblast-like synoviocytes (FLS) phenotype and might contribute to early stage disease pathogenesis. RA FLS are transformed from quiescent to aggressive and metabolically active cells and several works have shown that glucose metabolism is increased in activated FLS. Glycolytic inhibitors reduce not only FLS aggressive phenotype in vitro but also decrease bone and cartilage damage in several murine models of arthritis. Essential glycolytic enzymes, including hexokinase 2 (HK2) and 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase (PFKFB) enzymes, have important roles in FLS behavior. Of interest, HK2 is an inducible enzyme present only in the inflamed rheumatic tissues compared to osteoarthritis synovium. It is a contributor to glucose metabolism that could be selectively targeted without compromising systemic homeostasis as a novel approach for combination therapy independent of systemic immunosuppression. More information about metabolic targets that do not compromise global glucose metabolism in normal cells is needed.

Can Metabolic Pathways Be Therapeutic Targets in Rheumatoid Arthritis?

The metabolic rewiring of tumor cells and immune cells has been viewed as a promising source of novel drug targets. Many of the molecular pathways implicated in rheumatoid arthritis (RA) directly modify synovium metabolism and transform the resident cells, such as the fibroblast-like synoviocytes (FLS), and the synovial tissue macrophages (STM), toward an overproduction of enzymes, which degrade cartilage and bone, and cytokines, which promote immune cell infiltration. Recent studies have shown metabolic changes in stromal and immune cells from RA patients. Metabolic disruption in the synovium provide the opportunity to use in vivo metabolism-based imaging techniques for patient stratification and to monitor treatment response. In addition, these metabolic changes may be therapeutically targetable. Thus, resetting metabolism of the synovial membrane offers additional opportunities for disease modulation and restoration of homeostasis in RA. In fact, rheumatologists already use the antimetabolite methotrexate, a chemotherapy agent, for the treatment of patients with inflammatory arthritis. Metabolic targets that do not compromise systemic homeostasis or corresponding metabolic functions in normal cells could increase the drug armamentarium in rheumatic diseases for combination therapy independent of systemic immunosuppression. This article summarizes what is known about metabolism in synovial tissue cells and highlights chemotherapies that target metabolism as potential future therapeutic strategies for RA.

Survival of lymphocytes is not restricted by IDO-expressing fibroblast from rheumatoid arthritis patients

Objective: Rheumatoid arthritis (RA) is characterized by expansion of fibroblast-like synoviocytes (FLS) in inflamed joints and activation of lymphocytes. Tryptophan (trp) is an essential amino acid indispensable for the biosynthesis of proteins and critical for survival of lymphocytes. Indoleamine 2,3-dioxygenase (IDO) that initiates the degradation of trp and tryptophanyl-tRNA synthetase (TTS) essential for tryptophan synthesis, regulate trp bioavailability. Here, we tested the hypothesis that triggered by cytokines, enhanced IDO activity modulate regulatory function of otherwise non-tolerogenic FLS isolated from RA patients. Materials and methods: IDO and TTS mRNA expression were evaluated by RT-PCR. IDO enzymatic activity was confirmed using HPLC. Resting or PHA-activated PBMC from healthy volunteers and RA patients were co-cultured with IDO expressing untreated (FLS_C) or IFNγ-treated (FLS_IFNγ) RA FLS. Lymphocyte survival and proliferation were evaluated by flow cytometry analysis and tritiated thymidine incorporation, respectively. Results: RA FLS_IFNγ produce functionally active IDO and constitutively express TTS. RA FLS_C and FLS_IFNγ increased survival of resting lymphocytes in both studied groups, and decreased proliferation of healthy, but not RA, PBMC. Only FLS_IFNγ diminished survival of activated CD3⁺CD4^-, but not CD3⁺CD4⁺, healthy T cells and similar tendency was observed in rheumatoid cells. Importantly, IDO inhibitor, 1-methyl-DL-tryptophan (1-MT), failed to reverse this effect. PBMC, irrespective of their state (resting versus activated) or origin (healthy or RA), expressed high level of TTS mRNA. Conclusions: We suggest that RA FLS express functionally active IDO but control survival and expansion of healthy cells in IDO-independent mechanism and exert weaker, if any, suppressive effect on rheumatoid cells.

Investigation of indolamine 2, 3 dioxygenase (IDO-1) gene expression by real-time PCR among patients with lung cancer

INTRODUCTION

The aim of this study was to evaluate the expression of IDO-1 gene and cancerous grades of non-small cell lung cancer (NSCLC) and its subclasses among patients with lung cancer using real-time polymerase chain reaction (PCR).

MATERIALS AND METHODS

A total of 35 clinical samples were collected from patients with NSCLC. To evaluate the IDO-1 gene after the extraction of RNA and complementary DNA (cDNA) synthesis using real-time PCR, the expression of the gene was investigated. The western blot analysis method was used for protein expression.

RESULTS

The highest grade, IIIa grade included six patients (17.1%). Approximately 74% of adenocarcinoma cases were in T-categories of lung cancer and 25% of patients in IIIa grade. Patients in the IIA and IIB categories belong to the SCC subclass. Results showed that the expression of INDO 5.22 fold gene was more common in patients with lung cancer than NSCLC. Protein expression in western blot analysis in patients compared with normal 3.22 fold change increased.

CONCLUSION

The evidence shows that IDO-1 is a key parameter that inhibits antitumor immune responses in humans. This study has added interesting data to the IDO community for analyzing the expression of cancerous human cancer cells and cancer tissue in humans. The results showed that IDO-1 not only participates in the process of escape from tumor immunity but can also contribute to the safety of the pretumor area. A wide variety of observed IDO-1 expression values ​​among patients may present serious barriers to the clinical performance of anti- IDO strategies at present.

PD-L1 and IDO expression in cervical and vulvar invasive and intraepithelial squamous neoplasias: implications for combination immunotherapy

AIMS

The immunoregulatory enzyme indoleamine dioxygenase 2,3 (IDO) has been implicated in cervical and vulvar squamous carcinomas (SCC) and may represent a mechanism of resistance to anti-PD-1/anti-PD-L1 therapy. However, the relationship between IDO and PD-L1 has not been well-investigated.

METHODS AND RESULTS

Sixty-five cases of cervical and vulvar intraepithelial neoplasia and SCC were assessed for IDO and PD-L1 expression. Overall, tumoral PD-L1 expression was seen in 72% of SCC, while 50% expressed IDO; co-expression was seen in 42%. Using the combined positive score (CPS) threshold of 1 to account for both tumoral and immune staining, 83% of SCC expressed PD-L1, 61% expressed IDO and 53% showed co-expression. Cervical SCCs were significantly more likely than human papillomavirus (HPV)-related vulvar SCCs to express tumoral IDO (75% versus 13%, P < 0.001) and demonstrate an IDO CPS ≥ 1 (88% versus 25%, P < 0.001); no significant differences were seen for PD-L1. Additionally, there were no significant differences in IDO and PD-L1 expression in dVIN-associated versus HPV-associated vulvar SCC. In contrast to SCC, the majority of intraepithelial lesions were entirely negative for tumoral PD-L1 and IDO and had a CPS score of <1.

CONCLUSIONS

In summary, IDO and PD-L1 co-expression is common in cervical SCCs and, to a lesser extent, vulvar SCCs. These data suggest a role for combination immunotherapy in a subset of cervical SCCs as well as select vulvar SCCs. Expression for both markers is less common in intraepithelial lesions, providing no strong support for this form of immunotherapy in the absence of invasion.

IDO expression in breast cancer: an assessment of 281 primary and metastatic cases with comparison to PD-L1

The immune inhibitory enzyme indoleamine 2,3-dioxygenase (IDO) has been associated with immune evasion in numerous malignancies and may mark these cancers as susceptible to anti-IDO therapies. We herein address IDO expression in breast cancers, examine the relationship between IDO and PD-L1, and investigate IDO fidelity across breast cancer primaries and metastases. IDO and PD-L1 expression was assessed in tissue microarrays containing 242 invasive primary breast cancers, 20 nodal metastases, and 19 distant metastases. IDO and PD-L1 were scored by extent in the tumor cells and immune infiltrate. Tumor IDO staining was seen in 14% of primaries including 38% of triple-negative cancers. IDO immune cell staining was seen in 14% of primaries and 29% of triple-negative cancers. Tumoral IDO and PD-L1 co-expression was seen in 8% of primaries, including 70% of tumoral PD-L1-positive cases. Immune IDO and PD-L1 co-expression was identified in 14% of primaries, including 48% of immune PD-L1-positive cases. Tumoral and immune cell IDO was conserved in 94% of matched primary/metastasis. In summary, IDO expression is common among high-grade, triple-negative breast cancers and is frequently associated with PD-L1 co-expression, suggesting that IDO might be a mechanism of anti-PD-1/PD-L1 immunotherapy resistance and that dual therapy may be of utility. Tumoral and immune cell IDO expression shows fidelity between primary and metastatic sites in treatment-naïve cancers, arguing against IDO as an independent driver for metastatic spread. Clinical trials are needed to assess the efficacy of IDO inhibition relative to IDO expression, as well as its possible role in combination with anti-PD-1/PD-L1 immunotherapy.

Indoleamine 2,3-dioxygenase in endometrial cancer: a targetable mechanism of immune resistance in mismatch repair-deficient and intact endometrial carcinomas

Mismatch repair-deficient endometrial carcinomas are optimal candidates for immunotherapy given their high neoantigen loads, robust lymphoid infiltrates, and frequent PD-L1 expression. However, co-opting the PD-1/PD-L1 pathway is just one mechanism that tumors can utilize to evade host immunity. Another immune modulatory molecule that has been demonstrated in endometrial carcinoma is indoleamine 2,3-dioxygenase (IDO). We herein evaluate IDO expression in 60 endometrial carcinomas and assess results in relation to PD-L1 and mismatch repair status. IDO immunohistochemistry was performed on 60 endometrial carcinomas (20 Lynch syndrome (LS)-associated, 20 MLH1 promoter hypermethylated, and 20 mismatch repair-intact). Eight-five percent of endometrial carcinomas showed IDO tumor staining in >1% of cells. Twenty-five percent were positive in >25% of tumor cells and only 7% exceeded 50% staining. Mismatch repair-deficient cancers were more likely than mismatch repair-intact cancers to be >25% IDO-positive (35% vs. 5% p = 0.024). Differences were amplified when Lynch syndrome-associated cases were evaluated in isolation (50% Lynch syndrome-associated vs. 10% mismatch repair-intact and MLH1-hypermethylated, p = 0.001). Of the four cases showing >50% staining, three were Lynch syndrome-associated and one was MLH1-hypermethylated; no mismatch repair-intact cases had >50% staining. Forty-three percent of IDO-positive tumors were also positive for PD-L1, whereas only two cases showed tumoral PD-L1 in the absence of IDO. In summary, IDO expression is prevalent in endometrial carcinomas and diffuse staining is significantly more common in mismatch repair-deficient cancers, particularly Lynch syndrome-associated cases. Given that the majority of PD-L1 positive cancers also express IDO, synergistic combination therapy with anti-IDO and anti-PD1/PD-L1 may be relevant in this tumor type. Furthermore, anti-IDO therapy may be an option for a small subset of mismatch repair-intact cancers.

The role of the kynurenine pathway of tryptophan metabolism in cardiovascular disease

Coronary heart disease and stroke, the deadliest forms of cardiovascular disease (CVD), are mainly caused by atherosclerosis, a chronic inflammatory disease of the artery wall driven by maladaptive immune responses in the vessel wall. Various risk factors for CVD influence this pathogenic process, including diabetes mellitus, hypertension, dyslipidaemia, and obesity. Indoleamine 2,3-dioxygenase (IDO), an enzyme catalyzing the rate-limiting step in the kynurenine pathway of tryptophan degradation, is strongly induced by inflammation in several tissues, including the artery wall. An increasing body of evidence indicates that IDO promotes immune tolerance, decreases inflammation, and functions as a homeostatic mechanism against excessive immune reactions.

This review provides an overview of the emerging field of the kynurenine pathway of tryptophan degradation in CVD, emphasizing the role of IDO-mediated tryptophan metabolism and its metabolites in the modulation of ‘classical’ cardiovascular risk factors, such as hypertension, obesity, lipid metabolism, diabetes mellitus, and in the development of atherosclerotic CVD.

In Vitro Induction of Human Regulatory T Cells Using Conditions of Low Tryptophan Plus Kynurenines

Thymic regulatory T cells (tTregs) and induced regulatory T cells (iTregs) suppress murine acute graft-versus-host disease (GVHD). Previously, we demonstrated that the plasmacytoid dendritic cell indoleamine 2,3-dioxygenase (IDO) fosters the in vitro development of human iTregs via tryptophan depletion and kynurenine (Kyn) metabolites. We now show that stimulation of naïve CD4⁺ T cells in low tryptophan (low Trp) plus Kyn supports human iTreg generation. In vitro, low Trp + Kyn iTregs and tTregs potently suppress T effector cell proliferation equivalently but are phenotypically distinct. Compared with tTregs or T effector cells, bioenergetics profiling reveals that low Trp + Kyn iTregs have increased basal glycolysis and oxidative phosphorylation and use glutaminolysis as an energy source. Low Trp + Kyn iTreg viability was reliant on interleukin (IL)-2 in vitro. Although in vivo IL-2 administration increased low Trp + Kyn iTreg persistence on adoptive transfer into immunodeficient mice given peripheral blood mononuclear cells to induce GVHD, IL-2-supported iTregs did not improve recipient survival. We conclude that low Trp + Kyn create suppressive iTregs that have high metabolic needs that will need to be addressed before clinical translation.

Hypoxia and Mucosal Inflammation

Sites of inflammation are defined by significant changes in metabolic activity. Recent studies have suggested that O2 metabolism and hypoxia play a prominent role in inflammation so-called "inflammatory hypoxia," which results from a combination of recruited inflammatory cells (e.g., neutrophils and monocytes), the local proliferation of multiple cell types, and the activation of multiple O2-consuming enzymes during inflammation. These shifts in energy supply and demand result in localized regions of hypoxia and have revealed the important function off the transcription factor HIF (hypoxia-inducible factor) in the regulation of key target genes that promote inflammatory resolution. Analysis of these pathways has provided multiple opportunities for understanding basic mechanisms of inflammation and has defined new targets for intervention. Here, we review recent work addressing tissue hypoxia and metabolic control of inflammation and immunity.

Intralymphatic Administration of Adipose Mesenchymal Stem Cells Reduces the Severity of Collagen-Induced Experimental Arthritis

Mesenchymal stem cells (MSCs) are multipotent stromal cells with immunomodulatory properties. They have emerged as a very promising treatment for autoimmunity and inflammatory diseases such as rheumatoid arthritis. Previous studies have demonstrated that MSCs, administered systemically, migrate to lymphoid tissues associated with the inflammatory site where functional MSC-induced immune cells with a regulatory phenotype were increased mediating the immunomodulatory effects of MSCs. These results suggest that homing of MSCs to the lymphatic system plays an important role in the mechanism of action of MSCs in vivo. Thus, we hypothesized that direct intralymphatic (IL) (also referred as intranodal) administration of MSCs could be an alternative and effective route of administration for MSC-based therapy. Here, we report the feasibility and efficacy of the IL administration of human expanded adipose mesenchymal stem cells (eASCs) in a mouse model of collagen-induced arthritis (CIA). IL administration of eASCs attenuated the severity and progression of arthritis, reduced bone destruction and increased the levels of regulatory T cells (CD25⁺Foxp3⁺CD4⁺ cells) and Tr1 cells (IL10⁺CD4⁺), in spleen and draining lymph nodes. Taken together, these results indicate that IL administration of eASCs is very effective in modulating established CIA and may represent an alternative treatment modality for cell therapy with eASCs.

Therapeutic antibody targeting of indoleamine-2,3-dioxygenase (IDO2) inhibits autoimmune arthritis

Rheumatoid arthritis (RA) is a debilitating inflammatory autoimmune disease with no known cure. Recently, we identified the immunomodulatory enzyme indoleamine-2,3-dioxygenase 2 (IDO2) as an essential mediator of autoreactive B and T cell responses driving RA. However, therapeutically targeting IDO2 has been challenging given the lack of small molecules that specifically inhibit IDO2 without also affecting the closely related IDO1. In this study, we develop a novel monoclonal antibody (mAb)-based approach to therapeutically target IDO2. Treatment with IDO2-specific mAb alleviated arthritis in two independent preclinical arthritis models, reducing autoreactive T and B cell activation and recapitulating the strong anti-arthritic effect of genetic IDO2 deficiency. Mechanistic investigations identified FcγRIIb as necessary for mAb internalization, allowing targeting of an intracellular antigen traditionally considered inaccessible to mAb therapy. Taken together, our results offer preclinical proof of concept for antibody-mediated targeting of IDO2 as a new therapeutic strategy to treat RA and other autoantibody-mediated diseases.

Exosomes: Nanoparticulate tools for RNA interference and drug delivery

Exosomes are naturally occurring extracellular vesicles released by most mammalian cells in all body fluids. Exosomes are known as key mediators in cell‐cell communication and facilitate the transfer of genetic and biochemical information between distant cells. Structurally, exosomes are composed of lipids, proteins, and also several types of RNAs which enable these vesicles to serve as important disease biomarkers. Moreover, exosomes have emerged as novel drug and gene delivery tools owing to their multiple advantages over conventional delivery systems. Recently, increasing attention has been focused on exosomes for the delivery of drugs, including therapeutic recombinant proteins, to various target tissues. Exosomes are also promising vehicles for the delivery of microRNAs and small interfering RNAs, which is usually hampered by rapid degradation of these RNAs, as well as inefficient tissue specificity of currently available delivery strategies. This review highlights the most recent accomplishments and trends in the use of exosomes for the delivery of drugs and therapeutic RNA molecules.

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

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

Dose-response characteristics of Clematis triterpenoid saponins and clematichinenoside AR in rheumatoid arthritis rats by liquid chromatography/mass spectrometry-based serum and urine metabolomics

Clematidis Radix et Rhizoma is a traditional Chinese medicine widely used for treating arthritic disease. Clematis triterpenoid saponins (TS) and clematichinenoside AR (C-AR) have been considered to be responsible for its antiarthritic effects. However, the underling mechanism is still unclear because of their low bioavailability. To address of this issue, metabolomics tools were performed to determine metabolic variations associated with rheumatoid arthritis (RA) and responses to Clematis TS, C-AR and positive drug (Triptolide, TP) treatments. This metabolomics investigation of RA was conducted in collagen-induced arthritis (CIA) rats. Liquid chromatography/mass spectrometry and multivariate statistical tools were used to identify the alteration of serum and urine metabolites associated with RA and responses to drug treatment. As a result, 45 potential metabolites associated with RA were identified. After treatment, a total of 24 biomarkers were regulated to normal like levels. Among these, PC(18:0/20:4), 9,11-octadecadienoic acid, arachidonic acid, 1-methyladenosine, valine, hippuric acid and pantothenic acid etc, were reversed in Clematis TS and C-AR groups. Tetrahydrocortisol was regulated to normal levels in Clematis TS and TP groups, while 3,7,12-trihydroxycholan-24-oic acid was regulated in C-AR and TP groups. Biomarkers like citric acid, p-cresol glucuronide, creatinine, cortolone were reversed in TP group.

IDO2: A Pathogenic Mediator of Inflammatory Autoimmunity

Indoleamine 2,3-dioxygenase 2 (IDO2), a homolog of the better-studied tryptophan-catabolizing enzyme IDO1, is an immunomodulatory molecule with potential effects on various diseases including cancer and autoimmunity. Here, we review what is known about the direct connections between IDO2 and immune function, particularly in relationship to autoimmune inflammatory disorders such as rheumatoid arthritis and lupus. Accumulating evidence indicates that IDO2 acts as a pro-inflammatory mediator of autoimmunity, with a functional phenotype distinct from IDO1. IDO2 is expressed in antigen-presenting cells, including B cells and dendritic cells, but affects inflammatory responses in the autoimmune context specifically by acting in B cells to modulate T cell help in multiple model systems. Given that expression of IDO2 can lead to exacerbation of inflammatory responses, IDO2 should be considered a potential therapeutic target for autoimmune disorders.

Metabolite and Lipid Profiling of Biobank Plasma Samples Collected Prior to Onset of Rheumatoid Arthritis

OBJECTIVE

The early diagnosis of rheumatoid arthritis (RA) is desirable to install treatment to prevent disease progression and joint destruction. Autoantibodies and immunological markers pre-date the onset of symptoms by years albeit not all patients will present these factors, even at disease onset. Additional biomarkers would be of high value to improve early diagnosis and understanding of the process, leading to disease development.

METHODS

Plasma samples donated before the onset of RA were identified in the Biobank of Northern Sweden, a collection within national health survey programs. Thirty samples from pre-symptomatic individuals and nineteen from controls were subjected to liquid chromatography-mass spectrometry (LCMS) metabolite and lipid profiling. Lipid and metabolite profiles discriminating samples from pre-symptomatic individuals from controls were identified after univariate and multivariate OPLS-DA based analyses.

RESULTS

The OPLS-DA models including pre-symptomatic individuals and controls identified profiles differentiating between the groups that was characterized by lower levels of acyl-carnitines and fatty acids, with higher levels of lysophospatidylcholines (LPCs) and metabolites from tryptophan metabolism in pre-symptomatic individuals compared with controls. Lipid profiling showed that the majority of phospholipids and sphingomyelins were at higher levels in pre-symptomatic individuals in comparison with controls.

CONCLUSIONS

Our LCMS based approach demonstrated that there are changes in small molecule and lipid profiles detectable in plasma samples collected from the pre-symptomatic individuals who subsequently developed RA, which point to an up-regulation of levels of lysophospatidylcholines, and of tryptophan metabolism, perturbation of fatty acid beta-oxidation and increased oxidative stress in pre-symptomatic individuals' years before onset of symptoms.

Immune cells from patients with psoriasis are defective in inducing indoleamine 2,3-dioxygenase expression in response to inflammatory stimuli

BACKGROUND

Indoleamine 2,3-dioxygenase (IDO) is an inducible enzyme that suppresses the immune response. The role of IDO as a negative regulator of inflammatory responses has been documented in several experimental autoimmune diseases.

OBJECTIVES

To explore the regulation of IDO by immune cells in psoriasis and its relation with disease severity.

METHODS

The expression and activity of IDO were assessed by reverse-transcriptase polymerase chain reaction, flow cytometry and high-performance liquid chromatography in peripheral blood of patients with moderate-to-severe plaque-type psoriasis. The ability of immune cells to express IDO in response to inflammatory stimuli was studied. The functional role of IDO expression was evaluated in a regulatory T cell (Treg) differentiation assay, using cocultures of immature monocyte-derived dendritic cells with autologous peripheral CD4⁺ T cells.

RESULTS

Analysis of the kynurenine-to-tryptophan ratio in serum samples indicated higher IDO activity in patients with psoriasis than in healthy controls. However, correlation studies showed lower IDO activity in those patients with higher Psoriasis Area and Severity Index (PASI). Although myeloid dendritic cells from patients with psoriasis expressed higher levels of IDO than those from healthy controls, these cells did not upregulate IDO in response to a combination of tumour necrosis factor-α, interleukin (IL)-1β and IL-6 cytokines. The defective expression of IDO correlated with PASI. Immature monocyte-derived dendritic cells from patients with psoriasis also expressed low levels of IDO and induced CD4⁺ Treg differentiation poorly.

CONCLUSIONS

Immune cells from patients with psoriasis have a defect in upregulating IDO in response to inflammation associated with the severity of psoriasis.

IDO2 Modulates T Cell-Dependent Autoimmune Responses through a B Cell-Intrinsic Mechanism

Mechanistic insight into how adaptive immune responses are modified along the self-nonself continuum may offer more effective opportunities to treat autoimmune disease, cancer, and other sterile inflammatory disorders. Recent genetic studies in the KRN mouse model of rheumatoid arthritis demonstrate that the immunomodulatory molecule IDO2 modifies responses to self-antigens; however, the mechanisms involved are obscure. In this study, we show that IDO2 exerts a critical function in B cells to support the generation of autoimmunity. In experiments with IDO2-deficient mice, adoptive transplant experiments demonstrated that IDO2 expression in B cells was both necessary and sufficient to support robust arthritis development. IDO2 function in B cells was contingent on a cognate, Ag-specific interaction to exert its immunomodulatory effects on arthritis development. We confirmed a similar requirement in an established model of contact hypersensitivity, in which IDO2-expressing B cells are required for a robust inflammatory response. Mechanistic investigations showed that IDO2-deficient B cells lacked the ability to upregulate the costimulatory marker CD40, suggesting IDO2 acts at the T-B cell interface to modulate the potency of T cell help needed to promote autoantibody production. Overall, our findings revealed that IDO2 expression by B cells modulates autoimmune responses by supporting the cross talk between autoreactive T and B cells.