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Indoleamine 2,3-dioxygenase 1 (IDO1), a tryptophan catabolising enzyme, is known as a tumour cell survival factor that causes immune escape in several types of cancer. Flavonoids of Sophora flavescens have a variety of biological benefits for humans; however, cancer immunotherapy effect has not been fully investigated. The flavonoids (1–6) isolated from S. flavescens showed IDO1 inhibitory activities (IC50 4.3–31.4 µM). The representative flavonoids (4–6) of S. flavescens were determined to be non-competitive inhibitors of IDO1 by kinetic analyses. Their binding affinity to IDO1 was confirmed using thermal stability and surface plasmon resonance (SPR) assays. The molecular docking analysis and mutagenesis assay revealed the structural details of the interactions between the flavonoids (1–6) and IDO1. These results suggest that the flavonoids (1–6) of S. flavescens, especially kushenol E (6), as IDO1 inhibitors might be useful in the development of immunotherapeutic agents against cancers.

Background: Prunella vulgaris L. (P. vulgaris) has traditionally been used to treat swelling and inflammation of the thyroid gland. This study aimed to evaluate the effects of P. vulgaris on experimental autoimmune thyroiditis (EAT) and explore the roles of indoleamine 2,3-dioxygenase 1 (IDO1) and regulatory T cells (Tregs) in these P. vulgaris-mediated effects. Methods: The main bioactive compounds in P. vulgaris were analysed by high-performance liquid chromatography. An EAT model was established by immunization of Lewis rats with thyroglobulin via subcutaneous injection. Thyroid volume was assessed by ultrasound, and lymphatic infiltration in the thyroid was evaluated by haematoxylin and eosin staining. The serum levels of thyroglobulin antibody (TgAb) and cytokines were measured by indirect enzyme-linked immunosorbent assay. The percentage of CD4+CD25+Foxp3+ Tregs was detected by flow cytometry. The mRNA and protein levels of IDO1 were measured by qRT-PCR and Western blotting, respectively. The levels of tryptophan (Trp) and kynurenine (Kyn) in serum and faecal samples were assessed with a fluorometric kit and spectrophotometry. Results: The main bioactive compound in P. vulgaris was rosmarinic acid. The TgAb level and thyroid volume in EAT rats were significantly decreased after administration of P. vulgaris (P < 0.01). The inflammation score in EAT rats that were administered P. vulgaris was significantly lower than that in the EAT controls (P < 0.01). In addition, P. vulgaris promoted the expansion of splenic Tregs and increased the production of IL-10 and TGF-β (P < 0.01) in EAT rats. Moreover, P. vulgaris induced IDO1 mRNA and protein expression in the spleen and intestine in P. vulgaris-treated EAT rats (P < 0.01). Finally, Trp levels were reduced and Kyn levels and the Kyn/Trp ratio were increased in the serum of P. vulgaris-treated EAT rats. Conclusion: We were the first to demonstrate the role of IDO1-induced Treg expansion in P. vulgaris-mediated attenuation of EAT. Our study provides insight into the immunopathogenesis of autoimmune thyroiditis and shows the potential therapeutic value of P. vulgaris.

l-tryptophan (Trp), an essential amino acid for mammals, is the precursor of a wide array of immunomodulatory metabolites produced by the kynurenine and serotonin pathways. The kynurenine pathway is a paramount source of several immunoregulatory metabolites, including l-kynurenine (Kyn), the main product of indoleamine 2,3-dioxygenase 1 (IDO1) that catalyzes the rate-limiting step of the pathway. In the serotonin pathway, the metabolite N-acetylserotonin (NAS) has been shown to possess antioxidant, antiinflammatory, and neuroprotective properties in experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS). However, little is known about the exact mode of action of the serotonin metabolite and the possible interplay between the 2 Trp metabolic pathways. Prompted by the discovery that NAS neuroprotective effects in EAE are abrogated in mice lacking IDO1 expression, we investigated the NAS mode of action in neuroinflammation. We found that NAS directly binds IDO1 and acts as a positive allosteric modulator (PAM) of the IDO1 enzyme in vitro and in vivo. As a result, increased Kyn will activate the ligand-activated transcription factor aryl hydrocarbon receptor and, consequently, antiinflammatory and immunoregulatory effects. Because NAS also increased IDO1 activity in peripheral blood mononuclear cells of a significant proportion of MS patients, our data may set the basis for the development of IDO1 PAMs as first-in-class drugs in autoimmune/neuroinflammatory diseases.

Knowledge of a protein's spatial dynamics at the subcellular level is key to understanding its function(s), interactions, and associated intracellular events. Indoleamine 2,3-dioxygenase 1 (IDO1) is a cytosolic enzyme that controls immune responses via tryptophan metabolism, mainly through its enzymic activity. When phosphorylated, however, IDO1 acts as a signaling molecule in plasmacytoid dendritic cells (pDCs), thus activating genomic effects, ultimately leading to long-lasting immunosuppression. Whether the two activities-namely, the catalytic and signaling functions-are spatially segregated has been unclear. We found that, under conditions favoring signaling rather than catabolic events, IDO1 shifts from the cytosol to early endosomes. The event requires interaction with class IA phosphoinositide 3-kinases (PI3Ks), which become activated, resulting in full expression of the immunoregulatory phenotype in vivo in pDCs as resulting from IDO1-dependent signaling events. Thus, IDO1's spatial dynamics meet the needs for short-acting as well as durable mechanisms of immune suppression, both under acute and chronic inflammatory conditions. These data expand the theoretical basis for an IDO1-centered therapy in inflammation and autoimmunity.

The kynurenine pathway (KP) is highly regulated in the immune system, where it promotes immunosuppression in response to infection or inflammation. Indoleamine 2,3-dioxygenase 1 (IDO1), the main enzyme of KP, has a broad spectrum of activity on immune cells regulation, controlling the balance between stimulation and suppression of the immune system at sites of local inflammation, relevant to a wide range of autoimmune and inflammatory diseases. Various autoimmune diseases, among them endocrinopathies, have been identified to date, but despite significant progress in their diagnosis and treatment, they are still associated with significant complications, morbidity, and mortality. The precise cellular and molecular mechanisms leading to the onset and development of autoimmune disease remain poorly clarified so far. In breaking of tolerance, the cells of the innate immunity provide a decisive microenvironment that regulates immune cells’ differentiation, leading to activation of adaptive immunity. The current review provided a comprehensive presentation of the known role of IDO1 and KP activation in the regulation of the innate and adaptive arms of the immune system. Significant attention has been paid to the immunoregulatory role of IDO1 in the most prevalent, organ-specific autoimmune endocrinopathies—type 1 diabetes mellitus (T1DM) and autoimmune thyroiditis.

Indoleamine 2,3-dioxygenase 1 (IDO1) - the enzyme catalyzing the rate-limiting step of tryptophan catabolism along the kynurenine pathway - belongs to the class of inhibitory immune checkpoint molecules. Such regulators of the immune system are crucial for maintaining self-tolerance and thus, when properly working, preventing autoimmunity. A dysfunctional IDO1 has recently been associated with a specific single nucleotide polymorphism (SNP) and with the occurrence of autoimmune diabetes and multiple sclerosis. Many genetic alterations of IDO1 have been proposed being related with dysimmune disorders. However, the molecular and functional meaning of variations in IDO1 exomes as well as the promoter region remains a poorly explored field. In the present study, we identified a rare missense variant (rs751360195) at the IDO1 gene in a patient affected by coeliac disease, thyroiditis, and selective immunoglobulin A deficiency. Molecular and functional studies demonstrated that the substitution of lysine (K) at position 257 with a glutamic acid (E) results in an altered IDO1 protein that undergoes a rapid protein turnover. This genotype-to-phenotype relation is produced by peripheral blood mononuclear cells (PBMCs) of the patient bearing this variation and is associated with a specific phenotype (i.e., impaired tryptophan catabolism and defective mechanisms of immune tolerance). Thus decoding functional mutations of the IDO1 exome may provide clinically relevant information exploitable to personalize therapeutic interventions.

Background Indoleamine 2,3-dioxygenase 1 (IDO1) catalyzes the first and rate-limiting step in converting tryptophan to kynurenine. Chimeric antigen receptor (CAR) T cells are T cells with recombinant receptors targeting tumor-associated antigens. The Food and Drug Administration has approved CAR T cells that target CD19 for treatment of advanced B cell leukemia and lymphoma. However, CAR T cell therapy in solid tumors has been hampered by multiple obstacles. Preclinical and clinical studies suggest that combinatorial immune checkpoint blockade and IDO1 inhibition provide durable therapeutic efficacy against cancer. Yet, the combination of IDO1 inhibition and CAR T has not been attempted. Methods We analyze IDO1 downregulation by miR-153 in colon cancer cells and the association of IDO1 and miR-153 expression with colorectal patient survival. We generate CAR T cells targeting the epidermal growth factor receptor variant III and measure their tumor killing effects against colon cancer cells with or without miR-153 overexpression by killing assays and in xenografts. Results IDO1 is highly expressed in colorectal tumors and is inversely associated with patient survival. miR-153 directly inhibits IDO1 expression by targeting its 3′ untranslated region in colon cancer cells; yet, miR-153 overexpression does not affect cancer cell survival, apoptosis, and colony formation. When colon cancer cells are targeted by CAR T cells, miR-153 overexpression within tumor cells significantly enhances T cell killing in vitro and suppresses xenograft tumor growth in mice. Conclusions These findings indicate that miR-153 inhibits IDO1 expression in colon cancer cells and is a tumor-suppressive miRNA that enhances CAR T cell immunotherapy. This study supports the combinatorial use of IDO1 inhibitors and CAR T cells in treating solid tumors. Electronic supplementary material The online version of this article (10.1186/s13045-018-0600-x) contains supplementary material, which is available to authorized users.

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.