Induction of hepatitis B virus surface antigen-specific cytotoxic T lymphocytes can be up-regulated by the inhibition of indoleamine 2, 3-dioxygenase activity

A gene expression profile-based approach to screen the occurrence and predisposed host characteristics of drug-induced liver injury: a case study of Psoralea corylifolia Linn

Drug-induced liver injury (DILI) is one of the most common causes of a drug being withdrawn, and identifying the culprit drugs and the host factors at risk of causing DILI has become a current challenge. Recent studies have found that immune status plays a considerable role in the development of DILI. In this study, DILI-related differentially expressed genes mediated by immunoinflammatory cytokines were obtained from the Gene Expression Omnibus (GEO) database to predict the occurrence of DILI (named the DILI predictive gene set, DILI_PGS), and the predictability of the DILI_PGS was verified using the Connectivity Map (CMap) and LiverTox platforms. The results obtained DILI_PGS from the GEO database could predict 81.25% of liver injury drugs. In addition, the Coexpedia platform was used to predict the DILI_PGS-related characteristics of common host diseases and found that the DILI_PGS mainly involved immune-related diseases and tumor-related diseases. Then, animal models of immune stress (IS) and immunosuppressive (IP) were selected to simulate the immune status of the above diseases. Meanwhile, psoralen, a main component derived from Psoralea corylifolia Linn. with definite hepatotoxicity, was selected as an experimental drug with highly similar molecular fingerprints to three idiosyncratic hepatotoxic drugs (nefazodone, trovafloxacin, and nimesulide) from the same DILI_PGS dataset. The animal experiment results found a single administration of psoralen could significantly induce liver injury in IS mice, while there was no obvious liver function change in IP mice by repeatedly administering the same dose of psoralen, and the potential mechanism of psoralen-induced liver injury in IS mice may be related to regulating the expression of the TNF-related pathway. In conclusion, this study constructed the DILI_PGS with high accuracy to predict the occurrence of DILI and preliminarily identified the characteristics of host factors inducing DILI.

Emerging Roles on Immunological Effect of Indoleamine 2,3-Dioxygenase in Liver Injuries

Indoleamine 2,3-dioxygenase (IDO) is one of the initial rate-limiting enzymes of the kynurenine pathway (KP), which causes immune suppression and induction of T cell anergy. It is associated with the imbalance of immune homeostasis in numerous diseases including cancer, chronic viral infection, allergy, and autoimmune diseases. Recently, IDO has extended its role to liver field. In this review, we summarize the dysregulation and potentials of IDO in the emerging field of liver injuries, as well as current challenges for IDO targets. In particular, we discuss unexpected conclusions against previous work published. IDO is induced by pro-inflammatory cytokines in liver dysfunction and exerts an immunosuppressive effect, whereas the improvement of liver injury may require consideration of multiple factors besides IDO.

Abatacept treatment for patients with severe acute hepatitis caused by hepatitis B virus infection-Pilot study

Although glucocorticoids have been used for immunosuppression of patients with primary hepatitis B virus (HBV) infection-induced severe hepatitis, the treatment is associated with a high frequency of adverse events. We conducted a pilot study for evaluating the efficacy and safety of abatacept, a cytotoxic T lymphocyte antigen-4 immunoglobulin (CTLA4), for acute hepatitis B. Five patients with severe acute hepatitis B (prothrombin activity ≤ 60%) were treated for immunosuppression by abatacept. Four patients received abatacept concurrently with methylprednisolone, and another patient was treated with abatacept alone. Rapid decrease in serum alanine aminotransferase levels, increase in prothrombin activity and improvement of general condition were obtained in four out of five patients. The patient with the most severe hepatitis underwent liver transplantation due to exacerbation of hepatitis in spite of treatment with both abatacept and methylprednisolone. None of the patients developed significant adverse events associated with the use of abatacept. Hepatitis B surface antigen (HBsAg) became negative in all five patients. The effect of abatacept and methylprednisolone for severe hepatitis B was compared using a mouse model. Rapid reduction in mouse serum HBV DNA and human albumin levels and elevation of serum interferon-gamma and granzyme A levels were observed in HBV-infected human hepatocyte-transplanted immunodeficient mice that were administered human peripheral blood mononuclear cells. These hepatocyte injuries were inhibited to a greater extent by abatacept compared to methylprednisolone. Abatacept might be an effective therapy alternative to methylprednisolone to reduce acute massive liver damage for patients with severe acute hepatitis caused by HBV infection.

Hepatitis B surface antigen seroclearance: Immune mechanisms, clinical impact, importance for drug development

HBsAg seroclearance occurs rarely in the natural history of chronic hepatitis B (CHB) infection and is associated with improved clinical outcomes. Many factors are associated with HBsAg seroconversion, including immune and viral factors. However, the immune mechanisms associated with HBsAg seroclearance are still difficult to elucidate. HBsAg seroclearance is the ideal aim of HBV treatment. Unfortunately, this goal is rarely achieved with current treatments. Understanding the mechanisms of HBsAg loss appears to be important for the development of curative HBV treatments. While studies from animal models give insights into the potential immune mechanisms and interactions occurring between the immune system and HBsAg, they do not recapitulate all features of CHB in humans and are subject to variability due to their complexity. In this article, we review recent studies on these immune factors, focusing on their influence on CHB progression and HBsAg seroconversion. These data provide new insights for the development of therapeutic approaches to partially restore the anti-HBV immune response. Targeting HBsAg will ideally relieve the immunosuppressive effects on the immune system and help to restore anti-HBV immune responses.

Designer Parasites: Genetically Engineered Plasmodium as Vaccines To Prevent Malaria Infection

A highly efficacious malaria vaccine that prevents disease and breaks the cycle of infection remains an aspirational goal of medicine. Whole parasite vaccines based on the sporozoite forms of the parasite that target the clinically silent pre-erythrocytic stages of infection have emerged as one of the leading candidates. In animal models of malaria, these vaccines elicit potent neutralizing Ab responses against the sporozoite stage and cytotoxic T cells that eliminate parasite-infected hepatocytes. Among whole-sporozoite vaccines, immunization with live, replication-competent whole parasites engenders superior immunity and protection when compared with live replication-deficient sporozoites. As such, the genetic design of replication-competent vaccine strains holds the promise for a potent, broadly protective malaria vaccine. In this report, we will review the advances in whole-sporozoite vaccine development with a particular focus on genetically attenuated parasites both as malaria vaccine candidates and also as valuable tools to interrogate protective immunity against Plasmodium infection.

Tim-3 blockade promotes iNKT cell function to inhibit HBV replication

Increased expression of T cell immunoglobulin and mucin domain-3 (Tim-3) on invariant natural killer T (iNKT) cells is reported in chronic hepatitis B virus (HBV) infection. However, whether Tim-3 regulates iNKT cells in chronic HBV condition remains unclear. In this study, our results showed that the expression of Tim-3 was up-regulated on hepatic iNKT cells from HBV-transgenic (Tg) mice or iNKT cells stimulated with α-galactosylceramide (α-Galcer). Compared with Tim-3- iNKT cells, Tim-3+ iNKT cells expressed more IFN-γ, IL-4 and CD107a, indicating a strong relationship between Tim-3 and iNKT cell activation. Constantly, treatment of Tim-3 blocking antibodies significantly enhanced the production of IFN-γ, TNF-α, IL-4 and CD107a in iNKT cells both in vivo and in vitro. This Tim-3- mediated suppression of iNKT cells was further confirmed in Tim-3 knockout (KO) mice. Moreover, Tim-3 blockade promoted α-Galcer-triggered inhibition of HBV replication, displaying as the decreased HBV DNA and HBsAg level in serum, and down-regulated pgRNA expression in liver tissues. Collectively, our data, for the first time, demonstrated the potential role of Tim-3 blockade in promoting iNKT cell-mediated HBV inhibition. Therefore, combination of α-Galcer with Tim-3 blockade might be a promising approach in chronic hepatitis B therapy.

The IDO1 selective inhibitor epacadostat enhances dendritic cell immunogenicity and lytic ability of tumor antigen-specific T cells

Epacadostat is a novel inhibitor of indoleamine-2,3-dioxygenase-1 (IDO1) that suppresses systemic tryptophan catabolism and is currently being evaluated in ongoing clinical trials. We investigated the effects of epacadostat on (a) human dendritic cells (DCs) with respect to maturation and ability to activate human tumor antigen-specific cytotoxic T-cell (CTL) lines, and subsequent T-cell lysis of tumor cells, (b) human regulatory T cells (Tregs), and (c) human peripheral blood mononuclear cells (PBMCs) in vitro. Simultaneous treatment with epacadostat and IFN-γ plus lipopolysaccharide (LPS) did not change the phenotype of matured human DCs, and as expected decreased the tryptophan breakdown and kynurenine production. Peptide-specific T-cell lines stimulated with DCs pulsed with peptide produced significantly more IFN-γ, TNFα, GM-CSF and IL-8 if the DCs were treated with epacadostat. These T cells also displayed higher levels of tumor cell lysis on a per cell basis. Epacadostat also significantly decreased Treg proliferation induced by IDO production from IFN-γ plus LPS matured human DCs, although the Treg phenotype did not change. Multicolor flow cytometry was performed on human PBMCs treated with epacadostat; analysis of 123 discrete immune cell subsets revealed no changes in major immune cell types, an increase in activated CD83⁺ conventional DCs, and a decrease in immature activated Tim3⁺ NK cells. These studies show for the first time several effects of epacadostat on human DCs, and subsequent effects on CTL and Tregs, and provide a rationale as to how epacadostat could potentially increase the efficacy of immunotherapeutics, including cancer vaccines.

The Inhibition of Indoleamine 2,3-Dioxygenase Accelerates Early Liver Regeneration in Mice After Partial Hepatectomy

BACKGROUND AND AIM

The inflammatory response accelerates early liver regeneration after liver injury and resection. Recent studies have demonstrated that indoleamine 2,3-dioxygenase-1 (IDO1) suppresses the activation of inflammatory cells and induces immune tolerance. In this study, we examined the role of IDO1 in liver regeneration after partial hepatectomy (PHx).

METHODS

WT or IDO1-knockout (IDO1-KO) mice received 70% PHx. The liver-body weight ratio after PHx was measured and hepatocyte growth was assessed by immunostaining. The expression of cell cycle genes and pro-inflammatory cytokines in the liver was analyzed by quantitative RT-PCR. In addition, 1-methyl-DL-tryptophan (1-MT), which is an IDO1 inhibitory agent, was given to WT mice and the liver-body weight ratio was measured after PHx.

RESULTS

The liver-body weight ratio was significantly increased in IDO1-KO mice compared with that in WT mice after PHx. More Ki-67-positive cells were present in IDO1-KO mice than in WT mice after PHx. The expression of cell cycle genes (cyclin D1, cyclin E) and pro-inflammatory cytokines (IL-1β, TNF-α and IL-6) was up-regulated in the remnant liver of IDO1-KO mice compared with WT mice. Moreover, treatment with 1-MT promoted liver regeneration.

CONCLUSION

IDO1 deficiency promoted early liver regeneration after PHx, indicating that IDO1 suppresses the production of inflammatory cytokines and subsequently inhibits hepatocyte proliferation during liver regeneration.

Biological effects of bone marrow mesenchymal stem cells on hepatitis B virus in vitro

The aim of the present study was to explore the effects of co‑culturing bone marrow‑derived mesenchymal stem cells (BM-MSCs) cultured with hepatitis B virus (HBV)‑infected lymphocytes in vitro. BM‑MSCs and lymphocytes from Brown Norway rats were obtained from the bone marrow and spleen, respectively. Rats were divided into the following five experimental groups: Group 1, splenic lymphocytes (SLCs); group 2, HepG2.2.15 cells; group 3, BM‑MSCs + HepG2.2.15 cells; group 4, SLCs + HepG2.2.15 cells; and group 5, SLCs + BM‑MSCs + HepG2.2.15 cells. The viability of lymphocytes and HepG2.2.15 cells was assessed using the MTT assay at 24, 48 and 72 h, respectively. Levels of supernatant HBV DNA and intracellular HBV covalently closed circular DNA (cccDNA) were measured using quantitative polymerase chain reaction. Supernatant cytokine levels were measured by enzyme‑linked immunosorbent assay (ELISA). T cell subsets were quantified by flow cytometry using fluorescence‑labeled antibodies. In addition, the HBV genome sequence was analyzed by direct gene sequencing. Levels of HBV DNA and cccDNA in group 5 were lower when compared with those in group 3 or group 4, with a significant difference observed at 48 h. The secretion of interferon‑γ was negatively correlated with the level of HBV DNA, whereas secretion of interleukin (IL)‑10 and IL‑22 were positively correlated with the level of HBV DNA. Flow cytometry demonstrated that the percentage of CD3+CD8+ T cells was positively correlated with the levels of HBV DNA, and the CD3+CD4+/CD3+CD8+ ratio was negatively correlated with the level of HBV DNA. Almost no mutations in the HBV DNA sequence were detected in HepG2.2.15 cells co‑cultured with BM‑MSCs, SLCs, or in the two types of cells combined. BM‑MSCs inhibited the expression of HBV DNA and enhanced the clearance of HBV, which may have been mediated by the regulation of the Tc1/Tc2 cell balance and the mode of cytokine secretion to modulate cytokine expression.

Induction of humoral and cellular immune response to hepatitis B virus (HBV) vaccine can be upregulated by CpG oligonucleotides complexed with Dectin-1 ligand

A persistent hepatitis B virus (HBV) infection is characterized by a lack of or a weak immune response to HBV, which may be reflective of tolerance to HBV. Efficient induction of HBV-specific immune response leads to the clearance of HBV in patients with a chronic HBV infection. CpG oligodeoxynucleotides (ODN) has a powerful adjuvant effect in HBV vaccination. A recent report demonstrated that the immunization by B/K CpG ODN (K3) wrapped by the nonagonistic Dectin-1 ligand, schizophyllan (SPG), namely K3-SPG, was more effective in the induction of antigen-specific immune response than that by K3. In this study, we examined the efficacy of K3-SPG as a HBV vaccine adjuvant. Wild-type (WT) mice and HBV transgenic (HBV-Tg) mice were subcutaneously immunized with hepatitis B surface antigen (HBsAg) alone, HBsAg and K3, or HBsAg and K3-SPG. The vaccination with HBsAg and K3-SPG significantly enhanced humoral and cellular immune response to HBV antigen compared to the other vaccinations in WT and HBV-Tg mice. K3-SPG induced the accumulation of dendritic cells (DCs) into draining lymph node and the activation of DCs. The expression of cytokines and chemokines related to Th1 and Th2 responses was upregulated after the vaccination including with K3-SPG. In conclusion, these results indicated that the vaccination using K3-SPG may overcome tolerance even in patients with chronic HBV infection.

Chronic hepatitis B surface antigen seroclearance-related immune factors

The ultimate aims of the treatment of hepatitis B virus infection are the loss of hepatitis B surface antigen (HBsAg) and hepatitis B surface antibody seroconversion. Unfortunately, these goals are rarely reached. Many factors are associated with HBsAg seroconversion, including genetic, immune, and viral factors. However, the mechanism of HBsAg seroclearance, and particularly the immune mechanism, is still difficult to elucidate. The immune factor interferon-α is currently the main antiviral therapy for chronic hepatitis B virus infection. However, a sustained shift from response of HBsAg to hepatitis B surface antibody seroconversion is rarely obtained. Recent studies have revealed that several of the newly identified immune factors are closely related to the removal of HBsAg. In this article, we review recent studies on these immune factors, their influence on hepatitis B progression, and HBsAg seroconversion.

Inhibition of iNOS activity enhances the anti-tumor effects of alpha-galactosylceramide in established murine cancer model

Alpha-garactosylceramide (GalCer) has been shown to have anti-tumor effect in the basic research and clinical studies. However, anti-tumor effect of GalCer is limited. The administration of GalCer increases the production of IFN-γ which is involved in the suppression of tumor growth. On the other hand, the enhancement of IFN-γ production increases immunosuppressive factors such as nitric oxide. This suppressive action might impair the anti-tumor effect of GalCer. In the present study, we evaluated the anti-tumor effect of GalCer in the absence of inducible nitric oxide synthase (iNOS). In lung metastatic model, the number of tumor nodules in the lung of iNOS-KO mice treated with GalCer was significantly reduced compared with that of WT mice treated with GalCer. Moreover, L-NAME, which is the inhibitor for iNOS, enhanced the anti-tumor effect of GalCer in lung metastatic model. The frequency of CD8+ cells in bronchoalveolar lavage fluid increased in iNOS-KO mice treated with GalCer. The administration of GalCer increased the frequency of myeloid-derived suppressor cells (MDSCs) in the lung from tumor-bearing WT mice, but the increase of MDSCs in the lung was not induced in iNOS-KO mice. The subcutaneous tumor experiments revealed that the administration of GalCer in the absence of iNOS expression significantly enhanced the induction of tumor antigen-specific response. Finally, our results indicated that the inhibition of iNOS expression could enhance the therapeutic efficacy of GalCer via the increase of tumor antigen-specific immune response and the suppression of MDSCs.

Role of invariant NKT cells in lipopolysaccharide-induced lethal shock during encephalomyocarditis virus infection

Viral infections can give rise to secondary bacterial infections. In the present study, we examined the role of invariant natural killer T (iNKT) cells in lipopolysaccharide (LPS)-induced lethal shock during encephalomyocarditis virus (EMCV) infection. Wild-type (WT) mice and Jα18 gene knockout (Jα18 KO) mice were inoculated with EMCV, 5days prior to challenging with LPS. The survival rate of Jα18 KO mice subjected to EMCV and LPS was significantly higher than that of WT mice. TNF-α and nitric oxide (NO) production were increased in WT mice, than that in Jα18 KO mice, after the administration of EMCV and LPS. EMCV infection increased the number of iNKT cells and IFN-γ production by iNKT cells in WT mice. Moreover, EMCV infection enhanced the expression of Toll-like receptor 4 (TLR4) in the lung and spleen. IFN-γ also increased the expression of TLR4 in splenocytes. These findings indicated that EMCV infection activated iNKT cells, and IFN-γ secreted from the iNKT cells up-regulated the expression of TLR4 in various tissues. As a result, EMCV-infected mice were susceptible to LPS and easily developed the lethal shock. In conclusion, iNKT cells were involved in the development of LPS-induced lethal shock during EMCV infection.

The Deficiency of Indoleamine 2,3-Dioxygenase Aggravates the CCl4-Induced Liver Fibrosis in Mice

In the present study, we examined the role of indoleamine 2,3-dioxygenase (IDO) in the development of CCl4-induced hepatic fibrosis. The liver fibrosis induced by repetitive administration with CCl4 was aggravated in IDO-KO mice compared to WT mice. In IDO-KO mice treated with CCl4, the number of several inflammatory cells and the expression of pro-inflammatory cytokines increased in the liver. In the results, activated hepatic stellate cells (HSCs) and fibrogenic factors on HSCs increased after repetitive CCl4 administration in IDO-KO mice compared to WT mice. Moreover, the treatment with l-tryptophan aggravated the CCl4-induced hepatic fibrosis in WT mice. Our findings demonstrated that the IDO deficiency enhanced the inflammation in the liver and aggravated liver fibrosis in repetitive CCl4-treated mice.

Differential effects of the two amino acid sensing systems, the GCN2 kinase and the mTOR complex 1, on primary human alloreactive CD4⁺ T-cells

Amino acid deprivation activates general control nonderepressible 2 (GCN2) kinase and inhibits mammalian target of rapamycin (mTOR), affecting the immune response. In this study, the effects of GCN2 kinase activation or mTOR inhibition on human alloreactive CD4+ T-cells were evaluated. The mixed lymphocyte reaction, as a model of alloreactivity, the GCN2 kinase activator, tryptophanol (TRP), and the mTOR complex 1 inhibitor, rapamycin (RAP), were used. Both TRP and RAP suppressed cell proliferation and induced cell apoptosis. These events were p53-independent in the case of RAP, but were accompanied by an increase in p53 levels in the case of TRP. TRP decreased the levels of the Th2 signature transcription factor, GATA-3, as RAP did, yet the latter also decreased the levels of the Th1 and Th17 signature transcription factors, T-bet and RORγt, whereas it increased the levels of the Treg signature transcription factor, FoxP3. Accordingly, TRP decreased the production of interleukin (IL)-4, as RAP did, but RAP also decreased the levels of interferon-γ (IFN-γ) and IL-17. Both TRP and RAP increased the levels of IL-10. As regards hypoxia-inducible factor-1α (HIF-1α), which upregulates the Th17/Treg ratio, its levels were decreased by RAP. TRP increased the HIF-1α levels, which however, remained inactive. In conclusion, our findings indicate that, in primary human alloreactive CD4+ T-cells, the two systems that sense amino acid deprivation affect cell proliferation, apoptosis and differentiation in different ways or through different mechanisms. Both mTOR inhibition and GCN2 kinase activation exert immunosuppressive effects, since they inhibit cell proliferation and induce apoptosis. As regards CD4+ T-cell differentiation, mTOR inhibition exerted a more profound effect, since it suppressed differentiation into the Th1, Th2 and Th17 lineages, while it induced Treg differentiation. On the contrary, the activation of GCN2 kinase suppressed only Th2 differentiation.

Presentation of hepatocellular antigens

The liver is an organ in which antigen-specific T-cell responses manifest a bias toward immune tolerance. This is clearly seen in the rejection of allogeneic liver transplants, and multiple other phenomena suggest that this effect is more general. These include tolerance toward antigens introduced via the portal vein, immune failure to several hepatotropic viruses, the lack of natural liver-stage immunity to malaria parasites, and the frequent metastasis of cancers to the liver. Here we review the mechanisms by which T cells engage with hepatocellular antigens, the context in which such encounters occur, and the mechanisms that act to suppress a full T-cell response. While many mechanisms play a role, we will argue that two important processes are the constraints on the cross-presentation of hepatocellular antigens, and the induction of negative feedback inhibition driven by interferons. The constant exposure of the liver to microbial products from the intestine may drive innate immunity, rendering the local environment unfavorable for specific T-cell responses through this mechanism. Nevertheless, tolerance toward hepatocellular antigens is not monolithic and under specific circumstances allows both effective immunity and immunopathology.

Host-targeting agents for treatment of hepatitis B virus infection

Hepatitis B virus (HBV) infection is a major cause of chronic liver disease, including liver cirrhosis, liver failure and hepatocellular carcinoma (HCC)-the second leading and fastest rising cause of cancer death world-wide. While de novo infection can be efficiently prevented by vaccination and chronic infection can be controlled using antivirals targeting the viral polymerase, the development of efficient antiviral strategies to eliminate the virus and thus to cure infection remains a key unmet medical need. The recent progress in the development of robust infectious HBV cell culture models now enables the investigation of the full viral life cycle, including a more detailed study of the molecular mechanisms of virus-host interactions responsible for viral persistence. The understanding of these virus-host interactions will be instrumental for the development of curative treatments. Host-dependency factors have recently emerged as promising candidates to treat and prevent infection by various pathogens. This review focuses on the potential of host-targeting agents (HTAs) as novel antivirals to treat and cure HBV infection. These include HTAs that inhibit de novo and re-infection, synthesis and spread of cccDNA as well as development of immune-based approaches eliminating or curing infected hepatocytes, including the eradication of viral cccDNA.

Tryptophan Catabolism in Chronic Viral Infections: Handling Uninvited Guests

l-Tryptophan (l-Trp) is an essential amino acid that possesses diverse metabolic, neurological, and immunological roles spanning from the synthesis of proteins, neurotransmitter serotonin, and neurohormone melatonin, to its degradation into immunosuppressive catabolites by indoleamine-2, 3-dioxygenase (IDO) in the kynurenine pathway (KP). Trp catabolites, by activating aryl hydrocarbon receptor (AhR), play an important role in antimicrobial defense and immune regulation. IDO/AhR acts as a double-edged sword by both depleting l-Trp to starve the invaders and by contributing to the state of immunosuppression with microorganisms that were not cleared during acute infection. Pathogens experiencing Trp deprivation by IDO-mediated degradation include certain bacteria, parasites, and less likely viruses. However, chronic viral infections highjack the host immune response to create a state of disease tolerance via kynurenine catabolites. This review covers the latest data involving chronic viral infections such as human immunodeficiency virus (HIV), hepatitis B virus (HBV), hepatitis C virus (HCV), herpes, and cytomegalovirus (CMV) and their cellular interplay with Trp catabolites. Strategies developed by viruses to escape immune control also represent new avenues for therapeutic interventions based on Trp metabolism.

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.

Inhibition of indoleamine 2,3-dioxygenase activity enhances the anti-tumour effects of a Toll-like receptor 7 agonist in an established cancer model

Toll-like receptor (TLR) agonists have been shown to have anti-tumour activity in basic research and clinical studies. However, TLR agonist monotherapy does not sufficiently eliminate tumours. Activation of the innate immune response by TLR agonists is effective at driving adaptive immunity via interleukin-12 (IL-12) or IL-1, but is counteracted by the simultaneous induction of immunosuppressive cytokines and other molecules, including IL-10, transforming growth factor-β, and indoleamine 2,3-dioxygenase (IDO). In the present study, we evaluated the anti-cancer effect of the TLR7 agonist, imiquimod (IMQ), in the absence of IDO activity. The administration of IMQ in IDO knockout (KO) mice inoculated with tumour cells significantly suppressed tumour progression compared with that in wild-type (WT) mice, and improved the survival rate. Moreover, injection with IMQ enhanced the tumour antigen-specific T helper type 1 response in IDO-KO mice with tumours. Combination therapy with IMQ and an IDO inhibitor also significantly inhibited tumour growth. Our results indicated that the enhancement of IDO expression with TLR agonists in cancer treatment might impair host anti-tumour immunity while the inhibition of IDO could enhance the therapeutic efficacy of TLR agonists via the increase of T helper type 1 immune response.

Tumor indoleamine 2,3-dioxygenase (IDO) inhibits CD19-CAR T cells and is downregulated by lymphodepleting drugs

Although T cells expressing CD19-specific chimeric antigen receptors (CARs) are a promising new therapy for B-cell malignancies, objective responses are observed at lower frequencies in patients with lymphoma than in those with acute B-cell leukemia. We postulated that the tumor microenvironment suppresses CAR-expressing T cells (CARTs) through the activity of indoleamine 2,3-dioxygenase (IDO), an intracellular enzyme that converts tryptophan into metabolites that inhibit T -: cell activity. To investigate the effects of tumor IDO on CD19-CART therapy, we used a xenograft lymphoma model expressing IDO as a transgene. CD19-CARTs inhibited IDO-negative tumor growth but had no effect on IDO-positive tumors. An IDO inhibitor (1-methyl-tryptophan) restored IDO-positive tumor control. Moreover, tryptophan metabolites inhibited interleukin (IL)-2-, IL-7-, and IL-15-dependent expansion of CARTs; diminished their proliferation, cytotoxicity, and cytokine secretion in vitro in response to CD19 recognition; and increased their apoptosis. Inhibition of CD19-CARTs was not mitigated by the incorporation of costimulatory domains, such as 4-1BB, into the CD19-CAR. Finally, we found that fludarabine and cyclophosphamide, frequently used before CART administration, downregulated IDO expression in lymphoma cells and improved the antitumor activity of CD19-CART in vivo. Because tumor IDO inhibits CD19-CARTs, antagonizing this enzyme may benefit CD19-CART therapy.

Inhibition of induced nitric oxide synthase enhances the anti-tumor effects on cancer immunotherapy using TLR7 agonist in mice

Toll-like receptor (TLR) agonists have been shown to have anti-tumor activity in basic research and clinical studies. However, TLR agonist monotherapy in cancer treatment dose not sufficiently eliminate tumors. Activation of the innate immune response by TLR agonists and other pathogen-associated molecular patterns is effective for driving adaptive immunity via interleukin (IL)-12 or IL-1, but is counteracted by the simultaneous induction of immunosuppressive cytokines and other molecules, including IL-10, tumor growth factor-β, and induced nitric oxide synthase (iNOS). In the present study, we evaluated the anticancer effect of the TLR7 agonist, imiquimod (IMQ), in the absence of iNOS. The administration of IMQ in iNOS-knockout (KO) mice implanted with tumor cells significantly suppressed tumor progression as compared to that in wild-type mice and improved the survival rate. Moreover, injection with IMQ enhanced the tumor antigen-specific Th1 response in iNOS-KO mice with tumors. The enhancement of the antigen-specific Th1 response was associated with an increase in IL-2 and IL-12b expressions in the tumor-draining lymph nodes. Combination therapy with IMQ and an iNOS inhibitor also significantly inhibited tumor growth in the established tumor model. Finally, our results indicated that the enhancement of iNOS expression through the administration with TLR agonists impairs host anti-tumor immunity, while the inhibition of iNOS could enhance the therapeutic efficacy of TLR agonists via the increase in Th1 immune response.

New Insights into IDO Biology in Bacterial and Viral Infections

Initially, indoleamine-2,3-dioxygenase (IDO) has been introduced as a bactericidal effector mechanism and has been linked to T-cell immunosuppression and tolerance. In recent years, evidence has been accumulated that IDO also plays an important role during viral infections including HIV, influenza, and hepatitis B and C. Moreover, novel aspects about the role of IDO in bacterial infections and sepsis have been revealed. Here, we review these recent findings highlighting the central role of IDO and tryptophan metabolism in many major human infections. Moreover, we also shed light on issues concerning human-specific and mouse-specific host–pathogen interactions that need to be considered when studying the biology of IDO in the context of infections.