Effect of indoleamine dioxygenase-1 deficiency and kynurenine pathway inhibition on murine cerebral malaria

Indoleamine and tryptophan 2,3-dioxygenases as important future therapeutic targets

Conversion of tryptophan to N-formylkynurenine is the first and rate-limiting step of the tryptophan metabolic pathway (i.e., the kynurenine pathway). This conversion is catalyzed by three enzyme isoforms: indoleamine 2,3-dioxygenase 1 (IDO1), indoleamine 2,3-dioxygenase 2 (IDO2), and tryptophan 2,3-dioxygenase (TDO). As this pathway generates numerous metabolites that are involved in various pathological conditions, IDOs and TDO represent important targets for therapeutic intervention. This pathway has especially drawn attention due to its importance in tumor resistance. Over the last decade, a large number of IDO and TDO inhibitors have been developed, many of which have entered clinical trials. Here, detailed structural comparisons of these three enzymes (with emphasis on their active sites), their involvement in cellular signaling, and their role(s) in pathological conditions are discussed. Furthermore, the most important recent inhibitors described in papers and patents and involved in clinical trials are reviewed, with a focus on both selective and multiple inhibitors. A short overview of the biochemical and cellular assays used for inhibitory potency evaluation is also presented. This review summarizes recent advances on IDO and TDO as potential drug targets, and provides the key features and perspectives for further research and development of potent inhibitors of the kynurenine pathway.

Kynurenine elevation correlates with T regulatory cells increase in acute Plasmodium vivax infection: A pilot study

BACKGROUND

Disease-tolerance mechanisms limit infection severity by preventing tissue damage; however, the underlying mechanisms in human malaria are still unclear. Tryptophan (TRP), an essential amino acid, is catabolized into tolerogenic metabolites, kynurenines (KYN), by indoleamine 2,3-dioxygenase 1 (IDO1), which can induce Foxp3+ T regulatory cells (Tregs). In this study, we evaluated the relationship of these metabolites with Treg-mediated tolerance induction in acute malaria infections.

METHODS

We performed a cross-sectional study that evaluated asymptomatic, symptomatic malaria patients and endemic control patient groups. We assessed plasmatic concentration of cytokines by ELISA. Plasmatic TRP and KYN levels were measured by HPLC. Peripheral T regulatory cells were measured and phenotyped by flow cytometry.

RESULTS

The KYN/TRP ratio was significantly elevated in asymptomatic and symptomatic Plasmodium infection, compared to healthy controls. Also, Th1 and Th2 cytokines were elevated in the acute phase of malaria disease. IFN-γ increase in acute phase was positively correlated with the KYN/TRP ratio and KYN elevation was positively correlated with the increase of peripheral FoxP3+ T regulatory cells.

CONCLUSIONS

Additional studies are needed not only to identify innate mechanisms that increase tryptophan catabolism but also the role of Tregs in controlling malaria-induced pathology and malaria tolerance by the host.

Malaria systems immunology: Plasmodium vivax induces tolerance during primary infection through dysregulation of neutrophils and dendritic cells

OBJECTIVES

To dissect the transcriptional networks underpinning immune cells responses during primary Plasmodium vivax infection of healthy human adults.

METHODS

We conducted network co-expression analysis of next-generation RNA sequencing data from whole blood from P. vivax and P. falciparum controlled human malaria infection (CHMI) of healthy naïve and malaria-exposed volunteers. Single cell transcription signatures were used to deconvolute the bulk RNA-Seq data into cell-specific signals.

RESULTS

Initial exposure to P. vivax induced activation of innate immunity, including efficient antigen presentation and complement activation. However, this effect was accompanied by strong immunosuppression mediated by dendritic cells via the induction of Indoleamine 2,3-Dioxygenase 1(IDO1) and Lymphocyte Activation Gene 3 (LAG3). Additionally, P. vivax induced depletion of neutrophil populations associated with down regulation of 3G-protein coupled receptors, CRXCR1, CXCR2 and CSF3R. Accordingly, in malaria-exposed volunteers the inflammatory response was attenuated, with a decreased class II antigen presentation in dendritic cells. While the immunosuppressive signalling was maintained between plasmodium species, response to P. falciparum was significantly more immunogenic.

CONCLUSIONS

In silico analyses suggest that primary infection with P. vivax induces potent immunosuppression mediated by dendritic cells, conditioning subsequent anti-malarial immune responses. Targeting immune evasion mechanisms could be an effective alternative for improving vaccine efficacy.

Lipopolysaccharide shock reveals the immune function of indoleamine 2,3-dioxygenase 2 through the regulation of IL-6/stat3 signalling

Indoleamine 2,3-dioxygenase 2 (Ido2) is a recently identified catalytic enzyme in the tryptophan-kynurenine pathway that is expressed primarily in monocytes and dendritic cells. To elucidate the biological role of Ido2 in immune function, we introduced lipopolysaccharide (LPS) endotoxin shock to Ido2 knockout (Ido2 KO) mice, which led to higher mortality than that in the wild type (WT) mice. LPS-treated Ido2 KO mice had increased production of inflammatory cytokines (including interleukin-6; IL-6) in serum and signal transducer and activator of transcription 3 (stat3) phosphorylation in the spleen. Moreover, the peritoneal macrophages of LPS-treated Ido2 KO mice produced more cytokines than did the WT mice. By contrast, the overexpression of Ido2 in the murine macrophage cell line (RAW) suppressed cytokine production and decreased stat3 expression. Finally, RAW cells overexpressing Ido2 did not alter nuclear factor κB (NF-κB) or stat1 expression, but IL-6 and stat3 expression decreased relative to the control cell line. These results reveal that Ido2 modulates IL-6/stat3 signalling and is induced by LPS, providing novel options for the treatment of immune disorders.

Major Developments in the Design of Inhibitors along the Kynurenine Pathway

Disrupted kynurenine pathway (KP) metabolism has been implicated in the progression of neurodegenerative disease, psychiatric disorders and cancer. Modulation of enzyme activity along this pathway may therefore offer potential new therapeutic strategies for these conditions. Considering their prominent positions in the KP, the enzymes indoleamine 2,3-dioxygenase, kynurenine 3-monooxygenase and kynurenine aminotransferase, appear the most attractive targets. Already, increasing interest in this pathway has led to the identification of a number of potent and selective enzyme inhibitors with promising pre-clinical data and the elucidation of several enzyme crystal structures provides scope to rationalize the molecular mechanisms of inhibitor activity. The field seems poised to yield one or more inhibitors that should find clinical utility.

Cerebrospinal fluid kynurenine and kynurenic acid concentrations are associated with coma duration and long-term neurocognitive impairment in Ugandan children with cerebral malaria

Background

One-fourth of children with cerebral malaria (CM) retain cognitive sequelae up to 2 years after acute disease. The kynurenine pathway of the brain, forming neuroactive metabolites, e.g. the NMDA-receptor antagonist kynurenic acid (KYNA), has been implicated in long-term cognitive dysfunction in other CNS infections. In the present study, the association between the kynurenine pathway and neurologic/cognitive complications in children with CM was investigated.

Methods

Cerebrospinal fluid (CSF) concentrations of KYNA and its precursor kynurenine in 69 Ugandan children admitted for CM to Mulago Hospital, Kampala, Uganda, between 2008 and 2013 were assessed. CSF kynurenine and KYNA were compared to CSF cytokine levels, acute and long-term neurologic complications, and long-term cognitive impairments. CSF kynurenine and KYNA from eight Swedish children without neurological or infectious disease admitted to Astrid Lindgren’s Children’s Hospital were quantified and used for comparison.

Results

Children with CM had significantly higher CSF concentration of kynurenine and KYNA than Swedish children (P < 0.0001 for both), and CSF kynurenine and KYNA were positively correlated. In children with CM, CSF kynurenine and KYNA concentrations were associated with coma duration in children of all ages (P = 0.003 and 0.04, respectively), and CSF kynurenine concentrations were associated with worse overall cognition (P = 0.056) and attention (P = 0.003) at 12-month follow-up in children ≥5 years old.

Conclusions

CSF KYNA and kynurenine are elevated in children with CM, indicating an inhibition of glutamatergic and cholinergic signaling. This inhibition may lead acutely to prolonged coma and long-term to impairment of attention and cognition.

Benzo[b]quinolizinium Derivatives Have a Strong Antimalarial Activity and Inhibit Indoleamine Dioxygenase

The heme-containing enzymes indoleamine 2,3-dioxygenase-1 (IDO-1) and IDO-2 catalyze the conversion of the essential amino acid tryptophan into kynurenine. Metabolites of the kynurenine pathway and IDO itself are involved in immunity and the pathology of several diseases, having either immunoregulatory or antimicrobial effects. IDO-1 plays a central role in the pathogenesis of cerebral malaria, which is the most severe and often fatal neurological complication of infection with Plasmodium falciparum. Mouse models are usually used to study the underlying pathophysiology. In this study, we screened a natural compound library against mouse IDO-1 and identified 8-aminobenzo[b]quinolizinium (compound 2c) to be an inhibitor of IDO-1 with potency at nanomolar concentrations (50% inhibitory concentration, 164 nM). Twenty-one structurally modified derivatives of compound 2c were synthesized for structure-activity relationship analyses. The compounds were found to be selective for IDO-1 over IDO-2. We therefore compared the roles of prominent amino acids in the catalytic mechanisms of the two isoenzymes via homology modeling, site-directed mutagenesis, and kinetic analyses. Notably, methionine 385 of IDO-2 was identified to interfere with the entrance of l-tryptophan to the active site of the enzyme, which explains the selectivity of the inhibitors. Most interestingly, several benzo[b]quinolizinium derivatives (6 compounds with 50% effective concentration values between 2.1 and 6.7 nM) were found to be highly effective against P. falciparum 3D7 blood stages in cell culture with a mechanism independent of IDO-1 inhibition. We believe that the class of compounds presented here has unique characteristics; it combines the inhibition of mammalian IDO-1 with strong antiparasitic activity, two features that offer potential for drug development.

The Janus-faced nature of IDO1 in infectious diseases: challenges and therapeutic opportunities

Inhibition of IDO1 is a strategy pursued to develop novel therapeutic treatments for cancer. Recent years have witnessed growing evidence that the enzyme plays a pivotal role in viral, bacterial and fungal infections. These studies have underscored the Janus-faced nature of IDO1 in the regulation of host-pathogen interactions and commensalism. Starting with an outlook on the advances in the structural features of IDO1, herein we report recent findings that pinpoint the involvement of IDO1 in infectious diseases. Then, we present an overview of IDO1 inhibitors that have been enrolled in clinical trials as well as other distinct modulators of the enzyme that may enable further investigations of IDO1 and its role in infectious disease.

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.

Cerebral malaria: gamma-interferon redux

There are two theories that seek to explain the pathogenesis of cerebral malaria, the mechanical obstruction hypothesis and the immunopathology hypothesis. Evidence consistent with both ideas has accumulated from studies of the human disease and experimental models. Thus, some combination of these concepts seems necessary to explain the very complex pattern of changes seen in cerebral malaria. The interactions between malaria parasites, erythrocytes, the cerebral microvascular endothelium, brain parenchymal cells, platelets and microparticles need to be considered. One factor that seems able to knit together much of this complexity is the cytokine interferon-gamma (IFN-γ). In this review we consider findings from the clinical disease, in vitro models and the murine counterpart of human cerebral malaria in order to evaluate the roles played by IFN-γ in the pathogenesis of this often fatal and debilitating condition.

Persistent infectious diseases say - IDO. Role of indoleamine-2,3-dioxygenase in disease pathogenesis and implications for therapy

Indoleamine-2,3-dioxygenase (IDO) is an enzyme that catabolises tryptophan - an essential amino acid critical for T cell proliferation. Initially recognized as a first line of host defense against infectious pathogens, IDO has been subsequently identified as an important immune-regulator inhibiting T-cell responses and promoting immune tolerance. Research over the past few years has demonstrated a crucial role for IDO in the pathogenesis of persistent infections that place an enormous burden on public health. In this review, we summarize current knowledge about IDO's role in causing pathogen persistence and progression to clinical disease. We conclude with a perspective on the potential benefits and risks of therapeutic IDO manipulation.

Elevated cerebrospinal fluid kynurenic acid levels in patients with tick-borne encephalitis

BACKGROUND

Kynurenic acid (KYNA) is a neuroactive metabolite of tryptophan that is thought to regulate cognitive functions. Previous studies have shown that levels of KYNA increase during virus infection and that this metabolite interacts with the immune system.

OBJECTIVE

The aim of the study was to investigate whether patients with tick-borne encephalitis (TBE), a viral infectious disease associated with long-term cognitive impairment, have increased levels of KYNA in the cerebrospinal fluid (CSF).

METHODS

CSF KYNA was analysed using high-performance liquid chromatography in 108 patients with TBE and 52 age-matched controls. Patients were classified according to the severity of TBE: mild (47%), moderate (44%) or severe (9%).

RESULTS

Concentrations of CSF KYNA were considerably higher in patients with TBE (5.3 nmol L(-1) ) than in control subjects (0.99 nmol L(-1) ). KYNA concentration in the CSF varied greatly amongst individuals with TBE and increased (P < 0.05) with the severity of disease.

CONCLUSIONS

This is the first study to demonstrate increased levels of CSF KYNA in patients with TBE. The importance of brain KYNA in both immune modulation and neurotransmission raises the possibility that abnormal levels of the compound in TBE might play a part in the pathophysiology of the disease. A detailed knowledge of endogenous brain KYNA during the course of CNS infection might yield further insights into the neuroimmunological role of the compound and may also provide new pharmacological approaches for the treatment of cognitive symptoms.

Kynurenines in the mammalian brain: when physiology meets pathology

The essential amino acid tryptophan is not only a precursor of serotonin but is also degraded to several other neuroactive compounds, including kynurenic acid, 3-hydroxykynurenine and quinolinic acid. The synthesis of these metabolites is regulated by an enzymatic cascade, known as the kynurenine pathway, that is tightly controlled by the immune system. Dysregulation of this pathway, resulting in hyper-or hypofunction of active metabolites, is associated with neurodegenerative and other neurological disorders, as well as with psychiatric diseases such as depression and schizophrenia. With recently developed pharmacological agents, it is now possible to restore metabolic equilibrium and envisage novel therapeutic interventions.

Apicomplexan parasite, Eimeria falciformis, co-opts host tryptophan catabolism for life cycle progression in mouse

The obligate intracellular apicomplexan parasites, e.g. Toxoplasma gondii and Plasmodium species, induce an IFNγ-driven induction of host indoleamine 2,3-dioxygenase (IDO), the first and rate-limiting enzyme of tryptophan catabolism in the kynurenine pathway. Induction of IDO1 supposedly depletes cellular levels of tryptophan in host cells, which is proposed to inhibit the in vitro growth of auxotrophic pathogens. In vivo function of IDO during infections, however, is not clear, let alone controversial. We show that Eimeria falciformis, an apicomplexan parasite infecting the mouse caecum, induces IDO1 in the epithelial cells of the organ, and the enzyme expression coincides with the parasite development. The absence or inhibition of IDO1/2 and of two downstream enzymes in infected animals is detrimental to the Eimeria growth. The reduced parasite yield is not due to a lack of an immunosuppressive effect of IDO1 in the parasitized IDO1(-/-) or inhibitor-treated mice because they did not show an accentuated Th1 and IFNγ response. Noticeably, the parasite development is entirely rescued by xanthurenic acid, a by-product of tryptophan catabolism inducing exflagellation in male gametes of Plasmodium in the mosquito mid-gut. Our data demonstrate a conceptual subversion of the host defense (IFNγ, IDO) by an intracellular pathogen for progression of its natural life cycle. Besides, we show utility of E. falciformis, a monoxenous parasite of a well appreciated host, i.e. mouse, to identify in vivo factors underlying the parasite-host interactions.

Inhibition of the kynurenine-NAD+ pathway leads to energy failure and exacerbates apoptosis in pneumococcal meningitis

Pneumococcal meningitis causes neurological sequelae, including learning and memory deficits in up to half of the survivors. In both humans and in animal models of the disease, there is apoptotic cell death in the hippocampus, a brain region involved in learning and memory function. We previously demonstrated that in an infant rat model of pneumococcal meningitis, there is activation of the kynurenine (KYN) pathway in the hippocampus, and that there was a positive correlation between the concentration of 3-hydroxykynurenine and the extent of hippocampal apoptosis. To clarify the role of the KYN pathway in the pathogenesis of hippocampal apoptosis in pneumococcal meningitis, we specifically inhibited 2 key enzymes of the KYN pathway and assessed hippocampal apoptosis, KYN pathway metabolites, and nicotinamide adenine dinucleotide (NAD) concentrations by high-performance liquid chromatography. Pharmacological inhibition of kynurenine 3-hydroxylase and kynureninase led to decreased cellular NAD levels and increased apoptosis in the hippocampus. The cerebrospinal fluid levels of tumor necrosis factor and interleukin-1α and -β were not affected. Our data suggest that activation of the KYN pathway in pneumococcal meningitis is neuroprotective by compensating for an increased NAD demand caused by infection and inflammation;this mechanism may prevent energy failure and apoptosis in the hippocampus.

Kynurenine is an endothelium-derived relaxing factor produced during inflammation

Control of blood vessel tone is central to vascular homeostasis. Here we show that metabolism of tryptophan to kynurenine by indoleamine 2,3-dioxygenase (Ido) expressed in endothelial cells contributes to arterial vessel relaxation and the control of blood pressure. Infection of mice with malarial parasites (Plasmodium berghei) or induction of endotoxemia in mice led to endothelial expression of Ido, decreased plasma tryptophan concentration, increased kynurenine concentration and hypotension. Pharmacological inhibition of Ido increased blood pressure in systemically inflamed mice but not in mice deficient in Ido or interferon-gamma, which is required for Ido induction. Both tryptophan and kynurenine dilated preconstricted porcine coronary arteries; the dilating effect of tryptophan required the presence of active Ido and an intact endothelium, whereas the effect of kynurenine was endothelium independent. The arterial relaxation induced by kynurenine was mediated by activation of the adenylate and soluble guanylate cyclase pathways. Kynurenine administration decreased blood pressure in a dose-dependent manner in spontaneously hypertensive rats. Our results identify tryptophan metabolism by Ido as a new pathway contributing to the regulation of vascular tone.

The crystal structure of human alpha-amino-beta-carboxymuconate-epsilon-semialdehyde decarboxylase in complex with 1,3-dihydroxyacetonephosphate suggests a regulatory link between NAD synthesis and glycolysis

The enzyme alpha-amino-beta-carboxymuconate-epsilon-semialdehyde decarboxylase (ACMSD) is a zinc-dependent amidohydrolase that participates in picolinic acid (PA), quinolinic acid (QA) and NAD homeostasis. Indeed, the enzyme stands at a branch point of the tryptophan to NAD pathway, and determines the final fate of the amino acid, i.e. transformation into PA, complete oxidation through the citric acid cycle, or conversion into NAD through QA synthesis. Both PA and QA are key players in a number of physiological and pathological conditions, mainly affecting the central nervous system. As their relative concentrations must be tightly controlled, modulation of ACMSD activity appears to be a promising prospect for the treatment of neurological disorders, including cerebral malaria. Here we report the 2.0 A resolution crystal structure of human ACMSD in complex with the glycolytic intermediate 1,3-dihydroxyacetonephosphate (DHAP), refined to an R-factor of 0.19. DHAP, which we discovered to be a potent enzyme inhibitor, resides in the ligand binding pocket with its phosphate moiety contacting the catalytically essential zinc ion through mediation of a solvent molecule. Arg47, Asp291 and Trp191 appear to be the key residues for DHAP recognition in human ACMSD. Ligand binding induces a significant conformational change affecting a strictly conserved Trp-Met couple, and we propose that these residues are involved in controlling ligand admission into ACMSD. Our data may be used for the design of inhibitors with potential medical interest, and suggest a regulatory link between de novo NAD biosynthesis and glycolysis.