Indoleamine 2, 3-Dioxygenase-Mediated Tryptophan Catabolism: A Leading Star or Supporting Act in the Tuberculosis and HIV Pas-de-Deux?

Tryptophan catabolism via the kynurenine pathway regulates infection and inflammation: from mechanisms to biomarkers and therapies

BACKGROUND

L-Tryptophan (L-Trp), an essential amino acid, is the only amino acid whose level is regulated specifically by immune signals. Most proportions of Trp are catabolized via the kynurenine (Kyn) pathway (KP) which has evolved to align the food availability and environmental stimulation with the host pathophysiology and behavior. Especially, the KP plays an indispensable role in balancing the immune activation and tolerance in response to pathogens.

SCOPE OF REVIEW

In this review, we elucidate the underlying immunological regulatory network of Trp and its KP-dependent catabolites in the pathophysiological conditions by participating in multiple signaling pathways. Furthermore, the KP-based regulatory roles, biomarkers, and therapeutic strategies in pathologically immune disorders are summarized covering from acute to chronic infection and inflammation.

MAJOR CONCLUSIONS

The immunosuppressive effects dominate the functions of KP induced-Trp depletion and KP-produced metabolites during infection and inflammation. However, the extending minor branches from the KP are not confined to the immune tolerance, instead they go forward to various functions according to the specific condition. Nevertheless, persistent efforts should be made before the clinical use of KP-based strategies to monitor and cure infectious and inflammatory diseases.

Derangements of immunological proteins in HIV-associated diffuse large B-cell lymphoma: the frequency and prognostic impact

Introduction

Diffuse large B-cell lymphoma (DLBCL) is an aggressive malignancy of B-cells frequently encountered among people living with HIV. Immunological abnormalities are common in immunocompetent individuals with DLBCL, and are often associated with poorer outcomes. Currently, data on derangements of immunological proteins, such as cytokines and acute phase reactants, and their impact on outcomes in HIV-associated DLBCL (HIV-DLBCL) is lacking. This study assessed the levels and prognostic relevance of interleukin (IL)-6, IL-10 and Transforming Growth Factor Beta (TGFβ), the acute phase proteins C-reactive protein (CRP) and ferritin; serum free light chains (SFLC) (elevation of which reflects a prolonged pro-inflammatory state); and the activity of the immunosuppressive enzyme Indoleamine 2,3-dioxygenase (IDO)in South African patients with DLBCL.

Methods

Seventy-six patients with incident DLBCL were enrolled, and peripheral blood IL-6, IL-10, TGFβ, SFLC and IDO-activity measured in selected patients. Additional clinical and laboratory findings (including ferritin and CRP) were recorded from the hospital records.

Results

Sixty-one (80.3%) of the included patients were people living with HIV (median CD4-count = 148 cells/ul), and survival rates were poor (12-month survival rate 30.0%). The majority of the immunological proteins, except for TGFβ and ferritin, were significantly higher among the people living with HIV. Elevation of IL-6, SFLC and IDO-activity were not associated with survival in HIV-DLBCL, while raised IL-10, CRP, ferritin and TGFβ were. On multivariate analysis, immunological proteins associated with survival independently from the International Prognostic Index (IPI) included TGFβ, ferritin and IL-10.

Conclusion

Derangements of immunological proteins are common in HIV-DLBCL, and have a differential association with survival compared to that reported elsewhere. Elevation of TGFβ, IL-10 and ferritin were associated with survival independently from the IPI. In view of the poor survival rates in this cohort, investigation of the directed targeting of these cytokines would be of interest in our setting.

The Kynurenine/Tryptophan Ratio as a Promising Metabolomic Biomarker for Diagnosing the Spectrum of Tuberculosis Infection and Disease

The metabolic system and immunology used to be seen as distinct fields of study. Recent developments in our understanding of how the immune system operates in health and disease have connected these fields to complex systems. An effective technique for identifying probable abnormalities of metabolic homeostasis brought on by disease is metabolomics, which is defined as the thorough study of small molecule metabolic intermediates within a biological system that collectively make up the metabolome. A prognostic metabolic biomarker with adequate prognostic accuracy for tuberculosis progression has recently been created. The rate-limiting host enzyme for the conversion of tryptophan to kynurenine, indoleamine 2,3-dioxygenase (IDO), is greatly elevated in the lungs of tuberculosis disease patients. Targeted study on tryptophan in tuberculosis disease indicates that such decreases may also resembled this upregulation. Although tuberculosis diagnosis has improved with the use of interferon release assay and tuberculosis nucleic acid amplification, tuberculosis control is made difficult by the lack of a biomarker to diagnose active tuberculosis disease. We hope that the reader of this work can develop an understanding of the advantages of metabolomics testing, particularly as a sort of testing that can be used for both diagnosing and monitoring a patient's response to treatment for tuberculosis.

Leishmania major centrin knock-out parasites reprogram tryptophan metabolism to induce a pro-inflammatory response

Leishmaniasis is a parasitic disease that is prevalent in 90 countries, and yet no licensed human vaccine exists against it. Toward control of leishmaniasis, we have developed Leishmania major centrin gene deletion mutant strains (LmCen-/-) as a live attenuated vaccine, which induces a strong IFN-γ-mediated protection to the host. However, the immune mechanisms of such protection remain to be understood. Metabolomic reprogramming of the host cells following Leishmania infection has been shown to play a critical role in pathogenicity and shaping the immune response following infection. Here, we applied untargeted mass spectrometric analysis to study the metabolic changes induced by infection with LmCen-/- and compared those with virulent L. major parasite infection to identify the immune mechanism of protection. Our data show that immunization with LmCen-/- parasites, in contrast to virulent L. major infection promotes a pro-inflammatory response by utilizing tryptophan to produce melatonin and downregulate anti-inflammatory kynurenine-AhR and FICZ-AhR signaling.

Tryptophan metabolism determines outcome in tuberculous meningitis: a targeted metabolomic analysis

Background

Cellular metabolism is critical for the host immune function against pathogens, and metabolomic analysis may help understand the characteristic immunopathology of tuberculosis. We performed targeted metabolomic analyses in a large cohort of patients with tuberculous meningitis (TBM), the most severe manifestation of tuberculosis, focusing on tryptophan metabolism.

Methods

We studied 1069 Indonesian and Vietnamese adults with TBM (26.6% HIV-positive), 54 non-infectious controls, 50 with bacterial meningitis, and 60 with cryptococcal meningitis. Tryptophan and downstream metabolites were measured in cerebrospinal fluid (CSF) and plasma using targeted liquid chromatography-mass spectrometry. Individual metabolite levels were associated with survival, clinical parameters, CSF bacterial load and 92 CSF inflammatory proteins.

Results

CSF tryptophan was associated with 60-day mortality from TBM (hazard ratio [HR] = 1.16, 95% confidence interval [CI] = 1.10-1.24, for each doubling in CSF tryptophan) both in HIV-negative and -positive patients. CSF tryptophan concentrations did not correlate with CSF bacterial load nor CSF inflammation but were negatively correlated with CSF interferon-gamma concentrations. Unlike tryptophan, CSF concentrations of an intercorrelating cluster of downstream kynurenine metabolites did not predict mortality. These CSF kynurenine metabolites did however correlate with CSF inflammation and markers of blood-CSF leakage, and plasma kynurenine predicted death (HR 1.54, 95% CI = 1.22-1.93). These findings were mostly specific for TBM, although high CSF tryptophan was also associated with mortality from cryptococcal meningitis.

Conclusions

TBM patients with a high baseline CSF tryptophan or high systemic (plasma) kynurenine are at increased risk of death. These findings may reveal new targets for host-directed therapy.

Funding

This study was supported by National Institutes of Health (R01AI145781) and the Wellcome Trust (110179/Z/15/Z and 206724/Z/17/Z).

Association between pre-eclampsia and HIV: a case-control study in urban South Africa

BACKGROUND

Preeclampsia is a considerable cause of maternal and infant morbidity and mortality. Although its etiology is unknown, preeclampsia has been described as a state of exaggerated maternal inflammatory response. Therefore, it has been hypothesized that preeclampsia would occur less commonly in states of immune deficiency.

OBJECTIVE

This study aimed to compare the prevalence of treated and untreated HIV infections among preeclamptic cases and controls, determine infant outcomes, and evaluate the association between HIV and preeclampsia after adjusting for known predictor variables, including maternal age, gravidity, body mass index, and smoking.

STUDY DESIGN

This case-control study investigated the association between preeclampsia and HIV infection using secondary data from an unrelated study. We defined preeclamptic cases as pregnant women who were normotensive until 20 weeks of gestation and thereafter had at least 1 high blood pressure measurement either before or at delivery and proteinuria, defined as protein excretion of ≥300 mg within 24 hours or >2 protein on dipstick urinalysis. The prevalence of HIV infection was compared between cases and controls. Multivariate logistic regression analysis was used to assess the association between preeclampsia and potential confounding variables and reported using odds ratios and 95% confidence intervals.

RESULTS

There were 571 cases with preeclampsia and 596 normotensive controls included in this study. The median age was 27 years for cases and 26 years for controls (P=.008). Most participants (69%) had ≥2 previous pregnancies with no difference between the cases and controls (P=.176). Overall, 43% of the participants were obese, with a mean body mass index of 29 (interquartile range, 24.5–34.2), with higher proportions of women who were overweight and obese in the group with preeclampsia (P=.031). The prevalence of HIV was significantly lower in cases than in controls (24% vs 30%, respectively; P=.014). Compared with 16% of infants born preterm to normotensive controls, 48% of infants were born preterm born to women with preeclampsia (P<.001). Compared with 14% of infants born with low birthweight to normotensive controls, 53% of infants were born with low birthweight to women with preeclampsia (P<.0001). Untreated HIV infection was negatively associated with preeclampsia (unadjusted odds ratio, 0.330; 95% confidence interval, 0.197–0.552; P<.0001), whereas factors associated with preeclampsia were advanced maternal age (odds ratio, 1.673; 95% confidence interval, 1.209–2.316; P=.002) and obesity (odds ratio, 1.611; 95% confidence interval, 1.023–2.537; P=.040). After adjusting for maternal age, gravidity, smoking, and body mass index in the multivariate regression, only obesity remained significantly associated with preeclampsia (adjusted odds ratio, 1.624; 95% confidence interval, 1.024–2.575; P=.039).

CONCLUSION

Before the large-scale rollout of antiretroviral therapy in a setting with a high burden of HIV and preeclampsia, untreated HIV infection was found to have a protective effect against preeclampsia. The protective effect against preeclampsia was not apparent for HIV infection treated with antiretroviral therapy.

IDO activity forecasts obesity in males and premenopausal females in a 10-year follow-up study:The Cardiovascular Risk in Young Finns Study

BACKGROUND AND AIMS

Indoleamine 2,3-dioxygenase (IDO) is an intracellular enzyme associated with artery wall inflammation. Previous studies have verified correlation between IDO activity and early signs of atherosclerosis especially in females. We aimed to elucidate the relationship between an estimate of IDO activity and atherosclerotic risk factors related to non-alchohol-fatty liver (NAFLD) in a 6- and 10-year follow-up.

METHODS

Estimates of IDO activity along with complete risk factor data were measured from females (n = 506; age 24-39) and males (n = 421; age 24-39) in 2001. Risk factor measurements were conducted again in 2007 and 2011. Statistical examinations were carried out by Pearson correlation and risk ratio analysis.

RESULTS

In females, age-adjusted IDO correlated with body mass index (BMI) (p = 0.0008), waist (p = 0.0009), C-reactive protein (CRP) (p = 0.0014) and logarithmically modified triglycerides (p = 0.0488) in 2007. Correlation remained significant with BMI (p = 0.0007) and waist (p = 0.0063) in 2011. In males, age-adjusted IDO correlated with waist (p = 0.0367) and high-density lipoprotein cholesterol (HDL-C) (p = 0.0489) in 2007. Correlation remained significant with HDL-C (p = 0.0348) in 2011. In risk ratio analysis, relationship between IDO and obesity was confirmed in females after 10 years (RR = 1.026, p = 0.0147, 95% CI) and in males after 6 and 10 years (RR = 1.019, p = 0.0091, 95% CI and RR = 1.015, p = 0.0404, 95% CI, respectively) when the data was adjusted for age and BMI.

CONCLUSIONS

IDO activity correlated with obesity and factors related to NAFLD, namely obesity of visceral type, hypertriglyceridemia and CRP (in females), well-characterized risk factors for diabetes and atherosclerosis in 6- and 10-year follow-up in males and premenopausal females.

Plasma Kynurenine-to-Tryptophan Ratio, a Highly Sensitive Blood-Based Diagnostic Tool for Tuberculosis in Pregnant Women Living With Human Immunodeficiency Virus (HIV)

BACKGROUND

For pregnant women living with human immunodeficiency virus (HIV), concurrent active tuberculosis (TB) disease increases the risk of maternal mortality and poor pregnancy outcomes. Plasma indoleamine 2,3-dioxygenase (IDO) activity measured by kynurenine-to-tryptophan (K/T) ratio has been proposed as a blood-based TB biomarker. We investigated whether plasma K/T ratio could be used to diagnose active TB among pregnant women with HIV.

METHODS

Using an enzyme-linked immunosorbent assay (ELISA), we measured K/T ratio in 72 pregnant women with and active TB and compared them to 117 pregnant women with HIB but without TB, matched by age and gestational age.

RESULTS

Plasma K/T ratio was significantly elevated during pregnancy compared to sampling done after pregnancy (P < .0001). Pregnant women who had received isoniazid preventive therapy (IPT) before enrollment had decreased plasma K/T ratio compared to those who had not received IPT (P = .0174). Plasma K/T ratio was elevated in women with active TB at time of diagnosis compared to those without TB (P < .0001). Using a cutoff of 0.100, plasma K/T ratio gave a diagnostic sensitivity of 94% (95% confidence interval [CI]: 82-95), specificity of 90% (95% CI: 80-91), positive predictive value (PPV) 85% and negative predictive value (NPV) 98%. A receiver operating characteristic curve (ROC) gave an area under the curve of 0.95 (95% CI: .92-.97, P < .0001).In conclusion, plasma K/T ratio is a sensitive blood-based diagnostic test for active TB disease in pregnant women living with HIV. Plasma K/T ratio should be further evaluated as an initial TB diagnostic test to determine its impact on patient care.

Evolutionary Views of Tuberculosis: Indoleamine 2,3-Dioxygenase Catalyzed Nicotinamide Synthesis Reflects Shifts in Macrophage Metabolism: Indoleamine 2,3-Dioxygenase Reflects Altered Macrophage Metabolism During Tuberculosis Pathogenesis

Indoleamine 2,3-dioxygenase (IDO) is the rate-limiting enzyme in conversion of tryptophan to kynurenines, feeding de novo nicotinamide synthesis. IDO orchestrates materno-foetal tolerance, increasing human reproductive fitness. IDO mediates immune suppression through depletion of tryptophan required by T lymphocytes and other mechanisms. IDO is expressed by alternatively activated macrophages, suspected to play a key role in tuberculosis (TB) pathogenesis. Unlike its human host, Mycobacterium tuberculosis can synthesize tryptophan, suggesting possible benefit to the host from infection with the microbe. Intriguingly, nicotinamide analogues are used to treat TB. In reviewing this field, it is postulated that flux through the nicotinamide synthesis pathway reflects switching between aerobic glycolysis and oxidative phosphorylation in M. tuberculosis-infected macrophages. The evolutionary cause of such shifts may be ancient mitochondrial behavior related to reproductive fitness. Evolutionary perspectives on the IDO pathway may elucidate why, after centuries of co-existence with the Tubercle bacillus, humans still remain susceptible to TB disease.

An Insight into the Roles of Dietary Tryptophan and Its Metabolites in Intestinal Inflammation and Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is complex, chronic, and relapsing gastrointestinal inflammatory disorders, which includes mainly two conditions, namely ulcerative colitis (UC) and Crohn's disease (CD). Development of IBD in any individual is closely related to his/her autoimmune regulation, gene-microbiota interactions, and dietary factors. Dietary tryptophan (Trp) is an essential amino acid for intestinal mucosal cells, and it is associated with the intestinal inflammation, epithelial barrier, and energy homeostasis of the host. According to recent studies, Trp and its three major metabolic pathways, namely kynurenine (KYN) pathway, indole pathway, and 5-hydroxytryptamine (5-HT) pathway, have vital roles in the regulation of intestinal inflammation by acting directly or indirectly on the pro/anti-inflammatory cytokines, functions of various immune cells, as well as the intestinal microbial composition and homeostasis. In this review, recent advances in Trp- and its metabolites-associated intestinal inflammation are summarized. It further discusses the complex mechanisms and interrelationships of the three major metabolic pathways of Trp in regulating inflammation, which could elucidate the value of dietary Trp to be used as a nutrient for IBD patients.

IDO Expression in Cancer: Different Compartment, Different Functionality?

Indoleamine 2,3-dioxygenase 1 (IDO1) is a cytosolic haem-containing enzyme involved in the degradation of tryptophan to kynurenine. Although initially thought to be solely implicated in the modulation of innate immune responses during infection, subsequent discoveries demonstrated IDO1 as a mechanism of acquired immune tolerance. In cancer, IDO1 expression/activity has been observed in tumor cells as well as in the tumor-surrounding stroma, which is composed of endothelial cells, immune cells, fibroblasts, and mesenchymal cells. IDO1 expression/activity has also been reported in the peripheral blood. This manuscript reviews available data on IDO1 expression, mechanisms of its induction, and its function in cancer for each of these compartments. In-depth study of the biological function of IDO1 according to the expressing (tumor) cell can help to understand if and when IDO1 inhibition can play a role in cancer therapy.

Diagnostic accuracy of plasma kynurenine/tryptophan ratio, measured by enzyme-linked immunosorbent assay, for pulmonary tuberculosis

INTRODUCTION

The World Health Organization has identified the need for a non-sputum-based test capable of detecting active tuberculosis (TB) as a priority. The plasma kynurenine-to-tryptophan (K/T) ratio, largely mediated by activity of the enzyme indoleamine 2,3-dioxygenase, may have potential as a suitable biomarker for active TB.

METHOD

We evaluated a commercial enzyme-linked immunosorbent assay (ELISA) in comparison to mass spectrometry for measuring the K/T ratio. We also used ELISA to determine the K/T ratio in plasma from patients with active TB compared to latently infected controls, with and without HIV.

RESULTS

The two methods showed good agreement, with a mean bias of 0.01 (limit of agreement from -0.06 to 0.10). Using ELISA, it was found that HIV-infected patients with active TB disease had higher K/T ratios than those without TB (median, 0.101 [interquartile range (IQR), 0.091-0.140] versus 0.061 [IQR, 0.034-0.077], P<0.0001). At a cutoff of 0.080, the K/T ratio produced a sensitivity of 90%, a specificity of 80%, a positive predictive value (PPV) of 82%, and a negative predictive value (NPV) of 90%. In a receiver operating characteristics analysis, the K/T ratio had an area under the curve of 0.93. HIV-uninfected patients with active TB also had higher K/T ratios than those with latent TB infections (median, 0.064 [IQR, 0.040-0.088] versus 0.022 [IQR, 0.016-0.027], P<0.0001). A cutoff of 0.040 gave a sensitivity of 85%, a specificity of 92%, a PPV of 91%, and an NPV of 84%.

CONCLUSION

The plasma K/T ratio is a sensitive biomarker for active TB. The K/T ratio can be measured from blood using ELISA. The K/T ratio should be evaluated as an initial test for TB.