Increased Indoleamine-2,3-Dioxygenase Activity Is Associated With Poor Clinical Outcome in Adults Hospitalized With Influenza in the INSIGHT FLU003Plus Study

Kynurenine Pathway in Respiratory Diseases

The kynurenine pathway is the primary route for tryptophan catabolism and has received increasing attention as its association with inflammation and the immune system has become more apparent. This review provides a broad overview of the kynurenine pathway in respiratory diseases, from the initial observations to the characterization of the different cell types involved in the synthesis of kynurenine metabolites and the underlying immunoregulatory mechanisms. With a focus on respiratory infections, the various attempts to characterize the kynurenine/tryptophan (K/T) ratio as an inflammatory marker are reviewed. Its implication in chronic lung inflammation and its exacerbation by respiratory pathogens is also discussed. The emergence of preclinical interventional studies targeting the kynurenine pathway opens the way for the future development of new therapies.

Associations of microbial and indoleamine-2,3-dioxygenase-derived tryptophan metabolites with immune activation in healthy adults

Background

Tryptophan (Trp) metabolites from intestinal bacteria (indole, indole acetic acid [IAA] and indole propionic acid [IPA]), and the Trp metabolite kynurenine (Kyn) from the indoleamine 2,3-dioxygenase (IDO) pathway, are aryl hydrocarbon receptor (AhR) agonists and thus, can regulate immune activity via the AhR pathway. We hypothesized that plasma concentrations of these metabolites would be associated with markers of immune activation in a cohort of healthy adults in a manner consistent with AhR-mediated immune-regulation. We also hypothesized that the plasma Kyn/Trp ratio, a marker of IDO activity, would be associated with immune markers reflecting IDO activation in innate immune cells. Finally, we hypothesized that some intestinal bacteria would be associated with plasma indole, IPA and IAA, and that these bacteria themselves would be associated with immune markers.

Methods

A novel set of 88 immune markers, and plasma Trp metabolites, were measured in 362 healthy adults. Bacterial taxa from stool were identified by 16S rRNA gene analysis. Multiple linear regression analysis was used to identify significant associations with immune markers.

Results

The sum of indole and IAA was positively associated with natural killer T-cells levels. Kyn and Kyn/Trp were positively associated with neopterin and IP-10, markers of type 1 immunity, and TNF-α and C-reactive protein (CRP), markers of the acute phase response, and the regulatory cytokine IL-10. Three bacteria negatively associated with Trp metabolites were associated with markers of immune activation: the family Lachnospiraceae with higher lymphocyte counts but lower level of activated CD4 T-cells, the genus Dorea with higher production of IFN-γ by T-cells in PBMC cultures, and the genus Ruminococcus with higher production IL-6 in PBMC cultures stimulated with bacterial lipopolysaccharide (LPS).

Conclusions

In this cohort of healthy adults bacterial Trp metabolites were not strongly associated with immune markers. Conversely, the Kyn/Trp ratio was strongly associated with markers of systemic inflammation and the acute phase response, consistent with IDO activation in innate immune cells. Finally, commensal bacteria associated with lower plasma (and perhaps intestinal) levels of bacterial Trp metabolites were associated with greater immune activation, possibly reflecting decreased regulatory immune activity related to lower intestinal levels of bacterial indole metabolites.

Spatial Metabolomics Reveals Localized Impact of Influenza Virus Infection on the Lung Tissue Metabolome

The influenza virus (IAV) is a major cause of respiratory disease, with significant infection increases in pandemic years. Vaccines are a mainstay of IAV prevention but are complicated by IAV’s vast strain diversity and manufacturing and vaccine uptake limitations. While antivirals may be used for treatment of IAV, they are most effective in early stages of the infection, and several virus strains have become drug resistant. Therefore, there is a need for advances in IAV treatment, especially host-directed therapeutics. Given the spatial dynamics of IAV infection and the relationship between viral spatial distribution and disease severity, a spatial approach is necessary to expand our understanding of IAV pathogenesis. We used spatial metabolomics to address this issue. Spatial metabolomics combines liquid chromatography-tandem mass spectrometry of metabolites extracted from systematic organ sections, 3D models, and computational techniques to develop spatial models of metabolite location and their role in organ function and disease pathogenesis. In this project, we analyzed serum and systematically sectioned lung tissue samples from uninfected or infected mice. Spatial mapping of sites of metabolic perturbations revealed significantly lower metabolic perturbation in the trachea compared to other lung tissue sites. Using random forest machine learning, we identified metabolites that responded differently in each lung position based on infection, including specific amino acids, lipids and lipid-like molecules, and nucleosides. These results support the implementation of spatial metabolomics to understand metabolic changes upon respiratory virus infection.

IMPORTANCE The influenza virus is a major health concern. Over 1 billion people become infected annually despite the wide distribution of vaccines, and antiviral agents are insufficient to address current clinical needs. In this study, we used spatial metabolomics to understand changes in the lung and serum metabolome of mice infected with influenza A virus compared to uninfected controls. We determined metabolites altered by infection in specific lung tissue sites and distinguished metabolites perturbed by infection between lung tissue and serum samples. Our findings highlight the utility of a spatial approach to understanding the intersection between the lung metabolome, viral infection, and disease severity. Ultimately, this approach will expand our understanding of respiratory disease pathogenesis.

Metabolomics as a Tool to Investigate HIV/TB Co-Infection

The HIV/AIDS (human immunodeficiency virus/acquired immunodeficiency syndrome) and tuberculosis (TB) pandemics are perpetuated by a significant global burden of HIV/TB co-infection. The synergy between HIV and Mycobacterium tuberculosis (Mtb) during co-infection of a host is well established. While this synergy is known to be driven by immunological deterioration, the metabolic mechanisms thereof remain poorly understood. Metabolomics has been applied to study various aspects of HIV and Mtb infection separately, yielding insights into infection- and treatment-induced metabolic adaptations experienced by the host. Despite the contributions that metabolomics has made to the field, this approach has not yet been systematically applied to characterize the HIV/TB co-infected state. Considering that limited HIV/TB co-infection metabolomics studies have been published to date, this review briefly summarizes what is known regarding the HIV/TB co-infection synergism from a conventional and metabolomics perspective. It then explores metabolomics as a tool for the improved characterization of HIV/TB co-infection in the context of previously published human-related HIV infection and TB investigations, respectively as well as for addressing the gaps in existing knowledge based on the similarities and deviating trends reported in these HIV infection and TB studies.

Nicotinamide pathways as the root cause of sepsis - an evolutionary perspective on macrophage energetic shifts

Divergent pathways of macrophage metabolism occur during infection, notably switching between oxidative phosphorylation and aerobic glycolysis (Warburg-like metabolism). Concurrently, macrophages shift between alternate and classical activation. A key enzyme upregulated in alternatively activated macrophages is indoleamine 2,3-dioxygenase, which converts tryptophan to kynurenine for de novo synthesis of nicotinamide. Nicotinamide can be used to replenish cellular NAD⁺ supplies. We hypothesize that an insufficient cellular NAD⁺ supply is the root cause of metabolic shifts in macrophages. We assert that manipulation of nicotinamide pathways may correct deleterious immune responses. We propose evaluation of nicotinamide (Vitamin B3) and analogues, including isoniazid, nicotinamide mononucleotide and nicotinamide riboside, as potential therapy for infectious causes of sepsis, including COVID-19.

Metabolite profiles associated with disease progression in influenza infection

BACKGROUND

We performed metabolomic profiling to identify metabolites that correlate with disease progression and death.

METHODS

We performed a study of adults hospitalized with Influenza A(H1N1)pdm09. Cases (n = 32) were defined by a composite outcome of death or transfer to the intensive care unit during the 60-day follow-up period. Controls (n = 64) were survivors who did not require transfer to the ICU. Four hundred and eight metabolites from eight families were measured on plasma sample at enrollment using a mass spectrometry based Biocrates platform. Conditional logistic regression was used to summarize the association of the individual metabolites and families with the composite outcome and its major two components.

RESULTS

The ten metabolites with the strongest association with disease progression belonged to five different metabolite families with sphingolipids being the most common. The acylcarnitines, glycerides, sphingolipids and biogenic metabolite families had the largest odds ratios based on the composite endpoint. The tryptophan odds ratio for the composite is largely associated with death (OR 17.33: 95% CI, 1.60-187.76).

CONCLUSIONS

Individuals that develop disease progression when infected with Influenza H1N1 have a metabolite signature that differs from survivors. Low levels of tryptophan had a strong association with death.

REGISTRY

ClinicalTrials.gov Identifier: NCT01056185.

Plasma Kynurenine to Tryptophan Ratio Is Negatively Associated with Linear Growth of Children Living in a Slum of Bangladesh: Results from a Community-Based Intervention Study

Chronic exposure to infectious agents results in environmental enteric dysfunction-a significant contributor to childhood stunting. Low plasma tryptophan (TRP), increased kynurenine (KYN), and KYN-TRP (KT) ratio are associated with infections and chronic immune activation. We postulated that both these conditions are interlinked, and therefore aimed to identify the association between KT ratio and the linear growth of Bangladeshi children. A total of 480 stunted and at risk of being stunted children aged 12-18 months were enrolled and provided nutrition intervention for 90 days. Plasma samples were assessed using liquid chromatography tandem mass spectrometry to measure TRP and KYN concentrations. Multivariable linear regression with generalized estimating equations was applied to analyze association between the KT ratio and linear growth. Tryptophan, KYN, and KT ratio were significantly higher in stunted children than in children at risk of being stunted both at baseline and at the end of nutrition intervention. Following intervention, the median (interquartile range [IQR]) KYN concentration was significantly reduced from 4.6 (3.6, 5.4) µmol/L to 3.9 (0.3, 7.6) µmol/L, and median (IQR) KT ratio decreased from 104 (80.9, 131) to 92.8 (6.6, 247) in stunted children. We also found KT ratio to be negatively associated (coefficient = -0.7; 95% CI = -1.13, -0.26; P-value = 0.002) with linear growth. In addition, KYN and KT ratio were positively correlated with fecal neopterin and plasma C-reactive protein, whereas TRP was negatively correlated with both of these biomarkers and alpha-1-acid glycoprotein. Our findings imply that KT ratio is associated in the pathophysiology of stunting as well as with biomarkers of inflammation in Bangladeshi children.

Serum kynurenine levels are a novel biomarker to predict the prognosis of patients with hepatocellular carcinoma

BACKGROUND

We examined serum kynurenine levels in patients with chronic hepatitis C virus infection, and the relationship between serum kynurenine and prognosis in patients with hepatocellular carcinoma (HCC) and chronic hepatitis C.

METHODS

We retrospectively analyzed 604 patients with HCC diagnosed between January 1999 and December 2015, and 288 patients without HCC who were seen at the National Hospital Organization Nagasaki Medical Center between October 2014 and November 2017. The association between serum kynurenine and prognosis was evaluated using the Cox's proportional hazards regression analysis.

RESULTS

Patients with HCC had significantly higher values of serum kynurenine than patients without HCC (median: 557.1 vs. 464.2 ng/mL, p<0.001). Five-year survival rates of HCC patients with serum kynurenine ≥900 (n = 65), 600-899 (n = 194), and <600 ng/mL (n = 345) were 30.6%, 47.4%, and 61.4%, respectively (p = 0.001, log-rank test). Multivariate analysis identified serum kynurenine as an independent predictor for prognosis of HCC patients. The hazard ratio of serum kynurenine ≥900, and 600-899 compared with serum kynurenine <600 ng/mL were 1.91 (p<0.001) and 1.37 (p = 0.015), respectively.

CONCLUSIONS

A high level of serum kynurenine correlated with poor prognosis of HCC. Serum kynurenine levels may be a novel biomarker to predict the prognosis of patients with HCC. The development of drugs that inhibit kynurenine production is expected to help improve the prognosis of patients with HCC.

Assessment of neopterin and indoleamine 2,3-dioxygenase activity in patients with seasonal influenza: A pilot study

BACKGROUND

Seasonal influenza is an important cause of morbidity and mortality worldwide. Immune activation after stimulation with interferon-gamma leads to increased production of neopterin but also results in increased tryptophan catabolism through indoleamine 2,3-dioxygenase (IDO). Our pilot study determined neopterin serum levels and IDO activity in patients with influenza infection and investigated whether neopterin is linked to clinical outcome parameters (mortality ≤30 days, acute cardiac events (ACE) length of hospitalization, ICU admission).

METHODS

Neopterin concentrations were analyzed in serum samples of 40 patients with a confirmed diagnosis of influenza infection and in-hospital treatment for >24 hours. Data were compared to values of 100 healthy blood donors and 48 age-matched pneumonia patients. In a subgroup of 14 patients, tryptophan and kynurenine concentrations, as well as kynurenine-to-tryptophan ratio, were analyzed.

RESULTS

In all influenza patients, neopterin concentrations were increased and significantly higher compared to those determined in patients with pneumonia and healthy controls. Positive correlations between the duration of hospitalization and neopterin were found. Significantly higher levels of kynurenine, kynurenine-to-tryptophan ratio, and lower levels of tryptophan were seen in influenza patients compared to healthy controls.

CONCLUSIONS

Neopterin seems to be related to the course of the disease and could be a valuable biomarker to identify patients at an elevated risk of a worsened outcome; however, further prospective validation studies are needed to support the here presented preliminary results.