Role of Kynurenine and Its Derivatives in Liver Diseases: Recent Advances and Future Clinical Perspectives

Liver health is integral to overall human well-being and the pathogenesis of various diseases. In recent years, kynurenine and its derivatives have gradually been recognized for their involvement in various pathophysiological processes, especially in the regulation of liver diseases, such as acute liver injury, non-alcoholic fatty liver disease, cirrhosis, and liver cancer. Kynurenine and its derivatives are derived from tryptophan, which is broken down by the enzymes indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO), converting the essential amino acid tryptophan into kynurenine (KYN) and other downstream metabolites, such as kynurenic acid (KYNA), 3-hydroxykynurenine (3-HK), xanthurenic acid (XA), and quinolinic acid (QA). In liver diseases, kynurenine and its derivatives can promote the activity of the transcription factor aryl hydrocarbon receptor (AhR), suppress T cell activity for immune modulation, inhibit the activation of inflammatory signaling pathways, such as NF-κB for anti-inflammatory effects, and inhibit the activation of hepatic stellate cells to slow down fibrosis progression. Additionally, kynurenine and other downstream metabolites can influence the progression of liver diseases by modulating the gut microbiota. Therefore, in this review, we summarize and explore the mechanisms by which kynurenine and its derivatives regulate liver diseases to help develop new diagnostic or prognostic biomarkers and effective therapies targeting the kynurenine pathway for liver disease treatment.

Rational design of 2-benzylsulfinyl-benzoxazoles as potent and selective indoleamine 2,3-dioxygenase 1 inhibitors to combat inflammation

Mimicking the transition state of tryptophan (Trp) and O2 in the enzymatic reaction is an effective approach to design indoleamine 2,3-dioxygenase 1 (IDO1) inhibitors. In this study, we firstly assembled a small library of 2-substituted benzo-fused five membered heterocycles and found 2-sulfinyl-benzoxazoles with interesting IDO1 inhibitory activities. Next the inhibitory activity toward IDO1 was gradually improved. Several benzoxazoles showed potent IDO1 inhibitory activity with IC50 of 82-91 nM, and exhibited selectivity between IDO1 and tryptophan 2,3-dioxygenase (TDO2). Enzyme binding studies showed that benzoxazoles are reversible type II IDO1 inhibitors, and modeling studies suggested that the oxygen atom of the sulfoxide in benzoxazoles interacts with the iron atom of the heme group, which mimics the transition state of Fe-O-O-Trp complex. Especially, 10b can effectively inhibit the NO production in lipopolysaccharides (LPS) stimulated RAW264.7 cells, and it also shows good anti-inflammation effect on mice acute inflammation model of croton oil induced ear edema.

Kynurenine Pathway Dysregulation and Pain Perception in Acute Pancreatitis: Has the Connection Unraveled?

AIM

Tryptophan (TRP), an essential amino acid, undergoes catabolism through various pathways. Notably, the kynurenine pathway (KP), constituting one of these pathways, exhibits a unidirectional impact on immune response and energy metabolism. Nonetheless, its influence on pain sensation is characterized by biphasic dynamics. This study aims to scrutinize the influence of the KP pathway on pain sensation, particularly within the context of pancreatic inflammation.

METHODS

Our prospective case-control study involved individuals diagnosed with acute pancreatitis and a control group matched for gender and age. The patient cohort was subsequently subdivided into severe and non-severe subgroups. To assess metabolites within KP, two blood samples were collected from the patient cohort, one at the time of diagnosis and another during the recovery phase. Furthermore, for pain quantification, daily pain scores utilizing the Visual Analog Scale (VAS) were extracted from the patients' medical records.

RESULTS

The study incorporated 30 patients along with an equivalent number of controls. A noticeable distinction was evident between the patient and control groups, characterized by an increase in kynurenine levels and a decrease in the tryptophan/kynurenine ratio. Throughout the process of disease recovery, a uniform decrease was observed in all KP metabolites, excluding 3-Hydroxykynurenine. Elevated levels of Kynurenic acid (KYNA) were correlated with increased pain scores. Critically, no apparent distinctions in KP metabolites were discerned concerning pain severity in patients with comorbidities characterized by neural involvement.

CONCLUSION

Based on our results, the kynurenine pathway (KP) is activated in instances of acute pancreatitis. Elevated levels of KYNA were found to be associated with heightened pain scores. The operative stages within the KP responsible for pain modulation are impaired in cases characterized by neuropathy-induced pain sensation.

Taking advantage of the interaction between the sulfoxide and heme cofactor to develop indoleamine 2, 3-dioxygenase 1 inhibitors

Taking advantage of key interactions between sulfoxide and heme cofactor, we used the sulfoxide as the anchor functional group to develop two series of indoleamine 2, 3-dioxygenase 1 (IDO1) inhibitors: 2-benzylsulfinylbenzoxazoles (series 1) and 2-phenylsulfinylbenzoxazoles (series 2). In vitro enzymatic screening shows that both series can inhibit the activity of IDO1 in low micromolar (series 1) or nanomolar (series 2) levels. They also show inhibitory selectivity between IDO1 and tryptophan 2, 3-dioxygenase 2. Interestingly, although series 1 is less potent IDO1 inhibitors of these two series, it exhibited stronger inhibitory activity toward kynurenine production in interferon-γ stimulated BxPC-3 cells. Enzyme kinetics and binding studies demonstrated that 2-sulfinylbenzoxazoles are non-competitive inhibitors of tryptophan, and they interact with the ferrous form of heme. These results demonstrated 2-sulfinylbenzoxazoles as type II IDO1 inhibitors. Furthermore, molecular docking studies supports the sulfoxide being of the key functional group that interacts with the heme cofactor. Compound 22 (series 1) can inhibit NO production in a concentration dependent manner in lipopolysaccharides (LPS) stimulated RAW264.7 cells, and can relieve pulmonary edema and lung injury in LPS induced mouse acute lung injury models.

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.

Neuroimmune modulation by tryptophan derivatives in neurological and inflammatory disorders

The nervous and immune systems are highly developed, and each performs specialized physiological functions. However, they work together, and their dysfunction is associated with various diseases. Specialized molecules, such as neurotransmitters, cytokines, and more general metabolites, are essential for the appropriate regulation of both systems. Tryptophan, an essential amino acid, is converted into functional molecules such as serotonin and kynurenine, both of which play important roles in the nervous and immune systems. The role of kynurenine metabolites in neurodegenerative and psychiatric diseases has recently received particular attention. Recently, we found that hyperactivity of the kynurenine pathway is a critical risk factor for septic shock. In this review, we first outline neuroimmune interactions and tryptophan derivatives and then summarized the changes in tryptophan metabolism in neurological disorders. Finally, we discuss the potential of tryptophan derivatives as therapeutic targets for neuroimmune disorders.

The Emerging Role of 3-Hydroxyanthranilic Acid on C. elegans Aging Immune Function

3-hydroxyanthranilic acid (3HAA) is considered to be a fleeting metabolic intermediate along tryptophan catabolism through the kynurenine pathway. 3HAA and the rest of the kynurenine pathway have been linked to immune response in mammals yet whether it is detrimental or advantageous is a point of contention. Recently we have shown that accumulation of this metabolite, either through supplementation or prevention of its degradation, extends healthy lifespan in C. elegans and mice, while the mechanism remained unknown. Utilizing C. elegans as a model we investigate how 3HAA and haao-1 inhibition impact the host and the potential pathogens. What we find is that 3HAA improves host immune function with aging and serves as an antimicrobial against gram-negative bacteria. Regulation of 3HAA's antimicrobial activity is accomplished via tissue separation. 3HAA is synthesized in the C. elegans hypodermal tissue, localized to the site of pathogen interaction within the gut granules, and degraded in the neuronal cells. This tissue separation creates a new possible function for 3HAA that may give insight to a larger evolutionarily conserved function within the immune response.

A tryptophan metabolite modulates the host response to bacterial infection via kainate receptors

Bacterial infection involves a complex interaction between the pathogen and host where the outcome of infection is not solely determined by pathogen eradication. To identify small molecules that promote host survival by altering the host-pathogen dynamic, we conducted an in vivo chemical screen using zebrafish embryos and found that treatment with 3-hydroxy-kynurenine protects from lethal gram-negative bacterial infection. 3-hydroxy-kynurenine, a metabolite produced through host tryptophan metabolism, has no direct antibacterial activity but enhances host survival by restricting bacterial expansion in macrophages by targeting kainate-sensitive glutamate receptors. These findings reveal new mechanisms by which tryptophan metabolism and kainate-sensitive glutamate receptors function and interact to modulate immunity, with significant implications for the coordination between the immune and nervous systems in pathological conditions.

Multi-omics analysis reveals changes in tryptophan and cholesterol metabolism before and after sexual maturation in captive macaques

Rhesus macaques (Macaca mulatta, RMs) are widely used in sexual maturation studies due to their high genetic and physiological similarity to humans. However, judging sexual maturity in captive RMs based on blood physiological indicators, female menstruation, and male ejaculation behavior can be inaccurate. Here, we explored changes in RMs before and after sexual maturation based on multi-omics analysis and identified markers for determining sexual maturity. We found that differentially expressed microbiota, metabolites, and genes before and after sexual maturation showed many potential correlations. Specifically, genes involved in spermatogenesis (TSSK2, HSP90AA1, SOX5, SPAG16, and SPATC1) were up-regulated in male macaques, and significant changes in gene (CD36), metabolites (cholesterol, 7-ketolithocholic acid, and 12-ketolithocholic acid), and microbiota (Lactobacillus) related to cholesterol metabolism were also found, suggesting the sexually mature males have stronger sperm fertility and cholesterol metabolism compared to sexually immature males. In female macaques, most differences before and after sexual maturity were related to tryptophan metabolism, including changes in IDO1, IDO2, IFNGR2, IL1Β, IL10, L-tryptophan, kynurenic acid (KA), indole-3-acetic acid (IAA), indoleacetaldehyde, and Bifidobacteria, indicating that sexually mature females exhibit stronger neuromodulation and intestinal immunity than sexually immature females. Cholesterol metabolism-related changes (CD36, 7-ketolithocholic acid, 12-ketolithocholic acid) were also observed in female and male macaques. Exploring differences before and after sexual maturation through multi-omics, we identified potential biomarkers of sexual maturity in RMs, including Lactobacillus (for males) and Bifidobacterium (for females) valuable for RM breeding and sexual maturation research.

A tryptophan metabolite prevents depletion of circulating endothelial progenitor cells in systemic low-grade inflammation

Background

Chronic systemic inflammation reduces the bioavailability of circulating endothelial progenitor cells (EPCs). Indoleamine 2,3-dioxygenase 1 (IDO1), a key enzyme of immune tolerance catalyzing the initial step of tryptophan degradation along the so-called l-kynurenine (l-kyn) pathway, that is induced by inflammatory stimuli and exerts anti-inflammatory effects. A specific relationship between IDO1 activity and circulating EPC numbers has not yet been investigated.

Methods

In this study, circulating EPCs were examined in mice treated with low doses of lipopolysaccharide (LPS) to mimic low-grade inflammation. Moreover, the association between IDO1 activity and circulating EPCs was studied in a cohort of 277 patients with variable systemic low-grade inflammation.

Results

Repeated low doses of LPS caused a decrease in circulating EPCs and l-kyn supplementation, mimicking IDO1 activation, significantly increased EPC numbers under homeostatic conditions preventing EPC decline in low-grade endotoxemia. Accordingly, in patients with variable systemic low-grade inflammation, there was a significant interaction between IDO1 activity and high-sensitivity C-reactive protein (hs-CRP) in predicting circulating EPCs, with high hs-CRP associated with significantly lower EPCs at low IDO1 activity but not at high IDO1 activity.

Interpretation

Overall, these findings demonstrate that systemic low-grade inflammation reduces circulating EPCs. However, high IDO1 activity and l-kyn supplementation limit circulating EPC loss in low-grade inflammation.

Blockade of Indoleamine 2,3-Dioxygenase attenuates lipopolysaccharide-induced kidney injury by inhibiting TLR4/NF-κB signaling

Blockade of indoleamine 2,3-dioxygenase (IDO) has been shown to alleviate lipopolysaccharide (LPS)-induced endotoxic shock and reduce sepsis mortality, but its effect on LPS-induced kidney damage has not been reported. Herein, we established a mouse kidney injury model by intraperitoneal injection of 10 mg/kg LPS and established an in vitro renal tubular epithelial cell injury model by stimulating TCMK-1 cells with 10 mg/L LPS. We found that pretreatment with 1-methyl tryptophan (1-MT), an IDO inhibitor, significantly improved LPS-induced mouse survival, and IDO knockout (KO) mice also had higher survival rates after LPS exposure than wild-type mice. At the same time, IDO KO or pretreatment with 1-MT not only reduced serum creatinine, blood urea nitrogen, renal tubular injury pathological score, but also inflammatory factors and oxidative stress status in serum or kidney of LPS-exposed mice. In vitro, blockade of IDO with 1-MT significantly inhibited LPS-induced apoptosis, inflammation and oxidative stress in TCMK-1 cells. In addition, blockade of IDO significantly inhibited LPS-activated TLR4/NF-κB signaling pathway in kidney of mice or in TCMK-1 cells. In conclusion, our results suggested that blockade of IDO attenuated kidney inflammation, apoptosis and oxidative stress to protect against LPS-induced septic kidney injury via inhibiting the TLR4/NF-κB signaling pathway.

Anti-E. coli Immunoglobulin Yolk (IgY): Reduction of pathogen receptors and inflammation factors could be caused by decrease in E. coli load

Graft versus host disease (GVHD) remains the major cause of morbidity and mortality after allogeneic stem cell transplantation, especially for intestinal GVHD, as steroid resistant GVHD results in high mortality. For this reason, new treatments of GVHD are needed. One approach is the reduction of pathogenic bacteria using anti-E. coli Immunoglobulin Yolk (IgY). In a haploidentical murine model, B6D2F1 mice conditioned with total body irradiation (TBI), received bone marrow cells (BM) and splenocytes (SC) from either syngeneic (Syn = B6D2F1) or allogeneic (Allo = C57BL/6) donors. Following this, animals received from day -2 until day +28 chow contained IgY or control chow. Thereafter the incidence and severity of aGVHD, the cytokines, chemokines, IDO1 and different pathogen-recognition receptors (PRR) were analyzed and compared to control animals (received chow without IgY). We found that animals receiving chow with IgY antibody showed reduced GVHD severity compared to control animals. On day28 after alloBMT, IDO, NOD2, TLR2, TLR4 and the inflammatory chemokine CCL3, were reduced in the colon and correlated with a significant decrease in E. coli bacteria. In summary chow containing chicken antibodies (IgY) improved GVHD via decrease in bacterial load of E coli conducting to reduction of pathogen receptors (NOD2, TLR2 and 4), IDO, chemokines and cytokines.

Activities of plasma indoleamine-2, 3-dioxygenase (IDO) enzyme in Nigerian patients with lung diseases: basis for tryptophan supplementation or IDO inhibitor use

Background

Clinical trial of IDO inhibitor or uses of micro-nutrient supplements during management of diseases is commonly done without having adequate basis for the practise. Tryptophan (Trp) is an essential amino acid needed for T-lymphocyte function, and indoleamine-2,3-dioxygenase (IDO) is a potent immunoregulatory molecule that catalyses the rate-limiting step of Trp degradation in the kynurenine (Kyn) pathway.

Materials and methods

Human IDO in the plasma samples was measured using ELISA in patients with non-infectious (asthma) and infectious diseases (pulmonary tuberculosis and COVID-19) compared with corresponding un-infected controls.

Results

Mean IDO activity in COVID-19 patients was significantly higher compared with corresponding control (p = 0.001) while mean IDO activity in pulmonary tuberculosis patients was non-significantly higher compared with corresponding control (p = 0.520), and mean IDO activity in asthma patients was non-significantly lower compared with corresponding control (p = 0.102).

Conclusion

Our data suggest that IDO activity as an innate immune factor is increased in infectious lung diseases (COVID-19 and pulmonary tuberculosis) but reduced in non-infectious disease (asthma) and that use of tryptophan supplementation or IDO inhibitor may not be necessary in all lung diseases.

An integrated cytokine and kynurenine network as the basis of neuroimmune communication

Two of the molecular families closely associated with mediating communication between the brain and immune system are cytokines and the kynurenine metabolites of tryptophan. Both groups regulate neuron and glial activity in the central nervous system (CNS) and leukocyte function in the immune system, although neither group alone completely explains neuroimmune function, disease occurrence or severity. This essay suggests that the two families perform complementary functions generating an integrated network. The kynurenine pathway determines overall neuronal excitability and plasticity by modulating glutamate receptors and GPR35 activity across the CNS, and regulates general features of immune cell status, surveillance and tolerance which often involves the Aryl Hydrocarbon Receptor (AHR). Equally, cytokines and chemokines define and regulate specific populations of neurons, glia or immune system leukocytes, generating more specific responses within restricted CNS regions or leukocyte populations. In addition, as there is a much larger variety of these compounds, their homing properties enable the superimposition of dynamic variations of cell activity upon local, spatially limited, cell populations. This would in principle allow the targeting of potential treatments to restricted regions of the CNS. The proposed synergistic interface of 'tonic' kynurenine pathway affecting baseline activity and the superimposed 'phasic' cytokine system would constitute an integrated network explaining some features of neuroimmune communication. The concept would broaden the scope for the development of new treatments for disorders involving both the CNS and immune systems, with safer and more effective agents targeted to specific CNS regions.

Polyphenols: Chemoprevention and therapeutic potentials in hematological malignancies

Polyphenols are one of the largest plant-derived natural product and they play an important role in plants' defense as well as in human health and disease. A number of them are pleiotropic molecules and have been shown to regulate signaling pathways, immune response and cell growth and proliferation which all play a role in cancer development. Hematological malignancies on the other hand, are cancers of the blood. While current therapies are efficacious, they are usually expensive and with unwanted side effects. Thus, the search for newer less toxic agents. Polyphenols have been reported to possess antineoplastic properties which include cell cycle arrest, and apoptosis via multiple mechanisms. They also have immunomodulatory activities where they enhance T cell activation and suppress regulatory T cells. They carry out these actions through such pathways as PI3K/Akt/mTOR and the kynurenine. They can also reverse cancer resistance to chemotherapy agents. In this review, i look at some of the molecular mechanism of action of polyphenols and their potential roles as therapeutic agents in hematological malignancies. Here i discuss their anti-proliferative and anti-neoplastic activities especially their abilities modulate signaling pathways as well as immune response in hematological malignancies. I also looked at clinical studies done mainly in the last 10-15 years on various polyphenol combination and how they enhance synergism. I recommend that further preclinical and clinical studies be carried out to ensure safety and efficacy before polyphenol therapies be officially moved to the clinics.

Kynurenine Pathway-An Underestimated Factor Modulating Innate Immunity in Sepsis-Induced Acute Kidney Injury?

Sepsis is a life-threatening organ dysfunction caused by a dysregulated host response to infection, and it accounts for about half of the cases of acute kidney injury (AKI). Although sepsis is the most frequent cause of AKI in critically ill patients, its pathophysiological mechanisms are not well understood. Sepsis has the ability to modulate the function of cells belonging to the innate immune system. Increased activity of indoleamine 2,3-dioxygenase 1 (IDO1) and production of kynurenines are the major metabolic pathways utilized by innate immunity cells to maintain immunological tolerance. The activation of the kynurenine pathway (KP) plays a dual role in sepsis—in the early stage, the induction of IDO1 elicits strong proinflammatory effects that may lead to tissue damage and septic shock. Afterwards, depletion of tryptophan and production of kynurenines contribute to the development of immunosuppression that may cause the inability to overpower opportunistic infections. The presented review provides available data on the various interdependencies between elements of innate immunity and sepsis-induced AKI (SAKI) with particular emphasis on the immunomodulatory significance of KP in the above processes. We believe that KP activation may be one of the crucial, though underestimated, components of a deregulated host response to infection during SAKI.

Unbalanced IDO1/IDO2 Endothelial Expression and Skewed Keynurenine Pathway in the Pathogenesis of COVID-19 and Post-COVID-19 Pneumonia

Despite intense investigation, the pathogenesis of COVID-19 and the newly defined long COVID-19 syndrome are not fully understood. Increasing evidence has been provided of metabolic alterations characterizing this group of disorders, with particular relevance of an activated tryptophan/kynurenine pathway as described in this review. Recent histological studies have documented that, in COVID-19 patients, indoleamine 2,3-dioxygenase (IDO) enzymes are differentially expressed in the pulmonary blood vessels, i.e., IDO1 prevails in early/mild pneumonia and in lung tissues from patients suffering from long COVID-19, whereas IDO2 is predominant in severe/fatal cases. We hypothesize that IDO1 is necessary for a correct control of the vascular tone of pulmonary vessels, and its deficiency in COVID-19 might be related to the syndrome’s evolution toward vascular dysfunction. The complexity of this scenario is discussed in light of possible therapeutic manipulations of the tryptophan/kynurenine pathway in COVID-19 and post-acute COVID-19 syndromes.

Impact of IDO1 and IDO2 on the B Cell Immune Response

Indoleamine-2,3-dioxygenase (IDO)1 and IDO2 are closely related tryptophan catabolizing enzymes that have immunomodulatory properties. Although initially studied as modifiers of T cell activity, emerging evidence suggests IDO1 and IDO2 also have important roles as modulators of B cell function. In this context, IDO1 and IDO2 appear to play opposite roles, with IDO1 inhibiting and IDO2 driving inflammatory B cell responses. In this mini review, we discuss the evidence for IDO1 and IDO2 modulation of B cell function, focusing on the effect of these enzymes on autoimmunity, allergic responses, protective immunity, and response to pathogens. We summarize strategies to target IDO1 and/or IDO2 as potential therapeutics for inflammatory autoimmune disease and highlight outstanding questions and areas that require future study.

Tryptophan and the innate intestinal immunity: Crosstalk between metabolites, host innate immune cells and microbiota

The intestinal mucosal barrier is critical for the absorption of nutrients and the health of both humans and animals. Recent publications from clinical and experimental studies have shown the importanceof the nutrients-bacteria-host interaction for the intestinal homeostasis. Dysfunction of these interactions has been reported to be associated with metabolic disorders and development of intestinal diseases, such as the irritable bowel syndrome and inflammatory bowel diseases. Tryptophan and its metabolites, including kynurenine, kynurenic acid, and 5-hydroxytrptamine, can influence the proliferation of enterocytes, intestinal integrity and immune response, as well as intestinal microbiota, therefore regulating and contributing to the intestinal health. In this review, we highlight recent findings on the effect of tryptophan and its metabolites on the mucosal barrier and intestinal homeostasis and its regulation of innate immune response. Moreover, we present the signaling pathways related to Trp metabolism, such as mammalian target of rapamycin, aryl hydrocarbon receptor, and pregnane X receptor, that contribute to the intestinal homeostasis and discuss future perspectives on spontaneous interference in host tryptophan metabolism as potential clinical strategies of intestinal diseases. This article is protected by copyright. All rights reserved.

Nociceptor-derived Reg3γ prevents endotoxic death by targeting kynurenine pathway in microglia

Nociceptors can fine-tune local or systemic immunity, but the mechanisms of nociceptive modulation in endotoxic death remain largely unknown. Here, we identified C-type lectin Reg3γ as a nociceptor-enriched hormone that protects the host from endotoxic death. During endotoxemia, nociceptor-derived Reg3γ penetrates the brain and suppresses the expression of microglial indoleamine dioxygenase 1, a critical enzyme of the kynurenine pathway, via the Extl3-Bcl10 axis. Endotoxin-administered nociceptor-null mice and nociceptor-specific Reg3γ-deficient mice exhibit a high mortality rate accompanied by decreased brain HK1 phosphorylation and ATP production despite normal peripheral inflammation. Such metabolic arrest is only observed in the brain, and aberrant production of brain quinolinic acid, a neurotoxic metabolite of the kynurenine pathway, causes HK1 suppression. Strikingly, the central administration of Reg3γ protects mice from endotoxic death by enhancing brain ATP production. By identifying nociceptor-derived Reg3γ as a microglia-targeted hormone, this study provides insights into the understanding of tolerance to endotoxic death.

The immune-kynurenine pathway in social anxiety disorder

BACKGROUND

The tryptophan-kynurenine pathway is of major interest in psychiatry and is altered in patients with depression, schizophrenia and panic disorder. Stress and immune alterations can impact this system, through cortisol- and cytokine-induced activation. In addition, there is emerging evidence that the kynurenine pathway is associated with suicidality. There have been no studies to date exploring the immune-kynurenine system in social anxiety disorder (SAD), and indeed very limited human studies on the kynurenine pathway in any clinical anxiety disorder.

METHODS

We investigated plasma levels of several kynurenine pathway markers, including kynurenine (KYN), tryptophan (TRYP) and kynurenic acid (KYNA), along with the KYN/TRYP and KYNA/KYN ratios, in a cohort of 32 patients with SAD and 36 healthy controls. We also investigated a broad array of both basal and lipopolysaccharide (LPS)-stimulated blood cytokine levels including IFN-γ, interleukin (IL)-10, IL-1β, IL-2, IL-4, IL-6, IL-8 and tumor necrosis factor (TNF)-α.

RESULTS

SAD patients had elevated plasma KYNA levels and an increased KYNA/KYN ratio compared to healthy controls. No differences in KYN, TRYP or the KYN/TRYP ratio were seen between the two groups. SAD patients with a history of past suicide attempt showed elevated plasma KYN levels and a higher KYN/TRYP ratio compared to patients without a history of suicide attempt. No differences were seen in basal or LPS-stimulated pro-inflammatory cytokine levels between the patients and controls. However, unstimulated IL-10, an anti-inflammatory cytokine, was significantly lower in the SAD group. A significant sex influence was evident with SAD males having lower levels of IL-10 compared to healthy males but no difference seen between SAD females and healthy females.

CONCLUSIONS

The peripheral kynurenine pathway is altered in SAD and preferentially directed towards KYNA synthesis. Additionally, kynurenine pathway activation, as evidenced by elevated KYN and KYN/TRYP ratio, is evident in SAD patients with a history of past suicide attempt. While no differences in pro-inflammatory cytokines is apparent in SAD patients, lower anti-inflammatory IL-10 levels are seen in SAD males. Further investigation of the role of the immune-kynurenine pathway in SAD and other clinical anxiety disorders is warranted.

Tryptophan: A Unique Role in the Critically Ill

Tryptophan is an essential amino acid whose metabolites play key roles in diverse physiological processes. Due to low reserves in the body, especially under various catabolic conditions, tryptophan deficiency manifests itself rapidly, and both the serotonin and kynurenine pathways of metabolism are clinically significant in critically ill patients. In this review, we highlight these pathways as sources of serotonin and melatonin, which then regulate neurotransmission, influence circadian rhythm, cognitive functions, and the development of delirium. Kynurenines serve important signaling functions in inter-organ communication and modulate endogenous inflammation. Increased plasma kynurenine levels and kynurenine-tryptophan ratios are early indicators for the development of sepsis. They also influence the regulation of skeletal muscle mass and thereby the development of polyneuromyopathy in critically ill patients. The modulation of tryptophan metabolism could help prevent and treat age-related disease with low grade chronic inflammation as well as post intensive care syndrome in all its varied manifestations: cognitive decline (including delirium or dementia), physical impairment (catabolism, protein breakdown, loss of muscle mass and tone), and mental impairment (depression, anxiety or post-traumatic stress disorder).

Lipocalin 2 regulates expression of MHC class I molecules in Mycobacterium tuberculosis-infected dendritic cells via ROS production

Background

Iron has important roles as an essential nutrient for all life forms and as an effector of the host defense mechanism against pathogenic infection. Lipocalin 2 (LCN2), an innate immune protein, plays a crucial role in iron transport and inflammation. In the present study, we examined the role of LCN2 in immune cells during Mycobacterium tuberculosis (Mtb) infection.

Results

We found that infection with Mtb H37Ra induced LCN2 production in bone marrow-derived dendritic cells (BMDCs). Notably, expression of MHC class I molecules was significantly reduced in LCN2^−/− BMDCs during Mtb infection. The reduced expression of MHC class I molecules was associated with the formation of a peptide loading complex through LCN2-mediated reactive oxygen species production. The reduced expression of MHC class I molecules affected CD8⁺ T-cell proliferation in LCN2^−/− mice infected with Mtb. The difference in the population of CD8⁺ effector T cells might affect the survival of intracellular Mtb. We also found a reduction of the inflammation response, including serum inflammatory cytokines and lung inflammation in LCN2^−/− mice, compared with wild-type mice, during Mtb infection.

Conclusions

These data suggest that LCN2-mediated reactive oxygen species affects expression of MHC class I molecules in BMDCs, leading to lower levels of CD8⁺ effector T-cell proliferation during mycobacterial infection.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-021-00686-2.

Tryptophan: From Diet to Cardiovascular Diseases

Cardiovascular disease (CVD) is one of the major causes of mortality worldwide. Inflammation is the underlying common mechanism involved in CVD. It has been recently related to amino acid metabolism, which acts as a critical regulator of innate and adaptive immune responses. Among different metabolites that have emerged as important regulators of immune and inflammatory responses, tryptophan (Trp) metabolites have been shown to play a pivotal role in CVD. Here, we provide an overview of the fundamental aspects of Trp metabolism and the interplay between the dysregulation of the main actors involved in Trp metabolism such as indoleamine 2, 3-dioxygenase 1 (IDO) and CVD, including atherosclerosis and myocardial infarction. IDO has a prominent and complex role. Its activity, impacting on several biological pathways, complicates our understanding of its function, particularly in CVD, where it is still under debate. The discrepancy of the observed IDO effects could be potentially explained by its specific cell and tissue contribution, encouraging further investigations regarding the role of this enzyme. Thus, improving our understanding of the function of Trp as well as its derived metabolites will help to move one step closer towards tailored therapies aiming to treat CVD.

The anti-inflammatory cytokine interleukin-37 is an inhibitor of trained immunity

Trained immunity (TI) is a de facto innate immune memory program induced in monocytes/macrophages by exposure to pathogens or vaccines, which evolved as protection against infections. TI is characterized by immunometabolic changes and histone post-translational modifications, which enhance production of pro-inflammatory cytokines. As aberrant activation of TI is implicated in inflammatory diseases, tight regulation is critical; however, the mechanisms responsible for this modulation remain elusive. Interleukin-37 (IL-37) is an anti-inflammatory cytokine that curbs inflammation and modulates metabolic pathways. In this study, we show that administration of recombinant IL-37 abrogates the protective effects of TI in vivo, as revealed by reduced host pro-inflammatory responses and survival to disseminated candidiasis. Mechanistically, IL-37 reverses the immunometabolic changes and histone post-translational modifications characteristic of TI in monocytes, thus suppressing cytokine production in response to infection. IL-37 thereby emerges as an inhibitor of TI and as a potential therapeutic target in immune-mediated pathologies.

Kynurenine Relaxes Arteries of Normotensive Women and Those With Preeclampsia

Supplemental Digital Content is available in the text.

Rationale:

Activation of the kynurenine pathway of tryptophan catabolism by infection and inflammation contributes to the development of systemic hypotension. Commercially-available kynurenine has direct vasorelaxant effects on arteries from several species and reduces systemic blood pressure when administered to normotensive or hypertensive rats.

Objectives:

To determine whether kynurenine promotes relaxation of human resistance arteries from normotensive and hypertensive pregnant women and to identify the vascular mechanism of its effects.

Methods and Results:

In isolated omental and myometrial resistance arteries from normotensive pregnant women, kynurenine (1 mmol/L) significantly reduced U46619-induced constriction (omentum N=14, P=2.4×10⁻³; myometrium N=21–25, P=2.6×10⁻⁴) and relaxed preconstricted arteries (N=53, P=1.0×10⁻¹¹; N=20, P=8.8×10⁻³). Vasorelaxation persisted following endothelium removal (N=7, P=1.6×10⁻⁴) but was completely prevented by inhibition of large-conductance Ca²⁺-activated K⁺ channels (BK_Ca) channels with iberiotoxin (N=9, P=5.7×10⁻⁴) or paxilline (N=10, P=2.1×10⁻¹⁷). Accordingly, in isolated vascular smooth muscle cells from omental arteries, kynurenine increased the BK_Ca current (n=5–8, P=0.022) and the amplitude of spontaneous transient outward currents (n=6, P=0.031) but did not affect spontaneous transient outward current frequency. Kynurenine also increased Ca²⁺ spark frequency of pressurized omental arteries (n=8, P=0.031). Vasorelaxant effects of kynurenine persisted following inhibition of ryanodine receptors (N=7, P=0.48) but were moderately reduced by inhibition of adenylate cyclase (N=9, P=0.024). In arteries from women with preeclampsia, kynurenine similarly attenuated vasoconstriction (N=15, P=1.3×10⁻⁵) and induced BK_Ca-mediated vasodilation (N=16, P=2.0×10⁻⁴). Vasorelaxation in response to kynurenine and a specific BK_Ca activator, NS11021, was absent in fetal-derived placental resistance arteries in normal pregnancy and preeclampsia.

Conclusions:

Kynurenine dilates systemic arteries from multiple territories via BK_Ca activation. Notably, the vasorelaxatory capacity of kynurenine is preserved in preeclampsia, suggesting this approach may have translational potential for the treatment of hypertension in pregnancy. The data warrant further investigation of the potential to exploit this endogenous vasorelaxant as a new treatment for hypertensive pathologies.

Regulation of vascular tone and blood pressure by singlet molecular oxygen in inflammation

PURPOSE OF REVIEW

The principle aim of this review is to prompt vascular researchers interested in vascular inflammation and oxidative stress to consider singlet molecular oxygen (1O2) as a potentially relevant contributor. A secondary goal is to propose novel treatment strategies to address haemodynamic complications associated with septic shock.

RECENT FINDINGS

Increased inflammation and oxidative stress are hallmarks of a range of vascular diseases. We recently showed that in systemic inflammation and oxidative stress associated with models of inflammation including sepsis, the tryptophan catabolizing enzyme indoleamine 2,3-dioxygenase-1 (Ido1) contributes to hypotension and decreased blood pressure through production of singlet molecular oxygen (1O2). Once formed, 1O2 converts tryptophan bound to Ido1 to a vasoactive hydroperoxide which decreases arterial tone and blood pressure via oxidation of a specific cysteine residue of protein kinase G1α.

SUMMARY

These works show, for the first time, that 1O2 contributes to arterial redox signalling and that Ido1 contributes to the regulation of blood pressure through production of a novel tryptophan-derived hydroperoxide, thus presenting a new signalling pathway as novel target in the treatment of blood pressure disorders such as sepsis.

The Kynurenine Pathway is Differentially Activated in Children with Lyme Disease and Tick-Borne Encephalitis

In children, tick-borne encephalitis and neuroborreliosis are common infections affecting the central nervous system. As inflammatory pathways including cytokine expression are activated in these children and appear to be of importance for outcome, we hypothesized that induction of the kynurenine pathway may be part of the pathophysiological mechanism. Inflammatory biomarkers were analyzed in cerebrospinal fluid from 22 children with tick-borne encephalitis (TBE), 34 children with neuroborreliosis (NB) and 6 children with no central nervous system infection. Cerebrospinal fluid levels of kynurenine and kynurenic acid were increased in children with neuroborreliosis compared to the comparison group. A correlation was seen between expression of several cerebrospinal fluid cytokines and levels of kynurenine and kynurenic acid in children with neuroborreliosis but not in children with tick-borne encephalitis. These findings demonstrate a strong induction of the kynurenine pathway in children with neuroborreliosis which differs from that seen in children with tick-borne encephalitis. The importance of brain kynurenic acid (KYNA) in both immune modulation and neurotransmission raises the possibility that abnormal levels of the compound in neuroborreliosis might be of importance for the pathophysiology of the disease. Drugs targeting the enzymes of this pathway may open the venue for novel therapeutic interventions.

Necroptosis is active and contributes to intestinal injury in a piglet model with lipopolysaccharide challenge

Necroptosis, a newly discovered form of programmed cell death that combines the features of apoptosis and necrosis, is important in various physiological and pathological disorders. However, the role of necroptosis on intestinal injury during sepsis has been rarely evaluated. This study aimed to investigate the presence of necroptosis in intestinal injury, and its contribution to intestinal injury in a piglet model challenged with Escherichia coli lipopolysaccharide (LPS). Firstly, a typical cell necrotic phenomenon was observed in jejunum of LPS-challenged pigs by transmission electron microscope. Protein expression of necroptosis signals including receptor-interacting protein kinase (RIP) 1, RIP3, and phosphorylated mixed-lineage kinase domain-like protein (MLKL), mitochondrial proteins including phosphoglycerate mutase family member 5 (PGAM5) and dynamin-related protein 1 (DRP1), and cytoplasmic high-mobility group box 1 (HMGB1) were time-independently increased in jejunum of LPS-challenged piglets, which was accompanied by the impairment of jejunal morphology, and digestive and barrier function indicated by lower activities of jejunal disaccharidases and protein expression of jejunal tight junction proteins claudin-1 and occludin. Pro-inflammatory cytokines including tumor necrosis factor-α (TNF-α), interleukin (IL)-1β, and IL-6 were also dynamically induced in serum and jejunum of piglets after LPS challenge. Moreover, pretreatment with necrostatin-1 (Nec-1), an specific inhibitor of necroptosis, inhibited necroptosis indicated by decreased necrotic ultrastructural changes and decreased protein expression of RIP1, RIP3, and phosphorylated MLKL as well as PGAM5, DRP1, and cytoplasmic HMGB1. Nec-1 pretreatment reduced jejunal morphological injury, and improved digestive and barrier function. Nec-1 pretreatment also decreased the levels of serum and jejunal pro-inflammatory cytokines and the numbers of jejunal macrophages and monocytes. These findings indicate for the first time that necroptosis is present and contributes to LPS-induced intestinal injury. Nec-1 may have a preventive effect on intestinal injury during sepsis.

The immunometabolic role of indoleamine 2,3-dioxygenase in atherosclerotic cardiovascular disease: immune homeostatic mechanisms in the artery wall

Coronary heart disease and stroke, the two most common cardiovascular diseases worldwide, are triggered by complications of atherosclerosis. Atherosclerotic plaques are initiated by a maladaptive immune response triggered by accumulation of lipids in the artery wall. Hence, disease is influenced by several non-modifiable and modifiable risk factors, including dyslipidaemia, hypertension, smoking, and diabetes. Indoleamine 2,3-dioxygenase (IDO), the rate-limiting enzyme in the kynurenine pathway of tryptophan (Trp) degradation, is modulated by inflammation and regarded as a key molecule driving immunotolerance and immunosuppressive mechanisms. A large body of evidence indicates that IDO-mediated Trp metabolism is involved directly or indirectly in atherogenesis. This review summarizes evidence from basic and clinical research showing that IDO is a major regulatory enzyme involved in the maintenance of immunohomeostasis in the vascular wall, as well as current knowledge about promising targets for the development of new anti-atherosclerotic drugs.

Epidermal Growth Factor Receptor-Responsive Indoleamine 2,3-Dioxygenase Confers Immune Homeostasis During Shigella flexneri Infection

The resolution of Shigella flexneri infection-associated hyperinflammation is crucial for host survival. Using in vitro and in vivo models of shigellosis, we found that S. flexneri induces the expression of indoleamine 2,3-dioxygenase 1 (IDO1) through the nucleotide oligomerization domain 2 (NOD2) and epidermal growth factor receptor (EGFR) signaling pathway. Congruently, abrogation of NOD2 or EGFR compromises the ability of S. flexneri to induce IDO1 expression. We observed that the loss of IDO1 function in vivo exacerbates shigellosis by skewing the inflammatory cytokine response, disrupting colon epithelial barrier integrity and consequently limiting the host life-span. Interestingly, administration of recombinant EGF rescued mice from IDO1 inhibition-driven aggravated shigellosis by restoring the cytokine balance and subsequently restricting bacterial growth. This is the first study that underscores the direct implication of the NOD2-EGFR axis in IDO1 production and its crucial homeostatic contributions during shigellosis. Together, these findings reveal EGF as a potential therapeutic intervention for infectious diseases.

Lithium attenuates d-amphetamine-induced hyperlocomotor activity in mice via inhibition of interaction between cyclooxygenase-2 and indoleamine-2,3-dioxygenase

In the present study, we investigated whether mood stabilizer lithium (Li) protects against d-amphetamine (AMP)-induced mania-like behaviours via modulating the novel proinflammatory potential. Repeated treatment with AMP resulted in significant increases in proinflammatory cyclooxygenase-2 (COX-2) and indolemaine-2,3-dioxygenase-1 (IDO)-1 expression in the prefrontal cortex (PFC) of mice. However, AMP treatment did not significantly change IDO-2 and 5-lipoxygenase (5-LOX) expression, suggesting that proinflammatory parameters such as COX-2 and IDO-1 are specific for AMP-induced behaviours. AMP-induced initial expression of COX-2 (15 minutes post-AMP) was earlier than that of IDO-1 (1 hour post-AMP). Mood stabilizer Li and COX-2 inhibitor meloxicam significantly attenuated COX-2 expression 15 minutes post-AMP, whereas IDO-1 inhibitor 1-methyl-DL-tryptophan (1-MT) did not affect COX-2 expression. However, AMP-induced IDO-1 expression was significantly attenuated by Li, meloxicam or 1-MT, suggesting that COX-2 is an upstream molecule for the induction of IDO-1 caused by AMP. Consistently, co-immunoprecipitation between COX-2 and IDO-1 was observed at 30 minutes, 1, 3, and 6 hours after the final AMP treatment. This interaction was also significantly inhibited by Li, meloxicam or 1-MT. Furthermore, AMP-induced hyperlocomotion was significantly attenuated by Li, meloxicam or 1-MT. We report, for the first time, that mood stabilizer Li attenuates AMP-induced mania-like behaviour via attenuation of interaction between COX-2 and IDO-1, and that the interaction of COX-2 and IDO-1 may be critical for the therapeutic intervention mediated by mood stabilizer.

Immunomodulatory Effects of Genetic Alterations Affecting the Kynurenine Pathway

Several enzymes and metabolites of the kynurenine pathway (KP) have immunomodulatory effects. Modulation of the activities and levels of these molecules might be of particular importance under disease conditions when the amelioration of overreacting immune responses is desired. Results obtained by the use of animal and tissue culture models indicate that by eliminating or decreasing activities of key enzymes of the KP, a beneficial shift in disease outcome can be attained. This review summarizes experimental data of models in which IDO, TDO, or KMO activity modulation was achieved by interventions affecting enzyme production at a genomic level. Elimination of IDO activity was found to improve the outcome of sepsis, certain viral infections, chronic inflammation linked to diabetes, obesity, aorta aneurysm formation, and in anti-tumoral processes. Similarly, lack of TDO activity was advantageous in the case of anti-tumoral immunity, while KMO inhibition was found to be beneficial against microorganisms and in the combat against tumors, as well. On the other hand, the complex interplay among KP metabolites and immune function in some cases requires an increase in a particular enzyme activity for the desired immune response modulation, as was shown by the exacerbation of liver fibrosis due to the elimination of IDO activity and the detrimental effects of TDO inhibition in a mouse model of autoimmune gastritis. The relevance of these studies concerning possible human applications are discussed and highlighted. Finally, a brief overview is presented on naturally occurring genetic variants affecting immune functions via modulation of KP enzyme activity.

Switch of NAD Salvage to de novo Biosynthesis Sustains SIRT1-RelB-Dependent Inflammatory Tolerance

A typical inflammatory response sequentially progresses from pro-inflammatory, immune suppressive to inflammatory repairing phases. Although the physiological inflammatory response resolves in time, severe acute inflammation usually sustains immune tolerance and leads to high mortality, yet the underlying mechanism is not completely understood. Here, using the leukemia-derived THP-1 human monocytes, healthy and septic human peripheral blood mononuclear cells (PBMC), we report that endotoxin dose-dependent switch of nicotinamide adenine dinucleotide (NAD) biosynthesis pathways sustain immune tolerant status. Low dose endotoxin triggered nicotinamide phosphoribosyltransferase (NAMPT)-dependent NAD salvage activity to adapt pro-inflammation. In contrast, high dose endotoxin drove a shift of NAD synthesis pathway from early NAMPT-dependent NAD salvage to late indoleamine 2,3-dioxygenase-1 (IDO1)-dependent NAD de novo biosynthesis, leading to persistent immune suppression. This is resulted from the IDO1-dependent expansion of nuclear NAD pool and nuclear NAD-dependent prolongation of sirtuin1 (SIRT1)-directed epigenetics of immune tolerance. Inhibition of IDO1 activity predominantly decreased nuclear NAD level, which promoted sequential dissociations of immunosuppressive SIRT1 and RelB from the promoter of pro-inflammatory TNF-α gene and broke endotoxin tolerance. Thus, NAMPT-NAD-SIRT1 axis adapts pro-inflammation, but IDO1-NAD-SIRT1-RelB axis sustains endotoxin tolerance during acute inflammatory response. Remarkably, in contrast to the prevention of sepsis death of animal model by IDO1 inhibition before sepsis initiation, we demonstrated that the combination therapy of IDO1 inhibition by 1-methyl-D-tryptophan (1-MT) and tryptophan supplementation rather than 1-MT administration alone after sepsis onset rescued sepsis animals, highlighting the translational significance of tryptophan restoration in IDO1 targeting therapy of severe inflammatory diseases like sepsis.

Inhibition of indoleamine 2,3-dioxygenase attenuates endotoxin-mediated tolerance in granulosa cells through kynurenine pathway

Ovarian granulosa cells (GCs) have been shown to have innate immune capabilities, which modulate their native endocrine functions through toll-like receptors (TLRs). We have recently shown that GCs exposed to lipopolysaccharide (LPS; 1.0 µg/mL) transiently regulate proinflammatory cytokine expression (interleukin 1β [IL-1β], IL-6, and tumor necrosis factor α) through chromatin remodeling. In the present study, we have demonstrated that GCs become tolerant to LPS on repeated exposure of LPS. To understand the mechanism of this endotoxin tolerance (ET) phenomenon in buffalo GCs, we have further studied the genome-wide transcriptomic analyses in buffalo GCs (unpublished data) and identified indoleamine 2,3-dioxygenase 1 (IDO1) gene, known to be involved in tryptophan catabolism, was found to be highly upregulated in endotoxin-tolerant GCs. Real-time gene expression analyses also showed similar results. Further analyses of tryptophan and tryptophan metabolite, kynurenine, showed that tryptophan was found to be depleted with the accumulation of kynurenine in the endotoxin-tolerant GCs. The effect of IDO1 induced ET was reversed when cells were pretreated with IDO1 inhibitor (1-methyl tryptophan, 1 mM). To the best of our knowledge, this is the first report describing the role of IDO1 gene in ET in GCs mimicked by repeated endotoxin exposure in vitro. In summary, the present study convincingly demonstrated that the tryptophan catabolism, through the kynurenine pathway, plays a crucial role as an immunomodulatory mechanism of ET in GCs. The finding could be exploited in developing potential therapeutics to treat impaired GCs function due to the ET underlying prolonged uterine or systemic infection leads to accumulation of endotoxin in follicular fluid.

Orchestration of Tryptophan-Kynurenine Pathway, Acute Decompensation, and Acute-on-Chronic Liver Failure in Cirrhosis

Systemic inflammation (SI) is involved in the pathogenesis of acute decompensation (AD) and acute-on-chronic liver failure (ACLF) in cirrhosis. In other diseases, SI activates tryptophan (Trp) degradation through the kynurenine pathway (KP), giving rise to metabolites that contribute to multiorgan/system damage and immunosuppression. In the current study, we aimed to characterize the KP in patients with cirrhosis, in whom this pathway is poorly known. The serum levels of Trp, key KP metabolites (kynurenine and kynurenic and quinolinic acids), and cytokines (SI markers) were measured at enrollment in 40 healthy subjects, 39 patients with compensated cirrhosis, 342 with AD (no ACLF) and 180 with ACLF, and repeated in 258 patients during the 28-day follow-up. Urine KP metabolites were measured in 50 patients with ACLF. Serum KP activity was normal in compensated cirrhosis, increased in AD and further increased in ACLF, in parallel with SI; it was remarkably higher in ACLF with kidney failure than in ACLF without kidney failure in the absence of differences in urine KP activity and fractional excretion of KP metabolites. The short-term course of AD and ACLF (worsening, improvement, stable) correlated closely with follow-up changes in serum KP activity. Among patients with AD at enrollment, those with the highest baseline KP activity developed ACLF during follow-up. Among patients who had ACLF at enrollment, those with immune suppression and the highest KP activity, both at baseline, developed nosocomial infections during follow-up. Finally, higher baseline KP activity independently predicted mortality in patients with AD and ACLF. Conclusion: Features of KP activation appear in patients with AD, culminate in patients with ACLF, and may be involved in the pathogenesis of ACLF, clinical course, and mortality.

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.

Effects of 1-Methyltryptophan on Immune Responses and the Kynurenine Pathway after Lipopolysaccharide Challenge in Pigs

An enhanced indoleamine 2,3-dioxygenase 1 (IDO1) activity is associated with an increased mortality risk in sepsis patients. Thus, the preventive inhibition of IDO1 activity may be a promising strategy to attenuate the severity of septic shock. 1-methyltryptophan (1-MT) is currently in the interest of research due to its potential inhibitory effects on IDO1 and immunomodulatory properties. The present study aims to investigate the protective and immunomodulatory effects of 1-methyltryptophan against endotoxin-induced shock in a porcine in vivo model. Effects of 1-MT were determined on lipopolysaccharide (LPS)-induced tryptophan (TRP) degradation, immune response and sickness behaviour. 1-MT increased TRP and its metabolite kynurenic acid (KYNA) in plasma and tissues, suppressed the LPS-induced maturation of neutrophils and increased inactivity of the animals. 1-MT did not inhibit the LPS-induced degradation of TRP to kynurenine (KYN)-a marker for IDO1 activity-although the increase in KYNA indicates that degradation to one branch of the KYN pathway is facilitated. In conclusion, our findings provide no evidence for IDO1 inhibition but reveal the side effects of 1-MT that may result from the proven interference of KYNA and 1-MT with aryl hydrocarbon receptor signalling. These effects should be considered for therapeutic applications of 1-MT.

An integrated metabolomic strategy for the characterization of the effects of Chinese yam and its three active components on septic cardiomyopathy

This integrated metabolomic approach interpreted the effects of Chinese yam on septic cardiomyopathy and the roles of its major active components.

Rotavirus Degrades Multiple Interferon (IFN) Type Receptors To Inhibit IFN Signaling and Protects against Mortality from Endotoxin in Suckling Mice

STAT1 phosphorylation in response to exogenous interferon (IFN) administration can be inhibited by rotaviral replication both in vitro and in vivo In addition many rotavirus strains are resistant to the actions of different IFN types. The regulation by rotaviruses (RVs) of antiviral pathways mediated by multiple IFN types is not well understood. In this study, we find that during infection in vitro and in vivo, RVs significantly deplete IFN type I, II, and III receptors (IFNRs). Regulation of IFNRs occurred exclusively within RV-infected cells and could be abrogated by inhibiting the lysosomal-endosomal degradation pathway. In vitro, IFNR degradation was conserved across multiple RV strains that differ in their modes of regulating IFN induction. In suckling mice, exogenously administered type I, II, or III IFN induced phosphorylation of STAT1-Y701 within intestinal epithelial cells (IECs) of suckling mice. Murine EW strain RV infection transiently activated intestinal STAT1 at 1 day postinfection (dpi) but not subsequently at 2 to 3 dpi. In response to injection of purified IFN-α/β or -λ, IECs in EW-infected mice exhibited impaired STAT1-Y701 phosphorylation, correlating with depletion of different intestinal IFNRs and impaired IFN-mediated transcription. The ability of EW murine RV to inhibit multiple IFN types led us to test protection of suckling mice from endotoxin-mediated shock, an outcome that is dependent on the host IFN response. Compared to mortality in controls, mice infected with EW murine RV were substantially protected against mortality following parenteral endotoxin administration. These studies identify a novel mechanism of IFN subversion by RV and reveal an unexpected protective effect of RV infection on endotoxin-mediated shock in suckling mice.IMPORTANCE Antiviral functions of types I, II, and III IFNs are mediated by receptor-dependent activation of STAT1. Here, we find that RV degrades the types I, II, and III IFN receptors (IFNRs) in vitro In a suckling mouse model, RV effectively blocked STAT1 activation and transcription following injection of different purified IFNs. This correlated with significantly decreased protein expression of intestinal types I and II IFNRs. Recent studies demonstrate that in mice lipopolysaccharide (LPS)-induced lethality is prevented by genetic ablation of IFN signaling genes such as IFNAR1 and STAT1. When suckling mice were infected with RV, they were substantially protected from lethal exposure to endotoxin. These findings provide novel insights into the mechanisms underlying rotavirus regulation of different interferons and are likely to stimulate new research into both rotavirus pathogenesis and endotoxemia.

The interplay between cytokines and the Kynurenine pathway in inflammation and atherosclerosis

The kynurenine pathway (KP) is the major metabolic route of tryptophan (Trp) metabolism. Indoleamine 2,3-dioxygenase (IDO1), the enzyme responsible for the first and rate-limiting step in the pathway, as well as other enzymes in the pathway, have been shown to be highly regulated by cytokines. Hence, the KP has been implicated in several pathologic conditions, including infectious diseases, psychiatric disorders, malignancies, and autoimmune and chronic inflammatory diseases. Additionally, recent studies have linked the KP with atherosclerosis, suggesting that Trp metabolism could play an essential role in the maintenance of immune homeostasis in the vascular wall. This review summarizes experimental and clinical evidence of the interplay between cytokines and the KP and the potential role of the KP in cardiovascular diseases.

Vasorelaxing Action of the Kynurenine Metabolite, Xanthurenic Acid: The Missing Link in Endotoxin-Induced Hypotension?

The kynurenine pathway of tryptophan metabolism is activated by pro-inflammatory cytokines. L-kynurenine, an upstream metabolite of the pathway, acts as a putative endothelium-derived relaxing factor, and has been hypothesized to play a causative role in the pathophysiology of inflammation-induced hypotension. Here, we show that xanthurenic acid (XA), the transamination product of 3-hydroxykynurenine, is more efficacious than L-kynurenine in causing relaxation of a resistance artery, but fails to relax pre-contracted aortic rings. In the mesenteric artery, XA enhanced activating phosphorylation of endothelial nitric oxide synthase (NOS), and the relaxing action of XA was abrogated by pharmacological inhibition of NOS and endothelial-derived hyperpolarizing factor. Systemic injection of XA reduced blood pressure in mice, and serum levels of XA increased by several fold in response to a pulse with the endotoxin, lipopolysaccharide (LPS). LPS-induced hypotension in mice was prevented by pre-treatment with the kynurenine monooxygenase (KMO) inhibitor, Ro-618048, which lowered serum levels of XA but enhanced serum levels of L-kynurenine. UPF 648, another KMO inhibitor, could also abrogate LPS-induced hypotension. Our data identify XA as a novel vasoactive compound and suggest that formation of XA is a key event in the pathophysiology of inflammation-induced hypotension.

Neoagarooligosaccharides prevent septic shock by modulating A20-and cyclooxygenase-2-mediated interleukin-10 secretion in a septic-shock mouse model

Analysis of the signaling mechanism triggered by endotoxin-mediated toll-like receptor-4 activation using immune cell systems or rodent models may help identify potential agents for the prevention of Gram-negative bacteria infection. β-agarase cleaves the β-1,4-linkages of agar to produce neoagarooligosaccharides (NAOs), which have various physiological functions. The aim of this study was to investigate the efficacy of NAOs in preventing experimental sepsis caused by the administration of endotoxin or Gram-negative bacteria. Organ damage and neutrophil infiltration in an endotoxemia and septic-shock mouse model were suppressed by NAOs. Pro-inflammatory cytokine level was decreased, but IL-10 level was increased by NAO-treatment. Further induction by NAOs in the presence of endotoxin was associated with a significant induction of A20 and cyclooxygenase (COX)-2 expressions. Our data suggest that NAOs have a beneficial preventive effect in septic shock correlated with the enhancement of IL-10 via the induction of A20 and COX-2.

The role of indoleamine 2,3-dioxygenase-aryl hydrocarbon receptor pathway in the TLR4-induced tolerogenic phenotype in human DCs

A controlled inflammatory response is required for protection against infection, but persistent inflammation causes tissue damage. Dendritic cells (DCs) have a unique capacity to promote both inflammatory and anti-inflammatory processes. One key mechanism involved in DC-mediated immunosuppression is the expression of tryptophan-metabolizing enzyme indoleamine 2,3-dioxygenase (IDO). IDO has been implicated in diverse processes in health and disease but its role in endotoxin tolerance in human DCs is still controversial. Here we investigated the role of IDO in shaping DCs phenotype and function under endotoxin tolerance conditions. Our data show that TLR4 ligation in LPS-primed DCs, induced higher levels of both IDO isoforms together with the transcription factor aryl-hydrocarbon receptor (AhR), compared to unprimed controls. Additionally, LPS conditioning induced an anti-inflammatory phenotype in DCs - with an increase in IL-10 and higher expression of programmed death ligand (PD-L)1 and PD-L2 - which were partially dependent on IDO. Furthermore, we demonstrated that the AhR-IDO pathway was responsible for the preferential activation of non-canonical NF-κB pathway in LPS-conditioned DCs. These data provide new insight into the mechanisms of the TLR4-induced tolerogenic phenotype in human DCs, which can help the better understanding of processes involved in induction and resolution of chronic inflammation and tolerance.

Co-delivery of indoleamine 2,3-dioxygenase prevents loss of expression of an antigenic transgene in dystrophic mouse muscles

A significant problem affecting gene therapy approaches aiming at achieving long-term transgene expression is the immune response against the protein product of the therapeutic gene, which can reduce or eliminate the therapeutic effect. The problem is further exacerbated when therapy involves targeting an immunogenic tissue and/or one with a pre-existing inflammatory phenotype, such as dystrophic muscles. In this proof-of-principle study, we co-expressed a model antigen, bacterial β-galactosidase, with an immunosuppressive factor, indoleamine 2,3-dioxygenase 1 (IDO1), in muscles of the mdx mouse model of Duchenne muscular dystrophy. This treatment prevented loss of expression of the transgene concomitant with significantly elevated expression of T-regulatory (Treg) markers in the IDO1-expressing muscles. Moreover, co-expression of IDO1 resulted in reduced serum levels of anti-β-gal antibodies. These data indicate that co-expression of genes encoding immunomodulatory enzymes controlling kynurenine pathways provide a viable strategy for preventing loss of transgenes targeted into dystrophic muscles with pre-existing inflammation.

Necrostatin-1 rescues mice from lethal irradiation

There is an emerging need in new medical products that can mitigate and/or treat the short- and long-term consequences of radiation exposure after a radiological or nuclear terroristic event. The direct effects of ionizing radiation are realized primarily via apoptotic death pathways in rapidly proliferating cells within the initial 1-2days after the exposure. However later in the course of the radiation disease necrotic cell death may ensue via direct and indirect pathways from increased generation of pro-inflammatory cytokines. Here we evaluated radiomitigative potential of necrostatin-1 after total body irradiation (TBI) and the contribution of necroptosis to cell death induced by radiation. Circulating TNFα levels were increased starting on d1 after TBI and associated with increased plasmalemma permeability in ileum of irradiated mice. Necrostatin-1 given iv. 48h after 9.5Gy TBI attenuated radiation-induced receptor interacting protein kinase 3 (RIPK3) serine phosphorylation in ileum and improved survival vs. vehicle. Utilizing apoptosis resistant cytochrome c(-/-) cells, we showed that radiation can induce necroptosis, which is attenuated by RNAi knock down of RIPK1 and RIPK3 or by treatment with necrostatin-1 or -1s whereas 1-methyl-L-tryptophan, an indoleamine-2,3-dioxygenase inhibitor, did not exhibit radiomitigative effect. This suggests that the beneficial effect of necrostatin-1 is likely through inhibition of RIPK1-mediated necroptotic pathway. Overall, our data indicate that necroptosis, a form of programmed necrosis, may play a significant role in cell death contributing to radiation disease and mortality. This study provides a proof of principle that necrostatin-1 and perhaps other RIPK1 inhibitors are promising therapeutic agents for radiomitigation after TBI.

Myeloid deletion of SIRT1 suppresses collagen-induced arthritis in mice by modulating dendritic cell maturation

The type III histone deacetylase silent information regulator 1 (SIRT1) is an enzyme that is critical for the modulation of immune and inflammatory responses. However, the data on its role in rheumatoid arthritis (RA) are limited and controversial. To better understand how SIRT1 regulates adaptive immune responses in RA, we evaluated collagen-induced arthritis (CIA) in myeloid cell-specific SIRT1 knockout (mSIRT1 KO) and wild-type (WT) mice. Arthritis severity was gauged on the basis of clinical, radiographic and pathologic scores. Compared with their WT counterparts, the mSIRT1 KO mice exhibited less severe arthritis, which was less destructive to the joints. The expression levels of inflammatory cytokines, matrix metalloproteinases and ROR-γT were also reduced in the mSIRT1 KO mice compared with the WT mice and were paralleled by reductions in the numbers of Th1 and Th17 cells and CD80- or CD86-positive dendritic cells (DCs). In addition, impaired DC maturation and decreases in the Th1/Th17 immune response were observed in the mSIRT1 KO mice. T-cell proliferation was also investigated in co-cultures with antigen-pulsed DCs. In the co-cultures, the DCs from the mSIRT1 KO mice showed decreases in T-cell proliferation and the Th1/Th17 immune response. In this study, myeloid cell-specific deletion of SIRT1 appeared to suppress CIA by modulating DC maturation. Thus, a careful investigation of DC-specific SIRT1 downregulation is needed to gauge the therapeutic utility of agents targeting SIRT1 in RA.