Liver grafts from CD39-overexpressing rodents are protected from ischemia reperfusion injury due to reduced numbers of resident CD4+ T cells

Understanding Liver Transplantation Outcomes Through the Lens of Its Tissue-resident Immunobiome

Tissue-resident lymphocytes (TRLs) provide a front-line immunological defense mechanism uniquely placed to detect perturbations in tissue homeostasis. The heterogeneous TRL population spans the innate to adaptive immune continuum, with roles during normal physiology in homeostatic maintenance, tissue repair, pathogen detection, and rapid mounting of immune responses. TRLs are especially enriched in the liver, with every TRL subset represented, including liver-resident natural killer cells; tissue-resident memory B cells; conventional tissue-resident memory CD8, CD4, and regulatory T cells; and unconventional gamma-delta, natural killer, and mucosal-associated invariant T cells. The importance of donor- and recipient-derived TRLs after transplantation is becoming increasingly recognized, although it has not been examined in detail after liver transplantation. This review summarizes the evidence for the roles of TRLs in liver transplant immunology, focusing on their features, functions, and potential for their harnessing to improve transplant outcomes.

Ectonucleotidases in Ischemia Reperfusion Injury: Unravelling the Interplay With Mitochondrial Dysfunction in Liver Transplantation

Ischemia-reperfusion injury (IRI) profoundly impacts organ transplantation, especially in orthotopic liver transplantation (OLT). Disruption of the mitochondrial respiratory chain during ischemia leads to ATP loss and ROS production. Reperfusion exacerbates mitochondrial damage, triggering the release of damage-associated molecular patterns (DAMPs) and inflammatory responses. Mitochondrial dysfunction, a pivotal aspect of IRI, is explored in the context of the regulatory role of ectonucleotidases in purinergic signaling and immune responses. CD39, by hydrolyzing ATP and ADP; and CD73, by converting AMP to adenosine, emerge as key players in mitigating liver IRI, particularly through ischemic preconditioning and adenosine receptor signaling. Despite established roles in vascular health and immunity, the impact of ectonucleotidases on mitochondrial function during hepatic IRI is unclear. This review aims to elucidate the interplay between CD39/73 and mitochondria, emphasizing their potential as therapeutic targets for liver transplantation. This article explores the role of CD39/73 in tissue hypoxia, emphasizing adenosine production during inflammation. CD39 and CD73 upregulation under hypoxic conditions regulate immune responses, demonstrating protective effects in various organ-specific ischemic models. However, prolonged adenosine activation may have dual effects, beneficial in acute settings but detrimental in chronic hypoxia. Herein, we raise questions about ectonucleotidases influencing mitochondrial function during hepatic IRI, drawing parallels with cancer cell responses to chemotherapy. The review underscores the need for comprehensive research into the intricate interplay between ectonucleotidases, mitochondrial dynamics, and their therapeutic implications in hepatic IRI, providing valuable insights for advancing transplantation outcomes.

Adenosinergic metabolism pathway: an emerging target for improving outcomes of solid organ transplantation

Extracellular nucleotides are widely recognized as crucial modulators of immune responses in peripheral tissues. Adenosine triphosphate (ATP) and adenosine are key components of extracellular nucleotides, the balance of which contributes to immune homeostasis. Under tissue injury, ATP exerts its pro-inflammatory function, while the adenosinergic pathway rapidly degrades ATP to immunosuppressive adenosine, thus inhibiting excessive and uncontrolled inflammatory responses. Previous reviews have explored the immunoregulatory role of extracellular adenosine in various pathological conditions, especially inflammation and malignancy. However, current knowledge regarding adenosine and adenosinergic metabolism in the context of solid organ transplantation remains fragmented. In this review, we summarize the latest information on adenosine metabolism and the mechanisms by which it suppresses the effector function of immune cells, as well as highlight the protective role of adenosine in all stages of solid organ transplantation, including reducing ischemia reperfusion injury during organ procurement, alleviating rejection, and promoting graft regeneration after transplantation. Finally, we discuss the potential for future clinical translation of adenosinergic pathway in solid organ transplantation.

Classification of regulatory T cells and their role in myocardial ischemia-reperfusion injury

Myocardial ischemia-reperfusion injury (MIRI) is closely related to the final infarct size in acute myocardial infarction (AMI). Therefore, reducing MIRI can effectively improve the prognosis of AMI patients. At the same time, the healing process after AMI is closely related to the local inflammatory microenvironment. Regulatory T cells (Tregs) can regulate various physiological and pathological immune inflammatory responses and play an important role in regulating the immune inflammatory response after AMI. However, different subtypes of Tregs have different effects on MIRI, and the same subtype of Tregs may also have different effects at different stages of MIRI. This article systematically reviews the classification and function of Tregs, as well as the role of various subtypes of Tregs in MIRI. A comprehensive understanding of the role of each subtype of Tregs can help design effective methods to control immune reactions, reduce MIRI, and provide new potential therapeutic options for AMI.

Sterile inflammation in liver transplantation

Sterile inflammation is the immune response to damage-associated molecular patterns (DAMPs) released during cell death in the absence of foreign pathogens. In the setting of solid organ transplantation, ischemia-reperfusion injury results in mitochondria-mediated production of reactive oxygen and nitrogen species that are a major cause of uncontrolled cell death and release of various DAMPs from the graft tissue. When properly regulated, the immune response initiated by DAMP-sensing serves as means of damage control and is necessary for initiation of recovery pathways and re-establishment of homeostasis. In contrast, a dysregulated or overt sterile inflammatory response can inadvertently lead to further injury through recruitment of immune cells, innate immune cell activation, and sensitization of the adaptive immune system. In liver transplantation, sterile inflammation may manifest as early graft dysfunction, acute graft failure, or increased risk of immunosuppression-resistant rejection. Understanding the mechanisms of the development of sterile inflammation in the setting of liver transplantation is crucial for finding reliable biomarkers that predict graft function, and for development of therapeutic approaches to improve long-term transplant outcomes. Here, we discuss the recent advances that have been made to elucidate the early signs of sterile inflammation and extent of damage from it. We also discuss new therapeutics that may be effective in quelling the detrimental effects of sterile inflammation.

Identification of distinct immune infiltration and potential biomarkers in patients with liver ischemia-reperfusion injury

AIMS

To identify alterations of specific gene expression, immune infiltration components, and potential biomarkers in liver ischemia-reperfusion injury (IRI) following liver transplantation (LT).

MATERIALS AND METHODS

GSE23649 and GSE151648 datasets were obtained from the Gene Expression Omnibus (GEO) database. To determine the differentially expressed genes (DEGs), we utilized the R package "limma". We also identify the infiltration of different immune cells through single-sample gene-set enrichment analysis (ssGSEA). Furthermore, we utilized LASSO logistic regression to select feature genes and Spearman's rank correlation analysis to determine the correlation between these genes and infiltrating immune cells. Finally, the significance of these feature genes was confirmed using a mouse model of hepatic IRI.

KEY FINDINGS

A total of 17 DEGs were acquired, most of which were associated with inflammation, apoptosis, cell proliferation, immune disorders, stress response, and angiogenesis. 28 immune cell types were determined using ssGSEA. 5 feature genes (ADM, KLF6, SERPINE1, SLC20A1, and HBB) were screened using LASSO analysis, but the HBB gene was ultimately excluded due to the lack of statistical significance in the GSE151648 dataset. These 4 feature genes were predominantly related to immune cells. Finally, 15 significantly distinctive types of immune cells between the control and IRI groups were verified.

SIGNIFICANCE

We unveiled that macrophages, dendritic cells (DCs), neutrophils, CD4 T cells, and other immune cells infiltrated the IRI that occurred after LT. Moreover, we identified ADM, KLF6, SERPINE1, and SLC20A1 as potential biological biomarkers underlying IRI post-transplant, which may improve the diagnosis and prognosis of this condition.

ATP Secretion and Metabolism in Regulating Pancreatic Beta Cell Functions and Hepatic Glycolipid Metabolism

Diabetes (DM), especially type 2 diabetes (T2DM) has become one of the major diseases severely threatening public health worldwide. Islet beta cell dysfunctions and peripheral insulin resistance including liver and muscle metabolic disorder play decisive roles in the pathogenesis of T2DM. Particularly, increased hepatic gluconeogenesis due to insulin deficiency or resistance is the central event in the development of fasting hyperglycemia. To maintain or restore the functions of islet beta cells and suppress hepatic gluconeogenesis is crucial for delaying or even stopping the progression of T2DM and diabetic complications. As the key energy outcome of mitochondrial oxidative phosphorylation, adenosine triphosphate (ATP) plays vital roles in the process of almost all the biological activities including metabolic regulation. Cellular adenosine triphosphate participates intracellular energy transfer in all forms of life. Recently, it had also been revealed that ATP can be released by islet beta cells and hepatocytes, and the released ATP and its degraded products including ADP, AMP and adenosine act as important signaling molecules to regulate islet beta cell functions and hepatic glycolipid metabolism via the activation of P2 receptors (ATP receptors). In this review, the latest findings regarding the roles and mechanisms of intracellular and extracellular ATP in regulating islet functions and hepatic glycolipid metabolism would be briefly summarized and discussed.

The role and function of CD4+ T cells in hepatic ischemia-reperfusion injury

INTRODUCTION

Hepatic ischemia-reperfusion injury (IRI) is a severe complication frequently encountered in liver surgery, seriously affecting the therapeutic effects, tissue function. Various immune cells are involved in hepatic IRI, including macrophages, NKT cells, DCs, CD4 + T cells, and CD8 + T cells, among which CD4 + T cells play a critical role in this process. This article aims to summarize the functions and changes in various CD4 + T cell type counts and related cytokine levels in hepatic IRI and to review the possible mechanisms of mutual conversion between T cell types.

AREAS COVERED

We have covered the functions and changes that occur in Th1, Th17, and Treg cells in liver IRI, as well as the pathways and factors associated with them. We also discuss the prospects of clinical application and future directions for therapeutic advances.

EXPERT OPINION

This section explores the current clinical trials involving CD4 + T cells, especially Tregs, explains the limitations of their application, and summarizes the future development trends of cell engineering and their combination with the CAT technology. We also provide new ideas and therapeutic targets for alleviating liver IRI or other liver inflammatory diseases.

Interleukin-27 in liver xenotransplantation: A rational target to mitigate ischemia reperfusion injury and increase xenograft survival

Transplantation of xenogeneic organs is an attractive solution to the existing organ shortage dilemma, thus, securing a clinically acceptable prolongation of xenograft survival is an important goal. In preclinical transplantation models, recipients of liver, kidney, heart, or lung xenotransplants demonstrate significant graft damages through the release of pro-inflammatory molecules, including the C-reactive protein, cytokines, and histone-DNA complexes that all foster graft rejection. Recent studies have demonstrated that mitigation of ischemia reperfusion injury (IRI) greatly improves xenograft survival. Organ IRI develops primarily on a complex network of cytokines and chemokines responding to molecular cues from the graft milieu. Among these, interleukin 27 (IL-27) plays an immunomodulatory role in IRI onset due to graft environment-dependent pro- and anti- inflammatory activities. This review focuses on the impact of IL-27 on IRI of liver xenotransplants and provides insights on the function of IL-27 that could potentially guide genetic engineering strategies of donor pigs and/or conditioning of organs prior to transplantation.

Ischemia-reperfusion Injury in Allogeneic Liver Transplantation: A Role of CD4 T Cells in Early Allograft Injury

BACKGROUND

A major discrepancy between clinical and most experimental settings of liver ischemia-reperfusion injury (IRI) is the allogenicity.

METHODS

In the current study, we first established a murine model of allogeneic orthotopic liver transplantation with extended cold ischemia time (18 h). Roles of CD4 T cells in the pathogenesis of IRI in liver allografts were determined using a depleting anti-CD4 antibody. The clinical relevance of CD4 as a marker of liver IRI was analyzed retrospectively in 55 liver transplant patients.

RESULTS

CD4 depletion in both donors and recipients resulted in the most effective protection of liver allografts from IRI, as measured by serum transaminase levels and liver histology. CD4 depletion inhibited IR-induced intragraft neutrophil/macrophage infiltration and proinflammatory gene expressions. Quantitative reverse-transcriptase polymerase chain reaction analysis of human liver biopsies (2 h postreperfusion) revealed that posttransplant, rather than pretransplant, CD4 transcript levels correlated positively with proinflammatory gene expression profile. When we divided patients into subgroups according to intragraft CD4 levels, the high CD4 cohort developed a more severe hepatocellular damage than that with low CD4 levels.

CONCLUSIONS

CD4 T cells play a key pathogenic role in IRI of allogeneic liver transplants, and intragraft CD4 levels in the early postreperfusion phase may serve as a potential biomarker and therapeutic target to ameliorate liver IRI and improve orthotopic liver transplantation outcomes.

Role of Purinergic Signalling in Endothelial Dysfunction and Thrombo-Inflammation in Ischaemic Stroke and Cerebral Small Vessel Disease

The cerebral endothelium is an active interface between blood and the central nervous system. In addition to being a physical barrier between the blood and the brain, the endothelium also actively regulates metabolic homeostasis, vascular tone and permeability, coagulation, and movement of immune cells. Being part of the blood–brain barrier, endothelial cells of the brain have specialized morphology, physiology, and phenotypes due to their unique microenvironment. Known cardiovascular risk factors facilitate cerebral endothelial dysfunction, leading to impaired vasodilation, an aggravated inflammatory response, as well as increased oxidative stress and vascular proliferation. This culminates in the thrombo-inflammatory response, an underlying cause of ischemic stroke and cerebral small vessel disease (CSVD). These events are further exacerbated when blood flow is returned to the brain after a period of ischemia, a phenomenon termed ischemia-reperfusion injury. Purinergic signaling is an endogenous molecular pathway in which the enzymes CD39 and CD73 catabolize extracellular adenosine triphosphate (eATP) to adenosine. After ischemia and CSVD, eATP is released from dying neurons as a damage molecule, triggering thrombosis and inflammation. In contrast, adenosine is anti-thrombotic, protects against oxidative stress, and suppresses the immune response. Evidently, therapies that promote adenosine generation or boost CD39 activity at the site of endothelial injury have promising benefits in the context of atherothrombotic stroke and can be extended to current CSVD known pathomechanisms. Here, we have reviewed the rationale and benefits of CD39 and CD39 therapies to treat endothelial dysfunction in the brain.

Understanding, predicting and achieving liver transplant tolerance: from bench to bedside

In the past 40 years, liver transplantation has evolved from a high-risk procedure to one that offers high success rates for reversal of liver dysfunction and excellent patient and graft survival. The liver is the most tolerogenic of transplanted organs; indeed, immunosuppressive therapy can be completely withdrawn without rejection of the graft in carefully selected, stable long-term liver recipients. However, in other recipients, chronic allograft injury, late graft failure and the adverse effects of anti-rejection therapy remain important obstacles to improved success. The liver has a unique composition of parenchymal and immune cells that regulate innate and adaptive immunity and that can promote antigen-specific tolerance. Although the mechanisms underlying liver transplant tolerance are not well understood, important insights have been gained into how the local microenvironment, hepatic immune cells and specific molecular pathways can promote donor-specific tolerance. These insights provide a basis for the identification of potential clinical biomarkers that might correlate with tolerance or rejection and for the development of novel therapeutic targets. Innovative approaches aimed at promoting immunosuppressive drug minimization or withdrawal include the adoptive transfer of donor-derived or recipient-derived regulatory immune cells to promote liver transplant tolerance. In this Review, we summarize and discuss these developments and their implications for liver transplantation.

Implications of CD39 in immune-related diseases

Extracellular adenosine triphosphate (eATP) mediates pro-inflammatory responses by recruiting and activating inflammatory cells. CD39 can hydrolyze eATP into adenosine monophosphate (AMP), while CD73 can convert AMP into the immunosuppressive nucleoside adenosine (ADO). CD39 is a rate-limiting enzyme in this cascade, which is regarded as an immunological switch shifting the ATP-mediated pro-inflammatory environment to the ADO- mediated anti-inflammatory status. The CD39 expression can be detected in a wide spectrum of immunocytes, which is under the influence of environmental and genetic factors. It is increasingly suggested that, CD39 participates in some pathophysiological processes, like inflammatory bowel disease (IBD), sepsis, multiple sclerosis (MS), allergic diseases, ischemia-reperfusion (I/R) injury, systemic lupus erythematosus (SLE), diabetes and cancer. Here, we focus on the current understanding of CD39 in immunity, and comprehensively illustrate the diverse CD39 functions within a variety of disorders.

Skewed CD39/CD73/adenosine pathway contributes to B-cell hyperactivation and disease progression in patients with chronic hepatitis B

Background

The mechanisms underlying B-cell hyperactivation in patients with chronic hepatitis B virus (HBV) infection remain largely undefined. The present study assessed the clinical characteristics of the CD39/CD73/adenosine pathway in patients with chronic hepatitis B (CHB).

Methods

We examined CD39 and CD73 expression and adenosine production by B-cells from 202 HBV-infected patients. B-cell-activation phenotypes were assessed by flow cytometry after CpG+CD40 ligand stimulation with or without blockade and activation of the adenosine pathway.

Results

CD39 and CD73 expression on circulating B-cells was decreased in CHB patients with high HBV DNA, HBeAg positivity, high HBsAg levels, and active liver inflammation, and was hierarchically restored in complete responders according to HBeAg seroconversion or HBsAg reduction. However, CD39 and CD73 expression on activated memory and tissue-like memory B-cell subsets in complete responders was not increased despite effective antiviral treatments. Furthermore, CD39 and CD73 expression on intra-hepatic B-cells was decreased in inflammatory livers. In vitro, B-cells from CHB patients showed a markedly reduced capacity to generate CD39/CD73-dependent extracellular adenosine and expressed increased levels of activation markers after adenosine-production blockade. Contrastingly, metformin significantly reduced activation-marker expression via regulating AMP-activated protein kinase.

Conclusions

The skewed CD39 and CD73 expression on B-cells was associated with a high viral burden, liver inflammation, and antiviral efficacy in CHB patients, and the skewed CD39/CD73/adenosine pathway contributed to B-cell hyperactivation. Regulation of the CD39/CD73/adenosine pathway using metformin may represent a therapeutic option to reverse HBV-induced immune pathogenesis.

Purinergic signalling in liver diseases: Pathological functions and therapeutic opportunities

Extracellular nucleotides, including ATP, are essential regulators of liver function and serve as danger signals that trigger inflammation upon injury. Ectonucleotidases, which are expressed by liver-resident cells and recruited immune cells sequentially hydrolyse nucleotides to adenosine. The nucleotide/nucleoside balance orchestrates liver homeostasis, tissue repair, and functional restoration by regulating the crosstalk between liver-resident cells and recruited immune cells. In this review, we discuss our current knowledge on the role of purinergic signals in liver homeostasis, restriction of inflammation, stimulation of liver regeneration, modulation of fibrogenesis, and regulation of carcinogenesis. Moreover, we discuss potential targeted therapeutic strategies for liver diseases based on purinergic signals involving blockade of nucleotide receptors, enhancement of ectonucleoside triphosphate diphosphohydrolase activity, and activation of adenosine receptors.

The role of the CD39-CD73-adenosine pathway in liver disease

Extracellular adenosine triphosphate (ATP) is a danger signal released by dying and damaged cells, and it functions as an immunostimulatory signal that promotes inflammation. The ectonucleotidases CD39/ectonucleoside triphosphate diphosphohydrolase-1 and CD73/ecto-5'-nucleotidase are cell-surface enzymes that breakdown extracellular ATP into adenosine. This drives a shift from an ATP-driven proinflammatory environment to an anti-inflammatory milieu induced by adenosine. The CD39-CD73-adenosine pathway changes dynamically with the pathophysiological context in which it is embedded. Accumulating evidence suggests that CD39 and CD73 play important roles in liver disease as critical components of the extracellular adenosinergic pathway. Recent studies have shown that the modification of the CD39-CD73-adenosine pathway alters the liver's response to injury. Moreover, adenosine exerts different effects on the pathophysiology of the liver through different receptors. In this review, we aim to describe the role of the CD39-CD73-adenosine pathway and adenosine receptors in liver disease, highlighting potential therapeutic targets in this pathway, which will facilitate the development of therapeutic strategies for the treatment of liver disease.

TIM-3: An emerging target in the liver diseases

T cell immunoglobulin domain and mucin domain-containing molecule 3 (TIM-3) is found expression in the surface of terminally differentiated T cells and belongs to the TIM family of type Ⅰ transmembrane proteins. It binds to the ligand Galectin-9 and mediates T cell apoptosis. As the research progresses, TIM-3 is also expressed in Th17, NK, monocyte, which binds to ligand and induce immune peripheral tolerance in both mice and man. Numerous researches have demonstrated that TIM-3 influences liver diseases, including liver-associated chronic viral infection, liver fibrosis, liver cancer et al and suggest new approaches to intervention. Currently, targeted therapy of TIM-3 is a new treatment in the field of immunization. Although many studies have proven that TIM-3 has an inhibitory effect in vivo, the specific mechanism is not clear. Herein, we summarize the important role of TIM-3 in the regulation of liver disease and prospects for future clinical research. TIM-3 will provide new targets for improving clinical liver disease.

Protective Role of mTOR in Liver Ischemia/Reperfusion Injury: Involvement of Inflammation and Autophagy

Liver ischemia/reperfusion (IR) injury is a common phenomenon after liver resection and transplantation, which often results in liver graft dysfunction such as delayed graft function and primary nonfunction. The mammalian target of rapamycin (mTOR) is an evolutionarily highly conserved serine/threonine protein kinase, which coordinates cell growth and metabolism through sensing environmental inputs under physiological or pathological conditions, involved in the pathophysiological process of IR injury. In this review, we mainly present current evidence of the beneficial role of mTOR in modulating inflammation and autophagy under liver IR to provide some evidence for the potential therapies for liver IR injury.

ATP and T-cell-mediated rejection

PURPOSE OF REVIEW

Purine nucleosides and nucleotides are released in the extracellular space following cell injury and act as paracrine mediators through a number of dedicated membrane receptors. In particular, extracellular ATP (eATP) significantly influences T-lymphocyte activation and phenotype. The purpose of this review is to discuss the role of ATP signaling in the T-cell-mediated alloimmune response.

RECENT FINDINGS

In various animal models of solid transplantation, the purinergic axis has been targeted to prevent acute rejection and to promote long-term graft tolerance. The inhibition of ATP-gated P2X receptors has been shown to halt lymphocyte activation, to downregulate both Th1 and Th17 responses and to promote T-regulatory (Treg) cell differentiation. Similarly, the inhibition of ATP signaling attenuated graft-versus-host disease in mice undergoing hematopoietic cell transplantation. Significantly, different drugs targeting the purinergic system have been recently approved for human use and may be a viable therapeutic option for transplant patients.

SUMMARY

The inhibition of eATP signaling downregulates the alloimmune response, expands Treg cells and promotes graft survival. This robust preclinical evidence and the recent advances in pharmacological research may lead to intriguing clinical applications.

Caffeic acid attenuates rat liver injury after transplantation involving PDIA3-dependent regulation of NADPH oxidase

The transplanted liver inevitably suffers from ischemia reperfusion (I/R) injury, which represents a key issue in clinical transplantation determining early outcome and long-term graft survival. A solution is needed to deal with this insult. This study was undertaken to explore the effect of Caffeic acid (CA), a naturally occurring antioxidant, on I/R injury of grafted liver and the mechanisms involved. Male Sprague-Dawley rats underwent orthotopic liver transplantation (LT) in the absence or presence of CA administration. In vitro, HL7702 cells were subjected to hypoxia/reoxygenation. LT led to apparent hepatic I/R injury, manifested by deteriorated liver function, microcirculatory disturbance and increased apoptosis, along with increased PDIA3 expression and nicotinamide adenosine dinucleotide phosphate (NADPH) oxidase activity, and membrane translocation of NADPH oxidase subunits. Treatment with CA attenuated the above alterations. siRNA/shRNA-mediated knockdown of PDIA3 in HL7702 cells and rats played the same role as CA not only in inhibiting ROS production and NADPH oxidase activity, but also in alleviating hepatocytes injury. CA protects transplanted livers from injury, which is likely attributed to its protection of oxidative damage by interfering in PDIA3-dependent activation of NADPH oxidase.

Interleukin 2 Promotes Hepatic Regulatory T Cell Responses and Protects From Biliary Fibrosis in Murine Sclerosing Cholangitis

In the multidrug resistance protein 2 (Mdr2)^-/- mouse model, low phospholipid bile instigates biliary epithelial injury, sterile inflammation, and fibrosis, thereby recapitulating disease mechanisms implicated in biliary atresia (BA) and primary sclerosing cholangitis. We hypothesize that T lymphocytes contribute to the biliary injury and fibrosis in murine sclerosing cholangitis (SC) and that they are susceptible to suppression by regulatory T cells (Tregs). In juvenile Mdr2^-/- mice, intrahepatic CD8+ lymphocytes were expanded, and contraction of intrahepatic Tregs coincided with rising serum alanine transferase and alkaline phosphatase (ALP) levels between days 14-30 of life. Antibody-mediated depletion of intrahepatic CD8+ lymphocytes during that time reduced ALP levels and the expression of osteopontin (Opn), a pro-fibrogenic cytokine. Depletion of intrahepatic Tregs with anti-CD25 antibody between days 7-30 increased intrahepatic CD8+ T cells, Opn expression, and fibrosis. Conversely, expansion of intrahepatic Tregs with interleukin 2/anti-interleukin 2 immune complexes (IL-2c) downregulated hepatic expression of Opn and Tnf, reduced frequency of intrahepatic CD8+ lymphocytes, and diminished biliary injury and fibrosis. Treatment with IL-2c upregulated hepatic Treg expression of CD39, an ectonucleotidase capable of hydrolyzing pro-inflammatory adenosine triphosphate. In vitro, Tregs expressing CD39 suppressed the proliferation of hepatic CD8+ lymphocytes from Mdr2^-/- mice more efficiently than those lacking CD39. In infants with BA, infiltration of interlobular bile ducts with CD8+ cells was associated with biliary expression of Opn and its transcription was negatively correlated with mRNA expression of Treg-associated genes. Conclusion: Hepatic CD8+ T lymphocytes drive biliary injury and fibrosis in murine SC. Their proliferation is controlled by hepatic Tregs through the purinergic pathway, which is responsive to IL-2c, suggesting that Treg-directed low-dose Il-2 treatment may be considered as therapy for SC.

CD39-adenosinergic axis in renal pathophysiology and therapeutics

Extracellular ATP interacts with purinergic type 2 (P2) receptors and elicits many crucial biological functions. Extracellular ATP is sequentially hydrolyzed to ADP and AMP by the actions of defined nucleotidases, such as CD39, and AMP is converted to adenosine, largely by CD73, an ecto-5'-nucleotidase. Extracellular adenosine interacts with P1 receptors and often opposes the effects of P2 receptor activation. The balance between extracellular ATP and adenosine in the blood and extracellular fluid is regulated chiefly by the activities of CD39 and CD73, which constitute the CD39-adenosinergic axis. In recent years, several studies have shown this axis to play critical roles in transport of water/sodium, tubuloglomerular feedback, renin secretion, ischemia reperfusion injury, renal fibrosis, hypertension, diabetic nephropathy, transplantation, inflammation, and macrophage transformation. Important developments include global and targeted gene knockout and/or transgenic mouse models of CD39 or CD73, biological or small molecule inhibitors, and soluble engineered ectonucleotidases to directly impact the CD39-adenosinergic axis. This review presents a comprehensive picture of the multiple roles of CD39-adenosinergic axis in renal physiology, pathophysiology, and therapeutics. Scientific advances and greater understanding of the role of this axis in the kidney, in both health and illness, will direct development of innovative therapies for renal diseases.

In situ targeting of dendritic cells sets tolerogenic environment and ameliorates CD4+ T-cell response in the postischemic liver

CD4⁺ T cells recruited to the liver play a key role in the pathogenesis of ischemia/reperfusion (I/R) injury. The mechanism of their activation during alloantigen-independent I/R is not completely understood. We hypothesized that liver-resident dendritic cells (DCs) interact with CD4⁺ T cells in the postischemic liver and that modulation of DCs or T-cell-DC interactions attenuates liver inflammation. In mice, warm hepatic I/R (90/120-240 min) was induced. Tolerogenic DCs were generated in situ by pretreatment of animals with the vitamin D analog paricalcitol. A mAb-CD44 was used for blockade of CD4⁺ T-cell-DC interactions. As shown by 2-photon in vivo microscopy as well as confocal microscopy, CD4⁺ T cells were closely colocalized with DCs in the postischemic liver. Pretreatment with paricalcitol attenuated I/R-induced maturation of DCs (flow cytometry), CD4⁺ T-cell recruitment into the liver (intravital microscopy), and hepatocellular/microvascular damage (intravital microscopy, alanine aminotransferase/aspartate aminotransferase, histology). However, interruption of T-cell-DC interaction increased proinflammatory DC maturation and even enhanced tissue damage. Simultaneous treatment with an anti-CD44mAb completely abolished the beneficial effect of paricalcitol on T-cell migration and tissue injury. Our study demonstrates for the first time that hepatic DCs interact with CD4⁺ T cells in the postischemic liver in vivo; modulation of DCs and/or generation of tolerogenic DCs attenuates intrahepatic CD4⁺ T-cell recruitment and reduces I/R injury; and interruption of CD44-dependent CD4⁺ T-cell-DC interactions enhances tissue injury by preventing the modulatory effect of hepatic DCs on T cells, especially type 1 T helper effector cells. Thus, hepatic DCs are strongly involved in the promotion of CD4⁺ T-cell-dependent postischemic liver inflammation.-Funken, D., Ishikawa-Ankerhold, H., Uhl, B., Lerchenberger, M., Rentsch, M., Mayr, D., Massberg, S., Werner, J., Khandoga, A. In situ targeting of dendritic cells sets tolerogenic environment and ameliorates CD4⁺ T-cell response in the postischemic liver.

Unlocking the Potential of Purinergic Signaling in Transplantation

Purinergic signaling has been recognized as playing an important role in inflammation, angiogenesis, malignancy, diabetes and neural transmission. Activation of signaling pathways downstream from purinergic receptors may also be implicated in transplantation and related vascular injury. Following transplantation, the proinflammatory "danger signal" adenosine triphosphate (ATP) is released from damaged cells and promotes proliferation and activation of a variety of immune cells. Targeting purinergic signaling pathways may promote immunosuppression and ameliorate inflammation. Under pathophysiological conditions, nucleotide-scavenging ectonucleotidases CD39 and CD73 hydrolyze ATP, ultimately, to the anti-inflammatory mediator adenosine. Adenosine suppresses proinflammatory cytokine production and is associated with improved graft survival and decreased severity of graft-versus-host disease. Furthermore, purinergic signaling is involved both directly and indirectly in the mechanism of action of several existing immunosuppressive drugs, such as calcineurin inhibitors and mammalian target of rapamycin inhibitors. Targeting of purinergic receptor pathways, particularly in the setting of combination therapies, could become a valuable immunosuppressive strategy in transplantation. This review focuses on the role of the purinergic signaling pathway in transplantation and immunosuppression and explores possible future applications in clinical practice.

CD39 overexpression does not attenuate renal fibrosis in the unilateral ureteric obstructive model of chronic kidney disease

Chronic kidney disease has multiple etiologies, but its single, hallmark lesion is renal fibrosis. CD39 is a key purinergic enzyme in the hydrolysis of ATP and increased CD39 activity on regulatory T cells (Treg) is protective in adriamycin-induced renal fibrosis. We examined the effect of overexpression of human CD39 on the development of renal fibrosis in the unilateral ureteric obstructive (UUO) model, a model widely used to study the molecular and cellular factors involved in renal fibrosis. Mice overexpressing human CD39 (CD39Tg) and their wild-type (WT) littermates were subjected to UUO; renal histology and messenger RNA (mRNA) levels of adenosine receptors and markers of renal fibrosis were examined up to 14 days after UUO. There were no differences between CD39Tg mice and WT mice in the development of renal fibrosis at days 3, 7, and 14 of UUO. Relative mRNA expression of the adenosine A_2A receptor and endothelin-1 were higher in CD39Tg than WT mice at day 7 post UUO, but there were no differences in markers of fibrosis. We conclude that human CD39 overexpression does not attenuate the development of renal fibrosis in the UUO model. The lack of protection by CD39 overexpression in the UUO model is multifactorial due to the different effects of adenosinergic receptors on the development of renal fibrosis.

Role of the CD39/CD73 Purinergic Pathway in Modulating Arterial Thrombosis in Mice

OBJECTIVE

Circulating blood cells and endothelial cells express ectonucleoside triphosphate diphosphohydrolase-1 (CD39) and ecto-5'-nucleotidase (CD73). CD39 hydrolyzes extracellular ATP or ADP to AMP. CD73 hydrolyzes AMP to adenosine. The goal of this study was to examine the interplay between CD39 and CD73 cascade in arterial thrombosis.

APPROACH AND RESULTS

To determine how CD73 activity influences in vivo thrombosis, the time to ferric chloride-induced arterial thrombosis was measured in CD73-null mice. In response to 5% FeCl3, but not to 10% FeCl3, there was a significant decrease in the time to thrombosis in CD73-null mice compared with wild-type mice. In mice overexpressing CD39, ablation of CD73 did not inhibit the prolongation in the time to thrombosis conveyed by CD39 overexpression. However, the CD73 inhibitor α-β-methylene-ADP nullified the prolongation in the time to thrombosis in human CD39 transgenic (hC39-Tg)/CD73-null mice. To determine whether hematopoietic-derived cells or endothelial cell CD39 activity regulates in vivo arterial thrombus, bone marrow transplant studies were conducted. FeCl3-induced arterial thrombosis in chimeric mice revealed a significant prolongation in the time to thrombosis in hCD39-Tg reconstituted wild-type mice, but not on wild-type reconstituted hCD39-Tg mice. Monocyte depletion with clodronate-loaded liposomes normalized the time to thrombosis in hCD39-Tg mice compared with hCD39-Tg mice treated with control liposomes, demonstrating that increased CD39 expression on monocytes protects against thrombosis.

CONCLUSIONS

These data demonstrate that ablation of CD73 minimally effects in vivo thrombosis, but increased CD39 expression on hematopoietic-derived cells, especially monocytes, attenuates in vivo arterial thrombosis.

Liver transplantation in the mouse: Insights into liver immunobiology, tissue injury, and allograft tolerance

The surgically demanding mouse orthotopic liver transplant model was first described in 1991. It has proved to be a powerful research tool for the investigation of liver biology, tissue injury, the regulation of alloimmunity and tolerance induction, and the pathogenesis of specific liver diseases. Liver transplantation in mice has unique advantages over transplantation of the liver in larger species, such as the rat or pig, because the mouse genome is well characterized and there is much greater availability of both genetically modified animals and research reagents. Liver transplant experiments using various transgenic or gene knockout mice have provided valuable mechanistic insights into the immunobiology and pathobiology of the liver and the regulation of graft rejection and tolerance over the past 25 years. The molecular pathways identified in the regulation of tissue injury and promotion of liver transplant tolerance provide new potential targets for therapeutic intervention to control adverse inflammatory responses/immune-mediated events in the hepatic environment and systemically. In conclusion, orthotopic liver transplantation in the mouse is a valuable model for gaining improved insights into liver biology, immunopathology, and allograft tolerance that may result in therapeutic innovation in the liver and in the treatment of other diseases.

Targeting TRAF3 signaling protects against hepatic ischemia/reperfusions injury

BACKGROUND & AIMS

The hallmarks of hepatic ischemia/reperfusion (I/R) injury, a common clinical problem that occurs during liver surgical procedures, include severe cell death and inflammatory responses that contribute to early graft failure and a higher incidence of organ rejection. Unfortunately, effective therapeutic strategies are limited. Tumor necrosis factor receptor (TNFR)-associated factor (TRAF) 3 transduces apoptosis and/or inflammation-related signaling pathways to regulate cell survival and cytokine production. However, the role of TRAF3 in hepatic I/R-induced liver damage remains unknown.

METHODS

Hepatocyte- or myeloid cell-specific TRAF3 knockdown or transgenic mice were subjected to an I/R model in vivo, and in vitro experiments were performed by treating primary hepatocytes from these mice with hypoxia/reoxygenation stimulation. The function of TRAF3 in I/R-induced liver damage and the potential underlying mechanisms were investigated through various phenotypic analyses and biological approaches.

RESULTS

Hepatocyte-specific, but not myeloid cell-specific, TRAF3 deficiency reduced cell death, inflammatory cell infiltration, and cytokine production in both in vivo and in vitro hepatic I/R models, whereas hepatic TRAF3 overexpression resulted in the opposite effects. Mechanistically, TRAF3 directly binds to TAK1, which enhances the activation of the downstream NF-κB and JNK pathways. Importantly, inhibition of TAK1 almost completely reversed the TRAF3 overexpression-mediated exacerbation of I/R injury.

CONCLUSIONS

TRAF3 is a novel hepatic I/R mediator that promotes liver damage and inflammation via TAK1-dependent activation of the JNK and NF-κB pathways. Inhibition of hepatic TRAF3 may represent a promising approach to protect the liver against I/R injury-related diseases.

Molecular pathways in protecting the liver from ischaemia/reperfusion injury: a 2015 update

Ischaemia/reperfusion injury is an important cause of liver damage during surgical procedures such as hepatic resection and liver transplantation, and represents the main cause of graft dysfunction post-transplantation. Molecular processes occurring during hepatic ischaemia/reperfusion are diverse, and continuously include new and complex mechanisms. The present review aims to summarize the newest concepts and hypotheses regarding the pathophysiology of liver ischaemia/reperfusion, making clear distinction between situations of cold and warm ischaemia. Moreover, the most updated therapeutic strategies including pharmacological, genetic and surgical interventions, as well as some of the scientific controversies in the field are described.

IRF-1 promotes liver transplant ischemia/reperfusion injury via hepatocyte IL-15/IL-15Rα production

Ischemia and reperfusion (I/R) injury following liver transplantation (LTx) is an important problem that significantly impacts clinical outcomes. IFN regulatory factor-1 (IRF-1) is a nuclear transcription factor that plays a critical role in liver injury. Our objective was to determine the immunomodulatory role of IRF-1 during I/R injury following allogeneic LTx. IRF-1 was induced in liver grafts immediately after reperfusion in both human and mouse LTx. IRF-1 contributed significantly to I/R injury because IRF-1-knockout (KO) grafts displayed much less damage as assessed by serum alanine aminotransferase and histology. In vitro, IRF-1 regulated both constitutive and induced expression of IL-15, as well as IL-15Rα mRNA expression in murine hepatocytes and liver dendritic cells. Specific knockdown of IRF-1 in human primary hepatocytes gave similar results. In addition, we identified hepatocytes as the major producer of soluble IL-15/IL-15Rα complexes in the liver. IRF-1-KO livers had significantly reduced NK, NKT, and CD8(+) T cell numbers, whereas rIL-15/IL-15Rα restored these immune cells, augmented cytotoxic effector molecules, promoted systemic inflammatory responses, and exacerbated liver injury in IRF-1-KO graft recipients. These results indicate that IRF-1 promotes LTx I/R injury via hepatocyte IL-15/IL-15Rα production and suggest that targeting IRF-1 and IL-15/IL-15Rα may be effective in reducing I/R injury associated with LTx.

Negative CD4 + TIM-3 signaling confers resistance against cold preservation damage in mouse liver transplantation

Ischemia-reperfusion injury (IRI), an innate immunity-driven local inflammation, remains the major problem in clinical organ transplantation. T cell immunoglobulin and mucin domain (TIM-3)-Galectin-9 (Gal-9) signaling regulates CD4+ Th1 immune responses. Here, we explored TIM-3-Gal-9 function in a clinically relevant murine model of hepatic cold storage and orthotopic liver transplantation (OLT). C57BL/6 livers, preserved for 20 h at 4°C in UW solution, were transplanted to syngeneic mouse recipients. Up-regulation of TIM-3 on OLT-infiltrating activated CD4+ T cells was observed in the early IRI phase (1 h). By 6 h of reperfusion, OLTs in recipients treated with a blocking anti-TIM-3 Ab were characterized by: (1) enhanced hepatocellular damage (sALT levels, liver Suzuki's histological score); (2) polarized cell infiltrate towards Th1/Th17-type phenotype; (3) depressed T cell exhaustion markers (PD-1, LAG3); and (4) elevated neutrophil and macrophage infiltration/activation. In parallel studies, adoptive transfer of CD4+ T cells from naïve WT, but not from TIM-3 Tg donors, readily recreated OLT damage in otherwise IR-resistant RAG(-/-) test recipients. Furthermore, pre-treatment of mice with rGal-9 promoted hepatoprotection against preservation-association liver damage, accompanied by enhanced TIM-3 expression in OLTs. Thus, CD4+ T cell-dependent "negative" TIM-3 costimulation is essential for hepatic homeostasis and resistance against IR stress in OLTs.

Recipient T cell TIM-3 and hepatocyte galectin-9 signalling protects mouse liver transplants against ischemia-reperfusion injury

BACKGROUND & AIMS

By binding to T cell immunoglobulin mucin-3 (TIM-3) on activated Th1 cells, galectin-9 (Gal-9) negatively regulates Th1-type alloimmunity. Although T cells contribute to hepatic ischemia-reperfusion injury (IRI), it is unknown whether negative T cell-dependent TIM-3 co-stimulation may rescue IR-stressed orthotopic liver transplants from innate immunity-driven inflammation.

METHODS

We used wild type (WT) and TIM-3 transgenic (Tg) mice (C57BL/6) as liver donors and recipients in a clinically-relevant model of hepatic cold storage (20 h at 4°C in UW solution) and syngeneic orthotopic liver transplantation (OLT).

RESULTS

Orthotopic liver transplants in WT or TIM-3Tg→TIM-3Tg groups were resistant against IR-stress, evidenced by preserved hepatocellular function (serum ALT levels) and liver architecture (Suzuki's score). In contrast, orthotopic liver transplants in WT or TIM-3Tg→WT groups were susceptible to IRI. TIM-3 induction in circulating CD4+ T cells of the recipient: (1) depressed T-bet/IFN-γ, while amplifying GATA3 and IL-4/IL-10 expression in orthotopic liver transplants; (2) promoted T cell exhaustion (PD-1, LAG-3) phenotype; and (3) depressed neutrophil and macrophage infiltration/function in orthotopic liver transplants. In parallel studies, we documented for the first time that Gal-9, a natural TIM-3 ligand, was produced primarily by and released from IR-stressed hepatocytes, both in vivo and in vitro. Moreover, exogenous recombinant Gal-9 (rGal-9) potentiated liver resistance against IRI by depressing T cell activation and promoting apoptosis of CD4+ T cells.

CONCLUSIONS

Harnessing TIM-3/Gal-9 signalling at the T cell-hepatocyte interface facilitates homeostasis in IR-stressed orthotopic liver transplants. Enhancing anti-oxidant hepatocyte Gal-9 potentiates liver IR-resistance. Negative regulation by recipient TIM-3+CD4+ cells provides evidence for cytoprotective functions of a discrete T cell subset, which should be spared when applying T cell-targeted immunosuppression in transplant recipients.

CD39 deficiency in murine liver allografts promotes inflammatory injury and immune-mediated rejection

Adenosine triphosphate (ATP), an essential metabolic energy source, is released following cell apoptosis or necrosis. It acts as a damage-associated molecule pattern to stimulate innate immune cells. The ectonucleotidase CD39 regulates immune activation by hydrolysis of extracellular ATP. We have shown previously that CD39 expression by donor livers helps protect syngeneic grafts with extended (24 hr) cold preservation time from ischemia reperfusion injury. Given its immune regulatory properties, we hypothesized that CD39 expression in donor livers might modulate transplant tolerance that occurs following mouse allogeneic liver transplantation (LTx). Livers from C57BL/6 (B6) wild-type (WT) or CD39 KO mice were transplanted into normal C3H recipients with minimal (approximately 1 hr) cold ischemia. Serum alanine aminotransferase levels at day 4 post LTx were significantly higher in animals given CD39KO compared with WT livers. Moreover, IFN-γ production by liver-infiltrating CD8⁺ T cells at day 4 was significantly higher in CD39KO than in WT grafts. Furthermore, splenic T cells from CD39KO liver recipients exhibited greater proliferative responses to donor alloantigens than those from mice given WT grafts. By contrast, there was a concomitant significant reduction in the frequency of regulatory T cells (Treg) in CD39KO than in WT livers. Whereas WT liver allografts survived > 100 days, no CD39KO grafts survived beyond 40 days (median survival time [MST]: WT: >100 days vs CD39KO: 8 days; p<0.01). In addition, soluble CD39 administration significantly prolonged CD39KO liver allograft survival (MST: 27.5 days). These novel data suggest that CD39 expression in liver allografts modulates tissue injury, inflammation, anti-donor effector T cell responses and Treg infiltration and can suppress transplant rejection.

Tumor necrosis factor-related apoptosis-inducing ligand on NK cells protects from hepatic ischemia-reperfusion injury

BACKGROUND

Ischemia-reperfusion injury (IRI) significantly contributes to graft dysfunction after liver transplantation. Natural killer (NK) cells are crucial innate effector cells in the liver and express tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), a potent inducer of hepatocyte cell death. Here, we investigated if TRAIL expression on NK cells contributes to hepatic IRI.

METHODS

The outcome after partial hepatic IRI was assessed in TRAIL-null mice and contrasted to C57BL/6J wild-type mice and after NK cell adoptive transfer in RAG2/common gamma-null mice that lack T, B, and NK cells. Liver IRI was assessed by histological analysis, alanine aminotransferase, hepatic neutrophil activation by myeloperoxidase activity, and cytokine secretion at specific time points. NK cell cytotoxicity and differentiation were assessed in vivo and in vitro.

RESULTS

Twenty-four hours after reperfusion, TRAIL-null mice exhibited significantly higher serum transaminases, histological signs of necrosis, neutrophil infiltration, and serum levels of interleukin-6 compared to wild-type animals. Adoptive transfer of TRAIL-null NK cells into immunodeficient RAG2/common gamma-null mice was associated with significantly elevated liver damage compared to transfer of wild-type NK cells. In TRAIL-null mice, NK cells exhibit higher cytotoxicity and decreased differentiation compared to wild-type mice. In vitro, cytotoxicity against YAC-1 and secretion of interferon gamma by TRAIL-null NK cells were significantly increased compared to wild-type controls.

CONCLUSIONS

These experiments reveal that expression of TRAIL on NK cells is protective in a murine model of hepatic IRI through modulation of NK cell cytotoxicity and NK cell differentiation.

The Role of Ectonucleotidases CD39 and CD73 and Adenosine Signaling in Solid Organ Transplantation

Extracellular adenosine is a potent immunomodulatory molecule that accumulates in states of inflammation. Nucleotides such as adenosine triphosphate and adenosine diphosphate are release from injured and necrotic cells and hydrolyzed to adenosine monophosphate and adenosine by the concerted action of the ectonucleotidases CD39 and CD73. Accumulating evidence suggest that purinergic signaling is involved in the inflammatory response that accompanies acute rejection and chronic allograft dysfunction. Modification of the purinergic pathway has been shown to alter graft survival in a number of solid organ transplant models and the response to ischemia–reperfusion injury (IRI). Furthermore, the purinergic pathway is intrinsically involved in B and T cell biology and function. Although T cells have traditionally been considered the orchestrators of acute allograft rejection, a role for B cells in chronic allograft loss is being increasingly appreciated. This review focuses on the role of the ectonucleotidases CD39 and CD73 and adenosine signaling in solid organ transplantation including the effects on IRI and T and B cell biology.

CD39 expression by hepatic myeloid dendritic cells attenuates inflammation in liver transplant ischemia-reperfusion injury in mice

Hepatic innate immune cells, in particular, interstitial dendritic cells (DCs), regulate inflammatory responses and may promote inherent liver tolerogenicity. After tissue injury, adenosine triphosphate (ATP) is released and acts as a damage-associated molecular pattern that activates innate immune cells by pattern recognition receptors. CD39 (ectonucleoside triphosphate diphosphohydrolase-1) rapidly hydrolyzes extracellular ATP to maintain physiological levels. We hypothesized that CD39 expression on liver DCs might contribute to regulation of their innate immune functions. Mouse liver conventional myeloid DCs (mDCs) were hyporesponsive to ATP, compared with their splenic counterparts. This disparity was ascribed to more efficient hydrolysis of ATP by higher expression of CD39 on liver mDCs. Human liver mDCs expressed greater levels of CD39 than those from peripheral blood. The comparatively high expression of CD39 on liver mDCs correlated strongly with both ATP hydrolysis and adenosine production. Notably, CD39(-/-) mouse liver mDCs exhibited a more mature phenotype, greater responsiveness to Toll-like receptor 4 ligation, and stronger proinflammatory and immunostimulatory activity than wild-type (WT) liver mDCs. To investigate the role of CD39 on liver mDCs in vivo, we performed orthotopic liver transplantation with extended cold preservation using CD39(-/-) or WT donor mouse livers. Compared to WT liver grafts, CD39(-/-) grafts exhibited enhanced interstitial DC activation, elevated proinflammatory cytokine levels, and more-severe tissue injury. Moreover, portal venous delivery of WT, but not CD39(-/-) liver mDCs, to donor livers immediately post-transplant exerted a protective effect against graft injury in CD39(-/-) to CD39(-/-) liver transplantation.

CONCLUSIONS

These data reveal that CD39 expression on conventional liver mDCs limits their proinflammatory activity and confers protective properties on these important innate immune cells against liver transplant ischemia/reperfusion injury.

Overexpression of CD39 protects in a mouse model of preeclampsia

CD39 (NTPDase1), a critical immune and vascular ecto-nucleotidase, hydrolyses pro-inflammatory and pro-thrombotic nucleotides (adenosine-5'-triphosphate (ATP) and adenosine diphosphate) to adenosine. In humans, CD39 is the dominant ecto-nucleotidase in placental trophoblastic tissues and modulates ATP-dependent trophoblastic functions. CD39 is an integral component of regulatory T cells (Treg), which are central to immunological tolerance and maintenance of normal pregnancy. We examined the impact of CD39 overexpression in a mouse model of preeclampsia. Matings were performed between virginal BALB/c female (wild-type (WT) or CD39 transgenic (CD39TG)) and C57BL/6 male mice. On days 10 and 12 of pregnancy BALB/c Th1-polarized cells were injected. Systolic blood pressure (SBP) was measured throughout pregnancy. Mice were sacrificed at day 15 of pregnancy. Following transfer of Th1-polarized cells, SBP of pregnant WT mice increased (118 ± 3 mmHg to 142 ± 5 mmHg). Although ultrastructural changes were evident in the kidney this was not accompanied by significant proteinuria. SBP remained unchanged (115 ± 2 mmHg to 114 ± 3 mmHg) in pregnant CD39TG mice without evidence of renal lesions. We conclude that gestational hypertension can be induced in mice following transfer of maternally derived Th1-polarized cells and that overexpression of CD39 is protective in this model.

Differential migration of passenger leukocytes and rapid deletion of naive alloreactive CD8 T cells after mouse liver transplantation

Donor passenger leukocytes (PLs) from transplanted livers migrate to recipient lymphoid tissues, where they are thought to induce the deletion of donor-specific T cells and tolerance. Difficulties in tracking alloreactive T cells and PLs in rats and in performing this complex surgery in mice have limited progress in identifying the contribution of PL subsets and sites and the kinetics of T cell deletion. Here we developed a mouse liver transplant model in which PLs, recipient cells, and a reporter population of transgenic CD8 T cells specific for the graft could be easily distinguished and quantified in allografts and recipient organs by flow cytometry. All PL subsets circulated rapidly via the blood as soon as 1.5 hours after transplantation. By 24 hours, PLs were distributed differently in the lymph nodes and spleen, whereas donor natural killer and natural killer T cells remained in the liver and blood. Reporter T cells were activated in both liver and lymphoid tissues, but their numbers dramatically decreased within the first 48 hours. These results provide the first unequivocal demonstration of the differential recirculation of liver PL subsets after transplantation, and show that alloreactive CD8 T cells are deleted more rapidly than initially reported. This model will be useful for dissecting early events leading to the spontaneous acceptance of liver transplants.

Biological implications of extracellular adenosine in hepatic ischemia and reperfusion injury

The purine nucleoside adenosine is clinically employed in the treatment of supraventricular tachycardia. In addition, it has direct coronary vasodilatory effects, and may influence platelet aggregation. Experimental observations mechanistically link extracellular adenosine to cellular adaptation to hypoxia. Adenosine generation has been implicated in several pathophysiologic processes including angiogenesis, tumor defenses and neurodegeneration. In solid organ transplantation, prolonged tissue ischemia and subsequent reperfusion injury may lead to profound graft dysfunction. Importantly, conditions of limited oxygen availability are associated with increased production of extracellular adenosine and subsequent tissue protection. Within the rapidly expanding field of adenosine biology, several enzymatic steps in adenosine production have been characterized and multiple receptor subtypes have been identified. In this review, we briefly examine the biologic steps involved in adenosine generation and chronicle the current state of adenosine signaling in hepatic ischemia and reperfusion injury.

The protective effects of CD39 overexpression in multiple low-dose streptozotocin-induced diabetes in mice

Islet allograft survival limits the long-term success of islet transplantation as a potential curative therapy for type 1 diabetes. A number of factors compromise islet survival, including recurrent diabetes. We investigated whether CD39, an ectonucleotidase that promotes the generation of extracellular adenosine, would mitigate diabetes in the T cell-mediated multiple low-dose streptozotocin (MLDS) model. Mice null for CD39 (CD39KO), wild-type mice (WT), and mice overexpressing CD39 (CD39TG) were subjected to MLDS. Adoptive transfer experiments were performed to delineate the efficacy of tissue-restricted overexpression of CD39. The role of adenosine signaling was examined using mutant mice and pharmacological inhibition. The susceptibility to MLDS-induced diabetes was influenced by the level of expression of CD39. CD39KO mice developed diabetes more rapidly and with higher frequency than WT mice. In contrast, CD39TG mice were protected. CD39 overexpression conferred protection through the activation of adenosine 2A receptor and adenosine 2B receptor. Adoptive transfer experiments indicated that tissue-restricted overexpression of CD39 conferred robust protection, suggesting that this may be a useful strategy to protect islet grafts from T cell-mediated injury.

CD39 and CD73 in immunity and inflammation

The enzymatic activities of CD39 and CD73 play strategic roles in calibrating the duration, magnitude, and chemical nature of purinergic signals delivered to immune cells through the conversion of ADP/ATP to AMP and AMP to adenosine, respectively. This drives a shift from an ATP-driven proinflammatory environment to an anti-inflammatory milieu induced by adenosine. The CD39/CD73 pathway changes dynamically with the pathophysiological context in which it is embedded. It is becoming increasingly appreciated that altering this catabolic machinery can change the course or dictate the outcome of several pathophysiological events, such as AIDS, autoimmune diseases, infections, atherosclerosis, ischemia-reperfusion injury, and cancer, suggesting these ectoenzymes are novel therapeutic targets for managing a variety of disorders.

Biological functions of ecto-enzymes in regulating extracellular adenosine levels in neoplastic and inflammatory disease states

When present in the extracellular environment, the nucleoside adenosine protects cells and tissues from excessive inflammation and immune-mediated damage while promoting healing processes. This role has been highlighted experimentally using distinct disease models, including those of colitis, diabetes, asthma, sepsis, and ischemic injury. Adenosine also suppresses immune responses, as in the tumor microenvironment, assisting immune evasion while promoting angiogenesis. The mechanisms involved in adenosine signaling are addressed elsewhere in this issue. Here, the authors specifically address the generation of adenosine from extracellular nucleotides. This process is catalyzed by a series of plasma membrane ectonucleotidases, with the focus in this article on members of the CD39, CD73, and CD38 families and on their role in inflammatory and neoplastic hematological diseases. Pharmacological modulation of adenosine generation by drugs that either have or modulate ectonucleotidase function might be exploited to treat these diverse conditions.

The CD39-adenosinergic axis in the pathogenesis of renal ischemia-reperfusion injury

Hypoxic injury occurs when the blood supply to an organ is interrupted; subsequent reperfusion halts ongoing ischemic damage but paradoxically leads to further inflammation. Together this is termed ischemia-reperfusion injury (IRI). IRI is inherent to organ transplantation and impacts both the short- and long-term outcomes of the transplanted organ. Activation of the purinergic signalling pathway is intrinsic to the pathogenesis of, and endogenous response to IRI. Therapies targeting the purinergic pathway in IRI are an attractive avenue for the improvement of transplant outcomes and the basis of ongoing research. This review aims to examine the role of adenosine receptor signalling and the ecto-nucleotidases, CD39 and CD73, in IRI, with a particular focus on renal IRI.