Mitochondrial Dysfunction, Depleted Purinergic Signaling, and Defective T Cell Vigilance and Immune Defense

Methods for studying P2X4 receptor ion channels in immune cells

The P2X4 receptor is a trimeric ligand-gated ion channel activated by adenosine 5'-triphosphate (ATP). P2X4 is present in immune cells with emerging roles in inflammation and immunity, and related disorders. This review aims to provide an overview of the methods commonly used to study P2X4 in immune cells, focusing on those methods used to assess P2RX4 gene expression, the presence of the P2X4 protein, and P2X4 ion channel activity in these cells from humans, dogs, mice and rats. P2RX4 gene expression in immune cells is commonly assessed using semi-quantitative and quantitative reverse-transcriptase-PCR. The presence of P2X4 protein in immune cells is mainly assessed using anti-P2X4 polyclonal antibodies with immunoblotting or immunochemistry, but the use of these antibodies, as well as monoclonal antibodies and nanobodies to detect P2X4 with flow cytometry is increasing. Notably, use of an anti-P2X4 monoclonal antibody and flow cytometry has revealed that P2X4 is present on immune cells with a rank order of expression in eosinophils, then neutrophils and monocytes, then basophils and B cells, and finally T cells. P2X4 ion channel activity has been assessed mainly by Ca2+ flux assays using the cell permeable Ca2+-sensitive dyes Fura-2 and Fluo-4 with fluorescence microscopy, spectrophotometry, or flow cytometry. However, other methods including electrophysiology, and fluorescence assays measuring Na+ flux (using sodium green tetra-acetate) and dye uptake (using YO-PRO-12+) have been applied. Collectively, these methods have demonstrated the presence of functional P2X4 in monocytes and macrophages, microglia, eosinophils, mast cells and CD4+ T cells, with other evidence suggestive of functional P2X4 in dendritic cells, neutrophils, B cells and CD8+ T cells.

Tissue- and temporal-specific roles of extracellular ATP on T cell metabolism and function

The activation and function of T cells is fundamental for the control of infectious diseases and cancer, and conversely can mediate several autoimmune diseases. Among the signaling pathways leading to T cell activation and function, the sensing of extracellular adenosine triphosphate (eATP) has been recently appreciated as an important component. Through a plethora of purinergic receptors, most prominently P2RX7, eATP sensing can induce a wide variety of processes in T cells, such as proliferation, subset differentiation, survival, or cell death. The downstream roles of eATP sensing can vary according to (a) the T cell subset, (b) the tissue where T cells are, and (c) the time after antigen exposure. In this mini-review, we revisit the recent findings on how eATP signaling pathways regulate T-cell immune responses and posit important unanswered questions on this field.

The role of extracellular ATP in homeostatic immune cell migration

Antigen stimulation induces adenosine triphosphate (ATP) release from naïve lymphocytes in lymphoid tissues. However, previous studies indicated that the non-lytic release of ATP also occurs in most tissues and cell types under physiological conditions. Here, we show that extracellular ATP (eATP) is indeed constitutively produced by naïve T cells in response to lymphoid chemokines in uninflamed lymph nodes and is involved in the regulation of immune cell migration. In this review, we briefly summarize the homeostatic role of extracellular ATP in immune cell migration in vivo.

P2 Receptors: Novel Disease Markers and Metabolic Checkpoints in Immune Cells

Extracellular ATP (eATP) and P2 receptors are novel emerging regulators of T-lymphocyte responses. Cellular ATP is released via multiple pathways and accumulates at sites of tissue damage and inflammation. P2 receptor expression and function are affected by numerous single nucleotide polymorphisms (SNPs) associated with diverse disease conditions. Stimulation by released nucleotides (purinergic signalling) modulates several T-lymphocyte functions, among which energy metabolism. Energy metabolism, whether oxidative or glycolytic, in turn deeply affects T-cell activation, differentiation and effector responses. Specific P2R subtypes, among which the P2X7 receptor (P2X7R), are either up- or down-regulated during T-cell activation and differentiation; thus, they can be considered indexes of activation/quiescence, reporters of T-cell metabolic status and, in principle, markers of immune-mediated disease conditions.

NcRNAs-mediated P2RX1 expression correlates with clinical outcomes and immune infiltration in patients with breast invasive carcinoma

The development of novel treatments for breast invasive carcinoma (BC) has been stagnant. P2RX1, a member of the purinergic receptor family, has been found to have a prognostic impact in several tumors. Therefore, we analyzed the expression pattern of P2RX1 in pan-cancers including BC and its impact on survival and found that the expression level of P2RX1 was lower in BC compared with para-cancerous tissues, and higher P2RX1 expression indicated better prognoses. But real-time quantitative reverse transcription PCR (RT-qPCR) and Western blot detected that the P2RX1 expression in normal mammary epithelial cells was lower than that in tumor cells. Then we comprehensively analyzed the regulatory mechanism and protein-protein interaction network, and found that P2RX1 was significantly positively linked with immune cell infiltration and immune checkpoints.

The Purinergic Receptor P2X4 Promotes Th17 Activation and the Development of Arthritis

Purinergic signaling plays a major role in T cell activation leading to IL-2 production and proliferation. However, it is unclear whether purinergic signaling contributes to the differentiation and activation of effector T cells. In this study, we found that the purinergic receptor P2X4 was associated with human Th17 cells but not with Th1 cells. Inhibition of P2X4 receptor with the specific antagonist 5-BDBD and small interfering RNA inhibited the development of Th17 cells and the production of IL-17 by effector Th17 cells stimulated via the CD3/CD28 pathway. Our results showed that P2X4 was required for the expression of retinoic acid-related orphan receptor C, which is the master regulator of Th17 cells. In contrast, inhibition of P2X4 receptor had no effect on Th1 cells and on the production of IFN-γ and it did not affect the expression of the transcription factor T-bet (T-box transcription factor). Furthermore, inhibition of P2X4 receptor reduced the production of IL-17 but not of IFN-γ by effector/memory CD4⁺ T cells isolated from patients with rheumatoid arthritis. In contrast to P2X4, inhibition of P2X7 and P2Y₁₁ receptors had no effects on Th17 and Th1 cell activation. Finally, treatment with the P2X4 receptor antagonist 5-BDBD reduced the severity of collagen-induced arthritis in mice by inhibiting Th17 cell expansion and activation. Our findings provide novel insights into the role of purinergic signaling in T cell activation and identify a critical role for the purinergic receptor P2X4 in Th17 activation and in autoimmune arthritis.

Extracellular ATP Limits Homeostatic T Cell Migration Within Lymph Nodes

Whereas adenosine 5'-triphosphate (ATP) is the major energy source in cells, extracellular ATP (eATP) released from activated/damaged cells is widely thought to represent a potent damage-associated molecular pattern that promotes inflammatory responses. Here, we provide suggestive evidence that eATP is constitutively produced in the uninflamed lymph node (LN) paracortex by naïve T cells responding to C-C chemokine receptor type 7 (CCR7) ligand chemokines. Consistently, eATP was markedly reduced in naïve T cell-depleted LNs, including those of nude mice, CCR7-deficient mice, and mice subjected to the interruption of the afferent lymphatics in local LNs. Stimulation with a CCR7 ligand chemokine, CCL19, induced ATP release from LN cells, which inhibited CCR7-dependent lymphocyte migration in vitro by a mechanism dependent on the purinoreceptor P2X7 (P2X7R), and P2X7R inhibition enhanced T cell retention in LNs in vivo. These results collectively indicate that paracortical eATP is produced by naïve T cells in response to constitutively expressed chemokines, and that eATP negatively regulates CCR7-mediated lymphocyte migration within LNs via a specific subtype of ATP receptor, demonstrating its fine-tuning role in homeostatic cell migration within LNs.

P2X4 and P2X7 are essential players in basal T cell activity and Ca2+ signaling milliseconds after T cell activation

Initial T cell activation is triggered by the formation of highly dynamic, spatiotemporally restricted Ca²⁺ microdomains. Purinergic signaling is known to be involved in Ca²⁺ influx in T cells at later stages compared to the initial microdomain formation. Using a high-resolution Ca²⁺ live-cell imaging system, we show that the two purinergic cation channels P2X4 and P2X7 not only are involved in the global Ca²⁺ signals but also promote initial Ca²⁺ microdomains tens of milliseconds after T cell stimulation. These Ca²⁺ microdomains were significantly decreased in T cells from P2rx4^-/- and P2rx7^-/- mice or by pharmacological inhibition or blocking. Furthermore, we show a pannexin-1-dependent activation of P2X4 in the absence of T cell receptor/CD3 stimulation. Subsequently, upon T cell receptor/CD3 stimulation, ATP release is increased and autocrine activation of both P2X4 and P2X7 then amplifies initial Ca²⁺ microdomains already in the first second of T cell activation.

Modulation of Cell Energy Metabolism by the P2X7 Receptor

For many years the P2X7 receptor (P2X7R) was considered the prototypic cytolytic receptor due to its ability to cause dramatic changes in plasma membrane permeability, eventually leading to cell death. However, later studies revealed that controlled P2X7R activation has beneficial effects on cell metabolism and nowadays our perception of the physiological role of this receptor has radically changed. Some of the biochemical pathways underlying the trophic effect of the P2X7R are being unveiled, thus disclosing an unanticipated role of P2X7Rs in mitochondrial and glycolytic metabolism. We provide here an update of the effects of the P2X7R on cell energy metabolism.

Synapomorphic features of hepatic and pulmonary vasculatures include comparable purinergic signaling responses in host defense and modulation of inflammation

Hepatosplanchnic and pulmonary vasculatures constitute synapomorphic, highly comparable networks integrated with the external environment. Given functionality related to obligatory requirements of "feeding and breathing," these organs are subject to constant environmental challenges entailing infectious risk, antigenic and xenobiotic exposures. Host responses to these stimuli need to be both protective and tightly regulated. These functions are facilitated by dualistic, high-low pressure blood supply of the liver and lungs, as well as tolerogenic characteristics of resident immune cells and signaling pathways. Dysregulation in hepatosplanchnic and pulmonary blood flow, immune responses, and microbiome implicate common pathogenic mechanisms across these vascular networks. Hepatosplanchnic diseases, such as cirrhosis and portal hypertension, often impact lungs and perturb pulmonary circulation and oxygenation. The reverse situation is also noted with lung disease resulting in hepatic dysfunction. Others, and we, have described common features of dysregulated cell signaling during liver and lung inflammation involving extracellular purines (e.g., ATP, ADP), either generated exogenously or endogenously. These metabokines serve as danger signals, when released by bacteria or during cellular stress and cause proinflammatory and prothrombotic signals in the gut/liver-lung vasculature. Dampening of these danger signals and organ protection largely depends upon activities of vascular and immune cell-expressed ectonucleotidases (CD39 and CD73), which convert ATP and ADP into anti-inflammatory adenosine. However, in many inflammatory disorders involving gut, liver, and lung, these protective mechanisms are compromised, causing perpetuation of tissue injury. We propose that interventions that specifically target aberrant purinergic signaling might prevent and/or ameliorate inflammatory disorders of the gut/liver and lung axis.

Adenosine 5'-Monophosphate Protects from Hypoxia by Lowering Mitochondrial Metabolism and Oxygen Demand

Ischemia and reperfusion injury following severe trauma or cardiac arrest are major causes of organ damage in intensive care patients. The brain is particularly vulnerable because hypoxia rapidly damages neurons due to their heavy reliance on oxidative phosphorylation. Therapeutic hypothermia can reduce ischemia-induced brain damage, but cooling procedures are slow and technically difficult to perform in critical care settings. It has been previously reported that injection of naturally occurring adenosine 5'-monophosphate (AMP) can rapidly induce hypothermia in mice. We studied the underlying mechanisms and found that AMP transiently reduces the heart rate, respiratory rate, body temperature, and the consciousness of adult male and female C57BL/6J mice. Adding AMP to mouse or human neuronal cell cultures dose-dependently reduced the membrane potential (ΔΨm) and Ca signaling of mitochondria in these cells. AMP treatment increased intracellular AMP levels and activated AMP-activated protein kinase, which resulted in the inhibition of mammalian target of rapamycin complex 1 (mTORC1) and of mitochondrial and cytosolic Ca signaling in resting and stimulated neurons. Pretreatment with an intraperitoneal injection of AMP almost doubled the survival time of mice under hypoxic (6% O2) or anoxic (<1% O2) conditions when compared to untreated mice. These findings suggest that AMP induces a hypometabolic state that slows mitochondrial respiration, reduces oxygen demand, and delays the processes that damage mitochondria in the brain and other organs following hypoxia and reperfusion. Further examination of these mechanisms may lead to new treatments that preserve organ function in critical care patients.

Identification of a subset of immunosuppressive P2RX1-negative neutrophils in pancreatic cancer liver metastasis

The immunosuppressive microenvironment that is shaped by hepatic metastatic pancreatic ductal adenocarcinoma (PDAC) is essential for tumor cell evasion of immune destruction. Neutrophils are important components of the metastatic tumor microenvironment and exhibit heterogeneity. However, the specific phenotypes, functions and regulatory mechanisms of neutrophils in PDAC liver metastases remain unknown. Here, we show that a subset of P2RX1-negative neutrophils accumulate in clinical and murine PDAC liver metastases. RNA sequencing of murine PDAC liver metastasis-infiltrated neutrophils show that P2RX1-deficient neutrophils express increased levels of immunosuppressive molecules, including PD-L1, and have enhanced mitochondrial metabolism. Mechanistically, the transcription factor Nrf2 is upregulated in P2RX1-deficient neutrophils and associated with PD-L1 expression and metabolic reprogramming. An anti-PD-1 neutralizing antibody is sufficient to compromise the immunosuppressive effects of P2RX1-deficient neutrophils on OVA-activated OT1 CD8+ T cells. Therefore, our study uncovers a mechanism by which metastatic PDAC tumors evade antitumor immunity by accumulating a subset of immunosuppressive P2RX1-negative neutrophils.

Pancreatic ductal adenocarcinoma (PDAC) is a highly aggressive metastatic disease characterized by an immunosuppressive microenvironment. Here the authors show that a subset of P2RX1-negative neutrophils with immunosuppressive properties accumulate in PDAC metastatic liver tissues and promote tumor growth.

P2X receptors in cancer growth and progression

It is increasingly appreciated that ion channels have a crucial role in tumors, either as promoters of cancer cell growth, or modulators of immune cell functions, or both. Among ion channels, P2X receptors have a special status because they are gated by ATP, a common and abundant component of the tumor microenvironment. Furthermore, one P2X receptor, i.e. P2X7, may also function as a conduit for ATP release, thus fuelling the increased extracellular ATP level in the tumor interstitium. These findings show that P2X receptors and extracellular ATP are indissoluble partners and key regulators of tumor growth, and suggest the exploitation of the extracellular ATP-P2X partnership to develop innovative therapeutic approaches to cancer.

Mitochondria Synergize With P2 Receptors to Regulate Human T Cell Function

Intracellular ATP is the universal energy carrier that fuels many cellular processes. However, immune cells can also release a portion of their ATP into the extracellular space. There, ATP activates purinergic receptors that mediate autocrine and paracrine signaling events needed for the initiation, modulation, and termination of cell functions. Mitochondria contribute to these processes by producing ATP that is released. Here, we summarize the synergistic interplay between mitochondria and purinergic signaling that regulates T cell functions. Specifically, we discuss how mitochondria interact with P2X1, P2X4, and P2Y11 receptors to regulate T cell metabolism, cell migration, and antigen recognition. These mitochondrial and purinergic signaling mechanisms are indispensable for host immune defense. However, they also represent an Achilles heel that can render the host susceptible to infections and inflammatory disorders. Hypoxia and mitochondrial dysfunction deflate the purinergic signaling mechanisms that regulate T cells, while inflammation and tissue damage generate excessive systemic ATP levels that distort autocrine purinergic signaling and impair T cell function. An improved understanding of the metabolic and purinergic signaling mechanisms that regulate T cells may lead to novel strategies for the diagnosis and treatment of infectious and inflammatory diseases.

Plasma Adenylate Levels are Elevated in Cardiopulmonary Arrest Patients and May Predict Mortality

Cerebral and cardiac dysfunction cause morbidity and mortality in postcardiac arrest syndrome (PCAS) patients. Predicting clinical outcome is necessary to provide the optimal level of life support for these patients. In this pilot study, we examined whether plasma ATP and adenylate levels have value in predicting clinical outcome in PCAS patients. In total, 15 patients who experienced cardiac arrest outside the hospital setting and who could be reanimated were enrolled in this study. Healthy volunteers (n = 8) served as controls. Of the 15 PCAS patients, 8 died within 4 days after resuscitation. Of the 7 survivors, 2 lapsed into vegetative states, 1 survived with moderate disabilities, and 4 showed good recoveries. Arterial blood samples were drawn immediately after successful resuscitation and return of spontaneous circulation (ROSC). The concentrations of ATP and other adenylates in plasma were assessed with high-performance liquid chromatography. PCAS patients had significantly higher ATP levels than healthy controls. Plasma ATP levels correlated with lactate levels, Acute Physiology and Chronic Health Evaluation II scores, and the time it took to ROSC (time-to-ROSC). Plasma adenylate levels in patients who died after resuscitation were significantly higher than in survivors. Based on our results and receiver-operating characteristic curve analysis, we conclude that plasma adenylate levels may help predict outcome in PCAS patients.

Septic Shock Alters Mitochondrial Respiration of Lymphoid Cell-Lines and Human Peripheral Blood Mononuclear Cells: The Role of Plasma

INTRODUCTION

In septic shock patients, postseptic immunosuppression state after the systemic inflammatory response syndrome is responsible for nosocomial infections, with subsequent increased mortality. The aim of the present study was to assess the underlying cellular mechanisms of the postseptic immunosuppression state, by investigating mitochondrial functions of peripheral blood mononuclear cells (PBMCs) from septic shock patients over 7 days.

MATERIALS AND METHODS

Eighteen patients admitted to a French intensive care unit for septic shock were included. At days 1 and 7, PBMCs were isolated by Ficoll density gradient centrifugation. Mitochondrial respiration of intact septic PBMCs was assessed versus control group PBMCs, by measuring O2 consumption in plasma, using high-resolution respirometry. Mitochondrial respiration was then compared between septic plasmas and control plasmas for control PBMCs, septic PBMCs, and lymphoid cell-line (CEM). To investigate the role of plasma, we measured several plasma cytokines, among them High-Mobility Group Box 1 (HMGB1), by enzyme-linked immunosorbent assays.

RESULTS

Basal O2 consumption of septic shock PBMCs was of 8.27 ± 3.39 and 10.48 ± 3.99 pmol/s/10 cells at days 1 and 7, respectively, significantly higher than in control PBMCs (5.37 ± 1.46 pmol/s/10 cells, P < 0.05). Septic patient PBMCs showed a lower response to oligomycin, suggesting a reduced ATP-synthase activity, as well as an increased response to carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP) suggesting an increased mitochondrial respiratory capacity. At 6 h, septic plasmas showed a decreased O2 consumption of CEM (4.73 ± 1.46 vs. 6.58 ± 1.53, P < 0.05) as well as in control group PBMCs (1.76 ± 0.36 vs. 2.70 ± 0.42, P < 0.05), and triggered a decreased ATP-synthase activity but an increased response to FCCP. These differences are not explained by different cell survival. High HMGB1 levels were significantly associated with reduced PBMCs mitochondrial respiration.

CONCLUSIONS

Septic plasma impairs mitochondrial respiration in immune cells, with a possible role of the proinflammatory protein HMGB1, leading to a subsequent compensation, probably by enzymatic activation. This compensation result is an improvement of global mitochondrial respiratory capacity, but without restoring ATP-synthase activity.

Increased oxidative phosphorylation in lymphocytes does not atone for decreased cell numbers after burn injury

The acute systemic inflammatory response syndrome (SIRS) and multiorgan dysfunction (MOD) that occur in large burn injuries may be attributed, in part, to immunosuppressive responses such as decreased lymphocytes. However, the mitochondrial bioenergetics of lymphocytes after severe burn injury are poorly understood. The purpose of this study was to examine mitochondrial function of lymphocytes following severe burns in a swine model. Anesthetized Yorkshire swine (n = 17) sustained 40% total body surface area full-thickness contact burns. Blood was collected at pre-injury (Baseline; BL) and at 24 and 48 h after injury for complete blood cell analysis, flow cytometry, cytokine analysis, and ficoll separation of intact lymphocytes for high-resolution mitochondrial respirometry analysis. While neutrophil numbers increased, a concomitant decrease was found in lymphocytes (P < 0.001) after burn injury, which was not specific to CD4⁺ or CD8⁺ lymphocytes. No changes in immune cell population were observed from 24 h to 48 h post-injury. IL 12-23 decreased while a transient increase in IL 4 was found from BL to 24h (P < 0.05). CRP progressively increased from BL to 24h (P < 0.05) and 48h (P < 0.001) post-injury. Routine and maximal mitochondrial respiration progressively increased from BL to 24h (P < 0.05) and 48 h post-injury (P < 0.001). No changes were found in leak respiration or residual oxygen consumption. When considering the reduction in lymphocyte number, the total peripheral lymphocyte bioenergetics per volume of blood significantly decreased from BL to 24h and 48h (P < 0.05). For the first time, we were able to measure mitochondrial activity in intact lymphocyte mitochondria through high-resolution respirometry in a severely burned swine model. Our data showed that the non-specific reduction in peripheral T cells after injury was larger than the increased mitochondrial activity in those cells, which may be a compensatory mechanism for the total reduction in lymphocytes. Additional studies in the metabolic activation of T cell subpopulations may provide diagnostic or therapeutic targets after severe burn injury.

Calcium signalling in T cells

Calcium (Ca²⁺) signalling is of paramount importance to immunity. Regulated increases in cytosolic and organellar Ca²⁺ concentrations in lymphocytes control complex and crucial effector functions such as metabolism, proliferation, differentiation, antibody and cytokine secretion and cytotoxicity. Altered Ca²⁺ regulation in lymphocytes leads to various autoimmune, inflammatory and immunodeficiency syndromes. Several types of plasma membrane and organellar Ca²⁺-permeable channels are functional in T cells. They contribute highly localized spatial and temporal Ca²⁺ microdomains that are required for achieving functional specificity. While the mechanistic details of these Ca²⁺ microdomains are only beginning to emerge, it is evident that through crosstalk, synergy and feedback mechanisms, they fine-tune T cell signalling to match complex immune responses. In this article, we review the expression and function of various Ca²⁺-permeable channels in the plasma membrane, endoplasmic reticulum, mitochondria and endolysosomes of T cells and their role in shaping immunity and the pathogenesis of immune-mediated diseases.

PPARα activation enhances the ability of nile tilapia (Oreochromis niloticus) to resist Aeromonas hydrophila infection

Peroxisome proliferator-activated receptor α (PPARα) plays critical physiological roles in energy metabolism, antioxidation and immunity of mammals, however, these functions have not been fully understood in fish. In the present study, Nile tilapia (Oreochromis niloticus) were fed with fenofibrate, an agonist of PPARα, for six weeks, and subsequently challenged with Aeromonas hydrophila. The results showed that PPARα was efficiently activated by fenofibrate through increasing mRNA and protein expressions and protein dephosphorylation. PPARα activation increased significantly mitochondrial fatty acid β-oxidation efficiency, the copy number of mitochondrial DNA and expression of monoamine oxidase (MAO), a marker gene of mitochondria. Meanwhile, PPARα activation also increased significantly the expression of NADH dehydrogenase [ubiquinone] 1α subcomplex subunit 9 (NDUFA9, complex I) and mitochondrial cytochrome c oxidase 1 (MTCO1, complex IV). The fenofibrate-fed fish had higher survival rate when exposed to A. hydrophila. Moreover, the fenofibrate-fed fish also had higher activities of immune and antioxidative enzymes, and gene expressions of anti-inflammatory cytokines, while had lower expressions of pro-inflammatory cytokine genes. Taken together, PPARα activation improved the ability of Nile tilapia to resist A. hydrophila, mainly through enhancing mitochondrial fatty acids β-oxidation, immune and antioxidant capacities, as well as inhibiting inflammation. This is the first study showing the regulatory effects of PPARα activation on immune functions through increasing mitochondria-mediated energy supply in fish.

What's Bred in the Bone: Calcium Channels in Lymphocytes

Calcium (Ca²⁺) is an important second messenger in lymphocytes and is essential in regulating various intracellular pathways that control critical cell functions. Ca²⁺ channels are located in the plasma membrane and intracellular membranes, facilitating Ca²⁺ entry into the cytoplasm. Upon Ag receptor stimulation, Ca²⁺ can enter the lymphocyte via the Ca²⁺ release-activated Ca²⁺ channel found in the plasma membrane. The increase of cytosolic Ca²⁺ modulates signaling pathways, resulting in the transcription of target genes implicated in differentiation, activation, proliferation, survival, and apoptosis of lymphocytes. Along with Ca²⁺ release-activated Ca²⁺ channels, several other channels have been found in the membranes of T and B lymphocytes contributing to key cellular events. Among them are the transient receptor potential channels, the P2X receptors, voltage-dependent Ca²⁺ channels, and the inositol 1,4,5-trisphosphate receptor as well as the N-methyl-d-aspartate receptors. In this article, we review the contributions of these channels to mediating Ca²⁺ currents that drive specific lymphocyte functions.

Associations of P2RX7 Functional Diplotypes with Localized Aggressive Periodontitis

AIM

The purpose of this study was to test for the role of the P2X7 receptor in localized aggressive periodontitis (LAP).

METHODS

Peripheral blood was obtained from 95 subjects with LAP and 76 healthy unrelated controls (HUCs). Three P2RX7 single-nucleotide polymorphisms (rs1718119, rs2230911, and rs3751143) were genotyped from these subjects, and their peripheral blood samples were stimulated with lipopolysaccharide (LPS) from Escherichia coli and tested for inflammatory markers. The 3 P2RX7 single-nucleotide polymorphisms were in found to be in perfect linkage disequilibrium, and a total of 4 haplotypes and 9 diplotypes were identified among all subjects. For both subject populations, the 9 diplotypes were grouped into 4 functional groups and tested for association with subject inflammatory response. To specifically study the effects of extrinsic activation of the P2X7 receptor in LAP, peripheral blood samples from were stimulated under 3 treatments: LPS, LPS + ATP, and LPS +ATP+ P2X7 selective inhibitor. The effects of these treatments on P2X7 receptor activity were measured through Luminex protein assay. Last, to test whether receptor stimulation was related to P2RX7 expression, relative mRNA levels of P2RX7 were quantified with real-time quantitative polymerase chain reaction.

RESULTS

Several associations between the P2RX7 diplotypes and LPS-stimulated blood chemokine/cytokine levels were found between the LAP and HUC populations (P < 0.05). P2X7 activation resulted in statistically significant differences in IL-1β and IL-12p40 concentrations for both subject populations. The relative P2RX7 mRNA levels increased significantly after addition of its inhibitor for both LAP and HUC populations.

CONCLUSIONS

This study detected an association between P2RX7 functional diplotypes and in vitro immune response of whole blood from subjects with LAP. In addition, we found that inhibition of the activated P2X7 receptor leads to increased P2RX7 mRNA levels, suggesting a feedback loop ( ClinicalTrials.gov NCT01330719).

KNOWLEDGE TRANSFER STATEMENT

The results of this study suggest that P2RX7 functional diplotypes are associated with LAP and their in vitro immune response to bacteria. Ongoing studies to uncover the mechanistic link between P2RX7 and LAP phenotypes could lead to the development of preventive approaches for susceptible subjects.

Autocrine stimulation of P2Y1 receptors is part of the purinergic signaling mechanism that regulates T cell activation

Previous studies have shown that T cell receptor (TCR) and CD28 coreceptor stimulation involves rapid ATP release, autocrine purinergic feedback via P2X receptors, and mitochondrial ATP synthesis that promote T cell activation. Here, we show that ADP formation and autocrine stimulation of P2Y1 receptors are also involved in these purinergic signaling mechanisms. Primary human CD4 T cells and the human Jurkat CD4 T cell line express P2Y1 receptors. The expression of this receptor increases following T cell stimulation. Inhibition of P2Y1 receptors impairs the activation of mitochondria, as assessed by mitochondrial Ca²⁺ uptake, and reduces cytosolic Ca²⁺ signaling in response to TCR/CD28 stimulation. We found that the addition of exogenous ADP or overexpression of P2Y1 receptors significantly increased IL-2 mRNA transcription in response to TCR/CD28 stimulation. Conversely, antagonists or silencing of P2Y1 receptors reduced IL-2 mRNA transcription and attenuated T cell functions. We conclude that P2Y1 and P2X receptors have non-redundant, synergistic functions in the regulation of T cell activation. P2Y1 receptors may represent potential therapeutic targets to modulate T cell function in inflammation and host defense.

P2X4 receptors, immunity, and sepsis

Sepsis is life-threatening systemic organ dysfunction caused by a deregulated host response to an infectious insult. Currently, the treatment of sepsis is limited to the use of antibiotics, fluids, and cardiovascular/respiratory support. Despite these interventions, septic mortality remains high, with reduced life quality in survivors. For this reason, the identification of novel drug targets is a pressing task of modern pharmacology. According to a recent research, it appears that P2 purinergic receptors, which can regulate the host's response to infections, have been identified as potential targets for the treatment of sepsis. Among P2 receptors, the P2X₄ receptor has recently captured the attention of the research community owing to its role in protecting against infections, inflammation, and organ injury. The present review provides an outline of the role played by P2X₄ receptors in the modulation of the host's response to sepsis and the promise that targeting this receptor holds in the treatment of sepsis.

The comparisons in protective mechanisms and efficiencies among dietary α-lipoic acid, β-glucan and l-carnitine on Nile tilapia infected by Aeromonas hydrophila

Dietary α-lipoic acid (LA), β-glucan (Gluc) and l-carnitine (L-Ca) are commonly used additives to promote fish growth and stress resistance in aquaculture production. However their mechanisms and efficiencies in helping fish to resist diseases have not been compared before. In this study, we fed Nile tilapia (Oreochromis niloticus) with diets containing appropriate doses of LA, Gluc and L-Ca for five weeks and further intraperitoneally injected the fish with Aeromonas hydrophila. After dietary treatment, none of the additives affected the fish growth, but dietary Gluc and L-Ca reduced protein and lipid body contents in fish, respectively. After A. hydrophila challenge, all fish treated with the three dietary additives showed higher survival rate, but those fed on dietary L-Ca had lower survival than those fed on LA and Gluc diets, indicating high protection efficiency of LA and Gluc. The protective mechanisms of the three feed additives were quite different under A. hydrophila infection. Dietary LA induced higher total antioxidant capacity and higher mRNA expression of anti-oxidative genes than other additives in liver and also activated partly the immune function in serum and spleen. Gluc largely increased the immune function by activating the immunity enzymes in serum, inducing inflammation in liver and increasing the expression of immune genes in spleen and head kidney. Gluc also increased partly the antioxidant capacity in serum and liver and lipid catabolism in liver. L-Ca largely increased lipid catabolism in liver while it increased partly the antioxidant capacities in serum and liver. Taken together, these results indicate that, dietary LA, Gluc and L-Ca have various protective mechanisms and differ in their efficiencies on resisting A. hydrophila infection in Nile tilapia.

Lipopolysaccharide suppresses T cells by generating extracellular ATP that impairs their mitochondrial function via P2Y11 receptors

T cell suppression contributes to immune dysfunction in sepsis. However, the underlying mechanisms are not well-defined. Here, we show that exposure of human peripheral blood mononuclear cells to bacterial lipopolysaccharide (LPS) can rapidly and dose-dependently suppress interleukin-2 (IL-2) production and T cell proliferation. We also report that these effects depend on monocytes. LPS did not prevent the interaction of monocytes with T cells, nor did it induce programmed cell death protein 1 (PD-1) signaling that causes T cell suppression. Instead, we found that LPS stimulation of monocytes led to the accumulation of extracellular ATP that impaired mitochondrial function, cell migration, IL-2 production, and T cell proliferation. Mechanistically, LPS-induced ATP accumulation exerted these suppressive effects on T cells by activating the purinergic receptor P2Y11 on the cell surface of T cells. T cell functions could be partially restored by enzymatic removal of extracellular ATP or pharmacological blocking of P2Y11 receptors. Plasma samples obtained from sepsis patients had similar suppressive effects on T cells from healthy subjects. Our findings suggest that LPS and ATP accumulation in the circulation of sepsis patients suppresses T cells by promoting inappropriate P2Y11 receptor stimulation that impairs T cell metabolism and functions. We conclude that inhibition of LPS-induced ATP release, removal of excessive extracellular ATP, or P2Y11 receptor antagonists may be potential therapeutic strategies to prevent T cell suppression and restore host immune function in sepsis.

A Mutation in the Mitochondrial Aspartate/Glutamate Carrier Leads to a More Oxidizing Intramitochondrial Environment and an Inflammatory Myopathy in Dutch Shepherd Dogs

BACKGROUND

Inflammatory myopathies are characterized by infiltration of inflammatory cells into muscle. Typically, immune-mediated disorders such as polymyositis, dermatomyositis and inclusion body myositis are diagnosed.

OBJECTIVE

A small family of dogs with early onset muscle weakness and inflammatory muscle biopsies were investigated for an underlying genetic cause.

METHODS

Following the histopathological diagnosis of inflammatory myopathy, mutational analysis including whole genome sequencing, functional transport studies of the mutated and wild-type proteins, and metabolomic analysis were performed.

RESULTS

Whole genome resequencing identified a pathological variant in the SLC25A12 gene, resulting in a leucine to proline substitution at amino acid 349 in the mitochondrial aspartate-glutamate transporter known as the neuron and muscle specific aspartate glutamate carrier 1 (AGC1). Functionally reconstituting recombinant wild-type and mutant AGC1 into liposomes demonstrated a dramatic decrease in AGC1 transport activity and inability to transfer reducing equivalents from the cytosol into mitochondria. Targeted, broad-spectrum metabolomic analysis from affected and control muscles demonstrated a proinflammatory milieu and strong support for oxidative stress.

CONCLUSIONS

This study provides the first description of a metabolic mechanism in which ablated mitochondrial glutamate transport markedly reduced the import of reducing equivalents into mitochondria and produced a highly oxidizing and proinflammatory muscle environment and an inflammatory myopathy.

Sera from Septic Patients Contain the Inhibiting Activity of the Extracellular ATP-Dependent Inflammasome Pathway

Immunoparalysis is a common cause of death for critical care patients with sepsis, during which comprehensive suppression of innate and adaptive immunity plays a significant pathophysiological role. Although the underlying mechanisms are unknown, damage-associated molecular patterns (DAMPs) from septic tissues might be involved. Therefore, we surveyed sera from septic patients for factors that suppress the innate immune response to DAMPs, including adenosine triphosphate (ATP), monosodium urate, and high mobility group box-1. Macrophages, derived from THP-1 human acute monocytic leukemia cells, were incubated with each DAMP, in the presence or absence of sera that were collected from critically ill patients. Secreted cytokines were then quantified, and cell lysates were assayed for relevant intracellular signaling mediators. Sera from septic patients who ultimately did not survive significantly suppressed IL-1β production only in response to extracellular ATP. This effect was most pronounced with sera collected on day 3, and persisted with sera collected on day 7. However, this effect was not observed when THP-1 cells were treated with sera from survivors of sepsis. Septic sera collected at the time of admission (day 1) also diminished intracellular levels of inositol 1,4,5-triphosphate and cytosolic calcium (P < 0.01), both of which are essential for ATP signaling. Finally, activated caspase-1 was significantly diminished in cells exposed to sera collected on day 7 (P < 0.05). In conclusion, the sera of septic patients contain certain factors that persistently suppress the immune response to extracellular ATP, thereby leading to adverse clinical outcomes.

Purinergic P2X4 receptors and mitochondrial ATP production regulate T cell migration

T cells must migrate in order to encounter antigen-presenting cells (APCs) and to execute their varied functions in immune defense and inflammation. ATP release and autocrine signaling through purinergic receptors contribute to T cell activation at the immune synapse that T cells form with APCs. Here, we show that T cells also require ATP release and purinergic signaling for their migration to APCs. We found that the chemokine stromal-derived factor-1α (SDF-1α) triggered mitochondrial ATP production, rapid bursts of ATP release, and increased migration of primary human CD4+ T cells. This process depended on pannexin-1 ATP release channels and autocrine stimulation of P2X4 receptors. SDF-1α stimulation caused localized accumulation of mitochondria with P2X4 receptors near the front of cells, resulting in a feed-forward signaling mechanism that promotes cellular Ca2+ influx and sustains mitochondrial ATP synthesis at levels needed for pseudopod protrusion, T cell polarization, and cell migration. Inhibition of P2X4 receptors blocked the activation and migration of T cells in vitro. In a mouse lung transplant model, P2X4 receptor antagonist treatment prevented the recruitment of T cells into allograft tissue and the rejection of lung transplants. Our findings suggest that P2X4 receptors are therapeutic targets for immunomodulation in transplantation and inflammatory diseases.

The purinergic receptor P2RX7 directs metabolic fitness of long-lived memory CD8+ T cells

Extracellular ATP (eATP) is an ancient 'danger signal' used by eukaryotes to detect cellular damage¹. In mice and humans, the release of eATP during inflammation or injury stimulates both innate immune activation and chronic pain through the purinergic receptor P2RX7^2-4. It is unclear, however, whether this pathway influences the generation of immunological memory, a hallmark of the adaptive immune system that constitutes the basis of vaccines and protective immunity against re-infection^5,6. Here we show that P2RX7 is required for the establishment, maintenance and functionality of long-lived central and tissue-resident memory CD8⁺ T cell populations in mice. By contrast, P2RX7 is not required for the generation of short-lived effector CD8⁺ T cells. Mechanistically, P2RX7 promotes mitochondrial homeostasis and metabolic function in differentiating memory CD8⁺ T cells, at least in part by inducing AMP-activated protein kinase. Pharmacological inhibitors of P2RX7 provoked dysregulated metabolism and differentiation of activated mouse and human CD8⁺ T cells in vitro, and transient P2RX7 blockade in vivo ameliorated neuropathic pain but also compromised production of CD8⁺ memory T cells. These findings show that activation of P2RX7 by eATP provides a common currency that both alerts the nervous and immune system to tissue damage, and promotes the metabolic fitness and survival of the most durable and functionally relevant memory CD8⁺ T cell populations.

Inhibition of MicroRNA-23b Attenuates Immunosuppression During Late Sepsis Through NIK, TRAF1, and XIAP

Background

microRNA-23b (miR-23b) is a multiple functional miRNA. We hypothesize that miR-23b plays a role in the pathogenesis of sepsis. Our study investigated the effect of miR-23b on sepsis-induced immunosuppression.

Methods

Mice were treated with miR-23b inhibitors by tail vein injection 2 days after cecal ligation puncture (CLP)-induced sepsis. Apoptosis in spleens and apoptotic signals were investigated, and survival was monitored. T-cell immunoreactivities were examined during late sepsis. Nuclear factor κB (NF-κB)-inducing kinase (NIK), tumor necrosis factor (TNF)-receptor associated factor 1 (TRAF1), and X-linked inhibitor of apoptosis protein (XIAP), the putative targets of miR-23b, were identified by a dual-luciferase reporter assay.

Results

miR-23b expression is upregulated and sustained during sepsis. The activation of the TLR4/TLR9/p38 MAPK/STAT3 signal pathway contributes to the production of miR-23b in CLP-induced sepsis. miR-23b inhibitor decreased the number of spleen cells positive by terminal deoxynucleotidyl transferase dUTP nick-end labeling and improved survival. miR-23b inhibitor restored the immunoreactivity by alleviating the development of T-cell exhaustion and producing smaller amounts of immunosuppressive interleukin 10 and interleukin 4 during late sepsis. We demonstrated that miR-23b mediated immunosuppression during late sepsis by inhibiting the noncanonical NF-κB signal and promoting the proapoptotic signal pathway by targeting NIK, TRAF1, and XIAP.

Conclusions

Inhibition of miR-23b reduces late-sepsis-induced immunosuppression and improves survival. miR-23b might be a target for immunosuppression.

P2X7 targeting inhibits growth of human mesothelioma

Malignant pleural mesothelioma (MPM) is an aggressive tumor refractory to anti-blastic therapy. MPM cells show several genetic and biochemical defects, e.g. overexpression of oncogenes, downregulation of onco-suppressor genes, dysregulation of microRNA, or alteration of intracellular Ca²⁺ homeostasis and of apoptosis. No information is as yet available on purinergic signalling in this tumor. Signalling via the P2×7 (P2RX7 or P2×7R) purinergic receptor is attracting increasing attention as a pathway involved in cancer cell death or proliferation. In this report we show that the P2×7R is expressed by three MPM cell lines established from MPM patients but not by mesothelial cells from healthy subjects (healthy mesothelial cells, HMCs). MPM cell proliferation was inhibited by in vitro incubation in the presence of selective P2×7R antagonists, as well as by stimulation with the P2×7R agonist BzATP. Systemic administration of the selective P2×7R blocker AZ10606120 inhibited in vivo growth of MPM tumors whether implanted subcutaneously (s.c.) or intraperitoneally (i.p.). Our findings suggest that the P2×7R might be a novel target for the therapy of mesothelioma.

The P2X7 Receptor in Infection and Inflammation

Adenosine triphosphate (ATP) accumulates at sites of tissue injury and inflammation. Effects of extracellular ATP are mediated by plasma membrane receptors named P2 receptors (P2Rs). The P2R most involved in inflammation and immunity is the P2X7 receptor (P2X7R), expressed by virtually all cells of innate and adaptive immunity. P2X7R mediates NLRP3 inflammasome activation, cytokine and chemokine release, T lymphocyte survival and differentiation, transcription factor activation, and cell death. Ten human P2RX7 gene splice variants and several SNPs that produce complex haplotypes are known. The P2X7R is a potent stimulant of inflammation and immunity and a promoter of cancer cell growth. This makes P2X7R an appealing target for anti-inflammatory and anti-cancer therapy. However, an in-depth knowledge of its structure and of the associated signal transduction mechanisms is needed for an effective therapeutic development.

Inhibited fatty acid β-oxidation impairs stress resistance ability in Nile tilapia (Oreochromis niloticus)

Energy metabolism plays important roles in stress resistance and immunity in mammals, however, such functions have not been established in fish. In the present study, Nile tilapia (Oreochromis niloticus) was fed with mildronate, an inhibitor of mitochondrial fatty acid (FA) β-oxidation, for six weeks subsequently challenged with Aeromonas hydrophila and ammonia nitrogen exposure. Mildronate treatment reduced significantly l-carnitine concentration and mitochondrial FA β-oxidation efficiency, while it increased lipid accumulation in liver. The fish with inhibited hepatic FA catabolism had lower survival rate when exposed to Aeromonas hydrophila and ammonia nitrogen. Moreover, fish fed mildronate supplemented diet had lower immune enzymes activities and anti-inflammatory cytokine genes expressions, but had higher pro-inflammatory cytokine genes expressions. However, the oxidative stress-related biochemical indexes were not significantly affected by mildronate treatment. Taken together, inhibited mitochondrial FA β-oxidation impaired stress resistance ability in Nile tilapia mainly through inhibiting immune functions and triggering inflammation. This is the first study showing the regulatory effects of lipid catabolism on stress resistance and immune functions in fish.

Regulation of the T Cell Response by CD39

The ectonucleoside triphosphate diphosphohydrolase 1 (ENTPD1, or CD39) catalyzes the phosphohydrolysis of extracellular ATP (eATP) and ADP (eADP) released under conditions of inflammatory stress and cell injury. CD39 generates AMP, which is in turn used by the ecto-5'-nucleotidase CD73 to synthesize adenosine. These ectonucleotidases have a major impact on the dynamic equilibrium of proinflammatory eATP and ADP nucleotides versus immunosuppressive adenosine nucleosides. Indeed, CD39 plays a dominant role in the purinergic regulation of inflammation and the immune response because its expression is influenced by genetic and environmental factors. We review the specific role of CD39 in the kinetic regulation of cellular immune responses in the evolution of disease. We focus on the effects of CD39 on T cells and explore potential clinical applications in autoimmunity, chronic infections, and cancer.

Purinergic Signaling and the Immune Response in Sepsis: A Review

PURPOSE

Sepsis remains an unresolved clinical problem with high in-hospital mortality. Despite intensive research over decades, no treatments for sepsis have become available. Here we explore the role of ATP in the pathophysiology of sepsis. ATP is not only a universal energy carrier but it also acts as an extracellular signaling molecule that regulates immune function. ATP stimulates a large family of purinergic receptors found on the cell surface of virtually all mammalian cells. In severe sepsis and septic shock, ATP is released in large amounts into the extracellular space where it acts as a "danger" signal. In this review, we focus on the roles of ATP as a key regulator of immune cell function and as a disruptive signal that contributes to immune dysfunction in sepsis.

METHODS

We summarized the current understanding of the pathophysiology of sepsis, with special emphasis on the emerging role of systemic ATP as a disruptive force that promotes morbidity and mortality in sepsis.

FINDINGS

Over the past two decades, the discovery that regulated ATP release and purinergic signaling represent a novel regulatory mechanism in immune cell physiology has opened up new possibilities in the treatment of sepsis. Immune cells respond to stimulation with the release of cellular ATP, which regulates cell functions in autocrine and paracrine fashions. In sepsis, large amounts of systemic ATP produced by tissue damage and inflammation disrupt these regulatory purinergic signaling mechanisms, leading to immune dysfunction that promotes the pathophysiologic processes involved in sepsis.

IMPLICATIONS

The knowledge of these ATP-dependent signaling processes is likely to reveal exciting new avenues in the treatment of the unresolved clinical problem of sepsis.

Cutting off the power: inhibition of leukemia cell growth by pausing basal ATP release and P2X receptor signaling?

T cells respond to antigen stimulation with the rapid release of cellular ATP, which stimulates an autocrine feedback mechanism that regulates calcium influx through P2X receptors. This autocrine purinergic feedback mechanism plays an essential role in the activation of T cells resulting in cell proliferation and clonal expansion. We recently reported that increases in mitochondrial ATP production drive this stimulation-induced purinergic signaling mechanism but that low-level mitochondrial ATP production fuels basal T cell functions required to maintain vigilance of unstimulated T cells. Here we studied whether defects in these purinergic signaling mechanisms are involved in the unwanted proliferation of leukemia T cells. We found that acute leukemia T cells (Jurkat) possess a larger number and more active mitochondria than their healthy counterparts. Jurkat cells have higher intracellular ATP concentrations and generat more extracellular ATP than unstimulated T cells from healthy donors. As a result, increased purinergic signaling through P2X1 and P2X7 receptors elevates baseline levels of cytosolic Ca(2+) in Jurkat cells. We found that pharmacological inhibition of this basal purinergic signaling mechanism decreases mitochondrial activity, Ca(2+) signaling, and cell proliferation. Similar results were seen in the leukemic cell lines THP-1, U-937, and HL-60. Combined treatment with inhibitors of P2X1 or P2X7 receptors and the chemotherapeutic agent 6-mercaptopurine completely blocked Jurkat cell proliferation. Our results demonstrate that increased mitochondrial metabolism promotes autocrine purinergic signaling and uncontrolled proliferation of leukemia cells. These findings suggest that deranged purinergic signaling can result in T cell malignancy and that therapeutic targeting aimed at purinergic signaling is a potential strategy to combat T cell leukemia.

Recent developments in severe sepsis research: from bench to bedside and back

Severe sepsis remains a worldwide threat, not only in industrialized countries, due to their aging population, but also in developing countries where there still are numerous cases of neonatal and puerperal sepsis. Tools for early diagnosis, a prerequisite for rapid and appropriate antibiotic therapy, are still required. In this review, we highlight some recent developments in our understanding of the associated systemic inflammatory response that help deciphering pathophysiology (e.g., epigenetic, miRNA, regulatory loops, compartmentalization, apoptosis and synergy) and discuss some of the consequences of sepsis (e.g., immune status, neurological and muscular alterations). We also emphasize the challenge to better define animal models and discuss past failures in clinical investigations in order to define new efficient therapies.