The P2 purinergic receptors of human dendritic cells: identification and coupling to cytokine release

Could P2X7 receptor be a potencial target in neonatal sepsis?

The United Nations Inter-Agency Group for Child Mortality Estimation (UNIGME) estimates that every year 2.5 million neonates die in their first month of life, accounting for nearly one-half of deaths in children under 5 years of age. Neonatal sepsis is the third leading cause of neonatal mortality. The worldwide burden of bacterial sepsis is expected to increase in the next decades due to the lack of effective molecular therapies to replace the administration of antibiotics whose efficacy is compromised by the emergence of resistant strains. In addition, prolonged exposure to antibiotics can have negative effects by increasing the risk of infection by other organisms. With the global burden of sepsis increasing and no vaccine nor other therapeutic approaches proved efficient, the World Health Organization (WHO) stresses the need for new therapeutic targets for sepsis treatment and infection prevention (WHO, A73/32). In response to this unresolved clinical issue, the P2X7 receptor (P2X7R), a key component of the inflammatory cascade, has emerged as a potential target for treating inflammatory/infection diseases. Indeed numerous studies have demonstrated the relevance of the purinergic system as a pharmacological target in addressing immune-mediated inflammatory diseases by regulating immunity, inflammation, and organ function. In this review, we analyze key features of sepsis immunopathophysiology focusing in neonatal sepsis and on how the immunomodulatory role of P2X7R could be a potential pharmacological target for reducing the burden of neonatal sepsis.

"Find Me" and "Eat Me" signals: tools to drive phagocytic processes for modulating antitumor immunity

Phagocytosis, a vital defense mechanism, involves the recognition and elimination of foreign substances by cells. Phagocytes, such as neutrophils and macrophages, rapidly respond to invaders; macrophages are especially important in later stages of the immune response. They detect "find me" signals to locate apoptotic cells and migrate toward them. Apoptotic cells then send "eat me" signals that are recognized by phagocytes via specific receptors. "Find me" and "eat me" signals can be strategically harnessed to modulate antitumor immunity in support of cancer therapy. These signals, such as calreticulin and phosphatidylserine, mediate potent pro-phagocytic effects, thereby promoting the engulfment of dying cells or their remnants by macrophages, neutrophils, and dendritic cells and inducing tumor cell death. This review summarizes the phagocytic "find me" and "eat me" signals, including their concepts, signaling mechanisms, involved ligands, and functions. Furthermore, we delineate the relationships between "find me" and "eat me" signaling molecules and tumors, especially the roles of these molecules in tumor initiation, progression, diagnosis, and patient prognosis. The interplay of these signals with tumor biology is elucidated, and specific approaches to modulate "find me" and "eat me" signals and enhance antitumor immunity are explored. Additionally, novel therapeutic strategies that combine "find me" and "eat me" signals to better bridge innate and adaptive immunity in the treatment of cancer patients are discussed.

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.

Post-Transplant Cyclophosphamide Combined with Brilliant Blue G Reduces Graft-versus-Host Disease without Compromising Graft-versus-Leukaemia Immunity in Humanised Mice

Allogeneic haematopoietic stem cell transplantation (HSCT) leads to the establishment of graft-versus-leukaemia (GVL) immunity, but in many cases also results in the development of graft-versus-host disease (GVHD). This study aimed to determine if P2X7 antagonism using Brilliant Blue G (BBG) could improve the beneficial effects of post-transplant cyclophosphamide (PTCy) in a humanised mouse model of GVHD, without comprising GVL immunity. NOD.Cg-Prkdcscid Il2rgtm1Wjl (NSG) mice were injected with human peripheral blood mononuclear cells (PBMCs) (Day 0), then with cyclophosphamide (33 mg/kg) on Days 3 and 4, and with BBG (50 mg/kg) (or saline) on Days 0-10. PTCy with BBG reduced clinical GVHD development like that of PTCy alone. However, histological analysis revealed that the combined treatment reduced liver GVHD to a greater extent than PTCy alone. Flow cytometric analyses revealed that this reduction in liver GVHD by PTCy with BBG corresponded to an increase in human splenic CD39+ Tregs and a decrease in human serum interferon-γ concentrations. In additional experiments, humanised NSG mice, following combined treatment, were injected with human THP-1 acute myeloid leukaemia cells on Day 14. Flow cytometric analyses of liver CD33+ THP-1 cells showed that PTCy with BBG did not mitigate GVL immunity. In summary, PTCy combined with BBG can reduce GVHD without compromising GVL immunity. Future studies investigating P2X7 antagonism in combination with PTCy may lead to the development of novel treatments that more effectively reduce GVHD in allogeneic HSCT patients without promoting leukaemia relapse.

A Species-Specific Anti-Human P2X7 Monoclonal Antibody Reduces Graft-versus-Host Disease in Humanised Mice

Graft-versus-host disease (GVHD) is a T cell-mediated inflammatory disorder that arises from allogeneic haematopoietic stem cell transplantation and is often fatal. The P2X7 receptor is an extracellular adenosine 5'-triphosphate-gated cation channel expressed on immune cells. Blockade of this receptor with small molecule inhibitors impairs GVHD in a humanised mouse model. A species-specific blocking monoclonal antibody (mAb) (clone L4) for human P2X7 is available, affording the opportunity to determine whether donor (human) P2X7 contributes to the development of GVHD in humanised mice. Using flow cytometric assays of human RPMI 8266 and murine J774 cells, this study confirmed that this mAb bound and impaired human P2X7. Furthermore, this mAb prevented the loss of human regulatory T cells (hTregs) and natural killer (hNK) T cells in vitro. NOD-scid IL2Rγnull mice were injected with 10 × 106 human peripheral blood mononuclear cells (Day 0) and an anti-hP2X7 or control mAb (100 μg i.p. per mouse, Days 0, 2, 4, 6, and 8). The anti-hP2X7 mAb increased hTregs and hNK cells at Day 21. Moreover, anti-hP2X7 mAb-treatment reduced clinical and histological GVHD in the liver and lung compared to the control treatment at disease endpoint. hTregs, hNK, and hNK T cell proportions were increased, and human T helper 17 cell proportions were decreased at endpoint. These studies indicate that blockade of human (donor) P2X7 reduces GVHD development in humanised mice, providing the first direct evidence of a role for donor P2X7 in GVHD.

Combining High-Z Sensitized Radiotherapy with CD73 Blockade to Boost Tumor Immunotherapy

Radiation therapy (RT) has the capacity to induce immunogenic death in tumor cells, thereby potentially inducing in situ vaccination (ISV) to prime systemic antitumor immune responses. However, RT alone is often faced with various limitations during ISV induction, such as insufficient X-ray deposition and an immunosuppressive microenvironment. To overcome these limitations, we constructed nanoscale coordination particles AmGd-NPs by self-assembling high-Z metal gadolinium (Gd) and small molecular CD73 inhibitor AmPCP. Then, AmGd-NPs could synergize with RT to enhance immunogenic cell death, improve phagocytosis, and promote antigen presentation. Additionally, AmGd-NPs could also gradually release AmPCP to inhibit CD73's enzymatic activity and prevent the conversion of extracellular ATP to adenosine (Ado), thereby driving a proinflammatory tumor microenvironment that promotes DC maturation. As a result, AmGd-NPs sensitized RT induced potent in situ vaccination and boosted CD8⁺ T cell-dependent antitumor immune responses against both primary and metastatic tumors, which could also be potentiated by immune checkpoint inhibitory therapy.

Purinergic signaling: Diverse effects and therapeutic potential in cancer

Regardless of improved biological insights and therapeutic advances, cancer is consuming multiple lives worldwide. Cancer is a complex disease with diverse cellular, metabolic, and physiological parameters as its hallmarks. This instigates a need to uncover the latest therapeutic targets to advance the treatment of cancer patients. Purines are building blocks of nucleic acids but also function as metabolic intermediates and messengers, as part of a signaling pathway known as purinergic signaling. Purinergic signaling comprises primarily adenosine triphosphate (ATP) and adenosine (ADO), their analogous membrane receptors, and a set of ectonucleotidases, and has both short- and long-term (trophic) effects. Cells release ATP and ADO to modulate cellular function in an autocrine or paracrine manner by activating membrane-localized purinergic receptors (purinoceptors, P1 and P2). P1 receptors are selective for ADO and have four recognized subtypes-A1, A2A, A2B, and A3. Purines and pyrimidines activate P2 receptors, and the P2X subtype is ligand-gated ion channel receptors. P2X has seven subtypes (P2X1-7) and forms homo- and heterotrimers. The P2Y subtype is a G protein-coupled receptor with eight subtypes (P2Y1/2/4/6/11/12/13/14). ATP, its derivatives, and purinoceptors are widely distributed in all cell types for cellular communication, and any imbalance compromises the homeostasis of the cell. Neurotransmission, neuromodulation, and secretion employ fast purinergic signaling, while trophic purinergic signaling regulates cell metabolism, proliferation, differentiation, survival, migration, invasion, and immune response during tumor progression. Thus, purinergic signaling is a prospective therapeutic target in cancer and therapy resistance.

Physiologic roles of P2 receptors in leukocytes

Since their discovery in the 1970s, purinergic receptors have been shown to play key roles in a wide variety of biologic systems and cell types. In the immune system, purinergic receptors participate in innate immunity and in the modulation of the adaptive immune response. In particular, P2 receptors, which respond to extracellular nucleotides, are widely expressed on leukocytes, causing the release of cytokines and chemokines and the formation of inflammatory mediators, and inducing phagocytosis, degranulation, and cell death. The activity of these receptors is regulated by ectonucleotidases-expressed in these same cell types-which regulate the availability of nucleotides in the extracellular environment. In this article, we review the characteristics of the main purinergic receptor subtypes present in the immune system, focusing on the P2 family. In addition, we describe the physiologic roles of the P2 receptors already identified in leukocytes and how they can positively or negatively modulate the development of infectious diseases, inflammation, and pain.

A Possible Causal Involvement of Neuroinflammatory, Purinergic P2X7 Receptors in Psychiatric Disorders

P2X7 receptors (Rs) are prominent members of the P2XR family, which after binding ATP, open non-selective cationic channels, thereby allowing the transmembrane passage of Na+, Ca²⁺, and K+. Long-lasting and repetitive stimulation of the receptor by its agonist leads to the formation of large membrane pores permeable for organic cations of up to 900 Da molecular size. These pores are believed to play a role in apoptosis and inflammation. P2X7Rs are located primarily at peripheral macrophages and microglial cells, the resident macrophages of the CNS. The coactivation of toll-like receptors 4 (TLR4) by lipopolysaccharide, a constituent of the cell membrane of gram-negative bacteria, and the P2X7R by ATP leads to the generation and release of the proinflammatory cytokines interleukin-1β (IL-1β), IL-6, and tumor necrosis factor-α. Together with the microglial release of chemokines, reactive oxygen and nitrogen species, proteases, and excitotoxic glutamate, these cytokines result in neurodegeneration. P2X7Rs were found not only to amplify various neurodegenerative illnesses, such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and multiple sclerosis, but also to participate in a range of psychiatric diseases, such as major depression, bipolar disorder, schizophrenia, and autism spectrum disorder. Based on the prevention/reversal of neuroinflammation, pharmacological antagonists of P2X7Rs and their genetic deletion in animal experiments counteract these deleterious psychiatric conditions. Hence, brain penetrant P2X7R antagonists are potential therapeutics for psychiatric diseases, although the available evidence still needs to be extended and validated by further clinical data.

Interactions between the NLRP3-Dependent IL-1β and the Type I Interferon Pathways in Human Plasmacytoid Dendritic Cells

Generally, a reciprocal antagonistic interaction exists between the antiviral type I interferon (IFN) and the antibacterial nucleotide-binding oligomerization domain (NOD)-like receptor pyrin domain containing 3 (NLRP3)-dependent IL-1β pathways that can significantly shape immune responses. Plasmacytoid dendritic cells (pDCs), as professional type I IFN-producing cells, are the major coordinators of antiviral immunity; however, their NLRP3-dependent IL-1β secretory pathway is poorly studied. Our aim was to determine the functional activity of the IL-1β pathway and its possible interaction with the type I IFN pathway in pDCs. We found that potent nuclear factor-kappa B (NF-κB) inducers promote higher levels of pro-IL-1β during priming compared to those activation signals, which mainly trigger interferon regulatory factor (IRF)-mediated type I IFN production. The generation of cleaved IL-1β requires certain secondary signals in pDCs and IFN-α or type I IFN-inducing viruses inhibit IL-1β production of pDCs, presumably by promoting the expression of various NLRP3 pathway inhibitors. In line with that, we detected significantly lower IL-1β production in pDCs of psoriasis patients with elevated IFN-α levels. Collectively, our results show that the NLRP3-dependent IL-1β secretory pathway is inducible in pDCs; however, it may only prevail under inflammatory conditions, in which the type I IFN pathway is not dominant.

Immunogenic Cell Death Enhances Immunotherapy of Diffuse Intrinsic Pontine Glioma: From Preclinical to Clinical Studies

Diffuse intrinsic pontine glioma (DIPG) is the most lethal tumor involving the pediatric central nervous system. The median survival of children that are diagnosed with DIPG is only 9 to 11 months. More than 200 clinical trials have failed to increase the survival outcomes using conventional cytotoxic or myeloablative chemotherapy. Immunotherapy presents exciting therapeutic opportunities against DIPG that is characterized by unique and heterogeneous features. However, the non-inflammatory DIPG microenvironment greatly limits the role of immunotherapy in DIPG. Encouragingly, the induction of immunogenic cell death, accompanied by the release of damage-associated molecular patterns (DAMPs) shows satisfactory efficacy of immune stimulation and antitumor strategies. This review dwells on the dilemma and advances in immunotherapy for DIPG, and the potential efficacy of immunogenic cell death (ICD) in the immunotherapy of DIPG.

Preparation of the Murine Anti-Human P2X7 Receptor Monoclonal Antibody (Clone L4)

The murine anti-human P2X7 receptor monoclonal antibody (mAb) (clone L4) has been used to study the expression and function of the P2X7 receptor on primary leukocytes, keratinocytes, osteoblasts and neuronal cells, as well as various cell lines. This antibody has also been used to characterize polymorphic variants and isoforms of the P2RX7 gene and P2X7 site-directed mutations, and to identify molecules coassociated with P2X7 in the plasma membrane. This chapter describes the maintenance and cryopreservation of the L4 hybridoma cell line, as well as the generation of tissue culture supernatant containing the anti-human P2X7 mAb, and its subsequent purification by Protein A chromatography and conjugation to DyLight™ 488. Moreover, this chapter describes flow cytometric assays to assess the blocking activity and binding of the anti-human P2X7 mAb against P2X7 on human RPMI 8226 multiple myeloma cells.

Molecular and Pharmacological Evidence for the Expression of Multiple Functional P2 Purinergic Receptors in Human Adipocytes

Extracellular ATP exerts important functions as an extracellular signaling molecule via the activation of specific P2 purinergic receptors (P2X and P2Y). We investigated the expression of the different P2 receptors and their possible functional activation in human adipocytes in primary culture. We performed molecular expression analysis of the P2 receptors in human mature adipocytes; examined their functional activation by different nucleotides evaluating [Ca²⁺]_i modifications and IL-6 secretion, and determined the ability of adipocytes to release ATP in the extracellular medium. Human adipocytes express different P2X and P2Y receptors. Extracellular ATP elicited a rise in [Ca²⁺]_i via the activation of P2X and P2Y receptor subtypes. Human adipocytes spontaneously released ATP in the extracellular medium and secreted IL-6 both at rest and after stimulation with ATP. This stimulatory effect of ATP on IL-6 secretion was inhibited by pre-incubation with apyrase, an ATP metabolizing enzyme. These results demonstrate that human adipocytes express different P2X and P2Y receptors that are functionally activated by extracellular nucleotides. Furthermore, human adipocytes spontaneously release ATP, which can act in an autocrine/paracrine fashion on adipocytes, possibly participating in the regulation of inflammatory cytokine release. Thus, P2 purinergic receptors could be a potential therapeutic target to contrast the inflammatory and metabolic complications characterizing obesity.

The role of Cl- and K+ efflux in NLRP3 inflammasome and innate immune response activation

Inflammation is part of innate immunity and is a natural response of the body to bacteria, virus, any other pathogen infections, or to damaged tissues. However, too much inflammation or chronic inflammation contributes to a wide variety of diseases such as inflammatory bowel disease, cancer, type 2 diabetes, heart disease, or autoimmune disease such as rheumatoid arthritis. Recent studies underscored the critical role of K⁺ and Cl^- efflux in the activation of the inflammasome. The NLRP3 inflammasome is a multiprotein complex that mediates the production of the proinflammatory cytokines IL-1β and IL-18 and initiates the inflammatory cell death or pyroptosis. The NLRP3 inflammasome can be activated by multiple stimuli such as extracellular ATP, microbial toxins, ROS, mitochondria DNA or particulate matter. Although the precise mechanisms of NLRP3 activation and regulation by these diverse agonists remain unclear, multiple reports indicate that all NLRP3 agonists ultimately lead to a drop in intracellular concentration of potassium (K⁺ efflux) and chloride (Cl^- efflux). The WNK-SPAK/OSR1-[N]KCC pathway plays a critical role maintaining K⁺ and Cl^- ions concentration in the cell. Recent advances indicate that the WNK-SPAK-[N]KCC pathway play a role in the activation of the innate immune response. This review highlights recent discoveries detailing how ion transport regulates innate immune cell response to inflammatory stimuli.

To inhibit or to boost the ATP/P2RX7 pathway to fight cancer-that is the question

Despite new biological insights and recent therapeutic advances, many tumors remain at baseline during treatments. Therefore, there is an urgent need to find new therapeutic strategies to improve the care of patients with solid tumors. P2RX7 receptor (P2XR7), an ATP-gated ion channel characterized by its ability to form large pore within the cell membrane, is described by most of the investigators as a "chef d'orchestre" of the antitumor immune response. The purpose of this review is to detail the recent information concerning different cellular mechanisms linking P2RX7 to hallmarks of cancer and to discuss different progresses in elucidating how activation of the ATP/P2RX7/NLRP3/IL-18 pathway is a very promising approach to fight cancer progression by increasing antitumor immune responses.

P2X7 receptor in multifaceted cellular signalling and its relevance as a potential therapeutic target in different diseases

P2X7 receptor, a purinergic receptor family member, is abundantly expressed on many cells, including immune, muscle, bone, neuron, and glia. It acts as an ATP-activated cation channel that permits the influx of Ca²⁺, Na⁺ and efflux of K⁺ ions. The P2X7 receptor plays crucial roles in many physiological processes including cytokine and chemokine secretion, NLRP3 inflammasome activation, cellular growth and differentiation, locomotion, wound healing, transcription factors activation, cell death and T-lymphocyte survival. Past studies have demonstrated the up-regulation and direct association of this receptor in many pathophysiological conditions such as cancer, diabetics, arthritis, tuberculosis (TB) and inflammatory diseases. Hence, targeting this receptor is considered a worthwhile approach to lessen the afflictions associated with the disorders mentioned above by understanding the receptor architecture and downstream signalling processes. Here, in the present review, we have dissected the structural and functional aspects of the P2X7 receptor, emphasizing its role in various diseased conditions. This information will provide in-depth knowledge about the receptor and help to develop apt curative methodologies for the betterment of humanity in the coming years.

P2RX7 at the Host-Pathogen Interface of Infectious Diseases

The P2X7 receptor (P2RX7) is an important molecule that functions as a danger sensor, detecting extracellular nucleotides from injured cells and thus signaling an inflammatory program to nearby cells. It is expressed in immune cells and plays important roles in pathogen surveillance and cell-mediated responses to infectious organisms. There is an abundance of literature on the role of P2RX7 in inflammatory diseases and the role of these receptors in host-pathogen interactions. Here, we describe the current knowledge of the role of P2RX7 in the host response to a variety of pathogens, including viruses, bacteria, fungi, protozoa, and helminths. We describe in vitro and in vivo evidence for the critical role these receptors play in mediating and modulating immune responses. Our observations indicate a role for P2X7 signaling in sensing damage-associated molecular patterns released by nearby infected cells to facilitate immunopathology or protection. In this review, we describe how P2RX7 signaling can play critical roles in numerous cells types in response to a diverse array of pathogens in mediating pathogenesis and immunity to infectious agents.

Extracellular ATP: A Feasible Target for Cancer Therapy

Adenosine triphosphate (ATP) is one of the main biochemical components of the tumor microenvironment (TME), where it can promote tumor progression or tumor suppression depending on its concentration and on the specific ecto-nucleotidases and receptors expressed by immune and cancer cells. ATP can be released from cells via both specific and nonspecific pathways. A non-regulated release occurs from dying and damaged cells, whereas active release involves exocytotic granules, plasma membrane-derived microvesicles, specific ATP-binding cassette (ABC) transporters and membrane channels (connexin hemichannels, pannexin 1 (PANX1), calcium homeostasis modulator 1 (CALHM1), volume-regulated anion channels (VRACs) and maxi-anion channels (MACs)). Extracellular ATP acts at P2 purinergic receptors, among which P2X7R is a key mediator of the final ATP-dependent biological effects. Over the years, P2 receptor- or ecto-nucleotidase-targeting for cancer therapy has been proposed and actively investigated, while comparatively fewer studies have explored the suitability of TME ATP as a target. In this review, we briefly summarize the available evidence suggesting that TME ATP has a central role in determining tumor fate and is, therefore, a suitable target for cancer therapy.

P2X7 Receptor at the Crossroads of T Cell Fate

The P2X7 receptor is a ligand-gated, cation-selective channel whose main physiological ligand is ATP. P2X7 receptor activation may also be triggered by ARTC2.2-dependent ADP ribosylation in the presence of extracellular NAD. Upon activation, this receptor induces several responses, including the influx of calcium and sodium ions, phosphatidylserine externalization, the formation of a non-selective membrane pore, and ultimately cell death. P2X7 receptor activation depends on the availability of extracellular nucleotides, whose concentrations are regulated by the action of extracellular nucleotidases such as CD39 and CD38. The P2X7 receptor has been extensively studied in the context of the immune response, and it has been reported to be involved in inflammasome activation, cytokine production, and the migration of different innate immune cells in response to ATP. In adaptive immune responses, the P2X7 receptor has been linked to T cell activation, differentiation, and apoptosis induction. In this review, we will discuss the evidence of the role of the P2X7 receptor on T cell differentiation and in the control of T cell responses in inflammatory conditions.

P2X7 in Cancer: From Molecular Mechanisms to Therapeutics

P2X7 is a transmembrane receptor expressed in multiple cell types including neurons, dendritic cells, macrophages, monocytes, B and T cells where it can drive a wide range of physiological responses from pain transduction to immune response. Upon activation by its main ligand, extracellular ATP, P2X7 can form a nonselective channel for cations to enter the cell. Prolonged activation of P2X7, via high levels of extracellular ATP over an extended time period can lead to the formation of a macropore, leading to depolarization of the plasma membrane and ultimately to cell death. Thus, dependent on its activation state, P2X7 can either drive cell survival and proliferation, or induce cell death. In cancer, P2X7 has been shown to have a broad range of functions, including playing key roles in the development and spread of tumor cells. It is therefore unsurprising that P2X7 has been reported to be upregulated in several malignancies. Critically, ATP is present at high extracellular concentrations in the tumor microenvironment (TME) compared to levels observed in normal tissues. These high levels of ATP should present a survival challenge for cancer cells, potentially leading to constitutive receptor activation, prolonged macropore formation and ultimately to cell death. Therefore, to deliver the proven advantages for P2X7 in driving tumor survival and metastatic potential, the P2X7 macropore must be tightly controlled while retaining other functions. Studies have shown that commonly expressed P2X7 splice variants, distinct SNPs and post-translational receptor modifications can impair the capacity of P2X7 to open the macropore. These receptor modifications and potentially others may ultimately protect cancer cells from the negative consequences associated with constitutive activation of P2X7. Significantly, the effects of both P2X7 agonists and antagonists in preclinical tumor models of cancer demonstrate the potential for agents modifying P2X7 function, to provide innovative cancer therapies. This review summarizes recent advances in understanding of the structure and functions of P2X7 and how these impact P2X7 roles in cancer progression. We also review potential therapeutic approaches directed against P2X7.

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.

Extracellular ATP and P2 purinergic signalling in the tumour microenvironment

Modulation of the biochemical composition of the tumour microenvironment is a new frontier of cancer therapy. Several immunosuppressive mechanisms operate in the milieu of most tumours, a condition that makes antitumour immunity ineffective. One of the most potent immunosuppressive factors is adenosine, which is generated in the tumour microenvironment owing to degradation of extracellular ATP. Accruing evidence over the past few years shows that ATP is one of the major biochemical constituents of the tumour microenvironment, where it acts at P2 purinergic receptors expressed on both tumour and host cells. Stimulation of P2 receptors has different effects depending on the extracellular ATP concentration, the P2 receptor subtype engaged and the target cell type. Among P2 receptors, the P2X purinergic receptor 7 (P2X7R) subtype appears to be a main player in host-tumour cell interactions. Preclinical studies in several tumour models have shown that P2X7R targeting is potentially a very effective anticancer treatment, and many pharmaceutical companies have now developed potent and selective small molecule inhibitors of P2X7R. In this Review, we report on the multiple mechanisms by which extracellular ATP shapes the tumour microenvironment and how its stimulation of host and tumour cell P2 receptors contributes to determining tumour fate.

Extracellular ATP signaling and clinical relevance

Since purinergic signaling was discovered in the early 1970s, it has been shown that extracellular nucleotides, and their derivative nucleosides, are released in a regulated or unregulated manner by cells in various challenging settings and then bind defined purinergic receptors to activate intricate signaling networks. Extracellular ATP plays a role based on different P2 receptor subtypes expressed on specific cell types. Sequential hydrolysis of extracellular ATP catalyzed by ectonucleotidases (e.g. CD39, CD73) is the main pathway for the generation of adenosine, which in turn activates P1 receptors. Many studies have demonstrated that extracellular ATP signaling functions as an important dynamic regulatory pathway to coordinate appropriate immune responses in various pathological processes, including intracellular infection, host-tumor interaction, pro-inflammation vascular injury, and transplant immunity. ATP receptors and CD39 also participate in related clinical settings. Here, we review the latest research in to the development of promising clinical treatment strategies.

ATP promotes the fast migration of dendritic cells through the activity of pannexin 1 channels and P2X7 receptors

Upon its release from injured cells, such as infected, transformed, inflamed, or necrotic cells, extracellular adenosine-5'-triphosphate (ATP) acts as a danger signal that recruits phagocytes, such as neutrophils, macrophages, and dendritic cells (DCs), to the site of injury. The sensing of extracellular ATP occurs through purinergic (P2) receptors. We investigated the cellular mechanisms linking purinergic signaling to DC motility. We found that ATP stimulated fast DC motility through an autocrine signaling loop, which was initiated by the activation of P2X₇ receptors and further amplified by pannexin 1 (Panx1) channels. Upon stimulation of the P2X₇ receptor by ATP, Panx1 contributed to fast DC motility by increasing the permeability of the plasma membrane, which resulted in supplementary ATP release. In the absence of Panx1, DCs failed to increase their speed of migration in response to ATP, despite exhibiting a normal P2X₇ receptor-mediated Ca²⁺ response. In addition to DC migration, Panx1 channel- and P2X₇ receptor-dependent signaling was further required to stimulate the reorganization of the actin cytoskeleton. In vivo, functional Panx1 channels were required for the homing of DCs to lymph nodes, although they were dispensable for DC maturation. These data suggest that P2X₇ receptors and Panx1 channels are crucial players in the regulation of DC migration to endogenous danger signals.

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.

E-NPP3 controls plasmacytoid dendritic cell numbers in the small intestine

Extracellular adenosine 5'-triphosphate (ATP) performs multiple functions including activation and induction of apoptosis of many cell types. The ATP-hydrolyzing ectoenzyme ecto-nucleotide pyrophosphatase/phosphodiesterase 3 (E-NPP3) regulates ATP-dependent chronic allergic responses by mast cells and basophils. However, E-NPP3 is also highly expressed on epithelial cells of the small intestine. In this study, we showed that E-NPP3 controls plasmacytoid dendritic cell (pDC) numbers in the intestine through regulation of intestinal extracellular ATP. In Enpp3-/- mice, ATP concentrations were increased in the intestinal lumen. pDC numbers were remarkably decreased in the small intestinal lamina propria and Peyer's patches. Intestinal pDCs of Enpp3-/- mice showed enhanced cell death as characterized by increases in annexin V binding and expression of cleaved caspase-3. pDCs were highly sensitive to ATP-induced cell death compared with conventional DCs. ATP-induced cell death was abrogated in P2rx7-/- pDCs. Accordingly, the number of intestinal pDCs was restored in Enpp3-/- P2rx7-/- mice. These findings demonstrate that E-NPP3 regulates ATP concentration and thereby prevents the decrease of pDCs in the small intestine.

Metabolites: deciphering the molecular language between DCs and their environment

Dendritic cells (DCs) determine the outcome of the immune response based on signals they receive from the environment. Presentation of antigen under various contexts can lead to activation and differentiation of T cells for immunity or dampening of immune responses by establishing tolerance, primarily through the priming of regulatory T cells. Infections, inflammation and normal cellular interactions shape DC responses through direct contact or via cytokine signaling. Although it is widely accepted that DCs sense microbial components through pattern recognition receptors (PRRs), increasing evidence advocates for the existence of a set of signals that can profoundly shape DC function via PRR-independent pathways. This diverse group of host- or commensal-derived metabolites represents a newly appreciated code from which DCs can interpret environmental cues. In this review, we discuss the existing information on the effect of some of the most studied metabolites on DC function, together with the implications this may have in immune-mediated diseases.

Adenosine diphosphate involvement in THP-1 maturation triggered by the contact allergen 1-fluoro-2,4-dinitrobenzene

Dendritic cells' (DC) activation is considered a key event in the adverse outcome pathway for skin sensitization elicited by covalent binding of chemicals to proteins. The mechanisms underlying DC activation by contact sensitizers are not completely understood. However, several "danger signals" are pointed as relevant effectors. Among these extra-cellular early danger signals, purines may be crucial for the development of xenoinflammation and several reports indicate their involvement in contact allergic reactions. In the present work we used the DC-surrogate monocytic cell line THP-1, cultured alone or co-cultured with the human keratinocyte cell line HaCaT, to explore the contribution of extracellular adenine nucleotides to THP-1 maturation triggered by the extreme contact sensitizer, 1-fluoro-2,4-dinitrobenzene (DNFB). We found that THP-1 maturation induced by DNFB is impaired after purinergic signaling inhibition, and that the transcription of the purinergic metabotropic receptors P2Y2 and P2Y11 is modulated by the sensitizer. We also detected that THP-1 cells only partially hydrolyse extracellular adenosine triphosphate, leading to accumulation of the mono-phosphate derivative, AMP. We detected different and non-overlapping activation patterns of mitogen activated protein kinases by DNFB and extracellular nucleotides. Overall, our results indicate that THP-1 maturation induced by DNFB is strongly modulated by extracellular adenine nucleotides through metabotropic purinergic receptors. This knowledge unveils a molecular toxicity pathway evoked by sensitizers and involved in THP-1 maturation, a DC-surrogate cell line thoroughly used in in vitro tests for the identification of skin allergens.

Role of Extracellular Adenosine Triphosphate in Human Skin

Background:

The nucleotide adenosine triphosphate (ATP) has long been known to drive and participate in countless intracellular processes. Extracellular ATP and its metabolite adenosine have also been shown to exert a variety of effects on nearly every cell type in human skin. Knowledge of the sources and effects of extracellular ATP in human skin may help shape new therapies for skin injury, inflammation, and numerous other cutaneous disorders.

Objective:

The objective of this review is to introduce the reader to current knowledge regarding the sources and effects of extracellular ATP in human skin and to outline areas in which further research is necessary to clarify the nature and mechanism of these effects.

Conclusion:

Extracellular ATP seems to play a direct role in triggering skin inflammatory, regenerative, and fibrotic responses to mechanical injury, an indirect role in melanocyte proliferation and apoptosis, and a complex role in Langerhans cell-directed adaptive immunity.

P2X7 receptor in skin biology and diseases

The P2X7 receptor is a trimeric ligand-gated cation channel present on immune and other cells. Activation of this receptor by its natural ligand extracellular adenosine triphosphate results in a variety of downstream responses, including the release of pro-inflammatory mediators and cell death. In normal skin, P2X7 is present on keratinocytes, Langerhans cells and fibroblasts, while the presence of this receptor on other cutaneous cells is mainly inferred from studies of equivalent cell types present in other tissues. Mast cells in normal skin however express negligible amounts of P2X7, which can be upregulated in cutaneous disease. This review discusses the potential significance of P2X7 in skin biology, and the role of this receptor in inflammatory skin disorders such as irritant and chronic dermatitis, psoriasis, graft-versus-host disease, as well is in wound healing, transplantation and skin cancer.

Regulation of allergic inflammation by the ectoenzyme E-NPP3 (CD203c) on basophils and mast cells

Adenosine 5'-triphosphate (ATP) is released from dying or damaged cells, as well as from activated cells. Once secreted, extracellular ATP induces several immune responses via P2X and P2Y receptors. Basophils and mast cells release ATP upon FcεRI-crosslinking, and ATP activates basophils and mast cells in an autocrine manner. Nucleotide-converting ectoenzymes, such as E-NTPD1, E-NTPD7, and E-NPP3, inhibit ATP-dependent immune responses by hydrolyzing ATP, thereby contributing to immune response regulation. E-NPP3 is a well-known activation marker for human basophils. E-NPP3's physiologic function has recently been disclosed in mice. E-NPP3 is rapidly induced on basophils and mast cells after FcεRI-crosslinking and hydrolyzes extracellular ATP on cell surfaces to prevent ATP-dependent excess activation of basophils and mast cells. In the absence of E-NPP3, basophils and mast cells are overactivated and mice suffer from severe chronic allergic inflammation. Thus, the ATP-hydrolyzing ectoenzymes E-NPP3 has a nonnegligible role in the regulation of basophil- and mast cell-mediated allergic responses.

ROLE OF PURINERGIC RECEPTORS IN IMMUNE RESPONSE

Purine receptors are located on immune and somatic cells of animal and human organisms. Summation of signals from purine and TOLL-like receptors takes place on the level of inflammasome formation and results in summation of the first and second signals of innate immunity. The first signal - from PAMPs (pathogen associated molecular patterns), the second - from DAMPs (danger associated molecular patterns). Adenosine triphosphate (ATP) is the most studied DAMP. ATP connects with purine receptors, which include P2 (P2X7 receptors are the best described), that results in opening of channels of these receptors and transit of ATP into the cell. In parallel exit of K+ from cells and entrance of Ca2+ and Na+ into the cells is observed, that is associated with activation of the immune competent cell. Damaged cells dying via necrosis or apoptosis are the source of extracellular ATP, as well as activated immunocytes. Signals from P2 and TOLL-like receptors are summarized in effectors of immune response, and activation of P2 receptors in lymphocytes makes a contribution into activation of cells, mediated by T-cell receptor. Negative side of purine receptor activation is a stimulating effect on proliferation and metastasis of malignant cells. The practical output of knowledge on functioning of purine receptors for clinical immunology is the application of agonists and antagonists of purine receptors, as well as explanation of effect of immune modulators from the position of launch of K+/Na+-pump, resulting in prolonged activation of immune competent cells.

The role of danger signals and ectonucleotidases in acute graft-versus-host disease

Allogeneic hematopoietic cell transplantation (allo-HCT) represents the only curative treatment approach for many patients with benign or malignant diseases of the hematopoietic system. However, post-transplant morbidity and mortality are significantly increased by the development of acute graft-versus-host disease (GvHD). While alloreactive T cells act as the main cellular mediator of the GvH reaction, recent evidence suggests a critical role of the innate immune system in the early stages of GvHD initiation. Danger-associated molecular patterns released from the intracellular space as well as from the extracellular matrix activate antigen-presenting cells and set pro-inflammatory pathways in motion. This review gives an overview about danger signals representing therapeutic targets with a clinical perspective with a particular focus on extracellular nucleotides and ectonucleotidases.

Purinergic signalling and immune cells

This review article provides a historical perspective on the role of purinergic signalling in the regulation of various subsets of immune cells from early discoveries to current understanding. It is now recognised that adenosine 5'-triphosphate (ATP) and other nucleotides are released from cells following stress or injury. They can act on virtually all subsets of immune cells through a spectrum of P2X ligand-gated ion channels and G protein-coupled P2Y receptors. Furthermore, ATP is rapidly degraded into adenosine by ectonucleotidases such as CD39 and CD73, and adenosine exerts additional regulatory effects through its own receptors. The resulting effect ranges from stimulation to tolerance depending on the amount and time courses of nucleotides released, and the balance between ATP and adenosine. This review identifies the various receptors involved in the different subsets of immune cells and their effects on the function of these cells.

Mesenchymal stem cells derived from human exfoliated deciduous teeth (SHEDs) induce immune modulatory profile in monocyte-derived dendritic cells

BACKGROUND

Mesenchymal stem cells have prominent immune modulatory properties, which may have clinical applications; however their major source, bone marrow, is of limited availability. On the other hand, mesenchymal stem cells derived from human exfoliated deciduous teeth (SHEDs) are readily accessible, but their immune regulatory properties have not been completely investigated. This study was designed, therefore, to evaluate the SHEDs influence on DCs differentiation, maturation, ability to activate T cells and to expand CD4(+)Foxp3(+) T cells.

METHODOLOGY/PRINCIPAL FINDINGS

The experiments were based in cellular co-culture during differentiation and maturation of monocyte derived-DCs (moDCs), with, or not, presence of SHEDs. After co-culture with SHEDs, (moDCs) presented lower expression of BDCA-1 and CD11c, in comparison to DC cultivated without SHEDs. CD40, CD80, CD83 and CD86 levels were also decreased in mature DCs (mDCs) after co-cultivation with SHEDs. To assess the ability of SHEDs-exposed moDCs to modulate T cell responses, the former were separated from SHEDs, and co-cultured with peripheral blood lymphocytes. After 5 days, the proliferation of CD4(+) and CD8(+) T cells was evaluated and found to be lower than that induced by moDCs cultivated without SHEDs. In addition, an increase in the proportion of CD4(+)Foxp3(+)IL-10(+) T cells was observed among cells stimulated by mature moDCs that were previously cultivated with SHEDs. Soluble factors released during co-cultures also showed a reduction in the pro-inflammatory cytokines (IL-2, TNF-α and IFN-γ), and an increase in the anti-inflammatory molecule IL-10.

CONCLUSION/SIGNIFICANCE

This study shows that SHEDs induce an immune regulatory phenotype in moDCs cells, evidenced by changes in maturation and differentiation rates, inhibition of lymphocyte stimulation and ability to expand CD4(+)Foxp3(+) T cells. Further characterization and validation of this phenomenon could support the use of SHEDs, directly or indirectly for immune modulation in the clinical practice.

The purinergic system in allotransplantation

The purine nucleotide adenosine triphosphate (ATP) is a universal source of energy for any intracellular reaction. Under specific physiological or pathological conditions, ATP can be released into extracellular spaces, where it binds and activates the purinergic receptors system (i.e. P2X, P2Y and P1 receptors). Extracellular ATP (eATP) binds to P2X or P2Y receptors in immune cells, where it mediates proliferation, chemotaxis, cytokine release, antigen presentation and cytotoxicity. eATP is then hydrolyzed by ectonucleotidases into adenosine diphosphate (ADP), which activates P2Y receptors. Ectonucleotidases also hydrolyze ADP to adenosine monophosphate and adenosine, which binds P1 receptors. In contrast to P2X and P2Y receptors, P1 receptors exert mainly an inhibitory effect on the immune response. In transplantation, a prominent role has been demonstrated for the eATP/P2X7R axis; the targeting of this pathway in fact is associated with long-term graft function and reduced graft versus host disease severity in murine models. Novel P2X receptor inhibitors are available for clinical use and are under assessment as immunomodulatory agents. In this review, we will focus on the relevance of the purinergic system and on the potential benefits of targeting this system in allograft rejection and tolerance.

The P2X7 receptor: a key player in immune-mediated bone loss?

Inflammatory diseases are often multiorganic diseases with manifestations not related directly to the primary affected organ. They are often complicated by a generalized bone loss that subsequently leads to osteoporosis and bone fractures. The exact mechanism for the accompanying bone loss is not understood in full detail, but factors such as glucocorticoid treatment, immobilization, malnutrition, and insufficient intake of vitamin D play a role. However, it has become evident that the inflammatory process itself is involved and the resulting bone loss is termed immune-mediated bone loss. It stems from an increase in bone resorption and the pro-inflammatory cytokines tumor necrosis factor alpha and interleukin 1 beta and has been shown to not only mediate the inflammatory response but also to strongly stimulate bone degradation. The purinergic P2X7 receptor is central in the processing of these two cytokines and in the initiation of the inflammatory response, and it is a key molecule in the regulation of both bone formation and bone resorption. The aim of this review is therefore to provide evidence-based novel hypotheses of the role of ATP-mediated purinergic signalling via the P2X7 receptor in immune-mediated bone loss and -osteoporosis.

Purinergic signaling in inflammatory cells: P2 receptor expression, functional effects, and modulation of inflammatory responses

Extracellular ATP and related nucleotides promote a wide range of pathophysiological responses via activation of cell surface purinergic P2 receptors. Almost every cell type expresses P2 receptors and/or exhibit regulated release of ATP. In this review, we focus on the purinergic receptor distribution in inflammatory cells and their implication in diverse immune responses by providing an overview of the current knowledge in the literature related to purinergic signaling in neutrophils, macrophages, dendritic cells, lymphocytes, eosinophils, and mast cells. The pathophysiological role of purinergic signaling in these cells include among others calcium mobilization, actin polymerization, chemotaxis, release of mediators, cell maturation, cytotoxicity, and cell death. We finally discuss the therapeutic potential of P2 receptor subtype selective drugs in inflammatory conditions.

P2X4 receptor regulates P2X7 receptor-dependent IL-1β and IL-18 release in mouse bone marrow-derived dendritic cells

Activation of P2X7 receptor of dendritic cells plays a significant role in inflammation through production of cytokines such as IL-1β, and recent studies have suggested structural and functional interactions of P2X7 receptor with P2X4 receptor in macrophages. However, it is unknown whether P2X4 receptor modulates P2X7 functions in dendritic cells. Here, we present evidence that expression of P2X4 receptor is required for P2X7 receptor-dependent IL-1β and IL-18 release in mouse bone marrow-derived dendritic cells (BMDCs). We confirmed expression of both P2X7 receptor and P2X4 receptor in BMDCs. Treatment of BMDCs with 3 mM ATP caused a transient, P2X4-dependent elevation, or spike, of intracellular Ca(2+) level [Ca(2+)]i, followed by the sustained P2X7-dependent increase of [Ca(2+)]i. We performed knockdown of P2X4 receptor in BMDCs by transfection with short hairpin RNA targeting this receptor. The ATP-induced initial peak of [Ca(2+)]i was decreased in P2X4-knockdown cells (P2X4-KD). Further, we found that ATP-induced IL-1β and IL-18 release from LPS-primed BMDCs was suppressed by pretreatment with P2X7 antagonist A438079 or P2X4 antagonist TNP-ATP. The P2X7-dependent IL-1β and IL-18 release was significantly lower in P2X4-KD cells. Chelation of intracellular Ca(2+) also caused suppression of ATP-induced IL-1β and IL-18 release. These results suggest that P2X4 receptor-induced Ca(2+) influx is required for effective production of IL-1β and IL-18 via activation of P2X7 receptor in BMDCs. We conclude that co-expression of P2X4 receptor with P2X7 receptor in dendritic cells leads to enhancement of inflammation through facilitation of P2X7-dependent release of pro-inflammatory cytokines.

P2 receptors and immunity

Immune cells express receptors for extracellular nucleotides named P2 receptors. P2 receptors transduce signals delivered by nucleotides present in the extracellular environment. Accruing evidence shows that purinergic signalling has a profound effect on multiple immune cell responses such as T lymphocyte proliferation, chemotaxis, cytokine release, phagocytosis, Ag presentation and cytotoxicity. This makes P2 receptors an attractive target for the therapy of immuno-mediated disease and cancer.

The sixth sense: hematopoietic stem cells detect danger through purinergic signaling

Over the past decade, extracellular nucleotides (such as ATP and UTP) have emerged as key immunomodulators. This family of molecules, already known for its key metabolic functions, has been the focus of intense investigation that has unambiguously shown its crucial role as mediators of cell-to-cell communication. More recently, in addition to its involvement in inflammation and immunity, purinergic signaling has also been shown to modulate BM-derived stem cells. Extracellular nucleotides promote proliferation, CXCL12-driven migration, and BM engraftment of hematopoietic progenitor and stem cells. In addition, purinergic signaling acts indirectly on hematopoietic progenitor and stem cells by regulating differentiation and release of proinflammatory cytokines in BM-derived human mesenchymal stromal cells, which are part of the hematopoietic stem cell (HSC) niche. HSC research has recently blended into the field of immunology, as new findings highlighted the role played by immunologic signals (such as IFN-α, IFN-γ, or TNF-α) in the regulation of the HSC compartment. In this review, we summarize recent reports unveiling a previously unsuspected ability of HSCs to integrate inflammatory signals released by immune and stromal cells, with particular emphasis on the dual role of extracellular nucleotides as mediators of both immunologic responses and BM stem cell functions.

Extracellular ATP exerts opposite effects on activated and regulatory CD4+ T cells via purinergic P2 receptor activation

It has been reported that ATP inhibits or stimulates lymphoid cell proliferation depending on the cellular subset analyzed. In this study, we show that ATP exerts strikingly opposite effects on anti-CD3/CD28-activated and regulatory CD4(+) T cells (T(regs)), based on nucleotide concentration. We demonstrate that physiological concentrations of extracellular ATP (1-50 nM) do not affect activated CD4(+) T cells and T(regs). Conversely, higher ATP concentrations have a bimodal effect on activated CD4(+) T cells. Whereas 250 nM ATP stimulates proliferation, cytokine release, expression of adhesion molecules, and adhesion, 1 mM ATP induces apoptosis and inhibits activated CD4(+) T cell functions. The expression analysis and pharmacological profile of purinergic P2 receptors for extracellular nucleotides suggest that activated CD4(+) T cells are induced to apoptosis via the upregulation and engagement of P2X7R and P2X4R. On the contrary, 1 mM ATP enhances proliferation, adhesion, migration, via P2Y2R activation, and immunosuppressive ability of T(regs). Similar results were obtained when activated CD4(+) T cells and T(regs) were exposed to ATP released by necrotized leukemic cells. Taken together, our results show that different concentrations of extracellular ATP modulate CD4(+) T cells according to their activated/regulatory status. Because extracellular ATP concentration highly increases in fast-growing tumors or hyperinflamed tissues, the manipulation of purinergic signaling might represent a new therapeutic target to shift the balance between activated CD4(+) T cells and T(regs).

Immunoregulation through extracellular nucleotides

Extracellular ATP (eATP), the most abundant among nucleotides, can act as a mediator during inflammatory responses by binding to plasmamembrane P2 purinergic receptors, which are widely expressed on cells of the immune system. eATP is generally considered as a classical danger signal, which stimulates immune responses in the presence of tissue damage. Converging evidence from several studies using murine models of chronic inflammation have supported this hypothesis; however, the role of eATP in the regulation of human immune function appears to be more complex. Chronic stimulation with micromolar eATP concentrations inhibits the proliferation of T and NK lymphocytes and enhances the capacity of dendritic cells to promote tolerance. The effect of eATP depends on multiple factors, such as the extent of stimulation, eATP concentration, presence/absence of other mediators in the microenvironment, and pattern of P2 receptor engagement. Small but significant differences in the pattern of P2 receptor expression in mice and humans confer the diverse capacities of ATP in regulating the immune response. Such diversity, which is often overlooked, should therefore be carefully considered when evaluating the role of eATP in human inflammatory and autoimmune diseases.

AMP affects intracellular Ca2+ signaling, migration, cytokine secretion and T cell priming capacity of dendritic cells

The nucleotide adenosine-5'-monophosphate (AMP) can be released by various cell types and has been shown to elicit different cellular responses. In the extracellular space AMP is dephosphorylated to the nucleoside adenosine which can then bind to adenosine receptors. However, it has been shown that AMP can also activate A(1) and A(2a) receptors directly. Here we show that AMP is a potent modulator of mouse and human dendritic cell (DC) function. AMP increased intracellular Ca(2+) concentration in a time and dose dependent manner. Furthermore, AMP stimulated actin-polymerization in human DCs and induced migration of immature human and bone marrow derived mouse DCs, both via direct activation of A(1) receptors. AMP strongly inhibited secretion of TNF-α and IL-12p70, while it enhanced production of IL-10 both via activation of A(2a) receptors. Consequently, DCs matured in the presence of AMP and co-cultivated with naive CD4(+)CD45RA(+) T cells inhibited IFN-γ production whereas secretion of IL-5 and IL-13 was up-regulated. An enhancement of Th2-driven immune response could also be observed when OVA-pulsed murine DCs were pretreated with AMP prior to co-culture with OVA-transgenic naïve OTII T cells. An effect due to the enzymatic degradation of AMP to adenosine could be ruled out, as AMP still elicited migration and changes in cytokine secretion in bone-marrow derived DCs generated from CD73-deficient animals and in human DCs pretreated with the ecto-nucleotidase inhibitor 5'-(alpha,beta-methylene) diphosphate (APCP). Finally, the influence of contaminating adenosine could be excluded, as AMP admixed with adenosine desaminase (ADA) was still able to influence DC function. In summary our data show that AMP when present during maturation is a potent regulator of dendritic cell function and point out the role for AMP in the pathogenesis of inflammatory disorders.