Adenosine from a biologic source regulates neutrophil extracellular traps (NETs)

Resveratrol prevents the release of neutrophil extracellular traps (NETs) by controlling hydrogen peroxide levels and nuclear elastase migration

Neutrophil extracellular traps (NETs) are defense mechanisms that trap and kill microorganisms and degrade cytokines. However, excessive production, dysregulation of suppression mechanisms, or inefficient removal of NETs can contribute to increased inflammatory response and the development of pathological conditions. Therefore, research has focused on identifying drugs that inhibit or delay the NET release process. Since reactive oxygen species (ROS) play a significant role in NET release, we aimed to investigate whether resveratrol (RSV), with a wide range of biological and pharmacological properties, could modulate NET release in response to different stimuli. Thus, human neutrophils were pretreated with RSV and subsequently stimulated with PMA, LPS, IL-8, or Leishmania. Our findings revealed that RSV reduced the release of NETs in response to all tested stimuli. RSV decreased hydrogen peroxide levels in PMA- and LPS-stimulated neutrophils, inhibited myeloperoxidase activity, and altered the localization of neutrophil elastase. RSV inhibition of NET generation was not mediated through A2A or A2B adenosine receptors or PKA. Based on the observed effectiveness of RSV in inhibiting NET release, our study suggests that this flavonoid holds potential as a candidate for treating NETs involving pathologies.

cAMP-PKA/EPAC signaling and cancer: the interplay in tumor microenvironment

Cancer is a complex disease resulting from abnormal cell growth that is induced by a number of genetic and environmental factors. The tumor microenvironment (TME), which involves extracellular matrix, cancer-associated fibroblasts (CAF), tumor-infiltrating immune cells and angiogenesis, plays a critical role in tumor progression. Cyclic adenosine monophosphate (cAMP) is a second messenger that has pleiotropic effects on the TME. The downstream effectors of cAMP include cAMP-dependent protein kinase (PKA), exchange protein activated by cAMP (EPAC) and ion channels. While cAMP can activate PKA or EPAC and promote cancer cell growth, it can also inhibit cell proliferation and survival in context- and cancer type-dependent manner. Tumor-associated stromal cells, such as CAF and immune cells, can release cytokines and growth factors that either stimulate or inhibit cAMP production within the TME. Recent studies have shown that targeting cAMP signaling in the TME has therapeutic benefits in cancer. Small-molecule agents that inhibit adenylate cyclase and PKA have been shown to inhibit tumor growth. In addition, cAMP-elevating agents, such as forskolin, can not only induce cancer cell death, but also directly inhibit cell proliferation in some cancer types. In this review, we summarize current understanding of cAMP signaling in cancer biology and immunology and discuss the basis for its context-dependent dual role in oncogenesis. Understanding the precise mechanisms by which cAMP and the TME interact in cancer will be critical for the development of effective therapies. Future studies aimed at investigating the cAMP-cancer axis and its regulation in the TME may provide new insights into the underlying mechanisms of tumorigenesis and lead to the development of novel therapeutic strategies.

Staphylococcus aureus suppresses the pentose phosphate pathway in human neutrophils via the adenosine receptor A2aR to enhance intracellular survival

IMPORTANCE

Staphylococcus aureus is one of the leading causes of antimicrobial-resistant infections whose success as a pathogen is facilitated by its massive array of immune evasion tactics, including intracellular survival within critical immune cells such as neutrophils, the immune system's first line of defense. In this study, we describe a novel pathway by which intracellular S. aureus can suppress the antimicrobial capabilities of human neutrophils by using the anti-inflammatory adenosine receptor, adora2a (A2aR). We show that signaling through A2aR suppresses the pentose phosphate pathway, a metabolic pathway used to fuel the antimicrobial NADPH oxidase complex that generates reactive oxygen species (ROS). As such, neutrophils show enhanced ROS production and reduced intracellular S. aureus when treated with an A2aR inhibitor. Taken together, we identify A2aR as a potential therapeutic target for combatting intracellular S. aureus infection.

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

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

Secretomes of M1 and M2 macrophages decrease the release of neutrophil extracellular traps

The release of neutrophil extracellular traps (NETs) can be either beneficial or detrimental for the host, thus it is necessary to maintain a balance between formation and clearance of NETs. Multiple physiological factors eliciting NET release have been identified, yet the studies on natural signals limiting NET formation have been scarce. Accordingly, our aim was to analyze whether cytokines or immune cells can inhibit NET formation. To that end, human granulocytes were incubated with interleukin (IL)-4, IL-10, transforming growth factor beta-2 or adenosine and then stimulated to release NETs. Additionally, neutrophils were cultured in the presence of natural killer (NK) cells, regulatory T cells (Tregs), pro-inflammatory or anti-inflammatory macrophages (M1 or M2 macrophages), or in the presence of NK/Tregs/M1 macrophages or M2 macrophages-conditioned medium and subsequently stimulated to release NETs. Our studies showed that secretome of M1 and M2 macrophages, but not of NK cells and Tregs, diminishes NET formation. Co-culture experiments did not reveal any effect of immune cells on NET release. No effect of cytokines or adenosine on NET release was found. This study highlights the importance of paracrine signaling at the site of infection and is the first to show that macrophage secretome can regulate NET formation.

Role of adenosine A2a receptor in cancers and autoimmune diseases

Adenosine receptors are P1 class of purinergic receptors that belong to G protein-coupled receptors. There are 4 subtypes of adenosine receptors, namely A1, A2A, A2B, and A3. A2AR has a high affinity for the ligand adenosine. Under pathological conditions or external stimuli, ATP is sequentially hydrolyzed to adenosine by CD39 and CD73. The combination of adenosine and A2AR can increase the concentration of cAMP and activate a series of downstream signaling pathways, and further playing the role of immunosuppression and promotion of tumor invasion. A2AR is expressed to some extent on various immune cells, where it is abnormally expressed on immune cells in cancers and autoimmune diseases. A2AR expression also correlates with disease progression. Inhibitors and agonists of A2AR may be potential new strategies for treatment of cancers and autoimmune diseases. We herein briefly reviewed the expression and distribution of A2AR, adenosine/A2AR signaling pathway, expression, and potential as a therapeutic target.

Hydrogel delivery of purinergic enzymes improves cardiac ischemia/reperfusion injury

RATIONALE

The innate immune response contributes to cardiac injury in myocardial ischemia/reperfusion (MI/R). Neutrophils are an important early part of the innate immune response to MI/R. Adenosine, an endogenous purine, is a known innate immune modulator and inhibitor of neutrophil activation. However, its delivery to the heart is limited by its short half-life (<30 s) and off-target side effects. CD39 and CD73 are anti-inflammatory homeostatic enzymes that can generate adenosine from phosphorylated adenosine substrate such as ATP released from injured tissue.

OBJECTIVE

We hypothesize that hydrogel-delivered CD39 and CD73 target the local early innate immune response, reduce neutrophil activation, and preserve cardiac function in MI/R injury.

METHODS AND RESULTS

We engineered a poly(ethylene) glycol (PEG) hydrogel loaded with the adenosine-generating enzymes CD39 and CD73. We incubated the hydrogels with neutrophils in vitro and showed a reduction in hydrogen peroxide production using Amplex Red. We demonstrated availability of substrate for the enzymes in the myocardium in MI/R by LC/MS, and tested release kinetics from the hydrogel. On echocardiography, global longitudinal strain (GLS) was preserved in MI/R hearts treated with the loaded hydrogel. Delivery of purinergic enzymes via this synthetic hydrogel resulted in lower innate immune infiltration into the myocardium post-MI/R, decreased markers of macrophage and neutrophil activation (NETosis), and decreased leukocyte-platelet complexes in circulation.

CONCLUSIONS

In a rat model of MI/R injury, CD39 and CD73 delivered via a hydrogel preserve cardiac function by modulating the innate immune response.

Glycolysis, monocarboxylate transport, and purinergic signaling are key events in Eimeria bovis-induced NETosis

The protozoan parasite Eimeria bovis is the causative agent of bovine coccidiosis, an enteric disease of global importance that significantly affects cattle productivity. Previous studies showed that bovine NETosis—an important early host innate effector mechanism of polymorphonuclear neutrophil (PMN)—is elicited by E. bovis stages. So far, the metabolic requirements of E. bovis-triggered NET formation are unknown. We here studied early glycolytic and mitochondrial responses of PMN as well as the role of pH, distinct metabolic pathways, P2 receptor-mediated purinergic signaling, and monocarboxylate transporters 1 and 2 (MCT1, MCT2) in E. bovis sporozoite-induced NET formation. Seahorse-based experiments revealed a rapid induction of both neutrophil oxygen consumption rate (OCR) and early glycolytic responses, thereby reflecting immediate PMN activation and metabolic changes upon confrontation with sporozoites. The impact of these metabolic changes on NET formation was studied via chemical inhibition experiments targeting glycolysis and energy generation by the use of 2-fluor-2-deoxy-D-glucose (FDG), 6-diazo-5-oxo-L-norleucin (DON), sodium dichloroacetate (DCA), oxythiamine (OT), sodium oxamate (OXA), and oligomycin A (OmA) to block glycolysis, glutaminolysis, pyruvate dehydrogenase kinase, pyruvate dehydrogenase, lactate dehydrogenase, and mitochondrial ATP-synthase, respectively. Overall, sporozoite-induced NET formation was significantly diminished via PMN pretreatments with OmA and OXA, thereby indicating a key role of ATP- and lactate-mediated metabolic pathways. Consequently, we additionally studied the effects of extracellular pH, MCT1, MCT2, and purinergic receptor inhibitors (AR-C141900, AR-C155858, theobromine, and NF449, respectively). Pretreatment with the latter inhibitors led to blockage of sporozoite-triggered DNA release from exposed bovine PMN. This report provides first evidence on the pivotal role of carbohydrate-related metabolic pathways and purinergic receptors being involved in E. bovis sporozoite-induced NETosis.

The immunomodulatory function of adenosine in sepsis

Sepsis is an unsolved clinical condition with a substantial mortality rate in the hospital. Despite decades of research, no effective treatments for sepsis exists. The role of adenosine in the pathogenesis of sepsis is discussed in this paper. Adenosine is an essential endogenous molecule that activates the A1, A2a, A2b, and A3 adenosine receptors to regulate tissue function. These receptors are found on a wide range of immune cells and bind adenosine, which helps to control the immune response to inflammation. The adenosine receptors have many regulatory activities that determine the onset and progression of the disease, which have been discovered via the use of animal models. A greater understanding of the role of adenosine in modulating the immune system has sparked hope that an adenosine receptor-targeted treatment may be used one day to treat sepsis.

Myeloid Poldip2 Contributes to the Development of Pulmonary Inflammation by Regulating Neutrophil Adhesion in a Murine Model of Acute Respiratory Distress Syndrome

Background Lung injury, a severe adverse outcome of lipopolysaccharide-induced acute respiratory distress syndrome, is attributed to excessive neutrophil recruitment and effector response. Poldip2 (polymerase δ-interacting protein 2) plays a critical role in regulating endothelial permeability and leukocyte recruitment in acute inflammation. Thus, we hypothesized that myeloid Poldip2 is involved in neutrophil recruitment to inflamed lungs. Methods and Results After characterizing myeloid-specific Poldip2 knockout mice, we showed that at 18 hours post-lipopolysaccharide injection, bronchoalveolar lavage from myeloid Poldip2-deficient mice contained fewer inflammatory cells (8 [4-16] versus 29 [12-57]×10⁴/mL in wild-type mice) and a smaller percentage of neutrophils (30% [28%-34%] versus 38% [33%-41%] in wild-type mice), while the main chemoattractants for neutrophils remained unaffected. In vitro, Poldip2-deficient neutrophils responded as well as wild-type neutrophils to inflammatory stimuli with respect to neutrophil extracellular trap formation, reactive oxygen species production, and induction of cytokines. However, neutrophil adherence to a tumor necrosis factor-α stimulated endothelial monolayer was inhibited by Poldip2 depletion (225 [115-272] wild-type [myePoldip2^+/+] versus 133 [62-178] myeloid-specific Poldip2 knockout [myePoldip2^-/-] neutrophils) as was transmigration (1.7 [1.3-2.1] versus 1.1 [1.0-1.4] relative to baseline transmigration). To determine the underlying mechanism, we examined the surface expression of β2-integrin, its binding to soluble intercellular adhesion molecule 1, and Pyk2 phosphorylation. Surface expression of β2-integrins was not affected by Poldip2 deletion, whereas β2-integrins and Pyk2 were less activated in Poldip2-deficient neutrophils. Conclusions These results suggest that myeloid Poldip2 is involved in β2-integrin activation during the inflammatory response, which in turn mediates neutrophil-to-endothelium adhesion in lipopolysaccharide-induced acute respiratory distress syndrome.

Adenosine and inflammation: it's time to (re)solve the problem

Resolution of inflammation requires proresolving molecular pathways triggered as part of the host response during the inflammatory phase. Adenosine and its receptors, which are collectively called the adenosine system, shape inflammatory cell activity during the active phase of inflammation, leading these immune cells toward a functional repolarization, thus contributing to the onset of resolution. Strategies based on the resolution of inflammation have shaped a new area of pharmacology referred to as 'resolution pharmacology' and in this regard, the adenosine system represents an interesting target to design novel pharmacological tools to 'resolve' the inflammatory process. In this review, we outline the role of the adenosine system in driving the events required for an effective transition from the proinflammatory phase to the onset and establishment of resolution.

A1 and A2A adenosine receptors play a protective role to reduce prevalence of autoimmunity following tissue damage

Adenosine is a potent modulator that has a tremendous effect on the immune system. Adenosine affects T cell activity, and is necessary in maintaining the T helper/regulatory T cell (Treg ) ratio. Adenosine signalling is also involved in activating neutrophils and the formation of neutrophil extracellular traps (NETs), which has been linked to autoimmune disorders. Therefore, adenosine, through its receptors, is extremely important in maintaining homeostasis and involved in the development of autoimmune diseases. In this study, we aim to evaluate the role of adenosine A1 and A2A receptors in involvement of autoimmune diseases. We studied adenosine regulation by NETosis in vitro, and used two murine models of autoimmune diseases: type I diabetes mellitus (T1DM) induced by low-dose streptozotocin and pristane-induced systemic lupus erythematosus (SLE). We have found that A1 R enhances and A2A R suppresses NETosis. In addition, in both models, A1 R-knock-out (KO) mice were predisposed to the development of autoimmunity. In the SLE model in wild-type (WT) mice we observed a decline of A1 R mRNA levels 6 h after pristane injection that was parallel to lymphocyte reduction. Following pristane, 43% of A1 R-KO mice suffered from lupus-like disease while WT mice remained without any sign of disease at 36 weeks. In WT mice, at 10 days A2A R mRNA levels were significantly higher compared to A1R-KO mice. Similar to SLE, in the T1DM model the presence of A1 R and A2A R was protective. Our data suggest that, in autoimmune diseases, the acute elimination of lymphocytes and reduction of DNA release due to NETosis depends upon A1 R desensitization and long-term suppression of A2A R.

Neutrophils: fast and furious-the nucleotide pathway

Nucleotide signaling is a key element of the neutrophil activation pathway. Neutrophil recruitment and migration to injured tissues is guided by purinergic receptor sensitization, mostly induced by extracellular adenosine triphosphate (ATP) and its hydrolysis product, adenosine (ADO), which is primarily produced by the CD39-CD73 axis located at the neutrophil cell surface. In inflammation unrelated to cancer, neutrophil activation via purinergic signaling aims to eliminate antigens and promote an immune response with minimal damage to healthy tissues; however, an antagonistic response may be expected in tumors. Indeed, alterations in purinergic signaling favor the accumulation of extracellular ATP and ADO in the microenvironment of solid tumors, which promote tumor progression by inducing cell proliferation, angiogenesis, and escape from immune surveillance. Since neutrophils and their N1/N2 polarization spectrum are being considered new components of cancer-related inflammation, the participation of purinergic signaling in pro-tumor activities of neutrophils should also be considered. However, there is a lack of studies investigating purinergic signaling in human neutrophil polarization and in tumor-associated neutrophils. In this review, we discussed the human neutrophil response elicited by nucleotides in inflammation and extrapolated its behavior in the context of cancer. Understanding these mechanisms in cancerous conditions may help to identify new biological targets and therapeutic strategies, particularly regarding tumors that are refractory to traditional chemo- and immunotherapy.

Integration analysis of GWAS and expression quantitative trait loci to identify candidate genes and pathways for clozapine-related neutropaenia

AIMS

Little is known about the genetic basis of clozapine-related neutropaenia. This study aims to explore candidate genes and pathways involved in clozapine-related neutropaenia.

METHODS

This study conducted a two-stage integrative analysis of the summary statistics from the genome-wide association study (GWAS, n = 552) of the lowest absolute neutrophil count (ANC) during clozapine treatment and the summary data of the expressed quantitative trait locus (eQTL). First, we use the probabilistic Mendelian randomization (PMR-Egger) to identify genes whose expression is causally related to ANC, and then use Bayesian co-localization analysis to investigate whether there are shared causal variants between them [posterior probability for hypotheses 4 (PP.H4) > 0.80]. Finally, gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis were conducted to explore the pathways that may be associated with ANC during clozapine treatment.

RESULTS

PMR-Egger analysis identified 146 genes that may be causally associated with ANC after Bonferroni correction (P-value < 3.25e-6). Bayesian co-localization analysis identified six further genes whose gene expression shared common variants with ANC, including NT5E (PP.H4 = 0.96), GLDC (PP.H4 = 0.82), NUDT17 (PP.H4 = 0.88), MSH4 (PP.H4 = 0.88), PTER (PP.H4 = 0.89) and SERPINB6 (PP.H4 = 0.83). Enrichment analysis identified 52 GO terms and seven pathways associated with ANC, such as NAD metabolic process, drug catabolic process and glyoxylate and dicarboxylate metabolism.

CONCLUSION

This study identified multiple candidate genes and pathways that may be involved in clozapine-related neutropaenia, providing novel clues for the mechanism of clozapine-related neutropaenia.

Neutrophil extracellular traps in cancer

Beyond their well-known functions in the acute phases of the immune response, neutrophils play important roles in the various phases of tumor initiation and progression, through the release of their stored or newly synthesized mediators. In addition to reactive oxygen species, cytokines, chemokines, granule proteins and lipid mediators, neutrophil extracellular traps (NETs) can also be released upon neutrophil activation. NET formation can be achieved through a cell-death process or in association with the release of mitochondrial DNA from viable neutrophils. NETs are described as extracellular fibers of DNA and decorating proteins responsible for trapping and killing extracellular pathogens, playing a protective role in the antimicrobial defense. There is increasing evidence, however, that NETs play multiple roles in the scenario of cancer-related inflammation. For instance, NETs directly or indirectly promote tumor growth and progression, fostering tumor spread at distant sites and shielding cancer cells thus preventing the effects of cytotoxic lymphocytes. NETs can also promote tumor angiogenesis and cancer-associated thrombosis. On the other hand, there is some evidence that NETs may play anti-inflammatory and anti-tumorigenic roles. In this review, we focus on the main mechanisms underlying the emerging effects of NETs in cancer initiation and progression.

Candida Extracellular Nucleotide Metabolism Promotes Neutrophils Extracellular Traps Escape

Host innate immunity is fundamental to the resistance against Candida albicans and Candida glabrata infection, two of the most important agents contributing to human fungal infections. Phagocytic cells, such as neutrophils, constitute the first line of host defense mechanisms, and the release of neutrophil extracellular traps (NETs) represent an important strategy to immobilize and to kill invading microorganisms, arresting the establishment of infection. The purinergic system operates an important role in the homeostasis of immunity and inflammation, and ectophosphatase and ectonucleotidase activities are recognized as essential for survival strategies and infectious potential of several pathogens. The expression and unique activity of a 3′-nucleotidase/nuclease (3′NT/NU), able to hydrolyze not only AMP but also nucleic acids, has been considered as part of a possible mechanism of microbes to escape from NETs. The aim of the present study was to evaluate if yeasts escape from the NET-mediated killing through their 3′NT/NU enzymatic activity contributing to NET-hydrolysis. After demonstrating the presence of 3′NT/NU activity in C. albicans, C. glabrata, and Saccharomyces cerevisiae, we show that, during neutrophils-Candida interaction, when NETs formation and release are triggered, NETs digestion occurs and this process of NETs disruption promoted by yeast cells was prevented by ammonium tetrathiomolybdate (TTM), a 3′NT/NU inhibitor. In conclusion, although the exact nature and specificity of yeasts ectonucleotidases are not completely unraveled, we highlight the importance of these enzymes in the context of infection, helping yeasts to overcome host defenses, whereby C. albicans and C. glabrata can escape NET-mediate killing through their 3′NT/NU activity.

Role of the purinergic signaling network in lung ischemia-reperfusion injury

PURPOSE OF REVIEW

Primary graft dysfunction (PGD) is the leading cause of early mortality following lung transplantation and is typically caused by lung ischemia-reperfusion injury (IRI). Current management of PGD is largely supportive and there are no approved therapies to prevent lung IRI after transplantation. The purinergic signaling network plays an important role in this sterile inflammatory process, and pharmacologic manipulation of said network is a promising therapeutic strategy. This review will summarize recent findings in this area.

RECENT FINDINGS

In the past 18 months, our understanding of lung IRI has improved, and it is becoming clear that the purinergic signaling network plays a vital role. Recent works have identified critical components of the purinergic signaling network (Pannexin-1 channels, ectonucleotidases, purinergic P1 and P2 receptors) involved in inflammation in a number of pathologic states including lung IRI. In addition, a functionally-related calcium channel, the transient receptor potential vanilloid type 4 (TRPV4) channel, has recently been linked to purinergic signaling and has also been shown to mediate lung IRI.

SUMMARY

Agents targeting components of the purinergic signaling network are promising potential therapeutics to limit inflammation associated with lung IRI and thus decrease the risk of developing PGD.

The adenosinergic pathway in mesenchymal stem cell fate and functions

Mesenchymal stem cells (MSCs) play an important role in tissue homeostasis and damage repair through their ability to differentiate into cells of different tissues, trophic support, and immunomodulation. These properties made them attractive for clinical applications in regenerative medicine, immune disorders, and cell transplantation. However, despite multiple preclinical and clinical studies demonstrating beneficial effects of MSCs, their native identity and mechanisms of action remain inconclusive. Since its discovery, the CD73/ecto-5'-nucleotidase is known as a classic marker for MSCs, but its role goes far beyond a phenotypic characterization antigen. CD73 contributes to adenosine production, therefore, is an essential component of purinergic signaling, a pathway composed of different nucleotides and nucleosides, which concentrations are finely regulated by the ectoenzymes and receptors. Thus, purinergic signaling controls pathophysiological functions such as proliferation, migration, cell fate, and immune responses. Despite the remarkable progress already achieved in considering adenosinergic pathway as a therapeutic target in different pathologies, its role is not fully explored in the context of the therapeutic functions of MSCs. Therefore, in this review, we provide an overview of the role of CD73 and adenosine-mediated signaling in the functions ascribed to MSCs, such as homing and proliferation, cell differentiation, and immunomodulation. Additionally, we will discuss the pathophysiological role of MSCs, via CD73 and adenosine, in different diseases, as well as in tumor development and progression. A better understanding of the adenosinergic pathway in the regulation of MSCs functions will help to provide improved therapeutic strategies applicable in regenerative medicine.

Modulation of myeloid cells by adenosine signaling

Hypoxia, metabolic activity, cell death and immune responses influence the adenosine concentrations in the extracellular space. Cellular responses to hypoxia and inflammation in myeloid cells promote activation of adenosine sensing circuit, which involves increased expression of ectoenzymes that converts phospho-nucleotides such as ATP to adenosine and increased expression of G protein-coupled adenosine receptors. Adenosine sensing circuitry also involves feedforward signaling, which leads to increased expression of hypoxia-inducible factor 1-alpha (HIF1 and feedback signaling, which leads to the suppression of inflammatory transcription factor, the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) activation. In this review we will discuss how different subsets of myeloid cells sense adenosine accumulation and how adenosine sensing by myeloid cells influence progression of different immune-related conditions including cancer.

Adenosine Signaling in Autoimmune Disorders

The molecular components of the purinergic system (i.e., receptors, metabolizing enzymes and membrane transporters) are widely expressed in the cells of the immune system. Additionally, high concentrations of adenosine are generated from the hydrolysis of ATP in any "danger" condition, when oxygen and energy availability dramatically drops. Therefore, adenosine acts as a retaliatory metabolite to counteract the nucleotide-mediated boost of the immune reaction. Based on this observation, it can be foreseen that the recruitment with selective agonists of the receptors involved in the immunomodulatory effect of adenosine might represent an innovative anti-inflammatory approach with potential exploitation in autoimmune disorders. Quite surprisingly, pro-inflammatory activity exerted by some adenosine receptors has been also identified, thus paving the way for the hypothesis that at least some autoimmune disorders may be caused by a derailment of adenosine signaling. In this review article, we provide a general overview of the roles played by adenosine on immune cells with a specific focus on the development of adenosine-based therapies for autoimmune disorders, as demonstrated by the exciting data from concluded and ongoing clinical trials.