Adenosine A2A Receptor Signaling Research Articles

Adenosine A2A Receptor as a Drug Discovery Target

The adenosine A2A receptor is a G-protein-coupled receptor (GPCR) that has been extensively studied during the past few decades because it offers numerous possibilities for therapeutic applications. Herein we describe adenosine A2A receptor distribution, signaling pathways, pharmacology, and molecular structure, followed by a summary and SAR discussion of the most relevant series of adenosine A2A agonists and antagonists. This review also provides an update of the A2A ligands that are undergoing or have undergone clinical studies, including the two currently marketed agonists adenosine and regadenoson.

Extracellular adenosine regulates naive T cell development and peripheral maintenance

Adenosine produced as a byproduct of metabolic activity is present in all tissues and produces dose-dependent suppression of TCR signaling. Naive T cell maintenance depends on inhibition of TCR signals by environmental sensors, which are yet to be fully defined. We produced mice with a floxed adenosine A2A receptor (A2AR) gene, Adora2a, and show that either global A2AR deletion or cre-mediated T cell deletion elicits a decline in the number of naive but not memory T cells. A2AR signaling maintains naive T cells in a quiescent state by inhibiting TCR-induced activation of the phosphatidylinositide 3-kinase (PI3K)-AKT pathway, thereby reducing IL-7Rα down-regulation and naive T cell apoptosis. Patterns of IL-7Rα expression on T cells in chimeric mice reconstituted with Adora2a(+/+) and Adora2a(-/-) bone marrow cells suggest that decreased IL-7Rα in naive T cells is a cell-intrinsic consequence of Adora2a deletion. In addition, A2AR expression increases in early thymic T cell development and contributes to progression of double-negative thymic precursors to single-positive thymocytes with increased IL-7Rα expression. Therefore, A2AR signaling regulates T cell development and maintenance to sustain normal numbers of naive T cells in the periphery.

A feedback loop in PPARγ–adenosine A2A receptor signaling inhibits inflammation and attenuates lung damages in a mouse model of LPS-induced acute lung injury

Although peroxisome proliferator-activated receptor-γ (PPARγ) and adenosine A2A receptor (A2AR) are reported to be anti-inflammatory factors in acute lung injury (ALI), their internal link and synergic or antagonistic effect after activation are poorly understood. Here, we found that PPARγ and A2AR could upregulate the mRNA and protein expressions of each other in lung tissues of LPS-induced mouse ALI model and murine macrophages. Further investigation demonstrated that PPARγ upregulated A2AR expression by directly binding to a DR10 response element (−218 to −197) within A2AR gene promoter region. Instead of directly interacting with PPARγ, A2AR stimulated PPARγ expression via protein kinase A (PKA)–cAMP response element binding protein (CREB) signaling by provoking the binding of CREB to a cAMP responsive element (CRE)-like site in PPARγ gene promoter region. In addition, combination of PPARγ and A2AR agonists was found to exert obviously better effect on suppressing neutrophil infiltration and inflammatory cytokine expressions, attenuating lung edema, pathological changes and improving lung function of blood gas exchange than their single application. These findings reveal a novel functional positive feedback loop between PPARγ and A2AR signaling to potentialize their effect on inhibiting inflammation and attenuating lung damages in ALI. It suggests that targeting this PPARγ–A2AR signaling rather than PPARγ or A2AR alone may be a more attractive and efficient potential therapeutic strategy for ALI.

Sinomenine Protects against Lipopolysaccharide-Induced Acute Lung Injury in Mice via Adenosine A2A Receptor Signaling

Sinomenine (SIN) is a bioactive alkaloid extracted from the Chinese medicinal plant Sinomenium acutum, which is widely used in the clinical treatment of rheumatoid arthritis (RA). However, its role in acute lung injury (ALI) is unclear. In this study, we investigate the role of SIN in lipopolysaccharide (LPS)-induced ALI in mice. After ALI, lung water content and histological signs of pulmonary injury were attenuated, whereas the PaO2/FIO2 (P/F) ratios were elevated significantly in the mice pretreated with SIN. Additionally, SIN markedly inhibited inflammatory cytokine TNF-α and IL-1β expression levels as well as neutrophil infiltration in the lung tissues of the mice. Microarray analysis and real-time PCR showed that SIN treatment upregulated adenosine A2A receptor (A2AR) expression, and the protective effect of SIN was abolished in A2AR knockout mice. Further investigation in isolated mouse neutrophils confirmed the upregulation of A2AR by SIN and showed that A2AR-cAMP-PKA signaling was involved in the anti-inflammatory effect of SIN. Taken together, these findings demonstrate an A2AR-associated anti-inflammatory effect and the protective role of SIN in ALI, which suggests a potential novel approach to treat ALI.

Activation of microglial cells triggers a release of brain-derived neurotrophic factor (BDNF) inducing their proliferation in an adenosine A2A receptor-dependent manner: A2A receptor blockade prevents BDNF release and proliferation of microglia

BackgroundBrain-derived neurotrophic factor (BDNF) has been shown to control microglial responses in neuropathic pain. Since adenosine A2A receptors (A2ARs) control neuroinflammation, as well as the production and function of BDNF, we tested to see if A2AR controls the microglia-dependent secretion of BDNF and the proliferation of microglial cells, a crucial event in neuroinflammation.MethodsMurine N9 microglial cells were challenged with lipopolysaccharide (LPS, 100 ng/mL) in the absence or in the presence of the A2AR antagonist, SCH58261 (50 nM), as well as other modulators of A2AR signaling. The BDNF cellular content and secretion were quantified by Western blotting and ELISA, A2AR density was probed by Western blotting and immunocytochemistry and cell proliferation was assessed by BrdU incorporation. Additionally, the A2AR modulation of LPS-driven cell proliferation was also tested in primary cultures of mouse microglia.ResultsLPS induced time-dependent changes of the intra- and extracellular levels of BDNF and increased microglial proliferation. The maximal LPS-induced BDNF release was time-coincident with an LPS-induced increase of the A2AR density. Notably, removing endogenous extracellular adenosine or blocking A2AR prevented the LPS-mediated increase of both BDNF secretion and proliferation, as well as exogenous BDNF-induced proliferation.ConclusionsWe conclude that A2AR activation plays a mandatory role controlling the release of BDNF from activated microglia, as well as the autocrine/paracrine proliferative role of BDNF.

When cytokinin, a plant hormone, meets the adenosine A2A receptor: a novel neuroprotectant and lead for treating neurodegenerative disorders?

It is well known that cytokinins are a class of phytohormones that promote cell division in plant roots and shoots. However, their targets, biological functions, and implications in mammalian systems have rarely been examined. In this study, we show that one cytokinin, zeatin riboside, can prevent pheochromocytoma (PC12) cells from serum deprivation-induced apoptosis by acting on the adenosine A2A receptor (A2A-R), which was blocked by an A2A-R antagonist and a protein kinase A (PKA) inhibitor, demonstrating the functional ability of zeatin riboside by mediating through A2A-R signaling event. Since the A2A-R was implicated as a therapeutic target in treating Huntington’s disease (HD), a cellular model of HD was applied by transfecting mutant huntingtin in PC12 cells. By using filter retardation assay and confocal microscopy we found that zeatin riboside reversed mutant huntingtin (Htt)-induced protein aggregations and proteasome deactivation through A2A-R signaling. PKA inhibitor blocked zeatin riboside-induced suppression of mutant Htt aggregations. In addition, PKA activated proteasome activity and reduced mutant Htt protein aggregations. However, a proteasome inhibitor blocked both zeatin riboside-and PKA activator-mediated suppression of mutant Htt aggregations, confirming mediation of the A2A-R/PKA/proteasome pathway. Taken together, zeatin riboside might have therapeutic potential as a novel neuroprotectant and a lead for treating neurodegenerative disorders.

A2A adenosine receptor signaling in lymphocytes and the central nervous system regulates inflammation during experimental autoimmune encephalomyelitis.

Extracellular adenosine has an important role in regulating the severity of inflammation during an immune response. Although there are four adenosine receptor (AR) subtypes, the A2AAR is both highly expressed on lymphocytes and known as a prime mediator of adenosine's anti-inflammatory effects. To define the importance of A2AAR signaling during neuroinflammatory disease progression, we used the experimental autoimmune encephalomyelitis (EAE) animal model for multiple sclerosis. In EAE induction experiments, A2AAR antagonist treatment protected mice from disease development and its associated CNS lymphocyte infiltration. However, A2AAR(-/-) mice developed a more severe acute EAE phenotype characterized by more proinflammatory lymphocytes and activated microglia/macrophages. Interestingly, very high levels of A2AAR were expressed on the choroid plexus, a well-established CNS lymphocyte entry point. To determine the contribution of A2AAR signaling in lymphocytes and the CNS during EAE, we used bone marrow chimeric mice. Remarkably, A2AAR(-/-) donor hematopoietic cells potentiated severe EAE, whereas lack of A2AAR expression on nonhematopoietic cells protected against disease development. Although no defect in the suppressive ability of A2AAR(-/-) regulatory T cells was observed, A2AAR(-/-) lymphocytes were shown to proliferate more and produced more IFN-γ following stimulation. Despite this more proinflammatory phenotype, A2AAR antagonist treatment still protected against EAE when A2AAR(-/-) lymphocytes were adoptively transferred to T cell-deficient A2AAR(+/+) mice. These results indicate that A2AAR expression on nonimmune cells (likely in the CNS) is required for efficient EAE development, while A2AAR lymphocyte expression is essential for limiting the severity of the inflammatory response.

Cell-intrinsic adenosine A2A receptor signaling is required for T cell homeostasis and control of tumor growth (47.1)

Abstract Adenosine generated as a result of normal metabolic activity is elevated in response to tissue stress, damage, or hypoxia. By stimulating adenosine A2A receptors (A2ARs) adenosine links metabolic activity to regulation of normal physiological functions such as sleep, blood flow, and inflammation. Although stimulation of A2ARs on activated T cells and APCs acutely suppresses inflammation and immune killing of tumor cells, global A2AR deletion has variable effects on the growth rate of syngeneic tumors, suggesting that A2AR signaling has opposing effects on immune function. Here we show that A2ARs are required for homeostatic maintenance of naïve T cells. In global A2AR-deficient mice naïve T cells undergo apoptosis, which drastically lower their number in blood and in peripheral lymph nodes. Lymphoid deletion of A2ARs, adoptive co-transfer and mixed bone-marrow reconstitution experiments showed that cell intrinsic A2AR signaling is required for T cell homeostasis. A2AR signaling is also required for interleukin 7 to optimally facilitate T cell survival ex vivo. In mice injected with syngeneic melanoma cells, lymphoid-specific A2AR deletion decreases T cell numbers in the periphery and in the tumor, reduces the proportion of tumor infiltrating T cells with an activated/memory phenotype, and markedly accelerates tumor growth. The results indicate that homeostatic adenosine production and A2AR signaling are required in naïve T cell maintenance and immune defense against tumors.

Intracellular adenosine inhibits IgE-dependent degranulation of human skin mast cells (177.19)

Abstract Adenosine is a purine nucleoside that modulates the immediate release (degranulation) of allergic mediators from mast cells. We show that adenosine at relatively high concentrations (IC50=245 µM) inhibits IgE-dependent degranulation of human skin mast cells by a non-toxic intracellular mechanism. We demonstrate that pre-treatment with the specific inhibitor of equilibrative nucleoside transporter 1 (ENT1/SLC29A1), NBMPR, dose-dependently prevented the inhibitory effect of adenosine. In contrast, blocking adenosine A2a receptor (A2aAR) signaling with the specific antagonist ZM241385 was ineffective. Thus, blocking the influx of extracellular adenosine, but not A2aAR signals, was necessary and sufficient to prevent the adenosine-mediated inhibition of degranulation. The inhibitory effect of adenosine was not due to acute toxicity since human skin mast cells cultured up to 3 days with adenosine at concentrations as high as 0.5 mM remained completely viable and activation-competent. Importantly, the inhibitory action of intracellular adenosine was specific for FcϵRI-induced signaling pathways since degranulation induced by Complement Factor 5a, Substance P, or calcium ionophore A23187 was not affected. Moreover, activation (phosphorylation) of the FcϵRI-proximal and critical signaling molecule Syk was significantly inhibited in adenosine-treated mast cells. Overall, these findings provide the first evidence of an inhibitory anti-allergic effect of intracellular adenosine.

Cell‐intrinsic adenosine A2A receptor signaling is required for T cell homeostasis and tumor surveillance

Adenosine A2A receptors (A2ARs) are highly expressed in certain neuronal and immune cells and regulate locomotor activity, sleep, and immune functions. Thus, A2ARs are promising pharmacological targets in a wide range of conditions such as sleep disorders, inflammatory disorders and cancer. Although it is known that stimulation of A2ARs on T cells inhibits their activation, global deletion of A2ARs does not affect the growth of weakly immunogenic tumors. To study the role of A2ARs on T cells we generated a mouse line with T cell‐specific A2AR deletion. Here, we report that A2AR signaling is required to maintain normal numbers of peripheral naïve CD4 and CD8 T cells. T cell‐specific A2AR deletion does not affect thymic T cell differentiation but inhibits naïve T cell survival in response to interleukin‐7 ex vivo. In mice injected with syngeneic B16F10 melanoma cells, T cell specific A2AR deletion decreases T cell numbers in the periphery and in the tumor, reduces the proportion of tumor infiltrating T cells with an activated/memory phenotype, and markedly accelerates tumor growth. The results indicate that A2AR signaling is required for T cell homeostasis and control of tumor growth.

The adenosine A2A receptor agonist regadenoson decreases activation of invariant NKT cells in human sickle cell subjects

Sickle cell disease (SCD) is associated with widely disseminated vaso‐occlusion caused by hyper‐adherent sickle red blood cells and leukocytes. Recurrent episodes of ischemia reperfusion‐injury in the vasculature of SCD individuals promote a pro‐inflammatory state. In a mouse model of SCD, adenosine A2A receptor (A2AR) activation significantly decreases pulmonary inflammation and dysfunction. Similar protective effects are produced by depleting invariant NKT (iNKT) cells or by blocking CD1d restricted activation of iNKT cells, indicating that A2AR agonists mediate their anti‐inflammatory effects primarily by inhibiting iNKT cell activation. We have initiated a clinical trial to investigate the safety and efficacy of a selective A2AR agonist, regadenoson, in human SCD subjects. Our findings indicate that iNKT cells in blood from normal subjects are fewer in number and less activated than from subjects with SCD. Furthermore, activated iNKT cells from SCD subjects are CD4+ and express elevated levels of phospho‐NFκB (P‐NFκB), CD69, and the A2AR. Most significantly, infused regadenoson was observed to diminish iNKT cell expression of P‐NFκB and other activation markers in a time dependent manner. These results imply that A2AR signaling and NFκB regulate iNKT cell activation and that selective A2AR agonists may be useful for treating acute vaso‐occlusion in SCD.

Synergistic Roles for G-protein γ3 and γ7 Subtypes in Seizure Susceptibility as Revealed in Double Knock-out Mice

The functions of different G-protein αβγ subunit combinations are traditionally ascribed to their various α components. However, the discovery of similarly diverse γ subtypes raises the possibility that they may also contribute to specificity. To test this possibility, we used a gene targeting approach to determine whether the closely related γ(3) and γ(7) subunits can perform functionally interchangeable roles in mice. In contrast to single knock-out mice that show normal survival, Gng3(-/-)Gng7(-/-) double knock-out mice display a progressive seizure disorder that dramatically reduces their median life span to only 75 days. Biochemical analyses reveal that the severe phenotype is not due to redundant roles for the two γ subunits in the same signaling pathway but rather is attributed to their unique actions in different signaling pathways. The results suggest that the γ(3) subunit is a component of a G(i/o) protein that is required for γ-aminobutyric acid, type B, receptor-regulated neuronal excitability, whereas the γ(7) subunit is a component of a G(olf) protein that is responsible for A(2A) adenosine or D(1) dopamine receptor-induced neuro-protective response. The development of this mouse model offers a novel experimental framework for exploring how signaling pathways integrate to produce normal brain function and how their combined dysfunction leads to spontaneous seizures and premature death. The results underscore the critical role of the γ subunit in this process.

Differential effects of ADORA2A gene variations in pre-attentive visual sensory memory subprocesses

The ADORA2A gene encodes the adenosine A2A receptor that is highly expressed in the striatum where it plays a role in modulating glutamatergic and dopaminergic transmission. Glutamatergic signaling has been suggested to play a pivotal role in cognitive functions related to the pre-attentive processing of external stimuli. Yet, the precise molecular mechanism of these processes is poorly understood. Therefore, we aimed to investigate whether ADORA2A gene variation has modulating effects on visual pre-attentive sensory memory processing. Studying two polymorphisms, rs5751876 and rs2298383, in 199 healthy control subjects who performed a partial-report paradigm, we find that ADORA2A variation is associated with differences in the efficiency of pre-attentive sensory memory sub-processes. We show that especially the initial visual availability of stimulus information is rendered more efficiently in the homozygous rare genotype groups. Processes related to the transfer of information into working memory and the duration of visual sensory (iconic) memory are compromised in the homozygous rare genotype groups. Our results show a differential genotype-dependent modulation of pre-attentive sensory memory sub-processes. Hence, we assume that this modulation may be due to differential effects of increased adenosine A2A receptor signaling on glutamatergic transmission and striatal medium spiny neuron (MSN) interaction.

Toxicological effect of emodin in mouse testicular gene expression profile

Emodin (1,3,8-trihydroxy-6-methyl-anthraquinone) is a herbal medicine extracted from the rhizomes of Rheum palmatum, and is known as an inhibitor of casein kinase II (CK2). The CK2α' knockout mice are known to be male-infertile; however, there have been no reports on the toxicity of emodin in male reproductive organs/tissues. To evaluate the toxicological effects of emodin on differential gene expression profiles of the testis as compared with acrylamide, mice were orally administered emodin and acrylamide for 5 days at a dose of 1000 and 50 mg kg(-1) per day, respectively, and euthanized 24 h after the final administration. Both chemicals induced hypospermatogenesis, eosinophilic change and apoptosis of germ cell. A DNA microarray analysis showed that the IGF-1 receptor signaling was most closely related to the above testicular toxicity induced by emodin, and the RhoA regulation, TGF/WNT and cytoskeletal remodeling, TNFR1 signaling and adenosine A2A receptor signaling were commonly associated with the two chemicals. We selected 36 genes associated with CK2, apoptosis and spermatogenesis and determined their expression by quantitative reverse transcription-polymerase chain reaction (qPCR). Both chemicals perturbed the expression of genes associated with CK2. Genes related to spermatogenesis were also affected, as evidenced by hypospermatogenesis, and eosinophilic change and apoptosis of germ cell. The results suggest that emodin causes testicular toxicity, including apoptosis with related the IGF-1 receptor signaling pathway, and the two chemicals commonly affect CK2, spermatogenesis and sperm motility via four pathways, such as TNFR1 signaling.

Adenosine A2A Receptor Signaling and Golf Assembly Show a Specific Requirement for the γ7 Subtype in the Striatum

The adenosine A(2A) receptor (A(2A)R) is increasingly recognized as a novel therapeutic target in Parkinson disease. In striatopallidal neurons, the G-protein α(olf) subtype is required to couple this receptor to adenylyl cyclase activation. It is now well established that the βγ dimer also performs an active role in this signal transduction process. In principal, sixty distinct βγ dimers could arise from combinatorial association of the five known β and 12 γ subunit genes. However, key questions regarding which βγ subunit combinations exist and whether they perform specific signaling roles in the context of the organism remain to be answered. To explore these questions, we used a gene targeting approach to specifically ablate the G-protein γ(7) subtype. Revealing a potentially new signaling paradigm, we show that the level of the γ(7) protein controls the hierarchial assembly of a specific G-protein α(olf)β(2)γ(7) heterotrimer in the striatum. Providing a probable basis for the selectivity of receptor signaling, we further demonstrate that loss of this specific G-protein heterotrimer leads to reduced A(2A)R activation of adenylyl cyclase. Finally, substantiating an important role for this signaling pathway in pyschostimulant responsiveness, we show that mice lacking the G-protein γ(7) subtype exhibit an attenuated behavioral response to caffeine. Collectively, these results further support the A(2A)R G-protein α(olf)β(2)γ(7) interface as a possible therapeutic target for Parkinson disease.

Mechanism of exaggerated inflammation in A2A adenosine receptor-deficient mice. (138.28)

Abstract Extracellular adenosine regulates inflammatory responses via A2A adenosine receptor (A2AR). Indeed, A2AR-deficiency results in much exaggerated inflammatory tissue damage, indicating non-redundancy of adenosine-A2AR pathway in the inhibitory mechanism of immune activation. In this study, we found that intravenous injection of concanavalin A into A2AR-deficient mice induced not only stronger hepatic damage than in wild-type mice but also higher levels of cytokines, i.e. TNF-α, IL-4 and IFN-γ. Since NKT cells are responsible for the early production of these cytokines after concanavalin A injection, NKT cells were suspected to be under physiological control of adenosine-A2AR pathway. To directly examine the role of A2AR in NKT cell activation, we specifically stimulated NKT cells with α-galactosylceramide in the presence of dendritic cells. As a result, cytokine production from NKT cells decreased in the presence of A2AR agonists, suggesting that A2AR signaling can down-regulate NKT cell activation. We further injected α-galactosylceramide into mice and observed higher cytokines production in A2AR-deficient mice comparing to wild-type mice. Correspondingly, α-galactosylceramide induced stronger liver damage in A2AR-deficient mice. These results strongly suggest that NKT cell activation is controlled by endogenous adenosine via A2AR, and this physiological regulatory mechanism of NKT cells is critical in the control of tissue-damaging inflammation.

Adenosinergic Modulation Of The Imidazoline I1‐Receptor‐Dependent Hypotensive Effect Of Ethanol In Acute Renal Failure

We recently reported that ethanol lowers blood pressure (BP) in rats with acute renal failure (ARF) via facilitation of sympathoinhibitory pathways of central imidazoline I1‐receptors. This study investigated whether adenosine receptor subtypes modulate the ethanol‐I1‐receptor interaction. The effect of selective blockade of adenosine A1, A2A, or A2B receptors on hemodynamic responses to consecutive administration of moxonidine, I1‐receptor agonist, and ethanol was determined in the glycerol model (10 ml/kg, i.m.) of ARF. Ethanol (1 g/kg i.v.) elicited significant decreases and increases in BP and heart rate (HR), respectively, in ARF rats. Prior exposure to moxonidine abolished the hypotensive but not the tachycardic effect of ethanol. The ability of moxonidine to abolish ethanol hypotension disappeared after selective blockade of adenosine A1 or A2A receptors with 8‐Cyclopentyl‐1,3‐dipropylxanthine and 8‐(3‐chlorostyryl) caffeine, respectively. In contrast, the moxonidine‐ethanol interaction was not affected in presence of alloxazine, a selective A2B antagonist or 8‐phenyltheophylline, a nonselective adenosine receptor antagonist. These findings suggest that alterations caused by moxonidine in adenosine A1 and A2A receptor signaling function to offset the I1‐receptor‐dependent hypotensive action of ethanol in ARF rats.

Improved tumor elimination by anti-tumor T cells that are resistant to adenosine-mediated immunosuppression in tumor microenvironment (95.1)

Abstract Tumors protect themselves from the anti-tumor immune response. One of the self-protective mechanisms is extracellular adenosine in tumor microenvironment, which inactivates anti-tumor T cells via A2A adenosine receptor (A2AR). Anti-tumor T cells that are resistant to adenosine-mediated immunosuppression should be of significant benefit to improve adoptive tumor immunotherapy. Our previous study demonstrated that addition of an adenosine receptor agonist NECA in CTL culture made the CTL resistant to adenosine-mediated immunosuppression. In this study, we evaluated the efficacy of NECA-treated anti-tumor T cells in MCA205 lung metastasis model. Culture-activated tumor-draining lymph node cells (TDLN) were used as anti-MCA205 effector cells. It was confirmed that NECA treatment of TDLN are less sensitive to A2AR signaling. We transferred either TDLN or NECA-treated effector cells (NECA-TDLN) into tumor-bearing mice 10 days after tumor inoculation. NECA pretreatment improved tumor rejection as indicated by the decreased number of tumor nodules from 118 ± 51 (TDLN) to 21 ± 33 (NECA-TDLN). T cell proliferation in vivo was analyzed after the adoptive transfer of CFSE-labeled effector cells. NECA-TDLN proliferated better than control TDLN in the lung (tumor site) suggesting that adenosine-resistant NECA-TDLN could activate well in tumor microenvironment. Induction of adenosine-resistance by NECA pretreatment might be a novel approach to overcome immunosuppression in tumors.

A2A Adenosine Receptor May Allow Expansion of T Cells Lacking Effector Functions in Extracellular Adenosine-Rich Microenvironments

Immunosuppressive signaling via the A2A adenosine receptor (A2AR) provokes a mechanism that protects inflamed tissues from excessive damage by immune cells. This mechanism is desirable not only for preventing uncontrolled tissue destruction by overactive immune responses, but also for protecting tumor tissues from antitumor immune responses. In aforementioned circumstances, T cell priming may occur in an environment containing high concentrations of extracellular adenosine. To examine qualitative changes in T cells activated in the presence of adenosine, we asked whether different functional responses of T cells are equally susceptible to A2AR agonists. In this study, we demonstrate that A2AR signaling during T cell activation strongly inhibited development of cytotoxicity and cytokine-producing activity in T cells, whereas the inhibition of T cell proliferation was only marginal. Both CD8(+) and CD4(+) T cells proliferated well in the presence of A2AR agonists, but their IFN-gamma-producing activities were susceptible to inhibition by cAMP-elevating A2AR. Importantly, the impaired effector functions were maintained in T cells even after removal of the A2AR agonist, reflecting T cell memory of the immunoregulatory effect of adenosine. Thus, although the adenosine-rich environment may allow for the expansion of T cells, the functional activation of T cells may be critically impaired. This physiological mechanism could explain the inefficiency of antitumor T cells in the tumor microenvironment.

The Role of Adenosine A2A Receptor Signaling in Bronchiolitis Obliterans

Binding of adenosine to the anti-inflammatory Gs-coupled adenosine 2A receptor (A(2A)R) inhibits the activity of most inflammatory cells. Extensive preclinical evidence exists for the use of A(2A)R agonists in the prevention of acute ischemia-reperfusion injury. Activation of A(2A)Rs on lymphocytes and antigen-presenting cells also attenuates the alloimmune response. Because ischemia-reperfusion injury and alloimmunity are risk factors for the development of bronchiolitis obliterans (BO), the objective of this study was to determine the effect of A(2A)R signaling on tracheal rejection in a mouse model of BO. A non-revascularized heterotopic tracheal model across a total alloantigenic mismatch was used to study A(2A)R signaling in a mouse model of BO. Tracheal transplants were performed using Balb/c donors into wild-type or A(2A)R knockout C57BL/6 recipient mice. Another group of Balb/c transplants into C57BL/6 recipients were also treated with a selective A(2A)R agonist. Tracheas were assessed at 3, 7, 12, 21, and 28 days after transplantation by hematoxylin and eosin staining, immunohistochemical staining, and collagen staining. Compared with allograft tracheas in wild-type recipients, allografts in A(2A)R knockout recipients had increased inflammation and more severe BO development. Recipient wild-type mice treated with a selective A(2A)R agonist were significantly protected from lymphocyte infiltration and luminal occlusion, but fibro-obliteration still developed by 28 days after transplantation. Endogenous adenosine signals through the A(2A)R to attenuate inflammatory and immune factors involved in BO development. Synthetic A(2A)R agonists may provide a novel treatment strategy to prevent BO.