The extracellular versus intracellular mechanisms of inhibition of TCR-triggered activation in thymocytes by adenosine under conditions of inhibited adenosine deaminase

Cataloguing the postnatal small intestinal transcriptome during the period of susceptibility to necrotizing enterocolitis

In the first postnatal month, the developing mouse intestine shifts from an immature to a mature intestine that will sustain the organism throughout the lifespan. Here, we surveyed the mouse intestine in C57Bl/6 mice by RNA-Seq to evaluate the changes in gene expression over time from the day of birth through 1 month of age in both the duodenum and ileum. We analyzed gene expression for changes in gene families that correlated with the periods of NEC susceptibility or resistance. We highlight that increased expression of DNA processing genes and vacuolar structure genes, tissue development and morphogenesis genes, and cell migration genes all correlated with NEC susceptibility, while increases in immunity gene sets, intracellular transport genes, ATP production, and intracellular metabolism genes correlated with NEC resistance. Using trends identified in the RNA analyses, we further evaluated expression of cellular markers and epithelial regulators, immune cell markers, and adenosine metabolism components. We confirmed key changes with qRT-PCR and immunofluorescence. In addition, we compared some findings to humans using human intestinal biopsies and organoids. This dataset can serve as a reference for other groups considering the role of single molecules or molecular families in early intestinal and postnatal development.

Metabolite and thymocyte development defects in ADA-SCID mice receiving enzyme replacement therapy

Deficiency of adenosine deaminase (ADA, EC3.5.4.4), a housekeeping enzyme intrinsic to the purine salvage pathway, leads to severe combined immunodeficiency (SCID) both in humans and mice. Lack of ADA results in the intracellular accumulation of toxic metabolites which have effects on T cell development and function. While untreated ADA-SCID is a fatal disorder, there are different therapeutic options available to restore ADA activity and reconstitute a functioning immune system, including enzyme replacement therapy (ERT). Administration of ERT in the form of pegylated bovine ADA (PEG-ADA) has proved a life-saving though non-curative treatment for ADA-SCID patients. However, in many patients treated with PEG-ADA, there is suboptimal immune recovery with low T and B cell numbers. Here, we show reduced thymus cellularity in ADA-SCID mice despite weekly PEG-ADA treatment. This was associated with lack of effective adenosine (Ado) detoxification in the thymus. We also show that thymocyte development in ADA-deficient thymi is arrested at the DN3-to-DN4 stage transition with thymocytes undergoing dATP-induced apoptosis rather than defective TCRβ rearrangement or β-selection. Our studies demonstrate at a detailed level that exogenous once-a-week enzyme replacement does not fully correct intra-thymic metabolic or immunological abnormalities associated with ADA deficiency.

ADA activity is decreased in lymphocytes from patients with advanced stage of lung cancer

Cigarette smoking is directly associated with lung cancer. Non-small cell lung carcinoma (NSCLC) represents approximately 80% from all types of lung cancer. This latter is hard to diagnose and to treat due to the lack of symptoms in early stages of the disease. The aim of this study was to evaluate ADA activity and the expression of P2X7, A1, and A2A receptors and in lymphocytes. In addition, the profile of pro-inflammatory and anti-inflammatory cytokines serum levels of patients with lung cancer in advanced stage was evaluated. Patients (n = 13) previously treated for lung cancer at stage IV (UICC) with chemotherapy had their blood collected. Cancer patients showed a decrease in ADA activity and an increase in A1 receptor expression in lymphocytes when compared to the control group. Moreover, patients exhibited an increase in IL-6 and TNF-α, while IL-17 and INF-ϒ serum levels were lower in patients with lung cancer. The decreased ADA activity and the increase in A1 receptor expression may contribute to adenosine pro-tumor effects by increasing IL-6 and TNF-α and decreasing IL-17 and INF-γ serum levels. Our data show an indirect evidence that purinergic signaling may have a role in promoting a profile of cytokines levels that favors tumor progression.

Modulation of neuroimmunity by adenosine and its receptors: metabolism to mental illness

Adenosine is a pleiotropic bioactive with potent neuromodulatory properties. Due to its ability to easily cross the blood-brain barrier, it can act as a signaling molecule between the periphery and the brain. It functions through four (A1, A2A, A2B, and A3) cell surface G protein-coupled adenosine receptors (ARs) that are expressed in some combination on nearly all cells types within the CNS. By regulating the activity of adenylyl cyclase and changing the intracellular concentration of cAMP, adenosine can alter neuronal function and neurotransmission. A variety of illnesses related to metabolic dysregulation, such as type 1 diabetes and Alzheimer's disease, are associated with an elevated serum concentration of adenosine and a pathogenesis rooted in inflammation. This review describes the accepted physiologic function of adenosine in neurological disease and explores its new potential as a peripheral to central danger signal that can activate the neuroimmune system and contribute to symptoms of sickness and psychopathologies.

Nutrient Sensing via mTOR in T Cells Maintains a Tolerogenic Microenvironment

We have proposed that tolerance can be maintained through the induction, by Treg cells, of a tolerogenic microenvironment within tolerated tissues that inhibits effector cell activity but which supports the generation of further Treg cells by “infectious tolerance.” Two important components of this tolerogenic microenvironment depend on metabolism and nutrient sensing. The first is due to the up-regulation of multiple enzymes that consume essential amino acids, which are sensed in naïve T cells primarily via inhibition of the mechanistic target of rapamycin (mTOR) pathway, which in turn encourages their further differentiation into FOXP3⁺ Treg cells. The second mechanism is the metabolism of extracellular ATP to adenosine by the ectoenzymes CD39 and CD73. These two enzymes are constitutively co-expressed on Treg cells, but can also be induced on a wide variety of cell types by TGFβ and the adenosine generated can be shown to be a potent inhibitor of T cell proliferation. This review will focus on mechanisms of nutrient sensing in T cells, how these are integrated with TCR and cytokine signals via the mTOR pathway, and what impact this has on intracellular metabolism and subsequently the control of differentiation into different effector or regulatory T cell subsets.

The mTOR pathway and integrating immune regulation

The mammalian target of rapamycin (mTOR) pathway is an important integrator of nutrient-sensing signals in all mammalian cells, and acts to coordinate the cell proliferation with the availability of nutrients such as glucose, amino acids and energy (oxygen and ATP). A large part of the immune response depends on the proliferation and clonal expansion of antigen-specific T cells, which depends on mTOR activation, and the pharmacological inhibition of this pathway by rapamycin is therefore potently immunosuppressive. It is only recently, however, that we have started to understand the more subtle details of how the mTOR pathway is involved in controlling the differentiation of effector versus memory CD8(+) T cells and the decision to generate different CD4(+) helper T-cell subsets. In particular, this review will focus on how nutrient sensing via mTOR controls the expression of the master transcription factor for regulatory T cells in order to maintain the balance between tolerance and inflammation.

Purinergic signaling on leukocytes infiltrating the LPS-injured lung

Extracellular nucleotides and nucleosides have been implicated as important signaling molecules in the pathogenesis of acute lung injury (ALI). While adenosine is known to inhibit T cell activation, little information is available as to ATP and NAD degrading enzymes, the expression of ATP and adenosine receptors/transporters in different T cell subsets. ALI was induced by challenging mice with intra-tracheal instillation of 60 µl (3 µg/g) LPS. After 3 d and 7 d blood, lung tissue and bronchoalveolar lavage was collected and immune cells were analyzed using flow cytometry. The transcriptional phenotype of T helper cells, cytotoxic and regulatory T cells sorted by FACS was assessed by measuring the expression profile of 28 genes related to purinergic signaling using TaqMan Array Micro Fluidic Cards. Catabolism of ATP, NAD and cAMP by activated CD4⁺ T cells was evaluated by HPLC. CD73 was found to be highly abundant on lymphoid cells with little abundance on myeloid cells, while the opposite was true for CD39. After ALI, the abundance of CD39 and CD73 significantly increased on all T cell subsets derived from lung tissue and bronchoalveolar space. Expression analysis in T cell subsets of the lung revealed ATP (Cd39, Cd73) and NAD (Cd38, Cd157, Cd296, Pc-1) degrading enzymes. However, only transcription of Cd38, Cd39, Cd73, Ent1 and A2a receptor was significantly upregulated after ALI in T helper cells. CD4⁺ T cells from injured lung rapidly metabolized extracellular ATP to AMP and adenosine but not NAD or cAMP. These findings show that lung T cells – the dominant cell fraction in the later phase of ALI – exhibit a unique expression pattern of purinergic signaling molecules. Adenosine is formed by T cells at an enhanced rate from ATP but not from NAD and together with upregulated A2a receptor is likely to modulate the healing process after acute lung injury.

NF-κB enhances hypoxia-driven T-cell immunosuppression via upregulation of adenosine A(2A) receptors

Hypoxia affects inflammation by modulating T-cell activation via the adenosinergic system. We supposed that, in turn, inflammation influences cell hypoxic behavior and that stimulation of T-cells in inflammatory conditions involves the concerted action of the nuclear factor κB (NF-κB) and the related hypoxia-inducible factor 1α (HIF-1α) on the adenosinergic system. We addressed this hypothesis by monitoring both transcription factors and four adenosinergic signaling parameters - namely adenosine, adenosine deaminase (ADA), adenosine A2A receptor (A2AR) and cAMP - in T-cells stimulated using phorbol myristate acetate and phytohemagglutinin and submitted to hypoxic conditions which were mimicked using CoCl2 treatment. We found that cell viability was more altered in stimulated than in resting cells under hypoxia. Detailed analysis showed that: i) NF-κB activation remained at basal level in resting hypoxic cells but greatly increased following stimulation, stimulated hypoxic cells exhibiting the higher level; ii) HIF-1α production induced by hypoxia was boosted via NF-κB activation in stimulated cells whereas hypoxia increased HIF-1α production in resting cells without further activating NF-κB; iii) A2AR expression and cAMP production increased in stimulated hypoxic cells whereas adenosine level remained unchanged due to ADA regulation; and iv) the presence of H2S, an endogenous signaling molecule in inflammation, reversed the effect of stimulation on cell viability by down-regulating the activity of transcription factors and adenosinergic immunosuppression. We also found that: i) the specific A2AR agonist CGS-21680 increased the suppressive effect of hypoxia on stimulated T-cells, the antagonist ZM-241385 exhibiting the opposite effect; and ii) Rolipram, a selective inhibitor of cAMP-specific phosphodiesterase 4, and 8-Br-cAMP, a cAMP analog which preferentially activates cAMP-dependent protein kinase A (PKA), increased T-cell immunosuppression whereas H-89, a potent and selective inhibitor of cAMP-dependent PKA, restored cell viability. Together, these data indicate that inflammation enhances T-cell sensitivity to hypoxia via NF-κB activation. This process upregulates A2AR expression and enhances cAMP production and PKA activation, resulting in adenosinergic T-cell immunosuppression that can be modulated via H2S.

Association between Related Purine Metabolites and Diabetic Retinopathy in Type 2 Diabetic Patients

Aims. The purpose of the study was to investigate the differences of adenosine, adenine, inosine, xanthine, hypoxanthine, and uric acid concentrations in patients with type 2 diabetes mellitus and diabetic retinopathy and assess the relationship between purine metabolites and disease. Materials and Methods. The study group consisted of 114 subjects which were divided into three groups: control (n = 40), type 2 diabetes without retinopathy (n = 35), and type 2 diabetes with retinopathy (n = 39). Levels of metabolites were measured in plasma of all participants. Results. There is a significant increase of levels of adenosine (0.94 ± 0.17 mg/L versus 0.17 ± 0.01 mg/L, P < 0.001), inosine (0.297 ± 0.078 mg/L versus 0.086 ± 0.010 mg/L, P < 0.001), xanthine (1.01 ± 0.21 mg/L versus 0.54 ± 0.05 mg/L, P = 0.009), and uric acid (70.55 ± 3.97 mg/L versus 53.81 ± 2.36 mg/L, P < 0.001) with diabetic retinopathy compared to diabetes mellitus. The levels of adenine, hypoxanthine, and xanthine oxidase did not change. Uric acid, xanthine, inosine, and adenosine correlated positively with systolic blood pressure and urea nitrogen. Conclusions. The levels of adenosine, inosine, uric acid, and xanthine may be useful for monitoring the progression of diabetic retinopathy and evaluating the treatment.

Antiproliferative effects of selective adenosine receptor agonists and antagonists on human lymphocytes: evidence for receptor-independent mechanisms

The effects of standard adenosine receptor (AR) agonists and antagonists on the proliferation of human T lymphocytes, unstimulated and phytohemagglutinin-stimulated human peripheral blood lymphocytes (PBL), and Jurkat T cells were investigated. Real-time PCR measurements confirmed the presence of all four AR subtypes on the investigated cells, although at different expression levels. A2A ARs were predominantly expressed in PBL and further upregulated upon stimulation, while malignant Jurkat T cells showed high expression levels of A1, A2A, and A2B ARs. Cell proliferation was measured by [(3)H]-thymidine incorporation assays. Several ligands, including the subtype-selective agonists CPA (A1), BAY60-6583 (A2B), and IB-MECA (A3), and the antagonists PSB-36 (A1), MSX-2 (A2A), and PSB-10 (A3) significantly inhibited cell proliferation at micromolar concentrations, which were about three orders of magnitude higher than their AR affinities. In contrast, further investigated AR ligands, including the agonists NECA (nonselective) and CGS21680 (A2A), and the antagonists preladenant (SCH-420814, A2A), PSB-1115 (A2B), and PSB-603 (A2B) showed no or only minor effects on lymphocyte proliferation. The anti-proliferative effects of the AR agonists could not be blocked by the corresponding antagonists. The non-selective AR antagonist caffeine stimulated phytohemagglutinin-activated PBL with an EC50 value of 104 μM. This is the first study to compare a complete set of commonly used AR ligands for all subtypes on lymphocyte proliferation. Our results strongly suggest that these compounds induce an inhibition of lymphocyte proliferation and cell death through AR-independent mechanisms.

Search for adenosine A2A spare receptors on peripheral human lymphocytes

Some ligand–receptor couples involve spare receptors, which are apparent when a maximal response is achieved with only a small fraction of the receptor population occupied. This situation favours cross-reactions with low-affinity ligands, which may be detrimental for cell signaling. In the case of the adenosine A_2A receptors (A_2AR), which have an immunosuppressive effect on lymphocytes through cAMP production, the presence of spare A_2AR remains to be established. We examined the situation using patients over-expressing lymphocyte A_2AR and an agonist-like mAb to A_2AR. We found that maximal mAb binding and functional response varied among the patients whereas the dissociation constant and half-maximal effective concentration had similar mean values (0.19 and 0.18 μM, respectively). Lymphocyte A_2AR expression was correlated to plasma adenosine level and A_2AR occupation but not to A_2AR response. These results are consistent with a lack of a reserve of functional A_2AR on human lymphocytes as a general rule and suggest that the amount and functional state of the expressed A_2AR determine the maximal level of the lymphocyte response to adenosine.

Autoimmune dysregulation and purine metabolism in adenosine deaminase deficiency

Genetic defects in the adenosine deaminase (ADA) gene are among the most common causes for severe combined immunodeficiency (SCID). ADA-SCID patients suffer from lymphopenia, severely impaired cellular and humoral immunity, failure to thrive, and recurrent infections. Currently available therapeutic options for this otherwise fatal disorder include bone marrow transplantation (BMT), enzyme replacement therapy with bovine ADA (PEG-ADA), or hematopoietic stem cell gene therapy (HSC-GT). Although varying degrees of immune reconstitution can be achieved by these treatments, breakdown of tolerance is a major concern in ADA-SCID. Immune dysregulation such as autoimmune hypothyroidism, diabetes mellitus, hemolytic anemia, and immune thrombocytopenia are frequently observed in milder forms of the disease. However, several reports document similar complications also in patients on long-term PEG-ADA and after BMT or GT treatment. A skewed repertoire and decreased immune functions have been implicated in autoimmunity observed in certain B-cell and/or T-cell immunodeficiencies, but it remains unclear to what extent specific mechanisms of tolerance are affected in ADA deficiency. Herein we provide an overview about ADA-SCID and the autoimmune manifestations reported in these patients before and after treatment. We also assess the value of the ADA-deficient mouse model as a useful tool to study both immune and metabolic disease mechanisms. With focus on regulatory T- and B-cells we discuss the lymphocyte subpopulations particularly prone to contribute to the loss of self-tolerance and onset of autoimmunity in ADA deficiency. Moreover we address which aspects of immune dysregulation are specifically related to alterations in purine metabolism caused by the lack of ADA and the subsequent accumulation of metabolites with immunomodulatory properties.

Immune cell regulation by autocrine purinergic signalling

Stimulation of almost all mammalian cell types leads to the release of cellular ATP and autocrine feedback through a diverse array of purinergic receptors. Depending on the types of purinergic receptors that are involved, autocrine signalling can promote or inhibit cell activation and fine-tune functional responses. Recent work has shown that autocrine signalling is an important checkpoint in immune cell activation and allows immune cells to adjust their functional responses based on the extracellular cues provided by their environment. This Review focuses on the roles of autocrine purinergic signalling in the regulation of both innate and adaptive immune responses and discusses the potential of targeting purinergic receptors for treating immune-mediated disease.

Adenosine A2A receptor activation protects CD4+ T lymphocytes against activation-induced cell death

Activation-induced cell death (AICD) is initiated by T-cell receptor (TCR) restimulation of already activated and expanded peripheral T cells and is mediated through Fas/Fas ligand (FasL) interactions. Adenosine is a purine nucleoside signaling molecule, and its immunomodulatory effects are mediated by 4 G-protein-coupled receptors: A(1), A(2A), A(2B), and A(3). In this study, we investigated the role of A(2A) receptors in regulating CD4(+) T lymphocyte AICD. Our results showed that the selective A(2A) receptor agonist CGS21680 (EC(50)=15.2-32.6 nM) rescued mouse CD4(+) hybridomas and human Jurkat cells from AICD and that this effect was reversed by the selective A(2A) receptor antagonist ZM241385 (EC(50)=2.3 nM). CGS21680 decreased phosphatidylserine exposure on the membrane, as well as the cleavage of caspase-3, caspase-8 and poly(ADP-ribose) polymerase indicating that A(2A) receptor stimulation blocks the extrinsic apoptotic pathway. In addition, CGS21680 attenuated both Fas and FasL mRNA expression. This decrease in FasL expression was associated with decreased activation of the transcription factor systems NF-kappaB, NF-ATp, early growth response (Egr)-1, and Egr-3. The antiapoptotic effect of A(2A) receptor stimulation was mediated by protein kinase A. Together, these results demonstrate that A(2A) receptor activation suppresses the AICD of peripheral T cells.

Adenosine A2A receptor activation limits graft-versus-host disease after allogenic hematopoietic stem cell transplantation

GVHD is a major barrier to broader use of allogenic HSCT for nonmalignancy clinical applications such as the treatment of primary immunodeficiencies and hemoglobinopathies. We show in a murine model of C57BL/6J (H2-k(b)) --> B6D2F1/J (H2-k(b/d)) acute GVHD that when initiated 2 days before transplant, the activation of the adenosine A(2A)R with the selective agonist ATL146e inhibits the weight loss and mortality associated with disease progression. Furthermore, circulating levels of proinflammatory cytokines and chemokines, including IFN-gamma, IL-6, CCL2, KC, and G-CSF, are reduced significantly by 14-day ATL146e treatment. The up-regulation of CD25, CD69, and CD40L expression by donor CD4(+) and CD8(+) T cells is inhibited by A(2A)R activation; fewer CD3(+) T cells are found in the liver, skin, and colon of ATL146e-treated mice as compared with vehicle-treated controls; and associated tissue injury is lessened. The delayed administration of ATL146e, beginning 9 days after HSCT, reverses GVHD-associated body weight loss successfully, and improvement is sustained for the duration of treatment. We conclude that the selective activation of the A(2A)R has therapeutic potential in the prevention and treatment of acute GVHD.

Adenosine deamination sustains dendritic cell activation in inflammation

Adenosine is a suppressive agent that protects the host from excessive tissue injury associated with strong inflammation. In tissue stress, higher levels of adenosine signal through adenosine receptors to exert strong anti-inflammatory effects, and thus protect host cells. Existing evidence also suggests that elevated adenosine potently down-regulates the activation of lymphocytes during inflammation. This notion, however, is in contrast with another basic observation that the immune system is highly activated precisely under the same circumstances against pathogens. In this study, we show that inflammatory responses of dendritic cells (DCs) are highly sensitive to adenosine suppression. However, they intrinsically carry high adenosine deaminase activity, which in turn degrades and removes adenosine from the surroundings, cutting off DCs from the suppression. This regulatory mechanism is important in DC responses to pathogen-associated molecular patterns and their activation of T cells. Our findings suggest a mechanism that DCs maintain their hyperreactive state in inflammation despite the general state of suppression, and reveal a regulatory role of adenosine deaminase in DC innate immune responses.

Hypertonic stress regulates T-cell function by the opposing actions of extracellular adenosine triphosphate and adenosine

Hypertonic saline (HS) treatment promotes interleukin (IL)-2 production and enhances T-cell activation by the release of cellular adenosine triphosphate (ATP) that activates P2 nucleotide receptors. Released ATP can be hydrolyzed to adenosine, which inhibits T-cell activation. We examined if adenosine affects the response of T cells to HS treatment, and found that the amount of ATP released from T cells is a function of the HS concentration and duration of HS exposure. Physiologically relevant HS concentrations (<40 mmol/L) induced rapid ATP release, with the highest ATP concentrations released within 1 min. The released ATP was converted to adenosine, which opposed the enhancing effects of HS on IL-2 production. We found that Jurkat and CD4+ primary human T cells express most abundantly the A2A and A2B adenosine receptor subtypes, which mediate the suppressive effects of adenosine, as the A2 receptor agonist CGS 21680 suppressed IL-2 production, whereas the A2 receptor antagonist 3,7-dimethyl-1-(2-propynyl)xanthine augmented the enhancing effect of HS on T-cell function. Elimination of extracellular adenosine by adding exogenous adenosine deaminase also increased the enhancing effects of HS. These data suggest that the effect of HS treatment on T-cell function can be modulated with pharmacological agents that abolish the suppressive effects of adenosine formed from the ATP that is released in response to HS treatment.

Adenosine 5'-triphosphate and adenosine as endogenous signaling molecules in immunity and inflammation

Human health is under constant threat of a wide variety of dangers, both self and nonself. The immune system is occupied with protecting the host against such dangers in order to preserve human health. For that purpose, the immune system is equipped with a diverse array of both cellular and non-cellular effectors that are in continuous communication with each other. The naturally occurring nucleotide adenosine 5'-triphosphate (ATP) and its metabolite adenosine (Ado) probably constitute an intrinsic part of this extensive immunological network through purinergic signaling by their cognate receptors, which are widely expressed throughout the body. This review provides a thorough overview of the effects of ATP and Ado on major immune cell types. The overwhelming evidence indicates that ATP and Ado are important endogenous signaling molecules in immunity and inflammation. Although the role of ATP and Ado during the course of inflammatory and immune responses in vivo appears to be extremely complex, we propose that their immunological role is both interdependent and multifaceted, meaning that the nature of their effects may shift from immunostimulatory to immunoregulatory or vice versa depending on extracellular concentrations as well as on expression patterns of purinergic receptors and ecto-enzymes. Purinergic signaling thus contributes to the fine-tuning of inflammatory and immune responses in such a way that the danger to the host is eliminated efficiently with minimal damage to healthy tissues.

Adenosine A2A agonists in development for the treatment of inflammation

Extracellular adenosine binds specifically to a family of four G protein-coupled cell-surface adenosine receptors (ARs). As the activation of the A2AAR modulates the activity of multiple inflammatory cells including neutrophils, macrophages and T lymphocytes, the receptor is considered to be a promising pharmacological target for the treatment of inflammatory disorders. Although adenosine binds nonselectively to all four AR subtypes, A2AAR selective agonists have been developed and shown to inhibit multiple manifestations of inflammatory cell activation including superoxide anion generation, cytokine production and adhesion molecule expression. A2AAR agonists are also vasodilators, but the inhibition of inflammation occurs at low doses that produce few or no cardiovascular side effects. Therefore, the selective activation of the A2AAR by these compounds holds significant potential in the treatment of inflammation.

Adenosine deaminase and 5'nucleotidase activities in peripheral blood T cells of multiple sclerosis patients

Multiple sclerosis (MS) is one of the most common causes of neurological disability in young and middle-aged adults and is thought to be mediated by autoreactive T cells. Activities of adenosine deaminase (ADA) and 5'(nucleotidase (5'NT), which are involved in the differentiation and maturation of the lymphoid system, were measured in peripheral blood T cells from 21 MS patients and in 23 age and sex matched healthy controls to determine whether an association existed between these enzyme abnormalities and cellular immune functions. ADA and 5'NT activities were found significantly decreased in MS patients (P < .001 and P < .01 respectively) when compared with controls. Low levels of ADA and 5'NT activities were found irrespective of whether patients had relapsing-remitting or chronic progressive MS. These findings suggest that low levels of these enzyme activities in T cells may be related to the persistent abnormalities in T cell function in the clinical course of MS.

A2A adenosine receptor induction inhibits IFN-gamma production in murine CD4+ T cells

Incubation of purified C57BL/6 murine CD4(+) T lymphocytes with anti-CD3 mAb serves as a model of TCR-mediated activation and results in increased IFN-gamma production and cell surface expression of CD25 and CD69. We demonstrate here that signaling through the TCR causes a rapid (4-h) 5-fold increase in A(2A) adenosine receptor (AR) mRNA, which is correlated with a significant increase in the efficacy of A(2A)AR-mediated cAMP accumulation in these cells. A(2A)AR activation reduces TCR-mediated production of IFN-gamma by 98% with a potency order of 4-{3-[6-amino-9-(5-ethylcarbamoyl-3,4-dihydroxytetrahydrofuran-2-yl)-9H-purin-2-yl]prop-2-ynyl}cyclohexanecarboxylic acid methyl ester (ATL146e; EC(50) = 0.19 +/- 0.03 nM) > 4-{3-[6-amino-9-(5-cyclopropyl-carbamoyl-3,4-dihydroxytetrahydrofuran-2-yl)-9H-purin-2-yl]prop-2-ynyl}piperidine-1-carboxylic acid methyl ester (ATL313; 0.43 +/- 0.06 nM) > 5'-N-ethylcarboxamidoadenosine (3.5 +/- 0.77 nM) > 2-[4-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamidoadenosine (CGS21680; 7.2 +/- 1.4 nM) >> N(6)-cyclohexyladenosine (110 +/- 33 nM) > 2-chloro-N(6)-(3-iodobenzyl)-5'-N-methylcarboxamide (390 +/- 160 nM), similar to the potency order to compete for radioligand binding to the recombinant murine A(2A)AR but not the A(3)AR. The selective A(2A)AR antagonist, 4-(2-[7-amino-2-[2-furyl][1,2,4]triazolo[2,3-a][1,3,5]triazin-5-yl-amino]ethyl)phenol (ZM241385), inhibits the effect of ATL146e with a pA(2) of 0.34 nM and also inhibits the effects of N(6)-cyclohexyl-adenosine and 2-chloro-N(6)-(3-iodobenzyl)-5'-N-methylcarboxamide. In CD4(+) T cells derived from A(2A)AR(-/-) and A(2A)AR(+/-) mice, the IFN-gamma release response to ATL146e is reduced by 100 and 50%, respectively, indicative of a gene dose effect. The response of T cells to the phosphodiesterase inhibitor, 4-(3'-cyclopentyloxy-4'-methoxyphenyl)-2-pyrrolidone (rolipram), is not affected by A(2A)AR deletion. We conclude that the rapid induction of the A(2A)AR mRNA in T cells provides a mechanism for limiting T cell activation and secondary macrophage activation in inflamed tissues.

New insights into adenosine-receptor-mediated immunosuppression and the role of adenosine in causing the immunodeficiency associated with adenosine deaminase deficiency

There is growing interest in manipulating adenosine (Ado) signal transduction to control inflammation and autoimmunity. This concept probably originated with the discovery of severe combined immunodeficiency disease (SCID) in infants with inherited deficiency of adenosine deaminase (ADA). However, the basis for immunosuppression by Ado has not been well defined, and effects of 2'-deoxyadenosine (dAdo), which does not activate Ado receptors, have also been implicated in causing SCID. Here I discuss recent evidence that Ado, acting through its A2A receptor, interferes with NF-kappa B activation in antigen-receptor-stimulated B and T lymphocytes. I also assess the relative contributions of Ado and dAdo to the pathogenesis of ADA-deficient SCID.

Adenosine and cAMP are potent inhibitors of the NF-kappa B pathway downstream of immunoreceptors

Anergic B lymphocytes exert compromised signal transduction towards the activation of NF-kappa B in response to B cell antigen receptor (BCR) triggering, whereas activation of the ERK pathway appears normal. How this differential down-regulation of the NF-kappa B pathway is regulated remains still elusive. Here, we demonstrate that stimuli known to enhance 3',5'-cyclic adenosine monophosphate (cAMP) are capable of selectively suppressing the activation both of NF-kappa B downstream of the BCR and Toll-like receptor 4 in splenic B lymphocytes and of the high-affinity receptor for IgE in BM-derived mast cells. This suppression is accomplished by blocking phosphorylation and subsequent degradation of the inhibitor of NF-kappa B. A cAMP-dependent protein kinase (PKA) inhibitor reverses this suppressive effect, indicating that PKA is a downstream effector of cAMP in this process. Importantly, not only drugs that artificially elevate intracellular cAMP levels, but also the nucleoside adenosine, which is known to be a mediator of cellular distress, inhibit the NF-kappa B pathway. This suggests that adenosine-mediated signals represent an important step in the molecular decision process controlling inflammation versus anergic immune responses.

Adenosine Deaminase Deficiency: Metabolic Basis of Immune Deficiency and Pulmonary Inflammation

Genetic deficiencies in the purine catabolic enzyme adenosine deaminase (ADA) in humans results primarily in a severe lymphopenia and immunodeficiency that can lead to the death of affected individuals early in life. The metabolic basis of the immunodeficiency is likely related to the sensitivity of lymphocytes to the accumulation of the ADA substrates adenosine and 2'-deoxyadenosine. Investigations using ADA-deficient mice have provided compelling evidence to support the hypothesis that T and B cells are sensitive to increased concentrations of 2'-deoxyadenosine that kill cells through mechanisms that involve the accumulation of dATP and the induction of apoptosis. In addition to effects on the developing immune system, ADA-deficient humans exhibit phenotypes in other physiological systems including the renal, neural, skeletal, and pulmonary systems. ADA-deficient mice develop similar abnormalities that are dependent on the accumulation of adenosine and 2'-deoxyadenosine. Detailed analysis of the pulmonary insufficiency seen in ADA-deficient mice suggests that the accumulation of adenosine in the lung can directly access cellular signaling pathways that lead to the development and exacerbation of chronic lung disease. The ability of adenosine to regulate aspects of chronic lung disease is likely mediated by specific interactions with adenosine receptor subtypes on key regulatory cells. Thus, the examination of ADA deficiency has identified the importance of purinergic signaling during lymphoid development and in the regulation of aspects of chronic lung disease.

Kinetic study of adenosine concentrations and the expression of adenosine deaminase in mononuclear cells during hemodialysis

BACKGROUND

We previously showed that high intralymphocytic adenosine (Ado) concentrations are found in hemodialyzed patients due to the reduced activity of mononuclear cell adenosine deaminase (MCADA). These abnormalities contribute to the immune defect observed in HD patients. The kinetics of these abnormalities and the causes of the low MCADA activity, however, have not been investigated. Here, we addressed this question. Since interferon gamma (IFNgamma) partially modulates MCADA, we also evaluated the effect of IFNgamma on MCADA activity in vitro.

METHODS AND RESULTS

The study included 12 patients (eight men and four women) who were observed from the first to the 36th hemodialysis (HD) session, and eight healthy subjects (controls). MCADA activity and Ado concentrations were normal before the first HD session. Ado concentrations progressively increased from the first (10.5 +/- 3.1 pmol/10(7) cells) to the fourth session (26.7 +/- 3 pmol/10(7) cells), before stabilizing at a high level. MCADA activity increased transiently until the second session (2.2 +/- 0.5 IU/10(7) cells before HD vs. 2.8 +/- 0.6 IU/10(7)cells), and then decreased and stabilized at a low level (1.0 +/- 0.5 IU/10(7)cells). The amount of MCADA mRNA transiently increased until the third session (mRNA MCADA/mRNA beta-actin: 0.6 +/- 0.2 vs. 0.8 +/- 0.2), and then decreased to 0.3 +/- 0.1 at the 36th session. MCADA activity underwent a dose-dependent increase after IFNgamma stimulation.

CONCLUSION

HD affects the transcription of the gene encoding MCADA after just three HD sessions, leading to decreased MCADA activity and increased plasma concentration of Ado.

PhysiologicalControl ofImmuneResponse andInflammatoryTissueDamage byHypoxia-InducibleFactors andAdenosineA2AReceptors

Immune cell-mediated destruction of pathogens may result in excessive collateral damage to normal tissues, and the failure to control activated immune cells may cause immunopathologies. The search for physiological mechanisms that downregulate activated immune cells has revealed a critical role for extracellular adenosine and for immunosuppressive A2A adenosine receptors in protecting tissue from inflammatory damage. Tissue damage-associated deep hypoxia, hypoxia-inducible factors, and hypoxia-induced accumulation of adenosine may represent one of the most fundamental and immediate tissue-protecting mechanisms, with adenosine A2A receptors triggering "OFF" signals in activated immune cells. In these regulatory mechanisms, oxygen deprivation and extracellular adenosine accumulation serve as "reporters," while A2A adenosine receptors serve as "sensors" of excessive tissue damage. The A2A receptor-triggered generation of intracellular cAMP then inhibits activated immune cells in a delayed negative feedback manner to prevent additional tissue damage. Targeting A2A adenosine receptors may have important clinical applications.

Insulin induces expression of adenosine kinase gene in rat lymphocytes by signaling through the mitogen-activated protein kinase pathway

The activity of adenosine kinase (AK) was significantly impaired in splenocytes isolated from diabetic rats. Administration of insulin to diabetic animals restored AK activity, protein, and mRNA levels in diabetic splenocytes. Experiments performed on cultured rat lymphocytes demonstrated that insulin did not change the stability of AK mRNA. Insulin induced AK gene expression in a dose- and time-dependent manner. Maximal increases in AK mRNA (3.9-fold) and activity level (3.7-fold) were observed at the fourth and fifth hours of cell incubation with 10 nM insulin, respectively. The insulin effect on AK expression was not influenced by dibutyryl cAMP (dcAMP). On the other hand dcAMP weakly increased (1.7-fold) basal expression of AK. Exposure of rat lymphocytes to wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3K), or rapamycin, an inhibitor of mTOR, did not affect the ability of insulin to stimulate expression of AK. Prior treatment of the cells with 10 microM PD98059, an inhibitor of mitogen-activated protein kinase (MAPK) kinase (MEK) completely blocked insulin-stimulated expression of AK gene. Insulin produced a significant transient increase in the tyrosine phosphorylation of ERK1/2, and PD98059 inhibited this phosphorylation. Furthermore exposure of cells to insulin has resulted in transient phosphorylation of Elk-1 on Ser-383 and sustained elevation of c-Jun and c-Fos protein. The maximal phosphorylation of Elk-1 was observed at 15 min, and was blocked by PD98059. We concluded that insulin stimulates AK gene expression through a series of events occurring sequentially. This includes activation of the MAPK cascade and subsequent phosphorylation of Elk-1 followed by increased expression of c-fos and c-jun genes.

Chondrocytes respond to adenosine via A(2)receptors and activity is potentiated by an adenosine deaminase inhibitor and a phosphodiesterase inhibitor

OBJECTIVE

To test the mechanisms by which adenosine and adenosine analogues stimulate adenylate cyclase and suppress lipopolysaccharide (LPS)-induced production of nitric oxide (NO) by chondrocytes.

METHODS

Primary chondrocytes isolated from equine articular cartilage were plated in monolayer. Intracellular cyclic-AMP (cAMP) accumulation was measured following exposure to medium containing adenosine, the non-hydrolyzable adenosine analogue N(6)-methyladenosine, the A(2A)specific agonist N(6)-(dimethoxyphenyl)-ethyl]adenosine (DPMA), the adenosine deaminase inhibitor erythro-9-(2-Hydroxy-3-nonyl)adenine hydrochloride (EHNA), or forskolin, a potent stimulator of adenylate cyclase. Regulation of NO production by LPS-stimulated chondrocytes, as determined by nitrite concentration, was assessed in the presence of adenosine, N(6)-methyladenosine, DPMA, the broad agonist 5'-N-ethylcarboxamidoadenosine (NECA), or forskolin. Alternatively, LPS-stimulated chondrocytes were exposed to EHNA or the phosphodiesterase inhibitor rolipram in the presence or absence of supplemental adenosine.

RESULTS

Adenosine, N(6)-methyladenosine, DPMA, and forskolin each increased intracellular cAMP accumulation in a concentration-dependent manner and suppressed NO production by LPS-stimulated chondrocytes. NECA also decreased NO production by chondrocytes stimulated with LPS. Incubation with EHNA, to protect endogenously produced adenosine, or rolipram, which prevents the degradation of cAMP, similarly suppressed LPS-stimulated NO production. The addition of exogenous adenosine with EHNA or rolipram further suppressed NO production.

CONCLUSIONS

This study documents functional responses to adenosine by articular chondrocytes. These responses are mimicked by the A(2A)receptor agonist, DPMA. Effects were enhanced by protecting adenosine using an adenosine deaminase inhibitor or by potentiating the cAMP response with rolipram. These experiments suggest that adenosine may play a physiological role in regulation of chondrocytes and that adenosine pathways could represent a novel target for therapeutic intervention.

Differential regulation of two alternatively spliced isoforms of hypoxia-inducible factor-1 alpha in activated T lymphocytes

Cell adaptation to hypoxia is partially accomplished by hypoxia-inducible transcription factor-1 (HIF-1). Here we report the hypoxia-independent up-regulation of HIF-1 alpha subunit in antigen receptor-activated T cells. This is explained by a selective up-regulation of alternatively spliced mRNA isoform I.1 that encodes the HIF-1 alpha protein without the first 12 N-terminal amino acids. We show that both short (I.1) and long (I.2) HIF-1 alpha isoforms display similar DNA binding and transcriptional activities. Major differences were observed between these two HIF-1 alpha isoforms in their expression patterns with respect to the resting and activated T lymphocytes in hypoxic and normoxic conditions. The T cell antigen receptor (TCR)-triggered activation of normal ex vivo T cells and differentiated T cells results in up-regulation of expression of I.1 isoform of HIF-1 alpha mRNA without an effect on constitutive I.2 HIF-1 alpha mRNA expression. The accumulation of I.1 HIF-1 alpha mRNA isoform in T lymphocytes is also demonstrated during cytokine-mediated inflammation in vivo, suggesting a physiological role of short HIF-1 alpha isoform in activated lymphocytes. The TCR-triggered, protein kinase C and Ca(2+)/calcineurin-mediated HIF-1 alpha I.1 mRNA induction is protein synthesis-independent, suggesting that the HIF-1 alpha I.1 gene is expressed as an immediate early response gene. Therefore, these data predict a different physiological role of short and long isoforms of HIF-1 alpha in resting and activated cells.

Differential effects of physiologically relevant hypoxic conditions on T lymphocyte development and effector functions

Direct measurements revealed low oxygen tensions (0.5-4.5% oxygen) in murine lymphoid organs in vivo. To test whether adaptation to changes in oxygen tension may have an effect on lymphocyte functions, T cell differentiation and functions at varying oxygen tensions were studied. These studies show: 1) differentiated CTL deliver Fas ligand- and perforin-dependent lethal hit equally well at all redox conditions; 2) CTL development is delayed at 2.5% oxygen as compared with 20% oxygen. Remarkably, development of CTL at 2.5% oxygen is more sustained and the CTL much more lytic; and 3) hypoxic exposure and TCR-mediated activation are additive in enhancing levels of hypoxia response element-containing gene products in lymphocyte supernatants. In contrast, hypoxia inhibited the accumulation of nonhypoxia response element-containing gene products (e.g., IL-2 and IFN-gamma) in the same cultures. This suggests that T cell activation in hypoxic conditions in vivo may lead to different patterns of lymphokine secretion and accumulation of cytokines (e.g., vascular endothelial growth factor) affecting endothelial cells and vascular permeabilization. Thus, although higher numbers of cells survive and are activated during 20% oxygen incubation in vitro, the CTL which develop at 2.5% oxygen are more lytic with higher levels of activation markers. It is concluded that the ambient 20% oxygen tension (plus 2-ME) is remarkably well suited for immunologic specificity and cytotoxicity studies, but oxygen dependence should be taken into account during the design and interpretation of results of in vitro T cell development assays and gene expression studies in vivo.

High adenosine and deoxyadenosine concentrations in mononuclear cells of hemodialyzed patients

Infections are one of the most important complications of hemodialysis (HD). The high concentrations of adenosine (Ado) and of its metabolites during HD may contribute to the dialysis-induced immune deficiency through their known ability to alter lymphocyte function. The influence of HD on Ado metabolism was assessed in mononuclear cells through the measurement of (1) the concentrations of nucleosides in mononuclear cells and (2) the activities of mononuclear cell Ado deaminase (MCADA) and Ado kinase, two enzymes involved in Ado concentration regulation. Nine end-stage renal failure hemodialyzed patients (five men and four women; mean age, 69 +/- 10 yr) and eight healthy volunteers (four men and four women; mean age, 53 +/- 19 yr) were included in the study. Before HD, Ado, deoxyadenosine, and inosine concentrations were respectively 2.9-, 2.5-, and 2.5-fold higher in mononuclear cells of patients than in healthy volunteers. During HD, Ado concentration decreased by 34%, whereas inosine concentration increased by 27%. Before HD, MCADA activity level was 2.1-fold lower in patients than in control subjects. After HD, MCADA activity increased by nearly 50% but remained lower than in control subjects. Ado kinase activity level of patients did not differ from that of control subjects and was unchanged by HD. The influence of Ado on in vitro mononuclear cell proliferation and interferon-gamma production also was evaluated. Ado inhibited cell proliferation and interferon-gamma production in a dose-dependent manner, and these inhibitions were stronger for patients than for healthy volunteers. The high concentrations of Ado and deoxyadenosine in mononuclear cells and the low MCADA activity level likely are involved in the immune defect of patients who are undergoing HD.

Adenosine deaminase deficiency increases thymic apoptosis and causes defective T cell receptor signaling

Adenosine deaminase (ADA) deficiency in humans results in a severe combined immunodeficiency (SCID). This immunodeficiency is associated with severe disturbances in purine metabolism that are thought to mediate lymphotoxicity. The recent generation of ADA-deficient (ADA(-/-)) mice has enabled the in vivo examination of mechanisms that may underlie the SCID resulting from ADA deficiency. We demonstrate severe depletion of T and B lymphocytes and defects in T and B cell development in ADA(-/-) mice. T cell apoptosis was abundant in thymi of ADA(-/-) mice, but no increase in apoptosis was detected in the spleen and lymph nodes of these animals, suggesting that the defect is specific to developing thymocytes. Studies of mature T cells recovered from spleens of ADA(-/-) mice revealed that ADA deficiency is accompanied by TCR activation defects of T cells in vivo. Furthermore, ex vivo experiments on ADA(-/-) T cells demonstrated that elevated adenosine is responsible for this abnormal TCR signaling. These findings suggest that the metabolic disturbances seen in ADA(-/-) mice affect various signaling pathways that regulate thymocyte survival and function. Experiments with thymocytes ex vivo confirmed that ADA deficiency reduces tyrosine phosphorylation of TCR-associated signaling molecules and blocks TCR-triggered calcium increases.

Gene dose effect reveals no Gs-coupled A2A adenosine receptor reserve in murine T-lymphocytes: studies of cells from A2A-receptor-gene-deficient mice

Agonist binding to extracellular A2A adenosine receptors (A2ARs) inhibits the activation of virtually all tested functions of T-cells and can induce apoptosis in thymocytes. The evaluation of levels of expression of these immunosuppressive receptors is expected to clarify whether the absence of spare A2ARs (no 'receptor reserve') might be one of the mechanisms of attenuation of the effects of extracellular adenosine on T-cells. A2A transcript is found in T-cells and functional receptors can be demonstrated, but the density of receptor on T-cells is too low to be detected by radioligand binding. Studies of direct radioligand binding to murine brain with the selective A2AR agonist [3H]CGS21680 (2-(4-[(2-carboxyethyl)-phenyl]ethylamino)-5'-N-ethylcarboxamidoadenosine) established that striata levels of A2AR are virtually absent from A2A knock-out mice. Mice that are heterozygous (A2AR+/-) for the A2AR express significantly decreased levels of A2AR. To test for the presence of spare receptors in T-cells we took advantage of this gene dose effect and examined whether the decrease in the number of receptors in thymocytes from A2AR+/- mice was proportionately reflected in a decrease in the functional cAMP response of T-cells to adenosine. cAMP accumulation and apoptosis induced by adenosine and by A2AR agonist are of a lower magnitude in T-cells from A2AR+/- heterozygous mice than in T-cells from A2AR+/+ littermate control mice. These results indicate that there is no A2AR reserve in murine T-cells. Strongly decreased adenosine-triggered cAMP increases were detected in thymocytes from A2AR-/- mice, suggesting that A2B adenosine receptors cannot fully compensate for the loss of A2ARs in murine T-cells. We conclude that the number of A2ARs is the limiting factor in determining the maximal cAMP response of T-lymphocytes to extracellular adenosine, thereby minimizing the immunosuppressive effects of extracellular adenosine.

Study of A(2A) adenosine receptor gene deficient mice reveals that adenosine analogue CGS 21680 possesses no A(2A) receptor-unrelated lymphotoxicity

Cell surface A(2A) adenosine receptor (A(2A)R) mediated signalling affects a variety of important processes and adenosine analogues possess promising pharmacological properties. Demonstrating the receptor specificity of potentially lymphotoxic adenosine-based drugs facilitates their development for clinical applications. To distinguish between the receptor-dependent and -independent lymphotoxicity and apoptotic activity of adenosine and its analogues we used lymphocytes from A(2A)R-deficient mice. Comparison of A(2A)R-expressing (+/+) and A(2A)R-deficient (-/-) cells in cyclic AMP accumulation assays confirmed that the A(2A)R agonist CGS 21680 is indeed selective for A(2A) receptors in T-lymphocytes. Incubation of A(2A)R-expressing thymocytes with extracellular adenosine or CGS 21680 in vitro results in the death of about 7-15% of thymocytes. In contrast, no death was induced in parallel assays in cells from A(2A)R-deficient mice, providing genetic evidence that CGS 21680 does not display adenosine receptor-independent intracellular cytotoxicity. The A(2A) receptor-specific lymphotoxicity of CGS 21680 is also demonstrated in a long-term (6-day) in vitro model of thymocyte positive selection where addition of A(2A)R antagonist ZM 241,385 did block the effects of CGS 21680, allowing the survival of T cells. The use of cells from adenosine receptor-deficient animals is proposed as a part of the screening process for potential adenosine-based drugs for their receptor-independent cytotoxicity and lymphotoxicity.

Tumor-promoting functions of adenosine

Tumor growth is a multifactorial process that, in addition to mutations leading to dysregulated expression of oncogenes and tumor suppressive genes, requires specific conditions that provide a supportive physiological environment at the primary and metastatic sites of the disease. Adenosine is one of the factors potentially contributing to tumor growth that thus far has not received adequate attention, despite evidence for a broad range of cytoprotective, growth-promoting, and immunosuppressive activities. Adenosine accumulates in solid tumors at high concentrations, and has been shown to stimulate tumor growth and angiogenesis and to inhibit cytokine synthesis, adhesion of immune cells to the endothelial wall, and the function of T-cells, macrophages, and natural killer cells. However, the mechanisms whereby adenosine accumulates in cancer and the specific effects that result from this accumulation are not well understood. This article surveys the available evidence that supports an important role of adenosine in cancer.

A2A receptor dependent and A2A receptor independent effects of extracellular adenosine on murine thymocytes in conditions of adenosine deaminase deficiency

Adenosine deaminase (ADA) deficiency causes severe combined immunodeficiency (SCID) and is accompanied by T-cell depletion and accumulation of both intracellular and extracellular adenosine (extAdo) and deoxyadenosine. To better understand the causes of T-cell depletion in vivo and to discriminate between extracellular and intracellular effects of exogenously added adenosine in vitro, we investigated mechanisms of 2 different effects of adenosine on murine thymocytes. These effects of adenosine include direct induction of apoptosis in about 6% to 15% thymocytes and inhibition of T-cell receptor (TCR)-induced activation of the majority of thymocytes with inhibited ADA. A2A adenosine receptors, but not A2B, A1, or A3 receptors, are shown to be mostly responsible for extAdo-triggered signaling (cyclic adenosine monophosphate [cAMP] accumulation) in murine thymocytes and this prompted studies of the effects of extAdo on thymocytes from A2AR gene-deficient mice. It is found that direct apoptotic effects of extAdo on CD4+CD8+ double positive (DP) thymocytes are completely accounted for by signaling through A2AR, with no contribution of intracellular lymphotoxicity or of compensating A2BRs because only A2AR +/+, but not A2AR −/− thymocytes were susceptible to apoptotic effects of extAdo. Studies of the effects of cAMP-raising agents support observations of extAdo/A2AR/cAMP–triggered apoptosis in DP thymocytes. Unexpectedly, the extAdo strongly inhibited TCR-triggered activation of both A2AR +/+ and A2AR −/− thymocytes in the presence of ADA inhibitors. This was confirmed with thymocytes from ADA gene-deficient mice, suggesting the existence of A2AR-independent effects of extAdo on thymocytes. The presented data raises questions about the identity and functional role of A2AR-expressing thymocytes in T-cell differentiation and of the role of TCR-antagonizing effects of extAdo in conditions of ADA SCID.

A2A receptor dependent and A2A receptor independent effects of extracellular adenosine on murine thymocytes in conditions of adenosine deaminase deficiency

Adenosine deaminase (ADA) deficiency causes severe combined immunodeficiency (SCID) and is accompanied by T-cell depletion and accumulation of both intracellular and extracellular adenosine (extAdo) and deoxyadenosine. To better understand the causes of T-cell depletion in vivo and to discriminate between extracellular and intracellular effects of exogenously added adenosine in vitro, we investigated mechanisms of 2 different effects of adenosine on murine thymocytes. These effects of adenosine include direct induction of apoptosis in about 6% to 15% thymocytes and inhibition of T-cell receptor (TCR)-induced activation of the majority of thymocytes with inhibited ADA. A2A adenosine receptors, but not A2B, A1, or A3 receptors, are shown to be mostly responsible for extAdo-triggered signaling (cyclic adenosine monophosphate [cAMP] accumulation) in murine thymocytes and this prompted studies of the effects of extAdo on thymocytes from A2AR gene-deficient mice. It is found that direct apoptotic effects of extAdo on CD4+CD8+ double positive (DP) thymocytes are completely accounted for by signaling through A2AR, with no contribution of intracellular lymphotoxicity or of compensating A2BRs because only A2AR +/+, but not A2AR −/− thymocytes were susceptible to apoptotic effects of extAdo. Studies of the effects of cAMP-raising agents support observations of extAdo/A2AR/cAMP–triggered apoptosis in DP thymocytes. Unexpectedly, the extAdo strongly inhibited TCR-triggered activation of both A2AR +/+ and A2AR −/− thymocytes in the presence of ADA inhibitors. This was confirmed with thymocytes from ADA gene-deficient mice, suggesting the existence of A2AR-independent effects of extAdo on thymocytes. The presented data raises questions about the identity and functional role of A2AR-expressing thymocytes in T-cell differentiation and of the role of TCR-antagonizing effects of extAdo in conditions of ADA SCID.