Sepsis expands a CD39+ plasmablast population that promotes immunosuppression via adenosine-mediated inhibition of macrophage antimicrobial activity

Sepsis results in elevated adenosine in circulation. Extracellular adenosine triggers immunosuppressive signaling via the A2a receptor (A2aR). Sepsis survivors develop persistent immunosuppression with increased risk of recurrent infections. We utilized the cecal ligation and puncture (CLP) model of sepsis and subsequent infection to assess the role of adenosine in post-sepsis immune suppression. A2aR-deficient mice showed improved resistance to post-sepsis infections. Sepsis expanded a subset of CD39^hi B cells and elevated extracellular adenosine, which was absent in mice lacking CD39-expressing B cells. Sepsis-surviving B cell-deficient mice were more resistant to secondary infections. Mechanistically, metabolic reprogramming of septic B cells increased production of ATP, which was converted into adenosine by CD39 on plasmablasts. Adenosine signaling via A2aR impaired macrophage bactericidal activity and enhanced interleukin-10 production. Septic individuals exhibited expanded CD39^hi plasmablasts and adenosine accumulation. Our study reveals CD39^hi plasmablasts and adenosine as important drivers of sepsis-induced immunosuppression with relevance in human disease.

The abnormal function of CD39+ regulatory T cells could be corrected by high-dose dexamethasone in patients with primary immune thrombocytopenia

Primary immune thrombocytopenia is an autoimmune disease, characterized with decreased platelet and increased risk of bleeding. Recent studies have shown the reduction and dysfunction of regulatory T (Treg) cells in ITP patients. CD39 is highly expressed on the surface of Treg cells. It degrades ATP to AMP and CD73 dephosphorylates AMP into adenosine. Then adenosine binds with adenosine receptor and suppresses immune response by activating Treg cells and inhibiting the release of inflammatory cytokines from effector T (Teff) cells. Adenosine receptor has several subtypes and adenosine A2A receptor (A2AR) plays a crucial role especially within lymphocytes. The CD39⁺ Treg cells and the expression of A2AR showed abnormality in some autoimmune disease. But knowledge of CD39⁺ Treg cells and A2AR which are crucial in the adenosine immunosuppressive pathway is still limited in ITP. Thirty-one adult patients with newly diagnosed ITP were enrolled in this study. CD39 and A2AR expression was measured by flow cytometry and RT-PCR. The function of CD39 was reflected by the change of ATP concentration detected by CellTiter-Glo Luminescent Cell Viability Assay. CD39 expression within CD4⁺CD25⁺ Treg cells in ITP patients was decreased compared to normal controls. After high-dose dexamethasone therapy, response (R) group showed increased CD39 expression within Treg cells while non-response (NR) group did not show any difference in contrast to those before treatment. The expression of A2AR in CD4⁺CD25^- Teff and CD4⁺CD25⁺ Treg cells was both lower in ITP patients than that of normal controls. After therapy, CD4⁺CD25^- Teff cells had higher A2AR expression while CD4⁺CD25⁺ Treg cells did not show any difference in comparison to that before treatment. The enzymatic activity of CD39 was damaged in ITP patients and improved after high-dose dexamethasone therapy. In ITP, there was not only numerical decrease but also impaired enzymatic activity in CD39⁺ Treg cells. After high-dose dexamethasone treatment, these two defects could be reversed. Our results also suggested that ITP patients had reduced A2AR expression in both CD4⁺CD25⁺ Treg cells and CD4⁺CD25^- Teff cells. CD4⁺CD25^- Teff cells had increased A2AR expression after treatment.

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

Neutrophil extracellular traps (NETs) are implicated in autoimmune, thrombotic, malignant, and inflammatory diseases; however, little is known of their endogenous regulation under basal conditions. Inflammatory effects of neutrophils are modulated by extracellular purines such as adenosine (ADO) that is inhibitory or ATP that generally up-regulates effector functions. In order to evaluate the effects of ADO on NETs, human neutrophils were isolated from peripheral venous blood from healthy donors and stimulated to make NETs. Treatment with ADO inhibited NET production as quantified by 2 methods: SYTOX green fluorescence and human neutrophil elastase (HNE)-DNA ELISA assay. Specific ADO receptor agonist and antagonist were tested for their effects on NET production. The ADO _2A receptor (A_2A R) agonist CSG21680 inhibited NETs to a similar degree as ADO, whereas the A_2A R antagonist ZM241385 prevented ADO's NET-inhibitory effects. Additionally, CD73 is a membrane bound ectonucleotidase expressed on mesenchymal stromal cells (MSCs) that allows manipulation of extracellular purines in tissues such as bone marrow. The effects of MSCs on NET formation were evaluated in coculture. MSCs reduced NET formation in a CD73-dependent manner. These results imply that extracellular purine balance may locally regulate NETosis and may be actively modulated by stromal cells to maintain tissue homeostasis.

Ability of γδ T cells to modulate the Foxp3 T cell response is dependent on adenosine

Whether γδ T cells inhibit or enhance the Foxp3 T cell response depends upon their activation status. The critical enhancing effector in the supernatant is adenosine. Activated γδ T cells express adenosine receptors at high levels, which enables them to deprive Foxp3⁺ T cells of adenosine, and to inhibit their expansion. Meanwhile, cell-free supernatants of γδ T cell cultures enhance Foxp3 T cell expansion. Thus, inhibition and enhancement by γδ T cells of Foxp3 T cell response are a reflection of the balance between adenosine production and absorption by γδ T cells. Non-activated γδ T cells produce adenosine but bind little, and thus enhance the Foxp3 T cell response. Activated γδ T cells express high density of adenosine receptors and have a greatly increased ability to bind adenosine. Extracellular adenosine metabolism and expression of adenosine receptor A2ARs by γδ T cells played a major role in the outcome of γδ and Foxp3 T cell interactions. A better understanding of the functional conversion of γδ T cells could lead to γδ T cell-targeted immunotherapies for related diseases.

The adenosinergic system: a potential player in the pathogenesis of ANCA-associated vasculitis?

Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is a potentially lethal autoimmune disease whose pathology comprises disturbed T cell differentiation and functionality accompanied by dysfunctional autoreactive immunoglobulin development, culminating in destructive innate immune response as well. Purines, adenine nucleotides and adenosine in particular, have been elucidated as potent extracellular mediators for fine adjustment of these pivotal processes establishing human immunity. Therefore, the extracellular purinergic microenvironment is under control of ectonucleotidases CD39 and CD73 degrading pro-inflammatory adenosine triphosphate (ATP) to anti-inflammatory adenosine as well as adenosine deaminase bound to CD26 deactivating adenosine. Accordingly, the ATP P2X7 receptor was elicited to be responsible for promotion of inflammation, while predominantly the adenosine A2A receptor demonstrated the opposite. Recent reports pointed at the adenosinergic system to be crucially involved in AAV pathogenesis. Here, experimental evidence on ecto-enzymes controlling extracellular adenine nucleotide concentrations and purinergic signaling in the immune system with respect to its contribution to the AAV pathomechanism is reviewed besides unsolved problems being identified that require further investigation in order to develop new treatment strategies for AAV.

Adenosine A2A receptor agonist ameliorates EAE and correlates with Th1 cytokine-induced blood brain barrier dysfunction via suppression of MLCK signaling pathway

INTRODUCTION

Multiple sclerosis (MS) disease activity is associated with blood-brain barrier (BBB) disruption, which is mediated by inflammatory cytokines released by CD4+ lymphocytes. To assess the effects of adenosine A2A receptors on BBB permeability in vitro and in vivo.

METHODS

A2A receptor expression was detected by immunostaining in experimental autoimmune encephalomyelitis (EAE) C57BL/6 mice immunized with myelin oligodendrocyte glycoprotein (MOG)35-55 , and human MS brain. F-actin and the tight junction protein Claudin-5 were assessed in endothelial cells treated with an A2A receptor specific agonist (CGS-21680) after Th1 cytokine stimulation. EAE mice were divided into control and CGS-21680 (50 µg/kg, i.p., daily) groups. Disease scores were recorded daily to evaluate neurological impairment. The effects of A2A receptor on inflammation and demyelination were assessed after euthanasia by immunostaining or histology; BBB permeability was measured by sodium fluoride (Na-F) and FITC-dextran amounts.

RESULTS

Endothelial A2A receptor was detected in demyelination areas of MS brain samples. In EAE lesions, A2A receptor was expressed in the endothelium in association with immune cell infiltration. Treatment with CGS-21680 counteracted the effects of Th1 cytokines on endothelial cells in vitro, preventing the reduction of tight junction protein expression and stress fiber formation. The effects of A2A receptor activation were correlated with MLCK phosphorylation signaling repression. In EAE, A2A receptor agonist decreased BBB permeability and inhibited EAE neurologic deficiency in mice.

CONCLUSIONS

A2A receptor activation at EAE onset helps reduce the effects of Th1 stimulation on BBB permeability, indicating that A2A receptor mediates BBB function in CNS demyelinated disease.

Adenosine receptors differentially regulate type 2 cytokine production by IL-33-activated bone marrow cells, ILC2s, and macrophages

Group 2 innate lymphoid cells (ILC2s) represent a rapid source of type 2 cytokines, such as IL-5 and IL-13, and play an important role in orchestrating type 2 immune response. Adenosine is an endogenous purine nucleoside, a catabolite of ATP that binds and activates ≥1 of 4 transmembrane G protein-coupled cell-surface adenosine receptors (ARs)-A1, A2A, A2B, and A3. Here, we studied the role of ARs in the regulation of cytokine production by ILC2s. We found that A2BARs suppress the production of both IL-5 and IL-13 by ILC2s, whereas A2AARs augment IL-5 production and fail to affect IL-13 release. Combined stimulation of all ARs led to the suppression of both IL-5 and IL-13 production, which indicated that A2BARs dominate A2AARs. Both pre- and post-transcriptional processes may be involved in the AR modulation of ILC2 IL-5 and IL-13 production. Thus, we identify adenosine as a novel negative regulator of ILC2 activation.-Csóka, B., Németh, Z. H., Duerr, C. U., Fritz, J. H., Pacher, P., Haskó, G. Adenosine receptors differentially regulate type 2 cytokine production by IL-33-activated bone marrow cells, ILC2s, and macrophages.

A2AR Adenosine Signaling Suppresses Natural Killer Cell Maturation in the Tumor Microenvironment

Extracellular adenosine is a key immunosuppressive metabolite that restricts activation of cytotoxic lymphocytes and impairs antitumor immune responses. Here, we show that engagement of A2A adenosine receptor (A2AR) acts as a checkpoint that limits the maturation of natural killer (NK) cells. Both global and NK-cell-specific conditional deletion of A2AR enhanced proportions of terminally mature NK cells at homeostasis, following reconstitution, and in the tumor microenvironment. Notably, A2AR-deficient, terminally mature NK cells retained proliferative capacity and exhibited heightened reconstitution in competitive transfer assays. Moreover, targeting A2AR specifically on NK cells also improved tumor control and delayed tumor initiation. Taken together, our results establish A2AR-mediated adenosine signaling as an intrinsic negative regulator of NK-cell maturation and antitumor immune responses. On the basis of these findings, we propose that administering A2AR antagonists concurrently with NK cell-based therapies may heighten therapeutic benefits by augmenting NK cell-mediated antitumor immunity.Significance: Ablating adenosine signaling is found to promote natural killer cell maturation and antitumor immunity and reduce tumor growth. Cancer Res; 78(4); 1003-16. ©2017 AACR.

Targeting the adenosine 2A receptor enhances chimeric antigen receptor T cell efficacy

Chimeric antigen receptor (CAR) T cells have been highly successful in treating hematological malignancies, including acute and chronic lymphoblastic leukemia. However, treatment of solid tumors using CAR T cells has been largely unsuccessful to date, partly because of tumor-induced immunosuppressive mechanisms, including adenosine production. Previous studies have shown that adenosine generated by tumor cells potently inhibits endogenous antitumor T cell responses through activation of adenosine 2A receptors (A2ARs). Herein, we have observed that CAR activation resulted in increased A2AR expression and suppression of both murine and human CAR T cells. This was reversible using either A2AR antagonists or genetic targeting of A2AR using shRNA. In 2 syngeneic HER2+ self-antigen tumor models, we found that either genetic or pharmacological targeting of the A2AR profoundly increased CAR T cell efficacy, particularly when combined with PD-1 blockade. Mechanistically, this was associated with increased cytokine production of CD8+ CAR T cells and increased activation of both CD8+ and CD4+ CAR T cells. Given the known clinical relevance of the CD73/adenosine pathway in several solid tumor types, and the initiation of phase I trials for A2AR antagonists in oncology, this approach has high translational potential to enhance CAR T cell efficacy in several cancer types.

Co-inhibition of CD73 and A2AR Adenosine Signaling Improves Anti-tumor Immune Responses

Preclinical studies targeting the adenosinergic pathway have gained much attention for their clinical potential in overcoming tumor-induced immunosuppression. Here, we have identified that co-blockade of the ectonucleotidase that generates adenosine CD73 and the A2A adenosine receptor (A2AR) that mediates adenosine signaling in leuokocytes, by using compound gene-targeted mice or therapeutics that target these molecules, limits tumor initiation, growth, and metastasis. This tumor control requires effector lymphocytes and interferon-γ, while antibodies targeting CD73 promote an optimal therapeutic response in vivo when engaging activating Fc receptors. In a two-way mixed leukocyte reaction using a fully human anti-CD73, we demonstrated that Fc receptor binding augmented the production of proinflammatory cytokines.

Immunosuppressive activities of adenosine in cancer

Multiple immunosuppressive mechanisms impede anti-tumor immunity. Among them, the accumulation of extracellular adenosine is a potent and widespread strategy exploited by tumors to escape immunosurveillance through the activation of purinergic receptors. In the immune system, engagement of A2a and A2b adenosine receptors is a critical regulatory mechanism that protects tissues against excessive immune reactions. In tumors, this pathway is hijacked and hinders anti-tumor immunity, promoting cancer progression. Different groups have highlighted the therapeutic potential of blocking CD73-dependent adenosine-mediated immunosuppression to reinstate anti-tumor immunity. Phase clinical trials evaluating anti-CD73 antibodies and A2a receptor antagonists in cancer patients are currently ongoing. We here review the recent literature on the immunosuppressive effects of extracellular adenosine and discuss the development of adenosine inhibitors.

Hostile, hypoxia-A2-adenosinergic tumor biology as the next barrier to overcome for tumor immunologists

Hypoxia-driven, A2A adenosine receptor (A2AR)-mediated (hypoxia-A2-adenosinergic), T-cell-autonomous immunosuppression was first recognized as critical and nonredundant in protecting normal tissues from inflammatory damage and autoimmunity. However, this immunosuppressive mechanism can be highjacked by bacteria and tumors to provide misguided protection for pathogens and cancerous tissues. Inhibitors of the hypoxia-A2-adenosinergic pathway represent a conceptually novel type of immunologic coadjuvants that could be combined with cancer vaccines, adoptive cell transfer, and/or blockade of negative immunologic regulators to further prolong patient survival and to minimize treatment-related side effects. In support of this approach are preclinical studies and findings that some human cancers are resistant to chemotherapies and immunotherapies due to the tumor-generated extracellular adenosine and A2AR on antitumor T and natural killer (NK) cells. Among the coadjuvants are (i) antagonists of A2AR, (ii) extracellular adenosine-degrading drugs, (iii) inhibitors of adenosine generation by CD39/CD73 ectoenzymes, and (iv) inhibitors of hypoxia-HIF-1α signaling. Combining these coadjuvants with CTLA-4 and/or PD-1 blockade is expected to have additive or even synergistic effects of targeting two different antitumor protective mechanisms. It is expected that even after multicombinatorial blockade of negative immunologic regulators, the antitumor T and NK cells would still be vulnerable to inhibition by hypoxia and A2AR. Yet to be tested is the potential capacity of coadjuvants to minimize the side effects of CTLA-4 and/or PD-1 blockade by decreasing the dose of blocking antibodies or by eliminating the need for dual blockade.

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.

The immunosuppressive role of adenosine A2A receptors in ischemia reperfusion injury and islet transplantation

Activation of adenosine A2A receptors (A2AR) reduces inflammation by generally inhibiting the activation of pro-inflammatory cells, decreasing endothelial adhesion molecule expression and reducing the release of proinflammatory cytokine mediators. Numerous preclinical studies using selective A2AR agonists, antagonists, A2AR knockout as well as chimeric mice have suggested the therapeutic potential of A2AR agonists for the treatment of ischemia reperfusion injury (IRI) and autoimmune diseases. This review summarizes the immunosuppressive actions of A2AR agonists in murine IRI models of liver, kidney, heart, lung and CNS, and gives details on the cellular effects of A2AR activation in neutrophils, macrophages, dendritic cells, natural killer cells, NKT cells, T effector cells and CD4+CD25+FoxP3+ T regulatory cells. This is discussed in the context of cytokine mediators involved in inflammatory cascades. Whilst the role of adenosine receptor agonists in various models of autoimmune disease has been well-documented, very little information is available regarding the role of A2AR activation in type 1 diabetes mellitus (T1DM). An overview of the pathogenesis of T1DM as well as early islet graft rejection in the immediate peri-transplantation period offers insight regarding the use of A2AR agonists as a beneficial intervention in clinical islet transplantation, promoting islet graft survival, minimizing early islet loss and reducing the number of islets required for successful transplantation, thereby increasing the availability of this procedure to a greater number of recipients. In summary, the use of A2AR agonists as a clinical intervention in IRI and as an adjunct to clinical immunesuppressive regimen in islet transplantation is highlighted.

Adenosine promotes alternative macrophage activation via A2A and A2B receptors

Adenosine has been implicated in suppressing the proinflammatory responses of classically activated macrophages induced by Th1 cytokines. Alternative macrophage activation is induced by the Th2 cytokines interleukin (IL)-4 and IL-13; however, the role of adenosine in governing alternative macrophage activation is unknown. We show here that adenosine treatment of IL-4- or IL-13-activated macrophages augments the expression of alternative macrophage markers arginase-1, tissue inhibitor of matrix metalloproteinase-1 (TIMP-1), and macrophage galactose-type C-type lectin-1. The stimulatory effect of adenosine required primarily A(2B) receptors because the nonselective adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA) increased both arginase activity (EC(50)=261.8 nM) and TIMP-1 production (EC(50)=80.67 nM), and both pharmacologic and genetic blockade of A(2B) receptors prevented the effect of NECA. A(2A) receptors also contributed to the adenosine augmentation of IL-4-induced TIMP-1 release, as both adenosine and NECA were less efficacious in augmenting TIMP-1 release by A(2A) receptor-deficient than control macrophages. Of the transcription factors known to drive alternative macrophage activation, CCAAT-enhancer-binding protein β was required, while cAMP response element-binding protein and signal transducer and activator of transcription 6 were dispensable in mediating the effect of adenosine. We propose that adenosine receptor activation suppresses inflammation and promotes tissue restitution, in part, by promoting alternative macrophage activation.

Ectonucleotidases CD39 and CD73 on OvCA cells are potent adenosine-generating enzymes responsible for adenosine receptor 2A-dependent suppression of T cell function and NK cell cytotoxicity

The ectonucleotidases CD39 and CD73 degrade immune stimulatory ATP to adenosine that inhibits T and NK cell responses via the A(2A) adenosine receptor (ADORA2A). This mechanism is used by regulatory T cells (T(reg)) that are associated with increased mortality in OvCA. Immunohistochemical staining of human OvCA tissue specimens revealed further aberrant expression of CD39 in 29/36 OvCA samples, whereas only 1/9 benign ovaries showed weak stromal CD39 expression. CD73 could be detected on 31/34 OvCA samples. While 8/9 benign ovaries also showed CD73 immunoreactivity, expression levels were lower than in tumour specimens. Infiltration by CD4(+) and CD8(+) T cells was enhanced in tumour specimens and significantly correlated with CD39 and CD73 levels on stromal, but not on tumour cells. In vitro, human OvCA cell lines SK-OV-3 and OaW42 as well as 11/15 ascites-derived primary OvCA cell cultures expressed both functional CD39 and CD73 leading to more efficient depletion of extracellular ATP and enhanced generation of adenosine as compared to activated T(reg). Functional assays using siRNAs against CD39 and CD73 or pharmacological inhibitors of CD39, CD73 and ADORA2A revealed that tumour-derived adenosine inhibits the proliferation of allogeneic human CD4(+) T cells in co-culture with OvCA cells as well as cytotoxic T cell priming and NK cell cytotoxicity against SK-OV3 or OAW42 cells. Thus, both the ectonucleotidases CD39 and CD73 and ADORA2A appear as possible targets for novel treatments in OvCA, which may not only affect the function of T(reg) but also relieve intrinsic immunosuppressive properties of tumour and stromal cells.

Adenosine A(2A) agonist administration improves islet transplant outcome: Evidence for the role of innate immunity in islet graft rejection

Activation of adenosine A(2A) receptors inhibits inflammation in ischemia/reperfusion injury, and protects against cell damage at the injury site. Following transplantation 50% of islets die due to inflammation and apoptosis. This study investigated the effects of adenosine A(2A) receptor agonists (ATL146e and ATL313) on glucose-stimulated insulin secretion (GSIS) in vitro and transplanted murine syngeneic islet function in vivo. Compared to vehicle controls, ATL146e (100 nM) decreased insulin stimulation index [SI, (insulin)(high glucose)/(insulin)(low glucose)] (2.36 +/- 0.22 vs. 3.75 +/- 0.45; n = 9; p < 0.05). Coculture of islets with syngeneic leukocytes reduced SI (1.41 +/- 0.17; p < 0.05), and this was restored by ATL treatment (2.57 +/- 0.18; NS). Addition of a selective A(2A)AR antagonist abrogated ATL's protective effect, reducing SI (1.11 +/- 0.42). ATL treatment of A(2A)AR(+/+) islet/A(2A)AR(-/-) leukocyte cocultures failed to protect islet function (SI), implicating leukocytes as likely targets of A(2A)AR agonists. Diabetic recipient C57BL/6 mice (streptozotocin; 250 mg/kg, IP) received islet transplants to either the renal subcapsular or hepatic-intraportal site. Recipient mice receiving ATL therapy (ATL 146e or ATL313, 60 ng/kg/min, IP) achieved normoglycemia more rapidly than untreated recipients. Histological examination of grafts suggested reduced cellular necrosis, fibrosis, and lymphocyte infiltration in agonist-treated animals. Administration of adenosine A(2A) receptor agonists (ATL146e or ATL313) improves in vitro GSIS by an effect on leukocytes, and improves survival and functional engraftment of transplanted islets by inhibiting inflammatory islet damage in the peritransplant period, suggesting a potentially significant new strategy for reducing inflammatory islet loss in clinical transplantation.

1,3,7-trimethylxanthine (caffeine) may exacerbate acute inflammatory liver injury by weakening the physiological immunosuppressive mechanism

The genetic elimination of A2A adenosine receptors (A2AR) was shown to disengage the critical immunosuppressive mechanism and cause the dramatic exacerbation of acute inflammatory tissue damage by T cells and myeloid cells. This prompted the evaluation of the proinflammatory vs the anti-inflammatory effects of the widely consumed behavioral drug caffeine, as the psychoactive effects of caffeine are mediated largely by its antagonistic action on A2AR in the brain. Because caffeine has other biochemical targets besides A2AR, it was important to test whether the consumption of caffeine during an acute inflammation episode would lead to the exacerbation of immune-mediated tissue damage. We examined acute and chronic treatment with caffeine for its effects on acute liver inflammation. It is shown that caffeine at lower doses (10 and 20 mg/kg) strongly exacerbated acute liver damage and increased levels of proinflammatory cytokines. Because caffeine did not enhance liver damage in A2AR-deficient mice, we suggest that the potentiation of liver inflammation was mediated by interference with the A2AR-mediated tissue-protecting mechanism. In contrast, a high dose of caffeine (100 mg/kg) completely blocked both liver damage and proinflammatory cytokine responses through an A2AR-independent mechanism. Furthermore, caffeine administration exacerbated liver damage even when mice consumed caffeine chronically, although the extent of exacerbation was less than in "naive" mice that did not consume caffeine before. This study suggests an unappreciated "man-made" immunological pathogenesis whereby consumption of the food-, beverage-, and medication-derived adenosine receptor antagonists may modify an individual's inflammatory status and lead to excessive organ damage during acute inflammation.

Pharmacologic characterization of novel adenosine A2A receptor agonists in equine neutrophils

OBJECTIVE

To evaluate anti-inflammatory effects of several novel adenosine receptor agonists and to determine their specificity for various adenosine receptor subtypes on neutrophils, cells heterologously expressing equine adenosine receptors, or equine brain membranes.

SAMPLE POPULATION

Neutrophils isolated from 8 healthy horses.

PROCEDURES

Radioligand binding experiments were performed to compare binding affinities of adenosine receptor agonists to equine adenosine A(1), A(2A), and A(3) receptor subtypes. Effects of these agonists on endotoxin-induced production of reactive oxygen species (ROS) by equine neutrophils and roles of specific adenosine receptor subtypes and cAMP production in mediating these effects were determined.

RESULTS

Radioligand binding experiments yielded a ranked order of affinity for the brain equine A(2A) receptor on the basis of 50% inhibitory concentrations (IC(50)) of the agonists as follows: ATL307 (IC(50) = 1.9nM) and ATL313 > ATL309 and ATL310 > ATL202 > 2-([p-2- carboxyethyl] phenylethylamino)-5'-N-ethylcarboxyamidoadenosine > 5'-N-ethylcarboxamidoadenosine. Furthermore, ATL313 had approximately 100-fold greater selectivity for A(2A) over A(1) and A(3) receptors. In functional assays with equine neutrophils, the compounds inhibited endotoxin-induced ROS production and stimulated production of cAMP with the same ranked order of potency. Results of experiments performed with selective adenosine receptor antagonists indicated that functional effects of ATL313 were via stimulation of A(2A) receptors.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that activation of A(2A) receptors exerted anti-inflammatory effects on equine neutrophils and that stable, highly selective adenosine A(2A) receptor agonists may be developed for use in management of horses and other domestic animals with septic and nonseptic inflammatory diseases.

Effect of adenosine receptor subtypes stimulation on mixed lymphocyte reaction

The cell-to-cell interaction through binding of intercellular adhesion molecule (ICAM)-1 on monocytes to their ligands lymphocyte function-associated antigen (LFA)-1 on T-cells plays important roles in cytokine production and T-cell proliferation. Interleukin (IL)-18, which plasma levels are elevated in patients during acute rejection following organ transplantation, induces the expression of ICAM-1 on monocytes, production of interferon (IFN)-gamma and IL-12 and lymphocyte proliferation during human mixed lymphocyte reaction. Activation of the adenosine A(2A) receptor on during reperfusion of various tissues has been found to markedly reduce ischemia-reperfusion injury. In the present study, we examined the effect of adenosine at increasing concentrations ranging from 0.1 to 100 microM on the IL-18-enhanced expression of ICAM-1, production of IFN-gamma and IL-12 and lymphocyte proliferation during human mixed lymphocyte reaction. Adenosine inhibited the IL-18-initiated immune responses. The IC(50) values of adenosine for inhibition of the IL-18-enhanced ICAM-1 expression, IFN-gamma production and lymphocyte proliferation were 20 microM, respectively. The actions of adenosine depended on the stimulation of adenosine A(2A) receptor. An inhibitor of protein kinase A (PKA) at 100 microM inhibited the actions of adenosine, suggesting that PKA might be involved in the actions of adenosine. On the other hand, the stimulation of adenosine A(1) and A(3) receptor blocked the actions of adenosine A(2A) receptor stimulation. These results suggest that adenosine inhibits the immune responses during mixed lymphocyte reaction via adenosine A(2A) receptor.

Enhanced airway reactivity and inflammation in A2A adenosine receptor-deficient allergic mice

A(2A) adenosine receptor (A(2A)AR) has potent anti-inflammatory properties, which may be important in the regulation of airway reactivity and inflammation. Inflammatory cells that possess A(2A)AR also produce nitrosative stress, which is associated with pathophysiology of asthma, so we hypothesized that A(2A)AR deficiency may lead to increased airway reactivity and inflammation through nitrosative stress. Thus the present study was carried out to investigate the role of A(2A)AR on airway reactivity, inflammation, NF-kappaB signaling, and nitrosative stress in A(2A)AR knockout (KO) and wild-type (WT) mice using our murine model of asthma. Animals were sensitized intraperitoneally on days 1 and 6 with 200 microg of ragweed, followed by aerosolized challenges with 0.5% ragweed on days 11, 12, and 13, twice a day. On day 14, airway reactivity to methacholine was assessed as enhanced pause (Penh) using whole body plethysmography followed by bronchoalveolar lavage (BAL) and lung collection for various analyses. Allergen challenge caused a significant decrease in expression of A(2A)AR in A(2A) WT sensitized mice, with A(2A)AR expression being undetected in A(2A) KO sensitized group leading to decreased lung cAMP levels in both groups. A(2A)AR deletion/downregulation led to an increase in Penh to methacholine and influx of total cells, eosinophils, lymphocytes, and neutrophils in BAL with highest values in A(2A) KO sensitized group. A(2A) KO sensitized group further had increased NF-kappaB expression and nitrosative stress compared with WT sensitized group. These data suggest that A(2A)AR deficiency leads to airway inflammation and airway hyperresponsiveness, possibly via involvement of nitrosative stress in this model of asthma.

Adenosine A2A receptor-mediated cell death of mouse thymocytes involves adenylate cyclase and Bim and is negatively regulated by Nur77

Adenosine is generated in the microenvironment of emerging thymocytes through normal mechanisms of lymphocyte selection. In a normal thymus, most of the adenosine is catabolized by adenosine deaminase; however, in an environment where up to 95% of the cells undergo programmed cell death, a sufficient amount of adenosine is accumulated to trigger cell surface adenosine receptors. Here we show that accumulated adenosine can induce apoptosis in immature mouse thymocytes, mostly via adenosine A(2A) receptors. The signaling pathway is coupled to adenylate cyclase activation, induction of the Nur77 transcription factor, Nur77-dependent genes, such as Fas ligand and TRAIL, and the pro-apoptotic BH3-only protein Bim. We analyzed several knockout and transgenic mouse lines and found that adenosine-induced killing of mouse thymocytes requires Bim, occurs independently of "death receptor" signaling and is inhibited by Bcl-2 and Nur77. Collectively our data demonstrate that adenosine-induced cell death involves signaling pathways originally found in negative selection of thymocytes and suggest a determining role of Bim and a regulatory role for Nur77.

Diabetes-induced decrease of adenosine kinase expression impairs the proliferation potential of diabetic rat T lymphocytes

The proliferative response of T lymphocytes is a crucial step in cell-mediated immunity. This study was undertaken to investigate the mechanisms leading to the impaired proliferative response of diabetic T lymphocytes. T cells that had been isolated from the spleen of normal rats and cultured in medium containing 20 mm glucose and no insulin displayed the same degree of proliferative impairment as cells isolated from diabetic rats. The rate of T-cell proliferation, when induced with concanavalin A or anti-CD3 and anti-CD28 antibodies, was not affected by the inhibition of nucleoside transporters. T cells cultured at high glucose concentrations in the absence of insulin displayed decreased expression of adenosine kinase, and released measurable extracellular quantities of adenosine. Under resting conditions, the level of cAMP was 5.9-fold higher in these cells compared to cells grown in low glucose and in the presence of insulin. Experiments with specific adenosine receptor agonists and antagonists showed that adenosine-induced suppression of diabetic T cell proliferation was mediated by the A2A adenosine receptor, but not by the A2B receptor. Treatment of diabetic T cells with 10 microm H-89, a specific protein kinase A inhibitor, restored T-cell proliferation. These results show that suppressed proliferation of diabetic T lymphocytes is evoked by the decreased expression of adenosine kinase, leading to the outflow of adenosine from the cell. Extracellular adenosine then stimulates the A2A receptor and induces cAMP production, leading to the activation of protein kinase A, and suppression of T-cell proliferation.

Stimulation of A2A-adenosine receptors after myocardial infarction suppresses inflammatory activation and attenuates contractile dysfunction in the remote left ventricle

Following myocardial infarction (MI), contractile dysfunction develops not only in the infarct zone but also in noninfarcted regions of the left ventricle remote from the infarct zone. Inflammatory activation secondary to MI stimulates inducible nitric oxide synthase (iNOS) induction with excess production of nitric oxide. We hypothesized that the anti-inflammatory effects of selective A2A-adenosine receptor (A2AAR) stimulation would suppress inflammation and preserve cardiac function in the remote zone early after MI. A total of 53 mice underwent 60 min of coronary occlusion followed by 24 h of reperfusion. The A2AAR agonist (ATL146e, 2.4 μg/kg) was administered intraperitoneally 1, 3, and 6 h postreperfusion. Because of the 1-h delay in treatment after MI, ATL146e had no effect on infarct size, as demonstrated by contrast-enhanced cardiac MRI ( n = 18) performed 24 h post-MI. ATL146e did however preserve global cardiac function at that time by limiting contractile dysfunction in remote regions [left ventricle wall thickening: 51 ± 4% in treated ( n = 9) vs. 29 ± 3% in nontreated groups ( n = 9), P < 0.01]. RT-PCR, immunohistochemistry, and Western blot analysis indicated that iNOS mRNA and protein expression were significantly reduced by ATL146e treatment in both infarcted and noninfarcted zones. Similarly, elevations in plasma nitrate-nitrite after MI were substantially blunted by ATL146e ( P < 0.01). Finally, treatment with ATL146e reduced NF-κB activation in the myocardium by over 50%, not only in the infarct zone but also in noninfarcted regions ( P < 0.05). In conclusion, A2AAR stimulation after MI suppresses inflammatory activation and preserves cardiac function, suggesting the potential utility of A2AAR agonists against acute heart failure in the immediate post-MI period.

Immunomodulatory impact of the A2A adenosine receptor on the profile of chemokines produced by neutrophils

In LPS-stimulated human neutrophils, engagement of the adenosine A2A receptor selectively prevented the expression and release of TNF-alpha, MIP-1alpha/CCL3, MIP-1beta/CCL4, MIP-2alpha/CXCL2, and MIP-3alpha/CCL20. In mice lacking the A2A receptor, granulocytes that migrated into the air pouch 4 h after LPS injection expressed higher mRNA levels of TNF-alpha, MIP-1alpha, and MIP-1beta than PMNs from wild-type mice. In mononuclear cells present in the air pouch 72 h after LPS injection, expression of IL-1beta, TNF-alpha, IL-6, and MCP-2/CCL6 was higher in A2AR knockout mice. In addition to highlighting neutrophils as an early and pivotal target for mediating adenosine anti-inflammatory activities, these results identify TNF-alpha and the MIP chemokine family as gene products whose expression is pivotally affected by activation of A2AR in LPS-activated PMNs. Modulation by A2AR in the production of inflammatory signals by PMNs may thus influence the evolution of an inflammatory response by reducing the activation status of inflammatory cells.

Potentiation of neutrophil cyclooxygenase-2 by adenosine: an early anti-inflammatory signal

Neutrophils, which are often the first to migrate at inflamed sites, can generate leukotriene B(4) from the 5-lipoxygenase pathway and prostaglandin E(2) through the inducible cyclooxygenase-2 pathway. Adenosine, an endogenous autacoid with several anti-inflammatory properties, blocks the synthesis of leukotriene B(4) while it potentiates the cyclooxygenase-2 pathway in fMLP-treated neutrophils, following activation of the A(2A) receptor. Using the murine air pouch model of inflammation, we observed that inflammatory leukocytes from mice lacking the A(2A) receptor have less cyclooxygenase-2 induction than wild-type animals. In human leukocytes, A(2A) receptor activation specifically elicited potentiation of cyclooxygenase-2 in neutrophils, but not in monocytes. Signal transduction studies indicated that the cAMP, ERK1/2, PI-3K and p38K intracellular pathways are implicated both in the direct upregulation of cyclooxygenase-2 and in its potentiation. Together, these results indicate that neutrophils are particularly important mediators of adenosine's effects. Given the uncontrolled inflammatory phenotype observed in knockout mice and in view of the potent inhibitory actions of prostaglandin E(2) on inflammatory cells, an increased cyclooxygenase-2 expression resulting from A(2A) receptor activation, observed particularly in neutrophils, may take part in an early modulatory mechanism promoting anti-inflammatory activities of adenosine.

Role of adenosine receptors in regulating chemotaxis and cytokine production of plasmacytoid dendritic cells

Plasmacytoid dendritic cells (PDCs) are potent regulators of immune function and the major source of type I interferon (IFN) following viral infection. PDCs are found at sites of inflammation in allergic reactions, autoimmune disorders, and cancer, but the mechanisms leading to the recruitment of PDCs to these sites remain elusive. During inflammation, adenosine is released and functions as a signaling molecule via adenosine receptors. This study analyzes adenosine receptor expression and function in human PDCs. Adenosine was found to be a potent chemotactic stimulus for immature PDCs via an A(1) receptor-mediated mechanism. The migratory response toward adenosine was comparable to that seen with CXCL12 (stromal-derived factor-1 alpha [SDF-1 alpha), the most potent chemotactic stimulus identified thus far for immature PDCs. Upon maturation, PDCs down-regulate the A(1) receptor, resulting in a loss of migratory function. In contrast, mature PDCs up-regulate the A(2a) receptor, which is positively coupled to adenylyl cyclase and has been implicated in the down-regulation of DC cytokine-producing capacity. We show that in mature PDCs adenosine reduces interleukin-6 (IL-6), IL-12, and IFN-alpha production in response to CpG oligodeoxynucleotides (ODN). These findings indicate that adenosine may play a dual role in PDC-mediated immunity by initially recruiting immature PDCs to sites of inflammation and by subsequently limiting the extent of the inflammatory response induced by mature PDCs by inhibiting their cytokine-producing capacity.