Critical role of hypoxia and A2A adenosine receptors in liver tissue-protecting physiological anti-inflammatory pathway

Whole body exposure of wild type control littermates and A2A adenosine receptor (A2AR) gene deleted mice to low oxygen containing inspired gas mixture allowed the investigation of the mechanism that controls inflammatory liver damage and protects the liver using a mouse model of T cell-mediated viral and autoimmune hepatitis. We tested the hypothesis that the inflammatory tissue damage-associated hypoxia and extracellular adenosine --> A2AR signaling plays an important role in the physiological anti-inflammatory mechanism that limits liver damage during fulminant hepatitis. After induction of T cell-mediated hepatitis, mice were kept in modular chambers either under normoxic (21% oxygen) or hypoxic (10% oxygen) conditions for 8 h. It was shown that the whole body exposure to hypoxic atmosphere caused tissue hypoxia in healthy animals as evidenced by a decrease in the arterial blood oxygen tension and increase of the plasma adenosine concentration (P < 0.05). This "hypoxic" treatment resulted in significantly reduced hepatocellular damage and attenuated levels of serum cytokines in mice with acute liver inflammation. The anti-inflammatory effects of hypoxia were not observed in the absence of A2AR in studies of A2AR gene-deficient mice or when A2AR have been pharmacologically antagonized with synthetic antagonist. The presented data demonstrate that total body hypoxia-triggered pathway provides protection in acute hepatitis and that hypoxia (upstream) and A2AR (downstream) function in the same immunosuppressive and liver tissue-protecting pathway.

A2A adenosine receptor protects tumors from antitumor T cells

The A2A adenosine receptor (A2AR) has been shown to be a critical and nonredundant negative regulator of immune cells in protecting normal tissues from inflammatory damage. We hypothesized that A2AR also protects cancerous tissues by inhibiting incoming antitumor T lymphocytes. Here we confirm this hypothesis by showing that genetic deletion of A2AR in the host resulted in rejection of established immunogenic tumors in approximately 60% of A2AR-deficient mice with no rejection observed in control WT mice. The use of antagonists, including caffeine, or targeting the A2 receptors by siRNA pretreatment of T cells improved the inhibition of tumor growth, destruction of metastases, and prevention of neovascularization by antitumor T cells. The data suggest that effects of A2AR are T cell autonomous. The inhibition of antitumor T cells via their A2AR in the adenosine-rich tumor microenvironment may explain the paradoxical coexistence of tumors and antitumor immune cells in some cancer patients (the "Hellstrom paradox"). We propose to target the hypoxia-->adenosine-->A2AR pathway as a cancer immunotherapy strategy to prevent the inhibition of antitumor T cells in the tumor microenvironment. The same strategy may prevent the premature termination of immune response and improve the vaccine-induced development of antitumor and antiviral T cells. The observations of autoimmunity during melanoma rejection in A2AR-deficient mice suggest that A2AR in T cells is also important in preventing autoimmunity. Thus, although using the hypoxia-->adenosine-->A2AR pathway inhibitors may improve antitumor immunity, the recruitment of this pathway by selective drugs is expected to attenuate the autoimmune tissue damage.

Cutting Edge: Physiologic Attenuation of Proinflammatory Transcription by the Gs Protein-Coupled A2A Adenosine Receptor In Vivo

The A2A adenosine receptor plays a critical role in the physiologic immunosuppressive pathway that protects normal tissues from excessive collateral damage by overactive immune cells and their proinflammatory cytokines. In this study, we examine and clarify the mechanism of tissue protection by extracellular adenosine using A2AR-deficient mice and show that the A2AR inhibits TLR-induced transcription of proinflammatory cytokines in vivo. The observed increase in proinflammatory cytokines mRNA in A2AR-deficient mice was associated with enhanced activity of the NF-kappaB transcription factor. These observations provide the genetic in vivo evidence for attenuation of proinflammatory transcriptional activity of NF-kappaB by a "metabokine" adenosine and point to the need to re-evaluate the regulation of other transcription factors in hypoxic and adenosine-rich microenvironments of inflamed normal tissues and solid tumors.

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.

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.

A(2A) receptor dependent and A(2A) 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. A(2A) adenosine receptors, but not A(2B), A(1), or A(3) 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 A(2A)R 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 A(2A)R, with no contribution of intracellular lymphotoxicity or of compensating A(2B)Rs because only A(2A)R +/+, but not A(2A)R -/- thymocytes were susceptible to apoptotic effects of extAdo. Studies of the effects of cAMP-raising agents support observations of extAdo/A(2A)R/cAMP-triggered apoptosis in DP thymocytes. Unexpectedly, the extAdo strongly inhibited TCR-triggered activation of both A(2A)R +/+ and A(2A)R -/- thymocytes in the presence of ADA inhibitors. This was confirmed with thymocytes from ADA gene-deficient mice, suggesting the existence of A(2A)R-independent effects of extAdo on thymocytes. The presented data raises questions about the identity and functional role of A(2A)R-expressing thymocytes in T-cell differentiation and of the role of TCR-antagonizing effects of extAdo in conditions of ADA SCID. (Blood. 2000;95:3859-3867)

Memory of extracellular adenosine A2A purinergic receptor-mediated signaling in murine T cells

Accumulation of extracellular and intracellular adenosine (Ado) under hypoxic conditions or in the absence of adenosine deaminase results in lymphocyte depletion and in severe combined immunodeficiency, which are currently explained by direct intracellular lymphotoxicity of Ado metabolites. In support of the alternative, "signaling" mechanism, we show that extracellular Ado (extAdo) suppresses all tested T cell receptor (TCR)-triggered effector functions of T lymphocytes including the TCR-triggered FasL mRNA up-regulation in cytotoxic T lymphocytes. Strong evidence against the intracellular lymphotoxicity of Ado (and in support of the signaling model) is provided by abrogation of TCR-triggered growth inhibition in Ado-exposed T cells. The brief exposure to Ado was sufficient to observe inhibition of TCR-triggered effector functions. The "memory" of T cells to exposure to extAdo is best explained by sustained increases in cAMP. Selective agonist (CGS21680) and antagonist (ZM241385) of A2A adenosine receptor were used in functional assays and cDNA probes for different sybtypes of adenosine receptors were used in Northern blot studies. A2A receptors are identified as the predominantly expressed subtype of Gs-coupled Ado receptors in T cells. The demonstration of cross-talk between the A2A receptors and TCR in both directions support the possible role of A2A receptors in mechanisms of extAdo-mediated immunosuppression in vivo under adenosine deaminase deficiency and hypoxic conditions in, e.g., solid tumors.

Phosphorylation of T-lymphocyte plasma membrane-associated proteins by ectoprotein kinases: implications for a possible role for ectophosphorylation in T-cell effector functions

Extracellular adenosine triphosphate (ATPo) has been suggested to play a role in lymphocyte effector functions. Recently, it has been suggested that MgATP2- may be the molecular species which is involved in modulating the lytic interaction between cytotoxic T-lymphocytes (CTL) and their target cells. In this study, we provide evidence that ATPo mediates the phosphorylation of extracellular proteins on T-lymphocytes through the action of ectoprotein kinases. The ectophosphorylation is temperature-dependent, supported by Mg2+ and Mn2+, and both ATP and GTP, whereas kinase activity and/or substrates were removed by pretreatment of intact lymphocytes with trypsin. We show the presence of extracellular ATP/GTP-binding sites, indicating the presence of ectoenzymes on intact lymphocytes. The major ectoprotein kinase was identified as a casein kinase II-like protein kinase and could be inhibited by heparin, whereas its activity was enhanced by spermine. The ectoprotein kinase showed remarkable substrate specificity, phosphorylating the serum protein vitronectin, but not fibronectin. In experiments with the cell-impermeable protein kinase inhibitor K-252b, we demonstrate the possible functional importance of ectoprotein kinase in CTL-mediated cytotoxicity, i.e., target cell death was completely blocked by K-252b without affecting intracellular phosphorylation. These results suggest that ectoprotein phosphorylation may possibly be an important event in immunologically relevant cell-cell interactions.

Role of A2a extracellular adenosine receptor-mediated signaling in adenosine-mediated inhibition of T-cell activation and expansion

Accumulation of adenosine and of deoxyadenosine in the absence of adenosine deaminase activity (ADA) activity results in lymphocyte depletion and in severe combined immunodeficiency (ADA SCID), which is currently explained by direct cell death-causing effects of intracellular products of adenosine metabolism. We explored the alternative mechanisms of peripheral T-cell depletion as due to inhibition of T-cell expansion by extracellular adenosine-mediated signaling through purinergic receptors. The strong inhibition of the T-cell receptor (TCR)-triggered proliferation and of upregulation of interleukin-2 receptor alpha chain (CD25) molecules, but not the direct lymphotoxicity, were observed at low concentrations of extracellular adenosine. These effects of extracellular adenosine (Ado) are likely to be mediated by A2a receptor-mediated signaling rather than by intracellular toxicity of adenosine catabolites, because (1) poorly metabolized adenosine analogs cause the accumulation of cAMP and strong inhibition of TCR-triggered CD25 upregulation; (2) the A2a, but not the A1 or A3, receptors are the major expressed and functionally coupled adenosine receptors in mouse peripheral T and B lymphocytes, and the adenosine-induced cAMP accumulation in lymphocytes correlates with the expression of A2a receptors; (3) the specific agonist of A2a receptor, CGS21680, induces increases in [cAMP]i in lymphocytes, whereas the specific antagonist of A2a receptor, CSC, inhibits the effects of Ado and CGS21680; and (4) the increases in [cAMP]i mimic the adenosine-induced inhibition of TCR-triggered CD25 upregulation and splenocyte proliferation. These studies suggest the possible role of adenosine receptors in the regulation of lymphocyte expansion and point to the downregulation of A2a purinergic receptors on T cells as a potentially attractive pharmacologic target.

Transient up-regulation of P2Y2 nucleotide receptor mRNA expression is an immediate early gene response in activated thymocytes

In studies designed to understand the roles of P2 nucleotide receptors in differentiation of T lymphocytes, we observed a transient and protein synthesis-independent enhancement of mRNA expression for the G protein-coupled P2Y2 receptor in mouse thymocytes after the addition of steroid hormone or T cell receptor (TCR) crosslinking by anti-TCR mAb. Conversely, dexamethasone-induced increases in mRNA expression for the ligand-gated ion channel P2X1 receptor was detected in rat, but not mouse, thymocytes, raising questions about the previously suggested role of P2X1 receptors in thymocyte apoptosis. Flow cytometry analysis of thymocyte subsets excluded the possibility that the observed increases in P2Y2 receptor mRNA expression were due to the enrichment of steroid-treated cells with an P2Y2 mRNA-rich thymocyte subset. Triggering of TCR-mediated intracellular signaling pathways through crosslinking of TCR or by addition of phorbol ester and Ca2+ ionophore also resulted in the up-regulation of P2Y2, but not P2X1, receptor mRNA. It is proposed that the rapid increase of P2Y2 receptor mRNA expression could be a common early event in responses of T cells to different activating stimuli. Taken together with the recently discovered ability of nucleotide receptor-initiated signaling to antagonize or enhance the effects of TCR crosslinking or steroids on thymocytes, the observed rapid up-regulation of P2Y2 receptor mRNA expression may reflect an immediate early gene response where newly expressed cell surface nucleotide receptors provide regulatory feedback signaling from extracellular ATP in the T cell differentiation process.

Murine T lymphocytes modulate activity of an ATP-activated P2Z-type purinoceptor during differentiation

Murine T, but not B, lymphocytes constitutively express a membrane receptor for adenosine nucleotides that opens a nonspecific pore that admits Ca2+ and ethidium (314 Da), but not propidium (415 Da) ions. ATP, ADP, and AMP show decreasing potency; UTP and adenosine are inactive. Nonhydrolyzable ATP analogues are completely ineffective. Oxidized ATP inhibits the response. Activity is detectable at ATP concentrations of 125 microM and peaks at 1 mM. The intracellular free Ca2+ ([Ca2+]i) rise is not reversed by removing ATP by centrifugation or apyrase. The kinetics, agonist and antagonist profiles, and the passage of ions as large as ethidium are the characteristics of a P2z-type purinoceptor. No expression of classical P2x-, P2u-, or P2Y-type purinoceptors can be detected. The [Ca2+]i elevating activity of the ATP receptor is modulated during T cell differentiation. CD4+8+ double-positive thymocytes are the least responsive. CD4-8+ single-positive thymocytes, CD8+ splenic T cells, CD4+8- single-positive thymocytes, and CD4+ splenic T cells show increasing reactivity. Measurement of P2Z expression by the rate of ethidium ion uptake correlates with the [Ca2+]i. The trimodal expression of P2Z by splenic CD4+ T cells correlates with the subsets defined by CD44 and CD45RB, differentiation Ags that distinguish memory cells: P2Zlow cells are CD44brightCD45RBbright; P2Zint are CD44dullCD45RBint; P2Zhigh are CD44brightCD45RBdull. It is suggested that P2Z receptor-mediated signaling could be involved in the regulation of differentiation and cell death in the thymus and peripheral T lymphocytes.

Murine T lymphocytes modulate activity of an ATP-activated P2Z-type purinoceptor during differentiation

Murine T, but not B, lymphocytes constitutively express a membrane receptor for adenosine nucleotides that opens a nonspecific pore that admits Ca2+ and ethidium (314 Da), but not propidium (415 Da) ions. ATP, ADP, and AMP show decreasing potency; UTP and adenosine are inactive. Nonhydrolyzable ATP analogues are completely ineffective. Oxidized ATP inhibits the response. Activity is detectable at ATP concentrations of 125 microM and peaks at 1 mM. The intracellular free Ca2+ ([Ca2+]i) rise is not reversed by removing ATP by centrifugation or apyrase. The kinetics, agonist and antagonist profiles, and the passage of ions as large as ethidium are the characteristics of a P2z-type purinoceptor. No expression of classical P2x-, P2u-, or P2Y-type purinoceptors can be detected. The [Ca2+]i elevating activity of the ATP receptor is modulated during T cell differentiation. CD4+8+ double-positive thymocytes are the least responsive. CD4-8+ single-positive thymocytes, CD8+ splenic T cells, CD4+8- single-positive thymocytes, and CD4+ splenic T cells show increasing reactivity. Measurement of P2Z expression by the rate of ethidium ion uptake correlates with the [Ca2+]i. The trimodal expression of P2Z by splenic CD4+ T cells correlates with the subsets defined by CD44 and CD45RB, differentiation Ags that distinguish memory cells: P2Zlow cells are CD44brightCD45RBbright; P2Zint are CD44dullCD45RBint; P2Zhigh are CD44brightCD45RBdull. It is suggested that P2Z receptor-mediated signaling could be involved in the regulation of differentiation and cell death in the thymus and peripheral T lymphocytes.

Cell-mediated cytotoxicity: contact and secreted factors

The list of cells with cytotoxic potential now may include small resting T cells, but the exact nature of 'lethal hit delivery' by cytotoxic T lymphocytes remains elusive. Cell-mediated cytotoxicity by cytotoxic T lymphocytes is a complex, multistep process which seems likely to be mediated by several different pathways. Recent experimental evidence for the functioning of a novel cytotoxic mechanism through a target cell's surface receptor illustrates and emphasizes the necessity to study the interactions of cytotoxic T lymphocytes and target cells as a whole. Progress is evident in the description of molecular requirements for triggering cytotoxicity, cell-cell contacts and the regulation of the effector responses of cytotoxic T lymphocytes by extracellular, intracellular and granular proteins. Extracellular Ca(2+)-dependent secretion of perforin and protease(s) may explain several aspects of cellular cytotoxicity, whereas the apoptosis-mediating cell surface Fas protein is now implicated in Ca(2+)-independent cytotoxicity.

Highly lytic CD8+, alpha beta T-cell receptor cytotoxic T cells with major histocompatibility complex (MHC) class I antigen-directed cytotoxicity in beta 2-microglobulin, MHC class I-deficient mice

Targeted disruption of the beta 2-microglobulin (beta 2m) gene results in major histocompatibility complex (MHC) class I deficiency and virtual disappearance of functional CD8+ cytotoxic T lymphocytes (CTLs) in beta 2m-deficient (beta 2m-/-) mice. We asked whether the beta 2m-/- mice are able to reject tumor cells injected i.p. and what is the cellular composition of peritoneal exudate leukocytes (PELs) from such mice. We found that beta 2m-/- mice do reject MHC class I-bearing tumor cells injected i.p. Surprisingly, analysis of PEL CTLs obtained from i.p. tumor-injected beta 2m -/- mice revealed the presence of a large proportion of functional, tumor-destroying CD8+, CD4-, alpha beta T-cell receptor-positive, CD3+, Thy-1+, MHC class I-negative CTLs with strong MHC class I-directed cytotoxic activity. These results call for careful studies of local accumulation of CD8+ CTLs in beta 2m -/- mouse models and suggest that the dramatic decrease in MHC class I expression caused by beta 2m gene disruption does not prevent CD8+/CD4- cell selection and expansion.