Demonstration of adenosine receptor on human lymphocytes in vitro and its possible role in the adenosine deaminase-deficient form of severe combined immunodeficiency

Purinergic signalling and immune cells

This review article provides a historical perspective on the role of purinergic signalling in the regulation of various subsets of immune cells from early discoveries to current understanding. It is now recognised that adenosine 5'-triphosphate (ATP) and other nucleotides are released from cells following stress or injury. They can act on virtually all subsets of immune cells through a spectrum of P2X ligand-gated ion channels and G protein-coupled P2Y receptors. Furthermore, ATP is rapidly degraded into adenosine by ectonucleotidases such as CD39 and CD73, and adenosine exerts additional regulatory effects through its own receptors. The resulting effect ranges from stimulation to tolerance depending on the amount and time courses of nucleotides released, and the balance between ATP and adenosine. This review identifies the various receptors involved in the different subsets of immune cells and their effects on the function of these cells.

The birth and postnatal development of purinergic signalling

The purinergic signalling system is one of the most ancient and arguably the most widespread intercellular signalling system in living tissues. In this review we present a detailed account of the early developments and current status of purinergic signalling. We summarize the current knowledge on purinoceptors, their distribution and role in signal transduction in various tissues in physiological and pathophysiological conditions.

Enzyme replacement and other biochemical approaches to the therapy of adenosine deaminase deficiency

Addition of adenosine deaminase (ADA) restored in vitro responses of lymphocytes from a patient with ADA deficiency and severe combined immunodeficiency (SCID). Enzyme replacement therapy, using red blood cells as a source of encapsulated human ADA, restored both T and B cell function in this patient. Ten other ADA--SCID patients have been treated with this form of enzyme replacement and five have responded to therapy. Lymphocytes from ADA--SCID patients treated with enzyme replacement become immunocompetent but remain enzyme deficient. Studies of these cells provide evidence supporting both cyclic AMP- and dATP-mediated immunosuppressive mechanisms in ADA--SCID. These observations suggest that inhibition of cyclic AMP synthesis and/or deoxycytidine (and possibly thymidine) supplementation may be useful new biochemical approaches to the therapy of ADA--SCID.

Buridan's ass: one man's view of the immune system

This essay offers a highly biased and prejudiced interpretation of the immune response. The major points of this interpretation are: B and T lymphocyte diversity is generated by rearrangements within and among existing receptorcoding genes; the T cell receptor is a molecule genetically more closely related to immunoglobulins than to the products of the major histocompatibility complex (MHC); the course of B and T lymphocyte differentiation is largely the same except for a final processing of T lymphocytes in the thymus; the processing is directed by MHC molecules of the thymus epithelium; Ir and Ia genes are identical; and the function of MHC polymorphism is to generate diversity at the population level.

Intracellular, nonreceptor-mediated signaling by adenosine: induction and prevention of neuronal apoptosis

Inhibitory effect of adenosine on the isolated heart muscle and vascular system were first described in 1929. Since then, numerous reviews have been published on the diverse actions of this nucleoside on a wide variety of cell types. Essentially all effects of adenosine in neurons and non-neuronal cells are mediated by activation of nucleoside membrane receptors coupled to specific intracellular second messenger pathways. This brief review describes two novel actions of adenosine in peripheral sympathetic neurons, which are not mediated by adenosine receptors. First is described how adenosine and related nucleosides are able to induce apoptosis during the initial stages of neuronal growth and development in vitro and in vivo. Second is discussed how adenosine is able to prevent or delay apoptosis in more mature sympathetic neurons subjected to nerve growth factor deprivation in culture. Both the induction and prevention of apoptosis are independent of receptor activation, and totally dependent on the intracellular accumulation and subsequent phosphorylation of adenosine. The physiological significance and mechanisms by which adenosine can induce apoptosis in one situation, and rescue from apoptosis in another, are described in this article.

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.

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

The absence or low levels of adenosine deaminase (ADA) in humans result in severe combined immunodeficiency (SCID), which is characterized by hypoplastic thymus, T lymphocyte depletion and autoimmunity. Deficiency of ADA causes increased levels of both intracellular and extracellular adenosine, although only the intracellular lymphotoxicity of accumulated adenosine is considered in the pathogenesis of ADA SCID. It is shown that extracellular but not intracellular adenosine selectively inhibits TCR-triggered up-regulation of activation markers and apoptotic events in thymocytes under conditions of ADA deficiency. The effects of intracellular adenosine are dissociated from effects of extracellular adenosine in experiments using an adenosine transporter blocker. We found that prevention of toxicity of intracellular adenosine led to survival of TCR-cross-linked thymocytes in long-term (4 days) assays, but it was not sufficient for normal T cell differentiation under conditions of inhibited ADA. Surviving TCR-cross-linked thymocytes had a non-activated phenotype due to extracellular adenosine-mediated, TCR-antagonizing signaling. Taken together the data suggest that both intracellular toxicity and signaling by extracellular adenosine may contribute to pathogenesis of ADA SCID. Accordingly, extracellular adenosine may act on thymocytes, which survived intracellular toxicity of adenosine during ADA deficiency by counteracting TCR signaling. This, in turn, could lead to failure of positive and negative selection of thymocytes, and to additional elimination of thymocytes or autoimmunity of surviving T cells.

Growth Inhibition of Granulocyte-Macrophage Colony-Forming Cells by Human Cytidine Deaminase Requires the Catalytic Function of the Protein

Previous studies have indicated that cytidine deaminase (CDD) is a potent growth inhibitor of granulocyte-macrophage colony-forming cells (GM-CFC). In this study, we have undertaken molecular cloning and purification of recombinant human CDD to elucidate the growth regulatory potential and mechanism behind the growth suppressive effect. The purified protein had a specific activity of 1.35 × 105 U/mg and a Km value of 30 μmol/L. In the GM-CFC assay, the recombinant protein was shown to reduce colony formation to 50% at 16 pmol/L concentration. Similarly, as was observed with CDD derived from granulocyte extract, the effect depended on the presence of thymidine (≥ 4 × 10-5 mol/L). These results imply that CDD is an extremely potent inhibitor of GM-CFC and that no additional factor from the granulocyte extract is required for the growth inhibitory effect. Modification of CDD by truncation from the C-terminal end, or by amino acid substitution of an active site glutamate residue, eliminated both the enzyme activity and the growth regulatory potential of CDD. Furthermore, CDD fromEscherichia coli was found to be even more effective than human CDD in growth suppression of GM-CFC, with 10-fold higher inhibitory activity corresponding to a 10-fold higher enzymatic activity. Taken together, these results show that the catalytic nucleoside deaminating function of the protein is essential for the growth suppressive effect of CDD. Most probably, CDD exerts growth inhibition by depleting the cytidine and deoxycytidine pool required for DNA synthesis, as addition of deoxycytidine monophosphate, which is not a substrate for CDD, neutralizes the inhibiting effect.

Growth Inhibition of Granulocyte-Macrophage Colony-Forming Cells by Human Cytidine Deaminase Requires the Catalytic Function of the Protein

Previous studies have indicated that cytidine deaminase (CDD) is a potent growth inhibitor of granulocyte-macrophage colony-forming cells (GM-CFC). In this study, we have undertaken molecular cloning and purification of recombinant human CDD to elucidate the growth regulatory potential and mechanism behind the growth suppressive effect. The purified protein had a specific activity of 1.35 × 105 U/mg and a Km value of 30 μmol/L. In the GM-CFC assay, the recombinant protein was shown to reduce colony formation to 50% at 16 pmol/L concentration. Similarly, as was observed with CDD derived from granulocyte extract, the effect depended on the presence of thymidine (≥ 4 × 10-5 mol/L). These results imply that CDD is an extremely potent inhibitor of GM-CFC and that no additional factor from the granulocyte extract is required for the growth inhibitory effect. Modification of CDD by truncation from the C-terminal end, or by amino acid substitution of an active site glutamate residue, eliminated both the enzyme activity and the growth regulatory potential of CDD. Furthermore, CDD fromEscherichia coli was found to be even more effective than human CDD in growth suppression of GM-CFC, with 10-fold higher inhibitory activity corresponding to a 10-fold higher enzymatic activity. Taken together, these results show that the catalytic nucleoside deaminating function of the protein is essential for the growth suppressive effect of CDD. Most probably, CDD exerts growth inhibition by depleting the cytidine and deoxycytidine pool required for DNA synthesis, as addition of deoxycytidine monophosphate, which is not a substrate for CDD, neutralizes the inhibiting effect.

Human basophil/mast cell releasability. VII. Heterogeneity of the effect of adenosine on mediator secretion

5'-N-ethylcarboxamideadenosine (NECA) greater than 2-chloroadenosine greater than adenosine greater than N6-(R-phenyl-isopropyl)-adenosine (R-PIA) inhibited in vitro anti-IgE-induced histamine and peptide leukotriene C4 (LTC4) release from human basophils in a concentration-dependent fashion. Micromolar concentrations of adenosine, NECA and R-PIA potentiated the anti-IgE-stimulated release of histamine and LTC4 from human lung parenchymal mast cells. Submillimolar concentrations of adenosine, NECA and R-PIA inhibited in a concentration dependent manner the release of histamine and prostaglandin D2 (PGD2) from skin mast cells challenged with anti-IgE. These results demonstrate marked heterogeneity of the modulatory effect exerted by adenosine on mediator release from human basophils and mast cells.

Activities of purine metabolising enzymes in lymphocytes of neonates and young children: correlates with immune function

Depressed activities of the following purine enzymes have been shown to result in immunodeficiencies: adenosine deaminase (ADA), hypoxanthine-guanine phosphoribosyltransferase (HGPRT), and purine nucleoside phosphorylase (PNP). These enzymes and adenosine kinase (AK) were measured in cord blood lymphocytes of premature and small-for-gestational age infants since they have partial immunodeficiencies of unknown biochemical etiology which can persist for many years. We also measured these enzymes in 3 infants with various immunodeficiencies. Activities were compared with appropriate matched control groups. The results indicated normal ADA and PNP but significantly depressed AK (P less than 0.05) and HGPRT (P less than 0.001) activities in 10 premature/SGA infants when compared to 35 full-term normal infants. In the 3 immunodeficient children the results were as follows: Child 1 had a 2- to 3-fold decrease in ADA with normal PNP and AK activities; Child 2 had a 2- to 3-fold decrease in AK, 4-fold decrease in HGPRT with normal PNP and ADA activities; Child 3 had confirmed AIDS and a 4-fold decrease in ADA, 6-fold decrease in HGPRT with normal PNP activity. The possible role of these depressed purine enzyme activities found in lymphocytes is discussed in relation to the imparied immunity seen in these infants.

Role of adenylate cyclase in human T-lymphocyte surface antigen capping

Our recent studies indicated that capping of T3, T4 and T8 surface antigens on human T lymphocytes is augmented by interaction of adenosine with a purinergic receptor. We suggested that the T-cell capping process was mediated by an adenylate cyclase-coupled purinergic receptor that resulted in the generation of cAMP and occupancy of cAMP receptors. The present study was undertaken to examine whether activation of adenylate cyclase in the absence of purinergic stimulation is sufficient to regulate surface antigen capping. Treatment of T lymphocytes with forskolin or cholera toxin caused activation of adenylate cyclase and occupancy of intracellular types I and II regulatory subunits of protein kinase by cAMP, as demonstrated by photoaffinity labeling with [8-3H]N3-cAMP. Such treatment augmented the rate of capping of the T3, T4, and T8 antigens, which resulted in a significant decrement in the elapsed time to half-maximal capping of each antigen. These observations support the proposition that the normal T-lymphocyte capping mechanism of both T3+, T4+ (inducer/helper) and T3+, T8+ (suppressor) subsets can be augmented by activation of adenylate cyclase.

Adenosine induced immunosuppression: the role of the adenosine receptor--adenylate cyclase interaction in the alteration of T-lymphocyte surface phenotype and immunoregulatory function

Adenosine (1 microM at 37 degrees C) rapidly modulates the expression of T-lymphocyte surface antigens (OKT4 and OKT8) and Fc gamma receptors, and increases T-suppressor activity for pokeweed mitogen driven in vitro immunoglobulin synthesis. The adenosine induced loss of OKT4 expressed was maximal within 5 min, while increased OKT8 expression developed more slowly; increased Fc gamma expression was maximal at 30 min after initiation of incubation with adenosine. Adenosine, 2-chloroadenosine and adenosine in the presence of nitrobenzylthioinosine (NBTI) induced the decrease in OKT4 expression. In contrast only adenosine induced the enhancement of OKT8 and Fc gamma receptor expression. Neither 2-chloroadenosine, a poorly transported analog, nor adenosine in the presence of the adenosine transport inhibitor NBTI were capable of enhancing OKT8 or Fc gamma receptor expression. Adenosine was shown to cause a rapid biphasic increase in cAMP, while 2-chloroadenosine and adenosine with NBTI induces a prolonged elevation in cAMP. Similarly isoproteranol which induces a sustained elevation in cAMP suppressed the adenosine induced increases in OKT8 and Fc gamma receptor expression. Incubation of T-helper/inducer lymphocytes with 1 microM adenosine for 30 min at 37 degrees C caused the loss of T-helper function; this loss of T-helper activity has previously been shown to result from the activation of T-suppressor cells. The loss of T-helper function was blocked by the simultaneous addition of isobutylmethylxanthine, an R site adenosine receptor antagonist, or NBTI, an adenosine transport inhibitor. Moreover, 2-chloroadenosine could not induce loss of T-helper function.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence against functional adenosine receptors on murine lymphocytes

The effects of adenosine, 2-chloroadenosine (2Cl Ado) and N6-phenylisopropyladenosine (PIA) were examined on peripheral blood and splenic lymphocytes from mice. Lectin-stimulated DNA synthesis was antagonized by adenosine and 2Cl Ado at high concentrations. Lower concentrations of all three nucleosides produced an enhancement of lectin-stimulated thymidine uptake in splenic lymphocytes. Peripheral blood lymphocytes were found to exhibit only inhibitions of mitogenic stimulation suggesting a difference in response to nucleoside exposure between spleen and peripherally circulating cells. The synthesis of antibody to sheep red blood cells was inhibited in a non-cytotoxic manner by 2Cl Ado and PIA while adenosine was without effects. The receptor antagonist 8-phenyltheophylline was found to block nucleoside increases in thymidine uptake and low concentrations of 2Cl Ado with regard to antibody production. The effects of high concentrations of 2Cl Ado or PIA on humoral responses were not antagonized by receptor blockade. The data suggest that functional alterations of lymphocyte responses to nucleoside exposure are not a consequence of surface receptors for adenosine nucleosides in murine cells.

Immune regulation: what immunodeficiency disease has taught us

"Immune regulation: what immunodeficiency disease has taught us" is reviewed by discussing three immuno-deficiency disorders. Hypogammaglobulinemia, the first documented primary immunodeficiency disorder, has a well defined and uniform clinical presentation which reflects a variety of underlying abnormalities involving the B cell, T cell, and monocyte. X-linked hypogammaglobulinemia, transient hypogammaglobulinemia of infancy common variable immunodeficiency, and their pathogenesis are discussed. Combined immunodeficiency with adenosine deaminase (ADA) deficiency first led to the now accepted concept that a biochemical abnormality may result in immunodeficiency. The clinical presentation, possible biochemical abnormalities resulting in the observed immunodeficiency, relative selectivity of the defect for the immune system, and potential applications of knowledge gained from the study of ADA deficiency are presented. Acquired immunodeficiency (AIDS) has resulted in the concept that a virus is cytopathic for a specific population of T cells and that this, at least in part, results in the immunodeficiency seen in AIDS.

Varicella pneumonia in a bone marrow-transplanted, immune-reconstituted adenosine deaminase-deficient patient with severe combined immunodeficiency disease

Bone marrow transplantation provides an important modality for "enzyme replacement" and the immune reconstitution of patients with adenosine deaminase (ADA) deficiency and severe combined immunodeficiency disease. We report a patient with ADA deficiency who develops severe varicella pneumonia 6 years after successful bone marrow transplantation and immune reconstitution. Marked abnormalities in T-cell mitogen responsiveness and pokeweed mitogen-induced polyclonal immunoglobulin synthesis occurred. Coculture experiments suggested the presence of increased suppressor activity. T-cell phenotyping showed decreased T3 and T4 subsets. These abnormalities slowly resolved over several months as the patient recovered from the varicella infection. ADA enzyme levels and metabolite concentrations in urine and erythrocytes remained unchanged. These findings, together with the chromosome and immune studies, suggested that the bone marrow graft remained intact. These studies indicate that immunologically reconstituted ADA-deficient patients may be at higher risk for complications related to varicella infection and suggest that the institution of preventive measures is important.

Evidence for an adenosine A2/Ra receptor on human basophils

5'-N-ethylcarboxamideadenosine (NECA) greater than 2-chloroadenosine greater than adenosine greater than (-)-N6-(R-phenyl-isopropyl)-adenosine greater than (+)-N6-(S-phenylisopropyl)-adenosine, in that order of potency, inhibited in vitro antigen-induced histamine release from human basophils in a dose-dependent fashion. Inhibition occurred only during the first stage of antigen-induced histamine release and the nucleosides failed to inhibit the release caused by the Ca2+ ionophore, A23187. 6-nitrobenzylthioinosine and dipyridamole, which inhibit adenosine uptake, and erythro-9-(2-hydroxy-3-nonyl)adenine, which blocks adenosine metabolism, did not impair the inhibition caused by NECA and adenosine. 8-phenyltheophylline and theophylline, two competitive antagonists of adenosine receptors, blocked the inhibition caused by NECA and adenosine. These data suggest that NECA and other adenosine analogs activate a specific cell surface adenosine receptor which possesses properties similar to those of an adenosine A2/Ra receptor.

Differential inhibition of lymphocyte function by 2-chloroadenosine

The effects of 2-chloroadenosine, a poorly metabolized adenosine analogue, on some human lymphocyte functions were studied. Mixed lymphocyte responses were strongly inhibited by very low concentrations of 2-chloroadenosine. The mitogen-induced proliferation of human lymphocytes was also generally suppressed by 2-chloroadenosine in a dose dependent manner. Blastogenesis induced by Con A and PWM was severely inhibited by low doses of 2-chloroadenosine while its inhibition of that induced by PHA was less marked. Natural killer cell activity was inhibited only about 55% by high concentrations of 2-chloroadenosine. These results suggested that many subsets of human lymphocytes are controlled by adenosine receptor.

Inhibition by xanthine derivatives of adenosine receptor-stimulated cyclic adenosine 3',5'-monophosphate accumulation in rat and guinea-pig thymocytes

The effect of stable adenosine analogues, including adenosine 5'-N-ethylcarboxamide (NECA) and N6-L-phenylisopropyl-adenosine (L-PIA), were studied on cyclic adenosine 3', 5'-monophosphate (cyclic AMP) accumulation in rat and guinea-pig thymocytes. NECA was approximately 10 times more potent than L-PIA, in thymocytes from both species. D-PIA was more potent in guinea-pig than in rat thymocytes. The effect of a number of adenosine analogues followed the order: NECA greater than 2-chloro-adenosine greater than L-PIA greater than N6-cyclohexyl-adenosine (CHA), an order of potency characteristic for adenosine receptors of the A2-subtype. Thymocytes may be used as a model system to study the pharmacology of such receptors. Several xanthines were studied as antagonists of the NECA (1 microM)-induced cyclic AMP accumulation. The order of potency was: 1,3-diethyl-8-phenylxanthine greater than 8-phenyl-theophylline greater than IBMX = 8-p-sulphophenyltheophylline = verrophylline greater than theophylline greater than caffeine greater than enprofylline greater than theobromine greater than pentoxiphylline. The pA2 value for 8-phenyltheophylline was 0.35 microM, and the antagonism was shown to be competitive. The order of potency of the xanthine is virtually identical to that found earlier in several other systems in which the receptors are of the A1-subtype. None of the xanthine derivatives tested thus seem to discriminate between A1 and A2-receptor-mediated adenosine actions.

Adenosine: a physiological modulator of superoxide anion generation by human neutrophils

The effects of adenosine were studied on human neutrophils with respect to their generation of superoxide anion, degranulation, and aggregation in response to soluble stimuli. Adenosine markedly inhibited superoxide anion generation by neutrophils stimulated with N-formyl methionyl leucyl phenylalanine (FMLP), concanavalin A (Con A), calcium ionophore A23187, and zymosan-treated serum; it inhibited this response to PMA to a far lesser extent. The effects of adenosine were evident at concentrations ranging from 1 to 1,000 microM with maximal inhibition at 100 microM. Cellular uptake of adenosine was not required for adenosine-induced inhibition since inhibition was maintained despite the addition of dipyridamole, which blocks nucleoside uptake. Nor was metabolism of adenosine required, since both deoxycoformycin (DCF) and erythro-9-(2-hydroxy-3-nonyl) adenine did not interfere with adenosine inhibition of superoxide anion generation. The finding that 2-chloroadenosine, which is not metabolized, resembled adenosine in its ability to inhibit superoxide anion generation added further evidence that adenosine metabolism was not required for inhibition of superoxide anion generation by neutrophils. Unexpectedly, endogenously generated adenosine was present in supernatants of neutrophil suspensions at 0.14-0.28 microM. Removal of endogenous adenosine by incubation of neutrophils with exogenous adenosine deaminase (ADA) led to marked enhancement of superoxide anion generation in response to FMLP. Inactivation of ADA with DCF abrogated the enhancement of superoxide anion generation. Thus, the enhancement was not due to a nonspecific effect of added protein. Nor was the enhancement due to the generation of hypoxanthine or inosine by deamination of adenosine, since addition of these compounds did not affect neutrophil function. Adenosine did not significantly affect either aggregation or lysozyme release and only modestly affected beta-glucuronidase release by neutrophils stimulated with FMLP. These data indicate that adenosine (at concentrations that are present in plasma) acting via cell surface receptors is a specific modulator of superoxide anion generation by neutrophils.

Cell-membrane receptors for purines. Review

Purines are involved in many aspects of cell chemistry - intermediary metabolism, nucleic acid synthesis, and the supply of high-energy phosphates to various active transport systems. In addition, however, there appear to be specific receptor molecules located within the plasma membrane of some cells, which mediate changes of cell function in response to purines present in the extracellular fluid. It is the purpose of this review to summarize the kind of functions subserved by those receptors as well as the basic structural requirements for their activation.

Effects of adenosine and deoxyadenosine on PHA-stimulation of lymphocytes of man, horse and pig

1. Adenosine inhibits thymidine and uridine incorporation of PHA-stimulated lymphocytes of man and horse at concentrations higher than 50 and 10 microM, respectively. Deoxyadenosine is inhibitory at concentrations higher than 100 microM. Thymidine and uridine incorporation of porcine lymphocytes are elevated 5-7-fold by 25-100 microM adenosine, deoxyadenosine, inosine and hypoxanthine. Leucine incorporation of PHA-stimulated lymphocytes was affected by adenosine and deoxyadenosine in the same way, but to a lower extent. 2. Effects of adenosine and deoxyadenosine were more pronounced at shorter cultivation times. 3. EHNA potentiated the effects of adenosine and deoxyadenosine on human and equine lymphocytes. With human lymphocytes inhibition by deoxyadenosine and EHNA was higher than by adenosine and EHNA. With porcine lymphocytes only the combination of deoxyadenosine and EHNA was inhibitory. 4. Homocysteine potentiated the inhibition of thymidine incorporation by the combination of adenosine and deoxyadenosine with equine lymphocytes, but not the inhibition of adenosine or deoxyadenosine alone. 5. Adenosine suppressed the PHA-stimulated elevation of PRPP concentrations. With porcine lymphocytes PRPP remained at the level of 0 hr, while with equine lymphocytes PRPP concentration decreased to below that level. 6. The various effects of adenosine and deoxyadenosine on lymphocytes of man, horse and pig can partially be related to differences in adenosine and deoxyadenosine metabolism.

Bone marrow transplantation only partially restores purine metabolites to normal in adenosine deaminase-deficient patients

To delineate the extent to which bone marrow transplantation provides "enzyme replacement therapy", we have determined metabolite concentrations in two patients with adenosine deaminase (ADA) deficiency treated with bone marrow transplants and rendered immunologically normal. 10 yr after engraftment of lymphoid cells, erythrocyte deoxy ATP was markedly decreased compared to the marked elevations of deoxy ATP observed in untreated patients, but was still significantly elevated (62 and 90 vs. normal of 6.0 +/- 6.0 nmol/ml packed erythrocytes). Similarly, deoxyadenosine and adenosine excretion were both markedly diminished compared to that of untreated patients but deoxyadenosine excretion was still clearly increased (20.1 and 38.6 vs. normal of less than 0.2 nmol/mg creatinine) while adenosine excretion was in the upper range of normal (7.0 and 8.1 vs. normal of 5.6 +/- 3.6 nmol/mg creatinine). Mononuclear cell deoxy ATP content was also elevated compared to normal (5.25 and 14.4 vs. 1.2 +/- 0.3). Separated mononuclear cells of bone marrow transplanted patients contain both donor lymphocytes and recipient monocytes. When mononuclear cells were depleted of the cells enriched for donor lymphocytes (i.e. monocyte depleted) was lower than that of the mixed mononuclear cells (2.2 vs. 5.26). Surprisingly, plasma adenosine was as high as in untreated ADA-deficient patients (3.2 and 1.5 vs. untreated of 0.3-3 microM). Consistent with the elevated plasma adenosine and urinary deoxyadenosine, erythrocyte S-adenosyl homocysteine hydrolase activity was diminished (0.88 and 1.02 vs. normal of 5.64 +/- 0.25). Thus, bone marrow transplantation of ADA-deficient patients not only provides lymphoid stem cells, but also partially, albeit incompletely, clears abnormally increased metabolites from nonlymphoid body compartments.

A microfluorometric mithramycin assay for quantitating the effects of immunotoxicants on lymphocyte activation

A semiautomated, microfluorometric assay has been developed for the detection of toxicant-induced changes in lymphocyte DNA content at standard intervals after mitogen activation. DNA is quantitated by solubilizing the cells and determining the fluorescence enhancement that results from formation of the highly specific mithramycin:DNA adduct. The limit of detection is 0.21 microgram (30,000 resting cell equivalents) per microtiter well. Correlation with the less sensitive, nonautomatable, diphenylamine DNA assay gives a correlation coefficient r = 0.91. Prototype substances representative of true immunotoxicants (prostaglandin E2) and common interfering substances (thymidine at less than 5 X 10(-4) M)m have been tested. The latter substance produces false positive results in the standard [3H]thymidine assay. The mithramycin assay does not inappropriately detect this interfering substance. It has the characteristics of a highly specific, accurate technique for screening and quantitating immunotoxic drugs, agents, and mediators in patient sera and other complex biological fluids.

Lymphocyte membrane adenosine receptors coupled to adenylate cyclase: properties and occurrence in various lymphocyte subclasses

Lymphocytes possess membrane adenosine receptors through which this nucleoside or analogs activate adenylate cyclase. Several adenosine analogs - 5'-N-ethylcarboxamide adenosine (NECA), N6-(L-2-phenyl-isopropyl) adenosine (PIA), 2-chloro adenosine (Cl-Ado) - were tested on the adenylate cyclase of mouse thymocyte homogenates; the order of potencies: NECA greater than Cl-Ado greater than PIA is consistent with the order which has been assumed to be characteristic of Ra type adenosine receptors. The stimulation of thymocyte adenylate cyclase by NECA was biphasic while the responses to Cl-Ado and PIA were not, which suggests the existence of Ra sub-types of adenosine receptors. Moreover, these adenosine receptors were found to be present in several lymphocyte subclasses: mature and immature mouse thymocytes, B and T mouse splenocytes, human peripheral blood lymphocytes, and also in lymphoblastoid cell lines. No crucial differences were observed between the adenylate cyclase stimulations of the various population by NECA.

The metabolism of adenosine and distribution of adenosine receptor lymphocytes in two human circulating T cell subsets

Human circulating E rosette forming cells (ERFC), rerosetted with sheep erythrocytes in the presence of adenosine, yielded two T-lymphocyte subpopulations: a major fraction forming E-rosettes (E resistant = ER) and a minor non-rosetting fraction (E sensitive - ES). Both T cell subpopulations converted adenosine mainly into inosine. However, ES cells metabolized adenosine more extensively than ER cells. Adenosine deaminase (ADA) activity was significantly higher in ES cells. Purine nucleoside phosphorylase (PNP) activity, as well as hypoxanthine guanosine phosphoribosyl transferase (HGPRT) activity were similar in both T cell subsets. The ratio of ADA/PNP in ES cells relative to ER cells was 1.8 suggesting that ES cells are at an earlier stage of differentiation. Enrichment of lymphocytes bearing a receptor for adenosine was demonstrated in ES cells.

Characterization of lithium effects on two aspects of T-cell function

Cell surface receptors receive, transduce and relay a variety of environmental signals. These phenomena, which have been extensively characterized in non-lymphoid cells, also appear to play a crucial role in dictating the degree of lymphocyte responsiveness. The nature of these regulatory events is only beginning to be unraveled but the adenylate cyclase-cyclic AMP axis appears to be one of the important controlling systems. Lithium appears to be as important a modulator of lymphocyte responsiveness as previously shown for a variety of other cells and the mechanism of action, in general, is consistent with its role as a putative blocker of adenylate cyclase activation. Indeed, lithium may exert its role as a regulator of lymphocyte responsiveness by acting on specific lymphocyte subpopulations. Direct proof for this is still wanting and consideration of its capacity for action as an imperfect substitute for normal extra- or intracellular cations or on the physiochemical state of the plasma membrane is necessary. Nevertheless, these studies indicate the validity of using lithium for assessing the role of the lymphocyte adenylate cyclase-cyclic AMP system in the generation and expression of regulatory signals leading to modulation of the immune system.

Enzymes of the purine interconversion system in chronic lymphatic leukemia: decreased purine nucleoside phosphorylase and adenosine deaminase activity

Activities of adenosine deaminase (ADA), adenosine kinase (AK), adenine phosphoribosyltransferase (APRT), hypoxanthine guanine phosphoribosyltransferase (HGPRT), and purine nucleoside phosphorylase (PNP), all enzymes of the purine interconversion system, were determined in lymphocytes of 25 patients with chronic lymphatic leukemia (CLL) and in 23 controls. A statistically significant decrease of PNP activities and a reduction of ADA activities at borderline levels were found in the patients, whereas for the other enzymes assayed no deviation from normal values was observed.

Adenosine-induced cyclic AMP increase in pig lymphocytes is not related to adenylate cyclase stimulation

Adenosine-cyclic AMP relationships have been studied in pig mesenteric lymph node lymphocytes. The early 2--3-fold increase in cyclic AMP accumulation elicited by adenosine and 2-chloroadenosine, an adenosine deaminase-resistant analogue, could not be correlated to similar effects on the adenylate cyclase activity of disrupted cell preparations, but rather to the competitive inhibition of the low Km (0.17 muM) cyclic AMP phosphodiesterase. The existence of adenosine receptors coupled to lymphocyte adenylate cyclase, which had been proposed by several authors, could not be confirmed by this study Adenosine-cyclic AMP relationships do not appear to be involved in concanavalin A stimulation of pig lymphocytes.