Dynamics of chemotactic peptide-induced superoxide generation by human monocytes

Adenosine and anergy

T cells must integrate multiple environmental cues when deciding whether to mount an immunogenic or tolerogenic response. Since not all self-reactive T cells are eliminated during thymic development, mechanisms of peripheral tolerance such as T cell anergy contribute to preventing autoimmunity. Recent studies have implicated extracellular adenosine and the adenosine A(2A) receptor as playing an important role in inhibiting T cell effector function. Herein, we review the current literature regarding T cell anergy and the emerging literature implicating the A(2A) receptor as critical regulator of immune activation. Finally, we present evidence to suggest a possible role for adenosine A(2A) receptor signaling in T cell anergy.

Adenosine in inflammatory joint diseases

Inflammatory joint diseases are a group of heterogeneous disorders with a variety of different etiologies and disease manifestations. However, there are features that are common to all of them: first, the recruitment of various inflammatory cell types that are attracted to involved tissues over the course of the disease process. Second, the treatments used in many of these diseases are commonly medications that suppress or alter immune function. The demonstration that adenosine has endogenous anti-inflammatory functions and that some of the most commonly used anti-rheumatic medications exert their therapeutic effects through stimulation of adenosine release suggest an important role for purinergic signaling in inflammatory rheumatic disorders.

Shaping of monocyte and macrophage function by adenosine receptors

Adenosine is an endogenous purine nucleoside that, following its release into the extracellular space, binds to specific adenosine receptors expressed on the cell surface. Adenosine appears in the extracellular space under metabolically stressful conditions, which are associated with ischemia, inflammation, and cell damage. There are 4 types of adenosine receptors (A(1), A(2A), A(2B) and A(3)) and all adenosine receptors are members of the G protein-coupled family of receptors. Adenosine receptors are expressed on monocytes and macrophages and through these receptors adenosine modulates monocyte and macrophage function. Since monocytes and macrophages are activated by the same danger signals that cause accumulation of extracellular adenosine, adenosine receptors expressed on macrophages represent a sensor system that provide monocytes and macrophages with information about the stressful environment. Adenosine receptors, thus, allow monocytes and macrophages to fine-tune their responses to stressful stimuli. Here, we review the consequences of adenosine receptor activation on monocyte/macrophage function. We will detail the effect of stimulating the various adenosine receptor subtypes on macrophage differentiation/proliferation, phagocytosis, and tissue factor (TF) expression. We will also summarize our knowledge of how adenosine impacts the production of extracellular mediators secreted by monocytes and macrophages in response to toll-like receptor (TLR) ligands and other inflammatory stimuli. Specifically, we will delineate how adenosine affects the production of superoxide, nitric oxide (NO), tumor necrosis factor-alpha, interleukin (IL)-12, IL-10, and vascular endothelial growth factor (VEGF). A deeper insight into the regulation of monocyte and macrophage function by adenosine receptors should assist in developing new therapies for inflammatory diseases.

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

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

Regulation of adenosine receptor subtypes during cultivation of human monocytes: role of receptors in preventing lipopolysaccharide-triggered respiratory burst

Adenosine is a potent anti-inflammatory agent that modulates the function of cells involved in the inflammatory response. Here we show that it inhibits lipopolysaccharide (LPS)-induced formation of reactive oxygen intermediates (ROI) in both freshly isolated and cultured human monocytes. Blocking of adenosine uptake and inactivation of the adenosine-degrading enzyme adenosine deaminase enhanced the inhibitory action of adenosine, indicating that both pathways regulate the extracellular adenosine concentration. Adenosine-mediated inhibition could be reversed by XAC (xanthine amine congener), an antagonist of the adenosine receptor A(2A), and MRS 1220 [N-9-chloro-2-(2-furanyl)[1, 2, 4]-triazolo[1,5-c]quinazolin-5-benzeneacetamide], an A(3) receptor antagonist, in both cell populations, while DPCPX (1,3-dipropyl-8-cyclopentylxanthine), an A(1) receptor antagonist, had no effect. Similar to what was seen with adenosine, CGS 21680, an A(2A) and A(3) receptor agonist, and IB-MECA, a nonselective A(1) and A(3) receptor agonist, dose dependently prevented ROI formation, indicating the involvement of A(3) and probably also A(2A) in the suppressive effect of adenosine. Pretreatment of monocytes with adenosine did not lead to changes in the LPS-induced increase in intracellular calcium levels ([Ca(2+)](i)). Thus, participation of [Ca(2+)](i) in the action of adenosine seems unlikely. The adenosine-mediated suppression of ROI production was found to be more pronounced when monocytes were cultured for 18 h, a time point at which changes in the mRNA expression of adenosine receptors were observed. Most prominent was the increase in the A(2A) receptor mRNA. These data demonstrate that cultivation of monocytes is accompanied by changes in the inhibitory action of adenosine mediated by A(3) and probably also the A(2A) receptor and that regulation of adenosine receptors is an integral part of the monocyte differentiation program.

Molecular action of methotrexate in inflammatory diseases

Despite the recent introduction of biological response modifiers and potent new small-molecule antirheumatic drugs, the efficacy of methotrexate is nearly unsurpassed in the treatment of inflammatory arthritis. Although methotrexate was first introduced as an antiproliferative agent that inhibits the synthesis of purines and pyrimidines for the therapy of malignancies, it is now clear that many of the anti-inflammatory effects of methotrexate are mediated by adenosine. This nucleoside, acting at one or more of its receptors, is a potent endogenous anti-inflammatory mediator. In confirmation of this mechanism of action, recent studies in both animals and patients suggest that adenosine-receptor antagonists, among which is caffeine, reverse or prevent the anti-inflammatory effects of methotrexate.

Adenosine A1 receptor promotion of multinucleated giant cell formation by human monocytes: a mechanism for methotrexate-induced nodulosis in rheumatoid arthritis

OBJECTIVE

To determine why methotrexate (MTX) exacerbates rheumatoid nodules in some patients, despite the effective suppression of synovial inflammation.

METHODS

Phorbol myristate acetate (PMA)-induced differentiation of monocytes into multinucleated giant cells was used as an in vitro model to study the effects of adenosine on nodulosis.

RESULTS

MTX at 200-2,000 nM or the adenosine A1 agonist N5-cyclopentyl adenosine (CPA) (10(-12) to 10(-9) M) or the A2 antagonist 3,7-dimethyl-1-propargylxanthine markedly enhanced giant cell formation, whereas the adenosine A1 antagonist 8-cyclopentyl-dipropylxanthine completely reversed these effects. PMA, CPA, and MTX induced adenosine release by cultured monocytes at concentrations consistent with those associated with predominantly A1 effects. Furthermore, surface expression of A1 receptors was found to remain unchanged on the differentiating cells throughout the culture period.

CONCLUSION

Agents that inhibit adenosine A1 receptors might be useful in the treatment of MTX-induced rheumatoid nodulosis, while still potentiating the A2-mediated antiinflammatory effects of MTX on synovitis.

Vascular responses to activated leukocytes after regression of atherosclerosis

Activation of leukocytes in vivo produces marked constriction of large arteries in atherosclerotic, but not in normal, monkeys. We tested the hypotheses that vasoconstrictor responses to activated leukocytes in vivo may be abnormal during hypercholesterolemia before the development of atherosclerotic lesions and that responses may return to normal after the regression of atherosclerosis. Leukocytes were activated by injection of the chemotactic peptide formyl-methionine-leucine-phenylalanine (fMLP) into the blood-perfused hind limb of four groups of cynomolgus monkeys: monkeys fed a normal diet (normal group, n = 18), monkeys fed an atherogenic diet for 3-4 months (hypercholesterolemic group, n = 6), monkeys fed an atherogenic diet for 20 months (atherosclerotic group, n = 19), and monkeys fed an atherogenic diet for 18 months, followed by a normal diet for 20 months (regression group, n = 14). Baseline resistance of large arteries was 1.5 +/- 0.2 (mean +/- SEM), 2.0 +/- 0.6, 3.5 +/- 0.4 (p less than 0.05 versus normal), and 1.7 +/- 0.2 mm Hg/ml/min per 100 g tissue for the normal, hypercholesterolemic, atherosclerotic, and regression groups, respectively. Injection of fMLP did not change resistance of large arteries in normal or hypercholesterolemic monkeys. Injection of fMLP increased resistance of large arteries by 3.0 +/- 0.7 mm Hg/ml/min per 100 g tissue in atherosclerotic monkeys and by 1.3 +/- 0.4 mm Hg/ml/min per 100 g tissue in regression monkeys (p less than 0.05 versus atherosclerotic and normal). Thus, abnormal vasoconstriction in response to activation of leukocytes persists, but to a lesser extent, after regression. In contrast, vasoconstrictor responses to serotonin, which were potentiated in atherosclerotic monkeys, were normal after regression.(ABSTRACT TRUNCATED AT 250 WORDS)

Methotrexate inhibits neutrophil function by stimulating adenosine release from connective tissue cells

Although commonly used to control a variety of inflammatory diseases, the mechanism of action of a low dose of methotrexate remains a mystery. Methotrexate accumulates intracellularly where it may interfere with purine metabolism. Therefore, we determined whether a 48-hr pretreatment with methotrexate affected adenosine release from [14C]adenine-labeled human fibroblasts and umbilical vein endothelial cells. Methotrexate significantly increased adenosine release by fibroblasts from 4 +/- 1% to 31 +/- 6% of total purine released (EC50, 1 nM) and by endothelial cells from 24 +/- 4% to 42 +/- 7%. Methotrexate-enhanced adenosine release from fibroblasts was further increased to 51 +/- 4% (EC50, 6 nM) and from endothelial cells was increased to 58 +/- 5% of total purine released by exposure to stimulated (fMet-Leu-Phe at 0.1 microM) neutrophils. The effect of methotrexate on adenosine release was not due to cytotoxicity since cells treated with maximal concentrations of methotrexate took up [14C]adenine and released 14C-labeled purine (a measure of cell injury) in a manner identical to control cells. Methotrexate treatment of fibroblasts dramatically inhibited adherence to fibroblasts by both unstimulated neutrophils (IC50, 9 nM) and stimulated neutrophils (IC50, 13 nM). Methotrexate treatment inhibited neutrophil adherence by enhancing adenosine release from fibroblasts since digestion of extracellular adenosine by added adenosine deaminase completely abrogated the effect of methotrexate on neutrophil adherence without, itself, affecting adherence. One hypothesis that explains the effect of methotrexate on adenosine release is that, by inhibition of 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) transformylase, methotrexate induces the accumulation of AICAR, the nucleoside precursor of which (5-aminoimidazole-4-carboxamide ribonucleoside referred to hereafter as acadesine) has previously been shown to cause adenosine release from ischemic cardiac tissue. We found that acadesine also promotes adenosine release from and inhibits neutrophil adherence to connective tissue cells. The observation that the antiinflammatory actions of methotrexate are due to the capacity of methotrexate to induce adenosine release may form the basis for the development of an additional class of antiinflammatory drugs.

Human monocyte chemoattractant protein-1 (MCP-1)

During the past three years great advances have been made in the chemistry and biology of chemoattractants for human leukocytes. Two chemoattractant cytokines have been isolated, sequenced and cloned, each with distinctive leukocyte attractant specificity. Monocyte chemoattractant protein 1 (MCP-1), the subject of this review by Edward Leonard and Teizo Yoshimura, is secreted by PHA-stimulated mononuclear cells and can be identified by northern blotting in response to LPS or PHA. It attracts monocytes but not neutrophils. In contrast, neutrophil attractant/activation protein (NAP-1) (also known as interleukin 8 (IL-8)) attracts and activates human neutrophils but it is not a chemoattractant for human monocytes. Based on amino acid sequence analysis, each of these attractants has been assigned to one of two distinct families of cytokines that are thought to participate in host defense and inflammatory responses.

Vascular responses to leukocyte products in atherosclerotic primates

Little is known about the possible role of leukocytes in the pathogenesis of vasospasm. We hypothesized that vasoactive products released by leukocytes might produce constriction of atherosclerotic arteries. To test this hypothesis, we infused fmet-leu-phe (fMLP), a peptide that activates leukocytes to release their vasoactive products, into the perfused hind limb of normal and atherosclerotic cynomolgus monkeys. Infusion of fMLP did not change resistance of large arteries in normal monkeys. In contrast, fMLP produced pronounced constriction of large arteries in atherosclerotic monkeys. To determine whether leukotrienes, platelet-activating factor, or prostaglandin E2 (PGE2), which are released by leukocytes, may contribute to leukocyte-induced vasoconstriction in atherosclerotic monkeys, we injected leukotriene D4, platelet-activating factor, and PGE2 intra-arterially into the perfused hind limb. Leukotriene D4 and platelet-activating factor had minimal effects on large arteries in both normal and atherosclerotic monkeys. PGE2 produced marked constriction of large arteries in atherosclerotic, but not normal, monkeys. Thus, pronounced constriction in atherosclerotic, but not normal, arteries during infusion of fMLP suggests that products released by leukocytes may mediate vasoconstriction in atherosclerotic vessels. Vasoconstrictor responses to PGE2 are profoundly potentiated by atherosclerosis, which suggests that PGE2 may contribute to leukocyte-induced vasoconstriction.

Interactions of formylmethionyl-leucyl-phenylalanine, adenosine, and phosphodiesterase inhibitors in human monocytes. Effects on superoxide release, inositol phosphates and cAMP

Cessation of the fMLF-induced burst of human monocyte superoxide release was associated with a rise in cAMP. This was not due to inhibition of phosphodiesterase (PDE), the major form of which was the PDE IV isozyme. The action of burst inhibitors did not correlate with cAMP levels: Rolipram, a PDe IV inhibitor, increased cAMP 6-fold, with minimal effects on the burst; whereas theophylline increased cAMP less than 2-fold but decreased the burst to less than half. Although theophylline and the adenylate cyclase activator, adenosine, inhibited fMLF-induced superoxide release, they did not inhibit production of inositol phosphates. Thus, these studies on inhibition of superoxide release implicated neither cAMP nor inositol phosphates.