Adenosine Receptor A3

Abstract: Inflammatory bowel disease (IBD), including Crohn’s disease (CD) and ulcerative colitis (UC), is thought to result from a dysregulated immune response to commensal enteric bacterial antigens.1,2 The chronic nature of inflammation has been attributed to disturbed immune homeostasis due to polarized, dysregulated immune responses (Th1, Th17, and/or Th2) that potentiate inflammation.3–9 Because the underlying mechanism driving intestinal inflammation remains elusive, pharmacotherapy targeting downstream effects remains the mainstay of therapy. Traditional therapies result in nonspecific immune suppression (corticosteroids) or targeting of immune cell function (e.g., T and B lymphocytes, NK cells, etc.) by antimetabolites (e.g., azathioprine, methotrexate). Recent advances have focused on targeted biologic therapy (e.g., anti-TNF mAb) based on our evolving understanding of the immunopathogenesis of IBD. Adenosine deaminase (ADA) is a ubiquitous enzyme involved in purine salvage and recycling. It is concentrated in lymphoid tissue with heightened activity in proliferating cells found in inflamed, injured, or ischemic remodeling tissue.10–13 The congenital absence in humans results in Severe Combined Immunodeficiency syndrome (SCID) with resultant lymphopenia. Accumulation of 2-deoxyadenosine appears to be instrumental in lymphopenia, likely through phosphorylation to dATP and subsequent allosteric inhibition of ribonucleotide reductase resulting in impaired DNA synthesis in dividing cells (e.g., effector sites within an active immune response).14 2-Deoxyadenosine also inhibits S-adenosyl homocysteine hydrolase, which modulates APO-1/FAS-mediated cell death.15 In addition, ADA deficiency/inhibition results in accumulation of adenosine. Adenosine is a nucleoside essential to DNA synthesis, but also functions as a signaling molecule through 4 described G-protein-coupled receptors, where it has been described to modulate immune responses.16,17 Receptor A2A signaling appears to impart an antiinflammatory effect. Its activation attenuates immune responses in models of tissue-specific immune activation such as colitis, asthma, diabetes, and others.18–20 A2A activation attenuates ischemia/reperfusion injury in the kidney, liver, heart, and lung.21–24 More important to IBD, selective A2A activation reduces murine intestinal inflammation through downregulation of lymphocyte function.25 In vitro data suggest that A2A activation downregulates proinflammatory cytokine secretion in both lymphocytes and macrophages.26–29 Additionally, adenosine receptor A1 null mice are more susceptible to kidney reperfusion injury, with the presence of A1 conferring a protective effect through modification of proinflammatory cytokine, in particular TNF.30 Adenosine receptor A3 agonism is protective in both dextran sulfate sodium-induced colitis and IL-10−/− colitis.31 Thus, signaling through adenosine receptors may diminish autoimmune inflammation in a wide range of tissues. We hypothesized that the combination of focal lymphocyte toxicity within areas of active immune response (due to 2-deoxyadenosine accumulation) and downregulation of the proinflammatory response (due to adenosine accumulation) makes adenosine deaminase an attractive therapeutic target in IBD, where rapid induction of disease remission is desirable. The aim of the present study was to examine whether inhibition of ADA attenuates experimental colitis by blunting expansion of disease-causing effector T cells while reducing proinflammatory cytokine responses. We postulated that these effects would combine to reestablish regulated mucosal immune environments. To test this hypothesis we used a specific commercially available ADA inhibitor, pentostatin (2-deoxycorformycin [Nipent]), currently indicated for use in hairy cell leukemia.32 To determine the relationship between ADA inhibition and colitis we examined the effects of pentostatin on colitis activity as well as mucosal lymphocyte subset numbers and immune cell function.