Extracellular adenosine has been widely implicated in adaptive responses to hypoxia. The generation of extracellular adenosine involves phosphohydrolysis of adenine nucleotide intermediates, and is regulated by the terminal enzymatic step catalyzed by ecto-5′-nucleotidase (CD73). Guided by previous work indicating that hypoxia-induced vascular leakage is, at least in part, controlled by adenosine, we generated mice with a targeted disruption of the third coding exon of Cd73 to test the hypothesis that CD73-generated extracellular adenosine functions in an innate protective pathway for hypoxia-induced vascular leakage. Cd73 −/− mice bred and gained weight normally, and appeared to have an intact immune system. However, vascular leakage was significantly increased in multiple organs, and after subjection to normobaric hypoxia (8% O2), Cd73 −/− mice manifested fulminant vascular leakage, particularly prevalent in the lung. Histological examination of lungs from hypoxic Cd73 −/− mice revealed perivascular interstitial edema associated with inflammatory infiltrates surrounding larger pulmonary vessels. Vascular leakage secondary to hypoxia was reversed in part by adenosine receptor agonists or reconstitution with soluble 5′-nucleotidase. Together, our studies identify CD73 as a critical mediator of vascular leakage in vivo.

Background— Ecto-5′-nucleotidase (CD73)–dependent adenosine generation has been implicated in tissue protection during acute injury. Once generated, adenosine can activate cell-surface adenosine receptors (A 1 AR, A 2A AR, A 2B AR, A 3 AR). In the present study, we define the contribution of adenosine to cardioprotection by ischemic preconditioning. Methods and Results— On the basis of observations of CD73 induction by ischemic preconditioning, we found that inhibition or targeted gene deletion of cd73 abolished infarct size-limiting effects. Moreover, 5′-nucleotidase treatment reconstituted cd73 −/− mice and attenuated infarct sizes in wild-type mice. Transcriptional profiling of adenosine receptors suggested a contribution of A 2B AR because it was selectively induced by ischemic preconditioning. Specifically, in situ ischemic preconditioning conferred cardioprotection in A 1 AR −/− , A 2A AR −/− , or A 3 AR −/− mice but not in A 2B AR −/− mice or in wild-type mice after inhibition of the A 2B AR. Moreover, A 2B AR agonist treatment significantly reduced infarct sizes after ischemia. Conclusions— Taken together, pharmacological and genetic evidence demonstrate the importance of CD73-dependent adenosine generation and signaling through A 2B AR for cardioprotection by ischemic preconditioning and suggests 5′-nucleotidase or A 2B AR agonists as therapy for myocardial ischemia.

CD73, originally defined as a lymphocyte differentiation antigen, is thought to function as a cosignaling molecule on T lymphocytes and an adhesion molecule that is required for lymphocyte binding to endothelium. We show here that CD73 is widely expressed on many tumor cell lines and is upregulated in cancerous tissues. Because the ecto-5′-nucleotidase activity of CD73 catalyzes AMP breakdown to immunosuppressive adenosine, we hypothesized that CD73-generated adenosine prevents tumor destruction by inhibiting antitumor immunity. We confirmed this hypothesis by showing that combining tumor CD73 knockdown and tumorspecific T-cell transfer cured all tumor-bearing mice. In striking contrast, there was no therapeutic benefit of adoptive T-cell immunotherapy in mice bearing tumors without CD73 knockdown. Moreover, blockade of the A2A adenosine receptor with a selective antagonist also augmented the efficacy of adoptive T-cell therapy. These findings identify a potential mechanism for CD73-mediated tumor immune evasion and point to a novel cancer immunotherapy strategy by targeting the enzymatic activity of tumor CD73. Cancer Res; 70(6); 2245-55. ©2010 AACR.

Nucleotides and nucleosides influence nearly every aspect of physiology and pathophysiology. Extracellular nucleotides are metabolized through regulated phosphohydrolysis by a series of ecto-nucleotidases. The formation of extracellular adenosine from adenosine 5 0 -monophosphate is accomplished primarily through ecto-5 0 -nucleotidase (CD73), a glycosyl phosphatidylinositol-linked membrane protein found on the surface of a variety of cell types. Recent in vivo studies implicating CD73 in a number of tissue protective mechanisms have provided new insight into its regulation and function and have generated considerable interest. Here, we review contributions of CD73 to cell and tissue stress responses, with a particular emphasis on physiologic responses to regulated CD73 expression and function, as well as new findings utilizing Cd73-deficient animals.

Circulating or locally released nucleotides are rapidly metabolized by surface ectoenzymes (1). Ecto-5′nucleotidase (CD73) is a membrane-bound glycoprotein that functions to hydrolyze extracellular nucleotides into bioactive nucleoside intermediates (2). Surfacebound CD73 converts AMP to adenosine, which, when released, can activate seven transmembrane-spanning adenosine receptors (3, 4) or can be internalized through dipyridamole-sensitive carriers (5). These pathways have been shown to activate such diverse endpoints as adenine nucleotide recycling during cellular hypoxia (6), stimulation of epithelial electrogenic chloride secretion (responsible for mucosal hydration) (7), regulation of lymphocyte-epithelial adhesion (8), and promotion of endothelial and epithelial barrier function (4). Rather little is known about the regulation of CD73, and whether regulated expression provides a physiologic role. A number of studies have suggested that CD73 contributes to the protective aspects of adenine nucleotide metabolism during hypoxia and ischemia (9). For example, brief periods of ischemia preceding sustained ischemia, termed ischemic preconditioning, appear to result in large part from adenosine metabolism via increased CD73 activity (9). Increased CD73 activity in ischemic preconditioning has been attributed to adenosine receptor activation, protein kinase C activation, and α 1-adrenoreceptor activation (10). Few studies have addressed whether the CD73 gene is transcriptionally regulated. The cloned CD73 gene promoter contains a cAMP response element (CRE) (11), i.e., consensus DNA

IntroductionPrevious studies have implicated extracellular nucleotide metabolites, predominantly adenosine, as triggers of endogenous protective mechanisms in a number of acute injury models. [1][2][3][4][5][6][7] Extracellular adenosine is derived primarily through phosphohydrolysis of adenosine 5Ј-monophosphate (AMP). Ecto-5Ј-nucleotidase (CD73), a ubiquitously expressed ectoenzyme, is the pacemaker of this reaction. 8 Studies on the role of CD73 in tissue-injury showed that cd73 Ϫ/Ϫ mice develop profound vascular leakage and pulmonary edema upon hypoxia exposure. 8 Once generated into the extracellular space, adenosine can signal through any of 4 G-protein coupled adenosine-receptors (ARs: A1AR/A2AAR/A2BAR/A3AR). All of these receptors are expressed on vascular endothelia 9 and have been implicated in tissue-protection in different models of injury. [1][2][3]7,[10][11][12][13][14][15][16][17][18] Changes in vascular barrier function closely coincide with tissue injury of many etiologies, and result in fluid loss, edema, and organ dysfunction. [19][20][21] The predominant barrier (ϳ90%) to movement of macromolecules across a blood vessel wall is presented by the vascular endothelium. 20,22 Under physiologic conditions, macromolecules such as albumin (molecular weight ϳ70 kD) can cross the endothelial monolayer via a paracellular route (eg, by passing between adjacent endothelia) with some contribution of transcellular passage. 23,24 Endothelial barrier function correlates inversely with the size of molecules that can gain entry into tissues and differs between tissues of different origins. Endothelial permeability is highly regulated and may increase markedly upon exposure to inflammatory stimuli (eg, lipopolysaccharide, bacteria, bacterial compounds, prostaglandins, reactive oxygen species, leukotrienes) or adverse conditions such as ischemia or hypoxia. 18,20,[25][26][27][28][29] Given that activation of ARs can lead to an elevation of intracellular cAMP, and that elevated cAMP in endothelia promotes barrier function, 20,30 we considered the possibility of endothelial AR-signaling to regulate vascular permeability. In contrast to previous studies that found tissue protection during hypoxia or inflammation through signaling pathways involving the A2AAR, 1,3,7,31,32 the present studies conclude that the A2BAR is central to the control of vascular leak in hypoxia. Methods Cell cultureHuman microvascular endothelial cells (HMEC)-1 were cultured as described previously. 9,18 For preparation of experimental HMEC-1 monolayers, confluent endothelial cells were seeded at approximately less than 10 5 cells/cm 2 onto either permeable polycarbonate inserts or 100-mm Petri dishes. Endothelial cell purity was assessed by phase microscopic "cobblestone" appearance and uptake of fluorescent acetylated low-density lipoprotein. Stable repression of AR expression by siRNAWith the help of the siRNA Wizard (www.sirnawizard.com; InvivoGen, San Diego, CA) the following primer sequences were chosen within the coding region of the g...

Hypoxia is a well-documented inflammatory stimulus and results in tissue polymorphonuclear leukocyte (PMN) accumulation. Likewise, increased tissue adenosine levels are commonly associated with hypoxia, and given the anti-inflammatory properties of adenosine, we hypothesized that adenosine production via adenine nucleotide metabolism at the vascular surface triggers an endogenous anti-inflammatory response during hypoxia. Initial in vitro studies indicated that endogenously generated adenosine, through activation

CD73 is overexpressed in many types of human and mouse cancers and is implicated in the control of tumor progression. However, the specific contribution from tumor or host CD73 expression to tumor growth remains unknown to date. Here, we show that host CD73 promotes tumor growth in a T cell-dependent manner and that the optimal antitumor effect of CD73 blockade requires inhibiting both tumor and host CD73. Notably, enzymatic activity of CD73 on nonhematopoietic cells limited tumor-infiltrating T cells by controlling antitumor T cell homing to tumors in multiple mouse tumor models. In contrast, CD73 on hematopoietic cells (including CD4 + CD25 + Tregs) inhibited systemic antitumor T cell expansion and effector functions. Thus, CD73 on hematopoietic and nonhematopoietic cells has distinct adenosinergic effects in regulating systemic and local antitumor T cell responses. Importantly, pharmacological blockade of CD73 using its selective inhibitor or an anti-CD73 mAb inhibited tumor growth and completely restored efficacy of adoptive T cell therapy in mice. These findings suggest that both tumor and host CD73 cooperatively protect tumors from incoming antitumor T cells and show the potential of targeting CD73 as a cancer immunotherapy strategy.