tumor immunology is an intensely investigated field of research , even though its clinical applications in the field of cancer treatment are currently limited . it is now well established that the molecular mechanisms leading to cell transformation and cancer generation induce the appearance of neoantigens and danger signals . these molecules give rise to the immune response which drives tumor rejection ( a phenomenon called immunosurveillance ) , but some cancer cells escape this rejection by limiting tumor antigen expression ( a phenomenon called immunoediting ) mainly by inducing active immune tolerance mechanisms . these mechanisms include the proliferation and local accumulation of immunosuppressive cells , including regulatory t cells ( tregs ) , th17 cells , and myeloid - derived immunosuppressive cells ( mdscs ) . this tolerance ( a phenomenon called immunoescape ) prevents cancer rejection by the immune system and blunts the efficacy of immunotherapy . all these events have been clearly demonstrated in mice models for years .   in humans , recent data demonstrate that infiltration of the tumor bed by cd8 and memory t cells correlates with good outcomes , while tumor - bed infiltration by immunosuppressive cells correlates with poor outcomes [ 35 ] . . we may wonder whether therapies that shift immune tolerance towards the antitumor immune response could lead to tumor eradication . chemotherapies such as cyclophosphamide , 5-fluoro ruracil , and gemcitabine [ 69 ] by their capacity to eliminate immunosuppressive cells such as tregs and mdscs can restore the antitumor immune response . on the other hand , it is now widely accepted that the antitumor efficacy of many chemotherapy drugs is in part due to their induction of antitumor immune responses [ 1012 ] . in addition , drugs , like anti - ctla-4 mab and anti - pd1 mab , that directly target immune suppression , have either been approved by fda or are under clinical investigation in many cancer types with very impressive clinical results . one is to target adenosine ( a purine nucleoside ) or enzymes that catalyze the generation of adenosine , namely , ectonucleotidase molecules cd39 and cd73 . in this paper , we will propose a synthetic focus on the impact of this pathway on the antitumor immune response and its therapeutic potential . for this , we will describe not only the effect of adenosine on cancer cells , immune cells , and endothelial cells , but also how adenosine is produced by ectonucleotidase expressing cells . adenosine is constitutively present in the extracellular media at a very low concentration , but its concentration increases in many metabolically stressful conditions , notably in the tumor microenvironment [ 14 , 15 ] . following its release , adenosine binds to membranous adenosine receptors , which belong to a family of g - protein - coupled receptors . this family is composed of four different members called adenosine a1 , a2a , a2b , and a3 receptors , which mediate different cellular pathways through adenosine binding . a1 and a3 receptors induce a decrease in intracellular camp , while a2a and a2b receptors induce activation of adenylate cyclase resulting in increased intracellular levels of camp . a1 and a3 receptors also induce the activation of phosphatidyl - inositol 3 kinase ( pi3k ) and protein kinase c ( pkc ) . at low concentrations of adenosine , only high - affinity a1 , a2a , and a3 receptors are involved , whereas at high concentrations , like those observed in the tumor microenvironment , the low - affinity a2b receptor is involved in the signaling . because adenosine receptors are widely expressed , adenosine can influence immune , cancer , and endothelial cell functions ( figure 1 ) . taking into account the different affinities between adenosine and its receptors and the fact that adenosine receptors are differentially expressed depending on the cell type , adenosine has the ability to act variably on immune cells . adenosine binding to a1 or a2b receptors on neutrophils thus induces their activation , promotes their inflammatory activity , and induces chemotaxis and adherence of neutrophils to endothelial cells . in the context of cancer , neutrophil activation may be deleterious notably because neutrophils are able to produce metalloproteases , which foster matrix modification and promote metastases . neutrophils can also promote chronic inflammation , which promotes tumor growth [ 18 , 19 ] . this switch needs the previous activation of macrophages by tlr ( toll - like receptor ) agonists to upregulate the a2a receptor . this event enhances the capacity of macrophages to produce vegf ( vascular endothelial growth factor ) and il-10 , two cytokines that promote tumor growth . adenosine has been shown to promote mdsc functions in an a2b receptor - dependent manner . indeed , adenosine leads to mdsc expansion and may promote tumor tolerance in this way . adenosine could act on the a2a receptor of natural killer ( nk ) cells and could blunt their capacity to produce tumor necrosis factor- ( tnf- ) and interferon- ( ifn- ) . in addition , increased levels of adenosine in the tumor microenvironment inhibit the lytic activity of nk cells in an a2a receptor - dependent manner . adenosine also inhibits both perforin and fasl cytotoxic molecules , thus , limiting the ability of nk cells to mediate the lysis of tumor cells . adenosine could also modify nkt cell response by increasing their production of il-4 and transforming growth factor- ( tgf- ) , while decreasing their production of ifn- . dendritic cells ( dcs ) are a critical component of the immune response and are aimed at controlling t - cell polarization . during tumor growth , dcs invade the tumor bed and differentiate under hypoxic and inflamed conditions . in this context , dcs are in contact with high concentrations of adenosine . the stimulation of adenosine receptors skews dc differentiation towards a distinct cell population characterized by the expression of both dc and macrophage cell surface markers . pharmacologic analysis identified the a2b receptor as the mediator of adenosine 's effects on dcs . unlike normal myeloid dcs , adenosine - differentiated dcs have impaired allostimulatory activity and express high levels of angiogenic , proinflammatory , immune suppressor , and tolerogenic factors , including vegf , il-8 , il-6 , il-10 , cyclo - oxygenase-2 ( cox-2 ) , tgf- , and indoleamine 2,3-dioxygenase ( ido ) . in addition . however , the overall effect of a2b activation on dcs is not fully understood because some other reports suggest that adenosine could increase il-6 production and favor th17 responses . signaling through the tcr ( t cell receptor ) causes a rapid increase in a2a receptor mrna levels , which correlate with a significant increase in camp accumulation in these cells . in vitro , antigen recognition in the setting of a2a receptor activation by specific agonists induces t - cell anergy , even in the presence of costimulation such as cd28 triggering . t cells initially stimulated in the presence of an a2a receptor agonist also fail to proliferate and to produce il-2 and ifn- after restimulation . engagement of an a2a receptor in vivo inhibits il-6 expression while enhancing the production of tgf-. tgf- in the absence of il-6 promotes the differentiation of naive t cells into treg cells . consequently , treating mice with adenosine agonists not only inhibits th1 effector cell generation but also promotes the generation of tregs . in conclusion , exposure to adenosine during t cell activation promotes long - term t - cell anergy and the induction of tregs , both of which lead to a drastically impaired antitumor immune response . adenosine may affect cancer growth through direct binding on its specific receptors expressed at the cell surface of tumor cells ( figure 2 ) . more particularly , a1 receptor is mainly involved in tumor cell proliferation and induces activation of the cell cycle . a3 receptor is expressed in many cancers and seems to be overexpressed in cancer cells compared with normal cells . adenosine , in an a3-dependent manner , increases hypoxia - inducible factor-1 ( hif-1 ) protein expression in response to hypoxia in human melanoma , glioblastoma , and colon cancer cells [ 3133 ] . adenosine also mediates the production of vegf and angiopoietin by tumor cells in an a3-dependent manner . some reports have also demonstrated that human chronic lymphocytic leukemia ( cll ) , myeloma   and melanoma cells   express functional a2a receptors . the a2a receptor also increases erythropoietin ( epo ) production in hepatocellular carcinoma ( hep3b ) cells . adenosine does not always induce cancer cell proliferation ; some reports mentioned that adenosine could also induce cancer cell death or inhibit cell proliferation . a2b receptors are expressed in human neoangiogenic endothelial cells , where they play a role in the regulation of the expression of angiogenic factors like vegf , interleukin-8 ( il-8 ) , and fgf ( basic fibroblast growth factor ) [ 41 , 42 ] . a2b receptors are also involved in mrna and protein increases of il-6 in human astrocytoma cells , thus , promoting stat3 ( signal transducer and activator of transcription 3 ) mediated angiogenesis . the dominant pathway leading to extracellular adenosine production is the extracellular dephosphorylation of atp by ectonucleotidases . this degradation requires two enzymes called cd39 ( ectonucleoside triphosphate  diphosphohydrolase 1-entpd1 ) and cd73 ( ecto-5-nucleotidase - nt5e ) . cd39 hydrolyzes atp and adp to amp , which is further hydrolyzed to adenosine by cd73 . the conversion of atp into amp by cd39 is reversible by the action of extracellular kinases such as adenylate kinase . by contrast , the conversion of amp into adenosine by cd73 is reversible only after the transport of adenosine into cells where it can be converted into amp by adenosine kinase . cd39 is expressed on endothelial cells   and on many types of activated hematopoietic cells such as b cells , nk cells , and activated t cell subsets and also on monocytes / macrophages and dendritic cells [ 45 , 46 ] . cd39 degrades atp produced by activated platelets and thus inhibits the generation of thrombi , and may act on tumor angiogenesis by this pathway . cd39 expression on leukocytes is indispensable for the generation of adenosine and consequently dictates their immunosuppressive functions . cd73 catalyzes the dephosphorylation of purine and pyrimidine ribo- and deoxyribonucleoside monophosphates to the corresponding nucleosides . this antigen is expressed on some immune cells such as activated b but not on naive t cells [ 50 , 51 ] , endothelial cells , follicular dendritic cells , epithelial cells , and fibroblasts . in the tumor microenvironment in addition , some factors found in the tumor microenvironment such as tnf- , il-1 , prostaglandin e2 ( pge2 ) , tgf- , and il-6   could also upregulate ectonucleotidase expression , whereas ifn- and il-4 have been shown to downregulate cd73 expression [ 59 , 62 ] . many studies on cd39 or cd73-deficient animals have shown that the expression of ectonucleotidase on cancer cells and on host cells ( hematopoietic and nonhematopoietic cells ) is involved in tumor progression [ 6368 ] . ectonucleotidase expression has been observed in many human cancer types such as melanoma , breast cancer , colon cancer , glioma , leukemia , gastric , and head and neck cancers . the prognostic role of ectonucleotidase expression in cancer cells remains largely unclear . in a small cohort of breast cancer patients , expression of cd73 in cancer cells seemed to correlate with survival . in a cohort of colorectal cancer patients , high expression of cd73 was observed in about 50% of tumor samples and this overexpression correlated significantly with poor tumor differentiation , nodal status , and a high t stage . overall survival in patients with high expression of cd73 was poorer than in patients with low expression of cd73 . the expression of ectonucleotidases on chronic lymphocytic leukemia cells correlates with a better prognosis . in another study , cd39 was found to be widely expressed in cll lymph nodes , whereas cd73 is only expressed in proliferative centers . ectonucleotidase - expressing llc cells produce adenosine , which mediates drug - induced resistance via an ampc - dependent autocrine loop . for example , immunoediting may select cancer cells that highly express ectonucleotidases ; that is , cells better armed to fight against the antitumor immune response . hif induced under hypoxia could increase ectonucleotidase expression through direct binding on response elements located within the ectonucleotidase promoters . ectonucleotidase expression on tumor cells and on tumor exosomes ( small vesicles secreted by cancer cells ) may increase local concentrations of adenosine and could blunt the antitumoral immune response [ 73 , 74 ] . indeed , blockade of the a2a receptor on cd8 t cells inhibits the growth of strongly immunogenic melanomas . a2b receptor blockade acts on dc subsets and enhances tumor ag presentation and cytokine - mediated t cell activation . adenosine - receptor promoters contain an hif-1 response element that drives expression of these receptors in hypoxic cells including endothelial cells , cancer cells , and dcs . in fact , adenosine receptors have been found in many cancer types in mice and humans . thus hypoxia induces a vicious circle involving the adenosine pathway , by enhancing the production of adenosine via the upregulation of both ectonucleotidases and adenosine receptors . tregs are one of the key immunosuppressive cells in the context of cancer . regulatory t cells ( tregs ) were initially identified in both mice and humans as cd4 + t cells constitutively expressing cd25 and inhibiting the immune response of effector t cells . in cancer - bearing animals or patients , tregs expand , migrate to tumor sites , and suppress the antitumor immune response mediated by nk cells , cd4 , cd8 t cells , and myeloid cells , through different molecular mechanisms . in experimental tumor models as well as in cancer - bearing patients , treg accumulation was first described among peripheral blood leukocytes in cancer - bearing patients , in the spleen of tumor - bearing rodents and also in the tumor itself , where a high infiltration of tregs correlates with a poor prognosis in most cancer types . therefore , tregs are usually considered a major cell population involved in immune tolerance , which protects cancer cells from antitumor immunity . the mechanism of treg - mediated immunosuppression remains unclear , and many mechanisms of action have been proposed . recently , murine tregs were shown to express membranous cd73 and cd39 and to be able to transform atp into adenosine . functionally , the coexpression of cd39 and cd73 with the pericellular generation of adenosine dictates the suppressive functions of treg cells on a2a - positive effector t cells . in humans , tregs could be induced from naive t cells by tcr triggering in the presence of tgf-. a recent report demonstrated that triggering of the tcr induced expression of cd39 and cd73 on these cells . finally cd39 expression on tregs has also been shown to inhibit nk cell activity and to promote hepatic metastasis in a murine melanoma cancer model   and t cell anergy in human follicular lymphoma . however , the molecular mechanism that leads to ectonucleotidase expression in tregs and their role in the control of tumor growth remains unclear . th17 cells are cd4 t cells developed by tcr triggering with a combination of il-6 and tgf-. after induction , il-23 , an il-12 family member , maintains th17 cell polarization . th17 cells have emerged as key participants in a wide range of autoimmune disorders , including inflammatory bowel disease , psoriasis , and ankylosing spondylitis . th17 expansion has been shown in the blood , bone marrow , and spleen of tumor - bearing mice . th17 cell expansion has also been observed in human cancers such as melanoma , prostate cancer , fibrosarcoma , andadvanced head and neck cancers [ 8890 ] , and th17 infiltration is associated with a poor outcome in colon and liver cancers [ 91 , 92 ] . il-17 produced by th17 has been shown to promote angiogenesis and inflammation through stat3 signaling and mdsc mobilization [ 93 , 94 ] . on the other hand , the adoptive transfer of tumor - specific th17 cells could control tumor growth as a result of their ability to promote cytotoxic t cell activation [ 95 , 96 ] . these studies suggest that th17 cells may exert regulatory or inflammatory functions in the context of cancer depending on the cytokine microenvironment . we made the seminal observation that in vitro th17 cells generated with il-6 and tgf- express cd39 and cd73 , while inflammatory th17 cells generated with il-6 , il-1 , and il-23 do not . the expression of ectonucleotidases was dependent on il-6-driven stat3 activation and tgf--mediated downregulation of zinc finger protein growth factor independent-1 ( gfi-1 ) , both of which transcriptionally regulate ectonucleotidase expression during th17 cell differentiation . ectonucleotidase expression on th17 cells is relevant in the context of tumor growth as wild - type th17 cells promote tumor growth while th17 cells obtained from cd39-deficient mice remain unable to affect tumor growth . thus , our data suggest that the expression of ectonucleotidases dictates the immunosuppressive fate of th17 cells in cancer . myeloid - derived suppressor cells ( mdscs ) have been identified in cancer patients and in tumor - bearing mice as a population of immature myeloid cells with the ability to suppress t cell activation . in mice , mdscs are uniformly characterized by the expression of the cell - surface antigens gr1 , ly-6c / g , and cd11b , while in humans , mdscs are typically found in the lincd11bcd33hla - drsubset . given that mdscs from naive mice were generally found to lack immunosuppressive properties , it has been shown that mdscs require activation signals , such as cytokines or exosome membrane - bound hsp72 , from tumor cells to support their suppressive functions on t cells [ 97 , 98 ] . recently , ryzhov et al . have shown that cd11bgr1 ly-6 g cells express high levels of cd73 at the cell surface . this correlates with high ecto-5-nucleotidase enzymatic activity , which contributes to the expansion and the immunosuppressive properties of mdscs . the relevance of these observations in the control of tumor progression needs to be established . on the assumption that adenosine production promotes tumor proliferation , neoangiogenesis , and directly blunts antitumor effector cells , and that ectonucleotidases are highly expressed on tumor cells ( and correlate with a poorer overall survival rate ) and on immunosuppressive cells , it should be of great interest to inhibit the adenosine receptor on target cells or adenosine production by ectonucleotidases to promote the antitumor response .   even though targeting adenosine receptors seems to be relevant for example , it has been shown that targeting a2a receptors could dampen etoposide - mediated cll cell death , while enhancing the effects of melphalan , lenalidomide , bortezomib , and doxorubicin on multiple myeloma . inhibition of cd39 activity by polyoxometalate-1 ( pom-1 ) , a pharmacologic inhibitor of nucleoside triphosphate diphosphohydrolase activity , abrogated melanoma tumor growth in wild - type mice but not in cd39-null animals indicating a specific effect of pom-1 on host cd39 . a specific blocking antibody suppressed the growth of established 3-methylcholanthrene - induced tumors or prostate tumors and inhibited the development of lung metastases . moreover the inhibitor ,-methyleneadenosine 5-diphosphate ( apcp ) also affected thymoma or ovarian tumor growth and b16f10 lung metastasis formation [ 6668 ] . recent reports demonstrate that chemotherapies , in addition to their cytotoxic effects , mediate an immune effect via the release of atp , emphasizing the importance of inhibiting ectonucleotidases [ 10 , 100 ] . the use of ectonucleotidase inhibitors or blocking antibodies in association with chemotherapies that facilitate atp production could thus be focused on patients with ectonucleotidase - overexpressing tumors . for this , patients should be screened for ectonucleotidase expression on cancer cells and on tumor infiltrating cells , and atp producing chemotherapeutic drugs should be selected , notably thanks to the in vivo imaging method that allows the real - time measurement of atp within the tumor interstitium developed by the di virgilio team . clear understanding of the mechanisms involved in tumor - induced tolerance is a capital objective to develop effective antitumor immunotherapies . it is clear that ectonucleotidase expression on cancer cells as well as immune cells that infiltrate the tumor bed facilitates tumor development . adenosine production that results from the transformation of extracellular atp by ectonucleotidases promotes tumor cell proliferation , neoangiogenesis and blunts antitumor effector cells . thus a promising strategy to simultaneously reduce these effects would be to target ectonucleotidases using blocking antibodies or inhibitors of ectonucleotidases or adenosine receptors . this inhibition could be a new avenue to explore to improve the efficacy of classical cytotoxic agents by enhancing extracellular atp levels which would sustain the antitumor immune response .
it is now well known that tumor immunosurveillance contributes to the control of cancer growth . 
 many mechanisms can be used by cancer cells to avoid the antitumor immune response . 
 one such mechanism relies on the capacity of cancer cells or more generally of the tumor microenvironment to generate adenosine , a major molecule involved in antitumor t cell response suppression . 
 adenosine is generated by the dephosphorylation of extracellular atp released by dying tumor cells . 
 the conversion of atp into adenosine is mediated by ectonucleotidase molecules , namely , cd73 and cd39 . 
 these molecules are frequently expressed in the tumor bed by a wide range of cells including tumor cells , regulatory t cells , th17 cells , myeloid cells , and stromal cells . 
 recent evidence suggests that targeting adenosine by inhibiting ectonucleotidases may restore the resident antitumor immune response or enhance the efficacy of antitumor therapies . 
 this paper will underline the impact of adenosine and ectonucleotidases on the antitumor response .