Here we present a new mechanism of tumor-induced immunosuppression: CD73 expressed on tumors negatively modulates tumor antigen-specific T cell immunity. Extracellular adenosine derived from AMP/ATP is generated primarily through the combined action of CD39 (ecto-nucleoside triphosphate diphosphohydrolase-1 which converts ATP and ADP to adenosine) and ecto-5′-nucleotidase (CD73) found on the surface of a variety of cell types. Although upregulation of CD73 expression was found in many human tumors, the function of tumor CD73 remains unclear to date. Recently, the contribution of CD73-mediated generation of extracellular adenosine to host defense systems has been widely explored because adenosine is a well-known anti-inflammatory mediator (39
). Interestingly, more recent studies led to the identification of the A2AAR-mediated so-called “adenosinergic pathway” as critical in physiological regulation of immune responses in vivo
). Thus, CD73 overexpression on cancer cells prompted us to examine its role in immune modulation in cancer.
Cancer cells having CD39 (data not shown) and CD73 ecto-nucleotidases on the cell surface possess the capacity both to generate immunosuppressive adenosine and to clear the extracellular proinflammatory factor ATP. Among its distinct anti-inflammatory roles, it is believed that adenosine modulates T cell responses primarily by binding to A2AAR on T cells. A2AAR engagement suppresses T cell proliferation, inflammatory cytokine secretion, and reduces surface expression of cytokine receptors by elevating the intracellular levels of cyclic (c)AMP through adenylyl cyclase stimulation (42
). Indeed, increased intracellular cAMP triggers protein kinase A-mediated phosphorylation and activation of carboxy-terminal Src kinase (Csk), which in turn abrogates TCR signaling and IFN-γ production by inhibiting Lck (43
). Signaling through the A2AAR and/or A2BAR have been reported to inhibit various aspects of CD8+
T cell responses involved in antitumor T cell immunity (21
) including activation and proliferation (16
), lethal hit delivery (44
), Fas ligand up-regulation (45
), and secretion of effector cytokines such as IFN-γ (46
). Therefore, adenosinergic signaling in cancer is most likely immunosuppressive (47
We highlight the major role of tumor CD73-generated adenosine that prevents tumor destruction from antitumor T cells and raise the feasibility of new strategies to overcome this tumor-induced immunosuppressant by genetic ablation or pharmacological inhibition of CD73 activity. Our findings suggest future studies to establish whether CD73 can serve as a primary trigger of tumor-protecting immunosuppressive molecules. Our data are in further agreement with the recent view stated by Ohta et al.
) that targeting the adenosine-A2AAR pathway is a cancer immunotherapy strategy to prevent inhibition of antitumor T cells in the tumor microenvironment. Hypoxia is associated with accumulation of extracellular adenosine that may inhibit antitumor CD8+
T cells by increasing their immunosuppressive intracellular cAMP levels (49
). More importantly, we believe that extracellular adenosine accumulated in tumor microenvironment is likely in large part produced by CD39/CD73 expressed on tumor cells. Targeting A2AAR either by siRNA or pharmacological antagonists has limitations because activation of the other adenosine receptors in addition to A2AR may account for CD8+
T cell failure to destroy tumor (48
). Indeed, only about 60% of mice with genetically ablated A2AR reject tumors (21
). Furthermore, global inhibition of A2AAR may have unwanted side effects. Moreover, CD73 has been directly involved in cancer cell growth and invasion (31
). Thus, targeting the enzymatic activity of tumor CD73 appears to have more therapeutic benefits for the tumor-bearing host. We expect that blocking A2AAR signals on T cells and targeting CD73 on tumor cells could improve therapeutic efficacy beyond that achievable with either alone. Whether additional benefits would be obtained by inhibiting the expression of CD39 remains to be explored.
Of note, inhibiting CD73 alone fails to cure cancer despite increasing host survival and inhibiting tumor growth. This is likely due to the insufficient number of effector antitumor T cells in the tumor microenvironment that are not only unable to control the cancer but which are also readily influenced by other immunosuppressive mechanisms. Interestingly, inhibiting CD73 remarkably improves the therapeutic antitumor effectors of transferred tumor-specific T cells. Therefore, the optimal strategy to counteract immunosuppressive adenosinergic effects of CD73 in the tumor microenvironment is complementary to other approaches directed at improving the development and function of antitumor T cells such as adoptive T cell therapy and dendritic cell vaccines.
In summary, our data show that extracellular adenosine generated by CD73 on tumor cells negatively regulates both the activation phase and effector phase of the antitumor T cell response and promotes T cell apoptosis. We propose that CD73 on tumor cells could be a therapeutic target for the prevention of tumor-induced immunosuppression although future studies investigating the role of CD73 in endogenous tumors may be needed.