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Hypoxia-Adenosinergic Immunosuppression: Tumor Protection by T Regulatory Cells and Cancerous Tissue Hypoxia

Authors' Affiliation: New England Inflammation and Tissue Protection Institute, Northeastern University, Boston, Massachusetts

Requests for reprints: Michail V. Sitkovsky, New England Inflammation and Tissue Protection Institute, 113 MU, Northeastern University, 360 Huntington Avenue, Boston, MA 02115. Phone: 617-373-4157; E-mail: m.sitkovsky{at}neu.edu.

Abstract

Cancerous tissue protection from tumor-recognizing CD8⁺ and CD4⁺ T cells (antitumor T cells) limits the therapeutic potential of immunotherapies. We propose that tumor protection is to a large extent due to (a) inhibition of antitumor T cells by hypoxia-driven accumulation of extracellular adenosine in local tumor microenvironment and due to (b) T regulatory cell-produced extracellular adenosine. The adenosine triggers the immunosuppressive signaling via intracellular cyclic AMP–elevating A2A adenosine receptors (A2AR) on antitumor T cells. In addition, the activated antitumor T cells in hypoxic tumor microenvironment could be inhibited by elevated levels of immunosuppressive hypoxia-inducible factor-1. Complete rejection or tumor growth retardation was observed when A2AR has been genetically eliminated or antagonized with synthetic drug or with natural A2AR antagonist 1,3,7-trimethylxanthine (caffeine). The promising strategy may be in combining the anti-hypoxia-adenosinergic treatment that prevents inhibition of antitumor T cells by tumor-produced and T regulatory cell-produced adenosine with targeting of other negative regulators, such as CTL antigen-4 blockade. Observations of tumor rejection in mice and massive prospective epidemiologic studies support the feasibility of anti-hypoxia-adenosinergic combined immunotherapy.

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