Adenosine acts through an A₃ receptor to prevent the induction of murine anti-CD3-activated killer T cells

Adenosine, a purine nucleoside found at high levels in solid tumors, is able to suppress the recognition/adhesion and effector phases of killer lymphocyte-mediated tumor cell destruction. Here, we demonstrate that adenosine, at concentrations that are typically present in the extracellular fluid of solid tumors, exerts a profound inhibitory effect on the induction of mouse cytotoxic T cells, without substantially affecting T-cell viability. T-cell proliferation in response to mitogenic anti-CD3 antibody was impaired in the presence of 10 μM adenosine (plus coformycin to inhibit endogenous adenosine deaminase). Antigen-specific T-cell proliferation was similarly inhibited by adenosine. Anti-CD3-activated killer T (AK-T) cells induced in the presence of adenosine exhibited reduced major histocompatibility complex-unrestricted cytotoxicity against P815 mastocytoma cells in JAM and ⁵¹Cr-release assays. Diminished tumoricidal activity correlated with reduced expression of mRNAs coding for granzyme B, perforin, Fas ligand and tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL), as well as with diminished Nα-CBZ-L-lysine thiobenzylester (BLT) esterase activity. Interleukin-2 and interferon-γ synthesis by AK-T cells was also inhibited by adenosine. AK-T cells express mRNA coding for A_2A, A_2B and A₃ receptors, but little or no mRNA coding for A₁ receptors. The inhibitory effect of adenosine on AK-T cell proliferation was blocked by an A₃ receptor antagonist (MRSI191) but not by an A₂ receptor antagonist (3,7-dimethyl-1-propargylxanthine [DMPX]). The A₃ receptor agonists (N⁶-2-(4-aminophenyl)ethyladenosine [APNEA] and N⁶-benzyl-5′N-ethylcarboxamidoadenosine [N⁶-benzyl-NECA]) also inhibited AK-T cell proliferation. Adenosine, therefore, acts through an A₃ receptor to prevent AK-T cell induction. Tumor-associated adenosine may act through the same mechanism to impair the development of tumor-reactive T cells in cancer patients.