Abstract
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 ^51^Cr‐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 __N__α‐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 A~3~ receptors, but little or no mRNA coding for A~1~ receptors. The inhibitory effect of adenosine on AK‐T cell proliferation was blocked by an A~3~ receptor antagonist (MRS1191) but not by an A~2~ receptor antagonist (3,7‐dimethyl‐1‐propargylxanthine [DMPX]). The A~3~ receptor agonists (N^6^‐2‐(4‐aminophenyl)ethyladenosine [APNEA] and N^6^‐benzyl‐5′‐N‐ethylcarboxamidoadenosine [N^6^‐benzyl‐NECA]) also inhibited AK‐T cell proliferation. Adenosine, therefore, acts through an A~3~ 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. © 2002 Wiley‐Liss, Inc.