2-Chloroadenosine inhibits the MHC-unrestricted cytolytic activity of anti-cd3-activated killer cells: Evidence for the involvement of a non-A₁/A₂ cell-surface adenosine receptor

Adenosine is likely to be a frequent constituent of the tumor microenvironment since this purine nucleoside is produced in quantity by hypoxic cells such as those found in the interior of poorly vascularized solid tumors. In this study we show that 2-chloroadenosine (2CA), a stable analogue of adenosine, inhibits, in a dose-dependent fashion, MHC-unrestricted killing of P815 tumor target cells by anti-CD3-activated killer (AK) lymphocytes. 2CA mediates this effect by interfering with the recognition/adhesion phase of cytolysis. Blocking cellular uptake of 2CA with dipyridamole, rather than attenuating the inhibitory effect, potentiated the inhibition of cytolysis, indicating the involvement of a cell-surface receptor. However, neither the A₁ receptor antagonist DPCPX, nor the A₂ receptor antagonist DMPX were able to block the inhibitory effect of 2CA on AK lymphocyte function. Similarly, the nonselective A₁ and A₂ receptor antagonists, theophylline and 8-phenyltheophylline, had no effect on 2CA-mediated inhibition of AK cell activity. Taken together, these data provide evidence that 2CA inhibits the cytolytic activity of AK lymphocytes by interacting with a novel non-A₁/A₂ cell-surface receptor. A similar effect mediated in vivo by tumor-elaborated adenosine may be involved in tumor-associated immunosuppression.