A(2A) receptor dependent and A(2A) receptor independent effects of extracellular adenosine on murine thymocytes in conditions of adenosine deaminase deficiency

Adenosine deaminase (ADA) deficiency causes severe combined immunodeficiency (SCID) and is accompanied by T-cell depletion and accumulation of both intracellular and extracellular adenosine (extAdo) and deoxyadenosine. To better understand the causes of T-cell depletion in vivo and to discriminate between extracellular and intracellular effects of exogenously added adenosine in vitro, we investigated mechanisms of 2 different effects of adenosine on murine thymocytes. These effects of adenosine include direct induction of apoptosis in about 6% to 15% thymocytes and inhibition of T-cell receptor (TCR)-induced activation of the majority of thymocytes with inhibited ADA. A(2A) adenosine receptors, but not A(2B), A(1), or A(3) receptors, are shown to be mostly responsible for extAdo-triggered signaling (cyclic adenosine monophosphate [cAMP] accumulation) in murine thymocytes and this prompted studies of the effects of extAdo on thymocytes from A(2A)R gene-deficient mice. It is found that direct apoptotic effects of extAdo on CD4(+)CD8(+) double positive (DP) thymocytes are completely accounted for by signaling through A(2A)R, with no contribution of intracellular lymphotoxicity or of compensating A(2B)Rs because only A(2A)R +/+, but not A(2A)R -/- thymocytes were susceptible to apoptotic effects of extAdo. Studies of the effects of cAMP-raising agents support observations of extAdo/A(2A)R/cAMP-triggered apoptosis in DP thymocytes. Unexpectedly, the extAdo strongly inhibited TCR-triggered activation of both A(2A)R +/+ and A(2A)R -/- thymocytes in the presence of ADA inhibitors. This was confirmed with thymocytes from ADA gene-deficient mice, suggesting the existence of A(2A)R-independent effects of extAdo on thymocytes. The presented data raises questions about the identity and functional role of A(2A)R-expressing thymocytes in T-cell differentiation and of the role of TCR-antagonizing effects of extAdo in conditions of ADA SCID. (Blood. 2000;95:3859-3867)