Gene dose effect reveals no Gs-coupled A2A adenosine receptor reserve in murine T-lymphocytes:: studies of cells from A2A-receptor-gene-deficient mice

Agonist binding to extracellular A(2A) adenosine receptors (A(2A)Rs) inhibits the activation of virtually all tested functions of T-cells and can induce apoptosis in thymocytes. The evaluation of levels of expression of these immunosuppressive receptors is expected to clarify whether the absence of spare A(2A)Rs (no 'receptor reserve') might be one of the mechanisms of attenuation of the effects of extracellular adenosine on T-cells. A(2A) transcript is found in T-cells and functional receptors can be demonstrated, but the density of receptor on T-cells is too low to be detected by radioligand binding. Studies of direct radioligand binding to murine brain with the selective A(2A)R agonist [H-3]CGS21680 (2-{4-[(2-carboxyethyl)-phenyl]ethylamino}-5'-N-ethylcarbox-amidoadenosine) established that striata levels of A(2A)R are virtually absent from A(2A) knock-out mice. Mice that are heterozygous (A(2A)R(+/-)) for the A(2A)R express significantly decreased levels of A(2A)R. To test for the presence of spare receptors in T-cells we took advantage of this gene dose effect and examined whether the decrease in the number of receptors in thymocytes from A(2A)R(+/-) mice was proportionately reflected in a decrease in the functional cAMP response of T-cells to adenosine. cAMP accumulation and apoptosis induced by adenosine and by A(2A)R agonist are of a lower magnitude in T-cells from A(2A)R(+/-) heterozygous mice than in T-cells from A(2A)R(+/+) littermate control mice. These results indicate that there is no A(2A)R reserve in murine T-cells. Strongly decreased adenosine-triggered cAMP increases were detected in thymocytes from A(2A)R(-/-) mice, suggesting that A(2A) adenosine receptors cannot fully compensate for the loss of A(2A)Rs in murine T-cells. We conclude that the number of A(2A)Rs is the limiting factor in determining the maximal cAMP response of T-lymphocytes to extracellular adenosine, thereby minimizing the immunosuppressive effects of extracellular adenosine.