Gene dose effect reveals no G(s)-coupled A(2A) adenosine receptor reserve in murine T-lymphocytes: studies of cells from A(2A)-receptor-gene-deficient mice

Agonist binding to extracellular A(2A) adenosine receptors (A(2A)Rs) inhibi ts the activation of virtually all tested functions of T-cells and can indu ce 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 att enuation of the effects of extracellular adenosine on T-cells. A(2A) transc ript 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 se lective A(2A)R agonist [H-3]CGS21680 (2-{4-[(2-carboxyethyl)-phenyl]ethylam ino}-5'-N-ethylcarbox-amidoadenosine) established that striata levels of A( 2A)R are virtually absent from A(2A) knock-out mice. Mice that are heterozy gous (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 adv antage of this gene dose effect and examined whether the decrease in the nu mber of receptors in thymocytes from A(2A)R(+/-) mice was proportionately r eflected in a decrease in the functional cAMP response of T-cells to adenos ine. cAMP accumulation and apoptosis induced by adenosine and by A(2A)R ago nist 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 in dicate that there is no A(2A)R reserve in murine T-cells. Strongly decrease d adenosine-triggered cAMP increases were detected in thymocytes from A(2A) R(-/-) mice, suggesting that A(2A) adenosine receptors cannot fully compens ate 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 immunos uppressive effects of extracellular adenosine.