Extracellular adenosine activates P1 receptors (A(1), A(2A), A(2B), A(3)) on cellular membranes. Here, we investigated the involvement of P1 receptor-mediated signaling in differentiation to regulatory T cells (Treg). Treg were induced in vitro by incubating isolated CD4(+)CD62L(+) naive murine T cells under Treg-skewing conditions. Antagonists of A(1) and A(2B) receptors suppressed the expression of Foxp3, a specific marker of Treg, and the production of IL-10, suggesting the involvement of A(1) and A(2B) receptors in differentiation to Treg. We also investigated the effect of these antagonists on T cell activation, which is essential for differentiation to Treg, and found that A(1) antagonist, but not A(2B) antagonist, suppressed T cell activation. We conclude that A(1) and A(2B) receptors are both involved in differentiation to Treg, but through different mechanisms. Since A(2B) antagonist blocked differentiation to Treg without suppressing T cell activation, it is possible that blockade of A(2B) receptor would facilitate tumor immunity. (C) 2011 Elsevier Inc. All rights reserved.