Abstract
Extracellular adenosine has an important role in regulating the severity of inflammation during an immune response. Although there are four adenosine receptor (AR) subtypes, the A2AAR is both highly expressed on lymphocytes and known as a prime mediator of adenosine's anti-inflammatory effects. To define the importance of A2AAR signaling during neuroinflammatory disease progression, we used the experimental autoimmune encephalomyelitis (EAE) animal model for multiple sclerosis. In EAE induction experiments, A2AAR antagonist treatment protected mice from disease development and its associated CNS lymphocyte infiltration. However, A2AAR(-/-) mice developed a more severe acute EAE phenotype characterized by more proinflammatory lymphocytes and activated microglia/macrophages. Interestingly, very high levels of A2AAR were expressed on the choroid plexus, a well-established CNS lymphocyte entry point. To determine the contribution of A2AAR signaling in lymphocytes and the CNS during EAE, we used bone marrow chimeric mice. Remarkably, A2AAR(-/-) donor hematopoietic cells potentiated severe EAE, whereas lack of A2AAR expression on nonhematopoietic cells protected against disease development. Although no defect in the suppressive ability of A2AAR(-/-) regulatory T cells was observed, A2AAR(-/-) lymphocytes were shown to proliferate more and produced more IFN-γ following stimulation. Despite this more proinflammatory phenotype, A2AAR antagonist treatment still protected against EAE when A2AAR(-/-) lymphocytes were adoptively transferred to T cell-deficient A2AAR(+/+) mice. These results indicate that A2AAR expression on nonimmune cells (likely in the CNS) is required for efficient EAE development, while A2AAR lymphocyte expression is essential for limiting the severity of the inflammatory response.