The Pten tumor suppressor gene is critical for normal intrathymic development of T cells; however, its role in mature antigen-activated T cells is less well defined. A genetically crossed mouse line, Pten^fl/flGBC, in which Pten gene deletions could be primarily confined to antigen-activated CD8⁺ T cells, enabled us to evaluate the consequences of Pten loss on the course of experimental autoimmune encephalomyelitis. Compared with Pten^fl/fl controls, myelin oligodendrocyte glycoprotein (MOG) peptide-immunized Pten^fl/flGBC mice developed more severe and protracted disease. This was accompanied by increased spinal cord white matter myelin basic protein depletion and axonal damage, as well as a striking persistence of macrophage and granzyme B-expressing cellular neuroinfiltrates in the chronic phase of the disease. This persistence may be explained by the observation that anti-CD3 activated Pten^fl/flGBC T cells were more resistant to proapoptotic stimuli. Consistent with the predicted consequences of Pten loss, purified CD8⁺ T cells from Pten^fl/flGBC mice displayed augmented proliferative responses to anti-T-cell receptor stimulation, and MOG-primed Pten^fl/flGBC T cells exhibited a reduced activation threshold to MOG peptide. Pten^fl/flGBC mice also developed atypical central nervous system disease, manifested by prominent cervical cord and forebrain involvement. Collectively, our findings indicate that the phosphatidylinositol 3-kinase signaling pathway is an essential regulator of CD8⁺ T-cell effector function in experimental autoimmune encephalomyelitis.