CTLA-4 appears to be a negative regulator of T cell activation and is implicated in T cell-mediated autoimmune diseases.Experimental autoimmune myasthenia gravis (EAMG), induced by immunization of C57BL/6 mice with acetylcholine receptor(AChR) in adjuvant, is an autoantibody-mediated disease model for human myasthenia gravis (MG). The production of anti-AChR Abs in MG and EAMG is T cell dependent. In the present study, we demonstrate that anti-CTLA-4 Ab treatment enhancesT cell responses to AChR, increases anti-AChR Ab production, and provokes a rapid onset and severe EAMG. To address possible mechanisms underlying the enhanced autoreactive T cell responses after anti-CTLA-4 Ab treatment, mice were immunized withthe immunodominant peptide a146–162 representing an extracellular sequence of the AChR. Anti-CTLA-4 Ab, but not control Ab,treatment subsequent to peptide immunization results in clinical EAMG with diversification of the autoantibody repertoire as wellas enhanced T cell proliferation against not only the immunizing a146–162 peptide, but also against other subdominant epitopes.Thus, treatment with anti-CTLA-4 Ab appears to induce determinant spreading, diversify the autoantibody repertoire, andenhance B cell-mediated autoimmune disease in this murine model of MG.