Graves' disease is a thyroid-specific autoimmune disease mediated by stimulatory autoantibodies against the TSH receptor (TSHR). We have previously shown in our mouse model with adenovirus expressing the TSHR that antibody-mediated depletion of CD4⁺CD25⁺ regulatory T cells (Treg) enhances incidence and severity of hyperthyroidism in resistant and susceptible, respectively, mouse strains. These data indicate that balance between effector T cells and Treg is critical for disease development. This study was designed to evaluate the role played by recently identified another type of Treg, CD8⁺CD122⁺ T cells, in our mouse model to delineate the significance of different types of Treg in Graves' disease. Flow cytometry analysis showed that CD4⁺CD25⁺ and CD8⁺CD122⁺ T cells are distinct cell types, and anti-CD122 antibody effectively and selectively depleted CD8⁺CD122⁺ T cells. As for CD4⁺CD25⁺ Treg, CD8⁺CD122⁺ T cell depletion increased incidence of hyperthyroidism both in resistant and susceptible mice. Of interest, intrathyroidal lymphocytic infiltration was observed in some CD8⁺CD122⁺ T cell-depleted, hyperthyroid resistant mice. These results indicate that in addition to CD4⁺CD25⁺ T cells, CD8⁺CD122⁺ T cells also play a crucial role in disease susceptibility in mouse Graves' disease. Thus different types of Treg appear to be involved in tolerance to a self-antigen, the TSHR.