SUMMARYAgonistic antibodies against CD137 act as costimulators in the activation of CD8 T cells. They enhance the immune response against syngeneic tumour grafts and suppress T cell-dependent humoral immune responses in vivo. The present study was undertaken to determine whether suppression of antibody production by anti-CD137 mAb affects the development of collageninduced arthritis (CIA). Male DBA ⁄1J mice were immunized with bovine collagen II (CII) and treated with an agonistic anti-CD137 mAb or an isotype-matched control mAb. Mice were assessed regularly for macro-and microscopic signs of arthritis and for the appearance of collagen-specific antibody production. Interferon (IFN)-c determination, FACS analysis of splenocytes and histopathological joint examinations were performed after the animals were killed. Administration of anti-CD137 mAb at the time of collagen immunization blocked the development of disease and inhibited the humoral immune response against CII. Agonistic anti-CD137 mAb exhibited therapeutic efficacy even after the immune response to CII had succeeded and the disease became apparent. Furthermore, it induced a protective memory in the animals, enabling resistance to subsequent challenges with the pathogenic antigen. Our results suggest a key role for CD137 in the pathogenesis of CIA. This model provides insights into immunoregulatory conditions that control the pathogenesis of autoimmune diseases.