GITR but not OX40 triggering is able to revert suppression of activated T reg's. (A) CD4⁺CD25⁺ cells were purified and cultured in the presence of anti-CD3 (1 μg/mL), IL-2 (20 U/mL), and irradiated splenocytes for 3 days. mAbs to OX40 were added during the 3-day culture (Act+Ab 3 days; ▪) to washed, preactivated T reg's for 2 hours and then rewashed before adding effector T cells (Act+Ab 2hr; ▦) to washed, preactivated T reg's together with CD4⁺CD25^- effector cells (Act then Ab; ▨). The CD25^-/CD25⁺ ratio is indicated. (B) The same experiment as in panel A was performed using anti-GITR mAb in place of anti-OX40 mAb. Proliferations were measured after 72 hours and pulsed with ³[H]TdR for the last 10 hours. Shown are the means (±SD) from 1 representative out of 3 independent experiments. The significance of the data was evaluated by Student t test (^**P < .01).