The ability of CTLA-4 to activate T cells correlates with its interaction with PP2A. Stably transfected Jurkat T cells were induced overnight with doxycycline (1 μg/ml) to express A) WT CTLA-4, B) KLESS CTLA-4, C) Y165F CTLA-4, D) PRO- CTLA-4, E) Y182F CTLA-4 or F) Y165F/Y182F CTLA-4. Cells were stimulated in triplicate in the presence of doxycycline with the indicated concentrations of 24:26 for 48 hours at 37°C. Supernatants were harvested and IL-2 was measured by ELISA. All data are representative of three independent experiments. **, p < 0.01; ***, p < 0.001.