C, D WT and ADAP−/− OT-I CD8+ CTLs were stimulated with 10 nM OVA257-264-pulsed or unpulsed EL-4 cells for 4 h to examine surface expression and the mRNA levels of PD-1 (C), in vitro cytotoxicity at different Effector:Target ratios (D; mean of triplicates ± SD). Graphs are representative of at least three independent experiments. 