E WT, ADAP−/−, or SKAP55−/− OT-I CTLs were pretreated with the DMSO control, 10 μM PP2 or 75 μM 2APB, then stimulated with OVA257-264-pulsed EL-4 cells for 4 h to examine surface PD-1 expression (mean of triplicates ± SD). 