F Naïve CD8+ T cells were isolated from WT, SKAP55−/−, ADAP−/−, and SKAP55−/−ADAP−/− splenocytes, then stimulated with plate-bound anti-CD3/CD28 for 12 h to assess PD-1 mRNA levels, and for 48 h to check surface PD-1 expression (mean of triplicates ± SD). 