Specific lysis of CTLs against target cells. Target cells were incubated with 51Cr (100 μCi per 1 × 106 cells) for 2 h in a 37 °C water bath. After incubation with 51Cr, target cells were washed three times with PBS, resuspended in RPMI-1640 medium, and mixed with effector cells at a 25:1, 50:1 or 100:1 of effector to target (E/T) ratio. After a 4-h incubation, the supernatants were harvested, and the amount of released 51Cr was measured with a gamma counter. Compared with controls, P < 0.05