Cytotoxicity assay
To evaluate the levels of CTL activity, a standard 4-h 51Cr release assay was used. Briefly, 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. Assays were performed in triplicate for each sample at each ratio in a 96-well round-bottomed plate. After a 4-h incubation, the supernatants were harvested, and the amount of released 51Cr was measured with a gamma counter. The percent specific lysis was calculated according to the following formula:
Specific lysis = (experimental release − spontaneous release)/(maximal release − spontaneous release) × 100 %