Cytolytic activity of T-bet^–/– Vα14i NKT cells upon re-expression of T-bet. (A) T-bet^–/– Vα14i NKT cells retrovirally transduced with either an empty GFP control retrovirus or the T-bet–GFP retrovirus were used as effector cells. They were added in the indicated ratios to wells containing both CFSE-labeled A20 mCD1 target cells loaded with 4-deoxyGalCer and CMTMR-labeled A20 mCD1 target cells without antigen. After a 2.5-hour incubation period, each well was independently analyzed by flow cytometry and the ratio of cells loaded with antigen (Ag⁺) to cells not loaded with antigen (Ag^–) was calculated and the percentage is indicated in each plot. (B) Percentage of specific lysis was determined as described in “Materials and methods,” using the analysis of triplicate wells to determine standard error for each condition. Data are representative of 2 independent experiments.