Sequential analysis of spontaneous apoptosis of EBV-infected NK leukemia cells in vitro. NK leukemia cells (NKL; •) from cases no. 1 through 7 and normal NK cells from 6 normal controls (□) were cultured in vitro, and percentages of viable cells (A) and percentages of apoptotic cells (B) were sequentially determined on a trypan blue dye exclusion test and on morphology under an microscope, respectively. The NK leukemia cells underwent apoptotic cell death immediately after initiation of the culture. Note that apoptotic cell death reached a plateau approximately 48 hours after initiation of the culture. The addition of IFN-γ (100 U/mL) to the culture significantly inhibited the progression of spontaneous apoptosis of NK leukemia cells on both tests (NKL+IFNγ; ○). Data are shown as the mean ± SD (error bars).