Hepatic NK cell-induced apoptosis in CC531s cells as observed by fluorescence microscopy (A, C, E) and light microscopy (B, D, F). CC531s cells were coincubated with hepatic NK cells at an E:T ratio of 10:1 for 3 hours. Cells were stained with Hoechst 33342/propidium iodide. The thick arrows indicate apoptotic CC531s cells with fragmented nuclei. The small cells are hepatic NK cells (thin arrows indicate examples). (A, B), Coincubation of CC531s cells with hepatic NK cells in medium. (C, D), CC531s cells were pretreated with anti-MHC I mAb OX18. (E, F), CC531s cells were pretreated with anti-MHC I mAb OX18 and hepatic NK cells were pretreated with DCI. The number of apoptotic CC531s cells dramatically decreased. Bar = 10 μm. (G), The percentage of apoptotic CC531s cells was determined in preparations by counting at least 300 cells per sample. EGTA was present during the coincubation. *p < 0.05, **p < 0.01 vs. the corresponding control (LSD test).