Evidences of apoptotic cell death in EBV-infected NK leukemia cells. (A) Apoptotic NK leukemia cells 48 hours after the in vitro culture. Morphology of the apoptotic cells with May-Grünwald-Giemsa staining (original magnification × 1,000) (a) and with an electron microscopy (original magnification × 15,000) (b). (B) Fragmentation of DNA from NK leukemia cells (case no. 1) cultured in vitro. DNA ladder gradually became evident during the cultures in the absence of cytokines. The addition of either IFN-γ or IL-2 significantly inhibited the amounts of DNA fragmentation.