Constitutive expression of IFN-γ in the EBV-infected NK leukemia cells. (A) Concentrations of IFN-γ in 24-hour culture supernatants of NK leukemia cells. The culture supernatants of NK leukemia cells (open circles 1 through 7) contained significant levels of IFN-γ, whereas those of normal NK cells did not (•). (B) Intracytoplasmic staining of IFN-γ in the NK leukemia cells on flow cytometric analysis. The cells were incubated with monensin (2 μmol/L) for 3 hours before the analysis. The majority of NK leukemia cells expressed intracytoplasmic IFN-γ protein. (C) Concentrations of IFN-γ in patients’ sera. Significant levels of serum IFN-γ were seen in all NK leukemia patients (○), whereas in normal controls (•), levels of IFN-γ were below detectable levels (<0.1 U/mL).