IL-18–conditioned NK cells exert a helper function during MLR. MLR were carried out with DC and allogeneic naive CD4⁺ T cells in the absence or in the presence of NK cells during the MLR. (A) IFN-γ was measured in MLR supernatants by ELISA. □: DC with T cells; ■: DC with T and NK cells. (B) Viable IFN-γ–secreting cells were detected by FACS after 5 days of coculture. The mean fluorescence intensity is stated on the upper left corner. The percentage of IFN-γ⁺ cells is shown in parentheses.