Surface density expression of the (A) TNF-αR p80 and (B) TNF-αR p60 on fresh human NK cells by flow cytometry. Shaded area in each histogram represents isotype control antibody binding. Experiments were conducted with fresh NK cells from a single donor and are representative of two separate experiments. (C) Requirement for components of the IL-2R and IL-12R activation for the induction of NK cell apoptosis. Purified NK cells were cultured for 3 days in the presence of indicated cytokines at the following concentrations: IL-15 (15 ng/mL), TNF-α (5,000 U/mL), IFN-γ (10 ug/mL), and IL-12 (3 ng/mL). Results are expressed as the percent of cells undergoing apoptosis as determined by nuclear PI staining and flow cytometry. Standard deviation of the mean from duplicate wells was less than 2%. Results are representative of three individual experiments.