Minimal requirements for receptor-ligand interactions for secretion of cytokines and chemokines by NK cells. Resting NK cells were mixed with S2 cells expressing ligands for NK-cell receptors, as indicated, and incubated for 6 hours at 37°C. For some stimulations, S2 cells were preincubated with diluted anti–S2 cell serum (+ IgG). Supernatants were harvested, and the concentrations of cytokines and chemokines were determined by a multiplex immunoassay. Values represent mean ± SD of 5 different donors. Data are representative of 3 independent experiments. For clarity, selected statistical analyses are indicated. *P < .05, ***P < .001.