Poly I:C–induced augmentation of NK cytotoxic activity is time- and protein synthesis–dependent. (A-C) Highly purified short-term cultured NK cells (3 × 10⁶/mL [A]) and NK92 (0.2 × 10⁶/mL [B]) and NKL (0.4 × 10⁶/mL [C]) cell lines were left untreated (NT, dashed line) or stimulated with 100 μg/mL poly I:C (solid line) for 24 hours and then subjected to cytotoxicity assay against K562 and 721.221 target cells; P815 target cell lysis was assayed in the presence (•) or absence of anti-CD16 (B73.1) mAb (A). The data shown are representative of at least 5 independent experiments. (D-E) Cytotoxic activity against K562 target cells was tested in a ⁵¹Cr release assay. Highly purified polyclonal NK cells (left) and the NK92 cell line (right) were left untreated (dashed line) or stimulated with poly I:C (100 μg/mL) (open symbols) for different times (D); highly purified human polyclonal short-term cultured NK cells were stimulated with poly I:C (closed symbols) or left untreated (open symbols) for 24 hours in the presence (triangles) or not (circles) of cycloheximide (CHX, 10 μg/mL) (E). These results are representative of 1 out of 2 independent experiments.