Comparison of cytotoxic properties of NK cells expanded with PBMC, K562, and Jurkat feeder cells. Expanded NK cells were tested for cytotoxicity against K562, Jurkat, Hep3B, Raji, MCF-7, and Ramos. (A) Cytotoxic activity of K562-NK cells was measured according to culture duration. (B) Cancer cell lines (6×10⁴) and NK cells (1.8×10⁵) were added on to the plates and incubated for 4 hours at 37℃. NK cell cytotoxicity towards cancer cell lines was measured using LDH release assay. The data from four independent experiments performed in triplicate are expressed as mean±SD.