NK cell cytotoxicity assay
NK cell cytotoxicity assay was performed as previously described (16, 17). The peripheral blood mononuclear cells were used as the effector cells and were isolated by density gradient centrifugation using Ficoll-Paque Lymphoprep (Axis-Shield PoC As, Oslo, Norway). K562 leukemia cells were used as the target cell and were stained with DIO after washing in phosphate-buffered saline (PBS). The two types of cells were co-cultured in 96-well Costar plates (Corning Incorporated, Corning, NY, USA) at a effector:target ratio of 12.5:1, 25:1, 50:1, and incubated at 37 °C and 5% CO2 for 4 h. PI was added into each well, NK cell cytotoxicity was analyzed by flow cytometry and the death rate of K562 cells was calculated.