NK-mediated lysis of MSCs. Results of cytotoxicity assays in which NK cells were used as effector cells against ⁵¹Cr-labeled MSCs at different E/T ratios. (A-C) NK cells derived from 3 different donors were tested as freshly isolated cells (▴) or after 20-hour (▪) or 7-day (□) culture in 100 U/mL IL-2 for cytolytic activity against 3 different allogeneic MSC populations. (D-F) Results of 3 independent experiments in which different MSC populations were used as targets while either autologous (squares) or allogeneic (triangles) IL-2-activated NK cells were used as effectors. Data are expressed as percentage of lysis.