ULBP2 overexpression enhances NK cell–mediated cell lysis of 293T tumor cells. (A) Soluble FITC-labeled NKG2D receptor was used in flow cytometry analysis to detect ULBP2 on 293T cells that were transfected either with the expression vector p3.1 (filled) or with p3.1-ULBP2 (open). (B) The binding of antibodies recognizing NKG2D (marked with an arrow), CD56, or CD16 on primary NK cells was visualized with FITC-coupled goat anti–mouse IgG. Note that NK cells used for the cytotoxicity experiments expressed NKG2D and CD56 and lack CD16 expression. (C) 293T cells transfected with p3.1 (○) or p3.1-ULBP2 (•) were incubated with purified NK cells in different ratios for 2 hours and NK cell–mediated lysis of the target cells was determined in a europium release assay. The target cells were incubated with 10 μg/mL soluble NKG2D receptor to block NKG2D dependent lysis of p3.1-ULBP2 transfected target cells (▿). Average NK lysis and SD (n = 3) are indicated. The lysis increase of p3.1-ULBP2 overexpressing cells is significant in all effector-target ratios (P = .04) and was estimated with the paired t test using GraphPadPrism software.