Vpr-mediated up-regulation of NKG2D ligands in target cells promotes NK cell–mediated killing. (A) CEM.NKR T cells were transduced with lentiviral vectors WPI or WPI-VprWT and then exposed to primary NK cells in a 4-hour ⁵¹Cr release assay 48 hours after transduction, in the presence of soluble NKG2D-IgG Fc fusion proteins or matched-IgG Fc fusion molecules, as indicated. (B) CEM.NKR T cells were transduced with lentiviral vectors WPI-VprWT or WPI-VprR80A and then assessed for cell lysis by primary NK cells in a ⁵¹Cr release assay 48 hours after transduction. (C) CEM.NKR T cells were treated or not with APC (4μM) and analyzed, as in panel A, 24 hours after treatment. Primary NK cells used in panels A and C were isolated from the same donor. Error bars represent SEM. (D) Cell-surface expression of ULBP-2 was monitored on GFP⁺ and GFP⁻ CEM.NKR T cells 48 hours after transduction with lentiviral vectors expressing GFP alone (WPI) or coexpressing GFP and VprWT (WPI-VprWT) as indicated. MFI values were calculated by subtracting the corresponding isotype control values (dashed line). Results shown are representative of the data obtained from at least 2 independent experiments.