NK cell–mediated cytolytic activity is selectively impaired on HCMV-induced down-regulation of NKG2D. Mock or TB40/E-treated PBMCs were harvested at 72 hours and incubated with different target cells. NK cell cytotoxic activity was assessed by CD107a expression in CD56⁺CD3⁻ cells as described in “Functional assays.” Results obtained with 3 different donors are shown.