Impairment of cytolytic activity of fresh and IL-2–stimulated NK cells in HPS2 patients. Defective cytolytic activity of HPS2 patients against NK-susceptible target cells. Freshly isolated PBMCs derived from Pt 1 (•) or Pt 2 (▪) were tested against the K562 target cells either before (A) or after overnight incubation in the presence of rIL-2 (B) and compared with a representative healthy donor (▴) at different E/T ratios. Purified polyclonal rIL-2–activated NK cells, derived from Pt 1 (•) or Pt 2 (▪) or a healthy donor (▴), were tested against the NK-susceptible LCL 721.221 target cells at different E/T ratios (C). These experiments are representative of 6 independent experiments. Defective killing of autologous or heterologous PHA blasts by HPS2 patients' NK cells. The cytolytic activity of purified polyclonal rIL-2–activated NK cells was evaluated against autologous or heterologous PHA blasts derived from Pt 1 (D) or Pt 2 (E) or a representative healthy donor (F) at an E/T ratio of 25:1, either in the absence (□) or in the presence (▪) of anti-HLA class I mAb. These experiments are representative of 10 independent experiments. Data indicate the mean ± SD of 3 replicates.