Perforin is required for specific cytotoxicity.

Representative perforin and CMV A2 tetramer dot plots and cytotoxicity assays against CMV A2-restricted peptide (NLVPMVATL)–loaded cells in samples enriched for tetramer-positive cells. In the top row, 25% of CMV tetramer-positive cells from HIV-seropositive donor 307 PBMCs stain for perforin, whereas in the bottom row only 2.5% of cells reacting with the same tetramer from healthy donor HD2 are perforin positive. On the right, cytotoxicity assays were performed after PBMCs were enriched for tetramer-positive cells (Tables 4-5). The ratio of tetramer-positive cells to target cells is shown on the x-axis of the cytotoxicity graphs. Only the sample with perforin-staining, tetramer-positive cells has significant cytotoxic activity against peptide-loaded cells (▪) compared with control cells (■).