CMV-specific CTLs activated on AAPCs kill CMV-infected fibroblasts and recognize endogenously processed pp65. (A) Immunofluorescence staining for pp65 of uninfected control A2.1⁺ MRC5 fibroblasts (MRC5, left panel) or MRC5 infected with the CMV strain AD169 (MRC5/CMV, middle panel). Original magnification, × 40. More than 90% of MRC5/CMV stain positive for pp65, indicated by the green fluorescence. Staining was done immediately prior to cytotoxicity assays. Right panel shows cytotoxicity assays performed against MRC5 fibroblasts. CTLs activated on AAPC^A2pp65 specifically lyse MRC5/CMV but not uninfected MRC5. ♦ indicates uninfected MRC5; ▪, MRC5 infected with CMV. (B) pp65 immunofluorescence staining in A2.1⁺ BLCLs transduced with the irrelevant flu peptide (BLCL^flu, left panel) or with pp65 (BLCL^pp65, middle panel). Original magnification, × 20. More than 95% of the BLCL^pp65 stain positive. Right panel shows cytotoxicity assays performed against BLCLs shown on the left. CTLs specifically lyse BLC^pp65 but not BLCL^flu. ♦ indicates BLCL^flu; ▪, BLCL^pp65. Similar results were obtained for 3 different donors. A representative experiment is shown. Values represent average of triplicates.