Recognition of the BHRF1 122-133 epitope by the CD4 T-cell clone 4.19 is HLA-DR*0401-restricted. (A) Clone 4.19 cells were cocultured with autologous B-LCL loaded with PYY peptide at 10 μM. Inhibition with anti-DR (L243), anti-DP (B7.21), or anti–class I (W6.32) antibodies was performed by addition of mAb at 3 different concentrations (0.5 to 50 μg/mL) during the coculture. Percentage specific lysis was determined in a standard chromium release assay at an effector-to-target ratio of 10:1. Recognition of the autologous B-LCL was blocked by an anti-DR Ab but not by anti-DP or anti–class I Ab. (B) Autologous and allogeneic DR4⁺ B-LCLs were precoated with peptide PYYVVDLSVRGM (10 μM final) and then exposed to clone 4.19. Clone 4.19 recognized efficiently DR*0401 and DR*0404 B-LCLs loaded with PYY. A DR*0406 B-LCL was a weakly recognized peptide but DR*0402, DR*0405, and DR*0407 B-LCLs were not.