The potency of the alloreactivity exerted by virus-specific T cells. (A) To investigate the allo-HLA–reactive cytotoxic capacity of virus-specific T cells, 6 virus-specific cell clones were tested in cytotoxicity assays against 2 EBV-LCLs expressing the recognized allo-HLA molecules and 2 EBV-LCLs negative for the allo-HLA molecules. EBV-LCLs expressing the virus-specific restriction molecule were loaded with the viral peptide and used as positive control for cytotoxicity. (B) To compare the affinities of the allo-HLA–reactive response and the virus-specific response, the allo–HLA-A30/A31–reactive EBV-EBNA3A/HLA-A3–specific clone 19 was tested against the HLA-A*0301⁺ EBV-LCL AST transduced with a retrovirus encoding for EBNA3A and against 2 HLA-A*3101⁺ EBV-LCLs GGT and DSP. To compare the kinetics of the 2 responses, the clone was tested against the EBV-LCLs in different effector-stimulator ratios. (C) To extrapolate the results obtained with the EBV-LCLs and K562 cells to the recognition of normal cell subsets in vivo, we tested virus-specific T-cell clones against allo-HLA–expressing B cells, CD40 ligand–activated B cells (B APC), T cells, PHA blasts, monocytes, monocyte-derived DCs, and fibroblasts with and without IFNγ pretreatment. Experiments were performed in duplicate, mean values are shown ± SD.