Characterization of the EBV specific T-cell lines. (A) Phenotype of the EBV specific T-cell lines used for the treatment of EBV-LPD in patients who responded (closed symbols) and did not respond (open symbols) to the T-cell therapy. All EBV-CTL lines infused contained more than 90% of CD3⁺ cells with predominance of CD8⁺ T cells. However, 3 CTL lines contained predominantly CD4⁺ T cells. Infusion of the CD4⁺ T cell–predominant lines achieved CR in 2 of the 3 patients infused. All T-cell lines were equally depleted of NK and B cells. (B) Cytotoxic activity of the EBV-CTL lines used for the treatment of EBV-LPD of those patients who responded (closed symbols) and those who did not respond (open symbols) to the T-cell therapy. (C) Frequencies of EBV-specific T cells (black symbols) and alloreactive (gray symbols) detected by limiting dilution analysis in EBV-specific T-cell lines and DLI products before their use for the treatment of EBV-LPD demonstrate higher frequencies of EBV-specific T cells and lower frequencies of alloreactive T cells in the EBV-specific T-cell lines than in the unstimulated donor leukocytes. There were no differences in the frequencies of EBV-specific and alloreactive cells between the cell products used in responders and nonresponders. (D) All EBV-CTL lines infused exhibited exclusively EBV-specific cytotoxicity without any activity against recipient PHA-activated blasts, mismatched EBV-BLCLs, and K562 (a target for NK cells).