CEM-C7H2 T-ALL cells are killed by B-CLL cells through the Fas/FasL pathway. (A) T-ALL cells carrying the relevant vector control (VC) were incubated with 10 μCi/mL [³H]-thymidine for 16 hours and cocultivated with the indicated B-CLL cells EHEB (□). For blocking experiments, crmA-expressing CEM-C7H2 cells (CRM) were used as target cells. Alternatively, the labeled T-cells (VC) were coincubated with 0.25 μg/mL of the FasR blocking ZB4 MoAb. In a control experiment the FasR– neoplastic B-cell line U266 was used as a target (▨). Cocultivation of cells was performed for 72 hours at 37°C. The reduction of DNA-incorporated radioactivity was used to calculate the percentages of B-CLL–mediated target cell death (n = 12) and is presented in a box-blot and whiskers model. Statistical analysis of the blocking experiments showed the following: specific killing by EHEB: 30% mean inhibition by ZB4 (P < .0001), 66% mean inhibition by crmA expression (P < .0001). (B) CEM-C7H2 T-ALL cells are eliminated by CD19⁺ cells of B-CLL patients through the Fas/FasL pathway. PBMC of 5 B-CLL patients were depleted of T cells by magnetic cell separation. The resulting CD19⁺ B-cell fractions (purity > 98%) were cocultivated with [³H]-thymidine–prelabeled CEM-C7H2 target cells for 72 hours at 37°C in a effector/target ratio of 10:1 (four replicates). The reduction of DNA-incorporated radioactivity in the target cells was used to calculate specific target cell killing by CD19⁺B-CLL cells, which was blocked by preincubation with the antagonistic anti-FasR MoAb ZB4 (0.25 mg/mL, ▧) or the inhibitory anti-FasL MoAb NOK-2 (0.75 mg/mL, ▨). Results of the statistical analysis: 44% mean inhibition by ZB4 MoAb (.0001 < P < .002), 74% mean inhibition by NOK-2 MoAb (P < .0001).