CTL-mediated induction of apoptosis in B-CLL is contact dependent. Fresh or frozen 2.5 × 10⁶ B-CLL cells obtained from patients in stage IV were cultured alone (□) or with 5 × 10⁵ allogeneic anti-third-party CTLs. The cocultures were incubated in regular (▪) or in transwell plates (▧). Three days later, different wells were sampled and stained with the corresponding fluorescent Abs as described in “Materials and methods.” Subsequently, samples were analyzed by FACS for annexin staining. Values are expressed as annexin-positive cells from total CD19⁺ cells (n = 6). Error bars indicate SD.