Effect of microenvironmental factors on B-PAC-1–induced apoptosis in CLL lymphocytes. Freshly isolated primary CLL cells were incubated in suspension or on NKTert bone marrow stromal cells for 24 hours (n = 4; A, termed preincubation) or with IL-6 or IgM (n = 11; B) for 48 hours. Cells were then incubated with DMSO or with 5 or 10 µM B-PAC-1 for an additional 24 hours. Apoptosis was measured by Annexin V/PI staining assay and unstained cells were considered viable cells; time-matched DMSO-treated cells served as control.