CD178/TRAIL-induced apoptosis is dependent upon caspase activation and CD95 triggering. CLL B cells were cocultured for 24 hours with HeLa cells expressing CD154 as described in “Materials and methods.” After CD40 activation the CLL cells (n = 3) were cocultured with CHO effector cells expressing CD178, TRAIL, or both CD178 and TRAIL, as indicated on the left side. Thirty minutes before the initiation of each coculture, the various CHO–effector-cell populations were incubated with blocking mAbs specific for CD178 or TRAIL, or both blocking mAbs for CD178 and TRAIL, as indicated. In some cultures, we added zVAD to the CLL cells 30 minutes before the initiation of the coculture, as indicated on the left. A 100% value to the level of apoptosis achieved using CHO-CD178/TRAIL effector cells was used to compare the levels of CLL cell killing with that observed using other CHO effector cells and/or other culture conditions. Apoptosis induced by the other treatment was calculated relative as (percent apoptosis of treatment divided by percent apoptosis of CHO-CD178/TRAIL × 100). The error bars indicate the standard deviation of the mean.