Caspase-8 does not associate with death receptors in cells undergoing macrophagic differentiation. U937 cells were exposed to 20 nM TPA for indicated times (hours) before lysis. These lysates were used for immunoblotting before (Lysates) or after (IP:casp-8) immunoprecipitation with an anti–caspase-8 antibody. As positive controls, U937 cells were treated with 500 ng/mL TRAIL for 30 minutes (A) or 100 ng/mL CH11 Fas antibody plus 0.8 μg/mL CHX for 30 minutes (B) or 500 ng/mL of TNF-α for 3 hours (C). Molecular weights are in kilodaltons. The asterisk indicates cleavage products. Beads are negative controls without antibody for immunoprecipitation. One representative experiment is shown.