TTP plasma activates procaspase-8 in dermal MVECs. Dermal MVECs were exposed to 2% plasma from 2 healthy donors and 5 patients with acute TTP for 2 hours, followed by cell lysis and assessment of caspase-8 activity by a colorimetric assay. Caspase-8 activity, expressed as percent over baseline optical density values, was increased significantly by TTP versus normal plasma (P < .009).