Differential domain-specific antibody immunoreactivities suggesting PECAM-1 cell surface shedding at the site of thrombosis. The left side of the panel shows trichrome stains of 2 cases of nonresolving human deep vein thrombus (specimen A-B, A1, and B1) and the vessel wall from a human saphenous vein (C1). These tissue samples were immunhistochemically stained using domain-specific antibodies directed against the extracellular domain (N-terminal) of PECAM-1 (ePECAM-1: A2, B2, and C2) and intracellular portion (C-terminal) of PECAM-1 (iPECAM-1: A3, B3, and C3). Cleaved PECAM-1 is characterized by the presence of immunoreactivity derived from the antibody against ePECAM-1 in the absence of immunoreactivity derived from the antibody against iPECAM-1. The area of discordant PECAM-1 expression (A2 vs A3; B2 vs B3) is highlighted with an asterisk (A3, B3). By contrast, the endothelial cell layer of saphenous vein wall stains positively with both antibodies, indicating full length PECAM-1 expression (C2-3, arrows). Scale bar, 100 μm.