Influence of platelet factor 4 on thrombomodulin-dependent APC generation by cultured endothelial monolayers. Confluent endothelial monolayers derived from human umbilical vein (A), human dermal microvasculature (B), and human peripheral blood progenitors (C) were incubated for 2 hours in the presence of 10 μg/mL protein C, 1 nM thrombin, and the indicated concentration of PF4. APC generation was quenched by addition of hirudin and aliquots of supernatant buffer assayed for APC using the chromogenic substrate S-2366. Each curve within the designated cell type represents a separate experiment done on a separate primary culture preparation using the designated cell type.