Endothelial activation enhances PF4 binding. HUVECs lining microfluidic wells were injured with hematoporphyrin and perfused with PF4^null murine blood supplemented with 10 or 100 µg/mL of hPF4 as indicated. (A) VWF exposure on the surface of endothelial cells showing the effects of the photochemical injury using binding of a labeled anti-VWF antibody. —, no injury; +++, photochemical injury. N = 4 separate experiments, each performed in duplicate; each experimental result is shown as well as the mean. P values were determined using analysis of variance comparing MFI of injured with noninjured endothelium. (B) Same as panel A, but for PF4 binding to endothelial cells detected by labeled KKO.