Expression of P-selectin of platelets and HUVEC. To determine the degree of platelet activation through the contact with apoptotic HUVEC, platelets were tested for the expression of P-selectin. After incubation of the platelets with HUVEC for 20 minutes, the cells (whether adherent or unbound) were stained separately with anti–P-selectin MoAb (WAPS 12.2) followed by an FITC-conjugated anti-mouse MoAb. Cells were then assayed by flow cytometry to determine P-selectin expression. As controls, nonactivated and activated platelets and HUVEC were also measured separately. A representative experiment of three performed is shown.