Platelets adhering to the vascular wall mediate postischemic leukocyte-endothelial cell interactions in retinal microcirculation.

PURPOSE Recent evidence suggests that platelets play a major role in ischemia-reperfusion injury, not only through thrombus formation but also through participation in inflammatory reactions with leukocytes. This study was designed to investigate the contribution of platelets in leukocyte recruitment to inflamed regions in vivo. METHODS Thrombocytopenia was produced in male Long-Evans rats by intravenous injection of anti-platelet serum at 4 hours before ischemia-reperfusion. Leukocyte behavior in retinal microcirculation was evaluated with acridine orange digital fluorography. Expression of P-selectin in the postischemia retina was investigated by reverse transcription polymerase chain reaction (RT-PCR) and immunohistochemistry. After 14 days of reperfusion, ischemia-induced retinal damage was evaluated histologically. RESULTS Leukocyte rolling along major retinal veins of thrombocytopenic rats was dramatically suppressed, and subsequent leukocyte accumulation in the postischemia retina was also significantly reduced (72.3%; P < 0.001) at 24 hours after reperfusion. Although RT-PCR revealed no significant reduction of P-selectin mRNA in platelet-depleted rat retina after transient ischemia, immunohistologic examination showed suppression of P-selectin expression on the vascular wall. Another immunologic examination using anti-platelet antibody detected adherent platelets, which can also express P-selectin on their surfaces, on postischemic vascular endothelium in vehicle-treated retina. Moreover, blockage of platelet glycoprotein IIb/IIIa resulted in substantial inhibition of leukocyte rolling. In addition, histologic examination showed the participation of platelets in retinal ischemia-reperfusion injury. CONCLUSIONS This study demonstrated that the expression of P-selectin on platelets may contribute to the recruitment of leukocytes to tissues after ischemia.