We investigated the effect of cyclosporine (CyA) on leukocyte adhesion to endothelium under flow conditions. Confluent human umbilical vein endothelial cells (HUVECs) were incubated for 24 hours with CyA (1, 5, and 10 micromol/L) and then exposed to a total human leukocyte suspension in a parallel plate flow chamber under laminar flow (1.5 dynes/cm2. Human umbilical vein endothelial cells stimulated with interleukin-1beta (20 U/mL) were used as a positive control. Adherent cells were measured by digital image analysis. Results showed that CyA dose-dependently increased the number of leukocytes adhering to HUVECs compared with control cells. Leukocyte adhesion markedly increased on HUVECs incubated with interleukin-lbeta, one of the most potent inducers of endothelial cell adhesiveness. Exposure of endothelial cells to CyA did not affect the number of rolling leukocytes, which was similar to control values. To examine the role of adhesion molecules in CyA-induced leukocyte adhesion, HUVECs were incubated with monoclonal antibodies against intercellular adhesion molecule-1 (ICAM-1), vascular adhesion molecule-1 (VCAM-1), and E-selectin before adhesion assay. Functional blocking of ICAM-1, VCAM-1, and E-selectin on endothelial cells significantly inhibited CyA (10 micromol/L)-induced leukocyte adhesion. Confocal fluorescence microscopy studies showed that CyA induced an increase in the endothelial surface expression of ICAM-1, VCAM-1, and E-selectin. Pretreatment of leukocytes with the platelet activating factor receptor antagonist L659,989 significantly reduced the number of leukocytes adhering to CyA-treated HUVECs. We suggest that CyA enhances leukocyte adhesion to endothelium by upregulating adhesive proteins on endothelial surface membrane. Blocking leukocyte receptor for platelet-activating factor partially prevents adhesion, suggesting a role for endothelial cell-associated platelet-activating factor in the interaction between leukocytes and CyA-treated endothelium.