APICAL TOPOGRAPHY AND MODULATION OF ICAM-1 EXPRESSION ON ACTIVATED ENDOTHELIUM

Leukocyte-endothelium interactions and general inflammatory responses are contributed by the regulated expression of intercellular adhesion molecule-1 (ICAM-1) on endothelium. It is now shown by confocal fluore scence microscopy and immunogold transmission electron microscoscopy t hat ICAM-1 was exclusively localized on the apical (luminal) membrane of cytokine-activated human umbilical vein endothelial cells, In contr ast, other cell adhesion-promoting molecules, including beta(1) integr ins, were expressed exclusively on the basolateral endothelial cell me mbrane, under the same experimental conditions. Kinetic binding studie s of a I-125-labeled monoclonal antibody to ICAM-1 revealed that simil ar to 8% of membrane ICAM-1 on cytokine-activated endothelium was inte rnalized in both coated and noncoated vesicles at 37 degrees C, with a t(1/2) of similar to 18 min and a rate of similar to 3200 molecules/m inute. This internalization pathway was directly dependent upon the le vel of ICAM-1 expression on the cell surface. Genetically engineered I CAM-1 transfectants, expressing a 10-fold higher receptor density than activated endothelium, internalized similar to 18% of membrane ICAM-1 at a rate of 75,000 molecules/minute with a t(1/2) of similar to 22 m in. These findings suggest that a combined pathway of polarized membra ne topography and receptor trafficking may regulate ICAM-1-dependent a dhesion at the site of vascular injury and endothelial cell activation .