Hydrogen peroxide activation of endothelial cell-associated MMPs during VCAM-1-dependent leukocyte migration.

Leukocyte migration from the blood into tissues is vital for immune surveillance and inflammation. Specificity for the site of leukocyte migration is determined by the combination and concentration of adhesion molecules, cytokines and chemokines in the microenvironment. Leukocytes bound at sites of extravasation migrate within minutes. We have focused on the function of the adhesion molecule VCAM-1 and have reported an active function for the endothelium during VCAM- 1-dependent leukocyte migration. VCAM-1 activates endothelial cell NADPH oxidase followed by the generation of 1 microM H2O2. This stimulates endothelial cell-associated matrix metalloproteinase (MMP) activity in minutes, consistent with the time for lymphocyte migration. The endothelial cell NADPH oxidase and endothelial cell MMP activities are required for VCAM-1-dependent lymphocyte migration as determined by scavenging of ROS, by pharmacologic or antisense inhibition of NADPH oxidase and by pharmacologic inhibition of endothelial cell MMPs. Furthermore, antioxidants block VCAM-1 activation of MMPs. In vivo, administration of the antioxidant bilirubin blocks VCAM-1-dependent leukocyte migration into the lung in experimental asthma. In summary, endothelial cells are not simply a scaffold for leukocyte adhesion. Instead, endothelial cells have an active function during VCAM-1-dependent leukocyte transendothelial migration.