Abstract
Integrin αDβ2 (CD11d/CD18), the most recently discovered member of the β2 sub-family of adhesion receptors, is strongly upregulated on macrophage foam cells which underscores its potential role in atherosclerosis. However, the contribution of αDβ2 to monocyte/macrophage adhesive reactions and the significance of its overexpression on these cells remain unknown. Recently we characterized αDβ2 as a multiligand receptor capable of binding many extracellular matrix proteins with the recognition specificity overlapping that of the major myeloid-specific integrin αMβ2 (Mac-1). We hypothesized that the αDβ2 ability to bind numerous ligands in the extracellular matrix and its capacity to be upregulated to high density on the surface of macrophages may modulate cell adhesiveness and, thus, affect migration. To evaluate the role of αDβ2 in migration, we generated model and natural cells expressing different densities of αDβ2 and tested their migration to different extracellular matrix proteins. In vitro studies demonstrated that αDβ2 expressed at low densities, either on the surface of HEK293 cells or the mouse macrophage cell line IC-21, supported migration which was partially inhibited by anti-αD function-blocking antibodies. Furthermore, β1 and β3 integrins expressed on HEK293 cells and IC-21 macrophages, respectively, contributed to migration because anti-β1 and anti-β3 antibodies inhibited migration. Increased expression of αDβ2 on the surface of HEK293 cells and its upregulation by PMA on IC-21 macrophages resulted in the inhibition of cell migration. Ligation of αDβ2 with anti-αD antibodies restored β1- and β3-driven cell migration by means of removing restraints imposed by the excess of the αDβ2-ligand adhesive bonds. To test the possibility that progressive upregulation of αDβ2 can block macrophage migration in vivo, we assessed the effect of anti-αD function blocking antibodies using the thioglycollate-induced peritonitis model. More than 4-fold upregulation of αDβ2 was detected on macrophages in 72 h after thioglycollate stimulation and, similar to in vitro studies, the numbers of migration macrophages increased in the presence of anti-αD antibodies. These results demonstrate that the density of αDβ2 can modulate cell migration and suggest that low levels of αDβ2 can contribute to monocyte migration while αDβ2 upregulation on differentiated macrophages might facilitate their retention at the site of inflammation.
Exploring the role of extracellular matrix proteins to develop biomarkers of plaque vulnerability and outcome
