The Role of Integrin α_Dβ₂ in Macrophage Migration.

Integrin α_Dβ₂ (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 α_Dβ₂ to monocyte/macrophage adhesive reactions and the significance of its overexpression on these cells remain unknown. Recently we characterized α_Dβ₂ as a multiligand receptor capable of binding many extracellular matrix proteins with the recognition specificity overlapping that of the major myeloid-specific integrin α_Mβ₂ (Mac-1). We hypothesized that the α_Dβ₂ 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 α_Dβ₂ in migration, we generated model and natural cells expressing different densities of α_Dβ₂ and tested their migration to different extracellular matrix proteins. In vitro studies demonstrated that α_Dβ₂ 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-α_D 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 α_Dβ₂ on the surface of HEK293 cells and its upregulation by PMA on IC-21 macrophages resulted in the inhibition of cell migration. Ligation of α_Dβ₂ with anti-α_D antibodies restored β₁- and β₃-driven cell migration by means of removing restraints imposed by the excess of the α_Dβ₂-ligand adhesive bonds. To test the possibility that progressive upregulation of α_Dβ₂ can block macrophage migration in vivo, we assessed the effect of anti-α_D function blocking antibodies using the thioglycollate-induced peritonitis model. More than 4-fold upregulation of α_Dβ₂ 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-α_D antibodies. These results demonstrate that the density of α_Dβ₂ can modulate cell migration and suggest that low levels of α_Dβ₂ can contribute to monocyte migration while α_Dβ₂ upregulation on differentiated macrophages might facilitate their retention at the site of inflammation.