Promotion of Leukocyte Adhesion by a Novel Interaction Between Vitronectin and the β2Integrin Mac-1 (αMβ2, CD11b/CD18)

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Abstract
Objective—The leukocyte integrin Mac-1 (αMβ2, CD11b/CD18) binds a number of ligands and counter-receptors and thereby is a major determinant in regulation of leukocyte adhesion and extravasation. Vitronectin (VN) is an adhesion-promoting factor that is abundantly present as matrix molecule in vascular diseases such as atherosclerosis. Until now, only an indirect interaction between Mac-1 and VN via the urokinase receptor (urokinase plasminogen activator receptor) was known. We now propose that Mac-1 and VN can directly interact with each other.Methods and Results—In an in vitro system with purified components, Mac-1 specifically bound the multimeric matrix form of VN but not the monomeric plasma form. Using various competitors, the interaction domains in Mac-1 and VN were localized. Mac-1–expressing but not untransfected Chinese hamster ovary cells adhered strongly on VN. Introduction of a GFFKR deletion in the αMsubunit of Mac-1, which increases the constitutive activation of the integrin, led to increased adhesion on VN. Peripheral human blood neutrophils adhered and migrated on multimeric VN in a Mac-1–dependent manner.Conclusions—These results show that there is a specific integrin-affinity–regulated interaction between Mac-1 and the matrix form but not the plasma form of VN that may significantly participate in leukocyte adhesion and extravasation.