Regulation of α₄ integrin-mediated adhesion of human eosinophils to fibronectin and vascular cell adhesion molecule-1

Eosinophils selectively accumulate at sites of allergic inflammation. Their recruitment is dependent on both the expression and functional activity of cell adhesion molecules. How the functional activity of cell adhesion molecules on eosinophils is regulated is poorly understood. Objective: Our objective was to examine the functional activity of α₄ integrins on human eosinophils and its regulation by various agents. Methods: Function of α₄ integrins on human eosinophils was examined by testing adhesion to immobilized fibronectin and vascular cell adhesion molecule-1 (VCAM-1) in the presence or absence of a monoclonal antibody (mAb) (8A2) that activates β₁ integrin functions. Results: Spontaneous eosinophil adhesion to VCAM-1 was enhanced by 8A2, but adhesion to fibronectin could only be detected in the presence of 8A2. Concentrations of 8A2 that were approximately 100-fold less than saturating induced maximal eosinophil adhesion. Adhesion to VCAM-1 in the presence of 8A2 was effectively inhibited by α₄ and β₁ integrin mAbs; β₇ mAb had partial inhibitory activity. Connecting segment-1 peptide and α₄ mAb blocked 8A2-dependent fibronectin binding; β₁, β₂, and β₇ integrin mAbs had partial inhibitory activity. Eosinophils obtained from bronchoalveolar lavage fluids and blood eosinophils stimulated with IL-5, platelet-activating factor, or RANTES displayed increased β₂ integrin- dependent, not α₄ integrin-dependent, attachment. Spontaneous adhesion of eosinophils to VCAM-1 was significantly reduced by the tyrosine kinase inhibitor tyrphostin B46 (inhibitory concentration of 50% α 20 μmol/L); this effect was reversed by 8A2. Conclusions: The functional activity of integrins on eosinophils can be positively and negatively regulated. Altered integrin avidity may influence eosinophil recruitment in vivo.