Integrin alpha(4)beta(7) mediates human eosinophil interaction with MAdCAM-1, VCAM-1 and fibronectin

We have investigated the contribution of integrin alpha(4) beta(7) to human peripheral blood eosinophil adhesive interactions. Immunofluorescence and flow cytometry demonstrated constitutive expression of alpha(4) beta(7) by eosinophils. Expression of alpha(4) beta(1) or alpha(4) beta(7) was not enhanced by eosinophil activation with platelet-activating factor (PAF). Expression of alpha(4) beta(7) was confirmed by immunoprecipitation of I-125-labelled lysates analysed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). Approximately 20% of unstimulated eosinophils were adherent to L1-2 cells transfected with vascular cell adhesion molecule-1 (VCAM-1) cDNA, while very few resting eosinophils adhered to mouse mucosal adressin cell adhesion molecule-1 (MAdCAM-1) transfectants. Binding of unstimulated eosinophils to VCAM-1 transfectants was inhibited by HP1/2 (an antibody that blocks both alpha(4) beta(1) and alpha(4) beta(7) functions), but not Act-1, an alpha(4) beta(7) monoclonal antibody (mAb). PAF stimulation resulted in increased binding of eosinophils to MAdCAM-1 transfectants, which was inhibited by both HP1/2 and Act-1. In contrast, PAF did not enhance binding to VCAM-1 transfectants, although binding of PAF-stimulated eosinophils to VCAM-1 could be partially inhibited by Act-1. Stimulation of eosinophils with the beta(1)-activating mAb TS2/16 resulted in enhanced binding of eosinophils to both VCAM-1 and MAdCAM-1 transfectants. The increased binding was largely alpha 4 beta(7)-dependent. Unstimulated eosinophils bound to soluble recombinant human (rh)VCAM-1 and fibronectin (Fn), coated on 96-well plates in a dose-dependent manner. Binding was inhibited by HP1/2 and 4b4, an anti-beta(1) mAb, but not by Act-1. TS2/16 treatment increased adherent cell numbers and this enhanced binding was inhibited by Act-1. We have therefore confirmed that alpha(4) beta(1) is functionally active on unstimulated eosinophils. In contrast, PAF-induced enhancement of eosinophils binding to VCAM-1 or MAdCAM-1 was alpha(4) beta(7)-dependent. In addition, treatment with TS2/16 resulted in a alpha(4) beta(7)-dependent enhancement of eosinophil binding to VCAM-1, MAdCAM-1 and Fn. We therefore hypothesize that alpha(4) beta(7) may have an important role in eosinophil localization in diseases such as asthma and inflammatory bowel disease.