Mechanisms of eosinophil adhesion to endothelial cells under flow conditions

Abstract

Eosinophils play an important role in allergic inflammatory diseases such as allergic asthma. Infiltrates of these cells are present in the interstitium and the lumen of the bronchi of asthmatic patients. Eosinophils must pass the endothelium to enter this site of inflammation. A widely accepted paradigm for leukocyte extravasation is the multistep model. In this model selectins mediate rolling interactions between leukocytes and endothelium and subsequently, activated integrins facilitate firm adhesion and extravasation of the cells. The interactions between eosinophils and activated endothelial cells have been studied under flow conditions. This has been done in a flow chamber model in which a blood vessel is simulated.

In chapter 1 (general introduction) the multistep model of extravasation is explained. Furthermore, eosinophils function and their role in allergic asthma has been described. In chapter 2 data are presented on the function of alpha 4 integrins and E-selectin in the initial attachment of eosinophils to TNF-? activated endothelial cells. In chapter 3 data are presented on the differences in adhesion behavior between eosinophils of healthy blood donors and of allergic asthmatic patients on activated endothelial cells under flow conditions. Remarkably, eosinophils of allergic asthmatic patients bound platelets in contrast to cells of healthy controls. The platelet-eosinophil interactions resulted in increased adhesion to activated endothelial cells via a process that is called “ secondary tethering”. In chapter 4 data are presented on the role of IL-8 on eosinophil arrest. IL-8 is a typical neutrophil activator but is not known to be a eosinophil activator. We showed that IL-8 can induce 1) an alpha 4- and beta 2-integrin dependent arrest of rolling eosinophils on activated endothelial cells and 2) an increase in intracellular Ca2+ concentration of eosinophils binding to fibronectin and activated endothelial cells. Thus, IL-8 is not only a neutrophil activator but also an eosinophil activator. In chapter 5 data are presented on the role of phospholipase C (PLC) in the chemokine-induced integrin activation of eosinophils. Three chemokines, IL-8, eotaxin and C5a were tested on their ability to induce a PLC dependent arrest of eosinophils. Eotaxin mediated a totally PLC-dependent arrest whereas IL-8 and C5a only induced a partial PLC-dependent arrest. In resting eosinophils PLC is also important because inhibition of PLC decreased the functionality of alpha 4 integrins. In chapter 6 data are presented on the migration behavior of eosinophils. Migrating cells display a typical tear-drop shape while migrating. The front is a broad protrusion (lamellipodium) whereas the back of the cell is a small point-like structure (uropod). The small GTP-ase RhoA was found to play an important role in the regulation of the movement of the uropod. In chapter 7 (general discussion) the data of chapter 2-6 are discussed. Special emphasis was give to the validation of the model, the possible working mechanisms of PLC in integrin regulation, the role of platelets in the binding of eosinophils of allergic asthmatic patients to activated endothelial cells and possible target molecules for therapy.