Adherence of eosinophils to vascular endothelium and their accumulation at sites of allergen challenge are hallmarks of allergic inflammation. However, the molecular mechanisms mediating eosinophil adhesion under conditions of blood flow are not well understood. The present studies were performed to identify the receptors on human eosinophils involved in initiating adhesion to activated endothelium at physiologic shear rates in vivo. We have compared the relative contribution of L-selectin, VLA-4 (CD49d), and CD18 integrins in mediating eosinophil adhesion to microvascular endothelial cells in the rabbit mesentery by using intravital video microscopy. Eosinophils were found to roll in venules, but not arterioles, and this rolling could be stimulated by activation of endothelium with IL-1. In contrast to neutrophil rolling, which is predominantly L-selectin-dependent, eosinophil rolling was mediated by L-selectin, and also VLA-4. mAbs to L-selectin and VLA-4 alpha, but not CD18, significantly inhibited eosinophil rolling in vivo. The inhibition of VLA-4-mediated eosinophil rolling was not caused by modulation of eosinophil L-selectin or CD18 expression. This inhibition also was not caused by nonspecific inhibitory effect of the Abs studied, because the anti-VLA-4 mAbs inhibited eosinophil (VLA-4+) but not neutrophil (VLA-4-) rolling in the mesenteric venules. These results demonstrate that early events of eosinophil adhesion, i.e., rolling, are mediated by multiple adhesion receptors, including L-selectin and VLA-4, at physiologic shear rates in vivo.

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