A role for β 2 integrin (CD11/CD18)‐mediated tyrosine signaling in extravasation of human polymorphonuclear neutrophils

During the recruitment of human polymorphonuclear neutrophils (PMN) to sites of inflammation, leukocyte adhesion molecules of the β₂ integrin (CD11/CD18) family mediate firm adhesion of these cells to the endothelial cell monolayer lining the vessel wall. This process is a prerequisite for shape change and spreading of PMN on the endothelium which eventually allows PMN emigration into the extravascular space. In order to elucidate the molecular mechanisms which mediate this sequence of events, intracellular protein tyrosine signaling was studied subsequent to β₂ integrin‐mediated ligand binding. Using western blotting technique, β₂ integrin‐mediated adhesion was found to induce tyrosine phosphorylation of different proteins. The effect was absent in PMN derived from CD18‐deficient mice which lack any β₂ integrin expression on the cell surface demonstrating the specificity of the observed response. Inhibition of β₂ integrin‐mediated tyrosine signaling by herbimycin A almost completely inhibited adhesion, shape change, and subsequent spreading of PMN. Herbimycin A also diminished chemotactic migration of these cells in response to the soluble mediator N‐formyl‐Met–Leu–Phe (fMLP). In contrast, treatment of PMN with cytochalasin D had no substantial effect on β₂ integrin‐mediated signaling or adhesion but inhibited shape change, spreading, and chemotactic migration of PMN. This suggests that the signaling capacity exerted by β₂ integrins upon ligand binding was independent of an intact cytoskeleton. Moreover, the β₂ integrin‐mediated activation of intracellular signal transduction pathways was critical for firm adhesion of PMN, the prerequisite subsequent shape change and spreading, which allows emigration of PMN into the extravascular space.