VLA-4 (α₄β₁) engagement defines a novel activation pathway for β₂ integrin-dependent leukocyte adhesion involving the urokinase receptor

During acute inflammatory processes, β₂ and β₁ integrins sequentially mediate leukocyte recruitment into extravascular tissues. We studied the influence of VLA-4 (very late antigen-4) (α₄β₁) engagement on β₂ integrin activation-dependent cell-to-cell adhesion. Ligation of VLA-4 by the soluble chimera fusion product vascular cell adhesion molecule-1 (VCAM-1)-Fc or by 2 anti-CD29 (β₁ chain) monoclonal antibodies (mAb) rapidly induced adhesion of myelomonocytic cells (HL60, U937) to human umbilical vein endothelial cells (HUVECs). Cell adhesion was mediated via β₂ integrin (LFA-1 and Mac-1) activation: induced adhesion to HUVECs was inhibited by blocking mAbs anti-CD18 (70%-90%), anti-CD11a (50%-60%), or anti-CD11b (60%-70%). Adhesion to immobilized ligands of β₂ integrins (intercellular adhesion molecule-1 [ICAM-1], fibrinogen, keyhole limpet hemocyanin) as well as to ICAM-1-transfected Chinese hamster ovary cells, but not to ligands of β₁ integrins (VCAM-1, fibronectin, laminin and collagen), was augmented. VCAM-1-Fc binding provoked the expression of the activation- dependent epitope CBRM1/5 of Mac-1 on leukocytes. Clustering of VLA-4 through dimeric VCAM-1-Fc was required for β₂ integrin activation and induction of cell adhesion, whereas monovalent VCAM-1 or Fab fragments of anti-β₁ integrin mAb were ineffective. Activation of β₂ integrins by α₄β₁ integrin ligation (VCAM-1-Fc or anti-β₁ mAb) required the presence of urokinase receptor (uPAR) on leukocytic cells, because the removal of uPAR from the cell surface by phosphatidylinositol-specific phospholipase C reduced cell adhesion to less than 40%. Adhesion was reconstituted when soluble recombinant uPAR was allowed to reassociate with the cells. Finally, VLA-4 engagement by VCAM-1-Fc or anti-β₁ integrin mAb induced uPAR- dependent adhesion to immobilized vitronectin as well. These results elucidate a novel activation pathway of β₂ integrin-dependent cell-to-cell adhesion that requires α₄β₁ integrin ligation for initiation and uPAR as activation transducer. (C) 2000 by The American Society of Hematology.