VLA-4 (alpha(4)beta(1)) engagement defines a novel activation pathway for beta(2) integrin-dependent leukocyte adhesion involving the urokinase receptor

During acute inflammatory processes, beta(2) and beta(1) integrins sequenti ally mediate leukocyte recruitment into extravascular tissues. We studied t he influence of VLA-4 (very late antigen-4) (alpha(4)beta(1)) engagement on pp integrin activation-dependent cell-to-cell adhesion. Ligation of VLA-4 by the soluble chimera fusion product vascular cell adhesion molecule-1 (VC AM-1)-Fc or by 2 anti-CD29 (beta(1) chain) monoclonal antibodies (mAb) rapi dly induced adhesion of myelomonocytic cells (HL60, U937) to human umbilica l vein endothelial cells (HUVECs), Cell adhesion was mediated via beta(2) i ntegrin (LFA-1 and Mac-1) activation: induced adhesion to HUVECs was inhibi ted by blocking mAbs anti-CD18 (70%-90%), anti-CD11a (50%-60%), or anti-CD1 1b (60%-70%), Adhesion to immobilized ligands of beta(2) integrins (interce llular adhesion molecule-1 [ICAM-1], fibrinogen, keyhole limpet hemocyanin) as well as to ICAM-1-transfected Chinese hamster ovary cells, but not to l igands of beta(1) integrins (VCAM-1, fibronectin, laminin, and collagen), w as augmented, VCAM-1-Fc binding provoked the expression of the activation-d ependent epitope CBRM1/5 of Mac-1 on leukocytes, Clustering of VLA-4 throug h dimeric VCAM-1-Fc was required for beta(2) integrin activation and induct ion of cell adhesion, whereas monovalent VCAM-1 or Fab fragments of anti-be ta(1) integrin mAb were ineffective. Activation of beta(2) integrins by alp ha(4)beta(1) integrin ligation (VCAM-1-Fc or anti-beta(1) mAb) required the presence of urokinase receptor (uPAR) on leukocytic cells. because the rem oval of uPAR from the cell surface by phosphatidylinositol-specific phospho lipase C reduced cell adhesion to less than 40%, Adhesion was reconstituted when soluble recombinant uPAR was allowed to reassociate with the cells. F inally, VLA-4 engagement by VCAM-1-Fc or anti-beta(1) integrin mAb Induced uPAR-dependent adhesion to immobilized vitronectin as well, These results e lucidate a novel activation pathway of beta(2) integrin-dependent cell-to-c ell adhesion that requires alpha(4)beta(1) integrin ligation for initiation and uPAR as activation (C) 2000 by The American Society of Hematology.