alpha(4)beta(1) integrin/VCAM-1 interaction activates alpha(L)beta(2) integrin-mediated adhesion to ICAM-1 in human T cells

Modulation of integrin affinity and/or avidity provides a regulatory mechan ism by which leukocyte adhesion to endothelium is strengthened or weakened at different stages of emigration. In this study, we demonstrate that bindi ng of high affinity alpha(4)beta(1) integrins to VCAM-1 strengthens alpha(L )beta(2) integrin-mediated adhesion. The strength of adhesion of Jurkat cel ls, a human leukemia T cell line, or MnCl(2-)treated peripheral blood T cel ls to immobilized chimeric human VCAM-1/Fc, ICAM-1/Fc, or both was quantifi ed using parallel plate how chamber leukocyte detachment assays in which sh ear stress was increased incrementally (0.5-30 dynes/cm(2)). The strength o f adhesion to VCAM-1 plus ICAM-1, or to a 40-kDa fragment of fibronectin co ntaining the CS-1 exon plus ICAM-1, was greater than the sum of adhesion to each molecule alone, Treatment of Jurkat or blood T cells with soluble cro ss-linked VCAM-1/Fc or HP2/1, a mAb to alpha(4), significantly increased ad hesion to ICAM-1. These treatments induced clustering of alpha(L)beta(2) in tegrins, but not the high-affinity beta(2) integrin epitope recognized by m Ab 24, Up-regulated adhesion to ICAM-1 was abolished by cytochalasin D, an inhibitor of cytoskeletal rearrangement. Taken together, our data suggest t hat the binding of VCAM-1 or fibronectin to alpha(4)beta(1) integrins initi ates a signaling pathway that increases beta(2) integrin avidity but not af finity. A role for the cytoskeleton is implicated in this process.