Activation State of Α4β1 Integrin Controls Capture, Immobilization and Migration of Flowing Lymphocytes on Purified VCAM-1
Vascular cell adhesion molecule-1 (VCAM-1) is expressed on endothelial cells following cytokine stimulation, and can bind leukocyte α4β1 integrin. Adhesion of flowing human lymphocytes to VCAM-1 was analyzed through use of purified recombinant VCAM-1 immobilized onto plastic in a flow chamber. Integrin-mediated attachment supported adhesion of lymphocytes at physiological shear rates, but the nature of attachment was dependent on the concentration of VCAM-1 (10–100 μg/ml). At low concentration most adherent cells were rolling (~75%) but this decreased to <5% at 100 μg/ml VCAM-1. Activation of α4β1 integrin by Mn++ or monoclonal antibodies 12G10 and TS2/16 induced significant increases in lymphocyte adhesion only at low VCAM-1 concentration, and converted rolling adhesion to the stationary form. Treatment with phorbol ester had a similar effect, but also induced adherent lymphocytes to flatten and spread over the VCAM-1. These results suggest that the adhesive behavior of lymphocytes on VCAM-1, i.e. rolling adhesion vs. stationary adhesion and spreading, is dependent on the activation state of VLA-4, and that ligation of high concentrations of VCAM-1 is sufficient to induce activation of this integrin. Pretreatment of lymphocytes with the metabolic inhibitor sodium azide, or chelation of intracelluar Ca++ caused lymphocytes to remain rolling on 100 µg/ml VCAM-1, supporting the concept of ligand-induced, energy-dependent signaling. This implies that regulation of VCAM-1 concentration and integrin activation in vivo could control the nature of lymphocyte adhesion, with a co-expressed stimulus probably required for subsequent migration.