Subsecond induction of alpha4 integrin clustering by immobilized chemokines stimulates leukocyte tethering and rolling on endothelial vascular cell adhesion molecule 1 under flow conditions.

Leukocyte recruitment to target tissue is initiated by weak rolling attachments to vessel wall ligands followed by firm integrin-dependent arrest triggered by endothelial chemokines. We show here that immobilized chemokines can augment not only arrest but also earlier integrin-mediated capture (tethering) of lymphocytes on inflamed endothelium. Furthermore, when presented in juxtaposition to vascular cell adhesion molecule 1 (VCAM-1), the endothelial ligand for the integrin very late antigen 4 (VLA-4, alpha4beta1), chemokines rapidly augment reversible lymphocyte tethering and rolling adhesions on VCAM-1. Chemokines potentiate VLA-4 tethering within <0.1 s of contact through Gi protein signaling, the fastest inside-out integrin signaling events reported to date. Although VLA-4 affinity is not altered upon chemokine signaling, subsecond VLA-4 clustering at the leukocyte-substrate contact zone results in enhanced leukocyte avidity to VCAM-1. Endothelial chemokines thus regulate all steps in adhesive cascades that control leukocyte recruitment at specific vascular beds.