AbstractThe state of integrin activation can be assessed by monoclonal antibodies (mAb) that selectively recognize integrins in their active form. We demonstrate herein that the expression of the epitope recognized by mAb HUTS‐21 is induced on T lymphoblasts upon binding of soluble vascular cell adhesion molecule (VCAM)‐1 and an 80‐kDa tryptic fragment of fibronectin (FN80) to the β1 integrins very late activation antigen (VLA)‐4 and VLA‐5, and that this effect is dependent on ligand concentration and is specific for β1 integrins. On T lymphoblasts adhering to immobilized fibronectin, the HUTS‐21 epitope localized exclusively to sites of integrin binding to fibronectin. These results indicate that mAb HUTS‐21 recognizes a ligand‐induced binding site (LIBS) on the common β1 subunit of VLA proteins. Engagement of β1 integrins through this LIBS epitope inhibited T lym‐phoblast movement on fibronectin, as determined by quantitative time‐lapse video microscopy studies. Furthermore, the HUTS‐21 mAb also prevented T lymphoblast‐directed migration through gradients of substratum‐immobilized β1 integrin ligands such as fibronectin or VCAM‐1, whereas it did not affect migration on intercellular adhesion molecule (ICAM)‐1. This anti‐LIBS mAb stimulated cell adhesion through postreceptor events, without affecting receptor affinity for ligand, and appears to interfere with cell migration by a mechanism distinct from that of