FUNCTIONAL RELEVANCE DURING LYMPHOCYTE MIGRATION AND CELLULAR-LOCALIZATION OF ACTIVATED BETA-1 INTEGRINS

The state of integrin activation can be assessed by monoclonal antibod ies (mAb) that selectively recognize integrins in their active form. W e demonstrate herein that the expression of the epitope recognized by mAb HUTS-21 is induced on T lymphoblasts upon binding of soluble vascu lar cell adhesion molecule (VCAM)-1 and an 80-kDa tryptic fragment of fibronectin (FN80) to the beta 1 integrins very late activation antige n (VLA)-4 and VLA-5, and that this effect is dependent on ligand conce ntration and is specific for beta 1 integrins. On T lymphoblasts adher ing to immobilized fibronectin, the HUTS-21 epitope localized exclusiv ely to sites of integrin binding to fibronectin. These results indicat e that mAb HUTS-21 recognizes a ligand-induced binding site (LIES) on the common beta 1 subunit of VLA proteins. Engagement of beta 1 integr ins through this LIES epitope inhibited T lymphoblast movement on fibr onectin, as determined by quantitative time-lapse video microscopy stu dies. Furthermore, the HUTS-21 mAb also prevented T lymphoblast-direct ed migration through gradients of substratum-immobilized beta 1 integr in ligands such as fibronectin or VCAM-1, whereas it did not affect mi gration on intercellular adhesion molecule (ICAM)-1. This anti-LIES mA b stimulated cell adhesion through postreceptor events, without affect ing receptor affinity for ligand, and appears to interfere with cell m igration by a mechanism distinct from that of other anti-beta 1 activa ting antibodies.