Integrin leukocyte function-associated antigen-1-mediated cell binding canbe activated by clustering of membrane rafts

The leukocyte function-associated antigen-1 (LFA-1) integrin (CD11a/CD18) i s an important adhesion molecule for lymphocyte migration and the initiatio n of an immune response, At the cell surface, LFA-1 activity can be regulat ed by divalent cations that enhance receptor affinity but also by membrane clustering induced by treatment of cells with substances such as phorbol es ters. Membrane clustering leads to increased LFA-1 avidity. We report here that LFA-1-mediated binding of mouse thymocytes or activated T lymphocytes to intercellular adhesion molecule 1 can be rapidly induced by clustering o f membrane rafts using antibodies to the glycosylphophatidylinositol-anchor ed molecule CD24 or cholera toxin (CTx). CD24 and CD18 were found to coloca lize in rafts and cross-linking with CTx lead to enhanced LFA-1 clustering. We observed that disruption of raft, integrity by lowering the membrane ch olesterol content abolished the CTx and the phorbol la-myristate 13-acetate -induced LFA-1 binding but left the ability to activate LFA-1 with Mg2+/EGT A unimpaired. In contrast to activation with Mg2+/EGTA, activation via raft clustering was dependent on PI3-kinase, required cytoskeletal mobility, an d was accompanied by Tyr phosphorylation of a 18-kDa protein. Our results s upport the notion that rafts as preformed adhesion platforms could be impor tant for the rapid regulation of lymphocyte adhesion.