Adhesion-activating phorbol ester increases the mobility of leukocyte integrin LFA-1 in cultured lymphocytes

Lymphocytes activate adhesion to intracellular adhesion molecule 1(ICAM-1) via leukocyte function-associated antigen (LFA-1), their major beta(2) integrin, in response to PMA (phorbol 12-myristate 13-acetate) without an increase in the number of receptors expressed, The molecular details of the mechanism are unknown, To determine the effect of PMA activation on LFA-1 movement within the plasma membrane, we used the single particle tracking technique to measure the diffusion rate of LFA-1 molecules on EBV-transformed B cells before and after PMA activation. Diffusion of LFA-1 on unactivated cells was restricted compared to CR1 (CD35), another transmembrane protein of equivalent size, PMA caused a 10-fold increase in the diffusion rate of LFA-1 without any effect on CD35, The increased LFA-1 motion induced by PMA was random, not directed, indicating that it was due to a release of constraints rather than the application of forces, The diffusion rates of LFA-1 are consistent with cytoskeletal attachment before and free diffusion after PMA, Cytochalasin D led to an equivalent increase in mobility and, at low doses, stimulated adhesion. We propose that the release of LFA-1 from cytoskeletal constraints is an important early step in activation of adhesion, implying that the nonadhesive state of LFA-1 is actively maintained by the lymphocyte cytoskeleton.