LFA-1–mediated Adhesion Is Regulated by Cytoskeletal Restraint and by a Ca2+-dependent Protease, Calpain

Abstract

The activity of integrins on leukocytes is kept under tight control to avoid inappropriate adhesion while these cells are circulating in blood or migrating through tissues. Using lymphocyte function-associated antigen-1 (LFA-1) on T cells as a model, we have investigated adhesion to ligand intercellular adhesion molecule-1 induced by the Ca2+ mobilizers, ionomycin, 2,5-di-t-butylhydroquinone, and thapsigargin, and the well studied stimulators such as phorbol ester and cross-linking of the antigen-specific T cell receptor (TCR)– CD3 complex. We report here that after exposure of T cells to these agonists, integrin is released from cytoskeletal control by the Ca2+-induced activation of a calpain-like enzyme, and adhesive contact between cells is strengthened by means of the clustering of mobilized LFA-1 on the membrane. We propose that methods of leukocyte stimulation that cause Ca2+ fluxes induce LFA-1 adhesion by regulation of calpain activity. These findings suggest a mechanism whereby engagement of the TCR could promote adhesion strengthening at an early stage of interaction with an antigen-presenting cell.

Vav1 transduces TCR signals required for LFA‐1 function and cell polarization at the immunological synapse

“…Some agonists such as Mg 2+ ions or chemokines induce an affinity change, switching LFA-1 from a low to a high affinity state and we show here that this route to adhesion is Vav1-independent [25,26]. In contrast, TCR signaling induces an avidity change by allowing LFA-1 clustering [27,28]. In the resting lymphocyte, LFA-1 is held uniformly dispersed around the cell by interactions with the actin cytoskeleton.…”

“…Firstly, we and others have shown that Vav1 transduces signals to the activation of PLC + 1 and thus to an intracellular calcium flux [6,10,11,13,14]. This flux in turn activates the calcium-dependent protease caspian, which is required for LFA-1 clustering [28]. Secondly, phorbol esters induce LFA-1 clustering, presumably via protein kinase C. This pathway may be defective in Vav1 -/-cells in view of the defective activation of PLC + 1.…”

“…In the resting lymphocyte, LFA-1 is held uniformly dispersed around the cell by interactions with the actin cytoskeleton. Inside-out signaling leads to release of LFA-1 from the cytoskeletal anchor, translocation to the IS, clustering and reallocation with the actin cytoskeleton [27][28][29][30].…”

Critical role of the calpain/calpastatin balance in acute allograft rejection

“…This process requires the cleavage of the auto-inhibitory domain of calcineurin [7,8] or that of cain/cabin1, an endogenous inhibitor of calcineurin [9]. These reports, together with the observation that the engagement of the TCR increases calpain expression and calpain-dependent processes in T cells [10,11], suggest the hypothesis that calpains are involved in the activation of both NF-kB and calcineurin/NFAT pathways in T cells and thereby in allograft rejection.…”

“…This process requires the cleavage of the auto-inhibitory domain of calcineurin [7,8] or that of cain/cabin1, an endogenous inhibitor of calcineurin [9]. These reports, together with the observation that the engagement of the TCR increases calpain expression and calpain-dependent processes in T cells [10,11], suggest the hypothesis that calpains are involved in the activation of both NF-kB and calcineurin/NFAT pathways in T cells and thereby in allograft rejection.In the present work we assessed both expression and role of calpains in allograft rejection. To examine the role of calpain, we used a fully allogenic skin allograft model and transgenic mice expressing high levels of calpastatin (CalpTG) recently generated in our laboratory, as there is no pharmacological tool yet allowing us to specifically suppress the activity of calpains [12,13].…”