The spontaneously adhesive leukocyte function-associated antigen-1 (LFA-1) integrin in effector T cells mediates rapid actin- and calmodulin-dependent adhesion strengthening to ligand under shear flow

Integrins in effector T cells are highly expressed and important for trafficking of these cells and for their effector functions. However, how integrins are regulated in effector T cells remains poorly characterized. Here, we have investigated effector T cell LFA-1 regulation in primary murine effector T cells. These cells have high LFA-1 integrin expression and display high spontaneous binding to ICAM-1 ligand under static conditions. In addition, these cells are able to migrate spontaneously on ICAM-1. Atomic force microscopy measurements showed that the force required for unbinding of integrin-ligand interactions increases over time (0.5s-20s contact time). The maximum unbinding force for this interaction was approximately 140pN at 0.5s contact time, increasing to 580pN at 20s contact time. Also the total work required to disrupt the interaction increased over the 20s contact time, indicating LFA-1-mediated adhesion strengthening in primary effector T cells over a very quick timeframe. Effector T cells adhered spontaneously to ICAM-1 under conditions of shear flow, in the absence of chemokine stimulation, and this binding was independent of protein kinase B/Akt and protein kinase C kinase activity, but dependent on calcium/calmodulin signaling and an intact actin cytoskeleton. These results indicate that effector T cell integrins are highly expressed and spontaneously adhesive in the absence of inside-out integrin signaling, but that LFA-1-mediated firm adhesion under conditions of shear flow requires downstream integrin signaling which is dependent on calcium/calmodulin and the actin cytoskeleton.