Opposing Actin Networks Modulate the Mechano-Activation of the Integrin LFA-1 During Immunological Synapse formation
Formation of a functional immune response requires the regulated transfer of information between T cells, and APCs, which leads to a variety of functional outcomes. In many cases this information transfer occurs at a regulated area of cell - cell contact termed the immunological synapse (IS). In T cells responding to APCs there is a robust accumulation of F-actin on the T cell side of the synapse. This F-actin response is characterized by robust polymerization at the periphery of the contact site followed by centripetal flow of the network towards the center of the IS. While it is well known that this F-actin flow is required for the maintenance of signaling at the IS, the mechanism behind this observation has remained elusive. Recent evidence has suggested that the TCR and the integrin LFA-1 are regulated through the application of force on the receptor - ligand bond. This raises the distinct possibility that the robust F-actin flow provides the mechanical force to activate these receptors, thereby enhancing and maintaining signaling at the IS. I demonstrate that the robust F-actin flow at the T cell side of the IS is required for LFA-1 accumulation to and maintenance of conformational changes associated with affinity maturation at the IS. Furthermore I find that full induction of LFA-1 conformational change requires binding to immobilized ligand, and is enhanced through the induced fit of ligand binding, alongside the enhanced tension provided by pulling on immobilized ICAM-1. Furthermore, I find that Dendritic Cells (DCs) are capable of modulating the lateral mobility of ICAM-1 on their plasma membrane during TLR induced maturation. Mature DCs greatly restrict the lateral mobility of ICAM through the increased expression and activity of the actin binding proteins moesin and α-actinin-1. The limitation of ICAM-1 mobility is critical for ICAM-1 mediated adhesion of T cells to DCs, through the regulation of LFA-1 affinity maturation, and is essential for full T cell proliferation. Our results therefore indicate that the actin cytoskeletons on both sides of the T cell - DC IS coordinately function to maximize conformational change of LFA-1 and promote synapse stability at the IS. Additionally, these data demonstrate that one role of the T cell F-actin centripetal flow may be to provide the force required for the activation of mechano-sensitive molecules, thus propagating signal transduction at the IS.
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