Actin assembly and inward flow in the plane of the immunological synapse (IS) drives the centralization of T cell receptor microclusters (TCR MCs) and the integrin LFA-1. Using structured-illumination microscopy (SIM), we show that actin arcs populating the medial, lamella-like region of the IS arise from linear actin filaments generated by one or more formins present at the IS distal edge. After traversing the outer, Arp2/3-generated, lamellipodia-like region of the IS, these linear filaments are organized by myosin II into anti-parallel concentric arcs. 3D-SIM shows that active LFA-1 often aligns with arcs while TCR MCs commonly reside between arcs, and TIRF-SIM shows TCR MCs being swept inward by arcs. Consistently, disrupting actin arc formation via formin inhibition results in less centralized TCR MCs, miss-segregated integrin clusters, decreased T: B cell adhesion, and diminished TCR signaling. Together, our results define the origin, organization, and functional significance of a major actomyosin contractile structure at the IS that directly propels TCR MC transport. T cell receptor (TCR) mechano-transduction is an emerging but poorly understood component of T cell activation. To define the contribution made by the cytoskeleton to TCR mechano-transduction, we investigated the role played by contractile actomyosin II arcs populating the pSMAC region of the immunological synapse (IS) in ligand discrimination. Using super resolution microscopy, we show that the generation of organized actomyosin arcs depends on ligand potency and the ability of myosin II to contract actin filaments. While weak ligands induce disorganized actomyosin arcs, strong ligands result in organized actomyosin arcs that correlate well with tension-sensitive CasL phosphorylation and with ligand accumulation at the IS center. Furthermore, the presence of ligand-dependent actomyosin arcs allows for a broad dynamic range in the phosphorylation of the early signaling molecules. Taken together, our correlative study implicates ligand-dependent actomyosin arc formation and contraction in a mechano-chemical feedback mechanism that amplifies the accumulation of critical signaling molecules at the IS.
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