Revealing the Secrets of T Cell Immunological Synapse Formation and Function using Super Resolution Imaging

Upon antigen recognition, actin assembly and inward flow in the plane of the radially symmetric immunological synapse (IS) drives the centralization of T cell receptor microclusters (TCR MCs) and the integrin LFA-1. Using two forms of structured-illumination microscopy (SIM), we have found 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 distal edge of the IS. After traversing the outer, Arp2/3-generated, lamellopodia-like region of the IS, these linear filaments are organized by myosin II into concentric arcs that should possess the anti-parallel organization required for contraction. Quantitative, fixed-cell 3D-SIM shows that open, active LFA-1 often aligns with arcs while TCR MCs commonly reside between arcs, and live-cell 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 frequency, and diminished proximal 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.