Tropomyosins are critical mediators of CD8+ T cell synaptic actomyosin organization.

Abstract CD8+ T cells are a critical arm of the adaptive immune system because they kill virally-infected and transformed cells. Their function is critically dependent on their ability to form stable interactions with antigen-presenting cells (APCs). These interactions are mediated by a highly-organized structure at the T cell: APC interface termed the immunological synapse (IS). The IS itself is organized largely by the underlying actin cytoskeleton. Perturbation of either the Arp2/3-dependent branched network of the distal region or of the formin-derived arc network of the peripheral region dampens TCR signaling and impairs subsequent T cell activation. Tropomyosins are actin-binding proteins that form head-to-tail polymers along the actin filament. Several tropomyosin isoforms associate preferentially with linear formin-generated filaments, such as those in the peripheral region of the IS, where they promote the recruitment and activation of myosin 2 and thwart cofilin-mediated severing. We have found that mouse CD8+ T cells express two prominent tropomyosin isoforms (Tpm3.1 and Tpm4.2) and that these isoforms are strongly upregulated following naïve cell activation. Interestingly, both of these isoforms have been associated with human pathology in proteomics experiments. Fluorescent-tagged versions of these tropomyosins colocalize with pSMAC actomyosin arcs and migrate toward the center of the IS. Importantly, treatment with an inhibitor of Tpm3.1, ATM3507, severely impacts the organization of T cell synaptic actin structures. Our aim is to examine the role that tropomyosins play in the organization of T cell synaptic actin and actomyosin structures and to determine their importance for effector function.