Lymphocyte migration is essential for the function of the adaptive immune system, and regulation of T cell entry into tissues is an effective therapy in autoimmune diseases. Little is known about the specific role of cytoskeletal effectors that mediate mechanical forces and morphological changes essential for migration in complex environments. We developed a new Formin-like-1 (FMNL1) knock-out mouse model and determined that the cytoskeletal effector FMNL1 is selectively required for effector T cell trafficking to inflamed tissues, without affecting naïve T cell entry into secondary lymphoid organs. Here, we identify a FMNL1-dependent mechanism of actin polymerization at the back of the cell that enables migration of the rigid lymphocyte nucleus through restrictive barriers. Furthermore, FMNL1-deficiency impairs the ability of self-reactive effector T cells to induce autoimmune disease. Overall, our data suggest that FMNL1 may be a potential therapeutic target to specifically modulate T cell trafficking to inflammatory sites.

During their lifespan, T cells are tasked with patrolling the body for potential pathogens. To do so, T cells migrate through numerous distinct anatomical sites and tissue environments with different biophysical characteristics. To migrate through these different environments, T cells use various motility strategies that rely on actin network remodeling to generate shape changes and mechanical forces. In this review we initially discuss the migratory journey of T cells, then cover the actin polymerization effectors at play in T cells, and finally we focus on the function of these effectors of actin cytoskeleton remodeling in mediating T cell migration through diverse tissue environments.Specifically, we will discuss the current state of the field pertaining to our understanding of the roles in T cell migration played by members of the three main families of actin polymerization machinery: the Arp2/3 complex; formin proteins; and Ena/VASP proteins.

To perform their functions, T cells migrate from the blood vasculature into tissues by traversing the vascular wall through a process known as transendothelial migration (TEM). While many of the extracellular cues that guide this process have been identified, the role of downstream cytoskeletal effectors that mediate force generation and morphological changes required for TEM remains an area of active research. Formin-like-1 (FMNL1) is a cytoskeletal effector that mediates cytoskeletal remodeling. However, the role of FMNL1 in T cell functions, and migration in particular, is mostly unknown. The goal of our study was to examine the role of FMNL1 in T cell extravasation and trafficking. We generated a new FMNL1 knock-out (KO) mouse and identified a novel role for FMNL1 in promoting T cell migration through restrictive endothelial barriers during TEM. In vitro, FMNL1-deficient T cells had reduced actin polymerization in response to chemokine signaling. Furthermore, FMNL1 KO T cells had a selective impairment in chemotaxis through restrictive 3μm pores vs permissive 5μm pores. Using time-lapse microscopy, we determined that FMNL1-deficient T cells are severely impaired in their capacity to complete TEM through endothelial barriers. In vivo, we found that FMNL1 was dispensable for T cell extravasation through permissive high endothelial venules and homeostatic trafficking to lymphoid organs. Instead, FMNL1 regulated the ability of activated T cells to extravasate into inflamed peripheral tissues and enabled self-reactive T cells to induce autoimmune disease. Overall, our data identify FMNL1 as a key regulator of lymphocyte migration through restrictive barriers and as a potential target for modulating effector T cell trafficking.

Naive T cell activation in secondary lymphoid organs such as lymph nodes (LNs) occurs upon recognition of cognate antigens presented by antigen presenting cells (APCs). T cell activation requires cytoskeleton rearrangement and sustained interactions with APCs. Ena/VASP proteins are a family of cytoskeletal effector proteins responsible for actin polymerization and are frequently found at the leading edge of motile cells. Ena/VASP proteins have been implicated in motility and adhesion in various cell types, but their role in primary T cell activation has not been explored. Our goal was to determine the contribution of Ena/VASP proteins to T cell activation and expansion in vivo. Our results showed that naïve T cells from Ena/VASP-deficient mice have a significant reduction in antigen-specific T cell accumulation following Listeria monocytogenes infection. The kinetics of antigen-specific T cell impairment were further confirmed in Ena/VASP-deficient T cells stimulated via dendritic cell immunization. To investigate the cause of this T cell expansion defect, we analyzed T cell-APC interactions in vivo by 2-photon microscopy and observed fewer Ena/VASP-deficient naïve T cells interacting with APCs in LNs during priming. We also found that Ena/VASP-deficient T cells formed conjugates with significantly less actin polymerization at the T cell-APC synapse, and that these conjugates were less stable than their WT counterparts. Thus, we conclude that Ena/VASP proteins contribute to T cell actin remodeling downstream of T-APC interactions required for the initiation of stable T cell conjugates during APC scanning and for efficient activation and expansion of T cells in vivo. Supported by NIH (R01AI125553; T32AI007405)

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