T Lymphocyte Migration: An Action Movie Starring the Actin and Associated Actors

Abstract

The actin cytoskeleton is composed of a dynamic filament meshwork that builds the architecture of the cell to sustain its fundamental properties. This physical structure is characterized by a continuous remodeling, which allows cells to accomplish complex motility steps such as directed migration, crossing of biological barriers, and interaction with other cells. T lymphocytes excel in these motility steps to ensure their immune surveillance duties. In particular, actin cytoskeleton remodeling is a key to facilitate the journey of T lymphocytes through distinct tissue environments and to tune their stop and go behavior during the scanning of antigen-presenting cells. The molecular mechanisms controlling actin cytoskeleton remodeling during T lymphocyte motility have been only partially unraveled, since the function of many actin regulators has not yet been assessed in these cells. Our review aims to integrate the current knowledge into a comprehensive picture of how the actin cytoskeleton drives T lymphocyte migration. We will present the molecular actors that control actin cytoskeleton remodeling, as well as their role in the different T lymphocyte motile steps. We will also highlight which challenges remain to be addressed experimentally and which approaches appear promising to tackle them.

Keywords: T lymphocytes; actin cytoskeleton; migration.

Figure 1

Actin cytoskeleton underlies T cell morphological changes during directional migration. (A) Snapshots of a movie showing a primary CD8⁺ human T cell expressing Dendra2-LifeAct moving along a CXCL12 gradient created in a collagen IV-coated Ibidi μ-Slide Chemotaxis^2D. The cell extends dynamic protrusions and moves toward the source of CXCL12 (top). See Movie S1 in Supplementary Material. The organization of the actin cytoskeleton in T lymphocytes can also be appreciated in previous reports on the dynamics of actin during scanning of target cells (40) and polarization in response to CXCL12 (41). (B) Velocity of the cell shown in (A), calculated for successive 12 s intervals on the basis of the tracking of the cell. (C) Morphology of the cell shown in (A), calculated as Aspect Ratio (length/width of a fitted ellipsis) for each frame.

Figure 2

Actin cytoskeleton organization at the two poles of a migrating T cell. Schematic representations of the ultrastructure of the actin cytoskeleton networks at the leading and trailing edges of the migrating T cell shown in Figure 1. At the leading edge, the T cell that migrates on a 2D surface emits a protrusion that alternates between a lamellipodium and a pseudopodium. It contains a very dynamical and highly branched actin meshwork. At the trailing edge, the T cell uropod is made of a network of parallel actin bundles that can slide along each other to generate contractile forces.

Figure 3

Migratory challenges faced by T cells during their journey through the organism. (A) The crossing of the endothelium barrier follows steps of T cell tethering and rolling on the luminal surface of the endothelial cells. The combination of chemokines and adhesion molecules triggers firm adhesion to the endothelial surface via the emission of filopodia-like protrusions. Depending on their activation state, T cells then either use the transcellular route by emitting invadopodia-like protrusions that go across the entire endothelial cell body, or the paracellular by squeezing through the junction between two adjacent endothelial cells. (B) Following the crossing of the endothelial barrier and underlying basal membrane, T cells undergo interstitial migration through the tissue they have entered in. Using an amoeboid mode of motility, they crawl and squeeze along and through extracellular matrix (ECM) fibers of various nature. In the lymph node cortex, T cells preferentially migrate along a network of fibroblastic reticular cells decorated with chemokines. (C) During the scanning of antigen-presenting cells (APC), T cells make multiple encounters of various duration and quality. Some contacts may last only a few minutes in the form of an immature immunological synapse. Upon recognition of an APC bearing specific antigens, the T cell stops migrating to assemble a long-lasting immunological synapse. In addition to controlling the interaction between the T cell and the APC, the dynamical actin cytoskeleton serves as a physical platform for numerous signaling events that take place at the immunological synapse to activate the T cell. After a few hours, the activated T cell detaches from its APC partner and regains its motility behavior.

Figure 4

Actin cytoskeleton architecture. (A) Actin cytoskeleton dynamics rely in part on the tightly controlled cycle of polymerization and depolymerization, also known as treadmilling. ATP-bound actin is added to the fast growing barbed end of filaments via the combined action of profilin, which prevents self-nucleation of actin monomers and actin-nucleating proteins such as the formin FMLN1 or WASP-family proteins, both of which are under the control of RhoGTPases. Depolymerization is promoted by cofilin, which stimulates dissociation of ADP-bound actin at the pointed end of filaments. The rate of cofilin-mediated depolymerization can be controlled by Rho via Rock and LimK. (B) In addition to be elongated by formins, actin filaments can build networks in multiple ways. Actin bundles or cables with parallel or anti-parallel orientation of actin filaments are assembled by cross-linking proteins such as fimbrin. Actin filaments can also be cross-linked in a non-parallel fashion via filamin to create a gelled network. Branched networks are promoted by the Arp2/3 complex that initiates nucleation of branched filaments on the side of existing ones. This activity is driven by WASP-family proteins and stabilized by HS-1. An additional important regulation of actin cytoskeleton networks is mediated by capping proteins such as gelsolin, which bind the plus end of actin filaments to prevent monomer exchange. (C) Actin filaments not only generate forces while they elongate. They also generate the cell contractile forces via the intercalation of the molecular motor myosin between parallel actin filaments, which results in filament sliding. Such process is regulated by the control of the myosin light chain phosphatase and kinase activities, as well as by the degree of actin cross-linking via α-actinin.

Figure 5

The different facets of actin cytoskeleton remodeling in migrating T cells. Represented at the center of the scheme (green zone) are the dominant receptors in the control of T cell motility. They include chemokine receptors, the TCR and integrins such as LFA-1, each being interconnected with the actin cytoskeleton with specific sets of signaling molecules. Ligand-mediated triggering of these receptors leads to the activation of the RhoGTPases Rac, Rho, and Cdc42 via GEFs and GAPs (purple zone). Such activation is highly controlled in time and space to orchestrate the assembly of distinct actin networks. Rac activates the WAVE complex, leading to Arp2/3-mediated actin polymerization at the leading edge to form a branched actin network. Cdc42 also favors membrane extension by activating the Arp2/3 complex via WASP. In addition to its major role at the uropod, Rho plays a dual role at the leading edge by promoting actin filament elongation via the formin mDia and by favoring membrane retraction via myosin activation. The left side of the scheme depicts the role of ERM proteins as anchors of the actin cytoskeleton in the plasma membrane (orange zone). The right side of the scheme illustrates the role of BAR-domain proteins as molecular links to guarantee local coordination of membrane curvature and actin polymerization (yellow zone). The actin meshwork is represented as blue filaments that are intertwined with the different signaling areas.