Origin, Organization, Dynamics, and Function of Actin and Actomyosin Networks at the T Cell Immunological Synapse

Abstract

The engagement of a T cell with an antigen-presenting cell (APC) or activating surface results in the formation within the T cell of several distinct actin and actomyosin networks. These networks reside largely within a narrow zone immediately under the T cell's plasma membrane at its site of contact with the APC or activating surface, i.e., at the immunological synapse. Here we review the origin, organization, dynamics, and function of these synapse-associated actin and actomyosin networks. Importantly, recent insights into the nature of these actin-based cytoskeletal structures were made possible in several cases by advances in light microscopy.

Keywords: T cell; actin; formin; immunological synapse; lytic granule; myosin.

Figure 1

The four actin networks at the T cell IS. (a) The three distinct functional zones within the mature IS. (b) The spatial relationships between these three IS zones and the four actin networks at the IS. (c) A SIM image at the bottom of an activated Jurkat T cell that was stained for F-actin using phalloidin (i, red) and myosin 2A (ii, green). Subpanel iii shows the overlaid image, while the magnified inset in subpanel iv shows in greater detail the lamellipodia-like branched actin network spanning the dSMAC, the lamella-like actomyosin arc network spanning the pSMAC, and the actin hypodense network in the cSMAC. The green doublets that are concentrated in the pSMAC region in subpanel iv are individual myosin 2 bipolar filaments (reproduced from Reference 48). Note that the colors used in the drawings in panels a and b are not meant to match the colors in the micrographs in panels c, d, and e. (d) A 3D SIM image of an activated Jurkat T cell stained for F-actin using phalloidin (i). The inset in subpanel ii shows in greater detail the three networks noted in panel c. Color coding the 3D projection of this cell according to Z position (iii) shows that these actin networks are largely confined to the plane of the IS (reproduced from Reference 48). These networks can be considered, therefore, as 2D structures (which makes live TIRF-SIM possible) (see also 122). That said, the branched actin network in the dSMAC can sometimes lift off the surface, as shown by Yi et al. (47) and, more recently, Fritzsche et al. (50). (e) A still image from a TIRF-SIM movie of an activated Jurkat T cell expressing the indirect F-actin reporter F-Tractin tagged with GFP (adapted from Reference 48; see also Supplemental Video 1). (f) A still image from a two-color TIRF-SIM movie of an activated Jurkat T cell expressing Td-Tomato-tagged F-Tractin (red) and GFP-tagged myosin 2A (green) (adapted from Reference 48; see also Supplemental Video 2). Abbreviations: cSMAC, central SMAC; dSMAC, distal SMAC; GFP, green fluorescent protein; IS, immunological synapse; pSMAC, peripheral SMAC; SIM, structured illumination microscopy; SMAC, supramolecular activation cluster; TIRF, total internal reflection.

Figure 2

Key characteristics of the four actin networks. (a) An expanded view of the four actin networks at the IS. It should be noted that while the boundary between the branched actin network and actomyosin arc network, as well as the boundary between the actomyosin network and the actin hypodense center of the IS, can be both sharp and symmetric (see, for example Figure 1c–f), they can be more graded in some cells. (b) Assembly sites for the four actin networks, along with the nucleation-promoting factors and nucleation machines that drive their formation. Also shown are the two major sites of actin filament disassembly, along with key players in the disassembly process. Many other actin regulators play important roles in creating, organizing, and disassembling these actin networks. Some of these important proteins are HS1, IQGAP, ezrin/moesin, coronin 1A, L-plastin, CARMIL, and GMF (–131). Due to space limitations, we could not address the roles of these proteins in depth. (c) Dynamics of the four actin networks at the IS. Note that while the actin in the cSMAC is static in terms of directional flow, it is dynamic on a nanoscale. (d) Distribution of integrins (purple) in the mature synapse, which accumulate across the actomyosin-arc-rich pSMAC and, most dramatically, at the pSMAC/cSMAC boundary (because the LFA-1:ICAM pairs are size-excluded from the cSMAC). The expanded view shows that the actin arcs (red) are decorated with open, active LFA-1 (purple) as well as with myosin 2 bipolar filaments (blue) (see 48). (e) Activity-dependent, nanoscale, actin filament dynamics and myosin 2 contractility within the cSMAC promote lytic granule secretion by increasing the size of pores in the fine actin meshwork that normally restricts granule access to the plasma membrane fusion machinery (the red and green circles indicate restricted and unrestricted granules, respectively; the view is from just the APC side of the T cell plasma membrane looking into the T cell). The actomyosin arc network in the pSMAC is the likely source of the T cell–based force that augments cytotoxicity by straining the target cell plasma membrane. Abbreviations: APC, antigen-presenting cell; cSMAC, central SMAC; dSMAC, distal SMAC; IS, immunological synapse; MC, microcluster; pSMAC, peripheral SMAC; SMAC, supramolecular activation cluster; TCR, T cell receptor.

Figure 3

Origins of the four actin networks. “Make-actin” signals drive the formation of the four actin networks at the IS. While the generation of these signals is in every case downstream of the engagement of the TCR (and its coreceptors), we do not attempt to include TCR-proximal signaling pathways in this figure. Abbreviations: IS, immunological synapse; TCR, T cell receptor.