Morphodynamics of T-lymphocytes: Scanning to spreading

Abstract

Binding of the T cell receptor complex to its ligand, the subsequent molecular rearrangement, and the concomitant cell-scale shape changes represent the very first steps of adaptive immune recognition. The first minutes of the interaction of T cells and antigen presenting cells have been extensively scrutinized; yet, gaps remain in our understanding of how the biophysical properties of the environment may impact the sequence of events. In particular, many pioneering experiments were done on immobilized ligands and gave major insights into the process of T cell activation, whereas later experiments have indicated that ligand mobility was of paramount importance, especially to enable the formation of T cell receptor clusters. Systematic experiments to compare and reconcile the two schools are still lacking. Furthermore, recent investigations using compliant substrates have elucidated other intriguing aspects of T cell mechanics. Here we review experiments on interaction of T cells with planar artificial antigen presenting cells to explore the impact of mechanics on adhesion and actin morphodynamics during the spreading process. We enumerate a sequence tracing first contact to final spread state that is consistent with current understanding. Finally, we interpret the presented experimental results in light of a mechanical model that captures all the different morphodynamic states.

Figure 1

Antigen-presenting surfaces discussed in this review. (A) Relevant molecules are anti-CD3, ICAM-1, and pMHC. ICAM-1 is a ligand for integrins, pMHC is recognized by TCR, and anti-CD3 binds to the CD3 domain of the TCR complex. (B) Different substrate types. From top to bottom: SLBs with mobile ligands, SLBs with immobile ligands, hard glass surface, PDMS surface with modulable compliance. (C) Each of the substrates shown in (B) can be coated with the color-coded ligands shown in (A). Top row depicts surfaces that are homogenously coated with either anti-CD3 alone or with a mixture of ICAM-1 and either anti-CD3 or pMHC. The bottom row shows nano-patterns discussed in this review, where the ligands of the TCR are clustered, and the background carries optional ICAM-1.

Figure 2

Sequence of ligand recognition and spreading phases. Schemes (not to scale) show T cell morphology, and micrographs show labeled actin cytoskeleton (GFP-lifeact, TIRF mode) as a T cell undergoes adhesion to a ligand-covered surface. The resting T cell, shown close to a flat surface, is round with spiky microvilli. The very first step consists of microvilli making contact with the surface. In the absence of cognate ligands, this “search” phase lasts forever. If cognate ligands are found, a formin-dependent step is initiated. The microvilli form lasting contacts, which in the next Arp2/3-dependent step lead to close contact over an extended zone. Use of pharmacological agents such as SMIFH2/CK666 can block the process in the first or second phase respectively. The outcome of the Arp2/3-dependent phase depends on the substrate properties. On optimally stiff substrates that display immobilized ligands for the TCR complex or those that display integrin ligands in addition, cells undergo actin polymerization-driven monotonous spreading and activation. On SLB with mobile ligands for TCR complex (or on soft substrates), the cells fail to spread in spite of actin polymerization-dependent lamellipodial activity, presumably due to lack of traction. The formation of TCR clusters is not hindered, and cells may even get activated. The presence of friction and/or integrin ligands leads to full spreading.