The structural basis of T-cell receptor (TCR) activation: An enduring enigma

Abstract

T cells are critical for protective immune responses to pathogens and tumors. The T-cell receptor (TCR)-CD3 complex is composed of a diverse αβ TCR heterodimer noncovalently associated with the invariant CD3 dimers CD3ϵγ, CD3ϵδ, and CD3ζζ. The TCR mediates recognition of antigenic peptides bound to MHC molecules (pMHC), whereas the CD3 molecules transduce activation signals to the T cell. Whereas much is known about downstream T-cell signaling pathways, the mechanism whereby TCR engagement by pMHC is first communicated to the CD3 signaling apparatus, a process termed early T-cell activation, remains largely a mystery. In this review, we examine the molecular basis for TCR activation in light of the recently determined cryoEM structure of a complete TCR-CD3 complex. This structure provides an unprecedented opportunity to assess various signaling models that have been proposed for the TCR. We review evidence from single-molecule and structural studies for force-induced conformational changes in the TCR-CD3 complex, for dynamically-driven TCR allostery, and for pMHC-induced structural changes in the transmembrane and cytoplasmic regions of CD3 subunits. We identify major knowledge gaps that must be filled in order to arrive at a comprehensive model of TCR activation that explains, at the molecular level, how pMHC-specific information is transmitted across the T-cell membrane to initiate intracellular signaling. An in-depth understanding of this process will accelerate the rational design of immunotherapeutic agents targeting the TCR-CD3 complex.

Keywords: T-cell receptor (TCR); allosteric regulation; cryo-electron microscopy; major histocompatibility complex (MHC); mechanotransduction; molecular dynamics; nuclear magnetic resonance (NMR); signal transduction.

Figure 1.

Mechanisms of TCR activation. A, in the aggregation model, pMHC binding induces oligomerization of TCR–CD3 complexes. This clustering could increase the proximity of associated Lck molecules, resulting in activation of receptors in the aggregate by trans-autophosphorylation. B, segregation model proposes that TCR binding to pMHC induces zones of close contact at the T-cell–APC interface from which molecules with large ectodomains, such as the inhibitory tyrosine phosphatase CD45, are excluded. Segregation of CD45 favors phosphorylation of CD3 ITAMs by Lck. C, in the mechanosensing model, sliding of the T cell and APC membranes over each other during immune surveillance generates a mechanical force tangential to the T-cell surface that leads to dissociation of CD3 ITAMs from the T-cell membrane, thereby exposing them to phosphorylation by Lck. D, allosteric model postulates that pMHC binding to TCR induces long-range changes in TCR dynamics and/or conformation (represented by color changes in the TCR–CD3 complex) that are transmitted to the cytoplasmic tails of CD3 to expose ITAMs for phosphorylation. This transmission is mediated by allosteric sites in the TCR Cα and Cβ domains. E, in a unified model of TCR activation that combines mechanosensing (C) and allostery (D), mechanical force induces allosteric changes in TCR dynamics and/or conformation that propagate to CD3. Force amplifies allosteric communication between TCR and CD3 following pMHC ligation.

Figure 2.

Structure of the TCR–CD3 complex. A, side views of the overall structure of the TCR–CD3 complex shown in ribbon representation (PDB code 6JXR) (23). B, top view of the TM segments of the TCR–CD3 complex. Side chains of acidic and basic residues forming ionic interactions within the membrane are shown. C, top view of the ECDs of the TCR–CD3 complex.

Figure 3.

Allosteric sites in the TCR constant domains. Three allosteric sites identified by NMR (16, 17) in the Cα and Cβ domains of TCR A6 (PDB code 1QRN) (49) are boxed. Inset 1 shows the Cα AB loop and its surroundings in the TCR–CD3 structure (PDB code 6JXR) (23). The side chains of contacting residues are drawn in stick representation. The Cα AB loop contacts the CD3ϵ subunit of CD3ϵδ. Inset 2 shows the Cβ FG loop and its surroundings in the TCR–CD3 complex. The Cβ FG loop is situated immediately above the CD3ϵγ dimer but makes no direct contacts. Inset 3 shows the Cβ αA helix and its surroundings in the TCR–CD3 structure. The Cβ αA helix contacts the CP of the TCR α chain.

Figure 4.

Molecular dynamics simulations of TCR in unbound and pMHC-bound states. A, ΔRMSF values for the TCR A6 α chain with positive values indicating regions that become more rigid upon binding Tax–HLA-A2 and negative values indicating increased TCR α chain flexibility (17). B, ΔRMSF values for the A6 β chain. C, ΔRMSF values mapped onto the X-ray structure of the TCR A6–Tax–HLA-A2 complex in surface representation (PDB code 1QRN) (49). Color coding is as follows: HLA-A2 (gray); Tax peptide (magenta); TCR A6 α chain (green); TCR A6 β chain (wheat); ΔRMSF >0.2 Å (blue); 0.1 < ΔRMSF <0.2 Å (light blue); −0.2 < ΔRMSF < −0.1 Å (orange), and ΔRMSF < −0.2 Å (red).

Figure 5.

Bent and extended conformations of the TCRα TM helix. A, NMR structure of the TCRα TM helix in the bent conformation (PDB code 6MF8) (30). In the TCR–CD3 structure (23), TCRα Arg-253 and Lys-258 form salt bridges with acidic residues in CD3 TM helices (Fig. 2B). B, extended-state structure of the TCRα TM helix. C, conformation of the CD3ϵ cytoplasmic domain relative to the membrane (PDB code 2K4F) (29). The aromatic side chains of residues Tyr-38 and Tyr-49 of the CD3ϵ ITAM are embedded in the lipid bilayer.