T cell antigen recognition at the cell membrane

Abstract

T cell antigen receptors (TCRs) on the surface of T cells bind specifically to particular peptide bound major histocompatibility complexes (pMHCs) presented on the surface of antigen presenting cells (APCs). This interaction is a key event in T cell antigen recognition and activation. Most studies have used surface plasmon resonance (SPR) to measure the in vitro binding kinetics of TCR-pMHC interactions in solution using purified proteins. However, these measurements are not physiologically precise, as both TCRs and pMHCs are membrane-associated molecules which are regulated by their cellular environments. Recently, single-molecule förster resonance energy transfer (FRET) and single-molecule mechanical assays were used to measure the in situ binding kinetics of TCR-pMHC interactions on the surface of live T cells. These studies have provided exciting insights into the biochemical basis of T cell antigen recognition and suggest that TCRs serially engage with a small number of antigens with very fast kinetics in order to maximize TCR signaling and sensitivity.

Figure 1. Important molecules on and underneath either the T cell or APC surface

TCRs recognize antigen pMHCs on the cell membrane with the help of co-receptors and accessory molecules. These molecules play critical roles in the T cell recognition and the function of each of which is discussed in the text. Also depicted are possible regulatory mechanisms of TCR-pMHC interaction by the T cell membrane and intracellular structures.

Figure 2. Schematic of SPR for measuring the in vitro 3D kinetics of TCR-pMHC interactions

Interaction between purified soluble TCRs in solution and immobilized pMHCs on a sensor chip is measured by SPR angle shifts when the mass of the surface layer changes.

Figure 3. 2D methods for measuring in situ TCR-pMHC interactions

(A) Single-molecule FRET. A TCR on the T cell membrane is labeled with a single chain antibody fragment conjugated with a FRET acceptor, and the peptide on a pMHC anchored to the lipid bilayer is labeled with a FRET donor. The TCR-pMHC interaction brings the donor and acceptor into close distance to trigger FRET. (B) Adhesion frequency assay. A T cell (right) is brought in and out of contact with a pMHC coated RBC (left) to estimate an adhesion frequency using micropipette manipulation. The presence or absence of RBC deformation signifies whether a TCR-pMHC bond is present on T cell retraction. (C) Thermal fluctuation assay. A T cell (right) is brought into close proximity to a pMHC coated bead attached to a RBC (left). Single bond association and dissociation events are detected by monitoring the thermal fluctuation amplitude of the bead that is either restrained by only the RBC or the combination of the RBC and the single TCR-pMHC bond.

Figure 4. A fast kinetics based serial triggering model

TCR recognition of an antigenic pMHC leads to CD3 ITAM phosphorylation and signaling. The TCR ligation signal promotes the formation of TCR protein islands/clusters on the T cell membrane. The fast kinetics of TCR-pMHC interaction allows the TCRs in the cluster to serially engage with a small number of antigens on the APC surface and maximize T cell signaling and activation.