Force and affinity in ligand discrimination by the TCR

Abstract

T cell recognition of antigen is a physical process that requires formation of a cell-cell junction that is rich in active force generation. Recently a biomolecular force probe was used to examine how the T cell receptor (TCR)-pMHC interaction responds to force and the consequences of force-dependent interactions for T cell activation. While adhesion and costimulatory molecules in the immunological synapse impact upon the overall force of the interaction, these results suggest that the TCR uses a force-dependent bond - a catch bond - and that it may therefore be important to consider the TCR-pMHC interaction in isolation in the early phases of the decision process. We discuss here these findings in the context of other work on the impact of forces on the TCR and the quantification of interaction in interfaces.

Figure 1

Biomembrane force probe experiment. (A) A 2 μm glass bead is affixed to an RBC held by a stationary micropipet. Oriented pMHC are linked to the bead through a streptavidin-biotin interaction. A live T cell help by a separate micropipette than can be moved with nm precision is docked to the bead to generate a small contact area, probably with microvilli studding the lymphocyte surface. The T cell profile is traced from a thin section transmission electron micrograph of a non-polarized lymphoblast. Microvilli cannot be resolved by light microscopy, but the averaged refractility of all the microvilli is detected and the bead is brought into firm contact with this boundary (B). At 10x higher magnification it is imagined that the cell-bead contact slightly deforms the microviilli and generates a contact area that is about 2% of the apparent contact area. (C) At the densities used for agonist pMHC there is a 10-20% chance that a single TCR-pMHC tether will form across all the microvilli (μvilli). So when the T cell is retracted after a 0.1 s touch there is a 10-20% chance that a tether will be detected based on damping of oscillations in the bead. The tether can them be positioned to allow the tether be exposed only to the BFP oscillations of ± 1 pN or be loaded with forces of up to 30 pN at a rate of 1000 pN/s. Holding the force at at 10 pN was the optimal conditions to observe catch-bond behavior in agonist pMHC.

Figure I

A. A slip bond with the potential to form only one interaction that is destabilized by applied force. B. Force exerted by the blue molecule upfolds one of the domains to expose a second binding site complementary to a site on the green ligand. The additional binding energy stabilizes the bond, but as force is increased both of these bonds eventually fail.