Mechanical forces acting on ligand-engaged T-cell receptors (TCRs) have been implicated in T-cell antigen recognition. To measure these molecular forces in real time, we devised a FRET-based force sensor that is equipped with a TCR-reactive single chain antibody fragment and tethered to planar supported lipid bilayers (SLBs) which serve as surrogate antigen presenting cell. When confronting T-cells with sensor-functionalized gel-phase SLBs, we measured 4.6 pN force-peaks for activated T-cells and 5.5 pN force-peaks for non-activated scanning T-cells, which vary in the range of seconds with force loads of 1.5 pN per second. When using more physiological fluid-phase SLBs, TCR-imposed forces were found to have a ~3-fold reduction for scanning T-cells, yet no longer detectable for activated T-cells. Hence, if they are essential for ligand discrimination and TCR-proximal signaling during initial antigen scanning, TCR-exerted pulling forces are already effective below 2pN. Once activated, T-cells appear to create a synaptic environment that is devoid of noticeable TCR-exerted mechanical forces.

Temporal Analysis of T-Cell Receptor-Imposed Forces via Quantitative Single Molecule FRET Measurements