Biophysical Forces Tune T Cell Activation: Dynamic T Cell Receptor Conformations Revealed by Single-Molecule Force Spectroscopy

Abstract

T cells are a critical component of the adaptive immune system that mount a response to specific antigens and retain memory to efficiently combat future assaults. T cell activation is finely tuned with exquisite sensitivity and specificity to discriminate self from non-self, otherwise autoimmunity or immunodeficiencies may result. There is a clear disparity, however, between T cell response and low binding affinity to cognate ligands (KD ~ µM). Mounting evidence supports the mechanosensing model for T cell activation in which T cells leverage forces experienced during immune surveillance for antigen discrimination. The FG loop motif in the constant domain of the T cell receptor (TCR) b chain (Cb) has been implicated in mechanosensing, but precise molecular mechanisms at the initial TCR binding event that ultimately triggers T cell response are poorly understood. This work uses optical tweezers (OT) to investigate the interaction between TCRs and cognate ligands (pMHC) at the single-molecule and single-cell levels to reveal critical structural features that modulate mechanosensing. First, it was demonstrated that gd T cells, unlike the far more abundant ab T cells, cannot recognize antigen by mechanosensing due to the lack of FG loop motif in the homologous Cb domain. A refined mechanosensing model was developed that we call the molecular resonance model, which identifies reversible conformational transitions within the TCR as a driver of highly sensitive T cell response. Force-dependent molecular motions were dissected using disulfide ‘staples’ to reveal structural features that impact transition distances and bond lifetimes in different force ranges. Altogether, these findings refine how force regulates T cell receptor allostery and provides a framework that links molecular mechanics to immune function. These results have broad implications for future vaccines and immunotherapies.