Cooperative cis-interactions between ectodomains of TCRαβ CD3 subunits enable mechanotransduction

TCR signaling poses a mechanical problem: pMHC binding occurs at the TCR{beta} ectodomain (ECD) head, whereas ITAM phosphorylation occurs on CD3 cytoplasmic tails. Chemistry cannot bridge this >10 nm gap, so the two events must be coupled through the TCR-CD3 interface, involving conformational allostery allowing force-based regulation. Although weak ECD cis-interactions between TCR{beta} and CD3 have been proposed to contribute to this coupling, their kinetics and mechanical competence remain elusive. Here, we quantify TCR{beta}-CD3 ECD cis-interactions in a pseudo-cis configuration by two-dimensional affinity and single-bond force spectroscopy, finding that TCR{beta}-CD3{gamma}{epsilon} and TCR{beta}-CD3{delta}{epsilon} bimolecular interactions have low affinity and rapid kinetics, yet form catch bonds. Critically, concurrent engagement of CD3{gamma}{epsilon} and CD3{delta}{epsilon} generates cis-cooperativity, yielding a stronger and longer-lasting CD3{gamma}{epsilon}-TCR{beta}-CD3{delta}{epsilon} trimolecular catch bond than the sum of the two dimeric bonds, with force-stabilized lifetimes matching those of agonist TCR-pMHC trans-interaction. Molecular dynamics simulations reveal an expanded, cooperative, and asymmetric contact network, making CD3{delta}{epsilon} more force-responsive and susceptible to conformational change than CD3{gamma}{epsilon}. Interface mutations do not alter force-free affinities but remodel force-dependent cis-bond profiles, leading to an inverse correlation with trans-bond profiles and T cell signaling. These results define cooperative ECD cis-interaction as a mechanically regulatable allosteric coupling element at the TCR-CD3 junction important to antigen recognition and signal initiation.