TCR-pMHC kinetics under force in a cell-free system show no intrinsic catch bond, but a minimal encounter duration before binding.

The T cell receptor (TCR)-peptide-MHC (pMHC) interaction is the only antigen-specific interaction during T lymphocyte activation. Recent work suggests that formation of catch bonds is characteristic of activating TCR-pMHC interactions. However, whether this binding behavior is an intrinsic feature of the molecular bond, or a consequence of more complex multimolecular or cellular responses, remains unclear. We used a laminar flow chamber to measure, first, 2D TCR-pMHC dissociation kinetics of peptides of various activating potency in a cell-free system in the force range (6 to 15 pN) previously associated with catch-slip transitions and, second, 2D TCR-pMHC association kinetics, for which the method is well suited. We did not observe catch bonds in dissociation, and the off-rate measured in the 6- to 15-pN range correlated well with activation potency, suggesting that formation of catch bonds is not an intrinsic feature of the TCR-pMHC interaction. The association kinetics were better explained by a model with a minimal encounter duration rather than a standard on-rate constant, suggesting that membrane fluidity and dynamics may strongly influence bond formation.