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

Abstract

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.

MHC Restriction: Where Are We Now?

“…One study found that the formation of TCR-pMHC-CD8 catch bonds was dependent on the kinase activity of Lck and its ability to localize to CD8 and CD3, with inhibition of Lck kinase activity and mutation of several CD3z ITAMS resulting in a reduced ability of TCR and CD8 to form a catch bond with pMHC (39). A more recent study showed, in a cell-free system that precluded the contribution of the cellular response to catch bond formation, that intrinsic catch bonds were not formed by any of the five agonist TCR-pMHC ligand pairs studied, and that off-rates of binding were the best predictor of activation potency (54). Thus, our understanding of how the TCR-pMHC-coreceptor bond formation is initiated and changes over the duration of the encounter, and how that drives (or is driven) by downstream signaling events continues to evolve.…”

“…However, a couple of very recent studies have called into question whether catch-versus slip-bond formation is a cause or a consequence of a productive TCR-pMHC interaction (39,54). One study found that the formation of TCR-pMHC-CD8 catch bonds was dependent on the kinase activity of Lck and its ability to localize to CD8 and CD3, with inhibition of Lck kinase activity and mutation of several CD3z ITAMS resulting in a reduced ability of TCR and CD8 to form a catch bond with pMHC (39).…”

Controlling T cells shape, mechanics and activation by micropatterning

“…Recently, and driven by technological, biophysical and biological developments, forces that may be at play at the interface of the T cell and APC while the former scans the latter have been proposed to be crucial for T cell recognition and subsequent activation [18][19][20][21] . Experiments at single molecule scale have tested if TCR/pMHC bond rupture forces 22 or lifetimes 23,24 depends on the quality of the peptide and have proposed that this bond could exhibit a behaviour more complex than expected, namely a catch bond behaviour 23 , where, depending on the peptide, the lifetime of the bond can be modulated and even extended for a range of forces compatible with the ones that cell protrusions may exert 25,26 . The geometry of the force application has also been investigated using refined micromanipulation methods [27][28][29] .…”

How drag sharpens a T cell’s view on antigen

“…Limozin et al (13) further juxtaposed the measured rupture times to EC 50 values measured in T cell activation assays. While they found only a weak correlation under force-free conditions, high correlation was observed under shear forces in the low piconewton regime.…”

“…By placing slip bonds back into the spotlight, the study by Limozin et al (13) adds important arguments to the ongoing discussion regarding the role of forces in T cell antigen recognition. Interestingly, experimental outcomes suggested a relevant force range between 5 pN and 10 pN, which is somewhat lower than was previously measured for the peak of typical TCR-pMHC catch bonds.…”