Cooperative binding of TCR and CD4 to pMHC enhances TCR sensitivity

Abstract

Antigen recognition of CD4+ T cells by the T cell receptor (TCR) can be greatly enhanced by the coreceptor CD4. Yet, understanding of the molecular mechanism is hindered by the ultra-low affinity of CD4 binding to class-II peptide-major histocompatibility complexes (pMHC). Using two-dimensional (2D) mechanical-based assays, we determined a CD4–pMHC interaction to have 3-4 logs lower affinity than cognate TCR–pMHC interactions, and to be susceptible to increased dissociation by forces (slip bond). In contrast, CD4 binds TCR-prebound pMHC at 5-6 logs higher affinity, forming TCR–pMHC–CD4 trimolecular bonds that are prolonged by force (catch bond) and modulated by protein mobility on the cell membrane, indicating profound TCR–CD4 cooperativity. Consistent with a tri-crystal structure, using DNA origami as a molecular ruler to titrate spacing between TCR and CD4 indicates 7-nm proximity enables optimal trimolecular bond formation with pMHC. Our results reveal how CD4 augments TCR antigen recognition.

Catch bond models may explain how force amplifies TCR signaling and antigen discrimination

“…We introduced the catch-bond intensity I as a dimensionless scaled metric for the bond lifetime vs force curve and generated four model parameters that describe the curve’s geometric features. Upon analyzing all catch-slip and slip-only bond profiles (a total of 48 curves) published to date by four independent laboratories 4–6,8,13,15,32,38,39 , we found that these quantities do a better job to predict TCR function than any other quantities. This finding explains how force amplifies TCR signaling and antigen discrimination, because I is defined by a force curve and n * and θ only predict signaling when they assume none-zero values at F > 0.…”

“…Another boundary condition to be considered by future studies is that imposed by the coreceptor CD4 and CD8, as the co-ligation of the coreceptor prolongs bond lifetimes, amplifies catch bonds, and may even changes slip-only bonds to catch-slip bonds 7,13,39 . Future studies should also consider how to extend the current models to pre-TCR catch and slip bonds with a broad range of ligands 37,41 .…”

“…All data are included in the article and Supplementary Materials. Previously published bond lifetime data were re-analyzed for models fitting 41–6,8,13,15,32,39 .…”

“…1, re-analyzed from Refs. 4,5,8,13,32,39 ): OT1 (a) or 2C (b) TCR expressed on CD8 + naïve T cells interacting with indicated p:H2-K b α3A2; WT or mutant 2C (c) or 1G4 (d) TCR expressed on hybridomas interacting with indicated peptides presented by WT or MT H2-K b or HLA-A2. B13.C1 and B17.C1 TCR expressed on hybridomas interacted with NP366 bound to the D227K MT of H-2D b to prevent CD8 binding (e).…”

“…The TCR–pMHC complexes in the middle and right panels are drawn based on cryoEM (6JXR for TCR-CD3 complex) or crystal (2CKB for TCR–pMHC complex) structures. b , c, Fitting of theoretical 1/ k ( F ) ( curves ) to experimental bond lifetime vs force data ( points , mean ± sem) of the following interactions: OT1 or 2C TCR respectively expressed on CD8 + naïve T cells ( red ) or CD8 − hybridomas ( green ) or coated on beads ( pink for OT1 and yellow-green for 2C) respectively interacting with OVA:H2-K b α3A2 or QL9:H2-L d (m3), both MHC class I molecules 38 (b) or of E8 TCR expressed on CD8 − Jurkat ( sky-blue ) or coated on beads ( blue ) interacting with TPI:HLA-DR1, a MHC class II 39 (c). d, Comparison of the catch bond intensity ( top ) and best-fit parameters ( middle ) and θ ( bottom ) evaluated from data of pMHC interactions with cell surface vs purified TCRs in b and c. See Supplementary Table 3 and 5 for lists of the interacting molecules.…”