Cooperative binding of T cell receptor and CD4 to peptide-MHC enhances antigen sensitivity

Abstract

Antigen recognition by the T cell receptor (TCR) of CD4+ T cells 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). Here we show, using two-dimensional (2D) mechanical-based assays, that the affinity of CD4–pMHC interaction is 3-4 logs lower than that of cognate TCR–pMHC interactions, and it is more susceptible to increased dissociation by forces (slip bond). In contrast, CD4 binds TCR-pre-bound pMHC at 3-6 logs higher affinity, forming TCR–pMHC–CD4 tri-molecular 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 we show that 7-nm proximity optimizes TCR–pMHC–CD4 tri-molecular bond formation with pMHC. Our results thus provide deep mechanistic insight into CD4 enhancement of TCR antigen recognition.

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

“…This excludes the Bell model and the DHS model because both of their γ functions depend on F monotonically. Our goal is to construct a biphasic γ ( F ) with appropriate structural-elastic dependency to account for TCR– pMHC conformational changes from the bound state to the transition state, as analyzed by the biomembrane force probe (BFP) and optical tweezers (OT) experiments where a tensile force is applied to its two ends to modulate bond dissociation 4-10,13 .…”

“…g The three model parameters in f for both the 3.L2 and 2B4 TCR systems are plotted vs the catch-bond intensity I and fitted by a straight line. We also added to each panel an additional point obtained from data and model fit of E8 TCR–TPI:HLA-DR1 interactions 13 . All error bars in e-g are fitting error (Supplementary Table 6).…”

“…Mechanical forces applied to αβTCR via engaged pMHC substantially increase antigen sensitivity and amplify antigen discrimination 4-10,12 . As a fundamental force-elicited characteristic, strong cognate pMHCs form catch-slip bonds with TCR where bond lifetimes increase with force until reaching a peak, and decrease as force increases further, whereas weak agonist and antagonist pMHCs form slip-only bonds with TCR where bond lifetimes decrease monotonically with increasing force 4-10,13 . However, the mechanism underlying the correlation between the force-lifetime pattern and the ability for force on TCR to induce T cell signaling remains unclear.…”

“…Second, TCR–pMHC interactions exhibiting catch-slip bonds often have longest lifetimes around 10-20 pN (Ref. 4-10,13 ) and it has been reported that upon engaging pMHC, T cells would exert 12-19 pN endogenous forces on the TCR in a signaling-dependent fashion 15 . However, the relevance of this force range to T cell signaling remains incompletely understood.…”

“…1b). We applied our models and a published model 27 to analyze 49 TCR–pMHC bond lifetime vs force datasets published to date in 9 papers by four laboratories measured using two different techniques 4-6,8,9,13,16,28,29 , plus 6 new datasets generated in this study, including 12 TCRs and their mutants expressed on the cell membrane or coated on beads interacting with corresponding panels of both classes of pMHCs without coreceptor engagement. This analysis demonstrated the superior power of our models’ structural and physical parameters to quantitatively integrate and classify a broad range of catch-slip and slip-only bond behaviors as well as their corresponding biological activities.…”