Cooperative binding of TCR and CD4 to pMHC enhances TCR sensitivity

Antigen recognition of CD4 + T cells by the T cell receptor (TCR) can be greatly enhanced by the coreceptor CD4 1–7 . Yet, understanding of the molecular mechanism is hindered by the ultra-low affinity of CD4 binding to class-II peptide-major histocompatibility complexes (pMHC) 1,7–10 . 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 8 , and to be susceptible to increased dissociation by forces (slip bond) 5,8,11 . In contrast, CD4 binds TCR-prebound pMHC at 3-6 logs higher affinity, forming TCR–pMHC–CD4 trimolecular bonds that are prolonged by force (catch bond) 5,8,11 and modulated by protein mobility on the cell membrane, indicating profound TCR-CD4 cooperativity. Consistent with a tri-crystal structure 12 , using DNA origami as a molecular ruler to titrate spacing between TCR and CD4 indicates that 7-nm proximity optimizes trimolecular bond formation with pMHC. Our results reveal how CD4 augments TCR antigen recognition.