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][1]–[7][2]. 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][1],[7][2]–[10][3]. 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][4], and to be susceptible to increased dissociation by forces (slip bond)[5][5],[8][4],[11][6]. 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][5],[8][4],[11][6] and modulated by protein mobility on the cell membrane, indicating profound TCR-CD4 cooperativity. Consistent with a tri-crystal structure[12][7], 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. ### Competing Interest Statement The authors have declared no competing interest. [1]: #ref-1 [2]: #ref-7 [3]: #ref-10 [4]: #ref-8 [5]: #ref-5 [6]: #ref-11 [7]: #ref-12