Molecular design of the ?d T cell receptor ectodomain encodes biologically fit ligand recognition in the absence of mechanosensing

High acuity aßT cell receptor (TCR) recognition of peptides bound to MHC molecules (pMHC) requires mechanosensing, a process whereby piconewton (pN) bioforces exert physical load on aßTCR-pMHC bonds to dynamically alter their lifetimes and foster digital sensitivity cellular signaling. While mechanotransduction is operative for both aßTCRs and preTCRs within the aßT-lineage, its role in ?dT cells is unknown. Here we show that the human DP10.7 ?dTCR specific for the sulfoglycolipid sulfatide bound to CD1d only sustains significant load and undergoes force-induced structural transitions when the binding interface-distal ?d constant domain (C) module is replaced with that of aß. The chimeric ?d-aßTCR also signals more robustly than the wild-type ?dTCR as revealed by RNA-seq analysis of TCR-transduced Rag2-/- thymocytes, consistent with structural, single molecule and molecular dynamics studies reflective of ?dTCRs as mediating recognition via a more canonical immunoglobulin-like receptor interaction. Absence of robust, force-related catch bonds as well as ?dTCR structural transitions implies that ?dT cells do not use mechanosensing for ligand recognition. This distinction is consonant with the fact that their innate-type ligands, including markers of cellular stress, are expressed at high copy number relative to sparse pMHC ligands of aßT cells arrayed on activating target cells. We posit that mechanosensing emerged over ~200 million years of vertebrate evolution to fulfill indispensable adaptive immune recognition requirements for pMHC in the aßT cell lineage that are unnecessary for the ?d T cell lineage mechanism of non-pMHC ligand detection. Overall design: Comparison of a native ?dTCR with a chimeric ?dTCR where the constant domains have been replaced with those of an aßTCR, focusing on signaling properties during the DN3 and DN4 stages of thymocyte development. 2 receptors (?dTCR, ?d/aßTCR) x 2 stages (DN3, DN4) x 2 signaling environments (control, +sulfatide/CD1d agonist) x 3 replicates (except for 2 samples with 2 replicates) = 22 libraries