Pre–T cell receptors topologically sample self-ligands during thymocyte β-selection

Abstract

Self-discrimination, a critical but ill-defined molecular process programmed during thymocyte development, requires myriad pre–T cell receptors (preTCRs) and αβTCRs. Using x-ray crystallography, we show how a preTCR applies the concave β-sheet surface of its single variable domain (Vβ) to “horizontally” grab the protruding MHC α2-helix. By contrast, αβTCRs purpose all six complementarity-determining region (CDR) loops of their paired VαVβ module to recognize peptides bound to major histocompatibility complex molecules (pMHCs) in “vertical” head-to-head binding. The preTCR topological fit ensures that CDR3β reaches the peptide’s featured C-terminal segment for pMHC sampling, establishing the subsequent αβTCR canonical docking mode. “Horizontal” docking precludes germline CDR1β- and CDR2β-MHC binding to broaden β-chain repertoire diversification before αβTCR-mediated selection refinement. Thus, one subunit successively attunes the recognition logic of related multicomponent receptors.

Pre-T cell receptor Self-MHC Sampling Restricts Thymocyte Dedifferentiation

“…Nonetheless, preTCR signalling is not entirely normal as evidenced by the failure to observe upregulation of Trav and Bcl2a1, in agreement with the suggestion that the narrow width of the MHC1b α1α2 presenting platform relative to that of MHC1b might attenuate preTCR signaling 8 .…”

“…In contrast, 5 of the 8 selected genes in the DN1/2a-ALL “early” panel were significantly upregulated, and the remainder all trended upwards, compatible with the cells dedifferentiating from a DN4 state back towards the early progenitor state. As no significant differences were noted in CDR3 length or hydropathy between DN4 Vβ clonotypes developing on MHC + versus MHC - stroma ( 8 and Supplemental Information File 3), the abnormal transcriptome likely emanates from lack of preTCR ligation by MHC and not aberrant preTCR sequences per se .…”

“…Recent structural and biophysical data, however, reveal direct interactions between preTCRs and pMHC ligands that utilize a horizontal binding mode compatible with facile mechanosensing 8, 10, 24 . Moreover, functional assays demonstrate both restricted proliferation and repertoire development in the absence of stromal MHCI and MHCII molecules 7, 8 . Together these findings necessitate re-examination of the earlier results.…”

Stepwise progression of β-selection during T cell development as revealed by histone deacetylation inhibition

“…Interestingly, it is now emerging that differential CD5 expression also marks the propensity for different fates in thymocytes and CD4 + T cells, and this reflects transcriptional and epigenetic differences between cells with high and low CD5 expression (Rogers et al, 2021). A role for tuning of the TCRβ repertoire during β selection is made relevant by findings that the pre-TCR can bind with low affinity to MHC-peptide (Li et al, 2021;Mallis et al, 2015). The pre-TCR also forms an immunological synapse with many similarities to the immunological synapse formed by the αβTCR, oriented towards MHC (Allam et al, 2021).…”

A Leucine Zipper Dimerization Strategy to Generate Soluble T Cell Receptors Using the Escherichia coli Expression System

“…Each αβ T cell contains a unique αβ T cell receptor (TCR) that engages with antigens and initiates signaling transduction [1][2][3]. T cells develop in the thymus, where they undergo TCR β-selection, positive selection, and negative selection before migrating to peripheral lymphoid tissues [4][5][6][7]. T cells remain naïve until they have encountered their specific antigens in the form of a peptide-bound major histocompatibility complex (pMHC) on the surface of professional antigen-presenting cells such as dendritic cells [8] and macrophages [9].…”