Pre-T Cell Receptors (Pre-TCRs) Leverage Vβ Complementarity Determining Regions (CDRs) and Hydrophobic Patch in Mechanosensing Thymic Self-ligands.

The pre-T cell receptor (pre-TCR) is a pTα-β heterodimer functioning in early αβ T cell development. Although once thought to be ligand-autonomous, recent studies show that pre-TCRs participate in thymic repertoire formation through recognition of peptides bound to major histocompatibility molecules (pMHC). Using optical tweezers, we probe pre-TCR bonding with pMHC at the single molecule level. Like the αβTCR, the pre-TCR is a mechanosensor undergoing force-based structural transitions that dynamically enhance bond lifetimes and exploiting allosteric control regulated via the Cβ FG loop region. The pre-TCR structural transitions exhibit greater reversibility than TCRαβ and ordered force-bond lifetime curves. Higher piconewton force requires binding through both complementarity determining region loops and hydrophobic Vβ patch apposition. This patch functions in the pre-TCR as a surrogate Vα domain, fostering ligand promiscuity to favor development of β chains with self-reactivity but is occluded by α subunit replacement of pTα upon αβTCR formation. At the double negative 3 thymocyte stage where the pre-TCR is first expressed, pre-TCR interaction with self-pMHC ligands imparts growth and survival advantages as revealed in thymic stromal cultures, imprinting fundamental self-reactivity in the T cell repertoire. Collectively, our data imply the existence of sequential mechanosensor αβTCR repertoire tuning via the pre-TCR.