Pre-TCR ligand binding impacts thymocyte development before αßTCR expression.

Adaptive cellular immunity requires accurate self- vs. nonself-discrimination to protect against infections and tumorous transformations while at the same time excluding autoimmunity. This vital capability is programmed in the thymus through selection of αßT-cell receptors (αßTCRs) recognizing peptides bound to MHC molecules (pMHC). Here, we show that the pre-TCR (preTCR), a pTα-ß heterodimer appearing before αßTCR expression, directs a previously unappreciated initial phase of repertoire selection. Contrasting with the ligand-independent model of preTCR function, we reveal through NMR and bioforce-probe analyses that the ß-subunit binds pMHC using Vß complementarity-determining regions as well as an exposed hydrophobic Vß patch characteristic of the preTCR. Force-regulated single bonds akin to those of αßTCRs but with more promiscuous ligand specificity trigger calcium flux. Thus, thymic development involves sequential ß- and then, αß-repertoire tuning, whereby preTCR interactions with self pMHC modulate early thymocyte expansion, with implications for ß-selection, immunodominant peptide recognition, and germ line-encoded MHC interaction.