Pre-TCR ligand binding impacts thymocyte development before αβTCR 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 alpha beta T-cell receptors (alpha beta TCRs) recognizing peptides bound to MHC molecules (pMHC). Here, we show that the pre-TCR (preTCR), a pT alpha-beta heterodimer appearing before alpha beta TCR 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 beta-subunit binds pMHC using V beta complementarity-determining regions as well as an exposed hydrophobic V beta patch characteristic of the preTCR. Force-regulated single bonds akin to those of alpha beta TCRs but with more promiscuous ligand specificity trigger calcium flux. Thus, thymic development involves sequential beta- and then, alpha beta-repertoire tuning, whereby preTCR interactions with self pMHC modulate early thymocyte expansion, with implications for beta-selection, immunodominant peptide recognition, and germ line-encoded MHC interaction.