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

Programming T lymphocytes to distinguish self from non-self is a vital, multi-step process arising in the thymus. Signalling through the pre-T cell receptor (preTCR), a CD3-associated heterodimer comprising an invariant pTa chain and a clone-specific ß chain, constitutes a critical early checkpoint in thymocyte development within the aß T-cell lineage. Recent work demonstrates that preTCRs arrayed on double negative (DN) thymocytes, like aß TCRs appearing on double positive (DP) thymocytes, ligate peptides bound to MHC molecules (pMHC) on thymic stroma but via a different molecular docking strategy. Here we show the consequences of those distinctive interactions for thymocyte progression, using synchronized fetal thymic progenitor cultures differing in the presence or absence of pMHC on support stroma, determining single cell transcriptomes at key thymocyte developmental transitions. Although MHC negative stroma fosters aß T lymphocyte differentiation, the absence of pMHC-preTCR interplay leads to deviant thymocyte transcriptional programming associated with de-differentiation. Highly proliferative DN and DP subsets with antecedent characteristics of T cell lymphoblastic and myeloid malignancies emerge. Thus, in addition to fostering ß chain repertoire broadening for subsequent aß TCR utilization, preTCR-pMHC interaction limits cellular plasticity to facilitate normal thymocyte differentiation and proliferation that, if absent, introduces significant developmental vulnerabilities. Overall design: Transcriptional profiling of early thymocytes developing in the presence or absence of peptide-MHC