Defined αβ T Cell Receptors with Distinct Ligand Specificities Do Not Require Those Ligands to Signal Double Negative Thymocyte Differentiation

During T cell development in the thymus, pre–T cell receptor (TCR) complexes signal CD4− CD8− (double negative [DN]) thymocytes to differentiate into CD4+ CD8+ (double positive [DP]) thymocytes, and they generate such signals without apparent ligand engagements. Although ligand-independent signaling is unusual and might be unique to the pre-TCR, it is possible that other TCR complexes such as αβ TCR or αγ TCR might also be able to signal the DN to DP transition in the absence of ligand engagement if they were expressed on DN thymocytes. Although αγ TCR complexes efficiently signal DN thymocyte differentiation, it is not yet certain if αβ TCR complexes are also capable of signaling DN thymocyte differentiation, nor is it certain if such signaling is dependent upon ligand engagement. This study has addressed these questions by expressing defined αβ TCR transgenes in recombination activating gene 2−/− pre-Tα−/− double deficient mice. In such double deficient mice, the only antigen receptors that can be expressed are those encoded by the αβ TCR transgenes. In this way, this study definitively demonstrates that αβ TCR can in fact signal the DN to DP transition. In addition, this study demonstrates that transgenic αβ TCRs signal the DN to DP transition even in the absence of their specific MHC–peptide ligands

Lck Availability during Thymic Selection Determines the Recognition Specificity of the T Cell Repertoire

SummaryThymic selection requires signaling by the protein tyrosine kinase Lck to generate T cells expressing αβ T cell antigen receptors (TCR). For reasons not understood, the thymus selects only αβTCR that are restricted by major histocompatibility complex (MHC)-encoded determinants. Here, we report that Lck proteins that were coreceptor associated promoted thymic selection of conventionally MHC-restricted TCR, but Lck proteins that were coreceptor free promoted thymic selection of MHC-independent TCR. Transgenic TCR with MHC-independent specificity for CD155 utilized coreceptor-free Lck to signal thymic selection in the absence of MHC, unlike any transgenic TCR previously described. Thus, the thymus can select either MHC-restricted or MHC-independent αβTCR depending on whether Lck is coreceptor associated or coreceptor free. We conclude that the intracellular state of Lck determines the specificity of thymic selection and that Lck association with coreceptor proteins during thymic selection is the mechanism by which MHC restriction is imposed on a randomly generated αβTCR repertoire

Novel MHC-Independent αβTCRs Specific for CD48, CD102, and CD155 Self-Proteins and Their Selection in the Thymus.

MHC-independent αβTCRs (TCRs) recognize conformational epitopes on native self-proteins and arise in mice lacking both MHC and CD4/CD8 coreceptor proteins. Although naturally generated in the thymus, these TCRs resemble re-engineered therapeutic chimeric antigen receptor (CAR) T cells in their specificity for MHC-independent ligands. Here we identify naturally arising MHC-independent TCRs reactive to three native self-proteins (CD48, CD102, and CD155) involved in cell adhesion. We report that naturally arising MHC-independent TCRs require high affinity TCR-ligand engagements in the thymus to signal positive selection and that high affinity positive selection generates a peripheral TCR repertoire with limited diversity and increased self-reactivity. We conclude that the affinity of TCR-ligand engagements required to signal positive selection in the thymus inversely determines the diversity and self-tolerance of the mature TCR repertoire that is selected