Developmental and Molecular Characterization of Emerging β- and γδ-Selected Pre-T Cells in the Adult Mouse Thymus

SummaryThe first checkpoint in T cell development, β selection, has remained incompletely characterized for lack of specific surface markers. We show that CD27 is upregulated in DN3 thymocytes initiating β selection, concomitant with intracellular TCR-β expression. Clonal analysis determined that CD27high DN3 cells generate CD4+CD8+ progeny with more than 90% efficiency, faster and more efficiently than the CD27low majority. CD27 upregulation also occurs in γδ-selected DN3 thymocytes in TCR-β−/− mice and in IL2-GFP transgenic reporter mice where GFP marks the earliest emerging TCR-γδ cells from DN3 thymocytes. With CD27 to distinguish pre- and postselection DN3 cells, a detailed gene expression analysis defined regulatory changes associated with checkpoint arrest, with β selection, and with γδ selection. γδ selection induces higher CD5, Egr, and Runx3 expression as compared to β selection, but it triggers less proliferation. Our results also reveal differences in Notch/Delta dependence at the earliest stages of divergence between developing αβ and γδ T-lineage cells.