Identification of a Late Stage of Small Noncycling pTα−  Pre-T Cells as Immediate Precursors of T Cell Receptor α/β+  Thymocytes

During thymocyte development, progression from T cell receptor (TCR)β to TCRα rearrangement is mediated by a CD3-associated pre-TCR composed of the TCRβ chain paired with pre-TCRα (pTα). A major issue is how surface expression of the pre-TCR is regulated during normal thymocyte development to control transition through this checkpoint. Here, we show that developmental expression of pTα is time- and stage-specific, and is confined in vivo to a limited subset of large cycling human pre-T cells that coexpress low density CD3. This restricted expression pattern allowed the identification of a novel subset of small CD3− thymocytes lacking surface pTα, but expressing cytoplasmic TCRβ, that represent late noncycling pre-T cells in which recombination activating gene reexpression and downregulation of T early α transcription are coincident events associated with cell cycle arrest, and immediately preceding TCRα gene expression. Importantly, thymocytes at this late pre-T cell stage are shown to be functional intermediates between large pTα+ pre-T cells and TCRα/β+ thymocytes. The results support a developmental model in which pre-TCR–expressing pre-T cells are brought into cycle, rapidly do