Visualization of the earliest steps of gammadelta T cell development in the adult thymus.

The checkpoint in cd cell development that controls successful T cell receptor (TCR) gene rearrangements remains poorly characterized. Using mice expressing a reporter gene ‘knocked into’ the Tcrd constant region gene, we have characterized many of the events that mark the life of early cd cells in the adult thymus. We identify the developmental stage during which the Tcrd locus ‘opens’ in early T cell progenitors and show that a single checkpoint controls cd cell development during the penultimate CD4 – CD8 – stage. Passage through this checkpoint required the assembly of cd TCR heterodimers on the cell surface and signaling via the Lat adaptor protein. In addition, we show that cd selection triggered a phase of sustained proliferation similar to that induced by the pre-TCR.