Human recent thymic emigrants-identification, expansion, and survival characteristics

This study shows that, in humans at birth, circulating T cells represent recent thymic emigrants (RTEs) as reflected in their high level of expression of TCR excision circles. RTEs express "thymocyte-like" characteristics with regard to rapid rate of apoptosis. In the presence of common gamma -chain cytokines, in particular IL-7, they show enhanced potential to survive, entry into cell cycle, and proliferation. Although common gamma -chain cytokines were also potent antiapoptotic stimuli for mature adult-derived naive CD4(+)C13145RA(+) T cells, these cells were refractory to IL-7-induced expansion in vitro. RTEs cultured with IL-7 could not reinduce recombination-activating gene-2 gene expression in vitro. These data suggest that postthymic naive T cells in the periphery during early life are at a unique stage in ontogeny as RTEs, during which they can undergo homeostatic regulation including expansion and survival in an Ag-independent manner while maintaining their preselected TCR repertoire. The Journal of Immunology, 2001.