CD4+CD8+ double-positive (DP) T cells represent a minor subpopulation of T 
lymphocytes found in the periphery of adult rats. In this study, we show that 
peripheral DP T cells appear among the first T cells that colonize the 
peripheral lymphoid organs during fetal life, and represent approximately 40% of 
peripheral T cells during the perinatal period. Later their proportion decreases 
to reach the low values seen in adulthood. Most DP T cells are small size 
lymphocytes that do not exhibit an activated phenotype, and their proliferative 
rate is similar to that of the other peripheral T cell subpopulations. Only 
30-40% of DP T cells expresses CD8beta chain, the remaining cells expressing 
CD8alphaalpha homodimers. However, both DP T cell subsets have an intrathymic 
origin since they appear in the recent thymic emigrant population after 
injection of FITC intrathymically. Functionally, although DP T cells are 
resistant to undergo apoptosis in response to glucocorticoids, they show poor 
proliferative responses upon CD3/TCR stimulation due to their inability to 
produce IL-2. A fraction of DP T cells are not actively synthesizing the CD8 
coreceptor, and they gradually differentiate to the CD4 cell lineage in 
reaggregation cultures. Transfer of DP T lymphocytes into thymectomized SCID 
mice demonstrates that these cells undergo post-thymic maturation in the 
peripheral lymphoid organs and that their CD4 cell progeny is fully 
immunocompetent, as judged by its ability to survive and expand in peripheral 
lymphoid organs, to proliferate in response to CD3 ligation, and to produce IL-2 
upon stimulation.