Thymic production of human FOXP3(+) regulatory T cells is stable but does not correlate with peripheral FOXP3 expression

In humans functionally mature FOXP3⁺ regulatory T (Treg) cells can be found already in the fetus, but the kinetics of their maturation is still unknown. Here, we show that from birth to until 10 years of age the thymic production of FOXP3⁺ Treg cells is very stable and correlates with T-lymphopoiesis in general. The level of FOXP3 expression in the blood was also very stable, even when children and adults were compared, but there was no correlation between thymic and peripheral FOXP3 levels. Analysis of the cell cycle-associated marker Ki67 showed that a substantial fraction of peripheral FOXP3⁺ cells is dividing. This characteristic was obtained in the periphery, since it was not observed in thymic CD4⁺ FOXP3⁺ cells. These data suggest that the thymic output of human Treg cells is intrinsically stable, while in the periphery the increased rate of proliferation severs the connection between production and homeostatic maintenance of the FOXP3⁺ Treg cell population.