ABSTRACT: The mechanism of egress of mature regulatory T cells (Tregs) from the thymus to the periphery remains enigmatic, as does the nature of those factors expressed in the thymic environment. Here, we examined the fate of thymic Tregs in TNFα/RelA double-knockout (TA-KO) mice, because TA-KO mice retain a Treg population in the thymus but have only a small Treg population at the periphery. Transplantation of whole TA-KO thymus to under the kidney capsule of Rag1 null mice failed to induce the production of donor-derived splenic Tregs expressing neuropilin-1 (Nrp1), which was reported to be a marker of naturally occurring Tregs, indicating that TA-KO thymic Tregs either do not leave the thymus or are lost at the periphery. We next transplanted enriched TA-KO thymic Tregs to the peripheries of TA-KO mice and traced mouse survival. Transplantation of TA-KO thymic Tregs rescued the lethality in TA-KO mice, demonstrating that TA-KO thymic Tregs remain functional at the periphery. The TA-KO thymic Treg population had highly demethylated CpG motifs in the foxp3 locus, indicating that the cells were arrested at a late-mature stage. Also, the population included a large subpopulation of Tregs expressing IL-7Rα, which is a possible marker of late-mature Tregs. Finally, TA-KO fetal liver chimeric mice developed an Nrp1+ splenic Treg population from TA-KO cells, suggesting that Treg arrest is caused by a lack of RelA in the thymic environment. Together, these results suggest that egress of mature Tregs from the thymus depends on RelA in the thymic environment. For the isolation of thymic Tregs, CD4+CD8α-CD25hi thymocytes were isolated from five 1.5- to 2-week-old TNFα-KO or TA-KO mice by using a FACSAria cell sorter. For the isolation of thymic stromal cells, 10 thymi from 1.5- to 2-week-old TNFα-KO or TA-KO mice were minced with scissors and treated with RPMI 1640 supplemented with 2% FCS, 0.2 mg/ml collagenase (Roche, Basel, Switzerland), 0.2 mg/ml dispase I (Roche), and 100 U/ml DNase I (Life Technologies) for 30 min with stirring. Digested thymi were centrifuged in a Percoll (GE Healthcare Bio-Sciences, Piscataway, NJ, USA) gradient (density, 1.115, 1.065, and PBS) at 1400g for 30 min. Cells in the upper layer were collected, and the CD45-EpCAM+ (thymic epithelial cells) and CD45+EpCAM- populations (enriched thymic stromal cells containing macrophages or dendritic cells) were sorted.
