Aire controls the recirculation of murine Foxp3 regulatory T-cells back to the thymus.

In the thymus, medullary thymic epithelial cells (mTEC) determine the fate of newly selected CD4 and CD8 single positive (SP) thymocytes. For example, mTEC expression of Aire controls intrathymic self-antigen availability for negative selection. Interestingly, alterations in both Foxp3 Regulatory T-cells (T-Reg) and conventional SP thymocytes in Aire mice suggest additional, yet poorly understood, roles for Aire during intrathymic T-cell development. To examine this, we analysed thymocytes from Aire mice using Rag2GFP and Foxp3 expression, and a recently described CD69/MHCI subset definition of post-selection CD4 conventional thymocytes. We show that while Aire is dispensable for de novo generation of conventional αβT-cells, it plays a key role in controlling the intrathymic T-Reg pool. Surprisingly, a decline in intrathymic T-Reg in Aire mice maps to a reduction in mature recirculating Rag2GFP T-Reg that express CCR6 and re-enter the thymus from the periphery. Furthermore, we show mTEC expression of the CCR6 ligand CCL20 is reduced in Aire mice, and that CCR6 is required for T-Reg recirculation back to the thymus. Collectively, our study re-defines requirements for late stage intrathymic αβT-cell development, and demonstrates that Aire controls a CCR6-CCL20 axis that determines the developmental makeup of the intrathymic T-Reg pool.