Foxp3+ regulatory T cells (Tregs) are potent suppressor cells, essential for the 
maintenance of immune homeostasis. Most Tregs develop in the thymus and are then 
released into the immune periphery. However, some Tregs populate the thymus and 
constitute a major subset of yet poorly understood cells. Here we describe a 
subset of thymus recirculating IL18R+ Tregs with molecular characteristics 
highly reminiscent of tissue-resident effector Tregs. Moreover, we show that 
IL18R+ Tregs are endowed with higher capacity to populate the thymus than their 
IL18R- or IL18R-/- counterparts, highlighting the key role of IL18R in this 
process. Finally, we demonstrate that IL18 signaling is critical for the 
induction of the key thymus-homing chemokine receptor - CCR6 on Tregs. 
Collectively, this study provides a detailed characterization of the mature Treg 
subsets in the mouse thymus and identifies a key role of IL18 signaling in 
controlling the CCR6-CCL20-dependent migration of Tregs into the thymus.