Recirculating IL-1R2 Tregs fine-tune intrathymic Treg development under inflammatory conditions.

The vast majority of Foxp3 regulatory T cells (Tregs) are generated in the thymus, and several factors, such as cytokines and unique thymic antigen-presenting cells, are known to contribute to the development of these thymus-derived Tregs (tTregs). Here, we report the existence of a specific subset of Foxp3 Tregs within the thymus that is characterized by the expression of IL-1R2, which is a decoy receptor for the inflammatory cytokine IL-1. Detailed flow cytometric analysis of the thymocytes from Foxp3xRAG1 reporter mice revealed that the IL-1R2 Tregs are mainly RAG1 and CCR6CCR7, demonstrating that these Tregs are recirculating cells entering the thymus from the periphery and that they have an activated phenotype. In the spleen, the majority of IL-1R2 Tregs express neuropilin-1 (Nrp-1) and Helios, suggesting a thymic origin for these Tregs. Interestingly, among all tissues studied, the highest frequency of IL-1R2 Tregs was observed in the thymus, indicating preferential recruitment of this Treg subset by the thymus. Using fetal thymic organ cultures (FTOCs), we demonstrated that increased concentrations of exogenous IL-1β blocked intrathymic Treg development, resulting in a decreased frequency of CD25Foxp3 tTregs and an accumulation of CD25Foxp3 Treg precursors. Interestingly, the addition of IL-1R2 Tregs, but not IL-1R2 Tregs, to reaggregated thymic organ cultures (RTOCs) abrogated the IL-1β-mediated blockade, demonstrating that these recirculating IL-1R2 Tregs can quench IL-1 signaling in the thymus and thereby maintain thymic Treg development even under inflammatory conditions.