Thymus-derived Foxp3⁺ regulatory T cells upregulate RORγt expression under inflammatory conditions

Foxp3⁺ regulatory T cells (Tregs) co-expressing the Th17-lineage specification factor RORγt represent a unique Treg subpopulation that has been reported to be induced upon response to gut microbiota within the intestinal immune system. Hence, RORγt⁺ Tregs are considered to solely consist of peripherally induced Foxp3⁺ Tregs (pTregs), and the possibility that also thymus-derived Treg (tTregs) can upregulate RORγt expression and contribute to the pool of RORγt⁺ Tregs was largely ignored. Here, we expand our knowledge on the origin of RORγt⁺ Tregs by demonstrating that also tTregs can attain RORγt expression. In transgenic Foxp3^∆CNS1-Cre reporter mice, a substantial fraction of CNS1-independent Tregs, predominantly consisting of tTregs, was found to co-express RORγt. In addition, genuine tTregs isolated from thymi of Foxp3^hCD2RAG^GFP reporter mice initiated RORγt expression both in vitro and in vivo, particularly under inflammatory conditions. In conclusion, our data demonstrate that tTregs can upregulate RORγt expression under inflammatory conditions and that hence RORγt⁺ Tregs can be regarded as a heterogeneous population consisting of both pTregs and tTregs. KEY MESSAGES: RORγt cannot be considered as a marker for pTregs. CNS1-independent tTregs within the colon display RORγt expression. RORγt can be induced in genuine tTregs, particularly under inflammatory conditions. RORγt⁺ Tregs are a heterogeneous population consisting of both pTregs and tTregs.