Suppressive IL-17A⁺ Foxp3⁺ and ex-Th17 IL-17A^neg Foxp3⁺ T_reg cells are a source of tumour-associated T_reg cells

Th17 and regulatory T (T_reg) cells are integral in maintaining immune homeostasis and Th17-T_reg imbalance is associated with inflammatory immunosuppression in cancer. Here we show that Th17 cells are a source of tumour-induced Foxp3⁺ cells. In addition to natural (n)T_reg and induced (i)T_reg cells that develop from naive precursors, suppressive IL-17A⁺ Foxp3⁺ and ex-Th17 Foxp3⁺ cells are converted from IL-17A⁺ Foxp3 neg cells in tumour-bearing mice. Metabolic phenotyping of Foxp3-expressing IL-17A⁺, ex-Th17 and iT_reg cells demonstrates the dissociation between the metabolic fitness and the suppressive function of Foxp3-expressing T_reg cell subsets. Although all Foxp3-expressing subsets are immunosuppressive, glycolysis is a prominent metabolic pathway exerted only by IL-17A⁺ Foxp3⁺ cells. Transcriptome analysis and flow cytometry of IL-17A⁺ Foxp3⁺ cells indicate that Folr4, GARP, Itgb8, Pglyrp1, Il1rl1, Itgae, TIGIT and ICOS are Th17-to-T_reg cell transdifferentiation-associated markers. Tumour-associated Th17-to-T_reg cell conversion identified here provides insights for targeting the dynamism of Th17-T_reg cells in cancer immunotherapy.