Clone gk1

For FACS analysis, single-cell suspensions were stained with the following antibodies: anti-CD4 (BioLegend, clone GK1.5, #100402, 1:400), anti-CD4 (BD Biosciences, clone RM4-5, #560468, 1:500), anti-CD19 (Thermo Fisher Scientific, clone eBio1D3, #45-0193-82, 1:400), and anti-CD8 (BD Biosciences, clone 53-6.7, #560776, 1:500). For intracellular staining, T cells were restimulated with 50 ng/ml PMA and 750 ng/ml ionomycin in the presence of 10 μg/ml brefeldin A (all from Sigma-Aldrich) for 4 h. After fixation and permeabilization, the cells were stained with anti-IL-17A (Thermo Fisher Scientific, clone eBio17B7, #17-7177-81, 1:200) and anti-IFN-γ (Thermo Fisher Scientific, clone XMG1.2, #17-7311-82, 1:500). For detection of apoptotic cells, the cultured Bregs were harvested, washed with HBSS buffer and resuspended in a HBSS solution containing FITC-labeled Annexin V (Thermo Fisher Scientific, #88-8005-72, 1:100). To measure the cell proliferation, the staining with dye carboxyfluorescein succinimidyl ester (5 µM CFSE, Sigma-Aldrich) was performed. The cells were analyzed using FACSCalibur cytometer or BD FACSAria III cell sorter (both BD Biosciences). Data were analyzed with FlowJo analysis software (TreeStar). All FACS sequential gating/sorting strategies for the analysis of T and B cells are provided in Supplementary Fig. 7.