library_construction_protocol: Single cell suspensions were prepared from either spleens (for Steady state) or cervical draining LNs (for Oxazolone challenge day 1) from ICOS WT and ICOS FC mice.  Cells were stained with viability dye, anti-CD4 and anti-TCRβ. Live Treg (CD4+TCRβ+ YFP+) cells were sorted using a BD FACSAria (BD Biosciences) with more than 95% purity. A total of 12,000 cells from ICOS WT and ICOS FC mice were prepared for the library. Libraries were generated at the IRCM Molecular Biology and Functional Genomics Core Facility using the following components from 10x Genomics: Chromium Next GEM Chip G Single Cell kit, Chromium Next GEM Single Cell 3' GEM, Library & Gel Bead kit v3.1, and Chromium i7 Multiplex kit.