Transcription factor TCF1 binds Rorgt and orchestrates a regulatory network that determines homeostatic Th17 cell state [ATAC-seq]
CD4+ T helper 17 (Th17) cells encompass a spectrum of cell states including homeostatic cells that maintain physiological functions such as barrier integrity and pathogenic cells that drive autoimmune tissue inflammation. Identifying the regulators that determine these cell states will provide means to control tissue inflammation without compromising the physiological functions of Th17 cells. Here, we identified TCF1 as the key regulator that determines Th17 cell state. IL-23, a cytokine critical for inducing pathogenic Th17 cells, decreased TCF1 expression. Consistent with this observation, conditional deletion of Tcf7/TCF1 in mature myelin-specific T cells conferred pathogenicity to homeostatic Th17 cells independent of IL-23. Conversely, sustained TCF1 expression impaired acquisition of pathogenicity. Integration of transcriptional and chromatin accessibility data showed that TCF1 maintained homeostatic state through a regulatory network involving ETS family transcription factors, EGR1, and FOXO1 and by binding to and interfering with ROR?t activity. Our findings provide mechanistic insight into how the homeostatic and pathogenic Th17 cell states are determined and into the association of genetic variants in TCF7 with susceptibility to autoimmunity. Overall design: ATAC-seq for 2D2 Th17cells between 2D2 dlck-Cre+Tcf7 fl/fl IL17aGFP+ and 2D2 dlck-Cre- Tcf7fl/fl IL17aGFP+  in mouse