Inverted chromatin immunoprecipitation identifies Tcf1 as a negative regulator of Foxp3 expression

Regulatory T (Treg) cells are important regulators of the immune system and have versatile functions for the homeostasis and repair of tissues. They express the forkhead box transcription factor Foxp3 as a lineage-defining protein. In mature Treg cells, the Foxp3 core promoter is unmethylated indicating that this area could harbor a transcription factor complex to initiate or repress gene expression, respectively. We used an unbiased method to identify Foxp3-promoter-binding transcription factors (TFs) by inverted chromatin immunoprecipitation (IP) followed by quantitative mass spectrometry. We identified several candidate factors which showed Foxp3-promoter suppressive capacity, one of which was T-cell factor 1 (Tcf1). Using viral overexpression and CRISPR/Cas knockout studies, we identified Tcf1 as a repressor of Foxp3 expression in primary conventional CD4 T cells (Tconv). In Tcf1-deficient animals, increased levels of Foxp3intermediateCD25negative T cells were identified in secondary lymphoid tissues, implicating that Tcf1 protects Foxp3-negative T cells from inadvertent Foxp3 expression.