Satb1 enforces CD4+ effector T cell lineage stability by repressing Foxp3 via DNA methylation.

Special AT-rich sequence binding protein 1 (Satb1) is a critical chromatin organizer that globally regulates the T lymphocyte transcriptome. While its influence on primary T cell development is well documented, defining its specific function in mature peripheral T cells has been complicated by the developmental defects inherent to early-stage deletion models. To bypass these limitations, we utilized a Thpok-cre conditional knockout system to delete Satb1 specifically within mature CD4 single-positive thymocytes. We demonstrate that the ablation of Satb1 results in the spontaneous emergence of an atypical CD25-FoxP3+ population from naïve peripheral CD4+ T cells. This aberrant de-repression of Foxp3 occurs independently of TGFβ signaling and is fundamentally driven by alterations in DNA methylation, requiring the activity of the TET2 and TET3 demethylases. Furthermore, we show that this Satb1-deficient Foxp3 expression is unstable during in vitro activation and is insufficient to confer full regulatory T cell (Treg) functionality. Collectively, our findings reveal a critical requirement for Satb1 in enforcing the lineage stability and functional fitness of conventional CD4+ T cells by maintaining the epigenetic silencing of Foxp3, providing vital insights into T cell homeostasis.