Temporal and Context-Dependent Requirements for the Transcription Factor Foxp3 in Regulatory T Cell Development and Maintenance [ATAC_bulk_adult]

Regulatory T (Treg) cells, characterized by expression of the lineage-defining transcription factor Foxp3, are critical gatekeepers of immune homeostasis. While Foxp3’s essential role in Treg cell development is well established, the precise mechanisms by which Foxp3 governs the Treg-specific transcriptional network remain incompletely understood. Here, we employed a novel chemogenetic model that enables inducible, time-controlled degradation of Foxp3 protein in vivo to dissect its stage-specific functions. We show that Foxp3 is indispensable for the establishment of the Treg transcriptional program and the acquisition of suppressive function during thymic differentiation and in newly generated peripheral Treg cells. In contrast, degradation of Foxp3 in fully differentiated, mature Treg cells resulted in surprisingly minimal transcriptional changes and largely preserved suppressive capacity, affecting only a small subset of genes highly enriched for direct Foxp3 targets. Strikingly, intratumoral Treg cells were uniquely sensitive to Foxp3 loss, leading to impaired suppressive function and enhanced tumor rejection. These findings uncover a differential, context-dependent requirement for Foxp3 across Treg cell ontogeny, highlighting a distinct, largely indirect mechanism by which Foxp3 establishes — and to a more limited extent maintains — Treg identity and function.