Distribution of genome-wide BACH2 or JunD binding sites identified by ChIP-Seq in iTreg cells

In various tissues, cellular homeostasis is achieved by functionally quiescent stem cells which selfrenew while giving rise to differentiated progeny. Regulatory T (Treg) cells are composed of functionally quiescent resting Treg (rTreg) cells which differentiate into activated Treg (aTreg) cells upon antigen stimulation. How rTreg cells remain quiescent despite chronic exposure to cognate self- and foreign antigens is unclear. The transcription factor BACH2 is critical for early Treg lineage specification but its function following lineage commitment is unresolved. Here, we show Bach2 is highly expressed in lineage-committed rTreg cells but is downregulated in aTreg cells, and upon inflammation. BACH2 binds to AP-1 sites within Treg genomes and its high expression within rTreg cells functions to restrain their TCR-driven activation and differentiation into aTreg cells. As a consequence, cell-autonomous BACH2 expression is required following Treg lineagecommitment for the functional quiescence and long-term maintenance of Treg cell populations and for immune homeostasis. ChIP-seq profiles from iTreg cells, purified by FACS from WT mice, were generated by deep sequencing using Illumina HiSeq2000.