FOXP3 Expression Depends on Cell-Type Specific Cis-Regulatory Element and Transcription Factor Circuitry [CRISPRKO]

FOXP3 is a lineage-defining transcription factor (TF) for immune-suppressive regulatory T cells (Tregs). While mice exclusively express FOXP3 in Tregs, humans also transiently express FOXP3 in stimulated conventional CD4+ T cells (Tconvs). Mechanisms governing these distinct expression patterns remain unknown. Here, we performed CRISPR screens tiling the FOXP3 locus and targeting TFs in human Tregs and Tconvs to discover cis-regulatory elements (CREs) and trans-regulators of FOXP3. Tconv FOXP3 expression depended on a subset of Treg CREs and Tconv-selective positive (TcNS+) and negative (TcNS-) CREs. Combinatorial silencing of Tconv CREs revealed their epistatic logic. The CREs are occupied and regulated by TFs we identified as critical regulators of FOXP3. Finally, mutagenesis of murine TcNS- revealed that it is critical for restriction of FOXP3 expression to Tregs. We discover CRE and TF circuitry controlling FOXP3 expression and reveal evolution of mechanisms regulating a gene indispensable to immune homeostasis. Overall design: Pooled CRISPR screens in human regulatory T cells (Tregs) and conventional T cells (Tconvs). The CRISPR KO TF library targets transcription factors and select chromatin modifiers and immune genes of interest. Tconv screens were conducted in 3 donors. Treg screens were conducted in 2 donors.