Immune homeostasis is dependent on tight control over the size of a population of regulatory T (T(reg)) cells capable of suppressing over-exuberant immune responses. The T(reg) cell subset is comprised of cells that commit to the T(reg) lineage by upregulating the transcription factor Foxp3 either in the thymus (tT(reg)) or in the periphery (iT(reg)). Considering a central role for Foxp3 in T(reg) cell differentiation and function, we proposed that conserved non-coding DNA sequence (CNS) elements at the Foxp3 locus encode information defining the size, composition and stability of the T(reg) cell population. Here we describe the function of three Foxp3 CNS elements (CNS1-3) in T(reg) cell fate determination in mice. The pioneer element CNS3, which acts to potently increase the frequency of T(reg) cells generated in the thymus and the periphery, binds c-Rel in in vitro assays. In contrast, CNS1, which contains a TGF-beta-NFAT response element, is superfluous for tT(reg) cell differentiation, but has a prominent role in iT(reg) cell generation in gut-associated lymphoid tissues. CNS2, although dispensable for Foxp3 induction, is required for Foxp3 expression in the progeny of dividing T(reg) cells. Foxp3 binds to CNS2 in a Cbf-beta-Runx1 and CpG DNA demethylation-dependent manner, suggesting that Foxp3 recruitment to this 'cellular memory module' facilitates the heritable maintenance of the active state of the Foxp3 locus and, therefore, T(reg) lineage stability. Together, our studies demonstrate that the composition, size and maintenance of the T(reg) cell population are controlled by Foxp3 CNS elements engaged in response to distinct cell-extrinsic or -intrinsic cues.
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