Transcriptional control of regulatory T cell development and function

Abstract

Regulatory T (Treg) cells differentiate from thymocytes or peripheral T cells in response to host and environmental cues, culminating in induction of the transcription factor forkhead box P3 (Foxp3) and the Treg cell-specific epigenome. An intermediate amount of antigen stimulation is required to induce Foxp3 expression by engaging T cell receptor (TCR)-activated [e.g., nuclear factor (NF)-κB] and TCR-inhibited (e.g., Foxo) transcription factors. Furthermore, Treg cell differentiation is associated with attenuated Akt signaling, resulting in enhanced nuclear retention of Foxo1, which is indispensable for Treg cell function. These findings reveal that Treg cell lineage commitment is not only controlled by genetic and epigenetic imprinting, but also modulated by transcriptional programs responding to extracellular signals.

Keywords: Foxo1; Foxp3; cell signaling; transcription factor.

Figure 1

Transcriptional regulation of regulatory T (Treg) cell development. (A) Thymus-derived Treg (tTreg) cell differentiation. In developing thymocytes, T cell receptor (TCR) gene rearrangement generates diverse TCRs that recognize self-peptide major histocompatibility complex (pMHC) ligands at various intensities and durations. Thymocytes bearing TCRs that fail to productively interact with pMHC die by neglect, whereas those with extremely high affinity are eliminated by negative selection. TCR stimulation with relatively high intensity induces forkhead box P3 (Foxp3) expression, which is mediated in part by transcription factors including nuclear factor (NF)-κB, Foxo and nuclear receptor Nr4a. TCR/self-ligand interaction with proper duration induces Treg cell-specific epigenome. In the thymus, transforming growth factor (TGF)-β signaling promotes tTreg cell generation by restraining negative selection rather than direct transcriptional control of Foxp3. (B) Peripherally generated Treg (pTreg) and in vitro differentiated Treg (iTreg) development. Signaling cascades downstream of antigen stimulation, TGF-β, and retinoic acid (RA) signaling regulate Foxp3 gene expression from naive CD4⁺ T cells in the periphery. Transcription factors binding to the conserved noncoding sequence (CNS)1 element of the Foxp3 locus, nuclear factor of activated T cells Smad (NFAT), activator protein (AP)-1, Smad and, RA receptor (RAR), are shown. Abbreviation: Tconv, conventional T cell.

Figure 2

Transcriptional control of regulatory T (Treg) cell function. (A) Forkhead box P3 (Foxp3), the Treg lineage-specifying factor, is essential for the maintenance of Treg cell suppressive function. In addition, transcriptional factors (TFs) modulated by T cell receptor (TCR) signaling, including nuclear factor (NF)-κB, NFAT, activator protein (AP)-1 and Foxo1, may regulate Treg cell function. Foxo1 is unique among these TFs in that TCR engagement negatively regulates its activity. (B) Foxp3 does not function alone. Instead, Foxp3 cooperates with other nuclear factors, such as Runx, Ets, NFAT, interferon regulatory factor (IRF)4 and Eos, to establish the molecular signature and function of Treg cells. (C) In immature thymocytes, the Foxo family proteins Foxo1 and Foxo3 are both highly expressed. They bind to the Foxp3 locus and cooperatively induce Foxp3 gene transcription. The transcript of Foxo1 is upregulated during T cell maturation, whereas that of Foxo3 is downregulated. The nonredundant role of Foxo1 in mature Treg cells has been studied. Foxp3 and Foxo1 regulate largely independent transcription programs; both indispensable for Treg cell function. Notably, Foxo1 activity is subject to modulation of Akt kinase signaling, and Treg cells have dampened Akt signaling in response to TCR stimulation compared with conventional T (Tconv) cells.

Figure 3

Forkhead box (Fox)o-dependent program in T regulatory (Treg) cells. Akt kinase residues in a major node of T cell signaling that senses diverse extracellular inputs, such as antigen, co-stimulation, cytokine, and complement. These stimuli signal through phosphoinositide 3-kinase (PI3K) and activate Akt, which is counteracted by PTEN (phosphatase and tension homolog). Ligation of Treg cell-expressed receptor neuropilin-1 (Nrp1) with semaphorin-4a (Sema4a) restrains Akt activation via recruitment of PTEN. Activated Akt can inhibit Foxo transcriptional activity through phosphorylation at three conserved residues, resulting in cytoplasmic retention of Foxo. In the context of Treg cells, nuclear Foxo1 represses transcription of the proinflammatory cytokine interferon (IFN)-γ and promotes expression of chemokine CC receptor (CCR)7, which may collectively promote Treg cell function in vivo.