The regulation and differentiation of regulatory T cells and their dysfunction in autoimmune diseases

Abstract

The discovery of FOXP3⁺ regulatory T (T_reg) cells as a distinct cell lineage with a central role in regulating immune responses provided a deeper understanding of self-tolerance. The transcription factor FOXP3 serves a key role in T_reg cell lineage determination and maintenance, but is not sufficient to enable the full potential of T_reg cell suppression, indicating that other factors orchestrate the fine-tuning of T_reg cell function. Moreover, FOXP3-independent mechanisms have recently been shown to contribute to T_reg cell dysfunction. FOXP3 mutations in humans cause lethal fulminant systemic autoinflammation (IPEX syndrome). However, it remains unclear to what degree T_reg cell dysfunction is contributing to the pathophysiology of common autoimmune diseases. In this Review, we discuss the origins of T_reg cells in the periphery and the multilayered mechanisms by which T_reg cells are induced, as well as the FOXP3-dependent and FOXP3-independent cellular programmes that maintain the suppressive function of T_reg cells in humans and mice. Further, we examine evidence for T_reg cell dysfunction in the context of common autoimmune diseases such as multiple sclerosis, inflammatory bowel disease, systemic lupus erythematosus and rheumatoid arthritis.

Fig. 1 ∣. FOXP3-centred gene regulatory network: epigenetic modulation of T_reg cell function and stability.

FOXP3 plays a central role in governing the regulatory T (T_reg) cell gene regulatory network through both direct and indirect manners. Depending on interacting cofactors, FOXP3 can act as an activator or a repressor. Environmental factors, such as inflammation or nutrient availability, affect the epigenetic regulation of genes (DNA methylation, histone modifications and 3D genomic conformational changes) and can thereby directly and indirectly affect the expression of FOXP3 target genes. The FOXP3-centred gene regulatory network reinforces the expression of core T_reg cell signature genes, ensuring their stability. FOXP3 can also suppress the differentiation of conventional T cells by downregulating transcription factors (TFs) that promote the differentiation of these cells. This dual activity of FOXP3 synergistically maintains T_reg cell lineage stability and function. This multilayered FOXP3-centred gene regulatory network is indispensable for maintaining T_reg cell functionality and stability. ac, acetylation; me, methylation.

Fig. 2 ∣. cis-Regulatory elements in the FOXP3 locus that control T_reg cell induction, maintenance, stability and function.

The FOXP3 locus contains several regulatory elements, such as the conserved non-coding sequences CNS0–3 and the FOXP3 promoter element. These regions are bound by transcription factors and complexes of transcription factors, and the binding of these factors is controlled by DNA CpG methylation (CpG-me) and methylated histone H3 Lys4 (H3K4me1), which, in turn, is determined by the balance between DNA methyl transferases (DNMTs) 1–3 and the demethylating enzymes ten–eleven translocations (TETs) 1–3. Shown are transcription factors and the complexes they form that have been reported to bind to regulatory elements. Different conserved non-coding sequence regions play a role at different stages of regulatory T (T_reg) cell differentiation. iT_reg cell, induced FOXP3⁺ T_reg-type cell; pT_reg cell, peripheral T_reg cell; tT_reg cell, thymic T_reg cell.

Fig. 3 ∣. Transcription factors that regulate T_reg cell differentiation and function in the periphery.

Naive CD62L⁺CD44⁻TCF1⁺ regulatory T (nT_reg) cells that are stimulated via T cell receptor (TCR) signalling in the presence of IL-2 become CD62L⁻CD44^mid/hiTCF1⁺ activated regulatory T (aT_reg) cells and then differentiate into CD62L⁻CD44^hiTCF1⁻ effector regulatory T (eT_reg) cells. Specific transcription factors that regulate different eT_reg cell subsets and the differentiation steps of eT_reg cells are shown. Cell surface markers and transcription factors that change according to differentiation state from nT_reg cell to eT_reg cell are also shown (bottom). T_H1 cell, T helper 1 cell; T_H2 cell, T helper 2 cell; T_H17 cell, T helper 17 cell.

Fig. 4 ∣. Activation of the SGK1–FOXO1 axis is common to dysfunctional T_reg cells and pathogenic T_H17 cells.

IL-12 stimulation In the presence of a high salt environment induces regulatory T (T_reg) cell dysfunction. The upregulation of the serine/threonine kinase SGK1 due to the activation of β-catenin and/or the PI3K–AKT pathway under high salt conditions leads to the phosphorylation of FOXO1, which induces the translocation of FOXO1 from the nucleus to the cytosol where it becomes inactivated. This leads to reduced FOXP3 induction, higher IFNγ production and loss of T_reg cell suppressive function. T_reg cells from patients with multiple sclerosis express higher levels of the short isoform of the transcription factor BLIMP1 compared with T_reg cells from healthy individuals. This can upregulate SGK1 and, potentially, enhance FOXO1 inactivation. During the differentiation of T helper 17 (T_H17) cells, a high sodium environment activates p38 MAPK and NFAT5, which results in the activation of SGK1 and subsequent FOXO1 phosphorylation. The inactivation of phosphorylated FOXO1 allows for the derepression of the transcription factor RORγt, which, in turn, induces IL-23R expression. This promotes the pathogenic T_H17 cell phenotype with higher IFNγ, IL-17 and GM-CSF production. IRF, interferon regulatory factor.