Molecular determinants of regulatory T cell development: the essential roles of epigenetic changes

Abstract

Regulatory T (Treg) cells constitute a distinct T cell subset, which plays a key role in immune tolerance and homeostasis. The transcription factor Foxp3 controls a substantial part of Treg cell development and function. Yet its expression alone is insufficient for conferring developmental and functional characteristics of Treg cells. There is accumulating evidence that concurrent induction of Treg-specific epigenetic changes and Foxp3 expression is crucial for lineage specification and functional stability of Treg cells. This review discusses recent progress in our understanding of molecular features of Treg cells, in particular, the molecular basis of how a population of developing T cells is driven to the Treg cell lineage and how its function is stably maintained.

Keywords: DNA methylation; Foxp3; adaptability; epigenetics; lineage specification; plasticity; regulatory T cells.

Figure 1

Various mechanisms of Foxp3-dependent gene regulation in Treg cells. Some genes are directly regulated by Foxp3 alone (A), while others require the protein complexes containing Foxp3 and its co-factors for transcriptional regulation. Foxp3 can interact with pre-existing transcription factors such as Runx1 and Ets-1 (B) or with direct targets of Foxp3-mediated gene regulation, such as GATA-3 (C) (Rudra et al., 2012). Furthermore, there are also genes regulated by both Foxp3 and epigenetic changes. For example, at Foxp3 locus, epigenetic modifications unveil normally hidden enhancer and allow the transcriptional activation by Foxp3 and its co-factors (D) (Floess et al., ; Schmidl et al., ; Zheng et al., 2010).

Figure 2

The roles of epigenetic changes in stabilizing Foxp3 expression. Epigenetic changes during Treg cell development are important for long-term stability of Treg phenotypes, particularly Foxp3 expression. Foxp3 CNS2 in naïve CD4⁺ T cells shows repressive histone markers, low accessibility for transcription factors, and methylated CpG residues, likely attracting methyl-CpG-binding domains (MBDs). In the course of Treg cell development, epigenetic changes take place and accessibility of CNS2 increases by DNA demethylation, histone modifications, and possibly nucleosome repositioning (Ohkura et al., ; Samstein et al., 2012). The CNS2 region serves as an enhancer for Foxp3 transcription and is bound by transcription factors such as Foxp3, Ets-1, and CREB. These epigenetic alterations are maintained irrespective of environmental changes and thus allow stable Foxp3 transcription by constitutively expressed transcription factors. In contrast, Foxp3 expression induced by TGF-β signaling and TCR stimulation in vitro is unstable. These signals induce transcription factors, such as NFAT, AP-1, and Smad3, which are capable of activating Foxp3 transcription, and TGF-β signaling can also alter histone modifications of the Foxp3 locus (Tone et al., 2008). However, these features cannot be maintained once TGF-β signaling and TCR stimulation are lost, resulting in loss of Foxp3 transcription (Ohkura et al., 2012).

Figure 3

Adaptability of Treg cells. Treg cells effectively regulate immune responses in various contexts by flexibly adapting to the environments. While most Treg cells are generated in the thymus, some are induced from Tconv cells in the periphery, particularly in the intestine, where they play vital roles in maintaining the immune homeostasis with commensal microbes. Recent findings show that in local tissues such as adipose tissues, Treg cells, either induced locally or migrating from the lymphoid organs, exhibit unique characteristics, allowing specialized immune regulation (Cipolletta et al., 2012). Furthermore, during inflammation, Treg cells respond to environmental stimuli and adopt certain features of helper T cell characteristics to facilitate the immune regulation (Koch et al., ; Zheng et al., 2009). However, there are accumulating findings suggesting that strong stimulation by cytokines such as IL-12 induces not only the additional transcription factors and chemokine receptors but also pro-inflammatory cytokines in Treg cells (Oldenhove et al., ; McClymont et al., ; Zhao et al., ; Koenecke et al., 2012). Given the effects of pro-inflammatory cytokines in amplifying inflammation, possible cytokine production by Treg cells present potential hazard and might have relevance to chronic inflammation.

Figure 4

DNA demethylation during hematopoietic cell differentiation. Differentially methylated regions (DMRs) tends to be detected within genes encoding molecules associated with lineage specification, such as Cxcr2 and Gadd45α in granulocyte/macrophage progenitors; Cd19, Irf8, and Cd79α in B cell lineage; and CD8a in CD8⁺ T cells (Ji et al., ; Bock et al., ; Lee et al., 2012). Similarly, within CD4⁺ T cell subsets, lineage-specific DNA demethylation occurs within genes encoding molecules involved in cell subset-specific functions (Lee et al., ; Ansel et al., ; Wilson et al., ; Cohen et al., ; Ohkura et al., ; Thomas et al., 2012). These findings suggest the involvement of epigenetic regulations during cell fate determination and linage commitment.