Plasticity of T helper cell subsets: Implications in periodontal disease.

T helper (Th) cells have an important role in host defence as well in the pathogenesis of periodontal disease. Th cells differentiate from naive cells into various subsets, each of which is associated with a set of inducing and effector cytokines. Previously, it was thought that this differentiation was an irreversible event. Recent evidence suggest that even differentiated Th cells, retain the flexibility to transform from one lineage to another, a phenomenon referred to as plasticity. This plasticity is thought to be brought about by epigenetic modifications that are regulated by external and internal signals in the micro-environment of these cells. The factors and mechanisms which affect the plasticity of these cells and their potential role in the etio-pathogenesis of periodontal disease has been described in this article.

INTRODUCTION

The present review is an extension of our earlier review regarding factors that influence lineage determination of helper T cells.[1] Briefly, T helper cells (Th) were delineated by Mossman and Coffman,[2] into (Th1) and Th2 based on their pattern of cytokine secretion. The Th1/Th2 paradigm was used to explain the pathogenic mechanisms involved in several inflammatory/immune disorders including periodontal disease.[3] In recent years, other Th subsets such as Th17 (interleukin-17 [IL-17]-producing Th cell) and iTreg (inducible regulatory T) cells that are also differentiated from naive CD4+ T cells have been reported.[456]

Traditionally, the characterization of various Th cells subsets was undertaken with the premise that each was associated with unique non-overlapping sets of inducer and effector cytokines. Diseases were thus slotted into rigid categories of being Th1, Th2, Treg or Th17 cell dominated diseases. The commitment of Th cells into a particular lineage is controlled by transcriptional regulation (activation or repression). Transcriptional activation is in turn controlled by extrinsic and intrinsic signals as discussed in our previous review.[1] This over simplification has been challenged in recent years, following the understanding that one Th subset may transform to another under suitable environmental conditions, a phenomenon described as plasticity.[789] This plasticity is brought about by preferential gene expression that are controlled by epigenetic modifications.[1011]

PLASTICITY OF Th CELL SUBSETS

Epigenetic modifications, may have a role in determining the stability or plasticity of CD4 + T cell phenotypes,[12] as a result of their involvement in active transcription of cytokine genes.[13] Th cell differentiation and stable phenotype formation was previously thought to be entirely dependent on the transcription, translation and post-translational modifications. The role of epigenetic modifications has been elucidated in greater detail, in recent years.

Epigenetic modifications refer to those genetic factors that control or regulate protein synthesis without altering the structure of the deoxyribonucleic acid (DNA). In eukaryotic state, DNA is bound tightly around histone proteins that are now known to exert epigenetic influences. The most common epigenetic process that bring about these modification include DNA methylation and post translational histone modifications. DNA methylation involves the addition of methyl group to the DNA molecule at cytosine-phosphate-guanosine islands in the promoter regions that result in inaccessibilty of the promoter region to transcription factors. Consequently, transcription factor binding to the promoter region is retarded, leading to repressive genetic activity. Histone acetylation involves acetylation of the histone tail, enabling the condensed chromatin to become loosely packed. This enables transcription factor binding to the promoter region, resulting in permissive genetic activity. On the other hand, histone deacetylation results in repressive effect due transcription inaccessibility. In this manner, epigenetic modifications influence the ability of transcription factors to bind to the promoter region, there by regulating gene function [Figure 1].[10]

Figure 1

Epigenetic Modification. Histone deacetylation causes the condensation of chromatin, making it inaccessible to transcription factors and the genes are therefore silenced. Chromatin containing acetylated histones (histone acetylation) are open and accessible to transcription factors and the genes are potentially active. This modification may be associated with deoxyribonucleic acid (DNA) methylation. DNA methylation involves methylation of cytosine-phosphate-guanosine islands at the promoter region, directly switching off gene expression by preventing transcription factors from binding to the promoter region

In addition, these modifications are also involved in activation of poised genes or modification of genes carrying bivalent marks. Bivalent marks are areas that can express active and inactive genes at the same gene locus.[14]

These epigenetic modifications are known to influence T cell behaviour as most developing T cells have lineage commitment genes that are in a poised state or carry bivalent marks. The Th1/Th2 model has been used as an example in this review. The differentiation of CD4+ cells into Th1 and Th2 lineages depends on the accessibility to interferon (IFN) and IL-4 gene for their remodelling at the Ifng locus and Il4 locus respectively. The accessibility is in turn influenced by methylation and histone post-translational modification. The DNA sequence remains unchanged, as a result, epigenetic modifications and the information that they encode are inherited, that is, they are passed on from parent to progeny. However, they retain the potential for being plastic, that is, the potential to erase modifications and inscribe new ones is retained.[15]

Plasticity of Th1/Th2 cells (early – two-way switch, late – one-way switch)

The inductive signals required for the development of Th1 cells include IL-12, IFN-α during early differentiation and later by IL-18. These cytokines activate signal transducer and activator of transcription (STAT-1), which in turn upregulates the master regulator of Th1 differentiation – T box expressed in T cell (T-bet). Th2 development is induced by production of IL-4, which activates STAT-6, STAT-5, which then up regulates the master switch of Th2 differentiation, GATA-binding protein-3 (GATA-3). The effector cytokines secreted by each of these Th cells suppress the activation of the other [Figure 2].[16]

Figure 2

Cytokine regulation of epigenetic modifications and T helper cell plastiticty

As described previously,[1] Th1 phenotypes are normally associated with inductive IL-12, STAT-1, T-bet signalling and the effector cytokine IFN-γ cytokines. Similarly, Th2 phenotypes are associated with inductive IL-4, STAT-5, GATA-3 signalling.[1] Recent, evidence suggests that the developing Th cell express both T-bet and GATA-3 and activation of either one of them is dictated by epigenetic modification brought about by signals provided by the micro-environment. In the early stages, there is a two way switch, meaning that both Th1 and Th2 cells demonstrate the ability to transform into each other. Results obtained from studies by Murphy[17] and Zhu[18] inferred that Th1/Th2 cells have the ability to exhibit plasticity, depended on the state of differentiation of these cells. They demonstrated that a partially differentiated Th2 cells can be induced by IL-12 to produce IFN-γ (Th1cytokine), where as, a partially differentiated Th1 cells (IFNγ+ IL-4+) retain their capability to become IL-4 producing cells. This is possible as Th1 cells could be made permissive to GATA-3 by IL-4 in the environment, while Th2 cells could express T-bet under the influence of IL-12. In late stages, however, once their phenotype is established, Th1 cells may still retain their ability to plasticise/transform to Th2 (T-bet is repressed while GATA-3 is activated) while the Th2 cells (expression of GATA-3) remains stable without any inter-convertibility.

Plasticity of Th17/Treg cells (early and late-two-way switch)

Antigen-activated naive CD4 T cells respond to Transforming growth factor-β (TGF-β) to transiently co-express retinoic acid-related orphan receptor (RORγt) and Forkhead box P3 (FoxP3), but differentiate into either Th17 cells or induced regulatory T cells depending on the presence of IL-6 or retinoic acid respectively. IL-21 is induced by IL-6 to up-regulate RORγt, leading to expression of IL-23 receptor conferring responsiveness of Th17 precursors to both IL-23 and IL-12. Depending on the balance of TGF-β, IL-23 and IL-12, Th17 precursors express high levels of IL17A and IL-17F or suppress IL-17A and IL-17F to express a Th1 pattern of cytokines dominated by IFNγ [Figure 2].[9]

Majority of the Th17 and T reg cells are plastic throughout their entire differentiation stage (both early and late stage) with regard to the cytokines they produce.

Plasticity of Th17 to Th1

Th1 and Th2 responses, such as those that mediate periodontal disease, autoimmune diseases and allergic reactions require Th17 cells, partly due to their role in neutrophil recruitment as well as tissue inflammation. There is good evidence to demonstrate that Th17 cells tend to convert into a Th1 cell but not vice versa. Fully polarized Th17 cells can be converted into Th1 cells by expression of T-bet and STAT4 via IL12, IL-23 signalling [Figure 2].[1920]

Plasticity of Treg to either Th1 or Th2 lineage

FOXP3 + T regulatory (Treg) cells can lose FOXP3 expression and take on an effector memory T cell phenotype, producing (IFN-γ) or IL-4 in the absence of TGF-β, in an inflammatory milieu dominated by inducers of Th1, Th2 expression [Figure 2].[2122]

IMPLICATIONS OF TH CELL PLASTICITY TO PERIODONTAL DISEASE PATHOGENESIS

Although, there is general agreement on the role of T cells in the pathogenesis of periodontal disease, there is considerable controversy over their exact role. While some authors have implicated Th2 cells in progressive disease, others have forcefully argued that Th2 lesions are stable and Th1 cells are associated with progressive destruction.[23] The plasticity of Th cells may offer an explanation for these contradictory results. Epigenetic modifications may allow for environmental influences to cause conversion of Th1 to Th2. There is, as yet, no documentary evidence for such an event occurring in periodontal disease, but the possibility cannot be ruled out.[24] The episodic nature of the disease, lack of predictive markers makes clinical staging of disease difficult. It is thus, difficult to investigate the cytokine profile in these sites that may influence permissive genes to convert Th1 to Th2 or vice versa.

THERAPEUTIC IMPLICATIONS

Host modulation therapies have been targeted at reversing the up regulated cytokine, matrix metalloproteinase and more recently transcription factors affecting immune-modulation.[2526] The reversal of DNA methylation and the inhibition of histone deacetylation, are potential targets for host modulation strategies. The potential advantage of using such strategies is that they can be employed without affecting the basic DNA structure. Thus, the effect of such strategies would be limited to diseased sites while sparing the uninvolved tissue.