Immunology

Conditional stability of T cells

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Data from several recent studies on the dynamics of regulatory T cells — which suppress excessive immune responses — do not add up. Collective analysis of the observations may reconcile the differences between them.

Regulatory T (Treg) cells — as their name suggests — are crucial for suppressing excessive immune responses that can harm the organism1. They also establish tolerance to non-self antigens such as organ transplants. Deficiency or dysfunction of these cells in rodents and humans causes various T-cell-mediated autoimmune diseases, allergies and immune disorders such as inflammatory bowel disease. To understand how Treg cells regulate immune responses, it is crucial to determine how they are produced and whether they are functionally stable under various conditions. A paper published in Science (Rubtsov et al.2) is the latest of several recent reports3,4,5,6 that address these questions. The papers find apparently conflicting results, suggesting that the Treg-cell population can be both stable and dynamic.

Treg cells specifically express the transcription factor Foxp3, and a cardinal feature of these cells is that, in the thymus, they are produced as a functionally mature T-cell population specializing in immune suppression1. Some Foxp3-expressing (Foxp3+) Treg cells can also differentiate from naive T cells, which do not express Foxp3, in the periphery — for example, in the mucosal layer of the small intestine, where immune responses to digested food or products of commensal microbes could be damaging to the host and so must be suppressed.

Differentiation of naive T cells to either Treg cells or effector T cells, such as helper T (TH) cells, is elaborately controlled by immune mediators called cytokines1,7 (Fig. 1). In vitro, for instance, one cytokine, TGF-β, mediates differentiation of naive T cells to Foxp3+ Treg cells in response to antigenic stimulation, and another cytokine, IL-6, hampers this differentiation; together, TGF-β and IL-6 facilitate T-cell differentiation into a type of mature TH cell that secretes IL-17 (TH17 cells) — a highly inflammatory cytokine. The cytokine IL-2 is indispensable for the maintenance of thymus-derived Foxp3+ Treg cells and augments TGF-β-dependent Treg-cell differentiation in the periphery; however, it inhibits differentiation of TH17 cells.

Figure 1: T-cell differentiation.
figure1

As well as Foxp3+ Treg cells, which suppress excessive immune responses, the thymus produces naive T cells (TH0 cells), which can differentiate into effector T cells, including TH1, TH2 and TH17 cells, following stimulation with antigens and in the presence of appropriate cytokines (blue). Although Rubtsov et al.2 find that Foxp3+ Treg cells are generally stable, under certain conditions at least some of these cells may stop expressing Foxp3 and convert into effector TH cells. IL-2 produced by activated non-Treg cells inhibits this conversion and is also required for the maintenance of Foxp3+ Treg cells.

Foxp3+ Treg cells can also differentiate 'sideways', halting expression of Foxp3 and differentiating into effector T cells that secrete pro-inflammatory cytokines. This, however, may happen only under certain conditions, such as genetically determined reduced expression of Foxp3; antigen stimulation of Treg cells in vitro in a TH-cell-driven pro-inflammatory-cytokine milieu; or in vivo transfer of Treg cells to a T-cell-deficient environment, in which they undergo homeostatic proliferation3,4,5,8,9. The conversion of Treg cells to other types of effector T cell — if it occurs readily and frequently — could be harmful, because the resulting effector T cells, like the Treg cells from which they are derived, are likely to recognize self antigens, and so may cause autoimmune disease.

Immunologists are on the quest to assess this possible danger. There are, however, conflicting results from experiments in mice that are engineered to express a reporter dye in their T cells whenever the cells express Foxp3; the dye-expressing T cells are then traced to determine whether they lose Foxp3 expression and become effector T cells. An earlier report6 showed that a significant fraction of Foxp3+ Treg cells (about 10%) stop expressing Foxp3 and differentiate into TH cells that secrete the pro-inflammatory cytokine IFN-γ and that mediate type 1 diabetes in a mouse model of autoimmune disease. Rubtsov et al.2, however, show that Treg cells are highly stable in terms of both Foxp3 expression and their suppressive function, with few cells converting into effector T cells even after exposure of the animals to an inflammatory cytokine milieu or to X-ray radiation, which causes a reduction in the level of circulating T cells.

Although apparently conflicting, these findings provide insights into how T-cell differentiation to and from Treg cells depends on Foxp3 expression, on the mix of cytokines and on the presence of other T cells (Fig. 1). For one thing, a paucity of IL-2 clearly plays a crucial part in Treg conversion: when Foxp3+ Treg cells are transferred to T-cell-deficient mice, the co-transfer of T cells that do not express Foxp3 or infusion of IL-2 prevents conversion of the Foxp+ Treg cells to effector T cells. And if Treg cells are transferred on their own, Foxp3-negative T cells that have formed from Foxp3+ Treg cells produce IL-2, which inhibits the conversion of further Foxp3+ Treg cells in a negative-feedback loop3. Similarly, in Rubtsov and colleagues' study2, the sublethal X-ray radiation given might reduce T-cell levels substantially but not completely, allowing residual T cells to secrete IL-2 and thereby inhibit Treg conversion.

Another possibility is that Foxp3+ T cells vary in functional stability and therefore in their susceptibility to conversion; that is, only a certain fraction of Foxp3+ Treg cells may be 'plastic'. In support of this notion, the regulatory regions of the Foxp3 gene are more extensively demethylated in thymus-derived Treg cells than in those induced by TGF-β10, offering an explanation for the functional instability of the latter population and its higher susceptibility to conversion. Even thymus-derived Foxp3+ Treg cells could vary in the methylation status of their Foxp3 gene, in their suppressive activity and in their eventual fate. Indeed, there is evidence11 for functional and phenotypic variability in human Foxp3+ T cells: Foxp3high cells are terminally differentiated to be highly suppressive, whereas some Foxp3low cells are non-suppressive and can secrete effector cytokines.

Finally, alterations in the level of Foxp3 expression may affect the functional stability of Treg cells and, possibly, their susceptibility to conversion. For example, mice that express a reporter dye along with Foxp3 in their Treg cells show a slight reduction in Foxp3 expression and an increased susceptibility to autoimmune disease12. This might explain the discrepancies in Treg-cell conversion between the two studies2,6, which used different genetic manipulation methods such that different levels of the reporter dye were expressed.

Thus, the plasticity of T cells to differentiate to and from Foxp3+ Treg cells is elaborately controlled by factors internal and external to these cells. Collectively, these factors ensure a remarkably constant number of Foxp3+ Treg cells in the immune system (about 10% of all T cells expressing the surface marker CD4), with a general increase only occurring at sites of inflammation. Further understanding of the molecular basis of functional stability and the plasticity of Treg and other T cells should facilitate safe and effective control of physiological and disease-associated immune responses.

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Sakaguchi, S. Conditional stability of T cells. Nature 468, 41–42 (2010) doi:10.1038/468041a

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