Transcriptional regulation of Th17 cell differentiation

Abstract

The paradigm of effector T helper cell differentiation into either Th1 or Th2 lineages has been profoundly shaken by the discovery of T cells that secrete IL-17 and other inflammatory cytokines. This subset, referred to as Th17, is centrally involved in autoimmune disease and is important in host defense at mucosal surfaces. In mouse, a series of cytokines, including IL-6, IL-21, IL-23, and TGF-beta, function sequentially or synergistically to induce the Th17 lineage. Other cytokines, including IL-2, IL-4, IFNgamma, and IL-27, inhibit differentiation of this lineage. Here we review how the nuclear orphan receptor RORgammat functions to coordinate the diverse cytokine-induced signals and thus controls Th17 cell differentiation.

Figure 1. Sequential roles of cytokines in the differentiation of the Th17 cell lineage

IL-6 induces IL-21, which in turn induces its own transcription in a STAT3-dependent manner. IL-23R expression induced by IL-21 (and IL-23) is dependent on both RORγt and STAT3. IL-21R and IL-23R signaling, in concert with TGF-β, program the naïve T cells into the Th17 lineage independently of IL-6. Full differentiation of Th17 cells requires the cooperative action of RORγt and STAT3. The induction of IL-17 by IL-6 in the absence of IL-21/IL-21R signaling is presented as a dashed arrow.

Figure 2. Cytokine pathways and transcription factors in murine Th17 and Treg cell differentiation

RORγt and Foxp3 are central factors in Th17 and Treg cell differentiation, respectively. Th17 inducing cytokines (in green) induce RORγt through STAT3 (IL-6, IL-21, and IL-23) and IRF4 (induced by TCR activation). However, none of the RORγt-inducing cytokines can induce Th17 differentiation in the absence of TGF-β signaling. Th17 suppressing cytokines (in red) act in several ways. IL-2 acts through STAT5, and IL-27 possibly through SOCS3 and STAT3, inhibiting RORγt-upregulation. Retinoic acid (RA), which promotes Treg development, and TGF-β by itself, which induces Foxp3 expression, decrease Th17 differentiation. In addition, Th1 and Th2 inducing cytokines, such as IL-4, IFNγ, and IL-25 also inhibit Th17 differentiation by unknown mechanisms. TGF-β thus plays a dual role by being absolutely required for Th17 cell differentiation and by also promoting Treg cell development.

Figure 3. The cytokine milieu influences the balance of differentiating inducible Treg and Th17 cells

Inductive effects of TGF-β alone (left), with IL-2 and RA (top), and with IL-6, IL-21 and IL-23 (bottom). The left panel represents a hypothetical metastable state, induced by TGF-β, which would allow the cell to differentiate into either the Th17 or Treg lineage depending on the associated cytokines. At the metastable state, TGF-β is able to induce both RORγt and Foxp3, however, RORγt activity is inhibited by Foxp3 through an interaction of the two proteins, which ensures no IL-17 expression. Upon continued stimulation with TGF-β in the absence of proinflammatory cytokines and in the presence of IL-2 and RA, cells will differentiate into the iTreg lineage. On the other hand, if proinflammatory cytokines are present, Foxp3 expression/activity will diminish and RORγt expression will increase, tipping the “balance” towards the Th17 lineage.

Figure 4. Th17 and Treg cell differentiation in the intestinal lamina propria

The local cytokine milieu governs the balance between Th17 and Treg cells in the intestine. Th17 lineage-inducing cytokines (e.g., IL-6, L-23) and iTreg cell-inducing cytokines (e.g., TGF-β, IL-10, retinoic acid (RA)) are produced by different subpopulations of antigen presenting cells in response to signals from the lumen of the intestine or from invading microorganisms. Lamina propria macrophages and CD103⁺ DCs produce Treg-inducing cytokines and CD11b⁺CD103⁻ DCs produce Th17-inducing cytokines. The Th17:Treg balance may also be influenced by other subpopulations of DCs in the lamina propria or DCs in organized lymphoid structures, such as cryptopatches, where they are closely associated with RORγt-producing lymphoid tissue inducer-like (LTi-like) cells, and isolated lymphoid follicles (ILFs), which contain RORγt⁺ LTi-like cells, surrounding a single B cell follicle. LP DCs produce cytokines in response to signals from the commensal intestinal flora and pathogenic organisms in the lumen in several ways. They can sample the lumen by extending dendrites through the epithelial layer. Antigens transported through M cells in Peyer’s patches and ILFs also activate the resident DC populations. In addition, epithelial cells have also been shown to participate in DC activation.