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Review
. 2013 Mar 28;121(13):2402-14.
doi: 10.1182/blood-2012-09-378653. Epub 2013 Jan 16.

Essentials of Th17 cell commitment and plasticity

Pawel Muranski  1 , Nicholas P Restifo
Affiliations
Review

Essentials of Th17 cell commitment and plasticity

Pawel Muranski et al. Blood. .

Abstract

CD4(+) T helper (Th) cells exist in a variety of epigenetic states that determine their function, phenotype, and capacity for persistence. These polarization states include Th1, Th2, Th17, and Foxp3(+) T regulatory cells, as well as the more recently described T follicular helper, Th9, and Th22 cells. Th17 cells express the master transcriptional regulator retinoic acid-related orphan receptor γ thymus and produce canonical interleukin (IL)-17A and IL-17F cytokines. Th17 cells display a great degree of context-dependent plasticity, as they are capable of acquiring functional characteristics of Th1 cells. This late plasticity may contribute to the protection against microbes, plays a role in the development of autoimmunity, and is necessary for antitumor activity of Th17 cells in adoptive cell transfer therapy models. Moreover, plasticity of this subset is associated with higher in vivo survival and self-renewal capacity and less senescence than Th1 polarized cells, which have less plasticity and more phenotypic stability. New findings indicate that subset polarization of CD4(+) T cells not only induces characteristic patterns of surface markers and cytokine production but also has a maturational aspect that affects a cell's ability to survive, respond to secondary stimulation, and form long-term immune memory.

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Figures

Figure 1
Figure 1
Schematic representation of signaling and transcriptional regulation of Th17 polarization. Th17 cells are induced upon T-cell receptor activation in the presence of cytokines that activate Stat3, including IL-6, IL-21, and IL-23. IL-12 and IFN-γ, which signal via Stat4 and Stat1, respectively, promote type 1 differentiation and inhibit Th17 polarization. However, IL-23 also activates Stat4 (not shown), and it remains perplexing that animals deficient in Stat4 have impaired functionality of Th17 cells. Similarly, IL-4 signaling via Stat6 inhibits Th17 polarization and promotes type 2 differentiation. Phosphorylated Stat3 (pStat3) binds to the promoter regions and activates transcription of genes encoding master regulators of Th17 polarization: rorc (encoding Rorγt) and rora (encoding Rora) transcription factors. Batf and Irf4 form a functional complex that plays a central role in Rorγt-mediated activation of the type 17 molecular signature. Together with pStat3, Rorγt and Rorα activate the expression of genes encoding canonical Th17-associated cytokines IL-17A and IL-17F, as well as IL-21, IL-22, and others. IL-6–induced activation of Stat3 also augments the expression of the IL-23 receptor (IL-23R), thus increasing the sensitivity of early Th17 cells to the polarizing effects of IL-23. pStat3 also induces expression of Hif1α, which inhibits FoxP3 and promotes Th17 differentiation. IL-21 secreted by early Th17 cells acts in a self-amplified autocrine loop via the IL-21 receptor. IL-1 promotes Th17 polarization via activities of p38 mitogen-activated protein kinase (MAPK) and the Akt/mTOR pathway. IL-1 also induces Irf4, which directly augments IL-21 secretion. Th17 polarization is also increased by activation of aryl hydrocarbon receptors (Ahr). TGF-β1 signals via Smads that most likely limit expression of genes encoding T-bet, Gata3, and other Th1- and Th2-associated factors, thus increasing Th17 differentiation. TGF-β1 signaling in conjunction with retinoic acid (RA) and IL-2–induced pStat5 promotes FoxP3 expression and Treg differentiation. RA has been shown to either limit or (in lower concentrations) augment Th17 polarization. pStat5 directly inhibits pStat3 binding to IL-17 promoter (not shown). Both FoxP3 and Rorγt form complexes with Runx1 and reciprocally regulate each other.
Figure 2
Figure 2
Epigenetic mechanisms explain the lineage relationship between Th17, Th1, and iTreg subsets. (A) Whether a particular gene is poised for expression or not is determined by the chromatin structure, as well as histone and DNA methylation states. Trimethylation of histone 3H on lysine 4 (H3K4me3) is considered permissive for gene expression, whereas trimethylation of lysine 27 on histone 3H (H3K27me3) is a marker of gene silencing. In some cases, both states can be found in a gene locus, thus making it susceptible for either expression or negative regulation. (B) Plasticity of Th1, Th17, and iTreg cells is constrained by the epigenetic status of genes encoding for the master transcriptional regulators of polarization and canonical cytokines. Th17 cells display permissive H3K4me3 histone modification over the rorc and il17 genes and bivalent, poised status over loci encoding for tbx21 and foxp3, thus substantiating the relative instability of this subset and its propensity to evolve into Th1 progeny. In contrast, Th1 cells display only repressive H3K27me3 methylation status over gene loci encoding for rorc and Il17, rendering them much more stable. iTregs are another relatively unstable subset that can acquire Th1 or Th17 properties, based on the poised bivalent status of type 1– or type 17–associated genes.
Figure 3
Figure 3
Relationship between Th1 and Th17 polarization and maturational stage of Th memory cells. The initial polarization of naïve CD4+ T cells into the Th1 or Th17 subset not only induces a canonical set of transcription factors and cytokines but also affects CD4+ T-cell plasticity and fate. Th17 cells are relatively more plastic and less terminally differentiated than their Th1 cell counterparts. This is reflected by the higher ability of Th17 cells to self-renew, generate a distinctive highly functional Th1-like Th17 progeny, and form long-term memory following the secondary antigen experience. Th1 cells rapidly acquire a senescent phenotype and molecular signature, are less functional, and display less ability for self-renewal and memory formation. The lower panel demonstrates a proposed relationship between polarized CD4+ Th cells and a model of linear CD8+ T-cell memory formation. Upon antigen stimulation CD8+, T cells progress from naïve via self-renewing early memory stem cells (TSCM), to central memory (TCM and TEM) cells, and to the senescent terminally differentiated cells with no self-renewal potential. In vivo antitumor efficacy of adoptively transferred cells used for immunotherapy of cancer inversely correlates with the maturational stage of T cells.
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