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Immunology and Cell Biology (2012) 90, 917–918; doi:10.1038/icb.2012.47; published online 25 September 2012

How epigenetic imprinting contributes to stabilizing the Th2 phenotype

Marcus Robinson1, Melanie J McConnell1 and Graham Le Gros1

1Malaghan Institute of Medical Research, PO Box 7060, Wellington, New Zealand

Correspondence: Graham Le Gros, E-mail: glegros@malaghan.org.nz

One of the most enthralling quests of modern T-lymphocyte biology is the ongoing investigation of naive CD4+ T-cell differentiation into diverse T-helper (Th) cell phenotypes. Recent key advances for studying post-translational chromatin modifications are beginning to fill some gaps, providing fascinating insight into what determines a permissive or repressive environment for gene expression. Although there are now several recognized or potential CD4+ Th subsets beyond the original paradigm-setting Th1 and Th2 subsets,1 the expression of interferon-gamma (IFN-γ) versus interleukin-4 (IL-4) by the Th1 and Th2 subsets, respectively, is the most intriguing and potentially the most important. In a groundbreaking new study, heritable silencing of Th1 gene expression in Th2 cells is shown to require the histone-modifying suv39h1/HP1α silencing pathway.2

We have known for some time that the expression of key Th1/Th2-associated genes is related to the chromatin state3 with histones being post-translationally modified at serine, threonine, lysine and arginine residues.4 Although the effects of these modifications are incompletely understood, they are controlled by three intertwined processes: epigenetic ‘writers’ deposit epigenetic marks on histones; ‘erasers’ remove such marks; and ‘readers’ recognize these marks or combinations of these marks, binding and recruiting the appropriate machinery to open or close chromatin.4 Together, these processes influence DNA accessibility and gene expression. The Suv39h methyltransferase pathway is a well-characterized writer/reader interaction. Suv39h1 multi-methylates lysine 9 of histone 3 (H3K9), permitting HP1 to bind to the modified residue, closing chromatin and transcriptionally silencing the associated genes.5, 6 Silencing mediated by Suv39h1 also likely requires histone deacetylase activity, and epigenetic marks on other histone residues influence HP1 binding to H3K9.7 The net result is that heavy H3K9 trimethylation corresponds to the repression of gene expression, whereas acetylated H3K9-associated genes are readily expressed. It has also been shown that the expression of the Th2 cell phenotype, namely the production of IL-4 and absence of IFN-γ, is correlated with highly acetylated H3K9 and H3K14 near an Il4 enhancer,8 while the Ifng gene-associated H3K9 is multi-methylated.2, 9

Allan et al.2 have taken this analysis further and have used suv39h1-deficient mice to follow the role of the methyltransferase pathway in a functional setting. Using well-established in vitro culture systems to generate Th2 cells, they asked whether these normally stable IL-4-producing, IFN-γ non-producing cells can retain this phenotype when introduced to Th1-permissive culture conditions.2, 3 The results clearly show that while suv39h1-deficient Th2 cells introduced to Th1-permissive conditions retained normal frequencies of IL-4 producers, the frequency of cells producing IFN-γ increased by threefold—in short, many suv39h1-deficient Th2 cells now had dual Th2/Th1 functionality (Figure 1).2 Further effects included similar increases in expression of the master Th1 transcription factor T-bet,1 without diminution of Th2-specifying GATA-3.1 Characterization of H3K9 in Suv39h1-deficient Th2 cells revealed high acetylation levels at Ifng promoter regions and the gene encoding T-bet, indicating these Th1 genes were not transcriptionally repressed. Allan et al.2 also demonstrated that Th1 conditioning did not require Suv39h1, as Suv39h1-deficient Th1 cells did not produce IL-4 or express GATA-3 when transferred to Th2 conditioning medium, and they further demonstrated that suppression of Th1 genes in Th2 cells required HP1α recruitment, as HP1α deficiency, but not HP1γ deficiency, caused a seemingly identical phenotype.

Figure 1.
Figure 1 - Unfortunately we are unable to provide accessible alternative text for this. If you require assistance to access this image, please contact help@nature.com or the author

The Suv39h1/HP1α silencing pathway suppresses Th1-associated genes in Th2 cells. When CD4+ T cells are activated in the presence of IL-4 (Th2 conditioning), they differentiate into IL-4-producing Th2 cells. In the nucleus, suv39h1, likely in collaboration with unknown deacetylases, causes H3K9 associated with the Ifng gene to become increasingly trimethylated. When these cells are transferred to medium containing IL-12 and anti-IL-4 (Th1 stimuli), the Ifng gene remains suppressed and restimulation triggers only IL-4 production. Th2-conditioned Suv39h1-deficient cells also develop into IL-4-producing Th2 cells, but they are unable to write silencing marks onto the H3K9 at Th1-associated gene loci, resulting in aberrant Th1-associated gene expression on subsequent exposure to Th1 stimuli and ultimately, failure to stably polarize to the Th2 subset.

Full figure and legend (102K)

The authors then extended their studies to an in vivo model of Th2-mediated allergic airways inflammation. Using either suv39h1-deficient mice, or mice treated with the Suv39h1-inhibiting drug chaetocin,10 dual IFN-γ/IL-4-producing CD4+ T cells were observed more frequently in a Th2 immune response context. Interestingly, in both approaches, the Th2-dependent lung eosinophilia was found to be less severe, mucus deposition was diminished, and IFN-γ-mediated antibody production was detected without reduction in IL-4-dependent IgG1. These findings provide compelling evidence for the notion that the suv39h1/HP1 silencing pathway is involved in ‘cementing’ the Th2 phenotype by ensuring that the Ifng gene locus is silenced, and that the absence of this pathway can blunt a Th2 response.

As with all breakthroughs, there are some caveats to the conclusions made from this data. First, the decision checkpoint to develop into a Th2 cell was Suv39h1 independent. Work by numerous groups has investigated how access to specific promoter/enhancer regions of the Il4 gene, and associated histone modifications, affect IL-4 production in specific contexts (for example8, 11). However, the full expression of a Th2 phenotype requires silencing of lineage-inappropriate cytokines,1 and the current study defines an epigenetic process that does this. Further examination of the complete ‘epigenetic landscape’1 of Th1-associated gene loci in in vivo-generated Suv39h1-deficient Th cells may help us understand how Th2 cells can acquire a dual Th2/Th1 phenotype in other contexts, such as that observed after experimental LCMV infection.12

Systemic IFN-γ administration can inhibit the production of IgG1 and IgE,13 so it is noteworthy that IgG1 production was not reduced in Suv39h1-deficient mice despite increased IFN-γ levels. This gives rise to the exciting possibility that IL-4 can override IFN-γ in the support of B-cell responses in vivo. Determining whether dual Th2/Th1 cells in Suv39h1-deficient mice reside in follicular regions or if Ifng de-repression impairs IgE production could also identify the subtle interactions that determine polarized class-switch isotype selection.

Perhaps the most important question that remains unanswered is how epigenetic writers and erasers are induced to modify certain loci and not others. In the case of Allan et al.,2 the Suv39h1/HP1α silencing pathway fills an unplugged gap in our understanding of Th2 polarization. Studies such as this are rapidly rewriting the model of what is required to initiate and maintain Th2 immunity. We look forward to the next instalment in this dynamically evolving field.

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