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. Author manuscript; available in PMC: 2011 Jul 5.
Published in final edited form as: Immunity. 2010 May 13;32(5):616–627. doi: 10.1016/j.immuni.2010.04.016

Epigenetic Instability of Cytokine and Transcription Factor Gene Loci Underlies Plasticity of the T helper 17 Cell Lineage

Ryuta Mukasa 1,2, Anand Balasubramani 1,3,*, Yun Kyung Lee 1,3,*, Sarah K Whitley 3, Benjamin T Weaver 1, Yoichiro Shibata 4, Gregory E Crawford 4, Robin D Hatton 1,#, Casey T Weaver 1,3,#
PMCID: PMC3129685  NIHMSID: NIHMS203822  PMID: 20471290

Abstract

Phenotypic plasticity of T helper 17 (Th17) cells suggests instability of chromatin structure of key genes of this lineage. Here we identify epigenetic modifications across the clustered Il17a and Il17f, and Ifng loci before and after differential IL-12 or TGFβ signaling, which induce divergent fates of Th17 cell precursors. We find that Th17 precursors have substantial remodeling of the Ifng locus but undergo critical additional modifications to enable high-level expression when stimulated by IL-12. Permissive modifications across the Il17a-Il17f locus are amplified by TGFβ signaling in Th17 cells, but are rapidly reversed downstream of IL-12–induced, STAT4– and T-bet–mediated silencing of the Rorc gene. These findings reveal substantial chromatin instability of key transcription factor and cytokine genes of Th17 cells and support a model of Th17 lineage plasticity in which cell-extrinsic factors modulate Th17 cell fates through differential effects on the epigenetic status of Th17 lineage factors.

INTRODUCTION

While lineage-specific cytokine and transcription factor networks are important in specifying effector CD4+ T cell subset differentiation, heritable and stable programs of gene expression are reinforced through epigenetic processes that include post-translational modifications of nucleosomal histones (eg, methylation, acetylation, phosphorylation, ubiquitylation), DNA methylation, and changes in higher-order chromatin structure (Ansel et al., 2006; Wilson et al., 2009). Although the potential diversity of histone and DNA modifications are great, cis-regulatory elements of active or recently transcribed genes are typically characterized by the acetylation of histones H3 and H4 at various amino acid residues (H3Ac and H4Ac), and by methylation of H3 lysine residue 4 (H3K4) with one, two, or three methyl groups (H3K4me1, H3K4me2, and H3K4me3). Conversely, transcriptionally silent genes are characterized by methylation of H3 lysine residue 27 (H3K27) with two or three methyl groups (H3K27me2 and H3K27me3), especially in facultative, but not constitutive, heterochromatin of tissue-specific gene loci (Wang et al., 2008; Wilson et al., 2009).

Th1, Th2, and Th17 cells show distinct epigenetic modifications at lineage-specific cytokine gene loci that are associated with the rapid and efficient production of effector cytokines in recall responses. At the Ifng locus, naïve CD4+ T cells acquire permissive H3K4me, H3Ac, and H4Ac modifications at the promoter and distal regulatory elements when they differentiate into Th1 cells, whereas Th2 and Th17 cells lack these permissive modifications, having instead, increased repressive H3K27me3 modifications (Akimzhanov et al., 2007; Chang and Aune, 2005; Hatton et al., 2006; Schoenborn et al., 2007; Wei et al., 2009). At the Il17a-Il17f locus, where the Il17a and Il17f genes are clustered on opposite DNA strands, Th17 cells show permissive H3K4 tmethylation (H3K4me3), but no repressive H3K27me3 modifications at the promoters of both genes, whereas Th1 and Th2 cells show the opposite pattern (Wei et al., 2009). Th17 cells are also reported to have increased permissive H3 acetylation at the Il17a and Il17f promoters and at several conserved non-coding sequences (CNSs) in the Il17a-Il17f locus in comparison to Th1 and Th2 cells (Akimzhanov et al., 2007).

The epigenetic modifications at the Il17a-Il17f and Ifng loci of Th17 cells described to date are consistent with their potential to produce high amounts of IL-17A and IL-17F, but limited IFN© upon restimulation (Wei et al., 2009). Nevertheless, recent reports indicate that there is substantial late developmental plasticity of Th17 cells (Lee et al., 2009; Lexberg et al., 2008). Thus, restimulation of in vitro-polarized Th17 cells by IL-12 induced rapid transition to a Th1-like phenotype marked by greatly enhanced production of IFN© and extinction of IL-17A and IL-17F (Lee et al., 2009; Lexberg et al., 2008). Similarly, conversion of Th17-polarized cells to a Th1-like phenotype was observed in vivo in a transfer model of colitis (Lee et al., 2009), an antigen-specific ocular inflammation model (Shi et al., 2008), and transfer models of type I diabetes (Bending et al., 2009; Martin-Orozco et al., 2009). Although mechanisms underlying the developmental plasticity of Th17 cells are incompletely understood, these findings suggest that the epigenetic modifications observed at the Il17a-Il17f and Ifng loci might be particularly unstable.

Here we have performed comparative long-range DNase I hypersensitivity (HS) and histone modification analyses of the Il17a-Il17f and Ifng loci in naïve, Th1 and Th17 cells, and in Th17 precursors restimulated with TGFβ to maintain their phenotype or restimulated with IL-12 to deviate them to a Th1-like phenotype (Lee et al., 2009). Our findings reveal heretofore underappreciated remodeling of the Ifng locus in Th17 cells. We also find substantial reversibility of the chromatin structure of the Il17a-Il17f locus in Th17 cells that appears to be linked to loss of RORγt expression downstream of IL-12-induced, STAT4- and T-bet-mediated silencing of the Rorc gene. These findings provide a basis for the phenotypic plasticity of the Th17 lineage as well as the resistance of conventional and Th17-derived Th1-like cells to induction of Il17a and Il17f expression.

RESULTS

Identification of cis-regulatory elements in the Ifng and Il17a-Il17f gene loci

Naïve CD4+ T cells differentiated under Th17 cell-polarizing conditions express low levels of the IL-12 receptor component, IL-12Rβ2, and transition to Th1-like cells following restimulation in the presence of IL-12 and absence of TGFβ (Lee et al., 2009; Lexberg et al., 2008). IL-12 stimulation of polarized Th17 cells rapidly up-regulates Ifng expression, with a concomitant down-regulation of Il17a and Il17f expression. Although this transition is STAT4– and T-bet–dependent, the mechanism by which this occurs is undefined. To address this, we identified potential regulatory elements at the Ifng and Il17a-Il17f loci by long-range mapping of DNase I hypersensitivity mapping of naïve, Th1 and Th17 cells as a basis for delineating key cis-regulatory sites that might be targets of epigenetic modifications that contribute to this phenotype shift (Figure 1).

Figure 1. DNase I HS maps of the Ifng and Il17a-Il17f loci.

Figure 1

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Naïve CD4+ T cells from OT-II TCR transgenic mice were isolated and cultured under Th1- or Th17-polarizing conditions. Cells were harvested without (rest) or with (stim) PMA+ionomycin stimulation and subjected to DNase I digestion and DNase-chip analysis. Panels show results of Ifng (A) and Il17a-Il17f (B) loci, respectively, aligned with corresponding VISTA plot (Frazer et al., 2004), where mouse sequence is shown on the x axis and percentage similarity to human on the y axis.

For the Ifng locus, we analyzed ~140 kb flanking the Ifng gene and bordered by CTCF consensus sequences thought to represent insulator elements (Hadjur et al., 2009; Wilson et al., 2009). DNase I hypersensitivity (HS) sites in activated Th1 cells co-localized well with CNS elements and the promoter (Figure 1A). Many HS sites were shared with resting Th1 cells, while those at CNS-54 and CNS-6 were strictly activation–dependent, and those at CNS-34 and CNS+54 were significantly enhanced by activation. Notably, naïve and Th17 cells, which don’t express substantial amounts of IFN©, shared some DNase I HS sites with Th1 cells. In particular, activated Th17 cells showed a HS site pattern very similar to activated Th1 cells, with the notable exceptions of CNSs -6 and +46, which were not hypersensitive in Th17 cells. The Ifng promoter of resting Th17 cells was DNase I–resistant, in accord with recently reported repressive histone modifications at this site (Wei et al., 2009). However, the promoter demonstrated substantial hypersensitivity following activation of Th17 cells. This suggests that the chromatin structure of Th17 cells is in a highly “poised” state for Ifng transcription, despite limited production of IFN© by polarized Th17 cells. Based on these data, and unpublished studies that have examined transcription factor binding to cis elements in the Ifng locus (Balasubramani et al., data not shown), we chose to further analyze eight CNSs and the promoter (Figure 1A).

The Il17a and Il17f genes are closely linked on opposite strands of mouse chromosome 1. Based on a DNase I scan of ~2.5 megabases of DNA flanking these genes in the indicated T cell subsets, we focused on an ~185 kb of DNA region flanked by the Pkhd1 and Mcm3 genes for further study due to the presence of HS sites unique to Th17 cells in this region (Figure 1B). Prominent DNase I HS peaks were limited to Th17 cells, consistent with their unique capacity to produce IL-17A and IL-17F. Several sites (eg, the Il17a and Il17f promoters, and HS sites -5 +10 and +23 kb relative to the predicted transcription start site of the Il17a gene) were either uniquely detected, or were markedly enhanced, following activation of Th17 cells. With few exceptions, HS sites correlated well with CNSs. In striking contrast to the Ifng locus in Th17 cells, the Il17a-Il17f locus in Th1 cells was essentially devoid of DNase I HS sites, consistent with the resistance of Th1 cells to express IL-17A or IL-17F even under Th17 repolarizing conditions (Lee et al., 2009). Based on these data, we chose for further analysis twelve sites, including the Il17a and Il17f promoters.

Rapid remodeling of Ifng locus in IL-12–stimulated Th17 cells

To define epigenetic changes that might accompany the divergence of Th17 cell precursors into distinct cytokine expression phenotypes, we analyzed histone modifications across the Ifng and Il17a-Il17f loci. Specifically, H3K4me and H3K27me3 changes were assessed as marks of permissive and repressive chromatin modifications, respectively. Th1 and Th17 cells were derived from naïve CD4+ T cells, and Th17 cells were further stimulated with the antigen in the presence of Th1 (IL-12)– or Th17 (TGFβ)–polarizing conditions. In agreement with our previous study (Lee et al., 2009), frequencies of IL-17A-expressing cells were sustained and increased in the presence of TGF® (Figure 2; Th17+TGF®). In contrast, a large majority of Th17 cells restimulated with IL-12 (Figure 2; Th17+IL-12) produced IFN©, while IL-17A expression was markedly decreased.

Figure 2. IL-12 and TGF β induce differential cytokine expression phenotype in Th17 precursor cells.

Figure 2

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(A) CD4+ T cells from OT-II mice were cultured under Th1 or Th17 cell-polarizing conditions. Th17 cells were harvested and restimulated in the presence of TGFβ or IL-12, anti-IL-4 and OVAp for an additional 6 days. Cells were stained intracellularly for IL-17A and IFNγ after PMA+ionomycin activation. Data in the quadrants are the frequencies of CD4+ cells. (B) Cumulative data for frequencies of cytokine-positive CD4+ T cells analyzed as (A). Th1 and Th17 cells were harvested on day5 for ChIP analysis. For Th17+IL-12 and Th17+TGFβ cells, Th17 cells were harvested on day 6 or 7 of the first polarization and restimulated in the presence of IL-12 or TGFβ for another 6 days (means ± SD).

Concordant with their capacity for high Ifng expression, Th1 cells showed substantially increased permissive H3K4me modifications across the Ifng locus compared to naïve CD4+ T cells, with notable exceptions at CNSs -34 and -22, where no significant changes in H3K4me marks were evident above the substantial marks already present at those sites in naïve T cells (Figure 3A) (Hatton et al., 2006; Schoenborn et al., 2007). Th17 cells showed less, but appreciable, H3K4 methylation compared to Th1 cells at several distal CNSs of the Ifng locus, with the notable exceptions of CNS-6 and the Ifng promoter, which, concordant with the DNase I HS data (Figure 1A), were negligible in Th17 cells. Th17+IL-12 cells demonstrated up-regulated H3K4 methylation across the locus that were comparable or increased relative to Th1 cells, concordant with their high expression of Ifng upon restimulation. Th17+TGF® cells, whose frequency of IFN©+ cells (22%) was much lower than Th1 (86%) or Th17+IL-12 (87%) cells, but appreciably higher than Th17 cells (3%) (Figure 2B), also showed higher H3K4 methylation than Th17 precursor cells at most sites in the Ifng locus. However, Th17+TGF® cells had very low amounts of H3K4me at CNS-6, comparable to that of Th17 precursor cells and much lower than that of Th1 or Th17+IL-12 cells. Although Th17 and Th17+TGF® cells showed slightly higher repressive H3K37me3 than Th1 or Th17+IL-12 cells at some sites, the differences among subsets were less pronounced than the differential H3K4me changes. The increased expression of IFN© in Th17 precursors restimulated with IL-12- or TGFβ was reflected in long-range DNase I HS of the Ifng locus in Th17+IL-12 and Th17+TGF® cells (Figure S1, supplementary data). Thus, Th17+IL-12 and Th17+TGF® cells demonstrated more prominent DNase I HS peaks at several CNSs in the Ifng locus compared to Th17 precursors, most notably at the Ifng promoter, which, at rest, was DNaseI hypersensistive both in the IL-12- and TGFβ-stimulated Th17 cell progeny, analogous to conventional Th1 cells (Figure 1). Both of these populations also demonstrated prominent activation-induced hypersensitivity at CNS -54, which was lacking in Th17 precursors. However, like Th17 precursors, Th17+TGFβ cells lacked the HS site at CNS-6, correlating well with the absence of H3K4me marks at this site (Figure 3).

Figure 3. Th17 cells undergo rapid epigenetic remodeling and acquire Th1-like histone modifications across the Ifng locus following restimulation with IL-12.

Figure 3

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(A) Th1, Th17, Th17+IL-12 and Th17+TGF β cells were derived from CD4+ T cells of OT-II TCR transgenic mice as in Figure 2. Cells were then processed for ChIP analysis using antibodies specific for H3K4me or H3K27me3. Data for H3K4me and H3K27me3 were normalized against the value for naïve CD4+ T cells and input DNA, respectively (means ± SD). (B) CD4+ T cells from OT-II TCR transgenic mice were cultured under Th17-polarizing conditions. Recovered cells were restimulated in the presence of anti-IL-4 and TGF® or IL-12 for the indicated times and processed for expression of Ifng mRNA after 2 h of PMA+ionomycin activation (means ± SD). (C) Th17 cells restimulated with anti-CD3 and anti-CD28 in the presence of IL-12 and anti-IL-4 for 6, 24, or 48 hr as in (B). Cells were processed for ChIP analysis with antibodies specific for H3K4me or H3K27me3. Data are normalized against cells without restimulation (0 hr; means ± SD).

Because in the foregoing experiments an appreciable fraction of T cells generated under Th17 cell-polarizing conditions did not express either IL-17A or IFN©, we repeated the analysis using Il17fThy1.1/Thy1.1 mice to identify and isolate by magnetic sorting committed Th17 cells as a starting population for divergence of late development and assessment of epigenetic changes (Lee et al., 2009). Although the increases in H3K4 methylation in Th17+IL-12 and Th17+TGFβ cells relative to Th17 precursor cells were similar to those when OTII mice-derived CD4+ T cells were used to polarize Th17 cells (Figure S1B), there were marked decreases in the H3K27me3 modifications at nearly all sites examined, likely reflecting the decreased fraction of unpolarized cells present in the unsorted Th17 precursor cells. Notably, however, IL-17F+ Th17 precursors and Th17+TGFβ cells showed similar deficiency of H3K4me at CNS-6 compared to Th17 precursor cells restimulated with IL-12 (Figure 3A). Together with the DNaseI HS data, these data indicate that the chromatin organization of the Ifng locus in Th17 precursor cells shares remarkable similarity to conventional Th1 cells, and is highly poised for transition to high-level transcription of Ifng expression following IL-12-induced signaling, but lacks epigenetic remodeling at key cis-elements – particularly the proximal promoter and CNS-6. Based on these results, we focused on H3K4me modification of CNS-6 and the promoter for further analyses.

Kinetics of histone modifications correlates with Ifng expression

To define the temporal relationship between induction of enhanced Ifng expression and epigenetic remodeling at the Ifng locus, we compared kinetics of histone modifications and Ifng mRNA expression in Th17 cells following IL-12 stimulation. Permissive H3K4me modifications at the Ifng locus were unchanged at 6 hr following IL-12 stimulation (Figure 3B), consistent with the absence of inducible Ifng mRNA expression at that time (Figure 36), but increased by more than two-fold at CNS-6 and the promoter by 24 hr and further increased at the promoter at 48 hr, correlating well with increases in Ifng mRNA expression at these times. In contrast, changes in H3K27 methylation were modest at these sites (Figure 3B). Analysis of H4 acetylation (H4Ac), another permissive histone modification, showed similar kinetics to that of the H3K4me changes (data not shown). Correlation of kinetics of histone modifications at key cis-elements in the Ifng locus with markedly increased IFN-© production is consistent with a role for rapid chromatin remodeling in the enhanced production of IFN© by Th17 cells deviated by IL-12 signaling.

Remodeling of the Ifng locus in transitioning Th17 cells is STAT4 and T-bet dependent

We have shown that the induction of IFN© associated with IL-12-induced transition of Th17 cells to Th1-like cells required both STAT4 and T-bet (Lee et al., 2009). We therefore examined whether STAT4 directly binds the Ifng locus following IL-12 stimulation of Th17 precursor cells. ChIP analysis demonstrated prominent binding of STAT4 to CNS-34, CNS-22, and CNS+46 at 24 hr (Figure 4A), similar to findings in conventional Th1 cells (Balasubramani et al., data not shown). STAT4 binding to these sites was also observed when pure Th17 precursor cells derived from Il17fThy1.1/Thy1.1 mice were restimulated for 24 hr in the presence of IL-12, but not in the presence of TGF® (Figure S2A). There was no detectable binding of STAT4 to sites in the Ifng locus in Th17+IL-12 cells derived from Stat4-/- mice (data not shown). Despite the rapid nuclear translocation of phosphorylated STAT4 induced by IL-12 (Figure 4B), STAT4 binding was negligible at 6 hr and reached a peak at 24 hr, mirroring the kinetics of histone modifications (Figures 4C and 3, and data not shown). STAT4 binding to cis-elements was generally transient, such that significant binding was retained only at CNS+46 by day 6 after restimulation with IL-12 (Figure S2B). These results establish that despite rapid nuclear localization of phosphorylated STAT4, its binding to multiple distal cis-elements and the induction of high-level Ifng expression downstream of IL-12 signaling were temporally associated with key histone modifications, consistent with a requirement for chromatin remodeling of these sites prior to STAT4 binding.

Figure 4. STAT4 and T-bet are necessary for epigenetic remodeling of the Ifng locus following IL-12 stimulation of Th17 cells.

Figure 4

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(A) OT-II Th17 cells were restimulated with anti-CD3 and anti-CD28 in the presence of IL-12 and anti-IL-4 for 24 hr and processed for STAT4 ChIP analysis. Data are normalized against input DNA (means ± SD). (B) Th17 cells were derived from Il17fThy1.1/Thy1.1 mice and Thy1.1+ (IL-17F+) Th17 cells isolated by magnetic sorting. Cells were restimulated with anti-CD3 and anti-CD28 in the presence of IL-12 or TGF® for the indicated times, and immunoblot analysis was performed on isolated nuclei using antibodies specific for phopho-STAT4, total STAT4 or control (c-Jun). (C) Th17 cells were restimulated with anti-CD3 and anti-CD28 in the presence of IL-12 and anti-IL-4 for 6, 24, or 48 hr and cells were processed for STAT4 ChIP analysis as in (A). Data are normalized against input DNA and represent the mean ± SD of pooled data from two to four separate experiments. (D) Th17 cells were derived from wild type (WT), Stat4-/-, and Tbx21-/- mice and restimulated with IL-12, anti-CD3, anti-IL-4, and anti-IFN© for 6 days. Th17 presursor cells (Th17) and cells recovered after 6 days restimulation with IL-12 (Th17+IL-12) were processed for H3K4me ChIP. Data are normalized against input DNA (means ± SD).

To determine whether STAT4 is required for epigenetic changes observed following IL-12 stimulation, the H3K4me status of the Ifng promoter and CNS-6 was analyzed in Th17 cells derived from wild type (WT) and Stat4-/- mice, before and after IL-12 stimulation. Because T-bet has been shown to direct chromatin remodeling at the Ifng locus in developing Th1 cells, we also analyzed cells derived from T-bet-deficient (Tbx21-/-) mice. In agreement with previous studies (Harrington et al., 2005; Lee et al., 2009), Th17 cells derived from WT, Stat4-/-, and Tbx21-/- mice had comparable frequencies of IL-17A+ cells, and induction of IFN© in Th17 cells after restimulation with IL-12 was strongly inhibited in cells derived from Stat4-/- and Tbx21-/- mice (Figure S4). ChIP analyses of these cells revealed that H3K4me modifications were significantly increased at CNS-6 in WT cells after IL-12 stimulation (Figure 4D), consistent with previous results (Figures 3A and S1B). In contrast, induction of H3K4 methylation was ablated in cells derived from Stat4-/- and Tbx21-/- mice. Induction of H3K4me was similarly STAT4– and T-bet–dependent at the Ifng promoter after IL-12 stimulation, whereas the modest increases in H3K4me at CNS+17-19 induced by IL-12 signaling were not significantly different in Th17 cells derived from WT, Stat4-/- and Tbx21-/- mice. Collectively, these data indicate that STAT4 and T-bet play critical roles in epigenetic remodeling of key cis-elements in the Ifng locus, and further highlight the importance of epigenetic changes at CNS-6 and the promoter in controlling high-level expression of Ifng.

Epigenetic remodeling accompanies IL-12 repression of Il17a and Il17f

In concert with increased expression of IFN©, IL-12-stimulated Th17 precursor cells down-regulated Il17a and Il17f expression. We therefore examined the stability of the chromatin structure of the Il17a-Il17f locus during transition. Naïve, Th1, Th17, Th17+IL-12, Th17+TGF® cells were prepared and H3K4me and H3K27me3 modifications were analyzed. Compared to naïve CD4+ T cells, Th17 cells demonstrated markedly increased H3K4me modifications at multiple HS sites across the Il17a-Il17f locus, whereas H3K27me3 modifications were minimally altered (Figures 5A and S3A). Th1 cells, in contrast, showed negligible induction of H3K4me modifications, whereas H3K27me3 modifications were substantially increased. Thus, Th1 and Th17 cells show differential repressive and permissive histone modifications across the extended the Il17a-Il17f locus, respectively, reflecting their differential production of IL-17A and IL-17F.

Figure 5. IL-12 induces repressive histone modification at Il17a-Il17f locus of Th17 cells.

Figure 5

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(A) Naïve, Th1, Th17, Th17+IL-12, Th17+TGF® cells were derived from OT-II CD4+ T cells as in Figure 3A then analyzed for H3K4me and H3K27me3 modification of the indicated sites by ChIP. Data are normalized against input DNA (means ± SD). (B) OT-II Th17 cells were restimulated with anti-CD3 and anti-CD28 in the presence of anti-IL-4 and TGF® or IL-12 for the indicated times and processed for expression of Il17a mRNA after 2 hr of activation with PMA+ionomycin (means ± SD). (C) Th17 cells were restimulated with anti-CD3 and anti-CD28 in the presence of IL-12 and anti-IL-4 for 6, 24, or 48 h as in (B). Cells were then processed for ChIP assay with antibody specific for H3K4me or H3K27me3. Data are normalized against cells without restimulation (0 hr) and represent means ± SD.

Th17+IL-12 cells demonstrated substantial increases in H3K27me3 modifications at most hypersensitive sites across the Il17a-Il17f locus. At several sites, repressive marks surpassed those found in conventional Th1 cells. Although, H3K4 methylation of these sites appeared relatively unchanged in Th17+IL-12 cells derived from Th17 precursor cells of normal mice (Figure 5A), this might to be due to contamination of this population with cells derived from uncommitted precursors, as use of pure Th17 precursor cells derived from Il17fThy1.1/Thy1.1 mice as precursors demonstrated significant decreases in permissive histone marks at many of these sites (Figure S3A), particularly at the Il17a and Il17f promoters and their most proximal upstream elements (HS sites Il17a-5 and Il17f-7). Notably, Th17 precursors restimulated in the presence of TGF® and absence of IL-12 demonstrated increased H3K4 methylation of nearly all sites examined, consistent with higher frequencies of IL-17A+ and IL-17F+ cells in this population relative to Th17 precursor cells. Although this was accompanied by increased repressive marks at some sites in Th17 cells derived from normal mice (Figure 5A), again, this appeared to be due to use of incompletely committed precursors; cells derived from Il17fThy1.1/Thy1.1 mice demonstrated no increases in the repressive histone methylation at these sites (Figure S3B). Similarly, the H3K27me3 marks found in Th17+IL-12 cells from IL-17F reporter mice were substantially increased across the Il17a-Il17f locus compared to non-enriched Th17+IL-12 cells. Collectively, these data establish that the chromatin structure of the Il17a-Il17f locus is remodeled over a considerable distance in developing Th17 cells, particularly at intergenic sites and sites upstream of the Il17a gene, but is unstable and undergoes rapid and extensive remodeling contingent upon cytokines that are dominant during restimulation.

In accordance with the observed transitions in histone methylations, DNase I HS analyses of the Il17a-Il17f locus of Th17+IL-12 and Th17+TGF® cells demonstrated substantial changes compared to Th17 precursor cells. Most notably, inducible HS sites at promoters of the Il17a and Il17f genes were reversed in Th17+IL-12 cells, as was the HS site at Il17aCNS-5 (Figure S3B). In Th17+TGFβ cells, the IL17a and Il17f promoters became substantially more hypersensitive at rest, as did intergenic sites at CNS Il17a+10 and Il17a+23, whereas CNS Il17a+36 became more accessible both at rest and following stimulation, correlating with substantially greater expression of IL-17A and IL-17F by these cells. Thus, rapid modifications of repressive and permissive histone methylations that were associated with the suppression of Il17a and Il17f expression following IL-12 signaling — or enhanced expression of both genes upon continued culture in TGFβ, were reflected in extensive chromatin remodeling that resulted in the reversal or enhancement, respectively, of DNase I HS sites across the locus.

To examine the temporal relationship between extinction of gene expression and epigenetic remodeling at the Il17a-Il17f locus, we compared the kinetics Il17a mRNA expression and histone modifications following IL-12 stimulation (Figure 5B, C). Because IL-12-induced changes in histone modifications in Th17 cells were observed at multiple sites across the Il17a-Il17f locus, we focused our analysis on H3K4me and H3K27 methylations at the Il17a and Il17f promoters and their nearest upstream cis-elements, where prominent changes in both permissive and repressive histone methylations distinguished IL-12 and TGFβ treated Th17 precursors. As shown in Figure 5C, H3K27 trimethylation at the promoter (Il17aP) and HS Il17a-5 were not significantly increased by 48 hr, whereas IL-17A expression was decreased by more than half (Figure 5B). In contrast, H3K27me3 at the Il17fP and HS Il17f-7 increased by 3-fold or more at 48 h. H3K4 methylation was also limited during the first 48 hr of IL-12 stimulation, indicating an absence of absolute correlation between histone methylation at these sites and the observed decreases in cytokine expression, and suggesting that other mechanisms of suppression of Il17a (and Il17f, data not shown) were contributory.

Il17a-Il17f silencing is associated with STAT4– and T-bet–dependent repression of Rorc

STAT4 is essential for extinction of Il17a and Il17f expression by Th17+12 cells (Lee et al., 2009). To determine whether this might result from direct effects of STAT4 on the Il17a-Il17f locus, ChIP analysis for STAT4 binding to the extended locus was performed on Th17 cells restimulated with IL-12 or TGF® (Figure S4). STAT4 binding was detected at intergenic CNSs Il17a+10 and Il17a+28 at the peak of STAT4 DNA binding (24 hr; Figure 4), but was undetectable at other sites and in Th17+TGF® cells, or in Th17+IL-12 cells derived from Stat4-/- mice (data not shown). Notably, however, although STAT4 binding to these sites achieved statistical significance, it was substantially weaker than that measured at the Ifng locus in the same cell population (Figure 4A).

The limited binding of STAT4 in Th17+IL-12 cells suggested that other STAT4-dependent mechanisms might contribute to silencing of the Il17a-Il17f locus. Down-regulation of Th17 lineage transcription factors observed previously in Th17+IL-12 cells suggested that IL-12-induced STAT4 activation might indirectly induce repressive chromatin changes at the Il17a-Il17f locus (Lee et al., 2009). We therefore analyzed the kinetics of mRNA expression of Th17 lineage-associated transcription factors, as well as the Th1 lineage-associated transcription factor T-bet, that followed IL-12-mediated deviation of Th17 precursor cells (Figure 6). Compared to Th17 cells restimulated with TGFβ, expression of Rora and Ahr transcripts were markedly reduced by 24 hr following IL-12 stimulation, and remained low over the 5 day period of analysis. While IL-12 also induced a substantial decline in Rorc (RORγt) transcript by 24 hr, it continued to decline with time, approaching background levels by day 5 (Figure 6A). Notably, no decrement in the expression of Irf4 transcripts was observed. In contrast, Tbx21 transcripts, which remained at background levels in Th17+TGF® cells, were rapidly and progressively increased in Th17+IL-12 cells, demonstrating a reciprocal pattern of expression to that of Rorc.

Figure 6. IL-12-induced extinction of the Il17a-Il17f locus in Th17 cells is associated with rapid modulation of expression of Th17- and Th1-lineage transcription factors.

Figure 6

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(A) Pure Th17 cells (Thy1.1+) were derived CD4+ T cells of from Il17fThy1.1/Thy1.1 mice and were restimulated with IL-12 or TGF® for 1, 3, or 5 days and processed for quantification of mRNA of the indicated genes by RT-PCR. Data are normalized to 18S rRNA and expressed as relative values to Thy1.1+ cells used before restimulation. The horizontal dotted lines indicate the values of each mRNA in naïve CD4+ T cells from Il17fThy1.1/Thy1.1 mice. Data are representative of two independent experiments. (B) Th17 precursor cells prepared from wild type (WT), Stat4-/-, and Tbx21-/- mice were harvested and divided into two fractions; one (Th17) processed for without further treatment for mRNA quantitation of the indicated transcripts, and the other (Th17+IL-12) restimulated with IL-12, anti-IL-4, and anti-IFN© for 6 days and processed identically. Data are normalized to ®2-microglobulin and expressed as relative values to WT Th17 cells (mean ± SD) and are representative of two similar experiments.

Given the requirement for STAT4 and, to a lesser extent, T-bet, for IL-12-induced repression of Il17a and Il17f (Figure S4), we examined whether IL-12 suppression of Th17 lineage factors was STAT4- or T-bet-dependent. Th17 cells derived from WT, Stat4-/-, and Tbx21-/- mice were restimulated with IL-12 and analyzed for expression of Rorc, Rora and Ahr transcripts. While the IL-12-driven decrease in Rorc transcripts was significantly impaired in the absence of STAT4 or T-bet, transcripts of Rora and Ahr were not, such that Th17 cells derived from WT, STAT4-deficient, and T-bet-deficient cells demonstrated comparable decreases in Rora and Ahr transcripts (Figure 6B). Thus, STAT4 and T-bet play an important role in IL-12-induced suppression of ROR©t in Th17 cells, whereas ROR⟨ and Ahr are suppressed by a mechanism that is STAT4- and T-bet-independent.

Epigenetic repression of Rorc correlates with silencing of Il17a-Il17f locus

The correlation of silencing of the Il17a-Il17f locus with suppression of Rorc expression suggested that ROR©t might play an important role in maintaining II17a and Il17f expression in committed Th17 cells. Thus, suppression of ROR©t by IL-12 might initiate locus remodeling with attendant extinction of II17a and Il17f. To test this possibility, the effect of ectopic ROR©t expression on the IL-12–induced transition of Th17 cells to Th1-like cells was evaluated. CD4+ T cells from Il17fThy1./Thy1.1 mice were transduced with a retrovirus that directs expression of ROR©t (ROR©t-GFP) or a control retrovirus (GFP) during Th17 cell differentiation. Committed (Thy1.1+/IL-17F+) Th17 cells were then isolated by magnetic sorting and restimulated in the presence of IL-12 or TGF® (Figure 7A).

Figure 7. Extinction of the Il17a-Il17f locus is linked to STAT4- and T-bet-induced epigenetic repression of the Rorc gene.

Figure 7

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(A) CD4+ T cells isolated from Il17fThy1.1/Thy1.1 mice were activated with anti-CD3+CD28 coated beads under Th17 cell-polarizing conditions on day 0 and were transduced with the retroviral vectors encoding IRES-GFP (GFP (control)) or ROR©t-IRES-GFP (ROR©t-GFP) on day 1. Th17 cells were harvested on day 6 and a fraction of cells were stained for Thy1.1 and intracellular IL-17A and IFN© after PMA+ionomycin activation. Thy1.1+ (IL-17F+) cells were isolated from a second fraction by magnetic sorting and restimulated with anti-CD3 and anti-CD28 in the presence of anti-IL-4, anti-IFN©, and IL-12 for 6 days. Recovered Th17+IL-12 cells were analyzed for intracellular IL-17A and IFN© after PMA+ionomycin activation. Thy1.1+GFP+ cells and GFP+ cells were gated for analysis of Th17 and Th17+IL-12 cells, respectively. (B) CD4+ T cells of Il17fThy1.1/Thy1.1 mice were cultured under Th17 cell-polarizing conditions in the presence of anti-IL-12p40 for 6 days, and Thy1.1+ (IL-17F+) cells were isolated by magnetic sorting. ChIP for H3K4me and H3K27me3 histone modifications was performed on this fraction of the isolated Thy1.1+ cells (Th17). Th17+IL-12 and Th17+TGFβ cells were generated from a second fraction of Thy1.1+ cells and also evaluated for H3K4me and H3K27me3 by ChIP. Data are normalized against input DNA (mean ± SD). (C) Th17 and Th17+IL-12 cells were derived from wild type (WT), Stat4-/-, and Tbx21-/- mice as in (B) and analyzed for H3K27me3 modification of Rorc+5.0 by ChIP. Data are normalized against input DNA (mean ± SD). (D) Th17+IL-12 and Th17+TGFβ cells were generated from Il17fThy1.1/Thy1.1 CD4+ T cells as in (B) and analyzed for binding of T-bet to the indicated sites in the Rorc locus by T-bet-specific ChIP. Data are normalized against input DNA (mean ± SD).

IL-12 stimulation of Thy1.1+ Th17 precursors transduced with the control retrovirus resulted in induction of IFN©, and extinction of IL-17A (Figure 7A) and IL-17F (Thy1.1) (Figure S5A), consistent with previous results (Figure 2A). In contrast, retrovirally expressed ROR©t, which was resistant to IL-12-mediated suppression, inhibited extinction of IL-17A expression (Figure 7A), whereas extinction of IL-17F expression was partially inhibited (Figure S5A). TGF® restimulation of Th17 precursors transduced with either the control or RORγt-expressing retrovirus retained comparable, high expression of IL-17A and IL-17F (Thy1.1) (Figure S5A, and data not shown). Notably, retroviral expression of ROR©t nearly completely inhibited IL-12-mediated IFN© induction in Th17 cells (Figure 7A).

To examine mechanisms by which enforced RORγt expression might inhibit IL-12-mediated induction of IFN© and downregulation of IL-17A in Th17 cells, IL-12–induced STAT4 activation and T-bet induction were examined in Th17 precursors transduced with RORγt or control retrovirus (Figure S5B). Both STAT4 phophorylation and induction of T-bet expression were markedly suppressed in cells transduced with the RORγt-expressing retrovirus (RORγt-GFP) compared to control virus. Thus, Th17 cells expressing high levels of RORγt that cannot be down-modulated in response to IL-12 retain Il17a-Il17f locus competency and resist expression of the Ifng locus.

A possible role for ROR©t in maintaining Il17a expression led us to examine IL-12-induced remodeling of the Rorc locus in Th17 cells. It was reported that Th17 cells have H3K4me3 marks around the ROR©t isoform-specific exon of the Rorc locus, whereas Th1 cells have H3K27me3 marks across the coding region and immediate upstream non-coding region of the Rorc locus (Wei et al., 2009). We therefore examined permissive and repressive histone modifications near the ROR©t isoform-specific exon of the Rorc locus (Rorc+5.0) and at an upstream site near the second exon of the Rorc gene (Rorc+2.7), before and after the IL-12–induced transition of Th17 cells. Th17 cells were restimulated with IL-12 or TGF® and analyzed for histone methylation changes. As shown in Figure 7B, IL-12 stimulation decreased H3K4 methyation both at Rorc+2.7 and Rorc+5.0, whereas TGF®-stimulated cells retained H3K4 methylation at both sites. Coincident with decreased permissive modifications, IL-12 induced reciprocal increases in H3K27 trimethylation at both sites, although more prominently at the Rorc+5.0 site, which has been implicated as a ROR©t-specific promoter (Eberl and Littman, 2003). In contrast, neither permissive nor repressive modifications at these sites were altered in Th17+TGF® cells, suggesting that IL-12 signaling suppresses Rorc expression in Th17 cells at least in part through repressive epigenetic remodeling of the Rorc locus.

We next determined whether IL-12-induced epigenetic remodeling at the Rorc locus in Th17 cells was dependent on STAT4 or T-bet, focusing on H3K27me3 changes at Rorc+5.0 where the greatest disparity between Th17 and Th17+IL-12 cells was observed (Figure 7B). Th17 cells derived from WT, Stat4-/-, and Tbx21-/- mice were restimulated with IL-12, and H3K27 trimethylation at Rorc+5.0 was assessed before and after the IL-12-induced transition (Figure 7C). Induction of H3K27me3 was almost completely inhibited in STAT4- or T-bet-deficient cells, consistent with STAT4- and Tbet-dependent suppression of Rorc transcripts (Figure 6B). Thus, IL-12-induced STAT4 and T-bet play a critical role in reversing permissive epigenetic modifications at the Rorc locus in Th17 cells, which appears linked to repression of Rorc and extinction Il17a and Il17f expression.

To determine whether STAT4 or T-bet binds directly to the Rorc locus, ChIP analysis was performed on Th17 precursors restimulated in the presence of TGFβ or IL-12. Among four sites examined in the Rorc locus, significant binding of T-bet was identified at Rorc+2.7 in Th17+IL-12 cells, but not in Th17+TGFβ cell controls (Figure 7D). None of these sites showed detectable binding of STAT4 (data not shown). Accordingly, IL-12 induces binding of T-bet to Rorc+2.7 in Th17 precursor cells, suggesting that down-modulation of Rorc expression is likely due, at least in part, to T-bet-dependent recruitment of histone-modifying enzymes to the Rorc locus.

DISCUSSION

Epigenetic modifications of cytokine gene loci are considered a basis for heritability of gene expression patterns acquired by each T cell subset (Ansel et al., 2003; Wilson et al., 2009). Here, we find that chromatin structure of the Il17a-Il17f, Ifng and Rorc loci in Th17 cells is not stable, but changes rapidly in response to cell-extrinsic factors. Whereas lineage-committed Th17 cells maintained with TGFβ demonstrated enhanced epigenetic modifications across the Il17a-Il17f locus in accord with their enhanced production of IL-17A and IL-17F (Lee et al., 2009), the same precursors deviated towards highly induced IFN© expression and IL-17A and IL-17F extinction by IL-12 demonstrated marked reciprocal H3 methylation changes at multiple CNS-HS elements across the Il17a-Il17f locus that mirrored STAT4- and T-bet-dependent remodeling of the Rorc locus and its loss of expression. This is consistent with a role for persistent RORγt expression in maintenance of transcriptional competence of the Il17a-Il17f gene cluster in Th17 cells. Thus, epigenetic modifications of key cytokine and transcription factor gene loci that accompany Th17 differentiation are subject to substantial, and rapid, reversibility, which corresponds with, and is likely a basis for, their phenotypic plasticity (Lee et al., 2009; Lexberg et al., 2008).

Although histone modifications of cytokine genes in Th17 cells have been reported previously (Akimzhanov et al., 2007; Wei et al., 2009), this is the first report to describe extensive DNase I hypersensitivity mapping at the Il17a-Il17f and Ifng loci of Th17 cells. A notable feature of the DNase I HS map of the Ifng locus of Th17 cells is its similarity to Th1 cells. Indeed, even activated naïve CD4+ T cells demonstrated substantial DNase I HS sites, indicative of a partially remodeled locus prior to effector T cell differentiation (Sekimata et al., 2009). Th17 cells acquired further DNase I sites, with a pattern quite similar to that of activated Th1 cells despite their development in the absence of Th1-specifying cytokines or expression of the canonical Th1 transcription factor, T-bet. However, Th17 cells lacked hypersensitivity at CNS-6 and the Ifng promoter, consistent with the observed histone modifications. Previous reports showed absence of H3Ac and H3K4me3, but substantial H3K27me3 within the promoter (Akimzhanov et al., 2007; Wei et al., 2009). Our study confirms these findings, but also identifies permissive histone modifications and DNase I accessibility at distal Ifng regulatory elements. Accordingly, Th17 cells have extensive chromatin remodeling of the Ifng locus, indicating a far more highly “poised” state than previously appreciated, and which occurs in the absence of STAT4 or T-bet signaling. This likely accounts for the rapid up-regulation of IFN© production by Th17 cells in response to IL-12, but also their capacity to express lower amounts of IFN© in the absence IL-12 (or IL-23) and presence of TGFβ. Thus, while Tbx21 was reported to have bivalent histone methylation marks at its promoter (Wei et al., 2009), and undergoes rapid IL-12-induced up-regulation that is important for enhancement of Ifng expression in transitioning Th17 cells (Lee et al., 2004), STAT4- and T-bet-independent mechanisms that are yet to be defined enable extensive remodeling of the Ifng locus in committed Th17 cells, and permit expression of Ifng in the absence of these factors. This is in contrast to virtual absence of DNase I HS sites or permissive histone modifications of the Il17a-Il17f locus in Th1 cells, and is in accord with Th1 cells’ resistance to the developmental plasticity that characterizes Th17 cells (Harrington et al., 2005; Lee et al., 2009).

The comparative findings of DNase I HS and histone modifications at CNS-6 in the Ifng locus point to a key role for this element in controlling high-level transcription of Ifng. CNS-6 was shown previously to bind T-bet (Hatton et al., 2006; Shnyreva et al., 2004) and function as an enhancer for Ifng expression (Lee et al., 2004; Schoenborn et al., 2007). Further, CNS-6 contains binding sites for NFAT and AP-1 and has been shown to bind NFAT-1 in activated Th1 cells (Lee et al., 2004). Collectively, these findings suggest that IL-12-induced histone modifications at CNS-6 enable acute transcription factors activated by TCR signaling to efficiently bind CNS-6 in Th17 cells deviated to a Th1-like phenotype.

Rapid binding of STAT4 to distal Ifng CNSs of Th17 cells following IL-12 stimulation occurred at pre-existent DNase I HS sites, but appeared insufficient for high-level induction of IFN© and epigenetic remodeling in the absence of T-bet. In view of role for T-bet in cooperating with CCCTC-binding factor (CTCF) to induce conformational changes in the Ifng locus that juxtapose distal enhancers with the Ifng promoter (Sekimata et al., 2009), a primary action of STAT4 at these distal CNSs could be recruitment of T-bet to these sites, which in turn recruits these elements to the promoter in concert with CTCF-containing complexes. This is supported by the finding that ectopic T-bet expression in Th17 cells was inefficient at inducing Ifng compared to IL-12 signaling, and suggests a model in which IL-12-induced STAT4 acts both to enhance Tbx21 expression and target its interactions with distal cis-regulatory elements to enable juxtaposition of these elements to the Ifng proximal promoter in cooperation with CTCF-containing architectural complexes.

Long-range HS mapping of the Il17a-I17f locus herein substantially extends previous studies and defines the extent of lineage-specific accessibility at over a dozen sites distributed across ~185 kb of flanking sequence. This includes, but is not limited to, cis-elements previously identified as binding sites for Runx1 and RORγt (Zhang et al., 2008), STAT3 (Chen et al., 2006) and BATF (Schraml et al., 2009). These findings are consonant with our epigenetic data, which establish permissive, Th17 cell-specific histone modifications at most of these sites and identify novel candidates for cis-regulation.

In contrast to the TGFβ-induced enhancement of Il17a and Il17f expression reflected in enhanced H3K4 methylation across the Il17a-Il17f locus, IL-12-induced extinction of Il17a and Il17f was characterized by rapid and extensive reciprocal H3K27 and H3K4 methylations that resulted in a pattern of histone modifications closely resembling that of Th1 cells. Like the extensive H3K27 trimethylation of cis-elements in Th1 cells, this was associated with irreversible silencing of the Il17a-Il17f locus ((Lee et al., 2009), and unpublished observations). Thus, once modified by a preponderance of repressive histone methylations, permissive remodeling of the locus was difficult or impossible. H3K27me3 has been shown to recruit histone-binding proteins that contain chromodomains, including Polycomb group proteins that are components of multiprotein complexes implicated in gene silencing (Ansel et al., 2006; Kouzarides, 2007; Schuettengruber et al., 2007), and we conjecture that recruitment of such remodeling complexes following the down-modulation of RORγt in transitioning Th17 precursors contributes to silencing of the Il17a-Il17f locus.

The rapid down-regulation of Rorc, Rora, and Ahr, but not Irf4, by IL-12 signaling implicated loss of these three transcription factors in extinction of the Il17a-Il17f locus. However, only transcripts encoding ROR©t were inhibited by STAT4 and T-bet; transcripts encoding ROR⟨ and Ahr were inhibited independently of STAT4 or T-bet. Coupled with a requirement for STAT4 and T-bet for optimal extinction of the locus, this is consistent with an important role for constitutive expression of ROR©t in maintenance of the Il17a-Il17f locus, despite contributions of the other factors in initial Th17 cell development (Huber et al., 2008; Ivanov et al., 2006; Kimura et al., 2008; Quintana et al., 2008; Veldhoen et al., 2008; Yang et al., 2008). This is supported by the observation that enforced ROR©t expression strongly inhibited IL-12-induced down-regulation of IL-17A, through a mechanism that blunted STAT4 activation and induction of T-bet expression. The finding that extinction of the Il17a-Il17f locus was more dependent on STAT4 than T-bet could reflect a role for direct STAT4 binding to two intergenic sites in locus silencing. In other studies, we have found that these same two intergenic sites bind STAT3 in Th17 cells, suggesting that STAT4 might bind weakly to the same cis-elements important for STAT3-mediated control of Il17a and Il17f expression (S.K.W., unpublished findings), although other mechanisms cannot be excluded and will require further study.

The extinction of Rorc in committed Th17 cells by IL-12-induced STAT4 and T-bet was linked to reversal of permissive histone modifications at two lineage-specific DNase I HS sites. This correlated with loss of DNase I accessibility at both of these sites and direct binding of T-bet to one (Rorc+2.7). Although more extensive analyses will be required to precisely define mechanisms by which STAT4 and T-bet participate in this process, the findings herein support a model of antagonism between the Th17- and Th1-specifying lineage factors in which STAT4 and T-bet are dominant through their capacity to suppress Rorc expression. In this regard it is notable that the up-regulation of Tbx21 observed in IL-12-stimulated Th17 cells agrees well with the previously reported “bivalency” of H3 methylation of Tbx21 gene in Th17 cells (Wei et al., 2009), whereas the Rorc gene in Th17 cells lacked repressive histone marks in that study and our own. Thus, if histone methylation bivalency is associated with induction of expression of key transcription factors in the context of lineage transitions, it does not appear to be a prerequisite for termination of transcription factor expression associated with lineage transitions, and the chromatin structure in the Rorc locus appears to be particularly susceptible to epigenetic remodeling in recently differentiated Th17 cells.

In summary, our findings substantially extend the long-range mapping of the Il17a-Il17f and Ifng gene loci in distinct T cell subsets. They further establish that the epigenetic modifications that underlie changes in cytokine and transcription factor gene expression early in Th17 lineage specification are particularly unstable, and provide a mechanism whereby the developmental plasticity that appears to be characteristic of the Th17 lineage is enabled by reversal and remodeling of chromatin structure in response to local environmental cues.

METHODS

Mice

Mice were purchased from the Jackson Laboratories and/or bred at our facility: BALB/cByJ (BALB/c), C57BL/6J (B6), B6.OT-II TCR transgenic mice (OT-II), B6.129S6-Tbx21tm1Glm/J (T-bet-deficient), and B6.129S1-Il12btm/Jm/J (IL-12p40-deficient). The generation of Il17fThy1.1/Thy1.1 reporter mice was described previously (Lee et al., 2009). B6.Stat4-/- mice used in some studies were a kind gift from Mark H. Kaplan (Indiana University School of Medicine). All animals were bred and maintained in accordance with Institutional Animal Care and Use Committee regulations.

CD4+ T cell preparation and culture

CD4+ T cells were purified from pooled spleen and lymph nodes by positive selection and cultured with specific antigen or anti-CD3 and IL-12p40-deficient (Il12b-/-) splenocytes as previously described (Lee et al., 2009). OT-II TCR transgenic CD4+ cells were activated with 5 μg/ml OVA peptide (OVAp), whereas non-transgenic cells were stimulated with 2.5 μg/ml anti-CD3 (clone 145-2C11). Thy1.1+ cells of Il17fThy1./Thy1.1 reporter mice were isolated on day 6 of Th17 cell cultures by magnetic sorting as reported (Lee et al., 2009). In restimulation cultures, recovered cells were activated as in the primary culture with addition of 10 ng/ml rmIL-12 (R&D Systems) or 5 ng/ml rhTGF-β1 (R&D Systems) where indicated. For some experiments, cells were restimulated with plate-bound anti-CD3 (clone 145-2C11, 10 μg/ml) and 2.5 μg/ml soluble anti-CD28 (clone 37.51, eBioscience) or anti-CD3/CD28 coated beads (Invitrogen, according to the manufacturer’s instructions).

Flow cytometric analyses

Intracellular staining (ICS) was performed as previously described (Lee et al., 2009; Maynard et al., 2007). All samples were acquired on an LSRII instrument (BD Biosciences) and data were analyzed using FlowJo software (Tree Star Inc.).

DNase hypersensitivity mapping

DNase I hypersensitivity sites were mapped as described (Crawford et al., 2006; Shibata and Crawford, 2009). Briefly, nuclei from CD4+ T cells were digested with DNase I, ligated to biotinylated linkers and sonicated. After streptavidin bead enrichment, digested and randomly sheared captured ends were amplified following ligation of a second set of linkers. DNase-digested and randomly sheared ends were labeled and hybridized to custom NimbleGen tiled microarrays. Regions of the genome with DNase I hypersensitivity were identified using the tiled array peak calling software, ACME.

Chromatin Immunoprecipitation (ChIP)

ChIP was performed as previously described (Hatton et al., 2006). Briefly, chromatin was prepared from 1×107 cells for each reaction using the Chromatin Immunoprecipitation (ChIP) Assay Kit from Millipore according to the manufacturer’s instructions. Immunoprecipitated DNA released from cross-linked proteins was quantitated by real-time PCR using Platinum Quantitative PCR SuperMix-UDG (Invitrogen) or SYBR GreenER qPCR SuperMix (Invitrogen) on a BioRad iQ system. PCR primers and probes are included in the Supplemental Data. Unless otherwise indicated, data are expressed as relative values normalized to the input DNA samples (% input).

Gene expression analyses

RNA was extracted from T cells using TRIZOL (Invitrogen) and then treated with DNA-free (Ambion). cDNA synthesis was performed using Superscript III first-strand synthesis system (Invitogen). Real-time PCR was performed on a Bio-Rad iCycler with primer pairs specific for cDNAs of each mRNA transcript using SYBR GreenER qPCR SuperMix (Invitrogen). Primer sequences for Rora were as described previously (Lee et al., 2009). Additional primer sequences are found in Supplemental Data. Reactions were run in duplicate and normalized to 18S rRNA orβ2-microglobulin, which gave comparable results.

Retroviral transduction

The RORγt cDNA was PCR amplified and cloned into the bicistronic retroviral vector pMIB (Cotta et al., 2003; Thal et al., 2009), which contains IRES-regulated GFP and was kindly provided by Dr. Christopher A. Klug (University of Alabama at Birmingham). Briefly, CD4+ T cells were cultured under Th17 cell-polarizing conditions, transduced with retrovirus on day 1, and harvested on day 6 for ICS and restimulation cultures.

Immuno blotting

Immunoblotting was performed essentially as described previously (Mukasa et al., 2005). Nuclear and cytoplasmic extracts were prepared using NE-PER Nuclear and Cytoplasmic Extraction Reagents (Thermo Scientific) according to the manufacturer’s protocol. Anti-phosphorylated (Y693) STAT4 antibody were purchased from Invitrogen. Anti-STAT4 and anti-c-Jun antibodies were purchased from SantaCruz. Anti-β-actin antibody was purchased from Abcam.

Statistical analysis

Statistical significance was calculated by unpaired Student’s t test. All p values ≤ 0.05 are referred to as significant in the text, unless specifically indicated otherwise.

Supplementary Material

01

Acknowledgments

The authors are thankful to members of the Weaver laboratory for helpful comments and suggestions. We gratefully acknowledge M. H. Kaplan (University of Indiana) for provision of spleens from B6.Stat4-/- mice and C. A. Klug (UAB) for provision of the IRES-GFP retroviral vector. We also thank J. Oliver, C. Song, M. Blake, B.J. Parsons and S. Sinclair for expert technical assistance and G. Gaskins for editorial assistance, and acknowledge X. He and the UAB Epitope Recognition and Immunoreagent Core Facility for antibody preparations and the UAB Digestive Diseases Research Developmental Center (DDRDC) for generation and phenotyping of gene-targeted mice. This work was supported by grants from the NIH (C.T.W., R.D.H. and G.E.C.), Daichi-Sankyo Co. Ltd. (C.T.W. and R.M.), and the UAB Medical Scientist Training Program (S.K.W.).

Footnotes

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