Research ArticleIMMUNOLOGY

Homeostasis and transitional activation of regulatory T cells require c-Myc

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Science Advances  01 Jan 2020:
Vol. 6, no. 1, eaaw6443
DOI: 10.1126/sciadv.aaw6443

Abstract

Regulatory T cell (Treg) activation and expansion occur during neonatal life and inflammation to establish immunosuppression, yet the mechanisms governing these events are incompletely understood. We report that the transcriptional regulator c-Myc (Myc) controls immune homeostasis through regulation of Treg accumulation and functional activation. Myc activity is enriched in Tregs generated during neonatal life and responding to inflammation. Myc-deficient Tregs show defects in accumulation and ability to transition to an activated state. Consequently, loss of Myc in Tregs results in an early-onset autoimmune disorder accompanied by uncontrolled effector CD4+ and CD8+ T cell responses. Mechanistically, Myc regulates mitochondrial oxidative metabolism but is dispensable for fatty acid oxidation (FAO). Indeed, Treg-specific deletion of Cox10, which promotes oxidative phosphorylation, but not Cpt1a, the rate-limiting enzyme for FAO, results in impaired Treg function and maturation. Thus, Myc coordinates Treg accumulation, transitional activation, and metabolic programming to orchestrate immune homeostasis.

INTRODUCTION

Regulatory T cells (Tregs) play a crucial role in immune suppression and inhibition of autoimmunity (1). Integral to Treg-mediated maintenance of immune homeostasis during perinatal life and after acute inflammatory insults is the ability to expand in response to proinflammatory stimuli (2). This self-regulating, dynamic process is dependent on spatial and temporal signals, which influence proliferation, migration, and suppressive capacity of Tregs. It has become clear that Tregs are heterogeneous, with respect to the activation state (24). Central Treg cells (cTregs) represent a more quiescent, resting subpopulation, while effector Treg cells (eTregs) share qualities of more activated effector T cell subsets.

We and others have demonstrated that cellular metabolic regulation is interwoven with immune cell function and differentiation (58). Upon activation, T cells transition from quiescence to activation and effector differentiation by shifting from catabolic [i.e., fatty acid oxidation (FAO), autophagy, etc.] to anabolic (i.e., glycolysis, glutaminolysis, etc.) metabolism to generate sufficient energy and biosynthetic materials necessary for rapid proliferation and effector function (9). Previous work has shown that this metabolic reprogramming during lymphocyte activation, as well as in rapidly proliferating cancer cells, is dependent on the master transcriptional regulator c-Myc (Myc) (9, 10). This vital role of Myc has been described in conventional CD4+ and CD8+ T cells (9). However, the function of Myc in Tregs, a metabolically unique subset of T cells (5, 7, 11, 12), remains unclear.

We have recently reported that mitochondrial function is indispensable for eTreg generation and function in vivo (13). While multiple nutrient inputs can drive mitochondria-dependent oxidative metabolism, the prevailing view is that FAO is a major metabolic pathway for Tregs (14, 15) and other quiescent cell types, such as memory T cells (16). Moreover, Foxp3 has been shown to promote FAO and oxidative phosphorylation while dampening Myc expression and target gene expression (17), as well as PI3K (phosphatidylinositol 3-kinase) and anabolic metabolism (18). However, genetic models that target FAO reveal a disposable role of carnitine palmitoyltransferase (Cpt1a, the dominant isoform mediating FAO in immune cells) in T cell responses, unlike the effects observed with the pharmacological inhibitor etomoxir (19, 20). Considering our increasing knowledge on Treg metabolism and functional fitness during activation (5, 7, 11), Treg-specific roles of FAO and oxidative metabolism warrant further investigation.

Here, we show that Myc is vital for proper Treg function during the early stages of postbirth development and in response to acute inflammation. Myc is abundantly expressed in neonatal Tregs undergoing homeostatic expansion, and Treg-specific deletion of Myc results in a rapid, fatal autoimmune disorder characterized by systemic inflammation and tissue damage. Myc-deficient Tregs exhibit a cell-intrinsic activation defect and are unable to undergo expansion or develop into eTregs in response to induced inflammation in vivo. The functional necessity of Myc in Tregs appears to be temporally specific, as we unexpectedly find that eTreg status is unaffected by induced Myc deletion in vivo at steady state. Although Myc is essential for regulating mitochondrial function in Tregs, this effect is not linked to changes in FAO. Mice with Cpt1a-deficient Tregs display no signs of defective Treg function or activation in vivo, while Tregs with disrupted oxidative phosphorylation are impaired in suppressive function and eTreg differentiation. Together, our results highlight the importance of activation-induced Myc function and metabolic reprogramming for orchestrating Treg-suppressive activity in the establishment of immune homeostasis and tolerance.

RESULTS

Myc is functionally enriched in neonatal Tregs and supports Treg accumulation

Shortly after birth, T cell pools expand and migrate to fill appropriate niches within the lymphopenic host to establish immune homeostasis and tolerance (21). Tregs play a vital role in this process, and Tregs formed during this neonatal period have distinguishable gene expression and effector function compared with those generated during full immune maturity (22). To determine important functional mediators of neonatal Tregs, we performed gene set enrichment analysis (GSEA) on a deposited dataset of gene profiling of neonatal and adult Tregs (22). Myc targets were among the most highly enriched gene sets in neonatal Tregs (Fig. 1A and fig. S1A), suggesting the preferential up-regulation of Myc function during this important developmental period. Myc expression in neonatal Tregs was associated with increased expression of Treg-associated effector molecules, such as ICOS, CTLA4, neuropilin-1 (Nrp-1), CD98, and the proliferative marker Ki-67 (fig. S1B). To further determine Myc expression in Tregs during early life, we crossed Myc–GFP (green fluorescent protein) reporter mice (23) to Foxp3–RFP (red fluorescent protein) reporter mice (24) and compared Tregs in different tissues of neonatal (5 to 10 days old) and adult (6 to 8 weeks old) mice. While neonatal mice exhibited decreased Treg frequency in most tissues (except for the liver), Myc-GFP expression was notably increased in neonatal Tregs regardless of tissue residence (Fig. 1B and fig. S1C).

Fig. 1 Myc is functionally enriched in Tregs during early immune development, and deficiency of Myc decreases Tregs in vivo.

(A) Gene set enrichment plot of Hallmark Myc targets identified in Tregs isolated from neonatal versus adult mice (22). (B) Flow cytometry analysis of Myc–GFP (green fluorescent protein) expression in CD4+Foxp3-RFP+ Tregs from indicated tissues in neonatal (5 to 10 days old) and adult (6 to 8 weeks old) Foxp3RFPMyc-GFP mice. (C) Flow cytometry analysis and quantification of frequency and number of Foxp3-YFP+ Tregs in the spleen of WT and Foxp3CreMycfl/fl mice. (D) Total splenic Treg numbers on days 7 to 21 after birth in WT and Foxp3CreMycfl/fl mice. (E) Flow cytometry analysis and quantification [shown as normalized mean fluorescence intensity (MFI) with the expression in WT set as 1] of indicated marker expression in Tregs in the spleen of WT and Foxp3CreMycfl/fl mice. (F) Flow cytometry analysis of Myc-deficient or WT (CD45.2+) and congenic (CD45.1+) Foxp3+ Tregs in mixed bone marrow (BM) chimeric mice. (G) Flow cytometry analysis of indicated marker expression in splenic CD45.2+ Tregs from mixed BM chimeric mice. (H and I) Flow cytometry analysis and quantification of proliferation marker Ki-67 expression in Tregs in the spleen from WT and Foxp3CreMycfl/fl (H) or mixed BM chimeric (I) mice. *P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001; unpaired Student’s t test. Data are representative of or pooled from 3 (B), 15 (C, E, and H), 4 (D), or 9 (F, G, and I) independent experiments, with one to four mice per group per experiment. Graphs show means ± SEM. FDR, false discovery rate; NES, normalized enrichment score; PLN, peripheral lymph nodes.

To characterize the in vivo role of Myc in Tregs, we generated mice with Treg-specific deletion of Myc by crossing mice bearing a Foxp3-driven Cre recombinase (25) with mice containing floxed Myc alleles (Foxp3CreMycfl/fl) (26). Deletion of Myc in Tregs from Foxp3CreMycfl/fl mice was confirmed by real-time polymerase chain reaction, and Myc-deficient Tregs did not display compensatory induction of other Myc family (Mycn or Mycl) genes (fig. S1D). Myc-deficient Tregs displayed a profound reduction in frequency and total numbers (Fig. 1C); this defect was apparent at 7 days of age and continued to increase throughout the early life of the mice (Fig. 1D). Further characterization of Tregs in Foxp3CreMycfl/fl mice demonstrated reduced expression of effector molecules in both adult (Fig. 1E) and neonatal animals (fig. S1E). To determine whether these effects were cell intrinsic, we generated mixed bone marrow (BM) chimeras with age-matched wild-type (WT) or Foxp3CreMycfl/fl BM cells and congenic CD45.1+ BM cells at a 1:1 ratio. As compared with WT BM–derived Tregs, those from Myc-deficient BM cells showed a drastic reduction in Tregs (Fig. 1F and fig. S1F) and defective expression of activation markers (Fig. 1G). Together, these results indicate a cell-intrinsic role of Myc in Treg accumulation and homeostasis.

The cellular mechanisms for loss of Tregs may be due to compromised cell survival, proliferation, or lineage stability. The reduction in Tregs in Foxp3CreMycfl/fl mice was not attributed to increased cell death, as evidenced by comparable expression of active caspase-3 and annexin V between WT and Myc-deficient Tregs (fig. S1G). In contrast, Myc-deficient Tregs had severely defective expression of Ki-67 in Foxp3CreMycfl/fl mice and mixed BM chimeras (Fig. 1, H and I). Metabolic dysregulation in Tregs can lead to decreased Foxp3 stability and loss of Treg identity (2729). To determine whether Myc deficiency was linked to Foxp3 stability, we crossed Foxp3CreMycfl/fl mice to mice bearing Rosa26-driven, STOP-“floxed” cassette followed by GFP (Foxp3CreMycfl/flR26GFP). This system allows for Treg lineage tracing and determination of “ex-Tregs” characterized by Foxp3Cre-recombinase–driven GFP expression and loss of Foxp3-YFP expression (i.e., GFP+Foxp3-YFP). No presence of ex-Tregs was observed in Foxp3CreMycfl/flR26GFP mice, indicating undisturbed stability (fig. S1H). Thus, Myc function is essential for Tregs during neonatal development, and Myc deficiency reduces Treg accumulation likely through defective expansion.

Tregs require Myc to control immune homeostasis

Tregs generated during neonatal life are critical for immune tolerance (22). Consistent with a functional defect of Myc-deficient Tregs generated in neonatal life, mice with Myc-deficient Tregs developed a severe, early-onset autoimmune disease with death starting to occur at approximately 1 month of age (Fig. 2A). Histological examination revealed an extensive lymphoid/myeloid inflammatory presence in peripheral tissues (Fig. 2B). Also, Foxp3CreMycfl/fl mice showed considerable disruption of T cell homeostasis, with substantially expanded effector (CD62LloCD44hi) CD4+ and CD8+ populations (Fig. 2C). Significant increases in T helper cell 1 (TH1; CD4+IFN-γ+), TH2 (CD4+IL-4+), and TH17 (CD4+IL-17+) cells, as well as IFN-γ+ CD8+ T cells, were observed in Foxp3CreMycfl/fl mice (Fig. 2D). This nonspecific increase in all TH subsets is in contrast to previous reports using Treg-specific deletion of certain metabolic signaling molecules, with such studies describing a bias in TH subset inflammation (13, 27, 28, 30). Last, the functional suppressive capacity of Myc-deficient Tregs was impaired in vitro (fig. S2A). Thus, Myc function is important for ubiquitous Treg-mediated immunosuppressive activity.

Fig. 2 Deletion of Myc in Tregs results in a fatal autoimmune disease and extensively elevated T cell and GC responses.

(A) Survival curves of Foxp3CreMycfl/fl (n = 24) and WT (Foxp3CreMycfl/+ and Foxp3CreMyc+/+) mice (n = 8). (B) Representative histopathological images from hematoxylin and eosin–stained sections of the indicated tissues (magnification, ×10). (C) Flow cytometry analysis of naïve and effector populations of non-Treg CD4+ (denoted as CD4+) and CD8+ T cells in the spleen of WT and Foxp3CreMycfl/fl mice. (D) Quantification of cytokine production in splenic T cells of WT and Foxp3CreMycfl/fl mice. (E to G) Flow cytometry analysis and quantification of frequencies and total numbers of CD4+PD-1hiCXCR5+ follicular helper T (TFH) cells (E) and B220+GL-7+Fas+ GC B cells (F) in the spleen of WT and Foxp3CreMycfl/fl mice, or follicular regulatory T (TFR) cells in mixed BM chimeric mice (G). (H) Flow cytometry analysis of Myc-GFP expression within indicated CD4+ subsets in Foxp3RFPMyc-GFP mice. *P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001; unpaired Student’s t test. Data are representative of or pooled from 15 (C), 5 (D and G), 7 (E and F), or 2 (H) independent experiments, with one to four mice per genotype per experiment. Graphs show means ± SEM.

Proper Treg effector function is required to restrain germinal center (GC) responses mediated by follicular helper T (TFH) cells (3133). Foxp3CreMycfl/fl mice showed increased TFH cells (PD-1hiCXCR5+) and GC B cells (B220+GL-7+Fas+) (Fig. 2, E and F). Moreover, mixed BM chimeric mice displayed a pronounced reduction in Myc-deficient follicular regulatory T (TFR) cells (Fig. 2G), despite the rescue of overall conventional T cell and GC responses due to the presence of CD45.1+-derived cells (fig. S2, B and C). Consistent with the crucial role of Myc in TFR cells, further examination of Myc expression using Myc-GFP reporter mice revealed higher expression levels in CD4+PD-1hiCXCR5+ T cells, with the highest expression observed in TFR cells (Fig. 2H). These results highlight the importance of Myc in Treg-mediated maintenance of GC homeostasis.

Treg maturation and effector programming depend on Myc

To explore the molecular programs controlled by Myc, we purified Tregs from WT and Foxp3CreMycfl/fl mice and performed transcriptome analysis. Differential expression analysis showed that there were 331 up-regulated and 159 down-regulated probe sets in Myc-deficient Tregs (Fig. 3A). As expected, GSEA revealed that Myc-deficient cells had reductions in Myc target and protein synthesis genes (Fig. 3B). In contrast, enrichment in Myc-deficient Tregs mainly included proinflammatory gene sets (Fig. 3B). These results suggest that Myc acts in Tregs to enforce Treg function and, ultimately, maintain a proper anti-inflammatory transcriptional signature.

Fig. 3 Loss of Myc dampens eTreg gene signature and generation.

(A) Heatmap of differentially expressed (DE; fold change, >2) genes in Tregs from WT and Foxp3CreMycfl/fl mixed BM chimeric mice. (B) Top five gene sets differentially enriched by normalized enrichment score (NES) in WT (red) or Myc-deficient (blue) Tregs. (C) Volcano plot of DE genes from WT and Myc-deficient Tregs with number of genes correlating with eTreg gene signature (genes up-regulated in eTregs, blue; genes down-regulated in eTregs, red) (37). (D) Flow cytometry analysis of cTregs (CD62LhiCD44lo) and eTregs (CD62LloCD44hi) (gated on CD4+Foxp3-YFP+) and quantification of frequency and number of splenic Tregs in WT and Foxp3CreMycfl/fl mice. (E) Flow cytometry analysis and quantification of KLRG1 expression on total Tregs in the spleen of WT and Foxp3CreMycfl/fl mice. (F) Flow cytometry analysis of cTregs and eTregs and quantification of frequency and number of splenic eTregs in mixed BM chimeras. **P ≤ 0.01; ***P ≤ 0.001; χ2 square test (C) or unpaired Student’s t test (D to F). Data are representative of or pooled from 15 (D) or 6 (E and F) independent experiments, with one to three mice per group per experiment. Graphs show means ± SEM.

Tregs can be classified as eTregs and cTregs (2, 34, 35) based on their expression of different suppressive and trafficking molecules. eTregs are antigen and activation experienced with enhanced suppressive function and are necessary for overall immune homeostasis (36, 37). We found that Myc-deficient Tregs selectively lost signatures associated with eTregs (Fig. 3C) (37). Consistent with this observation, detailed analysis of Tregs in Foxp3CreMycfl/fl mice revealed a marked decrease in eTregs (CD62LloCD44hi) (Fig. 3D). A similar observation was made for the expression of the Treg activation–associated marker KLRG1 (Fig. 3E). The reduction in Myc-deficient eTregs was prevalent in mixed BM chimeras (Fig. 3F), supportive of a cell-intrinsic mechanism.

Myc is involved in a vast array of important cellular processes, and its expression is tightly regulated. We observed that Myc expression was temporally regulated during development (Fig. 1B). To determine whether improper Myc regulation could affect Treg accumulation or function, we used mice harboring a Myc transgene preceded by a STOP-floxed cassette on the Rosa26 locus (38). When crossed with Foxp3Cre mice, this results in constitutive Myc transgene expression specifically in Tregs (Foxp3CreR26MYC). Unexpectedly, Foxp3CreR26MYC mice showed no noticeable differences in frequencies of total Tregs (fig. S3A) or eTregs (fig. S3B) at steady state. Treg effector molecule expression was largely unaltered, except for Ki-67, which was markedly elevated in Tregs from Foxp3CreR26MYC mice (fig. S3C). Myc overexpression in Tregs had no effect on CD4+ or CD8+ T cell homeostasis (fig. S3D) or GC responses (fig. S3E), consistent with normal suppressive capacity in vitro (fig. S3F). These data suggest a more nuanced, context-dependent role of Myc in Treg function and homeostasis; while deficiency of Myc impairs Treg function and eTreg accumulation in vivo, Myc overexpression alone is not sufficient to alter immune homeostasis.

Myc-deficient Tregs fail to expand and control acute inflammation

Inflammation causes Tregs to undergo a transient activation program that increases their suppressive activity and expansion (34, 39). Our previous work identified the important role of Myc in the activation of conventional CD4+ and CD8+ T cells (9). To investigate a role in Tregs, we performed GSEA on two published datasets containing acutely activated Tregs and resting Tregs (34, 39). Expression of Myc target genes was highly enriched in activated Tregs from both datasets (Fig. 4A), suggesting a role of Myc in supporting the activation/effector program of Tregs. This functional importance of Myc likely extends to various pathological conditions, as we observed an increase of Myc expression and Myc+ Tregs in the spinal cords of mice with experimental autoimmune encephalomyelitis (EAE) (fig. S4A) and also among tumor-infiltrating lymphocytes of mice inoculated with MC38 adenocarcinoma cells (fig. S4B).

Fig. 4 Myc-deficient Tregs fail to expand and control acute inflammation.

(A) Enrichment of Hallmark Myc target gene set in activated and resting Tregs from published datasets (34, 39). (B) Schematic of diphtheria toxin (DT)–mediated Treg depletion in Foxp3-DTR (DT receptor) mosaic mice. (C) Flow cytometry analysis and quantification of WT/Myc-deficient (YFP+) and DTR+ (GFP+) Tregs before and after DT treatment in mosaic mice. (D) Quantification of eTreg number. (E) Flow cytometry analysis of naïve and effector populations of non-Treg CD4+ and CD8+ T cells. (F) Quantification of cytokine production in TH subsets in the spleen. (G) Flow cytometry analysis and quantification of Ki-67 expression in CD4+Foxp3-YFP+ Tregs in DT-treated mosaic mice. *P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001; ns, not significant; unpaired Student’s t test. Data are representative of or pooled from two independent experiments, with three to four mice per group per experiment. Graphs show means ± SEM. FDR, false discovery rate; NES, normalized enrichment score.

To directly test how Myc-deficient Tregs respond to inflammatory stimuli, we used a well-characterized in vivo model of acute inflammation via transient Treg depletion (34, 40). In this system, mosaic female mice (see Materials and Methods) have ~50% Tregs that express the receptor for diphtheria toxin (DT) and ~50% Tregs that are Myc deficient (or WT Tregs as control). These mice do not have aberrant inflammation at steady state (fig. S4C). Upon DT injection, the DT receptor (DTR)–expressing Tregs are depleted, leaving the remaining Tregs to respond to the resulting inflammation (Fig. 4B). Upon DT injection, Myc-deficient Tregs (YFP+) failed to expand to the same extent as WT Tregs (Fig. 4C). Furthermore, eTreg accumulation was enhanced in WT Tregs but was markedly impaired in those lacking Myc (Fig. 4D). This impairment was concomitant with increases in effector CD4+ and CD8+ populations (Fig. 4E) and TH1, TH2, and TH17 responses (Fig. 4F). Expression of Treg effector molecules was variable in this model (fig. S4D), but Ki-67 showed consistently decreased expression in Myc-deficient Tregs (Fig. 4G). These results establish that Myc deficiency is detrimental for eTreg generation and suppressive function during acute inflammation.

Myc is required for transition to, but not maintenance of, eTregs

Tregs require continuous T cell receptor (TCR) signals to establish and maintain the eTreg program and immune tolerance (36, 37). Because Myc expression and transcriptional programs are induced by TCR-dependent signals (9, 41), we next determined the temporal requirements for Myc expression in eTreg accumulation. To this end, we crossed Mycfl/fl mice with a tamoxifen-inducible Foxp3-Cre recombinase [Foxp3Cre-ERT2Mycfl/fl (42)]. In this system, Tregs maintain “normal” Myc functional capacity until mice are injected with tamoxifen, which allows Cre-mediated gene deletion to occur. In contrast to the severe inflammatory phenotype of Foxp3CreMycfl/fl or DT-treated Foxp3Cre/DTRMycfl/fl mice, Foxp3Cre-ERT2Mycfl/fl mice showed no signs of aberrant inflammation following tamoxifen-induced Myc deletion in Tregs (fig. S4, E and F), which was not attributed to elevated expression of Mycn or Mycl (fig. S4E). Notably, induced deletion of Myc had no effect on eTreg percentage, although KLRG1+ Tregs trended slightly lower (Fig. 5A). These results were unexpected, given the drastic eTreg phenotype observed in the constitutive deletion model, Foxp3CreMycfl/fl mice.

Fig. 5 Acute deletion of Myc reveals the requirement of Myc for transition to, but not maintenance of, eTregs.

(A) Flow cytometry analysis of cTregs and eTregs, and quantification of frequency and number of eTregs and KLRG1 expression on Tregs in the spleen of Foxp3Cre-ERT2Mycfl/+ and Foxp3Cre-ERT2Mycfl/fl mice treated with tamoxifen (2 mg per injection every other day for a total of six injections; analysis was performed 21 days after the final injection). Plots are pregated on Foxp3-GFP+ Tregs also reporting YFP+ for successful Cre-mediated recombination (CD4+GFP+YFP+). (B) Schematic with representative and quantified results for in vitro activation of cTregs (CD4+GFP+YFP+CD62LhiCD44lo) sorted from 8-week-old Foxp3Cre-ERT2Mycfl/+ and Foxp3Cre-ERT2Mycfl/fl mice treated with tamoxifen as in (A). (C) Effects of Myc inhibitors JQ-1 (500 nM) and i-BET-762 (500 nM) on in vitro activation of WT cTregs (CD4+YFP+CD62LhiCD44lo) sort purified from 6- to 8-week-old mice. (D) In vitro activation of Myc-overexpressing cTregs from 6- to 8-week-old WT and Foxp3CreR26MYC mice. *P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001; ns, not significant; unpaired Student’s t test. Data are representative of or pooled from four (A, C, and D) or two (B) independent experiments, with one to three mice per group per experiment. Graphs show means ± SEM. Forward scatter area, FSC-A.

We hypothesized that Myc function may be more important for Treg activation (i.e., during transition from cTregs to eTregs) rather than for the maintenance of eTregs. To test this, we used a previously published model of in vitro Treg activation (13, 43). We again used tamoxifen-injected Foxp3Cre-ERT2Mycfl/fl mice to sort GFP+YFP+CD62LhiCD44lo cTregs, followed by 3 days of stimulation with anti-CD3/CD28 antibodies and interleukin-2 (IL-2) (Fig. 5B). Examination of activation-associated parameters CD44 (marker associated with eTreg generation in vitro) and cell size (forward scatter area; FSC-A) revealed an inability of Myc-deficient Tregs to increase both of these markers following stimulation (Fig. 5B). Furthermore, pharmacological inhibition of Myc with either JQ-1 or i-BET-762 (29) in stimulated cTregs from WT mice caused a similar blunting effect on Treg activation (Fig. 5C). Last, cTregs from Foxp3CreR26MYC mice (with constitutive Myc expression) showed a consistent increase in CD44 and cell size following in vitro stimulation (Fig. 5D). These results show that Treg use of Myc occurs primarily during activation, while Myc function is dispensable for eTreg maintenance.

FAO-independent oxidative metabolism contributes to Treg function and eTreg generation

Our data demonstrating the importance of Myc function in proper activation of Tregs led us to consider current paradigms of metabolic regulation in Tregs. We have recently shown an essential role of mitochondrial function for Treg suppressive activity (13), and GSEA of Myc-deficient Tregs showed a reduction for the Hallmark oxidative phosphorylation pathway (Fig. 6A). To investigate the direct activity of Myc in mitochondrial function, we overlaid Myc target genes [generated from a published chromatin immunoprecipitation sequencing (ChIP-seq) dataset in T cells (10)] and mitochondria-related genes [identified by the MitoCarta 2.0 database (44)], with differentially expressed genes between WT and Myc-deficient Tregs (fig. S5A). Direct Myc targets based on the ChIP-seq dataset represented 111 of 526 genes (~21%) that were down-regulated in Myc-deficient Tregs. Further, direct Myc targets represented 219 of 1158 (~19%) of the MitoCarta 2.0 gene set, with 19 of 53 of the mitochondrial genes that were down-regulated in Myc-deficient Tregs representing direct Myc targets. In contrast, there was little overlap between the direct Myc targets, MitoCarta 2.0 genes, and genes up-regulated in Myc-deficient Tregs. These data indicate that Myc can directly promote mitochondrial metabolism to support Treg function. Consistent with this notion, Myc function was important for mitochondrial function, as indicated by the reduced oxygen consumption rate (OCR) in Tregs activated in vitro in the presence of Myc inhibitors (Fig. 6B). In addition, extracellular acidification rate (ECAR) was decreased (fig. S5B). Direct perturbation of mitochondrial oxidative phosphorylation by deleting Cox10 (fig. S5, C and D) (45) further revealed the importance of this metabolic pathway in Treg function and eTreg generation. Specifically, Foxp3CreCox10fl/fl mice showed reduced proportions of Tregs (Fig. 6C) and increased activation (Fig. 6D) and cytokine production (fig. S5E) by CD4+ and CD8+ T cells. Cox10-deficient Tregs showed impaired suppression in vitro (fig. S5F). Reductions in overall proportions of Cox10-deficient Tregs and eTregs (Fig. 6E) were observed in mixed BM chimeras, supportive of a cell-intrinsic defect. Thus, Myc is an essential transcriptional regulator of mitochondrial metabolism in Tregs, and impaired mitochondrial oxidative phosphorylation in Tregs is sufficient to disrupt Treg accumulation and eTreg generation.

Fig. 6 Oxidative metabolism contributes to Treg function and eTreg generation, while FAO is dispensable.

(A) Negative enrichment of Hallmark oxidative phosphorylation gene set in Myc-deficient Tregs. (B) Oxygen consumption rate (OCR) in Tregs activated in the presence of Myc inhibitors JQ-1 (1 μM) or i-BET-762 (1 μM). (C and D) Flow cytometry analysis of splenic Tregs (C) and naïve and effector CD4+ and CD8+ T cells (D) in WT and Foxp3CreCox10fl/fl mice. (E) Flow cytometry analysis of total splenic Tregs and cTregs and eTregs in mixed BM chimeras. (F) Heatmap of expression of FAO-related genes. (G to J) Flow cytometry analysis of naïve and effector CD4+ and CD8+ T cells (G), GC responses (H), total Tregs, cTregs, and eTregs (I), and marker and Ki-67 expression in Tregs (J) in the spleen of WT and Foxp3CreCpt1afl/fl mice. *P ≤ 0.05; ***P ≤ 0.001; ns, not significant; unpaired Student’s t-test. Data are representative of five independent experiments, with one to two mice per group per experiment. FDR, false discovery rate; NES, normalized enrichment score.

On the basis of pharmacological studies, FAO is a preferred metabolic pathway for driving mitochondrial function in Tregs (14), yet the regulation and in vivo function of this pathway remain poorly understood. We found that Myc-deficient Tregs did not have alteration of FAO-related genes (Fig. 6F), suggesting that Myc likely regulates mitochondrial function independently of FAO. To investigate the role of FAO in Treg-mediated immune homeostasis in vivo, we crossed Foxp3Cre mice to mice bearing floxed Cpt1a alleles (46), where we observed a specific reduction in Cpt1a but no changes in Cpt1b or Cpt1c (fig. S5G). Tregs from Foxp3CreCpt1afl/fl mice were confirmed to have functionally defective FAO when using palmitate bovine serum albumin (BSA) as a substrate for oxidative phosphorylation (fig. S5H). Mice with Cpt1a-deficient Tregs did not show aberrant CD4+ or CD8+ T cell activation (Fig. 6G), cytokine production (fig. S5I), or GC responses (Fig. 6H and fig. S5J). Also, Foxp3CreCpt1afl/fl mice had undisturbed cellularity of total Tregs and eTregs (Fig. 6I), with normal Treg effector marker and Ki-67 expression (Fig. 6J). Last, Cpt1a deficiency in Tregs had no effect on in vitro suppressive capacity (fig. S5K). These data suggest that FAO is a dispensable component of Treg development and function.

DISCUSSION

Myc is one of the most comprehensively studied molecules in cancer biology due to its broad functional scope and ubiquitous expression among diverse cell types (47). In physiological contexts, namely, activation of conventional CD4+ and CD8+ T cells, Myc facilitates metabolic reprogramming necessary for exit from quiescence (9). However, in Tregs, a metabolically unique cell type from conventional T cells (5, 7, 11, 12), Myc expression and function have been shown to be actively repressed by Foxp3 to exert immune tolerance (17). Therefore, the functional role and regulation of Myc in Tregs remain uncertain.

Our data show that the Treg pool in Foxp3CreMycfl/fl mice does not adequately expand during early neonatal development, a critical tuning period of the immune system (21). Without an effective counterbalance, proinflammatory inflammation causes tissue damage and leads to early death in Foxp3CreMycfl/fl mice. Whereas Foxp3CreMycfl/fl mice contain Foxp3+ Tregs (albeit at much lower numbers), the severity of autoimmune disease, early onset of lethality, and aberrant GC responses are comparable to Foxp3 null, “Scurfy” mice that harbor no Tregs (32, 48). Furthermore, our in vitro Treg suppression data suggest that Myc-deficient Tregs are unable to control inflammation. Transcriptome analysis reveals that Myc-deficient Tregs have reduced expression of the eTreg gene signature, which is consistent with our flow cytometry findings. We also find that Myc function is important for Tregs during acute inflammatory responses. Unlike the comparison of WT and Foxp3CreMycfl/fl mice, the DT-mediated mosaic Treg depletion model illustrates a critical response-reactive mechanism wherein Tregs are “forced” to confront a proinflammatory environment within a physiological context. The failure of Myc-deficient Tregs to expand or undergo transitional activation to become eTregs, and the consequential inability to subdue effector T cell responses, is consistent with the enrichment of Myc target genes in activated Tregs. Together, these results suggest that Myc-dependent Treg activation and eTreg population establishment are crucial components of early immune development and acute inflammation.

We describe several negative findings in this study that were originally surprising. Our data show that Myc is required for cTreg transition into eTreg but is dispensable in maintaining eTreg identity, based on the analysis of the tamoxifen-treated Foxp3Cre-ERT2Mycfl/fl mouse model. Previous work in conventional T lymphocytes and embryonic stem cells has argued that Myc has no direct impact on specification nor reprogramming of cell differentiation and instead serves as an “amplifier” of predicated transcriptional programs (10), although this notion has been recently challenged (49, 50). We propose that upon activation, Tregs transiently express Myc to boost expression of genes involved in providing a bolus of nutrients and proteins, which facilitates exit from quiescence. Moreover, we observe no substantial alterations in immune homeostasis or Tregs (except for increased Ki-67 expression) in Foxp3CreR26MYC mice. However, Myc-overexpressing Tregs from these mice show enhanced activation in vitro, suggesting that Myc function in Tregs is highly context dependent (i.e., only during activation) and ectopic Myc expression is not sufficient to alter baseline function. In one previous study supporting this notion, enforced expression of Myc was oncogenic within regenerating (metabolically active) livers but not within fully grown (metabolic steady state) adult livers (51).

Our previous work described the important role of Myc in the metabolic reprogramming of naïve CD4+ and CD8+ T cells upon activation. In the absence of Myc, these cells are unable to up-regulate anabolic pathways, but quiescence-associated pathways such as FAO are unaffected (9). It has been proposed that Treg function is reliant on mitochondrial metabolism driven by FAO, akin to memory and naïve T cells with low metabolic activity (14). However, this idea has recently been disputed (19, 20). Treg-specific impairment of oxidative phosphorylation in Foxp3CreCox10fl/fl mice results in an autoimmune disease, consistent with cell-intrinsic decreases in eTregs and in vitro suppressive capacity. In contrast, we show here that mice with Treg-specific Cpt1a deficiency show no signs of abnormal immune regulation nor Treg homeostasis, in line with the model of Cpt1a deficiency in all T lymphocytes (19). Literature addressing the role of anabolic metabolism in Treg function has revealed a relationship that is more complex than previously thought (13, 18, 27, 28, 52). Increasing anabolic metabolism through enforced expression of Glut1 or constitutively active Akt (18), or deletion of PTEN (phosphatase and tensin homolog) (27, 28), results in Treg hyperproliferation but diminished suppressive function, owing to the impaired lineage stability. In contrast, decreased anabolic metabolism through deletion of Mtor (13) or Raptor (52) leads to defective proliferation and eTreg generation, which is phenotypically similar to Myc deficiency in Tregs. By analyzing Cox10-deficient Tregs, we further reveal a crucial requirement of oxidative phosphorylation for Treg function and eTreg generation. Future research is warranted to dissect the specific metabolic programs underpinning oxidative phosphorylation.

In summation, Myc function is central for proper Treg accumulation, activation, and effector function. The results of this current study highlight metabolic reprogramming as a major determinant of Treg functional potency in the contexts of inflammation and during early development.

MATERIALS AND METHODS

Mice

C57BL/6, CD45.1+, Cox10fl/fl, Rag1−/−, Foxp3RFP, Foxp3DTR-GFP, R26MYC, R26YFP reporter, and R26GFP reporter (a loxP site–flanked STOP cassette followed by the YFP- or GFP-encoding sequence inserted into the Rosa26 locus) mice were purchased from the Jackson laboratory. Foxp3YFP-Cre (25) and Foxp3Cre-ERT2 (42) mice were gifts from A. Rudensky. Mycfl/fl mice (9) were gifts from D.R. Green and F.W. Alt. Myc-GFP reporter mice (23) were gifts from B. Sleckman. Foxp3CreMycfl/fl mice were used at 2 to 3 weeks old, with age- and gender-matched control mice. Other mice were used at 8 to 10 weeks old, unless otherwise noted. Mixed BM chimeric mice were generated by adoptively transferring a 1:1 mix of CD45.1+ spike and CD45.2+ (Foxp3CreMycfl/+ or Foxp3CreMycfl/fl) T cell–depleted BM cells into sublethally irradiated (5.5 Gy) Rag1−/− mice, followed by at least 8 weeks of reconstitution. For Treg depletion experiments, Mycfl/+ and Mycfl/fl female mosaic mice (harboring a Foxp3DTR-GFP allele on one X chromosome and Foxp3Cre allele on the other X chromosome) were injected intraperitoneally with DT (50 μg kg−1; EMD Millipore) every other day for four total injections and then analyzed 3 days after the last injection. For tamoxifen administration, mice were injected intraperitoneally with tamoxifen (2 mg per mouse) in corn oil every other day for six total injections and then analyzed 3 weeks after the last injection. All mice were kept in a specific pathogen–free facility in the Animal Resource Center at St. Jude Children’s Research Hospital, and animal protocols were approved by the Institutional Animal Care and Use Committee.

Flow cytometry

For analysis of surface markers, cells were stained in phosphate-buffered saline containing 2% (w/v) BSA. The following fluorescent-labeled antibodies (purchased from Thermo Fisher Scientific, Tonbo, BD Biosciences, Cell Signaling Technology, and Sony Biotechnology) were used: anti-CD4 (RM4-5), anti-CD8 (53-6.7), anti-CD25 (PC61.5), anti-B220 (RA3-6B2), anti-CD62L (MEL-14), anti-CD44 (IM7), anti-Fas (Jo2), anti-GL7 (GL-7), anti–PD-1 (J43), anti-ICOS (C398.4A), anti–Nrp-1 (3DS304M), anti-CD98 (RL388), anti-CD45.1 (A20), anti-CD45.2 (104), anti-KLRG1 (2F1), and anti-TCRβ (H57–597). Biotin-conjugated anti-CXCR5 (2G8) antibody and phycoerythrin (PE)–labeled streptavidin from BD Biosciences were used for TFH staining. Active caspase-3 or annexin V staining was performed according to the manufacturer’s instructions (BD Biosciences). For intracellular staining, cells were fixed using the Foxp3 fixation buffer (Thermo Fisher Scientific) as per the manufacturer’s instructions. The following antibodies were used: anti-Foxp3 (FJK-16 s), anti-CTLA4 (UC10-4B9), anti–Ki-67 (SolA15), anti–c-Myc (D84C12), anti–interferon-γ (IFN-γ) (XMG1.2), anti–IL-17A (TC11-18H10.1), and anti–IL-4 (11B11). For intracellular cytokine staining, cells were stimulated for 4 to 5 hours with phorbol 12-myristate 13-acetate and ionomycin in the presence of monensin (BD Biosciences). Flow cytometry data were collected using LSRII or Fortessa (BD Biosciences) cytometers and analyzed with FlowJo v10 software (TreeStar). Fluorescence-activated cell sorting was performed using Synergy or Reflection instruments (Sony Biotechnology).

Histology

Tissue samples were fixed in 10% neutral-buffered formalin, paraffin embedded, sectioned, and then stained with hematoxylin and eosin. All analyses were performed by an experienced pathologist (P.V.) in a blinded manner.

Cell culture

For cTreg activation, sort-purified cells were cultured for 3 days in complete Click’s medium [10% fetal bovine serum (FBS), 1% penicillin/streptomycin + l-glutamine, β-mercaptoethanol] with anti-CD3 (5 μg ml−1; plate bound), anti-CD28 (5 μg ml−1), and IL-2 (100 U ml−1). In some experiments, pharmacological Myc inhibitor JQ-1 (500 nM) or i-BET-762 (500 nM) was added to the culture. For in vitro Treg suppression assays, purified Tregs were cocultured with naïve CD4+ T cells and irradiated splenocytes as previously described (13).

EAE model

Mice were subcutaneously immunized with 200 μg of myelin oligodendrocyte glycoprotein (amino acids 35 to 55) in a total of 200 μl of emulsified incomplete Freund’s adjuvant supplemented with 1 mg of Mycobacterium tuberculosis (Difco) (complete Freund’s adjuvant). Mice received intraperitoneal injections of 200 ng of pertussis toxin (List Biological Laboratories) at the time of immunization and 2 days later. Flow cytometry analysis was performed on cells isolated from indicated organs at day 16 after immunization.

MC38 tumor model

MC38 colon adenocarcinoma cells were cultured in DMEM (Dulbecco’s modified essential medium) (10% FBS, 1% penicillin/streptomycin). Mice were inoculated subcutaneously with 5 × 105 MC38 cells in the right flank. Tumor-infiltrating lymphocytes were prepared by mincing and digesting tumor tissues in collagenase IV (1 mg/ml; Roche) and DNase I (200 U ml−1; Sigma-Aldrich) for 1 hour at 37°C, followed by Percoll density gradient centrifugation.

Metabolic assays

Seahorse XF96 extracellular flux analyzer was used to measure OCRs and ECARs under basal conditions and in response to 1 μM oligomycin, 2 μM fluoro-carbonyl cyanide phenylhydrazone (FCCP), and 500 nM rotenone. Tregs were activated with anti-CD3 (5 μg ml−1; plate bound), anti-CD28 (5 μg ml−1), and IL-2 (100 U ml−1) for 6 hours before metabolic analysis. Palmitate BSA or BSA control substrate (Agilent) was used where indicated to measure exogenous FAO according to the manufacturer’s instructions.

Microarray and GSEA

RNA was extracted with an RNeasy kit (Qiagen) from Tregs sorted from WT and Foxp3CreMycfl/fl mice. Microarray analysis was performed, as previously described (13). Microarray data from this study have been deposited into the Gene Expression Omnibus (GEO) database with the accession GSE141499. GSEA was performed on publicly available datasets, including neonatal versus adult Tregs (GSE66332) (22) and activated Tregs versus resting Tregs from two different datasets [(GSE55753) (34); (GSE83315) (39)]. eTreg signatures were generated from GSE61077 (37).

Statistics

Graphical results (GraphPad Prism software) are presented as means ± SEM with n per group and number of experimental replicates indicated in the respective figure legends. Student’s t test, χ2 square test, or one-way analysis of variance (ANOVA) with Tukey’s multiple comparison test was used where appropriate to generate P values. P values < 0.05 were considered significant.

SUPPLEMENTARY MATERIALS

Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/1/eaaw6443/DC1

Fig. S1. Myc function is important for neonatal Treg function and accumulation.

Fig. S2. Impaired in vitro Treg suppression with Myc deficiency and rescue of immune homeostasis in mixed BM chimeric mice.

Fig. S3. Constitutive Myc expression in Tregs does not affect immune homeostasis.

Fig. S4. CD4+ and CD8+ T cell responses in Foxp3Cre/DTR mosaic mice and mice with tamoxifen-induced Myc deletion.

Fig. S5. Immune homeostasis in mice with Cox10- or Cpt1a-deficient Tregs.

This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.

REFERENCES AND NOTES

Acknowledgments: We acknowledge D.R. Green and F.W. Alt for the Myc conditional allele, M. Hendren and A. KC for animal colony management, St. Jude Immunology FACS core facility for cell sorting, and S. A. Lim for help with the EAE animal model. Funding: This work was supported by the U.S. NIH (NIH AI105887, AI131703, AI140761, AI150241, AI150514, and CA221290 to H.C.). Author contributions: J.S. designed, performed, and analyzed experiments and wrote the manuscript. H.Z. designed, performed, and analyzed experiments. Y.D. and G.N. performed bioinformatical analyses. D.B.B., T.-L.M.N., N.M.C., Y.W., S.L., J.L.R., and A.K. performed and analyzed experiments, and N.M.C. and Y.W. edited the manuscript. P.V. provided histopathological analyses. P.C. provided critical reagents and insights. H.C. designed experiments, revised the manuscript, and provided funding and overall direction. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Microarray data have been deposited into the GEO database with the accession GSE141499. Additional data related to this paper may be requested from the authors.
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