Yeonseok Chung

T Helper 17 Lineage Differentiation Is Programmed by Orphan Nuclear Receptors RORα and RORγ

T cell functional differentiation is mediated by lineage-specific transcription factors. T helper 17 (Th17) has been recently identified as a distinct Th lineage mediating tissue inflammation. Retinoic acid receptor-related orphan receptor gamma (ROR gamma) was shown to regulate Th17 differentiation; ROR gamma deficiency, however, did not completely abolish Th17 cytokine expression. Here, we report Th17 cells highly expressed another related nuclear receptor, ROR alpha, induced by transforming growth factor-beta and interleukin-6 (IL-6), which is dependent on signal transducer and activator of transcription 3. Overexpression of ROR alpha promoted Th17 differentiation, possibly through the conserved noncoding sequence 2 in Il17-Il17f locus. ROR alpha deficiency resulted in reduced IL-17 expression in vitro and in vivo. Furthermore, ROR alpha and ROR gamma coexpression synergistically led to greater Th17 differentiation. Double deficiencies in ROR alpha and ROR gamma globally impaired Th17 generation and completely protected mice against experimental autoimmune encephalomyelitis. Therefore, Th17 differentiation is directed by two lineage-specific nuclear receptors, ROR alpha and ROR gamma.

Bcl6 Mediates the Development of T Follicular Helper Cells

T Follicular Helper Cell Differentiation When B cells respond to an infection, they often require help from CD4+ T cells to mount a proper response. It is thought that a subset of CD4+ effector T cells, called T follicular helper cells (TFH), performs this function. Several subsets of effector CD4+ T cells arise, depending on the type of infection, which have distinct transcriptional programs driving their differentiation. Whether this is also the case for TFH cells has not been clear (see the Perspective by Awasthi and Kuchroo). Nurieva et al. (p. 1001, published online 23 July) and Johnston et al. (p. 1006; published online 16 July) now demonstrate that the transcription factor Bcl6 is both necessary and sufficient for TFH differentiation and subsequent B cell–mediated immunity, suggesting that it is a master regulator of this lineage. Johnston et al. also show that expression of Bcl6 and the transcription factor, Blimp-1, are reciprocally regulated in TFH cells and that, when ectopically expressed, Blimp-1 inhibits TFH development. The transcription factors that regulate follicular T helper cell differentiation are identified. A fundamental function of CD4+ helper T (TH) cells is the regulation of B cell–mediated humoral immunity. Development of T follicular helper (TFH) cells that provide help to B cells is mediated by the cytokines interleukin-6 and interleukin-21 but is independent of TH1, TH2, and TH17 effector cell lineages. Here, we characterize the function of Bcl6, a transcription factor selectively expressed in TFH cells. Bcl6 expression is regulated by interleukin-6 and interleukin-21. Bcl6 overexpression induced TFH-related gene expression and inhibited other TH lineage cell differentiation in a DNA binding–dependent manner. Moreover, Bcl6 deficiency in T cells resulted in impaired TFH cell development and germinal center reactions, and altered production of other effector T cell subsets. Our data thus illustrate that Bcl6 is required for programming of TFH cell generation.

Molecular Antagonism and Plasticity of Regulatory and Inflammatory T Cell Programs

Regulatory T (Treg) and T helper 17 (Th17) cells were recently proposed to be reciprocally regulated during differentiation. To understand the underlying mechanisms, we utilized a Th17 reporter mouse with a red fluorescent protein (RFP) sequence inserted into the interleukin-17F (IL-17F) gene. Using IL-17F-RFP together with a Foxp3 reporter, we found that the development of Th17 and Foxp3(+) Treg cells was associated in immune responses. Although TGF-beta receptor I signaling was required for both Foxp3 and IL-17 induction, SMAD4 was only involved in Foxp3 upregulation. Foxp3 inhibited Th17 differentiation by antagonizing the function of the transcription factors RORgammat and ROR*. In contrast, IL-6 overcame this suppressive effect of Foxp3 and, together with IL-1, induced genetic reprogramming in Foxp3(+) Treg cells. STAT3 regulated Foxp3 downregulation, whereas STAT3, RORgamma, and ROR* were required for IL-17 expression in Treg cells. Our data demonstrate molecular antagonism and plasticity of Treg and Th17 cell programs.

Generation of T Follicular Helper Cells Is Mediated by Interleukin-21 but Independent of T Helper 1, 2, or 17 Cell Lineages

After activation, CD4(+) helper T (Th) cells differentiate into distinct effector subsets. Although chemokine (C-X-C motif) receptor 5-expressing T follicular helper (Tfh) cells are important in humoral immunity, their developmental regulation is unclear. Here we show that Tfh cells had a distinct gene expression profile and developed in vivo independently of the Th1 or Th2 cell lineages. Tfh cell generation was regulated by ICOS ligand (ICOSL) expressed on B cells and was dependent on interleukin-21 (IL-21), IL-6, and signal transducer and activator of transcription 3 (STAT3). However, unlike Th17 cells, differentiation of Tfh cells did not require transforming growth factor beta (TGF-beta) or Th17-specific orphan nuclear receptors RORalpha and RORgamma in vivo. Finally, naive T cells activated in vitro in the presence of IL-21 but not TGF-beta signaling preferentially acquired Tfh gene expression and promoted germinal-center reactions in vivo. This study thus demonstrates that Tfh is a distinct Th cell lineage.

Follicular regulatory T cells expressing Foxp3 and Bcl-6 suppress germinal center reactions

Foxp3+ regulatory T (Treg) cells suppress different types of immune responses to help maintain homeostasis in the body. How Treg cells regulate humoral immunity, including germinal center reactions, is unclear. Here we identify a subset of Treg cells expressing CXCR5 and Bcl-6 that localize to the germinal centers in mice and humans. The expression of CXCR5 on Treg cells depends on Bcl-6. These CXCR5+Bcl-6+ Treg cells are absent in the thymus but can be generated de novo from CXCR5−Foxp3+ natural Treg precursors. A lack of CXCR5+ Treg cells leads to greater germinal center reactions including germinal center B cells, affinity maturation of antibodies and the differentiation of plasma cells. These results unveil a Bcl-6-CXCR5 axis in Treg cells that drives the development of follicular regulatory T (TFR) cells that function to inhibit the germinal center reactions.

T helper 17 cells promote cytotoxic T cell activation in tumor immunity.

Although T helper 17 (Th17) cells have been found in tumor tissues, their function in cancer immunity is unclear. We found that interleukin-17A (IL-17A)-deficient mice were more susceptible to developing lung melanoma. Conversely, adoptive T cell therapy with tumor-specific Th17 cells prevented tumor development. Importantly, the Th17 cells retained their cytokine signature and exhibited stronger therapeutic efficacy than Th1 cells. Unexpectedly, therapy using Th17 cells elicited a remarkable activation of tumor-specific CD8(+) T cells, which were necessary for the antitumor effect. Th17 cells promoted dendritic cell recruitment into the tumor tissues and in draining lymph nodes increased CD8 alpha(+) dendritic cells containing tumor material. Moreover, Th17 cells promoted CCL20 chemokine production by tumor tissues, and tumor-bearing CCR6-deficient mice did not respond to Th17 cell therapy. Thus, Th17 cells elicited a protective inflammation that promotes the activation of tumor-specific CD8(+) T cells. These findings have important implications in antitumor immunotherapies.

CCR6 Regulates the Migration of Inflammatory and Regulatory T Cells

Th17 and regulatory T (Treg) cells play opposite roles in autoimmune diseases. However, the mechanisms underlying their proper migration to inflammatory tissues are unclear. In this study, we report that these two T cell subsets both express CCR6. CCR6 expression in Th17 cells is regulated by TGF-β and requires two nuclear receptors, RORα and RORγ. Th17 cells also express the CCR6 ligand CCL20, which is induced synergistically by TGF-β and IL-6, which requires STAT3, RORγ and IL-21. Th17 cells, by producing CCL20, promote migration of Th17 and Treg cells in vitro in a CCR6-dependent manner. Lack of CCR6 in Th17 cells reduces the severity of experimental autoimmune encephalomyelitis and Th17 and Treg recruitment into inflammatory tissues. Similarly, CCR6 on Treg cells is also important for their recruitment into inflammatory tissues. Our data indicate an important role of CCR6 in Treg and Th17 cell migration.

Th17 cells promote pancreatic inflammation but only induce diabetes efficiently in lymphopenic hosts after conversion into Th1 cells

IDDM is characterized by leukocyte invasion to the pancreatic tissues followed by immune destruction of the islets. Despite the important function of Th17 cells in other autoimmune disease models, their function in IDDM is relatively unclear. In this study, we found association of elevated Th17 cytokine expression with diabetes in NOD mice. To understand the function of Th17 cells in IDDM, we differentiated islet‐reactive BDC2.5 TcR transgenic CD4+ cells in vitro into Th17 cells and transferred them into NOD.scid and neonate NOD mice. NOD.scid recipient mice developed rapid onset of diabetes with extensive insulitic lesions, whereas in newborn NOD mice, despite extensive insulitis, most recipient mice did not develop diabetes. Surprisingly, BDC2.5+ cells recovered from diabetic NOD.scid mice, in comparison with those from neonate NOD mice, showed predominant IFN‐γ over IL‐17 expression, indicating conversion of donor cells into Th1 cells. Moreover, diabetes progression in NOD.scid recipients was dependent on IFN‐γ while anti‐IL‐17 treatment reduced insulitic inflammation. These results indicate that islet‐reactive Th17 cells promote pancreatic inflammation, but only induce IDDM upon conversion into IFN‐γ producers.

Toll-like Receptor 2 Signaling in CD4+ T Lymphocytes Promotes T Helper 17 Responses and Regulates the Pathogenesis of Autoimmune Disease

Toll-like receptors (TLRs) have previously been shown to play critical roles in the activation of innate immunity. Here, we describe that T cell expression of TLR2 regulates T helper 17 (Th17) cell responses. Stimulation with TLR2 agonists promoted Th17 differentiation in vitro and led to more robust proliferation and Th17 cytokine production. Using the experimental autoimmune encephalomyelitis (EAE) model, we found that TLR2 regulated Th17 cell-mediated autoimmunity in vivo and that loss of TLR2 in CD4(+) T cells dramatically ameliorated EAE. This study thus reveals a critical role of a TLR in the direct regulation of adaptive immune response and pathogenesis of autoimmune diseases.

Toll-like receptor 4 signaling in T cells promotes autoimmune inflammation

Toll-like receptors (TLRs) are critical components of innate immunity and function as rapid pathogen sensors. TLR4 is expressed on CD4+ T cells as well, the functional significance of which is unclear. In this study, we analyzed the function of TLR4 in T cells but did not find a role in promoting T helper (Th) cell polarization. Instead, TLR4 ligation enhanced both CD4+ T-cell proliferation and survival in vitro. Using the experimental autoimmune encephalomyelitis (EAE) model, we found that the loss of TLR4 solely in CD4+ T cells almost completely abrogated disease symptoms, mainly through blunted Th17 and, to a lesser degree, Th1 responses. Moreover, Tlr4−/− γδ T cells were defective in IL-17 and IFN-γ production following EAE induction. This study supports an important role of this innate receptor in the direct regulation of T-cell activation and survival during autoimmune inflammation.