Bcl6 and Blimp-1 are reciprocal and antagonistic regulators of T follicular helper cell differentiation

Effective B cell-mediated immunity and antibody responses often require help from CD4+ T cells. It is thought that a distinct CD4+ effector T cell subset, called T follicular helper cells (T(FH)), provides this help; however, the molecular requirements for T(FH) differentiation are unknown. We found that expression of the transcription factor Bcl6 in CD4+ T cells is both necessary and sufficient for in vivo T(FH) differentiation and T cell help to B cells in mice. In contrast, the transcription factor Blimp-1, an antagonist of Bcl6, inhibits T(FH) differentiation and help, thereby preventing B cell germinal center and antibody responses. These findings demonstrate that T(FH) cells are required for proper B cell responses in vivo and that Bcl6 and Blimp-1 play central but opposing roles in T(FH) differentiation.

Bcl6 mediates the development of T follicular helper cells

A fundamental function of CD4+ helper T (T(H)) cells is the regulation of B cell-mediated humoral immunity. Development of T follicular helper (T(FH)) 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 T(FH) cells. Bcl6 expression is regulated by interleukin-6 and interleukin-21. Bcl6 overexpression induced T(FH)-related gene expression and inhibited other T(H) lineage cell differentiation in a DNA binding-dependent manner. Moreover, Bcl6 deficiency in T cells resulted in impaired T(FH) 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 T(FH) cell generation.

Foxp3+ follicular regulatory T cells control the germinal center response

Follicular helper (T(FH)) cells provide crucial signals to germinal center B cells undergoing somatic hypermutation and selection that results in affinity maturation. Tight control of T(FH) numbers maintains self tolerance. We describe a population of Foxp3(+)Blimp-1(+)CD4(+) T cells constituting 10-25% of the CXCR5(high)PD-1(high)CD4(+) T cells found in the germinal center after immunization with protein antigens. These follicular regulatory T (T(FR)) cells share phenotypic characteristics with T(FH) and conventional Foxp3(+) regulatory T (T(reg)) cells yet are distinct from both. Similar to T(FH) cells, T(FR) cell development depends on Bcl-6, SLAM-associated protein (SAP), CD28 and B cells; however, T(FR) cells originate from thymic-derived Foxp3(+) precursors, not naive or T(FH) cells. T(FR) cells are suppressive in vitro and limit T(FH) cell and germinal center B cell numbers in vivo. In the absence of T(FR) cells, an outgrowth of non-antigen-specific B cells in germinal centers leads to fewer antigen-specific cells. Thus, the T(FH) differentiation pathway is co-opted by T(reg) cells to control the germinal center response.

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

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

Blimp-1 orchestrates plasma cell differentiation by extinguishing the mature B cell gene expression program

Blimp-1, a transcriptional repressor, drives the terminal differentiation of B cells to plasma cells. Using DNA microarrays, we found that introduction of Blimp-1 into B cells blocked expression of a remarkably large set of genes, while a much smaller number was induced. Blimp-1 initiated this cascade of gene expression changes by directly repressing genes encoding several transcription factors, including Spi-B and Id3, that regulate signaling by the B cell receptor. Blimp-1 also inhibited immunoglobulin class switching by blocking expression of AID, Ku70, Ku86, DNA-PKcs, and STAT6. These findings suggest that Blimp-1 promotes plasmacytic differentiation by extinguishing gene expression important for B cell receptor signaling, germinal center B cell function, and proliferation while allowing expression of important plasma cell genes such as XBP-1.

ICOS receptor instructs T follicular helper cell versus effector cell differentiation via induction of the transcriptional repressor Bcl6

The nature of follicular helper CD4(+) T (Tfh) cell differentiation remains controversial, including the minimal signals required for Tfh cell differentiation and the time at which Tfh cell differentiation occurs. Here we determine that Tfh cell development initiates immediately during dendritic cell (DC) priming in vivo. We demonstrate that inducible costimulator (ICOS) provides a critical early signal to induce the transcription factor Bcl6, and Bcl6 then induces CXCR5, the canonical feature of Tfh cells. Strikingly, a bifurcation between Tfh and effector Th cells was measurable by the second cell division of CD4(+) T cells, at day 2 after an acute viral infection: IL2Rα(int) cells expressed Bcl6 and CXCR5 (Tfh cell program), whereas IL2Rα(hi) cells exhibited strong Blimp1 expression that repressed Bcl6 (effector Th cell program). Virtually complete polarization between Bcl6(+) Tfh cells and Blimp1(+) effector Th cell populations developed by 72 hr, even without B cells. Tfh cells were subsequently lost in the absence of B cells, demonstrating a B cell requirement for maintenance of Bcl6 and Tfh cell commitment via sequential ICOS signals.

Control of inflammation, cytokine expression, and germinal center formation by BCL-6

The gene encoding the BCL-6 transcriptional repressor is frequently translocated and mutated in diffuse large cell lymphoma. Mice with a disrupted BCL-6 gene developed myocarditis and pulmonary vasculitis, had no germinal centers, and had increased expression of T helper cell type 2 cytokines. The BCL-6 DNA recognition motif resembled sites bound by the STAT (signal transducers and activators of transcription) transcription factors, which mediate cytokine signaling. BCL-6 could repress interleukin-4 (IL-4)-induced transcription when bound to a site recognized by the IL-4-responsive transcription factor Stat6. Thus, dysregulation of STAT-responsive genes may underlie the inflammatory disease in BCL-6-deficient mice and participate in lymphoid malignancies.

Inactivating mutations of acetyltransferase genes in B-cell lymphoma

B-cell non-Hodgkin's lymphoma comprises biologically and clinically distinct diseases the pathogenesis of which is associated with genetic lesions affecting oncogenes and tumour-suppressor genes. We report here that the two most common types--follicular lymphoma and diffuse large B-cell lymphoma--harbour frequent structural alterations inactivating CREBBP and, more rarely, EP300, two highly related histone and non-histone acetyltransferases (HATs) that act as transcriptional co-activators in multiple signalling pathways. Overall, about 39% of diffuse large B-cell lymphoma and 41% of follicular lymphoma cases display genomic deletions and/or somatic mutations that remove or inactivate the HAT coding domain of these two genes. These lesions usually affect one allele, suggesting that reduction in HAT dosage is important for lymphomagenesis. We demonstrate specific defects in acetylation-mediated inactivation of the BCL6 oncoprotein and activation of the p53 tumour suppressor. These results identify CREBBP/EP300 mutations as a major pathogenetic mechanism shared by common forms of B-cell non-Hodgkin's lymphoma, with direct implications for the use of drugs targeting acetylation/deacetylation mechanisms.

BCL-6 represses genes that function in lymphocyte differentiation, inflammation, and cell cycle control

BCL-6, a transcriptional repressor frequently translocated in lymphomas, regulates germinal center B cell differentiation and inflammation. DNA microarray screening identified genes repressed by BCL-6, including many lymphocyte activation genes, suggesting that BCL-6 modulates B cell receptor signals. BCL-6 repression of two chemokine genes, MIP-1alpha and IP-10, may also attenuate inflammatory responses. Blimp-1, another BCL-6 target, is important for plasmacytic differentiation. Since BCL-6 expression is silenced in plasma cells, repression of blimp-1 by BCL-6 may control plasmacytic differentiation. Indeed, inhibition of BCL-6 function initiated changes indicative of plasmacytic differentiation, including decreased expression of c-Myc and increased expression of the cell cycle inhibitor p27kip1. These data suggest that malignant transformation by BCL-6 involves inhibition of differentiation and enhanced proliferation.

The BCL-6 proto-oncogene controls germinal-centre formation and Th2-type inflammation

Structural alterations of the promoter region of the BCL-6 proto-oncogene represent the most frequent genetic alteration associated with non-Hodgkin lymphoma, a malignancy often deriving from germinal-centre B cells. The BCL-6 gene encodes a zinc-finger transcriptional repressor normally expressed in both B cells and CD4+ T cells within germinal centres, but its precise function is unknown. We show that mice deficient in BCL-6 displayed normal B-cell, T-cell and lymphoid-organ development but have a selective defect in T-cell-dependent antibody responses. This defect included a complete lack of affinity maturation and was due to the inability of follicular B cells to proliferate and form germinal centres. In addition, BCL-6-deficient mice developed an inflammatory response in multiple organs characterized by infiltrations of eosinophils and IgE-bearing B lymphocytes typical of a Th2-mediated hyperimmune response. Thus, BCL-6 functions as a transcriptional switch that controls germinal centre formation and may also modulate specific T-cell-mediated responses. Altered expression of BCL-6 in lymphoma represents a deregulation of the pathway normally leading to B cell proliferation and germinal centre formation.

Discovery and characterization of super-enhancer-associated dependencies in diffuse large B cell lymphoma

Diffuse large B cell lymphoma (DLBCL) is a biologically heterogeneous and clinically aggressive disease. Here, we explore the role of bromodomain and extra-terminal domain (BET) proteins in DLBCL, using integrative chemical genetics and functional epigenomics. We observe highly asymmetric loading of bromodomain 4 (BRD4) at enhancers, with approximately 33% of all BRD4 localizing to enhancers at 1.6% of occupied genes. These super-enhancers prove particularly sensitive to bromodomain inhibition, explaining the selective effect of BET inhibitors on oncogenic and lineage-specific transcriptional circuits. Functional study of genes marked by super-enhancers identifies DLBCLs dependent on OCA-B and suggests a strategy for discovering unrecognized cancer dependencies. Translational studies performed on a comprehensive panel of DLBCLs establish a therapeutic rationale for evaluating BET inhibitors in this disease.

Transcription factor IRF4 controls plasma cell differentiation and class-switch recombination

B cells producing high-affinity antibodies are destined to differentiate into memory B cells and plasma cells, but the mechanisms leading to those differentiation pathways are mostly unknown. Here we report that the transcription factor IRF4 is required for the generation of plasma cells. Transgenic mice with conditional deletion of Irf4 in germinal center B cells lacked post-germinal center plasma cells and were unable to differentiate memory B cells into plasma cells. Plasma cell differentiation required IRF4 as well as the transcriptional repressor Blimp-1, which both acted 'upstream' of the transcription factor XBP-1. In addition, IRF4-deficient B cells had impaired expression of activation-induced deaminase and lacked class-switch recombination, suggesting an independent function for IRF4 in this process. These results identify IRF4 as a crucial transcriptional 'switch' in the generation of functionally competent plasma cells.

T follicular helper cells express a distinctive transcriptional profile, reflecting their role as non-Th1/Th2 effector cells that provide help for B cells

Effector T cell responses have long been viewed in the context of the Th1/Th2 paradigm. Recently, a third major subset of nonpolarized effector T cells that provides help to B cells has been identified. These T cells, termed T follicular helper (T(FH)) cells, home to the B cell areas of secondary lymphoid tissue, through interactions mediated via the chemokine receptor CXCR5 and its ligand CXCL13. Affymetrix microarrays were used to identify transcription factors, cytokines, and cell surface molecules that underlie the differentiation pathways and functional properties of the T(FH) subset. The transcriptional profile of human CXCR5(+) T(FH) cells was compared with that of Th1 and Th2 cells, which enabled the identification of numerous genes expressed preferentially by T(FH) cells, over the other effector subsets. Certain T(FH) genes were also expressed by B cells and thus appear to be particularly relevant for humoral immunity. Abs were used to confirm the expression of several factors. In particular, CD84 and CD200, the cytokine IL-21, and the transcription factor BCL6 were all strongly associated with T(FH) cells. Gene microarrays reveal a highly distinctive transcriptional profile for a third subset of effector T cells that differs markedly from Th1 and Th2 cells.

Alterations of a zinc finger-encoding gene, BCL-6, in diffuse large-cell lymphoma

The molecular pathogenesis of diffuse large-cell lymphoma (DLCL), the most frequent and clinically relevant type of lymphoma, is unknown. A gene was cloned from chromosomal translocations affecting band 3q27, which are common in DLCL. This gene, BCL-6, codes for a 79-kilodalton protein that is homologous with zinc finger-transcription factors. In 33 percent (13 of 39) of DLCL samples, but not in other types of lymphoid malignancies, the BCL-6 gene is truncated within its 5' noncoding sequences, suggesting that its expression is deregulated. Thus, BCL-6 may be a proto-oncogene specifically involved in the pathogenesis of DLCL.

Regulation of B Cell Differentiation and Plasma Cell Generation by IL-21, a Novel Inducer of Blimp-1 and Bcl-6

IL-21 is a type I cytokine whose receptor is expressed on T, B, and NK cells. Within the B cell lineage, IL-21 regulates IgG1 production and cooperates with IL-4 for the production of multiple Ab classes in vivo. Using IL-21-transgenic mice and hydrodynamics-based gene delivery of IL-21 plasmid DNA into wild-type mice as well as in vitro studies, we demonstrate that although IL-21 induces death of resting B cells, it promotes differentiation of B cells into postswitch and plasma cells. Thus, IL-21 differentially influences B cell fate depending on the signaling context, explaining how IL-21 can be proapoptotic for B cells in vitro yet critical for Ag-specific Ig production in vivo. Moreover, we demonstrate that IL-21 unexpectedly induces expression of both Blimp-1 and Bcl-6, indicating mechanisms as to how IL-21 can serve as a complex regulator of B cell maturation and terminal differentiation. Finally, BXSB-Yaa mice, which develop a systemic lupus erythematosus-like disease, have greatly elevated IL-21, suggesting a role for IL-21 in the development of autoimmune disease.

The BCL6 proto-oncogene suppresses p53 expression in germinal-centre B cells

The human proto-oncogene BCL6 encodes a BTB/POZ-zinc-finger transcriptional repressor that is necessary for germinal-centre formation and is implicated in the pathogenesis of B-cell lymphoma. The precise function of BCL6 in germinal-centre development and lymphomagenesis is unclear because very few direct BCL6 target genes have been identified. Here we report that BCL6 suppresses the expression of the p53 (also known as tp53) tumour suppressor gene and modulates DNA damage-induced apoptotic responses in germinal-centre B cells. BCL6 represses p53 transcription by binding two specific DNA sites within the p53 promoter region and, accordingly, p53 expression is absent in germinal-centre B cells where BCL6 is highly expressed. Suppression of BCL6 expression via specific short interfering RNA leads to increased levels of p53 messenger RNA and protein both under basal conditions and in response to DNA damage. Most notably, constitutive expression of BCL6 protects B cell lines from apoptosis induced by DNA damage. These results suggest that an important function of BCL6 is to allow germinal-centre B cells to tolerate the physiological DNA breaks required for immunoglobulin class switch recombination and somatic hypermutation without inducing a p53-dependent apoptotic response. These findings also imply that deregulated BCL6 expression contributes to lymphomagenesis in part by functional inactivation of p53.

Transcriptional repression of atherogenic inflammation: modulation by PPARdelta

The formation of an atherosclerotic lesion is mediated by lipid-laden macrophages (foam cells), which also establish chronic inflammation associated with lesion progression. The peroxisome proliferator-activated receptor (PPAR) gamma promotes lipid uptake and efflux in these atherogenic cells. In contrast, we found that the closely related receptor PPARdelta controls the inflammatory status of the macrophage. Deletion of PPARdelta from foam cells increased the availability of inflammatory suppressors, which in turn reduced atherosclerotic lesion area by more than 50%. We propose an unconventional ligand-dependent transcriptional pathway in which PPARdelta controls an inflammatory switch through its association and disassociation with transcriptional repressors. PPARdelta and its ligands may thus serve as therapeutic targets to attenuate inflammation and slow the progression of atherosclerosis.

Mutation of BCL-6 gene in normal B cells by the process of somatic hypermutation of Ig genes

Immunoglobulin (Ig) genes are hypermutated in B lymphocytes that are the precursors to memory B cells. The mutations are linked to transcription initiation, but non-Ig promoters are permissible for the mutation process; thus, other genes expressed in mutating B cells may also be subject to somatic hypermutation. Significant mutations were not observed in c-MYC, S14, or alpha-fetoprotein (AFP) genes, but BCL-6 was highly mutated in a large proportion of memory B cells of normal individuals. The mutation pattern was similar to that of Ig genes.

TXNIP regulates peripheral glucose metabolism in humans

BACKGROUND

Type 2 diabetes mellitus (T2DM) is characterized by defects in insulin secretion and action. Impaired glucose uptake in skeletal muscle is believed to be one of the earliest features in the natural history of T2DM, although underlying mechanisms remain obscure.

METHODS AND FINDINGS

We combined human insulin/glucose clamp physiological studies with genome-wide expression profiling to identify thioredoxin interacting protein (TXNIP) as a gene whose expression is powerfully suppressed by insulin yet stimulated by glucose. In healthy individuals, its expression was inversely correlated to total body measures of glucose uptake. Forced expression of TXNIP in cultured adipocytes significantly reduced glucose uptake, while silencing with RNA interference in adipocytes and in skeletal muscle enhanced glucose uptake, confirming that the gene product is also a regulator of glucose uptake. TXNIP expression is consistently elevated in the muscle of prediabetics and diabetics, although in a panel of 4,450 Scandinavian individuals, we found no evidence for association between common genetic variation in the TXNIP gene and T2DM.

CONCLUSIONS

TXNIP regulates both insulin-dependent and insulin-independent pathways of glucose uptake in human skeletal muscle. Combined with recent studies that have implicated TXNIP in pancreatic beta-cell glucose toxicity, our data suggest that TXNIP might play a key role in defective glucose homeostasis preceding overt T2DM.

The function of follicular helper T cells is regulated by the strength of T cell antigen receptor binding

How follicular helper T cells (T(FH) cells) differentiate to regulate B cell immunity is critical for effective protein vaccination. Here we define three transcription factor T-bet-expressing antigen-specific effector helper T cell subsets with distinguishable function, migratory properties and developmental programming in vivo. Expression of the transcriptional repressor Blimp-1 distinguished T zone 'lymphoid' effector helper T cells (CD62L(hi)CCR7(hi)) from CXCR5(lo) 'emigrant' effector helper T cells and CXCR5(hi) 'resident' T(FH) cells expressing the transcriptional repressor Bcl-6 (CD62L(lo)CCR7(lo)). We then show by adoptive transfer and intact polyclonal responses that helper T cells with the highest specific binding of peptide-major histocompatibility complex class II and the most restricted T cell antigen receptor junctional diversity 'preferentially' developed into the antigen-specific effector T(FH) compartment. Our studies demonstrate a central function for differences in the binding strength of the T cell antigen receptor in the antigen-specific mechanisms that 'program' specialized effector T(FH) function in vivo.

BCL-6 mutations in normal germinal center B cells: evidence of somatic hypermutation acting outside Ig loci

The molecular mechanism involved in the process of antigen-driven somatic hypermutation of Ig genes is unknown, but it is commonly believed that this mechanism is restricted to the Ig loci. B cell lymphomas commonly display multiple somatic mutations clustering in the 5'-regulatory region of BCL-6, a proto-oncogene encoding for a POZ/Zinc finger transcriptional repressor expressed in germinal center (GC) B cells and required for GC formation. To determine whether BCL-6 mutations represent a tumor-associated phenomenon or reflect a physiologic mechanism, we screened single human tonsillar GC B cells for mutations occurring in the BCL-6 5'-noncoding region and in the Ig variable heavy chain sequences. Thirty percent of GC B cells, but not naive B cells, displayed mutations in the 742 bp region analyzed within the first intron of BCL-6 (overall frequency: 5 x 10(-4)/bp). Accordingly, an expanded survey in lymphoid malignancies showed that BCL-6 mutations are restricted to B cell tumors displaying GC or post-GC phenotype and carrying mutated Ig variable heavy chain sequences. These results indicate that the somatic hypermutation mechanism active in GC B cells physiologically targets non-Ig sequences.

Antitumor activity of a small-molecule inhibitor of human silent information regulator 2 enzymes

SIRT1 and other NAD-dependent deacetylases have been implicated in control of cellular responses to stress and in tumorigenesis through deacetylation of important regulatory proteins, including p53 and the BCL6 oncoprotein. Hereby, we describe the identification of a compound we named cambinol that inhibits NAD-dependent deacetylase activity of human SIRT1 and SIRT2. Consistent with the role of SIRT1 in promoting cell survival during stress, inhibition of SIRT1 activity with cambinol during genotoxic stress leads to hyperacetylation of key stress response proteins and promotes cell cycle arrest. Treatment of BCL6-expressing Burkitt lymphoma cells with cambinol as a single agent induced apoptosis, which was accompanied by hyperacetylation of BCL6 and p53. Because acetylation inactivates BCL6 and has the opposite effect on the function of p53 and other checkpoint pathways, the antitumor activity of cambinol in Burkitt lymphoma cells may be accomplished through a combined effect of BCL6 inactivation and checkpoint activation. Cambinol was well tolerated in mice and inhibited growth of Burkitt lymphoma xenografts. Inhibitors of NAD-dependent deacetylases may constitute novel anticancer agents.

Opposing signals from the Bcl6 transcription factor and the interleukin-2 receptor generate T helper 1 central and effector memory cells

Listeria monocytogenes infection generates T helper 1 (Th1) effector memory cells and CC chemokine receptor 7 (CCR7)(+) cells resembling central memory cells. We tracked endogenous L. monocytogenes-specific CD4(+) T cells to determine how these memory cells are formed. Two effector cell populations were already present several days after infection. One highly expressed the T-bet transcription factor and produced Th1 memory cells in an interleukin-2 (IL-2) receptor-dependent fashion. The other resided in the T cell areas, expressed CCR7 and CXC chemokine receptor 5 (CXCR5), and like follicular helper cells depended on the Bcl6 transcription factor and inducible costimulator ligand on B cells. The CCR7(+)CXCR5(+) effector cells produced similar memory cells that generated diverse effector cell populations in a secondary response. Thus, Th1 effector memory and follicular helper-like central memory cells are produced from early effector cell populations that diverge in response to signals from the IL-2 receptor, Bcl6, and B cells.

BCL-6, a POZ/zinc-finger protein, is a sequence-specific transcriptional repressor

Approximately 40% of diffuse large cell lymphoma are associated with chromosomal translocations that deregulate the expression of the BCL6 gene by juxtaposing heterologous promoters to the BCL-6 coding domain. The BCL6 gene encodes a 95-kDa protein containing six C-terminal zinc-finger motifs and an N-terminal POZ domain, suggesting that it may function as a transcription factor. By using a DNA sequence selected for its ability to bind recombinant BCL-6 in vitro, we show here that BCL-6 is present in DNA-binding complexes in nuclear extracts from various B-cell lines. In transient transfectin experiments, BCL6 can repress transcription from promoters linked to its DNA target sequence and this activity is dependent upon specific DNA-binding and the presence of an intact N-terminal half of the protein. We demonstrate that this part of the BCL6 molecule contains an autonomous transrepressor domain and that two noncontiguous regions, including the POZ motif, mediate maximum transrepressive activity. These results indicate that the BCL-6 protein can function as a sequence-specific transcriptional repressor and have implications for the role of BCL6 in normal lymphoid development and lymphomagenesis.

Early Th1 cell differentiation is marked by a Tfh cell-like transition

Follicular helper T (Tfh) cells comprise an important subset of helper T cells; however, their relationship with other helper lineages is incompletely understood. Herein, we showed interleukin-12 acting via the transcription factor STAT4 induced both Il21 and Bcl6 genes, generating cells with features of both Tfh and Th1 cells. However, STAT4 also induced the transcription factor T-bet. With ChIP-seq, we defined the genome-wide targets of T-bet and found that it repressed Bcl6 and other markers of Tfh cells, thereby attenuating the nascent Tfh cell-like phenotype in the late phase of Th1 cell specification. Tfh-like cells were rapidly generated after Toxoplasma gondii infection in mice, but T-bet constrained Tfh cell expansion and consequent germinal center formation and antibody production. Our data argue that Tfh and Th1 cells share a transitional stage through the signal mediated by STAT4, which promotes both phenotypes. However, T-bet represses Tfh cell functionalities, promoting full Th1 cell differentiation.