Thymus-derived Tregs Research Articles

Kinetics of Helios(+) and Helios(-) T regulatory cell subsets in the circulation of healthy pregnant women.

Regulatory T cells (Tregs) play a critical role in the maintenance of a pregnancy. While the kinetics of the number of peripheral blood Tregs has been satisfactorily described in mouse models, analysis of these cell populations in human pregnancy is complicated by high variability in the quantity of Tregs and inconsistencies in the markers used for detecting different types of Treg. In the light of this, we set out to investigate the kinetics of various types of Treg, including CD45RA, GARP and PD-1(+) Tregs, in the peripheral blood of pregnant women in the first, second and third trimester, and at the time of delivery. Tregs, defined as a CD4(+)CD25(++)CD127(dim)Foxp3(+) population of leucocytes, were detected using flow cytometry. Natural thymus-derived Tregs and induced Tregs in the peripheral blood were distinguished by the expression or absence of a Helios marker, respectively. Our results showed that during normal pregnancy the sizes of various Treg subpopulations varied across women and also in an individual woman did not remain constant but varied significantly, most notable being the decrease observed at the time of delivery. Helios(-) cells were significantly less frequent in the peripheral blood of healthy pregnant women than Helios(+) cells, and the majority of Tregs were Helios(+)PD-1(+) Tregs.

Interleukin 2 modulates thymic-derived regulatory T cell epigenetic landscape

Foxp3+CD4+ regulatory T (Treg) cells are essential for preventing fatal autoimmunity and safeguard immune homeostasis in vivo. While expression of the transcription factor Foxp3 and IL-2 signals are both required for the development and function of Treg cells, the commitment to the Treg cell lineage occurs during thymic selection upon T cell receptor (TCR) triggering, and precedes the expression of Foxp3. Whether signals beside TCR contribute to establish Treg cell epigenetic and functional identity is still unknown. Here, using a mouse model with reduced IL-2 signaling, we show that IL-2 regulates the positioning of the pioneer factor SATB1 in CD4+ thymocytes and controls genome wide chromatin accessibility of thymic-derived Treg cells. We also show that Treg cells receiving only low IL-2 signals can suppress endogenous but not WT autoreactive T cell responses in vitro and in vivo. Our findings have broad implications for potential therapeutic strategies to reprogram Treg cells in vivo.

Impact of activated invariant natural killer T cells on the expansion of regulatory T cell precursors in murine thymocytes in vitro

Tolerance induction is a goal of clinical transplantation to prevent graft rejection without the lifelong use of immunosuppressive drugs. In a series of mouse studies, we previously reported that the establishment of mixed chimerism by treatment with a ligand for invariant natural killer T (iNKT) cells with CD40 signal blockade makes it possible to prevent allograft rejection without immunosuppressants, and this approach fails in thymectomized recipient mice. In this study, we showed that iNKT cells in murine thymocyte cultures are indispensable for the expansion of CD4+CD25+Foxp3+ regulatory T (Treg) cells as well as CD4+CD25+Foxp3− cells, which contained precursor Tregs (preTregs). After the culture of BALB/c mouse-derived thymocytes in the presence of α-galactosylceramide (α-GalCer), a representative ligand for iNKT cells, the ratio of CD4+CD25+Foxp3− preTregs to total CD4+CD8− T cells was much higher than that of CD4+CD25+Foxp3+ Treg cells, regardless of anti-CD40 L mAb treatment. The proliferation of CD4+CD25+Foxp3− cells, but not Treg cells, was significantly augmented, and the stability of Treg cells was not affected by α-GalCer. The expansion of thymocyte-derived Tregs was not inhibited by cytokine neutralization. However, in vitro thymus-derived CD4+CD25+Foxp3− cells expressed Foxp3 after IL-2 stimulation in a dose-dependent manner. These results collectively suggest that in vitro thymus-derived Treg cell expansion by α-GalCer treatment was caused by the proliferation of CD4+CD25+Foxp3− preTregs but not existing Treg cells.

Thymus-derived Foxp3+ regulatory T cells upregulate RORγt expression under inflammatory conditions.

Foxp3+ regulatory T cells (Tregs) co-expressing the Th17-lineage specification factor RORγt represent a unique Treg subpopulation that has been reported to be induced upon response to gut microbiota within the intestinal immune system. Hence, RORγt+ Tregs are considered to solely consist of peripherally induced Foxp3+ Tregs (pTregs), and the possibility that also thymus-derived Treg (tTregs) can upregulate RORγt expression and contribute to the pool of RORγt+ Tregs was largely ignored. Here, we expand our knowledge on the origin of RORγt+ Tregs by demonstrating that also tTregs can attain RORγt expression. In transgenic Foxp3∆CNS1-Cre reporter mice, a substantial fraction of CNS1-independent Tregs, predominantly consisting of tTregs, was found to co-express RORγt. In addition, genuine tTregs isolated from thymi of Foxp3hCD2RAGGFP reporter mice initiated RORγt expression both in vitro and in vivo, particularly under inflammatory conditions. In conclusion, our data demonstrate that tTregs can upregulate RORγt expression under inflammatory conditions and that hence RORγt+ Tregs can be regarded as a heterogeneous population consisting of both pTregs and tTregs. KEY MESSAGES: RORγt cannot be considered as a marker for pTregs. CNS1-independent tTregs within the colon display RORγt expression. RORγt can be induced in genuine tTregs, particularly under inflammatory conditions. RORγt+ Tregs are a heterogeneous population consisting of both pTregs and tTregs.

Cutting Edge: TGF-β and Phosphatidylinositol 3-Kinase Signals Modulate Distinct Metabolism of Regulatory T Cell Subsets.

Murine Foxp3+ regulatory T cells (Tregs) differentiated in vitro (induced Tregs [iTregs]) in the presence of anti-inflammatory cytokine TGF-β rely predominantly upon lipid oxidation to fuel mitochondrial oxidative phosphorylation. Foxp3 expression underlies this metabolic preference, as it suppresses glycolysis and drives oxidative phosphorylation. In this study, we show that in contrast to iTregs, thymic-derived Tregs (tTregs), engage in glycolysis and glutaminolysis at levels comparable to effector T cells despite maintained Foxp3 expression. Interestingly, exposure of tTregs to the anti-inflammatory cytokine TGF-β represses PI3K-mediated mTOR signaling, inhibits glucose transporter and Hk2 expression, and reprograms their metabolism to favor oxidative phosphorylation. Conversely, replicating the effects of inflammation via elevation of PI3K signaling has minimal effects on tTregs but dramatically enhances the glycolysis of normally oxidative iTregs, resulting in reduction of Foxp3 expression. Collectively, these findings suggest both extrinsic and intrinsic factors govern the unique metabolic signature of Treg subsets.

The Cytotoxic Activity of Natural Killer Cells Is Suppressed by IL-10+ Regulatory T Cells During Acute Retroviral Infection.

Natural killer (NK) cells play a key role in host defense against cancer and viral infections. It was shown that NK cells are important for the control of acute retroviral infections, but their antiviral activity depends on multiple parameters such as viral inoculation dose, interactions with myeloid cell types and the cytokine milieu. In addition, during an ongoing retroviral infection regulatory T cells (Tregs) can suppress NK cell functions. However, the precise role of Tregs on the initial NK cell response and their immediate antiviral activity after an acute retroviral infection is still unknown. Here we show that thymus-derived Tregs suppress the proliferation, effector functions and cytotoxicity of NK cells very early during acute Friend Retrovirus (FV) infection. Tregs exhibited an activated phenotype and increased the production of the immunosuppressive cytokines IL-10 and TGF-β after FV infection of mice. Neutralization of the immunosuppressive cytokine IL-10 resulted in a significant augmentation of NK cell functions. Although the activation of dendritic cells (DCs) and macrophages as well as the IL-15 cytokine levels were increased after Treg depletion, Tregs mainly affect the NK cell activity in an IL-10-regulated pathway. In this study we demonstrate an IL-10-dependent suppression of NK cells by activated Tregs during the first days of a retroviral infection.

“First-In-Human” Clinical Trial Employing Adoptive Transfer of Autologous Thymus-Derived Treg Cells (thyTreg) to Prevent Graft Rejection in Heart-Transplanted Children

“First-In-Human” Clinical Trial Employing Adoptive Transfer of Autologous Thymus-Derived Treg Cells (thyTreg) to Prevent Graft Rejection in Heart-Transplanted Children

Regulatory T cell subsets are differentially dependent on CD28 for their proliferation

It is thought that CD28 plays a crucial role in the maintenance of regulatory T cell (Treg) pool size through promoting the development and proliferation of these cells. However, recently we found that the dependency on CD28 co-stimulation for their development is different between Treg subsets, thymus-derived Tregs (tTregs, CD28-dependent) and peripherally-derived Tregs (pTregs, CD28-independent), suggesting that CD28 may also have differential influences on the homeostasis of each Treg subset. Here, we demonstrated that both Treg subsets were reduced in secondary lymphoid organs of CD28 deficient mice, and that this reduction was due to impaired proliferation in both Treg subsets by the intrinsic CD28 defect. However, we found that the massive proliferation of both Treg subsets under lymphopenic condition was regulated by CD28, whereas the proliferative activity of tTregs but not pTregs in the steady state was dependent on CD28. Also, experiments using mutant CD28 knock-in mice revealed that proliferation of pTregs under lymphopenic condition required only the Lck-NFκB pathway of CD28, whereas tTregs required an additional unknown pathway. These findings indicate that the dependency on CD28 for proliferation in each Treg subset differs depending on the environment.

MTOR coordinates transcriptional programs and mitochondrial metabolism of activated Treg subsets to protect tissue homeostasis

Regulatory T (Treg) cells derived from the thymus (tTreg) and periphery (pTreg) have central and distinct functions in immunosuppression, but mechanisms for the generation and activation of Treg subsets in vivo are unclear. Here, we show that mechanistic target of rapamycin (mTOR) unexpectedly supports the homeostasis and functional activation of tTreg and pTreg cells. mTOR signaling is crucial for programming activated Treg-cell function to protect immune tolerance and tissue homeostasis. Treg-specific deletion of mTOR drives spontaneous effector T-cell activation and inflammation in barrier tissues and is associated with reduction in both thymic-derived effector Treg (eTreg) and pTreg cells. Mechanistically, mTOR functions downstream of antigenic signals to drive IRF4 expression and mitochondrial metabolism, and accordingly, deletion of mitochondrial transcription factor A (Tfam) severely impairs Treg-cell suppressive function and eTreg-cell generation. Collectively, our results show that mTOR coordinates transcriptional and metabolic programs in activated Treg subsets to mediate tissue homeostasis.

Antigen-specific Helios- , Neuropilin-1- Tregs induce apoptosis of autoreactive B cells via PD-L1.

Regulatory T cells (Tregs) maintain self-tolerance and prevent autoimmunity by controlling autoreactive T cells. We recently demonstrated invivo that Tregs can directly suppress auto-reactive B cells via programmed death ligand 1 (PD-L1) that ligated PD-1 on B cells and caused them to undergo apoptosis. Here, we asked whether this mechanism is utilized by thymus-derived natural Tregs and/or by peripheral lymphoid tissue-induced Tregs. We first demonstrated that antigen-specific PD-L1-expressing Tregs were induced in the draining lymph node of autoantigen-expressing tissue and characterized them by their lack of the transcription factor Helios and of the surface marker Neuropilin-1 (Nrp-1). Next, we established an invitro co-culture system to study the interaction between B cells and Treg subsets under controlled conditions. We found that Nrp- Treg, but not Nrp+ Treg suppressed autoreactive B cells, whereas both were able to suppress T-helper cells. Such suppression was antigen-specific and was facilitated by PD-L1/PD-1-induced apoptosis. Furthermore, it required physical cell contact and was MHC II-restricted, providing an explanation for the antigen-specificity of peripherally-induced Tregs. These findings identify a role for peripherally induced Helios- Nrp-1- inducible Treg in controlling peripheral B-cell tolerance against tissue auto-antigens.

Aire-dependent thymic FoxP3+ regulatory T cells in Type 1 Diabetes

Abstract Aire expression within the specialized mTECs has been found to be important for enforcing negative selection of T cells, however, mounting evidence has demonstrated the importance of Aire in Treg differentiation and development. Autoimmune diseases like T1D arise from the failure to restrain immune responses against self. Utilizing a novel mouse model, where the Aire locus drives the selected expression of the high affinity DT receptor, revealed a significant reduction in the number of FoxP3+ Tregs within the thymus following treatment. Thus, we hypothesize that alterations in the thymic self-antigen expression and presentation via ablation of Aire-expressing mTECs, will affect the repertoire and function of Aire-dependent FoxP3+ Tregs. New high-throughput platforms and single-cell DNA barcoding technology will allow us to assess several thousand individual Tregs that have undergone Aire-dependent selection in the thymus. The identification of these unique clones and gene signatures will be further interrogated retrogenically, allowing for qualitative assessment of their function within the model of T1D. The antigen specificities of the Aire-dependent TCRs will be assessed via screening against peptide libraries using MHCII molecule strongly associated with NOD mouse T1D. We anticipate identification of novel sequences that are recognized by Tregs, which may lead to discovery of new self-peptides that contribute to T1D. Our proposed study aims to expand our knowledge on TCR repertoire of Aire-dependent FoxP3+ Tregs and the antigen-specificity in T1D, improving our understanding of the respective function of thymic-derived antigen-specific Treg suppression in peripheral tissues associated with autoimmune T1D.

IL-1 and IL-33 differentially regulate the functional specialization of mucosal Foxp3+ regulatory T cells.

Abstract CD4+ regulatory T (TREG) cells are critical mediators of peripheral immune tolerance and homeostasis, and express the forkhead box p3 (Foxp3) transcription factor, the master-regulator driving the programming of the TREG cell suppressive phenotype. TREG cells are abundant at mucosal surfaces, where they acquire tissue-specific adaptations. The biological consequences of these adaptations on the stability of thymic-derived TREG (tTreg) cells remain largely unknown. To determine the signals that drive the fate of TREG cells, we isolated and compared stable from unstable TREG cells using a TREG transfer model where we previously observed the functional reprogramming of Foxp3+ TREG cells into Th1/Th17 effector T cells. We identified the expression of the IL-33 receptor (IL-33R, ST2) on stable tTREG cells and the IL-1 receptor (IL1R1) on unstable exTREG cells undergoing functional reprogramming. We show that both TREG cell populations represent competing subsets in inflammatory conditions. This is further underlined by the fact that the absence of IL1R1 expression (IL1R1−/−) leads to the accumulation of ST2+ TREG cells at mucosal sites in vivo. In two distinct lung infection models, we demonstrate that ST2-expressing TREG cells express GATA3 and resist production of inflammatory cytokines, whereas IL1R1-expressing TREG cells express RORγT and lose Foxp3 expression in vivo. While IL-1 signaling impairs TREG cell suppressive function, ST2 is required for the maintenance of the lineage identity and suppressive function of TREG cells. These observations demonstrate that IL-1 and IL-33 produced during immune challenge exert distinct roles on the functional adaptation of Foxp3+ tTREG cells at mucosal surfaces.

Phenotypic and functional properties of in vitro-induced Helios+ regulatory T cells

Objective To investigate the phenotypic and functional properties of in vitro-induced Helios+ regulatory T cells (Helios+ iTreg) and to analyze the differences between Helios+ iTreg and Helios+ thymic-derived natural Treg (Helios+ nTreg). Methods CD4+ CD25- effector T cells (CD4+ CD25- Teff) that were separated from healthy subjects were cultured with anti-CD3 and anti-CD28 antibodies and stimulated with IL-2 and TGF-β to induce the generation of Helios+ iTreg. Flow cytometry and real-time PCR were respectively used to analyze the differences in phenotype and CpG methylation in Treg-specific demethylated region (TSDR) between Helios+ iTreg and Helios+ nTreg derived from the same donor. Results CD45RA was highly expressed on Helios+ iTreg, but not on Helios+ nTreg. Additionally, Helios+ iTreg expressed significantly higher levels of CD127, ICOS and PD-1, but lower levels of CXCR3, CCR4 and CD25 than Helios+ nTreg did. IFN-γ and IL-2 that expressed by Helios+ iTreg were more than those by Helios+ nTreg. It was also found that the level of CpG methylation in TSDR was much higher in Helios+ iTreg than in Helios+ nTreg. Conclusion Both nTreg and iTreg expressed Helios, but the phenotypic and functional properties of the two Treg subsets were different. It was suggested that Helios+ iTreg and Helios+ nTreg might play different roles in the immune system. Key words: Regulatory T cell; Helios; Interleukin-2; Immunophenotype; Methylation

Egress of murine regulatory T cells from the thymus requires TIPE2

Regulatory T cells (Tregs) can be divided into thymus-derived Treg (tTregs) and peripheral induced Tregs (pTregs) in vivo according to their origins and are essential for the maintenance of immune hemostasis and immune tolerance. Tumor necrosis factor-α-induced protein 8 like 2 (TIPE2) is expressed primarily by immune cells and is a negative regulator of the innate and adaptive immune response. Previous studies indicate that TIPE2 is required for the expression of Treg signature genes and promotes leading-edge formation in neutrophils through cytoskeleton remodeling. In the current study, we showed that TIPE2 deficient mice accumulate more Treg cells in the thymus. Further studies revealed that TIPE2 deficiency doesn't affect the development and apoptosis of tTregs. Instead, TIPE2 promotes the chemotaxis of tTregs in vitro, which may account for the accumulation of Tregs in the thymus of TIPE2 deficient mice. Mechanistic study revealed that TIPE2 promotes the polarization of pAKT and F-actin in tTregs undergoing directed migration. Taken together, these results demonstrated that TIPE2 enhances the cytoskeleton remodeling and promotes the thymus egress of tTregs, which may play an important role in the maintenance of self-tolerance.

Phenotypic and Functional Changes of Peripheral Ly6C+ T Regulatory Cells Driven by Conventional Effector T Cells.

A relatively high affinity/avidity of T cell receptor (TCR) recognition for self-peptide bound to major histocompatibility complex II (self-pMHC) ligands is a distinctive feature of CD4 T regulatory (Treg) cells, including their development in the thymus and maintenance of their suppressive functions in the periphery. Despite such high self-reactivity, however, all thymic-derived peripheral Treg populations are neither homogenous in their phenotype nor uniformly immune-suppressive in their function under steady state condition. We show here that based on the previously defined heterogeneity in the phenotype of peripheral Treg populations, Ly6C expression on Treg marks a lower degree of activation, proliferation, and differentiation status as well as functional incompetence. We also demonstrate that Ly6C expression on Treg in a steady state is either up- or downregulated depending on relative amounts of tonic TCR signals derived from its contacts with self-ligands. Interestingly, peripheral appearance and maintenance of these Ly6C-expressing Treg cells largely differed in an age-dependent manner, with their proportion being continuously increased from perinatal to young adult period but then being gradually declined with age. The reduction of Ly6C+ Treg in the aged mice was not due to their augmented cell death but rather resulted from downregulation of Ly6C expression. The Ly6C downregulation was accompanied by proliferation of Ly6C+ Treg cells and subsequent change into Ly6C− effector Treg with concomitant restoration of immune-suppressive activity. Importantly, we found that this phenotypic and functional change of Ly6C+ Treg is largely driven by conventional effector T cell population. Collectively, these findings suggest a potential cross-talk between peripheral Treg subsets and effector T cells and provides better understanding for Treg homeostasis and function on maintaining self-tolerance.

Type 3 regulatory T cells at the interface of symbiosis.

The mammalian gastrointestinal tract accommodates trillions of bacteria, many of which provide beneficial effects to the host, including protection from pathogenic microorganisms and essential metabolites. However, the intestinal immune system needs to adapt to the constantly fluctuating microbial environment at mucosal surfaces in order to maintain homeostasis. In particular, the gut microbiota induces the differentiation of effector Th17 cells and regulatory T cells (Tregs) that express RORγt, the master regulator of antimicrobial type 3 immunity. RORγt+ Tregs constitute a major population of colonic Tregs that is distinct from thymusderived Tregs and require bacterial antigens for differentiation. The balance between Th17 cells and RORγt+ Tregs, that is, the tone of the local type 3 immune response, is regulated by the vitamin A metabolite retinoic acid produced by the host. Furthermore, Th17 cells and RORγt+ Tregs regulate intestinal type 2 immune responses, explaining how bacteria block allergic reactions. Here, we review the cellular and molecular mechanisms involved in the differentiation, regulation and function of RORγt+ (type 3) Tregs, and discuss the multiple equilibria that exist between effector T cells and Tregs, as well as between different types of immune responses, which are necessary to maintain homeostasis and health.

Forkhead-Box-P3 Gene Transfer in Human CD4+ T Conventional Cells for the Generation of Stable and Efficient Regulatory T Cells, Suitable for Immune Modulatory Therapy.

The development of novel approaches to control immune responses to self- and allogenic tissues/organs represents an ambitious goal for the management of autoimmune diseases and in transplantation. Regulatory T cells (Tregs) are recognized as key players in the maintenance of peripheral tolerance in physiological and pathological conditions, and Treg-based cell therapies to restore tolerance in T cell-mediated disorders have been designed. However, several hurdles, including insufficient number of Tregs, their stability, and their antigen specificity, have challenged Tregs clinical applicability. In the past decade, the ability to engineer T cells has proven a powerful tool to redirect specificity and function of different cell types for specific therapeutic purposes. By using lentivirus-mediated gene transfer of the thymic-derived Treg transcription factor forkhead-box-P3 (FOXP3) in conventional CD4+ T cells, we converted effector T cells into Treg-like cells, endowed with potent in vitro and in vivo suppressive activity. The resulting CD4FOXP3 T-cell population displays stable phenotype and suppressive function. We showed that this strategy restores Treg function in T lymphocytes from patients carrying mutations in FOXP3 [immune-dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX)], in whom CD4FOXP3 T cell could be used as therapeutics to control autoimmunity. Here, we will discuss the potential advantages of using CD4FOXP3 T cells for in vivo application in inflammatory diseases, where tissue inflammation may undermine the function of natural Tregs. These findings pave the way for the use of engineered Tregs not only in IPEX syndrome but also in autoimmune disorders of different origin and in the context of stem cell and organ transplantation.

Cyclic AMP-Responsive Element-Binding Protein (CREB) is Critical in Autoimmunity by Promoting Th17 but Inhibiting Treg Cell Differentiation

The molecular mechanisms that govern differential T cell development into pro-inflammatory Th17 vs. regulatory T (Treg) cells remain unclear. Here, we show that selective deletion of CREB in T cells or Th17 cells impaired Th17 cell differentiation in vitro and in vivo, and led to resistance to autoimmune diseases. Mechanistically, CREB, activated by CD3-PKC-ϴ signaling, plays a key role in regulating Th17 cell differentiation, at least in part through directly binding to the Il17-Il17f gene locus. Unexpectedly, although dispensable for FOXP3 expression and for the homeostasis and suppressive function of thymus-derived Treg cells, CREB negatively regulates the survival of TGF-β-induced Treg cells, and deletion of CREB resulted in increased FOXP3+ Treg cells in the intestine and protection in a colitis model. Thus, CREB is critical in autoimmune diseases by promoting Th17 cell and inhibiting de novo Treg cell generation.

A new approach to the role of IL-7 and TGF-ß in the in vitro generation of thymus-derived CD4+CD25+Foxp3+ regulatory T cells

Thymus-derived regulatory T cells of CD4+CD25+Foxp3+ phenotype develop as a functional, mature population playing an essential role in self-tolerance and immune homeostasis, and exhibiting therapeutic potential to inhibit adverse immune response. Despite intensive research on thymus-derived Tregs, the knowledge about agents involved in their generation, survival, proliferation, and biological functions is still insufficient. In this research we have focused on the role of selected cytokines in previously developed in vitro model based on the application of anti-CD3 monoclonal antibodies. We have demonstrated an essential role of IL-7 and TGF-β in the generation of thymus-derived Tregs in the co-culture of thymocytes and JAWS II cells. In addition, in vitro generated Tregs exhibited their suppressive function similarly to Tregs sorted from freshly isolated thymus.

B Cell Requirement for Robust Regulatory T Cell Responses to Friend Retrovirus Infection.

ABSTRACTRegulatory T cells (Tregs) are immunosuppressive cells of the immune system that control autoimmune reactivity. Tregs also respond during immune reactions to infectious agents in order to limit immunopathological damage from potent effectors such as CD8+ cytolytic T lymphocytes. We have used the Friend virus (FV) model of retroviral infection in mice to investigate how viral infections induce Tregs. During acute FV infection, there is significant activation and expansion of thymus-derived (natural) Tregs that suppress virus-specific CD8+ T cell responses. Unlike conventional T cells, the responding Tregs are not virus specific, so the mechanisms that induce their expansion are of great interest. We now show that B cells provide essential signals for Treg expansion during FV infection. Treg responses are greatly diminished in B cell-deficient mice but can be restored by adoptive transfers of B cells at the time of infection. The feeble Treg responses in B cell-deficient mice are associated with enhanced virus-specific CD8+ T cell responses and accelerated virus control during the first 2 weeks of infection. In vitro experiments demonstrated that B cells promote Treg activation and proliferation through a glucocorticoid-induced receptor superfamily member 18 (GITR) ligand-dependent mechanism. Thus, B cells play paradoxically opposing roles during FV infection. They provide proliferative signals to immunsosuppressive Tregs, which slows early virus control, and they also produce virus-specific antibodies, which are essential for long-term virus control.