Cutting edge: in the absence of TGF-β signaling in T cells, fewer CD103+ regulatory T cells develop, but exuberant IFN-γ production renders mice more susceptible to helminth infection

Multiple factors control susceptibility of C57BL/6 mice to infection with the helminth Heligmosomoides polygyrus, including TGF-β signaling, which inhibits immunity in vivo. However, mice expressing a T cell-specific dominant-negative TGF-β receptor II (TGF-βRII DN) show dampened Th2 immunity and diminished resistance to infection. Interestingly, H. polygyrus-infected TGF-βRII DN mice show greater frequencies of CD4(+)Foxp3(+)Helios(+) Tregs than infected wild-type mice, but levels of CD103 are greatly reduced on both these cells and on the CD4(+)Foxp3(+)Helios(-) population. Although Th9 and Th17 levels are comparable between infected TGF-βRII DN and wild-type mice, the former develop exaggerated CD4(+) and CD8(+) T cell IFN-γ responses. Increased susceptibility conferred by TGF-βRII DN expression was lost in IFN-γ-deficient mice, although they remained unable to completely clear infection. Hence, overexpression of IFN-γ negatively modulates immunity, and the presence of Helios(+) Tregs may maintain susceptibility on the C57BL/6 background.     

Regulatory T cell: a protection for tumour cells

Characterized by immunosuppression regulatory T cells (Tregs) play a key role in maintaining immune tolerance. A growing number of tumours have been found with Tregs accumulating in microenvironment and patients with high density of Tregs in tumour stroma get a worse prognosis, which suggests that Tregs may inhibit anti-tumour immunity in stroma, resulting in a poor prognosis. In this paper, we demonstrate the accumulation of Tregs in tumour stroma and the possible suppressive mechanisms. We also state the immunotherapy that has being used in animal and clinical trials.     

Tim-3 pathway controls regulatory and effector T cell balance during hepatitis C virus infection

Hepatitis C virus (HCV) is remarkable at disrupting human immunity to establish chronic infection. Upregulation of inhibitory signaling pathways (such as T cell Ig and mucin domain protein-3 [Tim-3]) and accumulation of regulatory T cells (Tregs) play pivotal roles in suppressing antiviral effector T cell (Teff) responses that are essential for viral clearance. Although the Tim-3 pathway has been shown to negatively regulate Teffs, its role in regulating Foxp3(+) Tregs is poorly explored. In this study, we investigated whether and how the Tim-3 pathway alters Foxp3(+) Treg development and function in patients with chronic HCV infection. We found that Tim-3 was upregulated, not only on IL-2-producing CD4(+)CD25(+)Foxp3(-) Teffs, but also on CD4(+)CD25(+)Foxp3(+) Tregs, which accumulate in the peripheral blood of chronically HCV-infected individuals when compared with healthy subjects. Tim-3 expression on Foxp3(+) Tregs positively correlated with expression of the proliferation marker Ki67 on Tregs, but it was inversely associated with proliferation of IL-2-producing Teffs. Moreover, Foxp3(+) Tregs were found to be more resistant to, and Foxp3(-) Teffs more sensitive to, TCR activation-induced cell apoptosis, which was reversible by blocking Tim-3 signaling. Consistent with its role in T cell proliferation and apoptosis, blockade of Tim-3 on CD4(+)CD25(+) T cells promoted expansion of Teffs more substantially than Tregs through improving STAT-5 signaling, thus correcting the imbalance of Foxp3(+) Tregs/Foxp3(-) Teffs that was induced by HCV infection. Taken together, the Tim-3 pathway appears to control Treg and Teff balance through altering cell proliferation and apoptosis during HCV infection.     

Porphyromonas gingivalis Infection Reduces Regulatory T Cells in Infected Atherosclerosis Patients

Increasing evidence has shown periodontal pathogen Porphyromonas gingivalis (P.gingivalis) infection contributes to atherosclerosis (AS) progression. P.gingivalis fimbriae act as an important virulence factor in AS. Regulatory T cells (Tregs) may play a crucial role in autoimmune response during this process. However, whether P.gingivalis infection is associated with Tregs dysregulation during AS is still unknown and the prevalence of different P.gingivalis FimA genotypes during this process is unclear. Here we analyzed the distribution of Tregs and in P.gingivalis-infected atherosclerotic patients to reveal the relationship between P.gingivalis infection and Tregs reduction/dysfunction and to elucidate their role in periodontitis-AS interaction. FimA genotype was also examined to determine the prevalence of fimbriae. Our results showed that P.gingivalis infection reduced Tregs in atherosclerotic patients compared with non-atherosclerotic patients and health controls. Concentration of TGF-β1, which plays an important role in the development of Tregs, also decreased in P.gingivalis infected patients. Furthermore, type II FimA seems to show higher prevalence than the other five detected types. The population of Tregs further decreased in patients with type II FimA compared with the other types. P.gingivlias FimA genotype II was the dominant type associated with decreased Treg population. These results indicate that P.gingivalis infection may be associated with Tregs dysregulation in AS; type II FimA may be a predominant genotype in this process.     

Malaria parasites require TLR9 signaling for immune evasion by activating regulatory T cells

Malaria is still a life-threatening infectious disease that continues to produce 2 million deaths annually. Malaria parasites have acquired immune escape mechanisms and prevent the development of sterile immunity. Regulatory T cells (Tregs) have been reported to contribute to immune evasion during malaria in mice and humans, suggesting that activating Tregs is one of the mechanisms by which malaria parasites subvert host immune systems. However, little is known about how these parasites activate Tregs. We herein show that TLR9 signaling to dendritic cells (DCs) is crucial for activation of Tregs. Infection of mice with the rodent malaria parasite Plasmodium yoelii activates Tregs, leading to enhancement of their suppressive function. In vitro activation of Tregs requires the interaction of DCs with parasites in a TLR9-dependent manner. Furthermore, TLR9(-/-) mice are partially resistant to lethal infection, and this is associated with impaired activation of Tregs and subsequent development of effector T cells. Thus, malaria parasites require TLR9 to activate Tregs for immune escape.     

IL-2 immunotoxin therapy modulates tumor-associated regulatory T cells and leads to lasting immune-mediated rejection of breast cancers in neu-transgenic mice

Studies in cancer patients have suggested that breast tumors recruit regulatory T cells (Tregs) into the tumor microenvironment. The extent to which local Tregs suppress antitumor immunity in breast cancer is unknown. We questioned whether inhibiting systemic Tregs with an IL-2 immunotoxin in a model of neu-mediated breast cancer, the neu-transgenic mouse, could impact disease progression and survival. As in human breast cancer, cancers that develop in these mice attract Tregs into the tumor microenvironment to levels of approximately 10-25% of the total CD4+ T cells. To examine the role of Tregs in blocking immune-mediated rejection of tumor, we depleted CD4+CD25+ T cells with an IL-2 immunotoxin. The treatment depleted Tregs without concomitant lymphopenia and markedly inhibited tumor growth. Depletion of Tregs resulted in a persistent antitumor response that was maintained over a month after the last treatment. The clinical response was immune-mediated because adoptive transfer of Tregs led to a complete abrogation of the therapeutic effects of immunotoxin treatment. Further, Treg down-modulation was accompanied by increased Ag-specific immunity against the neu protein, a self Ag. These results suggest that Tregs play a major role in preventing an effective endogenous immune response against breast cancer and that depletion of Tregs, without any additional immunotherapy, may mediate a significant antitumor response.     

Characterization of the Treg Response in the Hepatitis B Virus Hydrodynamic Injection Mouse Model

Regulatory T cells (Tregs) play an important role in counter-regulating effector T cell responses in many infectious diseases. However, they can also contribute to the development of T cell dysfunction and pathogen persistence in chronic infections. Tregs have been reported to suppress virus-specific T cell responses in hepatitis B virus (HBV) infection of human patients as well as in HBV animal models. However, the phenotype and expansion of Tregs has so far only been investigated in other infections, but not in HBV. We therefore performed hydrodynamic injections of HBV plasmids into mice and analyzed the Treg response in the spleen and liver. Absolute Treg numbers significantly increased in the liver but not the spleen after HBV injection. The cells were natural Tregs that surprisingly did not show any activation or proliferation in response to the infection. However, they were able to suppress effector T cell responses, as selective depletion of Tregs significantly increased HBV-specific CD8+ T cell responses and accelerated viral antigen clearance. The data implies that natural Tregs infiltrate the liver in HBV infection without further activation or expansion but are still able to interfere with T cell mediated viral clearance.     

Cross-talk between virus and host innate immunity in pediatric HIV-1 infection and disease progression

Variability in the susceptibility to HIV-1 infection and disease progression depends on both virus and host determinants. Some exposed individuals remain HIV-1-uninfected and HIV-1-infected subjects develop disease at varying intervals with a small percentage remaining long-term non-progressors. As innate immunity is the earliest response to microbial entry and injury, host factors that impact innate immunity may play a role in viral infectivity and pathogenesis. In the pediatric population the interactions between the virus and the host may be of particular relevance due to the still developing adaptive immune system. Data indicate that genetic variants of defensins and Toll-Like Receptors (TLRs), key elements of innate immunity, play a role in mother-to-child transmission (MTCT) of HIV-1, and in the outcome of pediatric HIV-1 disease. Although the mechanisms by which these genetic variants influence HIV-1 interactions with the host are still largely unknown, defensins and TLRs, along with their link with regulatory T cells (Tregs), may play a critical role in the onset and persistence of immune activation, a hallmark of HIV-1 disease.     

CD4 regulatory T cells in human cancer pathogenesis

Over the past decade, there has been an accelerated understanding of immune regulatory mechanisms. Peripheral immune regulation is linked to a collection of specialized regulatory cells of the CD4⁺ T cell lineage (i.e., CD4⁺ Tregs). This collection consists of Tregs that are either thymically derived (i.e., natural) or peripherally induced. Tregs are important for controlling potentially autoreactive immune effectors and immunity to foreign organisms and molecules. Their importance in maintaining immune homeostasis and the overall health of an organism is clear. However, Tregs may also be involved in the pathogenesis of malignancies as now compelling evidence shows that tumors induce or recruit CD4⁺ Tregs to block immune priming and antitumor effectors. Efforts are underway to develop approaches that specifically inhibit the function of tumor-associated Tregs which could lead to an increased capability of the body’s immune system to respond to tumors but without off-target immune-related pathologies (i.e., autoimmune disease). In this review, the biology of human CD4⁺ Tregs is discussed along with their involvement in malignancies and emerging strategies to block their function.     

Expanded Regulatory T Cells in Chronically Friend Retrovirus-Infected Mice Suppress Immunity to a Murine Cytomegalovirus Superinfection

It is still unclear whether expanded and activated regulatory T cells (Tregs) in chronic viral infections can influence primary immune responses against superinfections with unrelated viruses. Expanded Tregs found in the spleens of chronically Friend virus (FV)-infected mice decreased murine cytomegalovirus (mCMV)-specific CD8⁺ T cell responses during acute mCMV superinfection. This suppression of mCMV-specific T cell immunity was found only in organs with FV-induced Treg expansion. Surprisingly, acute mCMV infection itself did not expand or activate Tregs.     

Rapid Rebound of the Treg Compartment in DEREG Mice Limits the Impact of Treg Depletion on Mycobacterial Burden,but Prevents Autoimmunity

The development of an effective vaccine against tuberculosis (Tb) represents one of the major medical challenges of this century. Mycobacterium bovis Bacille Calmette-Guerin (BCG), the only vaccine available at present, is mostly effective at preventing disseminated Tb in children, but shows variable protection against pulmonary Tb, the most common form in adults. The reasons for this poor efficacy are not completely understood, but there is evidence that T regulatory cells (Tregs) might be involved. Similarly, Tregs have been associated with the immunosuppression observed in patients infected with Tb and are therefore believed to play a role in pathogen persistence. Thus, Treg depletion has been postulated as a novel strategy to potentiate M. bovis BCG vaccination on one side, while on the other, employed as a therapeutic approach during chronic Tb infection. Yet since Tregs are critically involved in controlling autoimmune inflammation, elimination of Tregs may therefore also incur the danger of an excessive inflammatory immune response. Thus, understanding the dynamics and function of Tregs during mycobacterial infection is crucial to evaluate the potential of Treg depletion as a medical option. To address this, we depleted Tregs after infection with M. bovis BCG or Mycobacterium tuberculosis (Mtb) using DEREG mice, which express the diphtheria toxin (DT) receptor under the control of the FoxP3 locus, thereby allowing the selective depletion of FoxP3⁺ Tregs. Our results show that after depletion, the Treg niche is rapidly refilled by a population of DT-insensitive Tregs (diTregs) and bacterial load remains unchanged. On the contrary, impaired rebound of Tregs in DEREG × FoxP3^GFP mice improves pathogen burden, but is accompanied by detrimental autoimmune inflammation. Therefore, our study provides the proof-of-principle that, although a high degree of Treg depletion may contribute to the control of mycobacterial infection, it carries the risk of autoimmunity.     

The 15N and 46R Residues of Highly Pathogenic Porcine Reproductive and Respiratory Syndrome Virus Nucleocapsid Protein Enhance Regulatory T Lymphocytes Proliferation

Porcine reproductive and respiratory syndrome virus (PRRSV) negatively modulates host immune responses, resulting in persistent infection and immunosuppression. PRRSV infection increases the number of PRRSV-specific regulatory T lymphocytes (Tregs) in infected pigs. However, the target antigens for Tregs proliferation in PRRSV infection have not been fully understood. In this study, we demonstrated that the highly pathogenic PRRSV (HP-PRRSV) induced more CD4⁺CD25⁺Foxp3⁺ Tregs than classical PRRSV (C-PRRSV) strain. Of the recombinant GP5, M and N proteins of HP-PRRSV expressed in baculovirus expression systems, only N protein induced Tregs proliferation. The Tregs assays showed that three amino-acid regions, 15–21, 42–48 and 88–94, in N protein played an important role in induction of Tregs proliferation with synthetic peptides covering the whole length of N protein. By using reverse genetic methods, it was firstly found that the 15N and 46R residues in PRRSV N protein were critical for induction of Tregs proliferation. The phenotype of induced Tregs closely resembled that of transforming-growth-factor-β-secreting T helper 3 Tregs in swine. These data should be useful for understanding the mechanism of immunity to PRRSV and development of infection control strategies in the future.     

Reduction of Splenic Immunosuppressive Cells and Enhancement of Anti-Tumor Immunity by Synergy of Fish Oil and Selenium Yeast

Growing evidence has shown that regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs) abnormally increase in cancer cachectic patients. Suppressions of Tregs and MDSCs may enhance anti-tumor immunity for cancer patients. Fish oil and selenium have been known to have many biological activities such as anti-inflammation and anti-oxidation. Whether fish oil and/or selenium have an additional effect on population of immunosuppressive cells in tumor-bearing hosts remained elusive and controversial. To gain insights into their roles on anti-tumor immunity, we studied the fish oil- and/or selenium-mediated tumor suppression and immunity on lung carcinoma, whereof cachexia develops. Advancement of cachexia in a murine lung cancer model manifested with such indicative symptoms as weight loss, chronic inflammation and disturbed immune functionality. The elevation of Tregs and MDSCs in spleens of tumor-bearing mice was positively correlated with tumor burdens. Consumption of either fish oil or selenium had little or no effect on the levels of Tregs and MDSCs. However, consumption of both fish oil and selenium together presented a synergistic effect-The population of Tregs and MDSCs decreased as opposed to increase of anti-tumor immunity when both fish oil and selenium were supplemented simultaneously, whereby losses of body weight and muscle/fat mass were alleviated significantly.     

Physiologic control of the functional status of Foxp3+ regulatory T cells

Foxp3-lineage CD4 regulatory T cells (Tregs) were named for their ability to maintain self tolerance and suppress T cell immunity. However, resting Tregs from noninflamed tissues exhibit little suppressor activity, and must be stimulated to acquire such function. Conversely, under certain inflammatory conditions, Tregs may undergo rapid reprogramming to acquire helper/effector functions. In this Brief Review, we describe recent progress in elucidating physiologic processes that control the functional status of Foxp3-lineage Tregs. Emerging evidence suggests the surprising possibility that reprogrammed Tregs can be an indispensable source of helper activity in some physiologic settings, such as priming CD8(+) T cell responses. This suggests a novel paradigm in which Foxp3(+) Tregs intrinsically possess bifunctional potential, acting as a preformed pool of first-responder cells at sites of local inflammation that can either provide classical regulatory/suppressor activity, or rapidly reprogram to supply helper/effector activity, contingent on signals that manifest in local physiologic settings.     

Identification and in vitro expansion of functional antigen-specific CD25+ FoxP3+ regulatory T cells in hepatitis C virus infection

CD4(+)CD25(+) regulatory T cells (CD25(+) Tregs) play a key role in immune regulation. Since hepatitis C virus (HCV) persists with increased circulating CD4(+)CD25(+) T cells and virus-specific effector T-cell dysfunction, we asked if CD4(+)CD25(+) T cells in HCV-infected individuals are similar to natural Tregs in uninfected individuals and if they include HCV-specific Tregs using the specific Treg marker FoxP3 at the single-cell level. We report that HCV-infected patients display increased circulating FoxP3(+) Tregs that are phenotypically and functionally indistinguishable from FoxP3(+) Tregs in uninfected subjects. Furthermore, HCV-specific FoxP3(+) Tregs were detected in HCV-seropositive persons with antigen-specific expansion, major histocompatibility complex class II/peptide tetramer binding affinity, and preferential suppression of HCV-specific CD8 T cells. Transforming growth factor beta contributed to antigen-specific Treg expansion in vitro, suggesting that it may contribute to antigen-specific Treg expansion in vivo. Interestingly, FoxP3 expression was also detected in influenza virus-specific CD4 T cells. In conclusion, functionally active and virus-specific FoxP3(+) Tregs are induced in HCV infection, thus providing targeted immune regulation in vivo. Detection of FoxP3 expression in non-HCV-specific CD4 T cells suggests that immune regulation through antigen-specific Treg induction extends beyond HCV.     

Early skin immunological disturbance after Plasmodium-infected mosquito bites

Although the role of regulatory T cells (Tregs) during malaria infection has been studied extensively, such studies have focused exclusively on the role of Treg during the blood stage of infection; little is known about the detailed mechanisms of Tregs and sporozoite deposition in the dermis by mosquito bites. In this paper we show that sporozoites introduced into the skin by mosquito bites increase the mobility of skin Tregs and dendritic cells (DCs). We also show differences in MHC class II and/or CD86 expression on skin-resident dendritic cell subtypes and macrophages. From the observed decrease of the number of APCs into draining lymph nodes, suppression of CD28 expression in conventional CD4 T cells, and a low homeostatic proliferation of skin-migrated CD4 T found in nude mice indicate that Tregs may play a fundamental role during the initial phase of malaria parasite inoculation into the mammalian host.     

The phenotype and activation status of regulatory T cells during Friend retrovirus infection

The suppressive capacity of regulatory T cells (Tregs) has been extensively studied and is well established for many diseases. The expansion, accumulation, and activation of Tregs in viral infections are of major interest in order to find ways to alter Treg functions for therapeutic benefit. Tregs are able to dampen effector T cell responses to viral infections and thereby contribute to the establishment of a chronic infection. In the Friend retrovirus (FV) mouse model, Tregs are known to expand in all infected organs. To better understand the characteristics of these Treg populations, their phenotype was analyzed in detail. During acute FV-infection, Tregs became activated in the spleen and bone marrow, as indicated by various T cell activation markers, such as CD43 and CD103. Interestingly, Tregs in the bone marrow, which contains the highest viral loads during acute infection, displayed greater levels of activation than Tregs from the spleen. Treg expansion was driven by proliferation but no FV-specific Tregs could be detected. Activated Tregs in FV-infection did not produce Granzyme B (GzmB) or tumor necrosis factor α (TNFα), which are thought to be a potential mechanism for their suppressive activity. Furthermore, Tregs expressed inhibitory markers, such as TIM3, PD-1 and PD-L1. Blocking TIM3 and PD-L1 with antibodies during chronic FV-infection increased the numbers of activated Tregs. These data may have important implications for the understanding of Treg functions during chronic viral infections.     

B7-deficient autoreactive T cells are highly susceptible to suppression by CD4(+)CD25(+) regulatory T cells

CD4(+)CD25(+) regulatory T cells (Tregs) suppress immunity to infections and tumors as well as autoimmunity and graft-vs-host disease. Since Tregs constitutively express CTLA-4 and activated T cells express B7-1 and B7-2, it has been suggested that the interaction between CTLA-4 on Tregs and B7-1/2 on the effector T cells may be required for immune suppression. In this study, we report that autopathogenic T cells from B7-deficient mice cause multiorgan inflammation when adoptively transferred into syngeneic RAG-1-deficient hosts. More importantly, this inflammation is suppressed by adoptive transfer of purified wild-type (WT) CD4(+)CD25(+) T cells. WT Tregs also inhibited lymphoproliferation and acquisition of activation markers by the B7-deficient T cells. An in vitro suppressor assay revealed that WT and B7-deficient T cells are equally susceptible to WT Treg regulation. These results demonstrate that B7-deficient T cells are highly susceptible to immune suppression by WT Tregs and refute the hypothesis that B7-CTLA-4 interaction between effector T cells and Tregs plays an essential role in Treg function.     

Plasmodium falciparum soluble extracts potentiate the suppressive function of polyclonal T regulatory cells through activation of TGFβ-mediated signals

Increased numbers of T regulatory cells (Tregs), key mediators of immune homeostasis, were reported in human and murine malaria and it is current opinion that these cells play a role in balancing protective immunity and pathogenesis during infection. However, the mechanisms governing their expansion during malaria infection are not completely defined. In this article we show that soluble extracts of Plasmodium falciparum (PfSEs), but not equivalent preparation of uninfected erythrocytes, induce the differentiation of polyclonally activated CD4(+) cells in Tregs endowed with strong suppressive activity. PfSEs activate latent TGFβ bound on the membrane of Treg cells, thus allowing the cytokine interaction with TGFβ receptor, and inducing Foxp3 gene expression and TGFβ production. The activation of membrane-bound latent TGFβ by PfSEs is significantly reduced by a broad-spectrum metalloproteinases inhibitor with Zn(++) -chelating activity, and completely inhibited by the combined action of such inhibitor and antibodies to a P. falciparum thrombospondin-related adhesive protein (PfTRAP). We conclude that Pf-Zn(++) -dependent proteinases and, to a lesser extent, PfTRAP molecules are involved in the activation of latent TGFβ bound on the membrane of activated Treg cells and suggest that, in malaria infection, this mechanism could contribute to the expansion of Tregs with different antigen specificity.