CD25(+)CD4(+) regulatory T cells from the peripheral blood of asymptomatic HIV-infected individuals regulate CD4(+) and CD8(+) HIV-specific T cell immune responses in vitro and are associated with favorable clinical markers of disease status

Enhancement of human antigen-specific memory T-cell responses by interleukin-7 may improve accuracy in diagnosing tuberculosis

Children and immunocompromised adults are at an increased risk of tuberculosis (TB), but diagnosis is more challenging. Recently developed gamma interferon (IFN-gamma) release assays provide increased sensitivity and specificity for diagnosis of latent TB, but their use is not FDA approved in immunocompromised or pediatric populations. Both populations have reduced numbers of T cells, which are major producers of IFN-gamma. Interleukin 7 (IL-7), a survival cytokine, stabilizes IFN-gamma message and increases protein production. IL-7 was added to antigen-stimulated lymphocytes to improve IFN-gamma responses as measured by enzyme-linked immunosorbent assay (ELISA) and enzyme-linked immunospot (ELISPOT) assay. Antigens used were tetanus toxoid (n = 10), p24 (from human immunodeficiency virus [HIV], n = 9), and TB peptides (n = 15). Keyhole limpet hemocyanin was used as a negative control, and phytohemagglutinin was the positive control. IL-7 improved antigen-specific responses to all antigens tested including tetanus toxoid, HIV type 1 p24, and TB peptides (ESAT-6 and CFP-10) with up to a 14-fold increase (mean = 3.8), as measured by ELISA. Increased IFN-gamma responses from controls, HIV-positive patients, and TB patients were statistically significant, with P values of <0.05, 0.01, and 0.05, respectively. ELISPOT assay results confirmed ELISA findings (P values of <0.01, 0.02, and 0.03, respectively), with a strong correlation between the two tests (R(2) = 0.82 to 0.99). Based on average background levels, IL-7 increased detection of IFN-gamma by 39% compared to the level with antigen alone. Increased production of IFN-gamma induced by IL-7 improves sensitivity of ELISA and ELISPOT assays for all antigens tested. Further enhancement of IFN-gamma-based assays might improve TB diagnosis in those populations at highest risk for TB.

CD4+CD25+ regulatory T cells are infected and activated during acute FIV infection

HIV-induced AIDS may be mediated by the activation of immunosuppressive CD4+CD25+ T regulatory cells (Treg cells). Treg cells have been shown to regulate CD4+ and CD8+ immune responses to HIV and FIV antigens in vitro. We tested the hypothesis that Treg cells become infected and activated during the acute infection with FIV leading to the suppression of CD4+ T helper cell responses. Cats were experimentally infected with FIV-NCSU1 and blood and lymph node cells were collected at weekly intervals following inoculation. Real-time RT-PCR was used to determine plasma viremia and the relative expression of FIV, FoxP3, TGF-beta, and GAPDH mRNA copies in CD4+CD25+ and CD4+CD25- T cell subsets. Flow cytometry was used to assess the absolute numbers of each cell type and the expression of surface TGF-beta and intracellular FoxP3 in CD4+CD25+ and CD4+CD25- T cells at each time-point. Treg suppression of IL-2 production in CD4+ T helper cells was assessed by ELISPOT assays. Our results showed that peak viremia occurred at 2 weeks post infection and correlated with maximal infectivity in CD4+CD25+ T cell populations. FIV-gag-mRNA levels were higher in CD4+CD25+ T cells than CD4+CD25- T cells throughout the acute phase of infection. Induction of FoxP3 and TGF-beta indicated activation of Treg cells during the acute stage infection, which was confirmed by Treg cell suppression of IL-2 production by CD4+ Th cells in an ELISPOT assay. Our findings support the hypothesis that early activation of Treg immunosuppressor function may limit an effective anti-FIV response, contributing to the establishment of chronic infection and the immunodeficiency caused by this virus.

Circumvention of regulatory CD4(+) T cell activity during cross-priming strongly enhances T cell-mediated immunity

Immunization with purified antigens is a safe and practical vaccination strategy but is generally unable to induce sustained CD8(+) T cell-mediated protection against intracellular pathogens. Most efforts to improve the CD8(+) T cell immunogenicity of these vaccines have focused on co-administration of adjuvant to support cross-presentation and dendritic cell maturation. In addition, it has been shown that CD4(+) T cell help during the priming phase contributes to the generation of protective CD8(+) memory T cells. In this report we demonstrate that the depletion of CD4(+) T cells paradoxically enhances long-lasting CD8-mediated protective immunity upon protein vaccination. Functional and genetic in vivo inactivation experiments attribute this enhancement primarily to MHC class II-restricted CD4(+) regulatory T cells (Treg), which appear to physiologically suppress the differentiation process towards long-living effector memory T cells. Since, in functional terms, this suppression by Treg largely exceeds the positive effects of conventional CD4(+) T cell help, even the absence of all CD4(+) T cells or lack of MHC class II-mediated interactions on priming dendritic cells result in enhanced CD8(+) T cell immunogenicity. These findings have important implications for the improvement of vaccines against intracellular pathogens or tumors, especially in patients with highly active Treg.

Suppression of HIV-specific and allogeneic T cell activation by human regulatory T cells is dependent on the strength of signals

Regulatory T cells (Tregs) suppress immune responses against both self and non-self antigens. Tregs require activation through the T cell receptor (TCR) and IL-2 to exert their suppressive functions. However, how strength of TCR signals modulate the potency of Treg-mediated suppression of antigen-specific T cell activation remain unclear. We found that both strength of TCR signals and ratios of Tregs to target cells, either through superantigen, allogeneic antigens or HIV-specific peptides, modified the suppressive ability of Tregs. While human Tregs were able to mediate suppression in the presence of only autologous antigen-presenting cells, this was much less efficient as compared to when Tregs were activated by allogeneic dendritic cells. In another physiologically relevant system, we show that the strength of peptide stimulation, high frequency of responder CD8+ T cells or presence of high IL-2 can override the suppression of HIV-specific CD8+ T cells by Tregs. These findings suggest that ratios and TCR activation of human Tregs, are important parameters to overcome robust immune responses to pathogens or allogeneic antigens. Modulating the strength of T cell signals and selective enhancement or depletion of antigen-specific Tregs thus may have implications for designing potent vaccines and regulating immune responses during allogeneic transplantation and chronic infections.

Presence of suppressor HIV-specific CD8+ T cells is associated with increased PD-1 expression on effector CD8+ T cells

Mechanisms leading to the observed immune dysregulation in HIV-1 infection are not well understood. HIV-specific IL-10-positive CD8(+) T cells are increased in advanced HIV disease. We have previously reported that Gag-specific IL-10-positive CD8(+) T cells suppressed cytolysis. In this study we describe the suppressive effect of Nef-specific IL-10-positive CD8(+) T cells. Interestingly, simultaneous removal of both Gag- and Nef-specific IL-10-positive CD8(+) T cells led to higher HIV-specific cytolysis compared with the removal of Nef-specific IL-10-positive CD8(+) T cells alone. We also examined the level of programmed cell death-1 (PD-1) as a measure of immune dysfunction in association with IL-10-positive suppressor CD8(+) T cells. The level of PD-1 expression on CD107-positive effector CD8(+) T cells was significantly increased when IL-10-positive suppressor CD8(+) T cells were present (p < 0.05). Our results suggest that IL-10-positive suppressor CD8(+) T cells contribute to the immune dysfunction observed in advanced HIV infection and that the concomitant presence of multiple IL-10-positive CD8(+) T cell populations may have an additive suppressive effect.

Immune activation driven by CTLA-4 blockade augments viral replication at mucosal sites in simian immunodeficiency virus infection

The importance of chronic immune activation in progression to AIDS has been inferred by correlative studies in HIV-infected individuals and in nonhuman primate models of SIV infection. Using the SIV(mac251) macaque model, we directly address the impact of immune activation by inhibiting CTLA-4, an immunoregulatory molecule expressed on activated T cells and a subset of regulatory T cells. We found that CTLA-4 blockade significantly increased T cell activation and viral replication in primary SIV(mac251) infection, particularly at mucosal sites, and increased IDO expression and activity. Accordingly, protracted treatment with anti-CTLA-4 Ab of macaques chronically infected with SIV(mac251) decreased responsiveness to antiretroviral therapy and abrogated the ability of therapeutic T cell vaccines to decrease viral set point. These data provide the first direct evidence that immune activation drives viral replication, and suggest caution in the use of therapeutic approaches for HIV infection in vivo that increase CD4(+) T cell proliferation.

CD127 and CD25 expression defines CD4+ T cell subsets that are differentially depleted during HIV infection

Decreased CD4(+) T cell counts are the best marker of disease progression during HIV infection. However, CD4(+) T cells are heterogeneous in phenotype and function, and it is unknown how preferential depletion of specific CD4(+) T cell subsets influences disease severity. CD4(+) T cells can be classified into three subsets by the expression of receptors for two T cell-tropic cytokines, IL-2 (CD25) and IL-7 (CD127). The CD127(+)CD25(low/-) subset includes IL-2-producing naive and central memory T cells; the CD127(-)CD25(-) subset includes mainly effector T cells expressing perforin and IFN-gamma; and the CD127(low)CD25(high) subset includes FoxP3-expressing regulatory T cells. Herein we investigated how the proportions of these T cell subsets are changed during HIV infection. When compared with healthy controls, HIV-infected patients show a relative increase in CD4(+)CD127(-)CD25(-) T cells that is related to an absolute decline of CD4(+)CD127(+)CD25(low/-) T cells. Interestingly, this expansion of CD4(+)CD127(-) T cells was not observed in naturally SIV-infected sooty mangabeys. The relative expansion of CD4(+)CD127(-)CD25(-) T cells correlated directly with the levels of total CD4(+) T cell depletion and immune activation. CD4(+)CD127(-)CD25(-) T cells were not selectively resistant to HIV infection as levels of cell-associated virus were similar in all non-naive CD4(+) T cell subsets. These data indicate that, during HIV infection, specific changes in the fraction of CD4(+) T cells expressing CD25 and/or CD127 are associated with disease progression. Further studies will determine whether monitoring the three subsets of CD4(+) T cells defined based on the expression of CD25 and CD127 should be used in the clinical management of HIV-infected individuals.

Altered balance between Th17 and Th1 cells at mucosal sites predicts AIDS progression in simian immunodeficiency virus-infected macaques

Loss of CD4(+) T cells in the gut is necessary but not sufficient to cause AIDS in animal models, raising the possibility that a differential loss of CD4(+) T-cell subtypes may be important. We found that CD4(+) T cells that produce interleukin (IL)-17, a recently identified lineage of effector CD4(+) T-helper cells, are infected by SIV(mac251)in vitro and in vivo, and are found at lower frequency at mucosal and systemic sites within a few weeks from infection. In highly viremic animals, Th1 cells predominates over Th17 T cells and the frequency of Th17 cells at mucosal sites is negatively correlated with plasma virus level. Because Th17 cells play a central role in innate and adaptive immune response to extracellular bacteria, our finding may explain the chronic enteropathy in human immunodeficiency virus (HIV) infection. Thus, therapeutic approaches that reconstitute an adequate balance between Th1 and Th17 may be beneficial in the treatment of HIV infection.

Preservation of FoxP3+ regulatory T cells in the peripheral blood of human immunodeficiency virus type 1-infected elite suppressors correlates with low CD4+ T-cell activation

Elite suppressors (ES) are untreated human immunodeficiency virus type 1 (HIV-1)-infected individuals who maintain normal CD4(+) T-cell counts and control viremia to levels that are below the limit of detection of current assays. The mechanisms involved in long-term control of viremia have not been fully elucidated. CD4(+) CD25(+) regulatory T cells (Tregs) downmodulate chronic inflammation by suppressing the activation and proliferation of effector lymphocytes. We found that while Tregs were functional in ES and patients on highly active antiretroviral therapy (HAART), ES maintained high levels of Tregs in peripheral blood mononuclear cells whereas patients on HAART had evidence of Treg depletion. We also demonstrated that Tregs can serve as reservoirs for HIV-1 in vivo. These data suggest that both direct infection by HIV-1 and tissue redistribution are possible explanations for declining FoxP3(+) Tregs in progressive HIV-1 infection. Furthermore, the maintenance of Tregs may be one mechanism associated with the nonprogressive nature of HIV-1 infection in ES.

T regulatory cells: aid or hindrance in the clearance of disease?

CD4⁺ CD25⁺ T regulatory cells (Tregs) are classified as a subset of T cells whose role is the suppression and regulation of immune responses to self and non-self. Since their discovery in the early 1970s, the role of CD4⁺ CD25⁺ Tregs in both autoimmune and infectious disease has continued to expand. This review exam-ines the recent advances on the role CD4⁺ CD25⁺ Tregs may be playing in various diseases regarding pro-gression or protection. In addition, advances made in the purification and manipulation of CD4⁺ CD25⁺ Tregs using new cell markers, techniques and antibodies are discussed. Ultimately, an overall understanding of the exact mechanism which CD4⁺ CD25⁺ Tregs implement during disease progression will enhance our ability to manipulate CD4⁺ CD25⁺ Tregs in a clinically beneficial manner.

The role of regulatory T cells in chronic and acute viral infections

Regulatory T cells, a subset of CD4(+) T lymphocytes, play a pivotal role in the maintenance of the balance between the tissue-damaging and protective effects of the immune response. These cells have immunosuppressive function and have been intensely studied in the context of autoimmunity, cancer, allergies, asthma, and infectious diseases. Their role in chronic and persistent viral infections is well appreciated. In acute viral infections, the function of these cells is still unclear. The host and pathogen factors that control the generation and activity of regulatory T cells and the role of these cells in modulating expansion, contraction, and development of immune memory in acute respiratory virus infection need to be further elucidated.

Role of Foxp3-positive regulatory T cells during infection

Surviving an infection requires the generation of an immune response that controls the invading pathogen while limiting collateral damage to self tissues that may result from an exuberant immune response. Various populations of regulatory cells, including Foxp3+ Treg, have been shown to play a central role in the establishment of these controlled immune responses. In this review, I discuss current hypotheses and points of polemic associated with the origin, mode of action and antigen specificity of Foxp3+ Treg during infection.

Regulatory T cells in HIV infection: who's suppressing what?

The role of regulatory T cells (Treg) in HIV pathogenesis is not fully elucidated. Persistent antigens, such as HIV, are believed to promote the expansion and activation of antigen-specific Treg, and several reports have described beneficial and detrimental roles for Treg in HIV pathogenesis. These apparently contradictory observations may arise from imprecision in enumerating Treg and the lack of definition of Treg subsets. New markers allowing more precise identification and purification of Treg for functional studies have been described recently, and these may open avenues for efficient isolation of pure, homogenous populations of human Treg.

Disruption of the gamma c cytokine network in T cells during HIV infection

The common gamma chain (gammac)-sharing cytokines (IL's-2, 4, 7, 9, 15, and 21) play a vital role in the survival, proliferation, differentiation and function of T lymphocytes. As such, disruption of their signaling pathways would be expected to have severe consequences on the integrity of the immune system. Indeed, it appears that the signaling network of these cytokines is both disrupted and exploited by HIV at various stages of infection. IL-2 secretion and signaling downstream of its receptor are impaired in T cells from chronically-infected HIV+ patients. Elevated plasma IL-7 levels and decreased IL-7Ralpha expression in patient T cells results in significantly decreased responsiveness to this critical cytokine. Interestingly, IL-2 and IL-15 are also able to render CD4+ T cells permissive to HIV infection through their influence on the activity of the APOBEC3G deaminase enzyme. Herein, we describe the current state of knowledge on how the gammac cytokine network is affected during HIV infection, with a focus on how this impairs CD4+ and CD8+ T cell function while also benefiting the virus itself. We also address the use of cytokines as adjuncts to highly active antiretroviral therapy to bolster immune reconstitution in infected patients.

Transforming growth factor-beta/transforming growth factor-betaRII signaling may regulate CD4+CD25+ T-regulatory cell homeostasis and suppressor function in feline AIDS lentivirus infection

We have reported that CD4+CD25+ T-regulatory (Treg) cells are fully activated for suppressor function in feline immunodeficiency virus (FIV)-infected cats. Studies have suggested that surface transforming growth factor-beta (TGFbeta; membrane TGFbeta [mTGFbeta]) is a feature of activated CD4+CD25+ Treg cells and may play a role in Treg homeostasis and suppressor function. Herein, we explore the role of TGFbeta in feline Treg homeostasis and suppressor function and what effect FIV infection of cats might have on these processes. Stimulation of CD4+CD25- T helper (Th) cells with Concanavalin A (ConA) plus TGFbeta converts them to Treg-like cells capable of suppressor function. Reverse-transcription polymerase chain reaction and flow cytometry revealed that these ConA/TGFbeta-converted Treg cells upregulate Foxp3 and mTGFbeta. ConA stimulation of CD4+CD25- T cells upregulates TGFbeta receptor II (RII), and pretreatment of these cells with anti-TGFbeta-RII antibodies blocks the TGFbeta-induced conversion to Treg cells. Pretreatment of ConA/TGFbeta-converted Treg cells with anti-TGFbeta antibodies also abrogates their suppressor function, suggesting that Treg homeostasis and suppressor function may be mediated by mTGFbeta. Finally, we show that treatment of CD4+CD25+ mTGFbeta-positive Treg cells from FIV-infected cats with anti-TGFbeta antibodies or treatment of ConA-stimulated CD4+CD25- Th target cells with anti-TGFbeta-RII antibodies diminishes suppressor function. These data suggest that the recruitment of Treg cells from the Th pool and suppressor function of Treg cells are dependent on the TGFbeta/TGFbeta-RII signaling pathway and that this pathway is constitutively upregulated in asymptomatic chronically FIV-infected cats.

NK cells lyse T regulatory cells that expand in response to an intracellular pathogen

We evaluated the capacity of NK cells to influence expansion of CD4(+)CD25(+)FoxP3(+) regulatory T cells (Tregs) in response to microbial Ags, using Mycobacterium tuberculosis as a model. We previously found that Tregs expand when CD4(+) cells and monocytes are exposed to M. tuberculosis. Addition of NK cells that were activated by monokines (IL-12, IL-15, and IL-18) or by exposure to M. tuberculosis-stimulated monocytes reduced Treg expansion in response to M. tuberculosis. NK cell inhibition of Treg expansion was not mediated through IFN-gamma. Activated NK cells lysed expanded, but not freshly isolated Tregs. Although monokines increased NK cell expression of the activating receptors NKp46, NKG2D, 2B4, CD16, and DNAM-1, only anti-NKG2D and anti-NKp46 inhibited NK cell lysis of expanded Tregs. Of five NKG2D ligands, only UL16-binding protein 1 (ULBP1) was up-regulated on M. tuberculosis-expanded Tregs, and anti-ULBP1 inhibited NK cell lysis of expanded Tregs. M. tuberculosis-stimulated monocytes activated NK cells to lyse expanded Tregs, and this was also inhibited by anti-NKG2D and anti-ULBP1, confirming the physiological relevance of this effect. Our study identifies a potential new role for NK cells in maintaining the delicate balance between the regulatory and effector arms of the immune response.

Does CD4+CD25+foxp3+ cell (Treg) and IL-10 profile determine susceptibility to immune reconstitution inflammatory syndrome (IRIS) in HIV disease?

HIV-specific T-lymphocyte responses that underlie IRIS are incomplete and largely remain hypothetical. Of the several mechanisms presented by the host to control host immunological damage, Treg cells are believed to play a critical role. Using the available experimental evidence, it is proposed that enormous synthesis of conventional FoxP3_- Th cells (responsive) often renders subjects inherently vulnerable to IRIS, whereas that of natural FoxP3⁺ Treg cell synthesis predominate among subjects that may not progress to IRIS. We also propose that IRIS non-developers generate precursor T-cells with a high avidity to generate CD4+CD25+FoxP3+ Tregs whereas IRIS developers generate T-cells of intermediate avidity yielding Th0 cells and effector T-cells to mediate the generation of proinflammatory cytokines in response to cell-signaling factors (IL-2, IL-6 etc.). Researchers have shown that IL-10 Tregs (along with TGF-β, a known anti-inflammatory cytokine) limit immune responses against microbial antigens in addition to effectively controlling HIV replication, the prime objective of HAART. Although certain technical limitations are described herein, we advocate measures to test the role of Tregs in IRIS.

Lentivirus-induced immune dysregulation

FIV/HIV infections are associated with an early robust humoral and cellular anti-viral immune response followed by a progressive immune suppression that eventually results in AIDS. Several mechanisms responsible for this immune dysfunction have been proposed including cytokine dysregulation, immunologic anergy and apoptosis, and inappropriate activation of immune regulatory cells. Studies on FIV infection provide evidence for all three. Cytokine alterations include decreases in IL2 and IL12 production and increases in IFNgamma and IL10 in FIV(+) cats compared to normal cats. The elevated IL10:IL12 ratio is associated with the inability of FIV(+) cats to mount a successful immune response to secondary pathogens. Additionally, chronic antigenic (FIV) stimulation results in an increase in the percent of activated T cells expressing B7 and CTLA4 co-stimulatory molecules in infected cats. The expression of these molecules is associated with T cells that are undergoing apoptosis in the lymph nodes. As ligation of CTLA4 by B7 transduces a signal for induction of anergy, one can speculate that the activated T cells are capable of T cell-T cell interactions resulting in anergy and apoptosis. The inability of CD4(+) cells from FIV(+) cats to produce IL2 in response to recall antigens and the gradual loss of CD4(+) cell numbers could be due to B7-CTLA4 interactions. The chronic antigenemia may also lead to activation of CD4(+)CD25(+) T regulatory cells. Treg cells from FIV(+) cats are chronically activated and inhibit the mitogen-induced proliferative response of CD4(+)CD25(-) by down-regulating IL2 production. Although Treg cell activation can be antigen-specific, the suppressor function is not, and thus activated Treg cells would suppress responses to secondary pathogens as well as to FIV. Concomitant with the well-known virus-induced immune suppression is a progressive immune hyper-activation. Evidence for immune hyper-activation includes polyclonal B cell responses, gradual replacement of naïve CD4(+) and CD8(+) T cell phenotypes with activation phenotypes (CD62L(-), B7(+), CTLA4(+)), and the chronic activation of CD4(+)CD25(+) Treg cells. Thus lentivirus infections lead to severe immune dysregulation manifested as both chronic immune suppression and chronic immune activation. FIV infection of cats provides a number of advantages over other lentivirus infections as a model to study this immune dysregulation. It is a natural infection that has existed in balance with the cat's immune system for thousands of years. As such, the natural history and pathogenesis provides an excellent model to study the long-term relationships between AIDS lentivirus and host immune system function/dysregulation.

Role for plasmacytoid dendritic cells in anti-HIV innate immunity

The role of plasmacytoid dendritic cells (pDC) in anti-HIV immunity is mostly represented by the production of type I IFN in response to HIV infection in vitro and in vivo. This production is decreased in HIV-1 infected patients at the time of primary infection and during chronic disease in association with progression of disease. Circulating pDC counts are decreased concomitantly with type I IFN, and both factors correlate inversely overall with viral loads and positively with CD4+ T-cell counts. These parameters might be used in clinical immunology to monitor treatment and as predictive factors of immune control of HIV-1 replication to help decide whether to interrupt antiretroviral treatment. They may be related to control of HIV replication as well as to pathogenesis of infection, perhaps in setting the balance between immunity or tolerance to the virus. A better understanding of these parameters is required while attempts to use IFN-alpha or ligands of Toll-like receptors found on pDC are being made.

T-cell regulation by CD4 regulatory T cells during hepatitis B and C virus infections: facts and controversies

In the past few years, we have witnessed extraordinary advances in the understanding of the functions of regulatory T (Treg) cells in immunity against pathogens. However, controversy exists over the part that these cells play in determining the outcome of hepatitis B virus (HBV) and hepatitis C virus (HCV) infections, the two main causes of chronic liver inflammation worldwide. Treg-cell responses may be either beneficial or detrimental to those infected with HBV and HCV, by either limiting liver immunopathology or suppressing protective T-cell responses. We review the latest research on CD4 Treg cells, dissect much of the Treg-related HBV and HCV literature, and discuss how new insights in Treg immunobiology apply to human and primate models of HBV and HCV infections. Moreover, we discuss the limitations of the conclusions drawn from current studies on Treg cells, and suggest experimental approaches that can resolve current conflicts and improve our understanding of the roles of Treg-cell subsets in HBV and HCV infections.

Effect of low-dose IL-2 immunotherapy on frequency and phenotype of regulatory T cells and NK cells in HIV/HCV-coinfected patients

We evaluated the effect of low-dose IL-2 therapy (daily 1.2 MIU/m(2), subcutaneously) on the number and phenotype of regulatory T cells (T(regs)) and natural killer (NK) cells in HIV/HCV-coinfected patients taking antiretroviral therapy. The frequency and phenotype of circulating T(regs) (defined as CD3(+) CD4(+) CD25(high) or CD3(+) CD4(+) FOXP3(+)) and NK cells (CD3(-) CD16(+)/CD56(+)) were evaluated at baseline and after 12 weeks of treatment. The expression of CD25, CTLA-4, and granzymes A and B by CD4(+) FOXP3(+) cells, as well as the expression of KIR receptors (NKB1, CD158a, and NKAT2) on NK cells, was evaluated. Low doses of IL-2 resulted in the augmented frequency and absolute number of T(regs) in coinfected individuals. FOXP3 levels per cell as well as augmented CD25 and CTLA-4 expression by T(regs) suggested that IL-2 may lead to both expansion and activation of T(regs), although changes in the proportion of CD4(+) FOXP3(+) cells were not associated with changes in HCV viral load and CD4(+) cells between baseline and week 12. NK cell frequency also increased after IL-2 therapy. Interestingly, the pattern of expression of KIR receptors was changed by IL-2 treatment, since the frequency of NK cells expressing NKB1 augmented whereas the frequency of NK expressing CD158a and NKAT2 decreased.

Effects of cryopreservation on CD4+ CD25+ T cells of HIV-1 infected individuals

The role of T regulatory cells (Tregs) in human immunodeficiency virus (HIV)-1 infection, although not entirely clear, has recently been highlighted. Despite their lack of specificity, fluorochrome-labeled CD4 and CD25 antibodies are common flow cytometric reagents used to identify these cells with immunosuppressive potential. Cryopreservation has previously been shown to alter the proportions of lymphocytes with certain phenotypes expressed in peripheral blood mononuclear cells (PBMCs). The aim of this study was to assess the effect of cryopreservation on CD4+ CD25+ T cells in PBMCs from HIV-1+ individuals to guide the design of future studies on Tregs. We recruited 30 HIV-1+ individuals and nine healthy controls. CD25 expression in CD4+ T cells was compared between fresh and frozen/thawed PBMC samples from the same time point. In this study, cryopreservation significantly decreased the proportion of CD4+ CD25+ T cells in PBMC samples from HIV-1 infected subjects. This finding suggests that studies of CD4+ CD25+ T cells should be carried out on fresh samples to avoid bias introduced by cryopreservation.

Friend retrovirus infection of myeloid dendritic cells impairs maturation, prolongs contact to naïve T cells, and favors expansion of regulatory T cells

Retroviruses have developed immunmodulatory mechanisms to avoid being attacked by the immune system. The mechanisms of this retrovirus-associated immune suppression are far from clarified. Dendritic cells (DCs) have been attributed a decisive role in these pathogenic processes. We have used the Friend retrovirus (FV) mouse model in order to acquire further knowledge about the role of infection of DCs in virus-induced immunosuppression. About 20% of the myeloid DCs that were generated from the bone marrow of FV-infected mice carried FV proteins. The infection was productive, and infected DCs transmitted the virus in cell culture and in vivo. FV infection of DCs led to a defect in DC maturation, as infected cells expressed very little costimulatory molecules. Live imaging analysis of the cell contact between DCs and T cells revealed prolonged contacts of T cells with infected DCs compared with uninfected DCs. Although naive T cells were still activated by FV-infected DCs, this activation did not result in antigen-specific T-cell proliferation. Interestingly, infected DCs expanded a population of Foxp3+ regulatory T cells with immunosuppressive potential, suggesting that the contact between naive T cells and retrovirus-infected DCs results in tolerance rather than immunity. Thus, retroviral infection of DCs leads to an expansion of regulatory T cells, which might serve as an immune escape mechanism of the virus.

A phase I study of ultra low dose interleukin-2 and stem cell factor in patients with HIV infection or HIV and cancer

PURPOSE

Ultra low doses of interleukin-2 (IL-2) can activate the high-affinity IL-2 receptor constitutively expressed on CD56(bright) natural killer (NK) cells, the CD34+ NK cell precursor, and CD4+ CD25+ regulatory T cells (Tregs) in vivo. We have previously shown synergy between IL-2 and stem cell factor (SCF) in the generation of CD56(bright) NK cells from CD34+ hemopoietic progenitor cells in vitro and showed synergistic NK cell expansion in an in vivo preclinical model. To determine the safety, toxicity, and immune modulation of this combination of cytokines in vivo, we conducted a first-in-man phase I study.

EXPERIMENTAL DESIGN

A phase I dose escalation study was conducted using IL-2 at 900,000 or 650,000 IU/m2/d for 8 weeks with 5 or 10 microg/kg/d of SCF given thrice a week for 8 weeks in patients with HIV infection and/or cancer.

RESULTS

Ten of 13 patients completed therapy; four experienced the dose-limiting toxicities of grade 3 fatigue or urticaria. The maximum tolerated doses of IL-2 and SCF in combination is 650,000 IU/m2/d of IL-2 and 5 microg/kg/d thrice a week of SCF. NK cells were expanded over 2-fold on therapy; Tregs were expanded nearly 6-fold from baseline.

CONCLUSIONS

Administration of IL-2 with SCF is safe and well tolerated and leads to expansion of lymphocyte subsets in patients with HIV or HIV and cancer; however, the changes in NK cell and Treg expansion seen with this cytokine combination were no different than those seen with a similar dose of IL-2 alone.

Human T cell reconstitution in DiGeorge syndrome and HIV-1 infection

The thymus is essential for proper development and maintenance of a broad T cell repertoire capable of recognizing a wide-range of foreign antigens. Recent advances in multicolor flow cytometry, non-invasive imaging techniques, and molecular assessments of thymic function have enabled a more comprehensive characterization of human thymic output in clinical settings than in the past. These techniques have been particularly valuable in monitoring human T cells after therapeutic thymic grafting for complete DiGeorge syndrome and during HIV-1 infection and AIDS. By defining the degree and mechanisms of T cell reconstitution in these settings, clinical investigators and primary caregivers have been able to better diagnose, treat and care for individuals with congenital or acquired immune deficiencies associated with loss of thymic function.

Regulatory T cells and infection: a dangerous necessity

Surviving a given infection requires the generation of a controlled immune response. Failure to establish or restore homeostatic conditions during or following the onset of an infection can lead to tissue damage. Investigation of the immunoregulatory network that arises in response to the infectious process or that is induced by the pathogen itself should provide insight into therapeutic approaches for the control of infection and any subsequent immunopathology. In this Review, I discuss current hypotheses and points of polemic associated with the origin, mode of action and antigen specificity of the various populations of regulatory T cells that arise during infection.

In vivo depletion of CD4+CD25+ regulatory T cells in cats

To establish a characterized model of regulatory T cell (Treg) depletion in the cat we assessed the kinetics of depletion and rebound in peripheral and central lymphoid compartments after treatment with anti-CD25 antibody as determined by cell surface markers and FOXP3 mRNA expression. An 82% decrease in circulating CD4+CD25+ Tregs was observed by day 11 after treatment. CD4+CD25+ cells were also reduced in the thymus (69%), secondary lymphoid tissues (66%), and gut (67%). Although CD4+CD25+ cells rebound by day 35 post-treatment, FOXP3 levels remain depressed suggesting anti-CD25 antibody treatment has a sustainable diminutive effect on the Treg population. To determine whether CD25+ Treg depletion strategies also deplete activated CD25+ effector cells, cats were immunized with feline immunodeficiency virus (FIV) p24-GST recombinant protein, allowing them to develop a measurable memory response, prior to depletion with anti-CD25 antibody. Anti-FIV p24-GST effector cell activity in peripheral blood after depletion was sustained as determined by antigen-specific T cell proliferation and humoral responses against FIV p24-GST with an ELISA for antigen-specific feline IgG. Furthermore, development of an anti-mouse response in Treg-depleted cats was similar to control levels indicating the retained capacity to respond to a novel antigen. We conclude that despite alterations in CD25+ cell levels during depletion, the feline immune system remains functional. We demonstrate here a model for the study of disease pathogenesis in the context of reduced numbers of immunosuppressive CD4+CD25+ Tregs throughout the feline immune system.

Upregulation of CTLA-4 by HIV-specific CD4+ T cells correlates with disease progression and defines a reversible immune dysfunction

In progressive viral infection, antiviral T cell function is impaired by poorly understood mechanisms. Here we report that the inhibitory immunoregulatory receptor CTLA-4 was selectively upregulated in human immunodeficiency virus (HIV)-specific CD4(+) T cells but not CD8(+) T cells in all categories of HIV-infected subjects evaluated, with the exception of rare people able to control viremia in the absence of antiretroviral therapy. CTLA-4 expression correlated positively with disease progression and negatively with the capacity of CD4(+) T cells to produce interleukin 2 in response to viral antigen. Most HIV-specific CD4(+) T cells coexpressed CTLA-4 and another inhibitory immunoregulatory receptor, PD-1. In vitro blockade of CTLA-4 augmented HIV-specific CD4(+) T cell function. These data, indicating a reversible immunoregulatory pathway selectively associated with CD4(+) T cell dysfunction, provide a potential target for immunotherapy in HIV-infected patients.

FoxP3+ CD25+ CD8+ T-cell induction during primary simian immunodeficiency virus infection in cynomolgus macaques correlates with low CD4+ T-cell activation and high viral load

The early immune response fails to prevent the establishment of chronic human immunodeficiency virus (HIV) infection but may influence viremia during primary infection, thereby possibly affecting long-term disease progression. CD25(+) FoxP3(+) regulatory T cells may contribute to HIV/simian immunodeficiency virus (SIV) pathogenesis by suppressing efficient antiviral responses during primary infection, favoring high levels of viral replication and the establishment of chronic infection. In contrast, they may decrease immune activation during chronic infection. CD4(+) regulatory T cells have been studied in the most detail, but CD8(+) CD25(+) FoxP3(+) T cells also have regulatory properties. We monitored the dynamics of CD25(+) FoxP3(+) T cells during primary and chronic SIVmac251 infection in cynomolgus macaques. The number of peripheral CD4(+) CD25(+) FoxP3(+) T cells paralleled that of memory CD4(+) T cells, with a rapid decline during primary infection followed by a rebound to levels just below baseline and gradual depletion during the course of infection. No change in the proportion of CD25(+) FoxP3(+) T cells was observed in peripheral lymph nodes. A small number of CD4(+) CD25(+) FoxP3(+) T cells at set point was associated with a high plasma viral load. In contrast, peripheral CD8(+) CD25(+) FoxP3(+) T cells were induced a few days after peak plasma viral load during primary infection. The number of these cells was positively correlated with viral load and negatively correlated with CD4(+) T-cell activation, SIV antigen-specific proliferative responses during primary infection, and plasma viral load at set point, with large numbers of CD8(+) CD25(+) FoxP3(+) T cells being indicative of a poor prognosis.

Severe depletion of CD4+ CD25+ regulatory T cells from the intestinal lamina propria but not peripheral blood or lymph nodes during acute simian immunodeficiency virus infection

CD4+ CD25+ regulatory T cells (Tregs) suppress the activation and proliferation of effector lymphocytes. In human immunodeficiency virus type 1 (HIV-1) infection, Tregs play a significant role in controlling the apoptotic loss of uninfected CD4+ T cells resulting from high levels of generalized immune activation. During acute HIV-1 infection, more than 50% of CD4+ T cells are depleted from the gastrointestinal lamina propria. To elucidate the role of Tregs in HIV-1-induced depletion of CD4+ T cells in the gut-associated lymphoid tissue (GALT), we first determine the distribution of Tregs in a setting of acute infection using the simian immunodeficiency virus (SIV)/pigtailed macaque model of HIV-1 disease. CD4+ T cells from the GALT, lymph nodes, and peripheral blood were isolated from SIV-infected pigtailed macaques on days 4, 14, and 114 postinoculation. Quantitative real-time reverse transcription-PCR was used to quantitate FOXP3 copy numbers in SIV-infected and uninfected control macaques. Expression of FOXP3 in the ileal lamina propria was significantly decreased at all stages of infection compared to levels in uninfected control macaques. In addition, functional analysis of ileal CD4+ T cells from SIV-infected macaques revealed a lack of suppressive activity suggestive of the absence of Tregs in that compartment. These results indicate that Tregs are rapidly depleted in the GALT of SIV-infected macaques, defining a role for the loss of Treg-mediated suppression in early events in the pathogenesis of the disease.

Increased frequency of CD4(+)CD25(high) Treg cells inhibit BCG-specific induction of IFN-gamma by CD4(+) T cells from TB patients

Cell-mediated immunity plays a considerable role in the protection against Mycobacterium tuberculosis infection. The immune response to tuberculosis (TB) was dominated by both CD4(+) T cells with the T helper 1 type cytokines and CD8(+) T cells. Recent studies have suggested that the circumstances in which protective or tissue-damaging T cell responses to microbes are affected by the activity of Treg (CD4(+)CD25(high)) cells. In the present study, we demonstrated that the frequencies of CD4(+)CD25(+) and CD4(+)CD25(high) T cells in TB patients were significantly higher compared to normal individuals. These Treg cells expressed CTLA-4 and Foxp3 at protein level and displayed activation and memory phenotypes as assessed by flow cytometric analysis. The frequencies of CD4(+)CD25(high)CTLA-4(+) and CD4(+)CD25(high)Foxp3(+) T cells within the total CD4(+) T cell population were significantly increased in the blood of TB patients compared to healthy donors. Moreover, the expression of GITR on Treg cells was higher in TB patients than in normal donors. The phenotypic analysis demonstrated that CD4(+)CD25(high) Treg expressed higher levels of CD45RO and HLA-DR, and lower levels of CD45RA compared to CD4(+)CD25(low) and CD4(+)CD25(-) T cells. The addition of CD4(+)CD25(high) T cells back to cultures could significantly suppress the antigen-specific production of IFN-gamma induced by BCG-stimulated CD4(+)CD25(-) T cells, suggesting that Treg might play a key role in the control of cellular immune responses in TB infection.

Regulatory T cells and immunity to pathogens

Immune responses to pathogens are modulated by one or more types of cells that perform a regulatory function. Some cells with this function, such as CD4+ Foxp3+ natural regulatory T cells (nTreg), pre-exist prior to infections whereas others may be induced as a consequence of infection (adaptive Treg). With pathogens that have a complex pathogenesis, multiple types of regulatory cells could influence the outcome. One major property of Treg is to help minimize collateral tissue damage that can occur during immune reactions to a chronic infection. The consequence is less damage to the host but in such situations the pathogen is likely to establish persistence. In some cases, a fine balance is established between Treg responses, effector components of immunity and the pathogen. Treg responses to pathogens may also act to hamper the efficacy of immune control. This review discusses these issues as well as the likely mechanisms by which various pathogens can signal the participation of Treg during infection.

Regulatory T-cell markers, indoleamine 2,3-dioxygenase, and virus levels in spleen and gut during progressive simian immunodeficiency virus infection

High levels of viral replication occur in gut-associated lymphoid tissue (GALT) and other lymphoid tissues (LT) since the early phase of human/simian immunodeficiency virus (HIV/SIV) infection. Regulatory T cells (T(reg)), a subset of immunosuppressive T cells expressing CTLA-4 and the FoxP3 transcription factor, accumulate in LT during HIV/SIV infection. Here we show that FoxP3 and CTLA-4 mRNA are increased in leukocytes from the spleens, lymph nodes (LN), and mucosal sites of chronically SIV-infected macaques with high viremia (SIV(HI)) compared to animals with low viremia (SIV(LO)). FoxP3 and CTLA-4 correlated with SIV RNA levels in tissues; SIV virus levels in the spleen, inguinal LN, mesenteric LN, colon, and jejunum directly correlated with the plasma virus level. Importantly, CTLA-4 and FoxP3 mRNA were predominantly increased in the CD25(-) subpopulation of leukocytes from SIV(HI), further challenging the classical definition of T(reg) as CD4(+) CD25(+) T cells. Similar to CTLA-4 and FoxP3, expression of indoleamine 2,3-dioxygenase (IDO), an immunosuppressive enzyme induced by T(reg) in antigen-presenting cells, was increased in the spleens, mesenteric LN, colons, and jejuna from SIV(HI) compared to SIV(LO) and directly correlated to SIV RNA in the same tissues. Accordingly, plasma kynurenine/tryptophan, a marker for IDO enzymatic activity, was significantly higher in SIV(HI) compared to SIV(LO) and correlated with plasma viral levels. Increased T(reg) and IDO in LT of SIV-infected macaques may be the consequence of increased tissue inflammation and/or may favor virus replication during the chronic phase of SIV infection.

Endogenous TGF-beta activation by reactive oxygen species is key to Foxp3 induction in TCR-stimulated and HIV-1-infected human CD4+CD25- T cells

Background

CD4⁺CD25⁺ T regulatory cells (Tregs) play an important role in regulating immune responses, and in influencing human immune diseases such as HIV infection. It has been shown that human CD4⁺CD25⁺ Tregs can be induced in vitro by TCR stimulation of CD4⁺CD25^- T cells. However, the mechanism remains elusive, and intriguingly, similar treatment of murine CD4⁺CD25^- cells did not induce CD4⁺CD25⁺Foxp3⁺ Tregs unless exogenous TGF-β was added during stimulation. Thus, we investigated the possible role of TGF-β in the induction of human Tregs by TCR engagement. We also explored the effects of TGF-β on HIV-1 infection mediated induction of human Tregs since recent evidence has suggested that HIV-1 infection may also impact the generation of Tregs in infected patients.

Results

We show here that endogenous TGF-β is key to TCR induction of Foxp3 in human CD4⁺CD25^- T cells. These events involve, first, the production of TGF-β by TCR and CD28 stimulation and the activation of latent TGF-β by reactive oxygen species generated from the activated T cells. Biologically active TGF-β then engages in the induction of Foxp3. Neutralization of active TGF-β with anti-TGF-β antibody or elimination of ROS with MnTBAP abrogated Foxp3 expression. HIV-1 infection enhanced Foxp3 expression in activated CD4⁺CD25^- T cells; which was also abrogated by blockade of endogenous TGF-β.

Conclusion

Several conclusions can be drawn from this work: (1) TCR and CD28-induced Foxp3 expression is a late event following TCR stimulation; (2) TGF-β serves as a link in Foxp3 induction in human CD4⁺CD25^- T cells following TCR stimulation, which induces not only latent, but also active TGF-β; (3) the activation of TGF-β requires reactive oxygen species; (4) HIV infection results in an increase in Foxp3 expression in TCR-activated CD25^- T cells, which is also associated with TGF-β. Taken together, our findings reinforce a definitive role of TGF-β not only in the generation of Tregs with respect to normal immune responses, but also is critical in immune diseases such as HIV-1 infection.

Proportions of circulating T cells with a regulatory cell phenotype increase with HIV-associated immune activation and remain high on antiretroviral therapy

OBJECTIVE

To examine the relationships between blood CD4 natural regulatory T (Treg) cells, plasma HIV RNA level, CD4 T-cell count and immune activation in untreated HIV-infected patients and immunodeficient patients beginning antiretroviral therapy (ART), using a novel phenotype to define Treg cells (CD25CD127CD4). Data were compared with established Treg cell markers (FoxP3, CTLA-4 and GITR).

METHODS

Twenty-nine untreated HIV-infected patients with CD4 T-cell counts of < 300 or > 400/microl were compared in a cross-sectional study and 12 patients beginning combination ART with < 100 CD4 T cells/mul were followed for 1 year on therapy. Three- and four-colour flow cytometry was used to quantitate proportions of Treg cells.

RESULTS

In control donors and patients with high CD4 T-cell counts, 28-89% (median 60%) of CD25CD127CD4 cells were FoxP3, but < 10% expressed GITR or CTLA-4. Immunodeficient patients also had CD4-negative lymphocytes with the phenotype FoxP3CD127. Proportions of CD25CD127 cells and activated (HLA-DR) cells in the CD4 T-cell population were increased in patients with low CD4 T cell counts. The proportion of CD25CD127CD4 T cells correlated positively with plasma HIV RNA level and CD4 T-cell activation, but inversely with CD4 T-cell count. Longitudinal studies of 12 patients receiving ART in two distinct cohorts (Western Australia and Malaysia) showed that the proportion of CD25CD127CD4 cells decreased slightly over time, but remained above levels seen in non-HIV controls.

CONCLUSIONS

Proportions of circulating T cells with a regulatory cell phenotype increase with HIV-associated immune activation and remain high after 1 year on ART.

Immune factors and immunoregulation in tuberculosis

Pathogens causing tuberculosis and other chronic infectious diseases of major public health importance commonly have complex mechanisms involved in their persistence in the host despite specific and sometimes strong immune responses. These diseases are also associated with the lack of efficient vaccines, difficult therapeutics and a high mortality rate among susceptible individuals. Here, we will review features of the host immune response that contribute to the occurrence of disease. In addition, we propose that the immune responses observed in tuberculosis cannot be interpreted solely on the basis of a Th1-Th2 counter-regulatory paradigm since there is growing evidence that natural regulatory T cells may play an important role in the regulation of host immune responses against Mycobacterium tuberculosis. Thus, the development of more effective vaccines against this bacterial disease should take into account the role of natural regulatory T cells in the progression to severe disease and persistence of infection. Finally, new treatments based on manipulation of regulatory T cells should be investigated.

Antigen-induced regulatory T cells in HBV chronically infected patients

T cell response against HBV is vigorous in patients with acute hepatitis who clear the virus, whereas it is weak and narrowly focused in patients with chronic disease. We report that following incubation with HBcAg, a population of CD4+FoxP3+ cells expressing phenotypic markers of both natural and induced Tregs, can be antigen-induced from peripheral mononuclear cells. Conversely, naive and naturally immune subjects did not increase CD4+FoxP3+ Tregs following stimulation with HBcAg, supporting the idea that natural Tregs are able to respond specifically to HBV antigen. Furthermore, increased frequencies of antigen-induced CD4+FoxP3+IL-10+ Tregs correlated with viral load, suggesting that antigen-induced Tregs could contribute to an inadequate response against the virus, leading to chronic infection and support the view that specific natural Tregs may be implicated in host immune tolerance during HBV infection.

Regulatory T cells (Treg) and HIV/AIDS: summary of the September 7-8, 2006 workshop

A workshop entitled "Regulatory T cells (T(reg)) and HIV/AIDS" was held in Cincinnati, OH, September 7-8, 2006. This workshop was the first completely dedicated to T(reg) in HIV infection, and gathered investigators working on different aspects of T(reg) biology, and on HIV pathogenesis. We report here the major topics of discussion of this workshop, the goal of which was to summarize what is known and not known about the role of T(reg) in HIV immune responses and pathogenesis; and to foster discussion on the means of manipulating T(reg) in HIV-infected subjects. Workshop participants also debated the research priorities in the field, which emerged as follows: (1) to arrive at a consensus on T(reg) definition and to standardize assays aimed at characterizing T(reg) number and function; (2) to study T(reg) biology in tissues, notably in mucosal tissues, and at different stages of infection; (3) to examine T(reg) function in vivo in animal models, as well as to test strategies to target or modulate T(reg) cell function in these models; (4) to investigate the effect of viral factors on T(reg) biology, and conversely to study whether T(reg) activity affects the level of HIV replication; (5) to conduct longitudinal studies of T(reg) number and function; and (6) to determine the effect of coinfections on T(reg) biology.