Suppression of HIV-specific T cell activity by lymph node CD25+ regulatory T cells from HIV-infected individuals

Regulatory T cell suppression of Gag-specific CD8 T cell polyfunctional response after therapeutic vaccination of HIV-1-infected patients on ART

We tested the hypothesis that therapeutic vaccination against HIV-1 can increase the frequency and suppressive function of regulatory, CD4⁺ T cells (Treg), thereby masking enhancement of HIV-1-specific CD8⁺ T cell response. HIV-1-infected subjects on antiretroviral therapy (N = 17) enrolled in a phase I therapeutic vaccine trial received 2 doses of autologous dendritic cells (DC) loaded with HIV-1 peptides. The frequency of CD4⁺CD25^hiFOXP3⁺ Treg in blood was determined prior to and after vaccination in subjects and normal controls. Polyfunctional CD8⁺ T cell responses were determined pre- and post-vaccine (N = 7) for 5 immune mediators after in vitro stimulation with Gag peptide, staphylococcal enterotoxin B (SEB), or medium alone. Total vaccine response (post-vaccine–pre-vaccine) was compared in the Treg(+) and Treg-depleted (Treg-) sets. After vaccination, 12/17 subjects showed a trend of increased Treg frequency (P = 0.06) from 0.74% to 1.2%. The increased frequency did not correlate with CD8⁺ T cell vaccine response by enzyme linked immunosorbent assay for interferon γ production. Although there was no significant change in CD8⁺ T cell polyfunctional response after vaccination, Treg depletion increased the polyfunctionality of the total vaccine response (P = 0.029), with a >2-fold increase in the percentage of CD8⁺ T cells producing multiple immune mediators. In contrast, depletion of Treg did not enhance polyfunctional T cell response to SEB, implying specificity of suppression to HIV-1 Gag. Therapeutic immunization with a DC-based vaccine against HIV-1 caused a modest increase in Treg frequency and a significant increase in HIV-1-specific, Treg suppressive function. The Treg suppressive effect masked an increase in the vaccine-induced anti-HIV-1-specific polyfunctional response. The role of Treg should be considered in immunotherapeutic trials of HIV-1 infection.

Increased sensitivity of CD4+ T-effector cells to CD4+CD25+ Treg suppression compensates for reduced Treg number in asymptomatic HIV-1 infection

BACKGROUND

In HIV infection, uncontrolled immune activation and disease progression is attributed to declining CD4+CD25+FoxP3+ regulatory T-cell (Treg) numbers. However, qualitative aspects of Treg function in HIV infection, specifically the balance between Treg cell suppressive potency versus suppressibility of effector cells, remain poorly understood. This report addresses this issue.

METHODOLOGY/PRINCIPAL FINDINGS

A classic suppression assay to measure CD4+CD45RO+CD25hi Treg cells to suppress the proliferation of CD4+CD45RO+CD25- effectors cells (E) following CD3/CD28 polyclonal stimulation was employed to compare the suppressive ability of healthy volunteers (N = 27) and chronic, asymptomatic, treatment naïve, HIV-infected subjects (N = 14). HIV-infected subjects displayed significantly elevated Treg-mediated suppression compared to healthy volunteers (p = 0.0047). Cross-over studies comparing Treg cell potency from HIV-infected versus control subjects to suppress the proliferation of a given population of allogeneic effector cells demonstrated increased sensitivity of CD4+CD25- effector cells from HIV-infected subjects to be suppressed, associated with reduced production of the Treg counter-regulatory cytokine, IL-17, rather than an increase in the suppressive potential of their CD4+CD25+ Treg cells. However, compared to controls, HIV+ subjects had significantly fewer absolute numbers of circulating CD4+CD25+FoxP3+ Treg cells. In vitro studies highlighted that one mechanism for this loss could be the preferential infection of Treg cells by HIV.

CONCLUSIONS/SIGNIFICANCE

Together, novel data is provided to support the contention that elevated Treg-mediated suppression may be a natural host response to HIV infection.

Increased peripheral blood leukocyte subsets with regulatory phenotype in clinically stable long-term HIV-infected hemophilia patients on HAART may be beneficial and contribute to a decrease in autoimmunity

After initiation of highly-active antiretroviral therapy (HAART), long-term HIV-infected hemophilia patients have been shown to lose autoantibodies against CD4(+) peripheral blood leukocytes (PBL), suggesting that HAART induces autoimmunity-blocking mechanisms. We compared cytokine levels and subpopulations of lymphocytes and dendritic cells (DC) in the blood of 40 long-term HIV(+) patients with those of 13 long-term HIV(-) hemophilia patients; 23 HIV(+) patients had a detectable retroviral load. Cell subsets were determined using flow cytometry and cytokine levels were measured using ELISA. HIV(+) patients showed higher proportions of DC subpopulations with immunostimulatory phenotypes (p < 0.01), CD8(+) PBL (p < 0.001), and IL-2 (p < 0.001) and sIL-2R plasma levels (p = 0.002) than HIV(-) patients. They also exhibited increased proportions of T PBL with immunosuppressive phenotypes such as CD3(+)CD4(+)CD25(+)Foxp3(+) (p = 0.001), and CD3(+)CD8(+)CD28(-)Foxp3(+) PBL (p < 0.001), and a decreased IL-7R expression on CD3(+)CD8(+) PBL (p = 0.001) compared to HIV(-) patients. Frequencies of CD3(+)CD4(+)CD25(+) PBL producing IL-2, IL-4, IL-10, IL-12, and/or IFN-gamma, and of CD3(+)CD4(+)CD28(-) PBL secreting IL-2 and/or IL-4 were lower in HIV(+) than in HIV(-) patients (p <or= 0.02). Proportions of CD4(+) PBL coated with IgG, IgM, and C3d were similar in HIV(+) and HIV(-) patients (p = n.s.). However, the proportion of CD4(+)gp120(+) PBL was higher in HIV(+) patients (p = 0.002), and associated with low CD3(+)CD4(+)CD25(+)Foxp3(+) PBL (p = 0.012). We conclude that long-term HIV-infected hemophilia patients on HAART show an adaptive immune response, presumably against HIV, in the presence of upregulated immunosuppressive T PBL, downregulated cytokine-producing CD4(+) PBL, and downregulated IL-7R expression on CD8(+) PBL. Increased immunoregulatory T PBL might decrease autoimmunity, thereby contributing to immunological reconstitution and stabilization of long-term HIV-infected hemophilia patients on HAART.

Gut mucosal FOXP3+ regulatory CD4+ T cells and Nonregulatory CD4+ T cells are differentially affected by simian immunodeficiency virus infection in rhesus macaques

The gastrointestinal tract represents a major site for human and simian immunodeficiency virus (HIV and SIV) replication and CD4(+) T-cell depletion. Despite severe depletion of mucosal CD4(+) T cells, FOXP3(+) regulatory CD4(+) T cells (T(reg)) are highly increased in the gut mucosa of chronically HIV-infected individuals and may contribute to HIV pathogenesis, either by their immunosuppressive function or as a significant target cell population for virus production. Little is known about the susceptibility of mucosal T(reg) to viral infection and the longitudinal effect of HIV/SIV infection on T(reg) dynamics. In this study, we determined the level of SIV infection in T(reg) and nonregulatory CD4(+) T cells (non-T(reg)) isolated from the colon of SIV-infected rhesus macaques. The dynamics of mucosal T(reg) and alterations in the mucosal CD4(+) T-cell pool were examined longitudinally. Our findings indicate that mucosal T(reg) were less susceptible to productive SIV infection than non-T(reg) and thus were selectively spared from SIV-mediated cell death. In addition to improved survival, local expansion of T(reg) by SIV-induced proliferation of the mucosal CD4(+) T-cell pool facilitated the accumulation of mucosal T(reg) during the course of infection. High frequency of mucosal T(reg) in chronic SIV infection was strongly related to a reduction of perforin-expressing cells. In conclusion, this study suggests that mucosal T(reg) are less affected by productive SIV infection than non-T(reg) and therefore spared from depletion. Although SIV production is limited in mucosal T(reg), T(reg) accumulation may indirectly contribute to viral persistence by suppressing antiviral immune responses.

Dendritic cell immunotherapy for HIV infection: from theory to reality

Knowledge concerning the immunology of dendritic cells (DCs) accumulated over the last few decades and the development of methodologies to generate and manipulate these cells in vitro has made their therapeutic application a reality. Currently, clinical protocols for DC-based therapeutic vaccine in HIV-infected individuals show that it is a safe and promising approach. Concomitantly, important advances continue to be made in the development of methodologies to optimize DC acquisition, as well as the selection of safe, immunogenic HIV antigens and the evaluation of immune response in treated individuals.

Human regulatory T cells are targets for human immunodeficiency Virus (HIV) infection, and their susceptibility differs depending on the HIV type 1 strain

Regulatory T cells (Treg) are a subpopulation of CD4(+) T cells characterized by the suppressive activity they exert on effector immune responses, including human immunodeficiency virus (HIV)-specific immune responses. Because Treg express CXCR4 and CCR5, they represent potential targets for HIV; however, Treg susceptibility to HIV infection is still unclear. We therefore performed an extensive study of Treg susceptibility to HIV, using lab strains and primary isolates with either CCR5 or CXCR4 tropism. Furthermore, we quantified HIV infection at early and late time points of the virus life cycle. We found that Treg were clearly susceptible to HIV infection. Circulating Treg were not preferentially infected with HIV compared to effector T cells (Teff) in vivo. Conversely, in vitro infection with either CCR5-using (R5) or CXCR4-using (X4) viruses occurred with different dynamics. For instance, HIV infection by R5 viruses (lab strains and primary isolates) resulted in lower levels of infection in Treg compared with Teff at both early and late time points. In contrast, X4 viruses induced higher levels of infection in Treg compared to Teff at early time points, but this difference disappeared at the late time points of the virus life cycle. Our results suggest that the relative susceptibility of Treg to HIV infection compared to Teff varies, depending on both viral and host factors. These variations may play an important role in HIV pathogenesis.

Placental immunopathology in the FIV-infected cat: a role for inflammation in compromised pregnancy?

In utero transmission of feline immunodeficiency virus (FIV) occurs frequently in queens experimentally infected with FIV-B-2542 and other FIV isolates. Fetal infection has been detected as early as 3-4 weeks gestation, and the incidence of fetal infection increases with progressing gestation. Reproductive failure occurs commonly, including fetal resorptions and developmentally-arrested fetuses, demonstrating that fetal demise occurs early in gestation. Precise, temporal immunomodulation within the placenta is essential for successful pregnancy. Placental Th1 and Th2 cytokines must be appropriately balanced, typically favoring Th2 cytokines at the maternal-fetal interface. Abnormal inflammatory cytokine expression often accompanies miscarriage. Regulatory T cells (Tregs) play an essential role in maternal tolerance of the semi-allogeneic fetus by suppressing inflammation. We are using the FIV-infected cat to examine the relationship between lentivirus-induced placental immunopathology and reproductive outcome. Using TaqMan real time reverse transcriptase (RT)-PCR, we measured relative expression of key immunomodulators in the placentas of FIV-B-2542-infected and control cats, including placentas from both viable and nonviable pregnancies. Our data associate significantly-increased expression of inflammatory cytokines with failed pregnancies, identify Treg markers in the placentas, and provide preliminary evidence that Tregs or other cells bearing similar activation markers may be involved in pregnancy maintenance. Our data suggest that placental inflammation in the FIV-infected cat may compromise pregnancy.

The regulatory T-cell response during acute retroviral infection is locally defined and controls the magnitude and duration of the virus-specific cytotoxic T-cell response

Cytotoxic CD8(+) T cells control acute viremia in many viral infections. However, most viruses that establish chronic infections evade destruction by CD8(+) T cells, and regulatory T cells (Treg) are thought to be involved in this immune evasion. We have infected transgenic mice, in which Treg can be selectively depleted, with Friend retrovirus (FV) to investigate the influence of Treg on pathogen-specific CD8(+) T-cell responses in vivo. We observed that Treg expansion during acute infection was locally defined to organs with high viral loads and massive activation of virus-specific effector CD8(+) T cells. Experimental ablation of Treg resulted in a significant increase of peak cytotoxic CD8(+) T-cell responses against FV. In addition, it prevented the development of functional exhaustion of CD8(+) T cells and significantly reduced FV loads in lymphatic organs. Surprisingly, despite the massive virus-specific CD8(+) T-cell response after temporary Treg depletion, no evidence of immunopathology was found. These results demonstrate the important role of Treg in controlling acute retrovirus-specific CD8(+) T-cell responses, and suggest that temporary manipulation of Treg might be a possible therapeutic approach in chronic infectious diseases.

Treatment with anti-FasL antibody preserves memory lymphocytes and virus-specific cellular immunity in macaques challenged with simian immunodeficiency virus

Immune deficiency viruses such as SIV in macaques or HIV-1 in human beings have evolved mechanisms to defeat host immunity that also impact the efficacy of vaccines. A key factor for vaccine protection is whether immune responses elicited by prior immunization remain at levels sufficient to limit disease progression once a host is exposed to the pathogen. One potential mechanism for escaping pre-existing immunity is to trigger death among antigen-activated cells. We tested whether FasL/CD178 is involved in destroying preexisting immunity. Rhesus macaques were immunized with recombinant vesicular stomatitis virus vaccine expressing SIV Gag to elicit cellular immune responses, then treated with antibody that neutralizes FasL and challenged with intravenous SIVmac251. Compared with animals injected with control antibody, anti-FasL-treated macaques had superior preservation of central memory CD4(+) and CD8(+) cells and decreased regulatory T cells in the blood. The CD4(+) and CD8(+) lymphocytes from treated animals responded better to SIV Gag compared with controls, evidenced by higher cell-mediated immune responses to viral antigens for at least 17 weeks after SIV challenge. Anti-FasL treatment during the initial stages of acute SIV infection preserved the T-cell compartment and sustained cell-mediated immunity to SIV.

Redefining chronic viral infection

Viruses that cause chronic infection constitute a stable but little-recognized part of our metagenome: our virome. Ongoing immune responses hold these chronic viruses at bay while avoiding immunopathologic damage to persistently infected tissues. The immunologic imprint generated by these responses to our virome defines the normal immune system. The resulting dynamic but metastable equilibrium between the virome and the host can be dangerous, benign, or even symbiotic. These concepts require that we reformulate how we assign etiologies for diseases, especially those with a chronic inflammatory component, as well as how we design and interpret genome-wide association studies, and how we vaccinate to limit or control our virome.

CD4+ and CD8+ T cells expressing FoxP3 in HIV-infected patients are phenotypically distinct and influenced by disease severity and antiretroviral therapy

OBJECTIVES

Forkhead box P3 (FoxP3) is critical for the development of CD4 regulatory T (Treg) cells and is a useful marker to identify this population. Recently, expression of FoxP3 was reported in human CD8 T cells from the blood of untreated HIV-infected individuals. We assessed whether FoxP3 expression in CD8 T cells is associated with suppressive potential and/or with HIV-associated immune activation.

METHODS

FoxP3CD8 T cells in non-HIV donors and in untreated and treated HIV-infected patients were identified by flow cytometry, then examined for coexpression of other Treg cell-associated markers [cytotoxic T lymphocyte-associated antigen (CTLA)-4, GITR, and CD45RO], markers of activation [HLA-DR, Ki-67, and programmed death (PD)-1], and markers of senescence (CD57 without CD28).

RESULTS

Similar proportions of FoxP3-expressing CD4 and CD8 T cells coexpressed HLA-DR and Ki-67. However, compared with FoxP3CD4 cells, FoxP3CD8 cells expressed less CTLA-4, CD28, and CD45RO but more PD-1 and CD57. FoxP3-expressing CD4 and CD8 cells from untreated patients exhibited higher expression of HLA-DR, Ki-67, and PD-1 compared with non-HIV donors and treated patients.

CONCLUSIONS

FoxP3CD8 T cells are phenotypically distinct from FoxP3CD4 and FoxP3CD8 T cells. Expression of FoxP3 is associated with cellular activation in both CD4 and CD8 T cells.

Regulatory T cells in the control of host-microorganism interactions (*)

Each microenvironment requires a specific set of regulatory elements that are finely and constantly tuned to maintain local homeostasis. Various populations of regulatory T cells contribute to the maintenance of this equilibrium and establishment of controlled immune responses. In particular, regulatory T cells limit the magnitude of effector responses, which may result in failure to adequately control infection. However, regulatory T cells also help limit collateral tissue damage caused by vigorous antimicrobial immune responses against pathogenic microbes as well as commensals. In this review, we describe various situations in which the balance between regulatory T cells and effector immune functions influence the outcome of host-microorganism coexistence and discuss current hypotheses and points of polemic associated with the origin, target, and antigen specificity of both endogenous and induced regulatory T cells during these interactions.

Protection from graft-versus-host disease by HIV-1 envelope protein gp120-mediated activation of human CD4+CD25+ regulatory T cells

Naturally occurring CD4(+)CD25(+) regulatory T cells (Tregs) represent a unique T-cell lineage that is endowed with the ability to actively suppress immune responses. Therefore, approaches to modulate Treg function in vivo could provide ways to enhance or reduce immune responses and lead to novel therapies. Here we show that the CD4 binding human immunodeficiency virus-1 envelope glycoprotein gp120 is a useful and potent tool for functional activation of human Tregs in vitro and in vivo. Gp120 activates human Tregs by binding and signaling through CD4. Upon stimulation with gp120, human Tregs accumulate cyclic adenosine monophosphate (cAMP) in their cytosol. Inhibition of endogeneous cAMP synthesis prevents gp120-mediated Treg activation. Employing a xenogeneic graft versus host disease model that has been shown to be applicable for the functional analysis of human Tregs in vivo, we further show that a single dose of gp120 is sufficient to prevent lethal graft versus host disease and that the tolerizing effect of gp120 is strictly dependent on the presence of human Tregs and their up-regulation of cAMP upon gp120-mediated activation. Our findings demonstrate that stimulation via the CD4 receptor represents a T-cell receptor-independent Treg activating pathway with potential to induce immunologic tolerance in vivo.

Compartmentalization of immune responses in human tuberculosis: few CD8+ effector T cells but elevated levels of FoxP3+ regulatory t cells in the granulomatous lesions

Immune responses were assessed at the single-cell level in lymph nodes from children with tuberculous lymphadenitis. Tuberculosis infection was associated with tissue remodeling of lymph nodes as well as altered cellular composition. Granulomas were significantly enriched with CD68+ macrophages expressing the M. tuberculosis complex-specific protein antigen MPT64 and inducible nitric oxide synthase. There was a significant increase in CD8+ cytolytic T cells surrounding the granuloma; however, CD8+ T cells expressed low levels of the cytolytic and antimicrobial effector molecules perforin and granulysin in the granulomatous lesions. Quantitative real-time mRNA analysis revealed that interferon-gamma, tumor necrosis factor-alpha, and interleukin-17 were not up-regulated in infected lymph nodes, but there was a significant induction of both transforming growth factor-beta and interleukin-13. In addition, granulomas contained an increased number of CD4+FoxP3+ T cells co-expressing the immunoregulatory cytotoxic T-lymphocyte antigen-4 and glucocorticoid-induced tumor necrosis factor receptor molecules. Low numbers of CD8+ T cells in the lesions correlated with high levels of transforming growth factor-beta and FoxP3+ regulatory T cells, suggesting active immunosuppression at the local infection site. Compartmentalization and skewing of the immune response toward a regulatory phenotype may result in an uncoordinated effector T-cell response that reduces granule-mediated killing of M. tuberculosis-infected cells and subsequent disease control.

PD-1 blockade: A promising immunotherapy for HIV?

The progressive loss of effector function in the setting of chronic viral infections has been associated with the upregulation of programmed death 1 (PD-1), a negative regulator of activated T cells. In HIV infection, increased levels of PD-1 expression correlate with CD8(+) T cell exhaustion, which has been shown in vitro to be reversible with PD-1 blockade. Velu and colleagues recently reported the first in vivo study showing enhancement of a virus-specific immune response through PD-1 blockade using an anti-PD-1 antibody in an SIV-macaque model. Their results show an expansion of virus-specific, polyfunctional CD8(+) T cells. Anti-PD1 antagonists show promise as a novel immunotherapy for HIV. However, several issues including development of autoimmunity, regulatory T cells and multiple inhibitory receptors associated with CD8(+) T cell exhaustion should first be addressed to help ensure a successful response in chronic HIV infected patients.

New insights into the immunopathogenesis of chronic hepatitis C

Despite the high propensity of hepatitis C virus to establish chronic viral persistence, immune-mediated viral clearance occurs in some patients, fostering hopes that therapeutic induction of specific antiviral immune responses might be able to contribute to viral clearance in chronically infected patients. Indeed, recent clinical trials of therapeutic vaccination have provided clear proof of concept that specific immunotherapy can reduce the viral load in some patients. Further improvement of these strategies will depend on a detailed analysis of the immunopathogenesis of chronic hepatitis C. Recent advances in our understanding of the mechanisms of down-regulation of virus-specific immune responses during chronic infection, including the role of regulatory T cells and inhibitory molecules such as programmed death receptor 1, may open up new avenues for second-generation immunotherapeutic interventions.

HIV-1 binding to CD4 on CD4+CD25+ regulatory T cells enhances their suppressive function and induces them to home to, and accumulate in, peripheral and mucosal lymphoid tissues: an additional mechanism of immunosuppression

The establishment and persistence of many chronic infections have been demonstrated to depend on restraint of the vigor of the anti-microbial immune responses by CD4+CD25+ regulatory T (Treg) cells. In HIV-infected individuals, Treg cells suppress both HIV-specific and general CD4+ and CD8+ T cell responses. Increases of CD4+CD25+ Treg cell function during viral infections might be mediated by host-derived pro-inflammatory molecules or directly by viral infection or binding. We examined the effect HIV has upon binding to CD4+CD25+ Treg cells by exposing human purified CD4+CD25+ T cells from healthy donors to HIV-1 in vitro and assessing their Treg-associated functional marker profile and suppressive activities. We found that HIV-1 binding increased their suppressor activities by 2- to 5-fold, which was accompanied by enhanced expression of Treg-associated functional markers sCTLA-4, glucocorticoid-induced tumor necrosis factor receptor and FoxP3. Moreover, HIV-1 binding extended the survival of CD4+CD25+ Treg cells and up-regulated the expression of homing receptors CD62L and integrin alpha4beta7, which in turn would result in Treg cells migrating more rapidly to the peripheral lymph nodes and mucosal lymphoid tissues where anti-HIV immune responses are occurring. Importantly, CD4+CD25+ Treg cells exposed to HIV were not susceptible to homing-induced apoptosis like are other resting CD4+ cells following HIV-1 binding. We show that CD4+CD25+ Treg cells respond directly to HIV-1 itself through HIV gp120 interactions with CD4 molecules. Collectively, our findings explain a mechanism that contributes to the abnormal accumulation of intensified Treg cells in lymphoid and mucosal tissues in HIV patients, resulting in impairment of immune responses which would greatly help HIV persistence.

Dendritic cell-based human immunodeficiency virus vaccine

Dendritic cells (DC) have profound abilities to induce and coordinate T-cell immunity. This makes them ideal biological agents for use in immunotherapeutic strategies to augment T-cell immunity to HIV infection. Current clinical trials are administering DC-HIV antigen preparations carried out ex vivo as proof of principle that DC immunotherapy is safe and efficacious in HIV-infected patients. These trials are largely dependent on preclinical studies that will provide knowledge and guidance about the types of DC, form of HIV antigen, method of DC maturation, route of DC administration, measures of anti-HIV immune function and ultimately control of HIV replication. Additionally, promising immunotherapy approaches are being developed based on targeting of DC with HIV antigens in vivo. The objective is to define a safe and effective strategy for enhancing control of HIV infection in patients undergoing antiretroviral therapy.

Immune dysregulation in human immunodeficiency virus infection: know it, fix it, prevent it?

Infection of humans by the human immunodeficiency virus (HIV) causes a progressive, multifactorial impairment of the immune system eventually leading to the acquired immunodeficiency syndrome (AIDS). No cure or vaccine exists yet against HIV infection. More worrisome is the fact that despite having identified HIV as the cause of the AIDS, we still do not understand what pathogenic mechanisms lead to the debacle of the immune system. In this review we consider the extent and the limits of our knowledge of HIV pathogenesis, and how this knowledge may be used to design preventive and therapeutic approaches.

HIV-specific regulatory T cells are associated with higher CD4 cell counts in primary infection

OBJECTIVE

Expansion of regulatory T (Treg) cells has been described in chronically HIV-infected individuals. We investigated whether HIV-suppressive Treg could be detected during primary HIV infection (PHI).

METHODS

Seventeen patients diagnosed early after PHI (median: 13 days; 1-55) were studied. Median CD4 cell count was 480 cells/microl (33-1306) and plasma HIV RNA levels ranged between 3.3 and 5.7 log10 copies/ml. Suppressive capacity of blood purified CD4CD25 was evaluated in a coculture assay. Fox-p3, IL-2 and IL-10 were quantified by reverse transcriptase (RT)-PCR and intracellular staining of ex vivo and activated CD4+CD25 T cells.

RESULTS

The frequency of CD4CD127CD25 T cells among CD4 T cells was lower in patients with PHI compared with chronic patients (n = 19). They exhibited a phenotype of memory T cells and expressed constitutively FoxP3. Similar to chronic patients, Treg from patients with PHI inhibited the proliferation of purified tuberculin (PPD) and HIV p24 activated CD4CD25 T cells. CD4CD25 T cells from patients with PHI responded specifically to p24 stimulation by expressing IL-10. In untreated patients with PHI, the frequency as well as HIV-specific activity of Treg decreased during a 24-month follow-up. A positive correlation between percentages of Treg and both CD4 cell counts and the magnitude of p24-specific suppressive activity at diagnosis of PHI was found.

CONCLUSION

Our data showed that HIV drives Treg, as PHI and these cells persist throughout the course of the infection. A correlation between the frequency of Treg and CD4 T-cell counts suggest that these cells may impact on the immune activation set point at PHI diagnosis.

HIV-activated human plasmacytoid DCs induce Tregs through an indoleamine 2,3-dioxygenase-dependent mechanism

Plasmacytoid DCs (pDCs) have been implicated as crucial cells in antiviral immune responses. On recognizing HIV, they become activated, secreting large amounts of IFN-alpha and inflammatory cytokines, thereby potentiating innate and adaptive antiviral immune responses. Here, we have shown that HIV-stimulated human pDCs can also induce the differentiation of naive CD4+ T cells into Tregs with suppressive function. This differentiation was independent of pDC production of IFN-alpha and primarily dependent on pDC expression of indoleamine 2,3-dioxygenase, which was induced through the TLR/MyD88 pathway, following binding of HIV to CD4 and triggering of TLR7 by HIV genomic RNA. Functionally, the Tregs induced by pDCs were shown to inhibit the maturation of bystander conventional DCs. This study therefore reveals what we believe to be a novel mechanism by which pDC may regulate and potentially limit anti-HIV immune responses.

Regulatory T cells and HIV-1 infection

Recently, it has been emphasized that chronic generalized immune activation is a leading event in the pathogenesis of HIV-1 infection. Supporting evidence comes from observations that in cases of lack of activation, infected subjects maintain a high number of T cells and do not develop AIDS-related events. Despite intensive studies, the exact mechanisms of T-cell activation are still not well understood and options for their control are limited. Very promising in this direction is a recently described T-cell subpopulation--regulatory T cells. Their functional activity and vitality are strongly dependent on the presence of IL-2. Better understanding of the mechanisms of T-cell activation, as well as the contribution of regulatory T cells to its control will increase therapeutic options for HIV-1-infected subjects. The application of immune-based therapy together with highly active antiretroviral therapy will lend a helping hand to the natural regulatory mechanisms in the control of infection.

FOXP3 inhibits HIV-1 infection of CD4 T-cells via inhibition of LTR transcriptional activity

FOXP3 is a necessary transcription factor for the development and function of CD4+ regulatory T-cells (Tregs). The role of Tregs in HIV-1 infection remains unclear. Here, we show that expression of FOXP3 in primary human CD4 T-cells significantly inhibits HIV-1 infection. Since FOXP3 inhibits NFAT activity, and NFAT proteins contribute to HIV-1 transcription, we explore a transcriptional repressive function of HIV-1 LTR by FOXP3. Over-expression of FOXP3 in primary CD4 T-cells inhibits wild-type HIV-1 LTR reporter activity, and truncation mutants demonstrate that repression of the LTR by FOXP3 requires the dual proximal NF kappaB/NFAT binding sites. Interestingly, FOXP3 decreases binding of NFAT2 to the HIV-1 LTR in vivo. Furthermore, FOXP3 does not inhibit infection of HIV-1 NL4-3 which is mutated to disrupt transcription factor binding at either proximal NFAT or NF kappaB binding sites. These data suggest that resistance of Tregs to HIV-1 infection is due to inhibition of HIV-1 LTR transcription by FOXP3.

Resting regulatory CD4 T cells: a site of HIV persistence in patients on long-term effective antiretroviral therapy

Background

In HIV-infected patients on long-term HAART, virus persistence in resting long-lived CD4 T cells is a major barrier to curing the infection. Cell quiescence, by favouring HIV latency, reduces the risk of recognition and cell destruction by cytotoxic lymphocytes. Several cell-activation-based approaches have been proposed to disrupt cell quiescence and then virus latency, but these approaches have not eradicated the virus. CD4⁺CD25⁺ regulatory T cells (Tregs) are a CD4⁺ T-cell subset with particular activation properties. We investigated the role of these cells in virus persistence in patients on long-term HAART.

Methodology/Principal Findings

We found evidence of infection of resting Tregs (HLADR⁻CD69⁻CD25^hiFoxP3⁺CD4⁺ T cells) purified from patients on prolonged HAART. HIV DNA harbouring cells appear more abundant in the Treg subset than in non-Tregs. The half-life of the Treg reservoir was estimated at 20 months. Since Tregs from patients on prolonged HAART showed hyporesponsiveness to cell activation and inhibition of HIV-specific cytotoxic T lymphocyte-related functions upon activation, therapeutics targeting cell quiescence to induce virus expression may not be appropriate for purging the Treg reservoir.

Conclusions

Our results identify Tregs as a particular compartment within the latent reservoir that may require a specific approach for its purging.

HIV immunopathogenesis and strategies for intervention

Therapeutic options aimed at tackling the HIV pandemic face many obstacles. The lack of readily accessible and affordable therapies means that most of those affected go untreated. The array of escape mechanisms used by HIV has undermined the efficiency of many antiviral products and continually represents a barrier to the development of an effective vaccine. Recent developments have seen a shift away from a cytopathic viral model of HIV pathogenesis towards the crucial role of immunopathogenic features--notably generalised immune activation--in the development of AIDS. As conventional vaccine strategies have sought to promote viral neutralisation and suppressive cellular responses, novel strategies that aim to address HIV immunopathogenesis should be sought. We review current opinion on HIV-induced pathogenic immune activation and strategies aimed at eliminating HIV, including a potential role for non-neutralising antibodies as part of a therapeutic vaccine option.

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.

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.

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.

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.

HLA homology within the C5 domain promotes peptide binding by HIV type 1 gp120

The mechanisms by which HIV-1 induces chronic pathogenic immune activation associated with disease progression remain unclear despite many years of AIDS research. One proposal suggests that sequence and structural mimicry between gp120 and HLA may endow HIV with the capacity to arouse alloreactive and autoimmune responses within the susceptible host, fueling disease progression in a manner similar to graft-versus-host disease (GVHD). Both gp120 and HLA share a common functional interaction with CD4 but also demonstrate peptide binding properties. Here we report the conserved nature of this feature across HIV-1 envelopes, the crucial role of the HLA homologous C5 region for peptide interactions, and the elimination of this property through specific antibody targeting. Given that the C5 domain mimics a HLA activation domain and the reported clinical benefits associated with nonneutralizing antibodies against this region, targeting the C5 domain may have use as a therapeutic vaccine to protect against disease progression.

The double-edged sword of statin immunomodulation

Statin drugs are widely prescribed to achieve aggressive low-density lipoprotein lowering in order to decrease cardiovascular disease. Although some of the immunomodulatory effects of statins may stabilize atherosclerotic plaque, they may be harmful in certain segments of the population. Recently, statins have been shown to increase the concentration of regulatory T cells (Tregs), in vivo. There is evidence that this increases the risk of many cancers, particularly in the elderly. Furthermore, a statin induced increase in Tregs may be detrimental in neurodegenerative disorders, such as amyotrophic lateral sclerosis; and a myriad of infectious diseases. These include, but are not limited to, human immunodeficiency virus, hepatitis B virus, hepatitis C virus, and varicella zoster virus. These issues need our attention, and call for a heightened state of vigilance among those prescribing statins.

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.

Intracellular protease activation in apoptosis and cell-mediated cytotoxicity characterized by cell-permeable fluorogenic protease substrates

Over the past decade the importance of signaling from reporter molecules inside live cells and tissues has been clearly established. Biochemical events related to inflammation, tumor metastasis and proliferation, and viral infectivity and replication are examples of processes being further defined as more molecular tools for live cell measurements become available. Moreover, in addition to quantitating parameters related to physiologic processes, real-time imaging of molecular interactions that compose basic cellular activities are providing insights into understanding disease mechanisms as well as extending clinical efficacy of therapeutic regimens. In this review the use of highly cell-permeable fluorogenic substrates that report protease activities inside live cells is described; applications to defining the molecular events of two cellular processes, i.e., apoptosis and cell-mediated cytotoxicity, are then illustrated.

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.

CD8 T cell dysfunction during chronic viral infection

Clearance of primary infection often leads to the development of highly functional memory T cells capable of rapid and long-lasting protective immunity. By contrast, chronic infections can result in T cell dysfunction and poor pathogen control. In this review, we will discuss recent work that highlights two main types of T cell dysfunction during chronic infection: exhaustion of effector functions and altered memory T cell development.

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.