FoxP3 enhances HIV-1 gene expression by modulating NFkappaB occupancy at the long terminal repeat in human T cells

Characterizing the Latent HIV-1 Reservoir in Patients with Viremia Suppressed on cART: Progress, Challenges, and Opportunities

Modern combination antiretroviral therapy (cART) can bring HIV-1 in blood plasma to level undetectable by standard tests, prevent the onset of acquired immune deficiency syndrome (AIDS), and allow a near-normal life expectancy for HIV-infected individuals. Unfortunately, cART is not curative, as within a few weeks of treatment cessation, HIV viremia in most patients rebounds to pre-cART levels. The primary source of this rebound, and the principal barrier to a cure, is the highly stable reservoir of latent yet replication-competent HIV-1 proviruses integrated into the genomic DNA of resting memory CD4+ T cells. In this review, prevailing models for how the latent reservoir is established and maintained, residual viremia and viremic rebound upon withdrawal of cART, and the types and characteristics of cells harboring latent HIV-1 will be discussed. Selected technologies currently being used to advance our understanding of HIV latency will also be presented, as will a perspective on which areas of advancement are most essential for producing the next generation of HIV-1 therapeutics.

Schistosoma mansoni soluble egg antigen (SEA) and recombinant Omega-1 modulate induced CD4+ T-lymphocyte responses and HIV-1 infection in vitro

Parasitic helminths evade, skew and dampen human immune responses through numerous mechanisms. Such effects will likely have consequences for HIV-1 transmission and disease progression. Here we analyzed the effects that soluble egg antigen (SEA) from Schistosoma mansoni had on modulating HIV-1 infection and cytokine/chemokine production in vitro. We determined that SEA, specifically through kappa-5, can potently bind to DC-SIGN and thereby blocks DC-SIGN mediated HIV-1 trans-infection (p<0.05) whilst not interfering with cis-infection. DCs exposed to SEA whilst maturing under Th2 promoting conditions, will upon co-culture with naïve T-cells induce a T-cell population that was less susceptible to HIV-1 R5 infection (p<0.05) compared to DCs unexposed to SEA, whereas HIV-1 X4 virus infection was unaffected. This was not observed for DCs exposed to SEA while maturing under Th1 or Th1/Th2 (T_mix) promoting conditions. All T-cell populations induced by SEA exposed DCs demonstrate a reduced capacity to produce IFN-γ and MIP-1β. The infection profile of T-cells infected with HIV-1 R5 was not associated with down-modulation of CCR5 cell surface expression. We further show that DCs maturing under T_mix conditions exposed to plant recombinant omega-1 protein (rω-1), which demonstrates similar functions to natural ω-1, induced T-cell populations that were less sensitive for HIV-1 R5 infection (p<0.05), but not for X4 virus infection. This inhibition associated again with a reduction in IFN-γ and MIP-1β expression, but additionally correlated with reduced CCR5 expression. We have shown that SEA parasite antigens and more specifically rω-1 can modulate HIV-1 infectivity with the potential to influence disease course in co-infected individuals.

Parasitic helminths have developed a number of strategies to evade, skew and dampen human immune responses. Such effects will likely have consequences for HIV-1 transmission and disease progression. Here we analyzed the effect that soluble egg antigen (SEA) from Schistosoma mansoni had on HIV-1 infection in vitro. We determined that SEA, through kappa-5, can potently block DC-SIGN mediated HIV-1 trans-infection of CD4⁺ T-lymphocytes, but not block cis-infection. Dendritic cells (DC) exposed to SEA during maturation under Th2 skewing conditions, induce T-cell populations that are less susceptible to HIV-1 R5 infection compared to cells induced by unexposed DCs. HIV-1 X4 infection was unaffected. This restricted infection profile was not associated with down-modulation of CCR5 surface expression or observed differences in cytokine/chemokine production. Using recombinant omega-1, an abundant component of SEA, HIV-1 R5 infection was similarly inhibited with no effect on HIV-1 X4 infection levels. Hence SEA possesses antigens, namely omega-1, that can modulate HIV-1 infection and potentially influence disease course in co-infected individuals.

CD4+ T Cell Subsets and Pathways to HIV Latency

Latent infection of CD4⁺ T cells is the main barrier to eradicating HIV-1 infection from infected patients. The cellular and molecular mechanisms involved in the establishment and maintenance of latent infection are directly linked to the transcriptional program of the different CD4⁺ T cell subsets targeted by the virus. In this review, we provide an overview of how T cell activation, T cell differentiation into functional subsets, and the mode of initial viral infection influence HIV proviral transcription and entry into latency.

Regulatory T Cells Contribute to HIV-1 Reservoir Persistence in CD4+ T Cells Through Cyclic Adenosine Monophosphate-Dependent Mechanisms in Humanized Mice In Vivo

Background

Regulatory T cells (Tregs) suppress T-cell immune activation and human immunodeficiency virus type 1 (HIV-1) replication, but the role of Tregs in HIV-1 reservoir persistence is poorly defined.

Methods

Tregs were depleted by denileukin diftitox in humanized mice with chronic HIV-1 infection. Viral replication in lineage cells was determined by p24 expression. Levels of HIV-1 RNA and DNA in human cells, as well as replication-competent-virus-producing cells, were measured to quantified viral replication and reservoirs.

Results

Treg depletion resulted in a blip of HIV-1 replication in T cells but not in myeloid cells. The major activated reservoir cells were memory CD4+ T cells in vivo. Interestingly, the transient activation of viral replication led to HIV-1 reservoir reduction after viremia resuppression, as indicated by the quantity of HIV-1 DNA and replication-competent-virus-producing cells. Furthermore, we demonstrated that Tregs use cyclic adenosine monophosphate (cAMP)-dependent protein kinase A pathway to inhibit HIV-1 activation and replication in resting conventional T cells in vitro.

Conclusion

Tregs suppress HIV-1 replication in T cells and contribute to HIV-1 reservoir persistence. cAMP produced in Tregs is involved in their suppression of viral gene activation and expression. Treg depletion combined with combination antiretroviral therapy provides a novel strategy for HIV-1 cure.

Diversity of HIV-1 reservoirs in CD4+ T-cell subpopulations

PURPOSE OF REVIEW

HIV-1 is able to create lasting reservoirs of virally infected cells that persist life-long and are extremely difficult to eradicate, thus necessitating indefinite antiretroviral therapy. Large numbers of studies suggest that CD4 T cells represent the major, and possibly the only cell type supporting HIV-1 long-term persistence. However, the ability to serve as long-term viral reservoirs may be confined to certain subpopulations of CD4 T cells with specific functional and developmental characteristics that HIV-1 can selectively exploit to propagate long-term viral survival within the host. Identification of CD4 T-cell subtypes that serve as hotspots for viral persistence may be critical for designing strategies to purge the immune system of persisting viral reservoirs.

RECENT FINDINGS

Developmentally immature, long-lasting CD4 memory T-cell populations seem to contain the majority of latently HIV-1-infected cells that persist despite antiretroviral therapy in the peripheral blood. Emerging data suggest that functional polarization toward a T helper 17 (Th17), a T follicular helper cell or a regulatory T-cell lineage may also be associated with an increased ability to serve as a viral reservoir site. Atypical T cells such a γδ CD4 T cells or tissue-resident memory CD4 T cells may be predestined to serve as sites for HIV-1 persistence in specific tissues, but will require additional exploration in future studies.

SUMMARY

Recent advances have increased awareness for the profound diversity and complexity of CD4 T-cell subpopulations serving as sites for HIV-1 persistence. Continuous technological and methodological improvements to interrogate viral reservoirs in distinct CD4 T-cell subpopulations may allow to define a more complete landscape of the HIV-1 reservoir composition in different T-cell subpopulations.

Expansion and productive HIV-1 infection of Foxp3 positive CD4 T cells at pleural sites of HIV/TB co-infection

BACKGROUND

CD4 T-cells expressing Foxp3 are expanded systemically during active tuberculosis (TB) regardless of HIV-1 co-infection. Foxp3⁺ CD4 T cells are targets of HIV-1 infection. However, expansion of HIV-1 infected Foxp3⁺ CD4 T cells at sites of HIV/TB co-infection, and whether they contribute to promotion of HIV-1 viral activity is not known.

METHODS

Pleural fluid mononuclear cells (PFMC) from HIV/TB co-infected patients with pleural TB were characterized by immune-staining and FACS analysis for surface markers CD4, CD127, CCR5, CXCR4, HLA-DR and intracellular expression of Foxp3, HIVp24, IFN-γ and Bcl-2. Whole PFMC and bead separated CD4⁺CD25⁺CD127^- T cells were assessed for HIV-1 LTR strong stop (SS) DNA by real-time PCR, which represents viral DNA post cell entry and initiation of reverse transcription.

RESULTS

High numbers of HIV-1 p24 positive Foxp3⁺ and Foxp3⁺CD127^- CD4 T cells were identified in PFMC from HIV/TB co-infected subjects. CD4⁺Foxp3⁺CD127^- T cells displayed high expression of the cellular activation marker, HLA-DR. Further, expression of the HIV-1 co-receptors, CCR5 and CXCR4, were higher on CD4⁺Foxp3⁺T cells compared to CD4⁺Foxp3^- T cells. Purified CD4⁺CD25⁺CD127^- T cells isolated from PFMC of HIV/TB co-infected patients, were over 90% CD4⁺Foxp3⁺T cells, and exhibited higher HIV-1 SS DNA as compared to whole PFMC, and as compared to CD4⁺CD25⁺CD127^- T cells from an HIV-infected subject with pleural mesothelioma. HIV-1 p24⁺ Foxp3⁺ CD4⁺T cells from HIV/TB patients higher in Bcl-2 expression as compared to both HIV-1 p24⁺ Foxp3^- CD4 T cells, and Foxp3⁺ CD4⁺T cells without HIV-p24 expression.

CONCLUSION

Foxp3⁺ CD4 T cells in PFMC from HIV/TB co-infected subjects are predisposed to productive HIV-1 infection and have survival advantage as compared to Foxp3 negative CD4 T cells.

Th1/17 Polarization of CD4 T Cells Supports HIV-1 Persistence during Antiretroviral Therapy

The ability to persist long term in latently infected CD4 T cells represents a characteristic feature of HIV-1 infection and the predominant barrier to efforts aiming at viral eradication and cure. Yet, increasing evidence suggests that only small subsets of CD4 T cells with specific developmental and maturational profiles are able to effectively support HIV-1 long-term persistence. Here, we analyzed how the functional polarization of CD4 T cells shapes and structures the reservoirs of HIV-1-infected cells. We found that CD4 T cells enriched for a Th1/17 polarization had elevated susceptibilities to HIV-1 infection in ex vivo assays, harbored high levels of HIV-1 DNA in persons treated with antiretroviral therapy, and made a disproportionately increased contribution to the viral reservoir relative to their contribution to the CD4 T memory cell pool. Moreover, HIV-1 DNA levels in Th1/17 cells remained stable over many years of antiretroviral therapy, resulting in a progressively increasing contribution of these cells to the viral reservoir, and phylogenetic studies suggested preferential long-term persistence of identical viral sequences during prolonged antiretroviral treatment in this cell compartment. Together, these data suggest that Th1/17 CD4 T cells represent a preferred site for HIV-1 DNA long-term persistence in patients receiving antiretroviral therapy.

IMPORTANCE

Current antiretroviral therapy is very effective in suppressing active HIV-1 replication but does not fully eliminate virally infected cells. The ability of HIV-1 to persist long term despite suppressive antiretroviral combination therapy represents a perplexing aspect of HIV-1 disease pathogenesis, since most HIV-1 target cells are activated, short-lived CD4 T cells. This study suggests that CD4 T helper cells with Th1/17 polarization have a preferential role as a long-term reservoir for HIV-1 infection during antiretroviral therapy, possibly because these cells may imitate some of the functional properties traditionally attributed to stem cells, such as the ability to persist for extremely long periods of time and to repopulate their own pool size through homeostatic self-renewal. These observations support the hypothesis that HIV-1 persistence is driven by small subsets of long-lasting stem cell-like CD4 T cells that may represent particularly promising targets for clinical strategies aiming at HIV-1 eradication and cure.

CD4+CD25+CD127 regulatory cells play multiple roles in maintaining HIV-1 p24 production in patients on long-term treatment: HIV-1 p24-producing cells and suppression of anti-HIV immunity

BACKGROUND

A major question when attempting to eradicate and treat HIV-1 infection is how to reactivate latent proviruses. Stimulating HIV-1-specific cytolytic T lymphocytes (CTL) has been shown to facilitate the elimination of the latent viral reservoir after viral reactivation. Regulatory T (Treg) cells are known to be capable of lowering both HIV-specific immunoreactions and general immune activation during HIV-1 infection. It was hypothesized that the depletion of Treg cells could increase the HIV-1-specific cytolytic T lymphocyte response and reactivate HIV-1 p24 production.

METHODS

Treg cells were isolated by isolation kit according to the surface marker of Treg cells. Real-time PCR method was used to quantify HIV-1 DNA. P24 antigens in the cell culture supernatant was done by ELISA. Cells activation and HIV specific HIV-1 CD8+ T cells were analyses using a FACSCalibur flow cytometer and CELLQUEST software.

RESULTS

This study included both HIV-infected patients who were antiviral treatment-naïve and patients with sustained viral responses to antiretroviral therapy (ART) for 1 or 5 years. It was found that the HIV-DNA levels in Treg cells were approximately 10-fold higher than those in non-Treg CD4+ cells and that the depletion of Treg cells could enhance the frequency of HIV-1-specific CTL and immune activation after 5 years of effective ART.

CONCLUSIONS

CD4+CD25+CD127 regulatory cells play multiple roles in maintaining HIV-1 p24 production in long-term ART patients. Treg cells may be a target for eliminating the latent HIV reservoir after effective long-term ART.

Pim-2 Kinase Influences Regulatory T Cell Function and Stability by Mediating Foxp3 Protein N-terminal Phosphorylation

Regulation of the extent of immune responses is a requirement to maintain self-tolerance and limit inflammatory processes. CD4(+)CD25(+)Foxp3(+) regulatory T (Treg) cells play a role in regulation. The Foxp3 transcription factor is considered a dominant regulator for Treg cell development and function. Foxp3 function itself is directly regulated by multiple posttranslational modifications that occur in response to various external stimuli. The Foxp3 protein is a component of several dynamic macromolecular regulatory complexes. The complexes change constituents over time and through different signals to regulate the development and function of regulatory T cells. Here we identified a mechanism regulating Foxp3 level and activity that operates through discrete phosphorylation. The Pim-2 kinase can phosphorylate Foxp3, leading to decreased suppressive functions of Treg cells. The amino-terminal domain of Foxp3 is modified at several sites by Pim-2 kinase. This modification leads to altered expression of proteins related to Treg cell functions and increased Treg cell lineage stability. Treg cell suppressive function can be up-regulated by either pharmacologically inhibiting Pim-2 kinase activity or by genetically knocking out Pim-2 in rodent Treg cells. Deficiency of Pim-2 activity increases murine host resistance to dextran sodium sulfate-induced colitis in vivo, and a Pim-2 small molecule kinase inhibitor also modified Treg cell functions. Our studies define a pathway for limiting the regulation of Foxp3 function because the Pim-2 kinase represents a potential therapeutic target for modulating the Treg cell suppressive activities in controlling immune responses.

Sex-based differences in HIV type 1 pathogenesis

Human immunodeficiency virus (HIV) remains a global infectious diseases threat that disproportionally affects women. Beyond social and political factors, biological and genetic differences have been identified that lead to differential disease courses and outcomes in men and women. Following HIV type 1 (HIV-1) seroconversion, women have up to 40% lower HIV loads and higher CD4(+) T-cell counts than men. However, at the same level of viremia, progression to AIDS is faster in women. After adjustment for viral load, HIV-positive women also display increased levels of generalized immune activation and experience the consequences of elevated inflammatory activity more frequently than men. Part of these observations are linked to sex-based differences in innate immunity, in which the differential ability of plasmacytoid dendritic cells to produce interferon α following stimulation of Toll-like receptor 7 and upregulation of interferon-stimulated genes play a central role. Here, we review the current knowledge and remaining gaps therein regarding sex-based differences in HIV-1 pathogenesis.

Epigenetic modification of FOXP3 in patients with chronic HIV infection

OBJECTIVES

HIV-1 modulates host cell epigenetic machinery to control its own replication and induce immune suppression. HIV-1 infection leads to activation of T regulatory cell (T(reg)), but the mechanism underlying this immune modulation is unclear. T(reg) plays a prominent role in gut-mucosal immune tolerance by restraining excessive effector T-cell responses, a mechanism that is known to be disturbed in chronic HIV-1 infection. DNA methylation plays a major role in T(reg) lineage commitment and immune homeostasis, which may be regulated by HIV. To investigate the mechanisms of aberrant methylation of the T(reg) marker FOXP3 in HIV-1 infection, we evaluated the expression pattern of methylation-related enzymes and its correlation to FOXP3 methylation.

METHODS

FOXP3 promoter methylation in the colon mucosa and peripheral blood from HIV-infected patients and control subjects was measured using Pyrosequencing. Gene expression pattern of DNA methylation enzymes in the colon mucosa was investigated by Microarray and quantitative reverse transcriptase-polymerase chain reaction analysis in the same subjects.

RESULTS

FOXP3 promoter was significantly (P ≤ 0.0001) demethylated in HIV-infected patients compared with control subjects in both tissues. Expression of DNA methyltransferase 1 (DNAMT1), DNA methyltransferase 1-associated protein 1(DMAP1), methyltransferase-like 7B (METTL7B), and methyltransferase-like 10 (METTL10) were significantly down regulated in HIV-infected patients compared with controls and had a significant positive correlation to FOXP3 promoter methylation.

CONCLUSIONS

We present evidence suggesting that altered methylation pattern of FOXP3 and accordingly higher T(reg) frequency in gut mucosa of HIV-infected patients may be because of aberrant methylation processing in HIV.

Immune quiescence: a model of protection against HIV infection

Aberrant immune activation is a strong correlate of HIV disease progression, but little is known about how immune activation alters susceptibility to HIV infection. Susceptibility to HIV infection varies between individuals, but the immunological determinants of HIV transmission are not well understood. Here, we present evidence from studies of HIV transmission in the context of clinical trials and HIV-exposed seronegative (HESN) cohorts that implicates elevated immune activation as a risk factor for acquiring HIV. We propose a model of protection from infection based on a phenotype of low baseline immune activation referred to as immune quiescence. Immune quiescence is evidenced by reduced expression of T cell activation markers, low levels of generalized gene transcription and low levels of proinflammatory cytokine and chemokine production in the periphery and genital mucosa of HESN. Since HIV preferentially replicates in activated CD4+ T cells, immune quiescence may protect against infection by limiting HIV target cell availability. Although the determinants of immune quiescence are unclear, several potential factors have been identified that may be involved in driving this phenotype. HESN were shown to have elevated proportions of regulatory T cells (Tregs), which are known to suppress T cell activation. Likewise, proteins involved in controlling inflammation in the genital tract have been found to be elevated in HESN. Furthermore, expression of interferon regulatory factor 1 (IRF-1) is reduced in HESN as a consequence of genetic polymorphisms and differential epigenetic regulation. Since IRF-1 is an important regulator of immune responses, it may play a role in maintaining immune quiescence. Based on this model, we propose a novel avenue for HIV prevention targeted based on reducing host mucosal immune activation.

T Cell Transcription Factors and Their Impact on HIV Expression

By targeting CD4⁺ effector T cells, HIV has a dramatic impact on the depletion, expansion and function of the different polarized T cell subsets. The maturation of T cell lineages is in part driven by intrinsic transcription factors that potentially influence how efficiently HIV replicates. In this review, we explore whether transcription factors that are required for polarizing T cells influence HIV replication. In particular, we examine provirus transcription as well as the establishment and maintenance of HIV latency. Furthermore, it is suggested these factors may provide novel cell-specific therapeutic strategies for targeting the HIV latent reservoir.

Forkhead transcription factor FOXP3 upregulates CD25 expression through cooperation with RelA/NF-κB

Considerable evidence supports the prediction that CD25 is directly regulated by the forkhead transcription factor FOXP3. However, given that CD25 is normally upregulated in activated T cells, regardless of whether they express FOXP3, this issue has still to be definitively demonstrated. Here we describe that FOXP3, induced by CD28 signals in human CD4⁺CD25⁻ T lymphocytes, synergizes with RelA on a regulatory region of Cd25 promoter to mediate the transcriptional activation of Cd25 gene. We found that a striking feature of this regulatory region is the presence of a κB site and of two tandem copies of a non-consensus FOXP3 binding site separated at 5′ ends by 19 nucleotides that allow FOXP3 and RelA binding to DNA and their physical interaction. The occupancy of the two FOXP3 binding sites in conjunction with RelA binding site occupancy allows FOXP3 to function as a positive activator of Cd25 gene. Indeed mutations of both FOXP3 binding sites such as mutation of κB site on Cd25 promoter abolished FOXP3 activatory functions. Moreover, FOXP3 mutation ΔE251, that compromises FOXP3 homotypic interactions, failed to trans activate Cd25 promoter, suggesting that both FOXP3 DNA binding and dimerization are required to trans activate Cd25 promoter. These findings identify a novel mechanism by which RelA and FOXP3 cooperate to mediate transcriptional regulation of target genes and characterize a region on Cd25 promoter where FOXP3 dimer could bridge intramolecularly two DNA sites and trans activate Cd25 gene.

The split personality of regulatory T cells in HIV infection

Natural regulatory T cells (Tregs) participate in responses to various chronic infections including HIV. HIV infection is associated with a progressive CD4 lymphopenia and defective HIV-specific CD8 responses known to play a key role in the control of viral replication. Persistent immune activation is a hallmark of HIV infection and is involved in disease progression independent of viral load. The consequences of Treg expansion, observed in HIV infection, could be either beneficial, by suppressing generalized T-cell activation, or detrimental, by weakening HIV-specific responses and thus contributing to viral persistence. The resulting balance between Tregs contrasting outcomes might have critical implications in pathogenesis. Topics covered in this review include HIV-induced alterations of Tregs, Treg cell dynamics in blood and tissues, Treg-suppressive function, and the relationship between Tregs and immune activation. This review also provides a focus on the role of CD39(+) Tregs and other regulatory cell subsets. All these issues will be explored in different situations including acute and chronic infection, antiretroviral treatment-mediated viral control, and spontaneous viral control. Results must be interpreted with regard to both the Treg definition used in context and to the setting of the disease in an attempt to draw clearer conclusions from the apparently conflicting results.

Molecular and biological role of the FOXP3 N-terminal domain in immune regulation by T regulatory/suppressor cells

Regulatory T (Treg) cells are essential in preventing the host from developing certain autoimmune diseases and limiting excessive immune responses against pathogens. The normal function of most Treg cells requires sustained expression of functional FOXP3, a member of the FOXP family transcription factors. FOXP3 is distinct from other subfamily members because of its unique proline rich amino (N)-terminal domain. Mutations in this region are occasionally identified in certain patients with X-linked autoimmunity-allergic dysregulation syndrome (XLAAD) and similar mutations also increase susceptibility of autoimmune diseases in rodent models. Previous analyses of the FOXP3 N-terminal domain revealed a role in nuclear import, interaction with other transcription factors, and as sites of specific post-translational modifications of FOXP3 that contribute to FOXP3 stability.

Homeostasis and function of regulatory T cells in HIV/SIV infection

Regulatory T cells (Tregs) play a pivotal role in the maintenance of tolerance as well as in the control of immune activation, particularly during chronic infections. In the setting of HIV infection, the majority of studies have reported an increase in Treg frequency but a decrease in absolute number in all immune compartments of HIV-infected individuals. Several nonexclusive mechanisms have been postulated to explain this preferential Treg accumulation, including peripheral survival, increased proliferation, increased peripheral conversion, and tissue redistribution. The role played by Tregs during HIV infection is still poorly understood, as two opposing hypotheses have been proposed. A detrimental role of Tregs during HIV infection was suggested based on the evidence that Tregs suppress virus-specific immune responses. Conversely, Tregs could be beneficial by limiting immune activation, thus controlling the availability of HIV targets as well as preventing immune-based pathologies. Despite the technical difficulties, getting a better understanding of the mechanisms regulating Treg dynamics remains important, as it will help determine whether we can successfully manipulate Treg function or number to the advantage of the infected host. The aim of this review is thus to discuss the recent findings on Treg homeostasis and function in the setting of HIV infection.

Altered phenotype of regulatory T cells associated with lack of human immunodeficiency virus (HIV)-1-specific suppressive function

Mechanisms by which CD4+ regulatory T cells (T(regs)) mediate suppression of virus-specific responses remain poorly defined. Adenosine, mediated via CD39 and CD73, has been shown to play a role in the action of murine T(regs) . In this study we investigate the phenotype of T(regs) in the context of human immunodeficiency virus (HIV)-1 infection, and the function of these cells in response to HIV-1-Gag and cytomegalovirus (CMV) peptides. Phenotypic data demonstrate a decrease in forkhead box transcription factor 3 (FoxP3+) T(reg) numbers in the peripheral blood of HIV-1+ individuals compared to healthy controls, which is most pronounced in those with high HIV-1 RNA plasma load. Due to aberrant expression of CD27 and CD127 during HIV-1 disease, these markers are unreliable for T(reg) identification. The CD3+ CD4+ CD25(hi) CD45RO+ phenotype correlated well with FoxP3 expression in both the HIV-1+ and seronegative control cohorts. We observed expression of CD39 but not CD73 on T(regs) from HIV-1+ and healthy control cohorts. We demonstrate, through T(reg) depletion, the suppressive potential of T(regs) over anti-CMV responses in the context of HIV-1 infection; however, no recovery of the HIV-1-specific T cell response was observed indicating a preferential loss of HIV-1-specific T(reg) function. We propose that before immunotherapeutic manipulation of T(regs) is considered, the immunoregulatory profile and distribution kinetics of this population in chronic HIV-1 infection must be elucidated fully.

Foxp3 inhibits HDAC1 activity to modulate gene expression in human T cells

We have previously reported that HIV-1 preferentially infects Foxp3+ Treg cells in vitro and in vivo, and Foxp3 enhances the HIV-1 LTR expression through epigenetic mechanisms in T cells. We report here that histone deacetylase inhibitor (HDACi) failed to further enhance HIV gene expression in FoxP3+ T cells. We discovered that Foxp3 inhibited cellular HDAC activity in T cells, and mutations in the forkhead domain that ablate Foxp3 function also abolished its ability to inhibit HDAC. When co-expressed, Foxp3 specifically inhibited the deacetylase activity of HDAC1. We further showed that Foxp3 was associated with HDAC1, and mutations in the forkhead domain that ablate Foxp3 function in Treg cells also inhibited Foxp3 association with and inhibition of HDAC1. Finally, Foxp3 failed to enhance HIV-1 gene expression in human T cells expressing HDAC1-specific shRNA. We conclude that Foxp3 modulates gene expression in human T cells at least partly by inhibiting HDAC1 activity.

FoxP3 interacts with linker histone H1.5 to modulate gene expression and program Treg cell activity

The forkhead box transcription factor FoxP3 controls the development and function of CD4+CD25+ regulatory T (Treg) cell. FoxP3 modulates gene expression in Treg cells by multiple epigenetic mechanisms that are not clearly defined. We identified FoxP3-interacting proteins in human T cells by co-immunoprecipitation/MS. We discovered that FoxP3 interacted with linker histone H1.5 via the leucine zipper (LZ) domain. Two independent IPEX patient-derived single residue mutations in the LZ of FoxP3 both abrogated its interaction with H1.5. Functionally, FoxP3 and H1.5 cooperatively repressed interleukin-2 (IL-2) expression in human T cells; and silencing of H1.5 expression inhibited the ability of FoxP3 to suppress IL-2 expression. We show that FoxP3 specifically enhanced H1.5 association at the IL-2 promoter, but reduce its association at the CTLA4 promoter, correlated with higher or lower histone acetylation of the respective promoters. Finally, silencing of H1.5 expression in human Treg cells impaired the Treg function to suppress target T cells. We conclude that FoxP3 interacts with H1.5 to alter its binding to target genes to modulate their expression and to program Treg function.

Regulation of IL-2 gene expression by Siva and FOXP3 in human T cells

BACKGROUND

Severe autoinflammatory diseases are associated with mutations in the Foxp3 locus in both mice and humans. Foxp3 is required for the development, function, and maintenance of regulatory T cells (Tregs), a subset of CD4 cells that suppress T cell activation and inflammatory processes. Siva is a pro-apoptotic gene that is expressed across a range of tissues, including CD4 T cells. Siva interacts with three tumor necrosis factor receptor (TNFR) family members that are constitutively expressed on Treg cells: CD27, GITR, and OX40.

RESULTS

Here we report a biophysical interaction between FOXP3 and Siva. We mapped the interaction domains to Siva's C-terminus and to a central region of FOXP3. We showed that Siva repressed IL-2 induction by suppressing IL-2 promoter activity during T cell activation. Siva-1's repressive effect on IL-2 gene expression appears to be mediated by inhibition of NFkappaB, whereas FOXP3 repressed both NFkappaB and NFAT activity.

CONCLUSIONS

In summary, our data suggest that both FOXP3 and Siva function as negative regulators of IL-2 gene expression in Treg cells, via suppression of NFAT by FOXP3 and of NFkappaB by both FOXP3 and Siva. Our work contributes evidence for Siva's role as a T cell signalling mediator in addition to its known pro-apoptotic function. Though further investigations are needed, evidence for the biophysical interaction between FOXP3 and Siva invites the possibility that Siva may be important for proper Treg cell function.

Partial regulatory T cell depletion prior to acute feline immunodeficiency virus infection does not alter disease pathogenesis

Feline immunodeficiency virus (FIV) infection in cats follows a disease course similar to HIV-1, including a short acute phase characterized by high viremia, and a prolonged asymptomatic phase characterized by low viremia and generalized immune dysfunction. CD4(+)CD25(hi)FoxP3(+) immunosuppressive regulatory T (Treg) cells have been implicated as a possible cause of immune dysfunction during FIV and HIV-1 infection, as they are capable of modulating virus-specific and inflammatory immune responses. Additionally, the immunosuppressive capacity of feline Treg cells has been shown to be increased during FIV infection. We have previously shown that transient in vivo Treg cell depletion during asymptomatic FIV infection reveals FIV-specific immune responses suppressed by Treg cells. In this study, we sought to determine the immunological influence of Treg cells during acute FIV infection. We asked whether Treg cell depletion prior to infection with the highly pathogenic molecular clone FIV-C36 in cats could alter FIV pathogenesis. We report here that partial Treg cell depletion prior to FIV infection does not significantly change provirus, viremia, or CD4(+) T cell levels in blood and lymphoid tissues during the acute phase of disease. The effects of anti-CD25 mAb treatment are truncated in cats acutely infected with FIV-C36 as compared to chronically infected cats or FIV-naïve cats, as Treg cell levels were heightened in all treatment groups included in the study within two weeks post-FIV infection. Our findings suggest that the influence of Treg cell suppression during FIV pathogenesis is most prominent after Treg cells are activated in the environment of established FIV 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.

The biology of FoxP3: a key player in immune suppression during infections, autoimmune diseases and cancer

The Transcription factor FoxP3 belongs to the forkhead/winged-helix family of transcriptional regulators and shares general structural features with other FoxP family members. FoxP3 functions as a master of transcription for the development of regulatory T-cells (Treg cells) both in humans and in mice. Natural genetic mutations ofFoxP3 that disrupt its function in humans result in an autoimmune syndrome called Immune Polyendocrinopathy, Enteropathy, X-linked (IPEX) and in mice, its deletion causes the Scurfy phenotype, with similar pathology. The finding that FoxP3 is required for the development and function of Tregs has led to an explosion of research in determining its regulation and function in the immune system. Understanding the biological properties of FoxP3 has a wide range of implications for immune tolerance, autoimmune disorders, inflammation and immune response to infectious diseases and cancer.

Current humanized mouse models for studying human immunology and HIV-1 immuno-pathogenesis

A robust animal model for "hypothesis-testing/mechanistic" research in human immunology and immuno-pathology should meet the following criteria. First, it has well-studied hemato-lymphoid organs and target cells similar to those of humans. Second, the human pathogens establish infection and lead to relevant diseases. Third, it is genetically inbred and can be manipulated via genetic, immunological and pharmacological means. Many human-tropic pathogens such as HIV-1 fail to infect murine cells due to the blocks at multiple steps of their life cycle. The mouse with a reconstituted human immune system and other human target organs is a good candidate. A number of human-mouse chimeric models with human immune cells have been developed in the past 20 years, but most with only limited success due to the selective engraftment of xeno-reactive human T cells in hu-PBL-SCID mice or the lack of significant human immune responses in the SCID-hu Thy/Liv mouse. This review summarizes the current understanding of HIV-1 immuno-pathogenesis in human patients and in SIV-infected primate models. It also reviews the recent progress in the development of humanized mouse models with a functional human immune system, especially the recent progress in the immunodeficient mice that carry a defective gammaC gene. NOD/SCID/gammaC(-/-) (NOG or NSG) or the Rag2(-/-)gammaC(-/-) double knockout (DKO) mice, which lack NK as well as T and B cells (NTB-null mice), have been used to reconstitute a functional human immune system in central and peripheral lymphoid organs with human CD34(+) HSC. These NTB-hu HSC humanized models have been used to investigate HIV-1 infection, immuno-pathogenesis and therapeutic interventions. Such models, with further improvements, will contribute to study human immunology, human-tropic pathogens as well as human stem cell biology in the tissue development and function in vivo.

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.

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.

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.

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.

Foxp3 and Treg cells in HIV-1 infection and immuno-pathogenesis

FoxP3(+)CD4(+)CD25(+) regulatory T (Treg) cells are implicated in a number of pathologic processes including elevated levels in cancers and infectious diseases, and reduced levels in autoimmune diseases. Treg cells are activated to modulate immune responses to avoid over-reactive immunity. However, conflicting findings are reported regarding relative levels of Treg cells during HIV-1 infection and disease progression. The role of Treg cells in HIV-1 diseases (aberrant immune activation) is poorly understood due to lack of a robust model. We summarize here the regulation and function of Foxp3 in Treg cells and in modulating HIV-1 replication. Based on recent findings from SIV/monkey and HIV/humanized mouse models, a model of the dual role of Treg cells in HIV-1 infection and immuno-pathogenesis is discussed.

Structural aspects of the FOXP3 regulatory complex as an immunopharmacological target

The forkhead family transcription factor FOXP3 plays a fundamental role in immune homeostasis. FOXP3 dysfunction in regulatory T cells (Tregs) contributes to multiple disease processes such as autoimmunity, tumor development, and viral infection. FOXP3 cooperates and associates with a group of other transcriptional factors, co-repressors and co-activators in Tregs to form one or more dynamic regulatory complexes. These ensembles communicate with multiple key signaling pathways to either upregulate or downregulate the expression of downstream target genes such as cytokines and cell surface receptors, which are critical for the control of normal immune responses. Although the details of the underlying mechanism by which FOXP3 operates as a transcriptional repressor or an activator is largely undefined, FOXP3(+) Tregs based cellular therapies have been studied in animal models. Our recent studies concerning the FOXP3 complex ensemble provide structural and biochemical insights into FOXP3 function of Tregs, which are essential to the development of novel immunopharmacological agents for treating human immunological disease.

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.

FoxP3+CD4+ regulatory T cells play an important role in acute HIV-1 infection in humanized Rag2-/-gammaC-/- mice in vivo

The role of FoxP3(+)CD4(+) regulatory T (Treg) cells in HIV-1 disease in vivo is poorly understood due to the lack of a robust model. We report here that CD4(+)FoxP3(+) T cells are developed in all lymphoid organs in humanized Rag2(-/-)gammaC(-/-) (DKO-hu HSC) mice and they display both Treg phenotype and Treg function. These FoxP3(+) Treg cells are preferentially infected and depleted by a pathogenic HIV-1 isolate in HIV-infected DKO-hu HSC mice; and depletion of Treg cells is correlated with induction of their apoptosis in vivo. When CD4(+)CD25(+/hi) Treg cells are depleted with the IL-2-toxin fusion protein (denileukin diftitox), HIV-1 infection is significantly impaired. This is demonstrated by reduced levels of productively infected cells in lymphoid organs and lower plasma viremia. Therefore, FoxP3(+) Treg cells are productively infected and play an important role in acute HIV-1 infection in vivo. The DKO-hu HSC mouse will be a valuable model to study human Treg functions and their role in HIV-1 pathogenesis in vivo.

Endogenous IL-32 controls cytokine and HIV-1 production

IL-32, a proinflammatory cytokine that activates the p38MAPK and NF-kappaB pathways, induces other cytokines, for example, IL-1beta, IL-6, and TNF-alpha. This study investigated the role of endogenous IL-32 in HIV-1 infection by reducing IL-32 with small interfering (si)RNA in freshly infected PBMC and in the latently infected U1 macrophage cell line. When PBMC were pretreated with siRNA to IL-32 (siIL-32), IL-6, IFN-gamma, and TNF-alpha were reduced by 57, 51, and 36%, respectively, compared with scrambled siRNA. Cotransfection of NF-kappaB and AP-1 reporter constructs with siIL-32 decreased DNA binding of these transcription factors by 42 and 46%, respectively. Cytokine protein array analysis revealed that the inhibitory activity of siIL-32 primarily targeted Th1 and proinflammatory cytokines and chemokines, e.g., MIP-1alpha/beta. Unexpectedly, HIV-1 production (as measured by p24) increased 4-fold in these same PBMC when endogenous IL-32 was reduced. Because IFN-gamma was lower in siIL-32-treated PBMC, we blocked IFN-gamma bioactivity, which enhanced the augmentation of p24 by siIL-32. Furthermore, siIL-32 reduced the natural ligands of the HIV-1 coreceptors CCR5 (MIP-1alpha/beta and RANTES) and CXCR4 (SDF-1). Inhibition of endogenous IL-32 in U1 macrophages also increased HIV-1. When rhIL-32gamma was added to these cells, p24 levels fell by 72%; however, in the same cultures IFN-alpha increased 4-fold. Blockade of IFN-alpha/beta bioactivity in IL-32gamma-stimulated U1 cells revealed that IFN-alpha conveys the anti-HIV-1 effect of rhIL-32gamma. In summary, depletion of endogenous IL-32 reduced the levels of Th1 and proinflammatory cytokines but paradoxically increased p24, proposing IL-32 as a natural inhibitor of HIV-1.

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.

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.

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.