Imaging the function of regulatory T cells in vivo

Role of Regulatory T Cells in Intracerebral Hemorrhage

Intracerebral hemorrhage (ICH) is a common cerebrovascular disease that can lead to severe neurological dysfunction in surviving patients, resulting in a heavy burden on patients and their families. When ICH occurs, the blood‒brain barrier is disrupted, thereby promoting immune cell migration into damaged brain tissue. As important immunosuppressive T cells, regulatory T (Treg) cells are involved in the maintenance of immune homeostasis and the suppression of immune responses after ICH. Treg cells mitigate brain tissue damage after ICH in a variety of ways, such as inhibiting the neuroinflammatory response, protecting against blood‒brain barrier damage, reducing oxidative stress damage and promoting nerve repair. In this review, we discuss the changes in Treg cells in ICH clinical patients and experimental animals, the mechanisms by which Treg cells regulate ICH and treatments targeting Treg cells in ICH, aiming to support new therapeutic strategies for clinical treatment.

Regulatory T cells in retroviral infections

Tight regulation of immune responses is not only critical for preventing autoimmune diseases but also for preventing immunopathological damage during infections in which overactive immune responses may be more harmful for the host than the pathogen itself. Regulatory T cells (Tregs) play a critical role in this regulation, which was discovered using the Friend retrovirus (FV) mouse model. Subsequent FV studies revealed basic biological information about Tregs, including their suppressive activity on effector cells as well as the molecular mechanisms of virus-induced Treg expansion. Treg suppression not only limits immunopathology but also prevents complete elimination of pathogens contributing to chronic infections. Therefore, Tregs play a complex role in the pathogenesis of persistent retroviral infections. New therapeutic concepts to reactivate effector T-cell responses in chronic viral infections by manipulating Tregs also came from work with the FV model. This knowledge initiated many studies to characterize the role of Tregs in HIV pathogenesis in humans, where a complex picture is emerging. On one hand, Tregs suppress HIV-specific effector T-cell responses and are themselves targets of infection, but on the other hand, Tregs suppress HIV-induced immune hyperactivation and thus slow the infection of conventional CD4⁺ T cells and limit immunopathology. In this review, the basic findings from the FV mouse model are put into perspective with clinical and basic research from HIV studies. In addition, the few Treg studies performed in the simian immunodeficiency virus (SIV) monkey model will also be discussed. The review provides a comprehensive picture of the diverse role of Tregs in different retroviral infections and possible therapeutic approaches to treat retroviral chronicity and pathogenesis by manipulating Treg responses.

Regulatory T cells (Tregs) play a very complex role in retroviral infections, and the balance of beneficial versus detrimental effects from Tregs can change between the acute and chronic phase of infection. Therefore, the development of therapeutics to treat chronic retroviral infections via modulation of Tregs requires detailed information regarding both the positive and negative contributions of Tregs in a particular phase of a specific infection. Here, we review the molecular mechanisms that initiate and control Treg responses in retroviral infections as well as the target cells that are functionally manipulated by Tregs. Basic findings from the Friend retrovirus mouse model that initiated this area of research are put into perspective with clinical and basic research from HIV studies. The targeted manipulation of Treg responses holds a bright future for enhancing immune responses to infections, vaccine responses, and for cure or functional cure of chronic retroviral infections.

Phosphoproteomics Reveals Regulatory T Cell-Mediated DEF6 Dephosphorylation That Affects Cytokine Expression in Human Conventional T Cells

Regulatory T cells (Tregs) control key events of immune tolerance, primarily by suppression of effector T cells. We previously revealed that Tregs rapidly suppress T cell receptor (TCR)-induced calcium store depletion in conventional CD4⁺CD25⁻ T cells (Tcons) independently of IP₃ levels, consequently inhibiting NFAT signaling and effector cytokine expression. Here, we study Treg suppression mechanisms through unbiased phosphoproteomics of primary human Tcons upon TCR stimulation and Treg-mediated suppression, respectively. Tregs induced a state of overall decreased phosphorylation as opposed to TCR stimulation. We discovered novel phosphosites (T595_S597) in the DEF6 (SLAT) protein that were phosphorylated upon TCR stimulation and conversely dephosphorylated upon coculture with Tregs. Mutation of these DEF6 phosphosites abrogated interaction of DEF6 with the IP₃ receptor and affected NFAT activation and cytokine transcription in primary Tcons. This novel mechanism and phosphoproteomics data resource may aid in modifying sensitivity of Tcons to Treg-mediated suppression in autoimmune disease or cancer.

CD11c-Expressing Cells Affect Regulatory T Cell Behavior in the Meninges during Central Nervous System Infection

Regulatory T cells (Tregs) play an important role in the CNS during multiple infections, as well as autoimmune inflammation, but the behavior of this cell type in the CNS has not been explored. In mice, infection with Toxoplasma gondii leads to a Th1-polarized parasite-specific effector T cell response in the brain. Similarly, Tregs in the CNS during T. gondii infection are Th1 polarized, as exemplified by their T-bet, CXCR3, and IFN-γ expression. Unlike effector CD4⁺ T cells, an MHC class II tetramer reagent specific for T. gondii did not recognize Tregs isolated from the CNS. Likewise, TCR sequencing revealed minimal overlap in TCR sequence between effector T cells and Tregs in the CNS. Whereas effector T cells are found in the brain parenchyma where parasites are present, Tregs were restricted to the meninges and perivascular spaces. The use of intravital imaging revealed that activated CD4⁺ T cells within the meninges were highly migratory, whereas Tregs moved more slowly and were found in close association with CD11c⁺ cells. To test whether the behavior of Tregs in the meninges is influenced by interactions with CD11c⁺ cells, mice were treated with anti-LFA-1 Abs to reduce the number of CD11c⁺ cells in this space. The anti-LFA-1 treatment led to fewer contacts between Tregs and the remaining CD11c⁺ cells and increased the speed of Treg migration. These data suggest that Tregs are anatomically restricted within the CNS, and their interaction with CD11c⁺ populations regulates their local behavior during T. gondii infection.

Co-stimulate or Co-inhibit Regulatory T Cells, Which Side to Go?

Co-stimulatory and co-inhibitory molecules direct the "second signal," which largely determines the outcome of the "first signal" generated by the interaction of T cell receptor (TCR) with cognate MHC-peptide complex. The co-stimulatory and co-inhibitory signals are key mechanistic contributors to the regulation of adaptive immunity, especially the T cell-mediated immune response. Regulatory T cells (Tregs) are a special population of T cells, which unlike other T cells function as "attenuators" to suppress T cell immunity. Dysregulation of either the "second signal" or Tregs leads to an unbalanced immune system, which can result in a range of immune-related disorders, including autoimmune diseases, chronic infections, and tumors. In contrast, precise manipulation of these two systems offers tremendous clinical opportunities to treat these same diseases. Co-stimulatory and co-inhibitory molecules modulate immunity at molecular level, whereas Tregs delicately control the immune response at cellular level. Accumulating evidence has demonstrated that these two regulatory strategies converge and synergize with each other. This review discusses recent progress on the roles of co-stimulatory and co-inhibitory signals in the context of Tregs.

Regulatory T-Cell-Mediated Suppression of Conventional T-Cells and Dendritic Cells by Different cAMP Intracellular Pathways

Regulatory T-cells (Tregs) mediate their suppressive action by acting directly on conventional T-cells (Tcons) or dendritic cells (DCs). One mechanism of Treg suppression is the increase of cyclic adenosine 3′,5′-monophosphate (cAMP) levels in target cells. Tregs utilize cAMP to control Tcon responses, such as proliferation and cytokine production. Tregs also exert their suppression on DCs, diminishing DC immunogenicity by downmodulating the expression of costimulatory molecules and actin polymerization at the immunological synapse. The Treg-mediated usage of cAMP occurs through two major mechanisms. The first involves the Treg-mediated influx of cAMP in target cells through gap junctions. The second is the conversion of adenosine triphosphate into adenosine by the ectonucleases CD39 and CD73 present on the surface of Tregs. Adenosine then binds to receptors on the surface of target cells, leading to increased intracellular cAMP levels in these targets. Downstream, cAMP can activate the canonical protein kinase A (PKA) pathway and the exchange protein activated by cyclic AMP (EPAC) non-canonical pathway. In this review, we discuss the most recent findings related to cAMP activation of PKA and EPAC, which are implicated in Treg homeostasis as well as the functional alterations induced by cAMP in cellular targets of Treg suppression.

Bystander immunotherapy as a strategy to control allergen-driven airway inflammation

Allergic asthma is a chronic inflammatory disease characterized by airway hyperresponsiveness (AHR), lung infiltration of Th2 cells, and high levels of IgE. To date, allergen-specific immunotherapy (SIT) is the only treatment that effectively alleviates clinical symptoms and has a long-term effect after termination. Unfortunately, SIT is unsuitable for plurisensitized patients, and highly immunogenic allergens cannot be used. To overcome these hurdles, we sought to induce regulatory CD4(+) T cells (Treg) specific to an exogenous antigen that could be later activated as needed in vivo to control allergic responses. We have established an experimental approach in which mice tolerized to ovalbumin (OVA) were sensitized to the Leishmania homolog of receptors for activated c kinase (LACK) antigen, and subsequently challenged with aerosols of LACK alone or LACK and OVA together. Upon OVA administration, AHR and allergic airway responses were strongly reduced. OVA-induced suppression was mediated by CD25(+) Treg, required CTLA-4 and ICOS signaling and resulted in decreased numbers of migrating airway dendritic cells leading to a strong impairment in the proliferation of allergen-specific Th2 cells. Therefore, inducing Treg specific to a therapeutic antigen that could be further activated in vivo may represent a safe and novel curative approach for allergic asthma.

Neonatal regulatory T cells have reduced capacity to suppress dendritic cell function

Regulatory T cells (Treg cells) limit contact between dendritic cells (DCs) and conventional T cells (Tcons), decreasing the formation of aggregates as well as down-modulating the expression of co-stimulatory molecules by DCs, thus decreasing DC immunogenicity and abrogating T-cell activation. Despite the importance of this Treg-cell function, the capacity of Treg cells from term and preterm neonates to suppress DCs, and the suppressive mechanisms they use, are still undefined. We found that, relative to adult Treg cells, activated Treg cells from human neonates expressed lower FOXP3 and CTLA-4, but contained higher levels of cAMP. We developed an in vitro model in which Treg function was measured at a physiological ratio of 1 Treg for 10 Tcon and 1 monocyte-derived DC, as Treg target. Term and preterm Treg cells failed to suppress the formation of DC-Tcon aggregates, in contrast to naïve and memory Treg cells from adults. However, neonatal Treg cells diminished DC and Tcon activation as well as actin polymerization at the immunological synapses. In addition, CTLA-4 and cAMP were the main suppressive molecules used by neonatal Treg. Altogether, both preterm and term neonatal Treg cells appear less functional than adult Treg cells, and this defect is consistent with the general impairment of CD4 cell function in newborns.

Depletion of regulatory T cells augments a vaccine-induced T effector cell response against the liver-stage of malaria but fails to increase memory

Regulatory T cells (T_reg) have been shown to restrict vaccine-induced T cell responses in different experimental models. In these studies CD4⁺CD25⁺ T_reg were depleted using monoclonal antibodies against CD25, which might also interfere with CD25 on non-regulatory T cell populations and would have no effect on Foxp3⁺CD25⁻ T_reg. To obtain more insights in the specific function of T_reg during vaccination we used mice that are transgenic for a bacterial artificial chromosome expressing a diphtheria toxin (DT) receptor-eGFP fusion protein under the control of the foxp3 gene locus (depletion of regulatory T cell mice; DEREG). As an experimental vaccine-carrier recombinant Bordetella adenylate cyclase toxoid fused with a MHC-class I-restricted epitope of the circumsporozoite protein (ACT-CSP) of Plasmodium berghei (Pb) was used. ACT-CSP was shown by us previously to introduce the CD8⁺ epitope of Pb-CSP into the MHC class I presentation pathway of professional antigen-presenting cells (APC). Using this system we demonstrate here that the number of CSP-specific T cells increases when T_reg are depleted during prime but also during boost immunization. Importantly, despite this increase of T effector cells no difference in the number of antigen-specific memory cells was observed.

Human T cell priming assay: depletion of peripheral blood lymphocytes in CD25(+) cells improves the in vitro detection of weak allergen-specific T cells

To develop an in vitro assay that recapitulates the key event of allergic contact dermatitis (ACD), that is the priming of effector T cells by hapten-presenting dendritic cells, and then allows for the sensitive detection of chemical allergens represents a major challenge. Classical human T cell priming assays (hTCPA) that have been developed in the past, using hapten-loaded monocyte-derived dendritic cells (MDDCs) as antigen-presenting cells and peripheral blood lymphocytes (PBLs) as responding cells, were not efficient to prime T cells to common allergens with moderate/weak sensitizing properties. Recent progress in the understanding of the effector and regulatory mechanisms of ACD have shown that T cell priming requires efficient uptake of allergens by immunogenic DCs and that it is controlled by several subsets of regulatory cells including CD25(+) Tregs. We therefore analyzed various parameters involved in allergen-specific T cell activation in vitro and showed that priming of allergen-specific T cells is hampered by several subsets of immune cells comprising CD1a(neg) DCs, CD25(+) T cells, and CD56(+) regulatory cells.CD4(+)CD25(+)FoxP3(+) Tregs prevented the in vitro T cell priming to moderate/weak allergens, and depletion of human PBLs in CD25(+) cells significantly increased specific T cell proliferation and IFN-γ secretion. CD56(+) cells exerted an additional control of T cell priming since co-depletion of both CD56(+) and CD25(+) cells improved the magnitude of chemical-specific T cell activation. Finally, CD1a(low) MDDCs were able to inhibit T cell activation obtained by allergen-pulsed CD1a(high) MDDC. Moreover, we showed that uptake by DC of allergen-encapsulated nanoparticles significantly increased their activation status and their ability to prompt specific T cell activation. Hence, by combining the different strategies, i.e., depletion of CD25(+) and CD56(+) cells, use of CD1a(high) MDDC, and nanoparticle encapsulation of allergens, it was possible to induce T cell priming to most of the moderate/weak allergens, including lipophilic molecules highly insoluble in culture media. Therefore, the present optimized in vitro human T cell priming assay is a valuable method to detect the sensitizing properties of chemical allergens.

Imaging dynamics of CD11c⁺ cells and Foxp3⁺ cells in progressive autoimmune insulitis in the NOD mouse model of type 1 diabetes

AIMS/HYPOTHESIS

The aim of this study was to visualise the dynamics and interactions of the cells involved in autoimmune-driven inflammation in type 1 diabetes.

METHODS

We adopted the anterior chamber of the eye (ACE) transplantation model to perform non-invasive imaging of leucocytes infiltrating the endocrine pancreas during initiation and progression of insulitis in the NOD mouse. Individual, ACE-transplanted islets of Langerhans were longitudinally and repetitively imaged by stereomicroscopy and two-photon microscopy to follow fluorescently labelled leucocyte subsets.

RESULTS

We demonstrate that, in spite of the immune privileged status of the eye, the ACE-transplanted islets develop infiltration and beta cell destruction, recapitulating the autoimmune insulitis of the pancreas, and exemplify this by analysing reporter cell populations expressing green fluorescent protein under the Cd11c or Foxp3 promoters. We also provide evidence that differences in morphological appearance of subpopulations of infiltrating leucocytes can be correlated to their distinct dynamic behaviour.

CONCLUSIONS/INTERPRETATION

Together, these findings demonstrate that the kinetics and dynamics of these key cellular components of autoimmune diabetes can be elucidated using this imaging platform for single cell resolution, non-invasive and repetitive monitoring of the individual islets of Langerhans during the natural development of autoimmune diabetes.

Whole-body imaging of adoptively transferred T cells using magnetic resonance imaging, single photon emission computed tomography and positron emission tomography techniques, with a focus on regulatory T cells

Cell-based therapies using natural or genetically modified regulatory T cells (T(regs)) have shown significant promise as immune-based therapies. One of the main difficulties facing the further advancement of these therapies is that the fate and localization of adoptively transferred T(regs) is largely unknown. The ability to dissect the migratory pathway of these cells in a non-invasive manner is of vital importance for the further development of in-vivo cell-based immunotherapies, as this technology allows the fate of the therapeutically administered cell to be imaged in real time. In this review we will provide an overview of the current clinical imaging techniques used to track T cells and T(regs) in vivo, including magnetic resonance imaging (MRI) and positron emission tomography (PET)/single photon emission computed tomography (SPECT). In addition, we will discuss how the finding of these studies can be used, in the context of transplantation, to define the most appropriate T(reg) subset required for cellular therapy.

Evolving immune circuits are generated by flexible, motile, and sequential immunological synapses

The immune system is made up of a diverse collection of cells, each of which has distinct sets of triggers that elicit unique and overlapping responses. It is correctly described as a 'system' because its overall properties (e.g. 'tolerance', 'allergy') emerge from multiple interactions of its components cells. To mobilize a response where needed, the majority of the cells of the system are obligatorily highly motile and so must communicate with one another over both time and space. Here, we discuss the flexibility of the primary immunological synapse (IS) with respect to motility. We then consider the primary IS as an initiating module that licenses 'immunological circuits': the latter consisting of two or more cell-cell synaptic interactions. We discuss how two or three component immunological circuits interact might with one another in sequence and how the timing, stoichiometry, milieu, and duration of assembly of immunological circuits are likely to be key determinants in the emergent outcome and thus the system-wide immune response. An evolving consideration of immunological circuits, with an emphasis on the cell-cell modules that complement T-antigen-presenting cell interaction, provides a fundamental starting point for systems analysis of the immune response.

Molecular mechanisms of treg-mediated T cell suppression

CD4(+)CD25(high)Foxp3(+) regulatory T cells (Tregs) can suppress other immune cells and, thus, are critical mediators of peripheral self-tolerance. On the one hand, Tregs avert autoimmune disease and allergies. On the other hand, Tregs can prevent immune reactions against tumors and pathogens. Despite the importance of Tregs, the molecular mechanisms of suppression remain incompletely understood and controversial. Proliferation and cytokine production of CD4(+)CD25(-) conventional T cells (Tcons) can be inhibited directly by Tregs. In addition, Tregs can indirectly suppress Tcon activation via inhibition of the stimulatory capacity of antigen presenting cells. Direct suppression of Tcons by Tregs can involve immunosuppressive soluble factors or cell contact. Different mechanisms of suppression have been described, so far with no consensus on one universal mechanism. Controversies might be explained by the fact that different mechanisms may operate depending on the site of the immune reaction, on the type and activation state of the suppressed target cell as well as on the Treg activation status. Further, inhibition of T cell effector function can occur independently of suppression of proliferation. In this review, we summarize the described molecular mechanisms of suppression with a particular focus on suppression of Tcons and rapid suppression of T cell receptor-induced calcium (Ca(2+)), NFAT, and NF-κB signaling in Tcons by Tregs.

Regulatory T-cells: diverse phenotypes integral to immune homeostasis and suppression

Regulatory T-cells (T(REG)) are diverse populations of lymphocytes that regulate the adaptive immune response in higher vertebrates. T(REG) delete autoreactive T-cells, induce tolerance, and dampen inflammation. T(REG) cell deficiency in humans (i.e., IPEX [Immunodysregulation, Polyendocrinopathy and Enteropathy, X-linked syndrome]) and animal models (e.g., "Scurfy" mouse) is associated with multisystemic autoimmune disease. T(REG) in humans and laboratory animal species are similar in type and regulatory function. A molecular marker of and the cell lineage specification factor for T(REG) is FOXP3, a forkhead box transcription factor. CD4(+) T(REG) are either natural (nT(REG)), which are thymus-derived CD4(+)CD25(+)FOXP3(+) T-cells, or inducible (i.e., Tr1 cells that secrete IL-10, Th3 cells that secrete TGF-β and IL-10, and Foxp3(+) Treg). The proinflammatory Th17 subset has been a major focus of research. T(H)17 CD4(+) effector T-cells secrete IL-17, IL-21, and IL-22 in autoimmune and inflammatory disease, and are dynamically balanced with T(REG) cell development. Other lymphocyte subsets with regulatory function include: inducible CD8(+) T(REG), CD3(+)CD4(-)CD8(-) T(REG) (double-negative), CD4(+)Vα14(+) (NKT(REG)), and γδ T-cells. T(REG) have four regulatory modes of action: secretion of inhibitory cytokines (e.g., IL-10 and TGF-β), granzyme-perforin-induced apoptosis of effector lymphocytes, depriving effector T-cells of cytokines leading to apoptosis, or inhibition of dendritic cell function. The role of T(REG) in mucosal sites, inflammation/infection, pregnancy, and cancer as well as a review of T(REG) as a modulatory target in drug development will be covered.

In vivo SPECT reporter gene imaging of regulatory T cells

Regulatory T cells (Tregs) were identified several years ago and are key in controlling autoimmune diseases and limiting immune responses to foreign antigens, including alloantigens. In vivo imaging techniques including intravital microscopy as well as whole body imaging using bioluminescence probes have contributed to the understanding of in vivo Treg function, their mechanisms of action and target cells. Imaging of the human sodium/iodide symporter via Single Photon Emission Computed Tomography (SPECT) has been used to image various cell types in vivo. It has several advantages over the aforementioned imaging techniques including high sensitivity, it allows non-invasive whole body studies of viable cell migration and localisation of cells over time and lastly it may offer the possibility to be translated to the clinic. This study addresses whether SPECT/CT imaging can be used to visualise the migratory pattern of Tregs in vivo. Treg lines derived from CD4⁺CD25⁺FoxP3⁺ cells were retrovirally transduced with a construct encoding for the human Sodium Iodide Symporter (NIS) and the fluorescent protein mCherry and stimulated with autologous DCs. NIS expressing self-specific Tregs were specifically radiolabelled in vitro with Technetium-99m pertechnetate (^99mTcO₄⁻) and exposure of these cells to radioactivity did not affect cell viability, phenotype or function. In addition adoptively transferred Treg-NIS cells were imaged in vivo in C57BL/6 (BL/6) mice by SPECT/CT using ^99mTcO₄⁻. After 24 hours NIS expressing Tregs were observed in the spleen and their localisation was further confirmed by organ biodistribution studies and flow cytometry analysis. The data presented here suggests that SPECT/CT imaging can be utilised in preclinical imaging studies of adoptively transferred Tregs without affecting Treg function and viability thereby allowing longitudinal studies within disease models.

CD4+ CD25+ regulatory T cells prevent type 1 diabetes preceded by dendritic cell-dominant invasive insulitis by affecting chemotaxis and local invasiveness of dendritic cells

Development of type 1 diabetes (T1D) is preceded by invasive insulitis. Although CD4(+)CD25(+) regulatory T cells (nTregs) induce tolerance that inhibits insulitis and T1D, the in vivo cellular mechanisms underlying this process remain largely unclear. Using an adoptive transfer model and noninvasive imaging-guided longitudinal analyses, we found nTreg depletion did not affect systemic trafficking and tissue localization of diabetogenic CD4(+) BDC2.5 T (BDC) cells in recipient mice prior to development of T1D. In addition, neither the initial expansion/activation of BDC cells nor the number of CD11c(+) or NK cells in islets and pancreatic lymph nodes were altered. Unexpectedly, our results showed nTreg depletion led to accelerated invasive insulitis dominated by CD11c(+) dendritic cells (ISL-DCs), not BDC cells, which stayed in the islet periphery. Compared with control mice, the phenotype of ISL-DCs and their ability to stimulate BDC cells did not change during invasive insulitis development. However, ISL-DCs from nTreg-deficient recipient mice showed increased in vitro migration toward CCL19 and CCL21. These results demonstrated invasive insulitis dominated by DCs, not CD4(+) T cells, preceded T1D onset in the absence of nTregs, and suggested a novel in vivo function of nTregs in T1D prevention by regulating local invasiveness of DCs into islets, at least partly, through regulation of DC chemotaxis toward CCL19/CCL21 produced by the islets.

PD1 blockade reverses the suppression of melanoma antigen-specific CTL by CD4+ CD25(Hi) regulatory T cells

Regulatory CD4(+)CD25(Hi) T cells (Treg) and programmed death-1 (PD-1) molecule have emerged as pivotal players in immune regulation. However, the underlying mechanisms by which they impact antigen-specific CD8(+) immune responses in cancer patients and how they interact with each other under physiologic conditions remain unclear. Herein, we examined the relationship of PD-1 and its abrogation to the function of Treg in patients with melanoma using short-term in vitro assays to generate melanoma-specific T cells. We identified Treg in the circulation of vaccinated melanoma patients and detected PD-1 expression on vaccine-induced melanoma antigen-specific CTLs, as well as on and within Treg from patients' peripheral blood. Programmed death ligand (PD-L) 1 expression was also detected on patients' Treg. PD-1 blockade promoted the generation of melanoma antigen-specific CTLs and masked their inhibition by Treg. The mechanisms by which PD-1 blockade mediated immune enhancement included direct augmentation of melanoma antigen-specific CTL proliferation, heightening their resistance to inhibition by Treg and direct limitation of the inhibitory ability of Treg. PD-1 blockade reversed the increased expression of PD-1 and PD-L1 on melanoma antigen-specific CTL by Treg, rescued INF-gamma and IL-2 or INF-gamma and tumor necrosis factor-alpha co-expression and expression of IL-7 receptor by melanoma antigen-specific CTL which were diminished by Treg. PD-1 blockade also resulted in down-regulation of intracellular FoxP3 expression by Treg. These data suggest that PD-1 is importantly implicated in the regulation of Treg function in melanoma patients.

Regulating the regulators: costimulatory signals control the homeostasis and function of regulatory T cells

SUMMARY

Costimulation is a concept that goes back to the early 1980s when Lafferty and others hypothesized that cell surface and soluble molecules must exist that are essential for initiating immune responses subsequent to antigen exposure. The explosion in this field of research ensued as over a dozen molecules have been identified to function as second signals following T-cell receptor engagement. By 1994, it seemed clear that the most prominent costimulatory pathway CD28 and functionally related costimulatory molecules, such as CD154, were the major drivers of a positive immune response. Then the immunology world turned upside down. CD28 knockout mice, which were, in most cases, immunodeficient, led to increased autoimmunity when bred into the non-obese diabetic background. Another CD28 family member, cytotoxic T-lymphocyte-associated protein 4, which was presumed to be a costimulatory molecule on activated T cells, turned out to be critical in downregulating immunity. These results, coupled with the vast suppressor cell literature which had been largely rebuked, suggested that the immune system was not poised for response but controlled in such a way that regulation was dominant. Over the last decade, we have learned that these costimulatory molecules play a key role in the now classical CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs) that provide critical control of unwanted autoimmune responses. In this review, we discuss the connections between costimulation and Tregs that have changed the costimulation paradigm.

Regulatory T cells and EBV associated malignancies

It has been shown that the T-regulatory cells (Tregs) not only play a key role in the establishment and maintenance of peripheral tolerance to prevent the autoimmune disease, but also inhibit the anti-tumor immunity. Recently, many studies have demonstrated that cytotoxicity T cells (CTL) can control the growth of EBV-positive tumor cells in vitro, including Hodgkin's lymphoma (HL), nasopharyngeal carcinoma, posttransplantation lymphoproliferative disorders (PTLD), depending on the large mount of EBV antigens presented by MHC molecules on the surface of these malignant cells. However, limited benefit of CTL adoptive immunotherapy has been reported in the treatment of EBV positive HL and NPC, and Tregs are regarded as a critical hurdle in this issue. In the present review, we discuss the correlation of EBV antigens expression in the tumor cells and the induction of Tregs in tumor microenvironment. Treg subsets and its possible mechanism to attenuate the anti-tumor immunity in EBV associated malignancies are also discussed, following by the possible strategies of targeting Tregs in the future immunotherapy for EBV positive cancers.

OX40 triggering blocks suppression by regulatory T cells and facilitates tumor rejection

Regulatory T (T reg) cells are the major obstacle to cancer immunotherapy, and their depletion promptly induces conversion of peripheral precursors into T reg cells. We show that T reg cells can be functionally inactivated by OX40 triggering. In tumors, the vast majority of CD4⁺ T cells are Foxp3⁺ and OX40^bright. However, intratumor injection of the agonist anti-OX40 monoclonal antibody (mAb) OX86, but not anti-CD25 mAb, induces tumor rejection in 80% of mice, an effect that is abrogated by CD8 depletion. Upon intratumor OX40 triggering, increased numbers of infiltrating dendritic cells (DCs) migrate to draining lymph nodes and generate a new wave of tumor-specific cytotoxic T lymphocytes, as detected by tetramer and CD44 staining of node CD8⁺ T lymphocytes. Tumor-bearing Rag1-knockout (KO) mice reconstituted with OX40-deficient T reg cells and wild-type (WT) effector T cells, or the reciprocal combination, showed that both T reg and effector T cells must be triggered via OX40 for the tumor to be rejected. Accordingly, WT but not OX40-KO mice receiving intratumor coinjection of OX86 and ovalbumin protein were able to revert tumor-induced tolerization of adoptively transferred OX40-competent OTII T lymphocytes. In conclusion, OX40-mediated inactivation of T reg cell function unleashes nearby DCs, allowing them to induce an adaptive immune response. In addition, the known OX40-dependent delivery of fitness signals to activated T cells is boosted by concurrent T reg cell inhibition. OX40 triggering thus has multiple effects that converge to mediate tumor rejection.

Assessing the in vitro suppressive capacity of regulatory T cells

Regulatory T cells (Treg) play a vital role in controlling peripheral immune responses in order to prevent autoimmunity and control inflammation. Altered Treg activities have been associated with the pathogenesis of multiple disorders including autoimmunity, allergy, cancer, and infection with persistent pathogens. As such, a great deal of interest has recently been directed towards developing additional tools and methods to better understand the mechanisms of suppression employed by Treg. The in vitro suppression assay has emerged as a valuable means by which to assess the functional capacity and activity of Treg. In this review, we summarize the merits and limitations of the various in vitro assays that have been utilized to assess Treg activity and present a novel two color proliferation assay that allows simultaneous monitoring of both regulatory and effector T cell activity. As further immunomodulatory therapies are explored, the need for additional methodologies to understand the cellular and molecular mechanisms of immune regulation conferred by Treg will play an increasingly important role.

Dysfunction of CD4+CD25high T regulatory cells in patients with recurrent aphthous stomatitis

BACKGROUND

Recurrent aphthous stomatitis (RAS) is a chronic inflammatory disease of unknown etiology characterized by recurring formation of painful oral ulcers. RAS may result from oral epithelium damage caused by T-cell-mediated immune response. CD4(+)CD25(+) T regulatory (Treg) cells suppress proliferation and effector functions of other immune cells, and therefore are crucial in regulating the immune response.

METHODS

We tested the function of peripheral CD4(+)CD25(high) Treg cells in active RAS through their ability to inhibit proliferation and cytokine production of conventional CD4(+) T cells. We also attempted to detect the presence of FOXP3 and indoleamine 2,3-dioxygenase (IDO) mRNA in the lesional and non-lesional oral mucosa of RAS patients and healthy individuals using real-time PCR assay.

RESULTS

Treg cells derived from RAS patients were less efficient in the suppression of cytokine production of CD4(+) T effector cells than Treg cells from healthy individuals. Moreover, in RAS, Treg cells were nearly twice less potent in the inhibition of CD4(+)CD25(-) T cell proliferation than in healthy donors. Furthermore, we have demonstrated the decreased proportion of CD4(+)CD25(+)FOXP3(+) Treg cells in peripheral blood of RAS patients compared with controls. We failed to detect FOXP3 mRNA, while IDO mRNA expression was decreased in non-lesional mucosa biopsies from RAS patients compared with ulcer biopsies or normal mucosa from healthy donors.

CONCLUSIONS

These findings suggest that CD4(+)CD25(high) Treg cells are both functionally and quantitatively compromised in RAS and that decreased constitutive expression of IDO in oral mucosa in RAS may lead to the loss of local immune tolerance.

The Foxp3+ regulatory T cell: a jack of all trades, master of regulation

The function of regulatory T cells (T(reg) cells) has been attributed to a growing number of diverse pathways, molecules and processes. Seemingly contradictory conclusions regarding the mechanisms underlying T(reg) cell suppressive activity have revitalized skeptics in the field who challenge the core validity of the idea of T(reg) cells as central immune regulators. However, we note that a consensus may be emerging from the data: that multiple T(reg) cell functions act either directly or indirectly at the site of antigen presentation to create a regulatory milieu that promotes bystander suppression and infectious tolerance. Thus, the versatility and adaptability of the Foxp3+ T(reg) cells may in fact be the best argument that these cells are 'multitalented masters of immune regulation'.

Human regulatory T cells inhibit polarization of T helper cells toward antigen-presenting cells via a TGF-beta-dependent mechanism

The molecular mechanisms used by regulatory T cells (Treg) to inhibit the effector phase of adaptive immune responses are still elusive. In the present work, we investigated the possibility that Treg may interfere with a basic biological function of T helper cells (T(H)): polarization of secretory machinery for dedicated help delivery. To address this question, we visualized by confocal microscopy different parameters of activation in T(H) and Treg cells interacting simultaneously with individual antigen-presenting cells (APC). Our results show that, although productive TCR engagement in T(H)/APC conjugates was unaffected by the presence of adjacent Treg, the reorientation of T(H) secretory machinery toward APC was strongly inhibited. Blocking TGF-beta completely reverted Treg induced inhibition of T(H) polarization. Our results identify a previously undescribed mechanism by which Treg inhibit effector T cells. TGF-beta produced by adjacent Treg interferes with polarization of T(H) secretory machinery toward APC, thus affecting a crucial step of T(H)-mediated amplification of the immune response.

Immunotherapy with dendritic cells for cancer

Dendritic cells are professional antigen-presenting cells with a key role in both immunity induction and tolerance maintenance. Dendritic cells are highly specialized in antigen capture, processing and presentation, and express co-stimulation signals which activate T lymphocytes and NK cells. Dendritic cells generated in culture and loaded with an antigen efficiently induce antigen-specific immunity after injection. More recently, methods have been developed that target antigens to dendritic cells in vivo, bypassing the need for ex vivo cell manipulations. Numerous ongoing studies aim to evaluate the effectiveness of dendritic cell vaccines in preventing tumor relapses and extending patients' survival. Further implementation of this form of immunotherapy is expected following the identification of the mechanisms controlling dendritic cell immunogenicity, and from a better understanding of the cell dynamics whereby immune responses are orchestrated. Here, we discuss these new insights together with an overview of the dendritic cell-based clinical studies carried out to date.

Molecular pathology of malignant gliomas

Malignant gliomas, the most common type of primary brain tumor, are a spectrum of tumors of varying differentiation and malignancy grades. These tumors may arise from neural stem cells and appear to contain tumor stem cells. Early genetic events differ between astrocytic and oligodendroglial tumors, but all tumors have an initially invasive phenotype, which complicates therapy. Progression-associated genetic alterations are common to different tumor types, targeting growth-promoting and cell cycle control pathways and resulting in focal hypoxia, necrosis, and angiogenesis. Knowledge of malignant glioma genetics has already impacted clinical management of these tumors, and researchers hope that further knowledge of the molecular pathology of malignant gliomas will result in novel therapies.

Interplay of pathogens, cytokines and other stress signals in the regulation of dendritic cell function

Dendritic cells (DCs) are the only antigen-presenting cell capable of activating naïve T lymphocytes, and hence they play a crucial role in the induction of adaptive immunity. Immature DCs sample and process antigens, and efficiently sense a large variety of signals from the surrounding environment. Upon activation, they become capable to activate naïve T cells and to direct the differentiation and polarization of effector T lymphocytes. It is becoming increasingly clear that different signals are able to determine distinct programs of DC differentiation and different forms of immunity and tolerance. In the past few years many advances have been made in addressing the action exerted by pathogen-associated molecular patterns (PAMPs), cytokines, chemokines, and other less characterized stress molecules on the activity of DCs. In this review we focus on the multiplicity of innate signals able to modulate the functional profile of DCs.