CD28 costimulation regulates FOXP3 in a RelA/NF-κB-dependent mechanism

Osteoimmunology: The Crosstalk between T Cells, B Cells, and Osteoclasts in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is an ongoing inflammatory condition that affects the joints and can lead to severe damage to cartilage and bones, resulting in significant disability. This condition occurs when the immune system becomes overactive, causing osteoclasts, cells responsible for breaking down bone, to become more active than necessary, leading to bone breakdown. RA disrupts the equilibrium between osteoclasts and osteoblasts, resulting in serious complications such as localized bone erosion, weakened bones surrounding the joints, and even widespread osteoporosis. Antibodies against the receptor activator of nuclear factor-κB ligand (RANKL), a crucial stimulator of osteoclast differentiation, have shown great effectiveness both in laboratory settings and actual patient cases. Researchers are increasingly focusing on osteoclasts as significant contributors to bone erosion in RA. Given that RA involves an overactive immune system, T cells and B cells play a pivotal role by intensifying the immune response. The imbalance between Th17 cells and Treg cells, premature aging of T cells, and excessive production of antibodies by B cells not only exacerbate inflammation but also accelerate bone destruction. Understanding the connection between the immune system and osteoclasts is crucial for comprehending the impact of RA on bone health. By delving into the immune mechanisms that lead to joint damage, exploring the interactions between the immune system and osteoclasts, and investigating new biomarkers for RA, we can significantly improve early diagnosis, treatment, and prognosis of this condition.

Suppression of FOXP3 expression by the AP-1 family transcription factor BATF3 requires partnering with IRF4

FOXP3 is the lineage-defining transcription factor for Tregs, a cell type critical to immune tolerance, but the mechanisms that control FOXP3 expression in Tregs remain incompletely defined, particularly as it relates to signals downstream of TCR and CD28 signaling. Herein, we studied the role of IRF4 and BATF3, two transcription factors upregulated upon T cell activation, to the conversion of conventional CD4+ T cells to FOXP3+ T cells (iTregs) in vitro. We found that IRF4 must partner with BATF3 to bind to a regulatory region in the Foxp3 locus where they cooperatively repress FOXP3 expression and iTreg induction. In addition, we found that interactions of these transcription factors are necessary for glycolytic reprogramming of activated T cells that is antagonistic to FOXP3 expression and stability. As a result, Irf4 KO iTregs show increased demethylation of the critical CNS2 region in the Foxp3 locus. Together, our findings provide important insights how BATF3 and IRF4 interactions integrate activating signals to control CD4+ cell fate decisions and govern Foxp3 expression.

Premature Senescence of T-cells Favors Bone Loss During Osteolytic Diseases. A New Concern in the Osteoimmunology Arena

Cellular senescence is a biological process triggered in response to time-accumulated DNA damage, which prioritizes cell survival over cell function. Particularly, senescent T lymphocytes can be generated prematurely during chronic inflammatory diseases regardless of chronological aging. These senescent T lymphocytes are characterized by the loss of CD28 expression, a co-stimulatory receptor that mediates antigen presentation and effective T-cell activation. An increased number of premature senescent CD4+CD28- T lymphocytes has been frequently observed in osteolytic diseases, including rheumatoid arthritis, juvenile idiopathic arthritis, ankylosing spondylitis, osteopenia, osteoporosis, and osteomyelitis. Indeed, CD4+CD28- T lymphocytes produce higher levels of osteoclastogenic molecular mediators directly related to pathologic bone loss, such as tumor necrosis factor (TNF)-α, interleukin (IL)-17A, and receptor-activator of nuclear factor κB ligand (RANKL), as compared with regular CD4+CD28+ T lymphocytes. In addition, premature senescent CD8+CD28- T lymphocytes have been negatively associated with bone healing and regeneration by inhibiting osteoblast differentiation and mesenchymal stromal cell survival. Therefore, accumulated evidence supports the role of senescent T lymphocytes in osteoimmunology. Moreover, premature senescence of T-cells seems to be associated with the functional imbalance between the osteolytic T-helper type-17 (Th17) and bone protective T regulatory (Treg) lymphocytes, as well as the phenotypic instability of Treg lymphocytes responsible for its trans-differentiation into RANKL-producing exFoxp3Th17 cells, a key cellular phenomenon directly related to bone loss. Herein, we present a framework for the understanding of the pathogenic characteristics of T lymphocytes with a premature senescent phenotype; and particularly, we revise and discuss their role in the osteoimmunology of osteolytic diseases.

The many-sided contributions of NF-κB to T-cell biology in health and disease

T cells (or T lymphocytes) exhibit a myriad of functions in immune responses, ranging from pathogen clearance to autoimmunity, cancer and even non-lymphoid tissue homeostasis. Therefore, deciphering the molecular mechanisms orchestrating their specification, function and gene expression pattern is critical not only for our comprehension of fundamental biology, but also for the discovery of novel therapeutic targets. Among the master regulators of T-cell identity, the functions of the NF-κB family of transcription factors have been under scrutiny for several decades. However, a more precise understanding of their pleiotropic functions is only just emerging. In this review we will provide a global overview of the roles of NF-κB in the different flavors of mature T cells. We aim at highlighting the complex and sometimes diverging roles of the five NF-κB subunits in health and disease.

The NF-κB RelA Transcription Factor Is Critical for Regulatory T Cell Activation and Stability

Regulatory T cells (Tregs) play a major role in immune homeostasis and in the prevention of autoimmune diseases. It has been shown that c-Rel is critical in Treg thymic differentiation, but little is known on the role of NF-κB on mature Treg biology. We thus generated mice with a specific knockout of RelA, a key member of NF-κB, in Tregs. These mice developed a severe autoimmune syndrome with multi-organ immune infiltration and high activation of lymphoid and myeloid cells. Phenotypic and transcriptomic analyses showed that RelA is critical in the acquisition of the effector Treg state independently of surrounding inflammatory environment. Unexpectedly, RelA-deficient Tregs also displayed reduced stability and cells that had lost Foxp3 produced inflammatory cytokines. Overall, we show that RelA is critical for Treg biology as it promotes both the generation of their effector phenotype and the maintenance of their identity.

Signal Transduction Via Co-stimulatory and Co-inhibitory Receptors

T-cell receptor (TCR)-mediated antigen-specific stimulation is essential for initiating T-cell activation. However, signaling through the TCR alone is not sufficient for inducing an effective response. In addition to TCR-mediated signaling, signaling through antigen-independent co-stimulatory or co-inhibitory receptors is critically important not only for the full activation and functional differentiation of T cells but also for the termination and suppression of T-cell responses. Many studies have investigated the signaling pathways underlying the function of each molecular component. Co-stimulatory and co-inhibitory receptors have no kinase activity, but their cytoplasmic region contains unique functional motifs and potential phosphorylation sites. Engagement of co-stimulatory receptors leads to recruitment of specific binding partners, such as adaptor molecules, kinases, and phosphatases, via recognition of a specific motif. Consequently, each co-stimulatory receptor transduces a unique pattern of signaling pathways. This review focuses on our current understanding of the intracellular signaling pathways provided by co-stimulatory and co-inhibitory molecules, including B7:CD28 family members, immunoglobulin, and members of the tumor necrosis factor receptor superfamily.

Notch and NF-κB: Coach and Players of Regulatory T-Cell Response in Cancer

The Notch signaling pathway plays multiple roles in driving T-cell fate decisions, proliferation, and aberrant growth. NF-κB is a cell-context key player interconnected with Notch signaling either in physiological or in pathological conditions. This review focuses on how the multilayered crosstalk between different Notches and NF-κB subunits may converge on Foxp3 gene regulation and orchestrate CD4⁺ regulatory T (Treg) cell function, particularly in a tumor microenvironment. Notably, Treg cells may play a pivotal role in the inhibition of antitumor immune responses, possibly promoting tumor growth. A future challenge is represented by further dissection of both Notch and NF-κB pathways and consequences of their intersection in tumor-associated Treg biology. This may shed light on the molecular mechanisms regulating Treg cell expansion and migration to peripheral lymphoid organs thought to facilitate tumor development and still to be explored. In so doing, new opportunities for combined and/or more selective therapeutic approaches to improve anticancer immunity may be found.

An NF-κB Transcription-Factor-Dependent Lineage-Specific Transcriptional Program Promotes Regulatory T Cell Identity and Function

Both conventional T (Tconv) cells and regulatory T (Treg) cells are activated through ligation of the T cell receptor (TCR) complex, leading to the induction of the transcription factor NF-κB. In Tconv cells, NF-κB regulates expression of genes essential for T cell activation, proliferation, and function. However the role of NF-κB in Treg function remains unclear. We conditionally deleted canonical NF-κB members p65 and c-Rel in developing and mature Treg cells and found they have unique but partially redundant roles. c-Rel was critical for thymic Treg development while p65 was essential for mature Treg identity and maintenance of immune tolerance. Transcriptome and NF-κB p65 binding analyses demonstrated a lineage specific, NF-κB-dependent transcriptional program, enabled by enhanced chromatin accessibility. These dual roles of canonical NF-κB in Tconv and Treg cells highlight the functional plasticity of the NF-κB signaling pathway and underscores the need for more selective strategies to therapeutically target NF-κB.

CD28 Costimulation: From Mechanism to Therapy

Ligation of the CD28 receptor on T cells provides a critical second signal alongside T cell receptor (TCR) ligation for naive T cell activation. Here, we discuss the expression, structure, and biochemistry of CD28 and its ligands. CD28 signals play a key role in many T cell processes, including cytoskeletal remodeling, production of cytokines, survival, and differentiation. CD28 ligation leads to unique epigenetic, transcriptional, and post-translational changes in T cells that cannot be recapitulated by TCR ligation alone. We discuss the function of CD28 and its ligands in both effector and regulatory T cells. CD28 is critical for regulatory T cell survival and the maintenance of immune homeostasis. We outline the roles that CD28 and its family members play in human disease and we review the clinical efficacy of drugs that block CD28 ligands. Despite the centrality of CD28 and its family members and ligands to immune function, many aspects of CD28 biology remain unclear. Translation of a basic understanding of CD28 function into immunomodulatory therapeutics has been uneven, with both successes and failures. Such real-world results might stem from multiple factors, including complex receptor-ligand interactions among CD28 family members, differences between the mouse and human CD28 families, and cell-type specific roles of CD28 family members.

Silencing of Foxp3 delays the growth of murine melanomas and modifies the tumor immunosuppressive environment

Forkhead box p3 (Foxp3) expression was believed to be specific for T-regulatory cells but has recently been described in non-hematopoietic cells from different tissue origins and in tumor cells from both epithelial and non-epithelial tissues. The aim of this study was to elucidate the role of Foxp3 in murine melanoma. The B16F10 cell line Foxp3 silenced with small interference Foxp3 plasmid transfection was established and named B16F10.1. These cells had lower levels of Foxp3 mRNA (quantitative real-time reverse transcription-polymerase chain reaction [0.235-fold]), protein (flow cytometry [0.02%]), CD25⁺ expression (0.06%), cellular proliferation (trypan blue staining), and interleukin (IL)-2 production (enzyme-linked immunosorbent assay [72.35 pg/mL]) than those in B16F10 wild-type (WT) cells (P<0.05). Subcutaneous inoculation of the B16F10.1 cell line into C57BL/6 mice delayed the time of visible tumor appearance, increased the time of survival, and affected the weight of tumors, and also decreased the production of IL-10, IL-2, and transforming growth factor beta compared with mice inoculated with the B16F10 WT cell line. The B16F10.1 cells derived from tumors and free of T-cells (isolated by Dynabeads and plastic attachment) expressed relatively lower levels of Foxp3 and CD25⁺ than B16F10 WT cells (P<0.05) in a time-dependent manner. The population of tumor-infiltrating lymphocytes of T CD4⁺ cells (CD4⁺, CD4⁺CD25⁺, and CD4⁺CD25⁺Foxp3⁺) increased in a time-dependent manner (P<0.05) in tumors derived from B16F10 WT cells and decreased in tumors derived from B16F10.1 cells. Similar data were obtained from spleen cells. These results suggest that, in melanomas, Foxp3 partly induces tumor growth by modifying the immune system at the local and peripheral level, shifting the environment toward an immunosuppressive profile. Therapies incorporating this transcription factor could be strategies for cancer treatment.

TGF-β-induced IκB-ζ controls Foxp3 gene expression

Inhibitor of kappa B (IκB)-ζ, a member of the nuclear IκB family of proteins, is induced by the transforming growth factor (TGF)-β signaling pathway and plays a pivotal role in maintaining the balance of T helper (Th) cell subsets. IκB-ζ deficiency results in reduced percentages of Th17 cells and increased percentages of Th1 cells. In this study, the effects of IκB-ζ deficiency on T-cell subsets were examined further. The data showed that IκB-ζ-deficient T cells had a high capacity for generation of regulatory T cells (Tregs) when T cells were cultured under TGF-β stimulation in the presence of cytokine-neutralizing antibodies. Mechanistically, IκB-ζ itself negatively regulated activation of the Foxp3 promoter in a nuclear factor of kappaB-dependent manner. Thus, this study showed that IκB-ζ controlled Treg differentiation.

CD28 co-stimulation in T-cell homeostasis: a recent perspective

T-cells play a key role within the adaptive immune system mediating cellular immunity and orchestrating the immune response as a whole. Their activation requires not only recognition of antigen/major histocompatibility complexes by the T-cell receptor but in addition co-stimulation via the CD28 molecule through binding to CD80, CD86, or as recently discovered, inducible co-stimulator ligand expressed by antigen-presenting cells. Apart from tight control of the co-stimulatory signal by the T-cell receptor complex, expression of the inhibitory receptor cytotoxic T-lymphocyte antigen-4 (CTLA-4) sharing its ligands with CD28 is required to avoid inappropriate or prolonged T-cell activation. CD4(+) Foxp3(+) regulatory T (Treg) cells, which are crucial inhibitors of autoimmunity, add another level of complexity in that they differ from conventional non-regulatory CD4(+) T-cells by strongly depending on CD28 signaling for their generation and homeostasis. Moreover, CTLA-4 is constitutively expressed by Treg cells where it serves as a key mediator of suppression, while conventional CD4(+) T-cells express CTLA-4 only after activation. Here, we discuss recent insights into the molecular events underlying CD28-mediated co-stimulation, its impact on gene regulation, and the differential role of CD28 expression on Treg cells versus conventional CD4(+) and CD8(+) T-cells. Moreover, we summarize the exciting therapeutic options which have arisen from our current understanding of T-cell co-stimulation. Some of these have already been translated into the clinic, while others are expected to follow soon due to promising preclinical results. In particular, we discuss the failed 2006 trial of the CD28 superagonist TGN1412, and the return of this potent T-cell activator to clinical development.

The protein tyrosine phosphatase PTPN22 controls forkhead box protein 3 T regulatory cell induction but is dispensable for T helper type 1 cell polarization

Protein tyrosine phosphatases (PTPs) regulate T cell receptor (TCR) signalling and thus have a role in T cell differentiation. Here we tested whether the autoimmune predisposing gene PTPN22 encoding for a PTP that inhibits TCR signalling affects the generation of forkhead box protein 3 (FoxP3)(+) T regulatory (Treg ) cells and T helper type 1 (Th1) cells. Murine CD4(+) T cells isolated from Ptpn22 knock-out (Ptpn22(KO) ) mice cultured in Treg cell polarizing conditions showed increased sensitivity to TCR activation compared to wild-type (WT) cells, and subsequently reduced FoxP3 expression at optimal-to-high levels of activation. However, at lower levels of TCR activation, Ptpn22(KO) CD4(+) T cells showed enhanced expression of FoxP3. Similar experiments in humans revealed that at optimal levels of TCR activation PTPN22 knock-down by specific oligonucleotides compromises the differentiation of naive CD4(+) T cells into Treg cells. Notably, in vivo Treg cell conversion experiments in mice showed delayed kinetic but overall increased frequency and number of Treg cells in the absence of Ptpn22. In contrast, the in vitro and in vivo generation of Th1 cells was comparable between WT and Ptpn22(KO) mice, thus suggesting PTPN22 as a FoxP3-specific regulating factor. Together, these results propose PTPN22 as a key factor in setting the proper threshold for FoxP3(+) Treg cell differentiation.

Helper T-cell differentiation in graft-versus-host disease after allogeneic hematopoietic stem cell transplantation

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is an effective therapeutic option for many malignant diseases. However, the efficacy of allo-HSCT is limited by the occurrence of destructive graft-versus-host disease (GVHD). Since allogeneic T cells are the driving force in the development of GVHD, their activation, proliferation, and differentiation are key factors to understanding GVHD pathogenesis. This review focuses on one critical aspect: the differentiation and function of helper T (Th) cells in acute GVHD. We first summarize well-established subsets including Th1, Th2, Th17, and T-regulatory cells; their flexibility, plasticity, and epigenetic modification; and newly identified subsets including Th9, Th22, and T follicular helper cells. Next, we extensively discuss preclinical findings of Th-cell lineages in GVHD: the networks of transcription factors involved in differentiation, the cytokine and signaling requirements for development, the reciprocal differentiation features, and the regulation of microRNAs on T-cell differentiation. Finally, we briefly summarize the recent findings on the roles of T-cell subsets in clinical GVHD and ongoing strategies to modify T-cell differentiation for controlling GVHD in patients. We believe further exploration and understanding of the immunobiology of T-cell differentiation in GVHD will expand therapeutic options for the continuing success of allo-HSCT.

Peripheral and thymic foxp3(+) regulatory T cells in search of origin, distinction, and function

Over the past decade, much has been learnt and much more to discover about Foxp3(+) regulatory T cells (Tregs). Initially, it was thought that Tregs were a unique entity that originates in the thymus. It is now recognized that there is a fraternal twin sibling that is generated in the periphery. The difficulty is in the distinction between these two subsets. The ability to detect, monitor, and analyze these two subsets in health and disease will provide invaluable insights into their functions and purposes. The plasticity and mechanisms of action can be unique and not overlapping within these subsets. Therefore, the therapeutic targeting of a particular subset of Tregs might be more efficacious. In the past couple of years, a vast amount of data have provided a better understanding of the cellular and molecular components essential for their development and stability. Many studies are implicating their preferential involvement in certain diseases and immunologic tolerance. However, it remains controversial as to whether any phenotypic markers have been identified that can differentiate thymic versus peripheral Tregs. This review will address the validity and controversy regarding Helios, Lap/Garp and Neuropilin-1 as markers of thymic Tregs. It also will discuss updated information on distinguishing features of these two subsets and their critical roles in maternal-fetal tolerance and transplantation.

TLR5 signaling enhances the proliferation of human allogeneic CD40-activated B cell induced CD4hiCD25+ regulatory T cells

Although diverse functions of different toll-like receptors (TLR) on human natural regulatory T cells have been demonstrated recently, the role of TLR-related signals on human induced regulatory T cells remain elusive. Previously our group developed an ex vivo high-efficient system in generating human alloantigen-specific CD4^hiCD25⁺ regulatory T cells from naïve CD4⁺CD25⁻ T cells using allogeneic CD40-activated B cells as stimulators. In this study, we investigated the role of TLR5-related signals on the generation and function of these novel CD4^hiCD25⁺ regulatory T cells. It was found that induced CD4^hiCD25⁺ regulatory T cells expressed an up-regulated level of TLR5 compared to their precursors. The blockade of TLR5 using anti-TLR5 antibodies during the co-culture decreased CD4^hiCD25⁺ regulatory T cells proliferation by induction of S phase arrest. The S phase arrest was associated with reduced ERK1/2 phosphorylation. However, TLR5 blockade did not decrease the CTLA-4, GITR and FOXP3 expressions, and the suppressive function of CD4^hiCD25⁺ regulatory T cells. In conclusion, we discovered a novel function of TLR5-related signaling in enhancing the proliferation of CD4^hiCD25⁺ regulatory T cells by promoting S phase progress but not involved in the suppressive function of human CD40-activated B cell-induced CD4^hiCD25⁺ regulatory T cells, suggesting a novel role of TLR5-related signals in the generation of induced regulatory T cells.

A Bovine Lymphosarcoma Cell Line Infected with Theileria annulata Exhibits an Irreversible Reconfiguration of Host Cell Gene Expression

Theileria annulata, an intracellular parasite of bovine lymphoid cells, induces substantial phenotypic alterations to its host cell including continuous proliferation, cytoskeletal changes and resistance to apoptosis. While parasite induced modulation of host cell signal transduction pathways and NFκB activation are established, there remains considerable speculation on the complexities of the parasite directed control mechanisms that govern these radical changes to the host cell. Our objectives in this study were to provide a comprehensive analysis of the global changes to host cell gene expression with emphasis on those that result from direct intervention by the parasite. By using comparative microarray analysis of an uninfected bovine cell line and its Theileria infected counterpart, in conjunction with use of the specific parasitacidal agent, buparvaquone, we have identified a large number of host cell gene expression changes that result from parasite infection. Our results indicate that the viable parasite can irreversibly modify the transformed phenotype of a bovine cell line. Fifty percent of genes with altered expression failed to show a reversible response to parasite death, a possible contributing factor to initiation of host cell apoptosis. The genes that did show an early predicted response to loss of parasite viability highlighted a sub-group of genes that are likely to be under direct control by parasite infection. Network and pathway analysis demonstrated that this sub-group is significantly enriched for genes involved in regulation of chromatin modification and gene expression. The results provide evidence that the Theileria parasite has the regulatory capacity to generate widespread change to host cell gene expression in a complex and largely irreversible manner.

Foxp3+ regulatory T cells are activated in spite of B7-CD28 and CD40-CD40L blockade

Costimulatory signals are required for priming and activation of naive T cells, while it is less clear how they contribute to induction of regulatory T (Treg)-cell activity. We previously reported that the blockade of the B7-CD28 and CD40L-CD40 interaction efficiently suppresses allogeneic T-cell activation in vivo. This was characterized by an initial rise in Foxp3(+) cells, followed by depletion of host-reactive T cells. To further investigate effects of costimulatory blockade on Treg cells, we used an in vitro model of allogeneic CD4(+) cell activation. When CTLA-4Ig and anti-CD40L mAb (MR1) were added to the cultures, T-cell proliferation and IL-2 production were strongly reduced. However, Foxp3(+) cells proliferated and acquired suppressive activity. They suppressed activation of syngeneic CD4(+) cells much more efficiently than did freshly isolated Treg cells. CD4(+) cells activated by allogeneic cells in the presence of MR1 and CTLA-4Ig were hyporesponsive on restimulation, but their response was restored to that of naive CD4(+) cells when Foxp3(+) Treg cells were removed. We conclude that natural Treg cells are less dependent on B7-CD28 or CD40-CD40L costimulation compared with Foxp3(-) T cells. Reduced costimulation therefore alters the balance between Teff and Treg-cell activation in favor of Treg-cell activity.

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.

Cellular and molecular determinants for the development of natural and induced regulatory T cells

Regulation of immune responses to self and foreign antigens is critically dependent on suppressive CD4(+) T cells characterized by expression of Foxp3. The large majority of regulatory T (Treg) cells develop in the thymus as a stable suppressive lineage. However, under the proper physiological conditions, conventional peripheral CD4(+) T lymphocytes also develop into Treg cells, particularly in the gut mucosa and inflammatory tissue sites. This review will focus on our current understanding of the immunological and molecular signals controlling the development of thymic derived natural (n)Treg and peripheral converted induced (i)Treg cells. Given the importance of Foxp3 in the development of these cells, particular attention is placed on how such signals are integrated to induce and maintain the expression of this signature transcriptional regulator of Treg cells.

The molecular mechanisms of Foxp3 gene regulation

Induction of Foxp3 gene expression and acquisition of regulatory T cell fate is, understandably, a highly controlled process and one which many investigators want to illuminate. In studying the regulation of Foxp3 gene expression, several conserved non-coding regions have been identified and the role of various transcription factors at these sites has been explored. What emerges is that many factors, some positive, some negative, interact to collectively drive Foxp3 gene expression and then maintain its expression in Foxp3(+) regulatory T cells. TCR signaling is imperative for Foxp3 gene expression and TGF-β is a key cytokine for initiating Foxp3 gene expression in naïve T cells. But other signaling pathways are also known to play a role in properly orchestrating Foxp3 gene expression and regulatory T cell expansion. Here we review the recent progress in understanding the complex molecular events that drive Foxp3 gene expression and allow functional regulatory T cells to develop.

Molecular mechanisms underlying the regulation and functional plasticity of FOXP3(+) regulatory T cells

CD4(+) CD25(+) regulatory T (Treg) cells engage in the maintenance of immunological self-tolerance and homeostasis by limiting aberrant or excessive inflammation. The transcription factor forkhead box P3 (FOXP3) is critical for the development and function of Treg cells. The differentiation of the Treg cell lineage is not terminal, as developmental and functional plasticity occur through the sensing of inflammatory signals in the periphery. Here, we review the recent progress in our understanding of the molecular mechanisms underlying the regulation and functional plasticity of CD4(+) CD25(+) FOXP3(+) Treg cells, through the perturbation of FOXP3 and its complex at a transcriptional, translational and post-translational level.

Mechanisms underlying the induction of regulatory T cells and its relevance in the adaptive immune response in parasitic infections

To fulfill its function, the immune system must detect and interpret a wide variety of signals and adjust the magnitude, duration, and specific traits of each response during the complex host-parasite relationships in parasitic infections. Inflammation must be tightly regulated since uncontrolled inflammation may be as destructive as the triggering stimulus and leads to immune-mediated tissue injury.

During recent years, increasing evidence points to regulatory T cells (Tregs) as key anti-inflammatory cells, critically involved in limiting the inflammatory response. Herein, we review the published information on the induction of Tregs and summarize the most recent findings on Treg generation in parasitic diseases.

The dual role of the X-linked FoxP3 gene in human cancers

The FoxP3 (forkhead box P3) gene is an X-linked gene that is submitted to inactivation. It is an essential transcription factor in CD4(+)CD25(+)FoxP3 regulatory T cells, which are therapeutic targets in disseminated cutaneous melanoma. Moreover, FoxP3 is an important tumor suppressor gene in carcinomas and has putative cancer suppressor gene function in cutaneous melanoma as well. Therefore understanding the structure and function of the FoxP3 gene is crucial to gaining insight into the biology of melanoma to better develop immunotherapeutics and future therapeutic strategies.