Immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome: a model of immune dysregulation

A Guide to Nucleic Acid Vaccines in the Prevention and Treatment of Infectious Diseases and Cancers: From Basic Principles to Current Applications

Faced with the challenges posed by infectious diseases and cancer, nucleic acid vaccines present excellent prospects in clinical applications. Compared with traditional vaccines, nucleic acid vaccines have the characteristics of high efficiency and low cost. Therefore, nucleic acid vaccines have potential advantages in disease prevention and treatment. However, the low immunogenicity and instability of nucleic acid vaccines have limited their development. Therefore, a large number of studies have been conducted to improve their immunogenicity and stability by improving delivery methods, thereby supporting progress and development for clinical applications. This article mainly reviews the advantages, disadvantages, mechanisms, delivery methods, and clinical applications of nucleic acid vaccines.

Meta-analysis reveals significant association between FOXP3 polymorphisms and susceptibility to Graves' disease

OBJECTIVE

This meta-analysis aimed to determine the associations between the rs3761547, rs3761548, and rs3761549 single-nucleotide polymorphisms (SNPs) of the forkhead box P3 (FOXP3) gene and susceptibility to Graves' disease (GD).

METHODS

Case-control studies with information on the associations between the rs3761547, rs3761548, and rs3761549 FOXP3 SNPs and GD published before 01 May 2020 were identified in the PubMed, Embase, Web of Science, and China National Knowledge Infrastructure databases. Data from the studies were analyzed using RevMan version 5.3.

RESULTS

Seven independent case-control studies including 4051 GD patients and 4569 controls were included in the meta-analysis. The overall pooled analysis indicated that FOXP3/rs3761548 and FOXP3/rs3761549 polymorphisms were significantly associated with GD susceptibility (rs3761548: A vs. C, odds ratio [OR] = 1.32, 95% confidence interval [CI] 1.05-1.67; rs3761549: TT vs. CC, OR = 1.98, 95%CI 1.49-2.65; (TT + TC) vs. CC, OR = 1.44, 95%CI 1.11-1.88). In contrast, the FOXP3/rs3761547 polymorphism was not associated with GD susceptibility. Subgroup analysis according to ethnicity showed that rs3761548 was associated with GD in Asians but not in Caucasians, whereas rs3761549 was associated in both Asians and Caucasians.

CONCLUSION

This meta-analysis demonstrated that FOXP3/rs3761548 and FOXP3/rs3761549 SNPs were significantly associated with susceptibility to GD, at least in Asian populations.

The double-edged sword of Tregs in M tuberculosis, M avium, and M absessus infection

Immunity against different Mycobacteria species targeting the lung requires distinctly different pulmonary immune responses for bacterial clearance. Many parameters of acquired and regulatory immune responses differ quantitatively and qualitatively from immunity during infection with Mycobacteria species. Nontuberculosis Mycobacteria species (NTM) Mycobacterium avium- (M avium), Mycobacterium abscessus-(M abscessus), and the Mycobacteria species Mycobacterium tuberculosis-(Mtb). Herein, we discuss the potential implications of acquired and regulatory immune responses in the context of animal and human studies, as well as future directions for efforts to treat Mycobacteria diseases.

Histone Deacetylation Inhibitors as Modulators of Regulatory T Cells

Regulatory T cells (T_regs) are important mediators of immunological self-tolerance and homeostasis. Being cluster of differentiation 4⁺Forkhead box protein3⁺ (CD4⁺FOXP3⁺), these cells are a subset of CD4⁺ T lymphocytes and can originate from the thymus (tT_regs) or from the periphery (pT_regs). The malfunction of CD4⁺ T_regs is associated with autoimmune responses such as rheumatoid arthritis (RA), multiple sclerosis (MS), type 1 diabetes (T1D), inflammatory bowel diseases (IBD), psoriasis, systemic lupus erythematosus (SLE), and transplant rejection. Recent evidence supports an opposed role in sepsis. Therefore, maintaining functional T_regs is considered as a therapy regimen to prevent autoimmunity and allograft rejection, whereas blocking T_reg differentiation might be favorable in sepsis patients. It has been shown that T_regs can be generated from conventional naïve T cells, called iT_regs, due to their induced differentiation. Moreover, T_regs can be effectively expanded in vitro based on blood-derived tT_regs. Taking into consideration that the suppressive role of T_regs has been mainly attributed to the expression and function of the transcription factor Foxp3, modulating its expression and binding to the promoter regions of target genes by altering the chromatin histone acetylation state may turn out beneficial. Hence, we discuss the role of histone deacetylation inhibitors as epigenetic modulators of T_regs in this review in detail.

The Dual Role of Treg in Cancer

Regulatory T cells (Tregs) represent a small subpopulation of CD4⁺ cells. Tregs are characterized by the expression of transcription factor Forkhead box protein 3 (FoxP3), also known as scurfin. Tregs are modulators of adaptive immune responses and play an important role in maintaining tolerance to self-antigens, providing the suppression associated with tumour microenvironment as well. These immunomodulatory properties are the main reason for the development of numerous therapeutic strategies, designed to inhibit the activity of cancer cells. However, due to Treg subpopulation diversity and its many functional pathways, the role of these cells in the cancer development and progression is still not fully understood.

Regulation of Immunity to Tuberculosis

Immunity against Mycobacterium tuberculosis requires a balance between adaptive immune responses to constrain bacterial replication and the prevention of potentially damaging immune activation. Regulatory T (Treg) cells express the transcription factor Foxp3+ and constitute an essential counterbalance of inflammatory Th1 responses and are required to maintain immune homeostasis. The first reports describing the presence of Foxp3-expressing CD4+ Treg cells in tuberculosis (TB) emerged in 2006. Different Treg cell subsets, most likely specialized for different tissues and microenvironments, have been shown to expand in both human TB and animal models of TB. Recently, additional functional roles for Treg cells have been demonstrated during different stages and spectrums of TB disease. Foxp3+ regulatory cells can quickly expand during early infection and impede the onset of cellular immunity and persist during chronic TB infection. Increased frequencies of Treg cells have been associated with a detrimental outcome of active TB, and may be dependent on the M. tuberculosis strain, animal model, local environment, and the stage of infection. Some investigations also suggest that Treg cells are required together with effector T cell responses to obtain reduced pathology and sterilizing immunity. In this review, we will first provide an overview of the regulatory cells and mechanisms that control immune homeostasis. Then, we will review what is known about the phenotype and function of Treg cells from studies in human TB and experimental animal models of TB. We will discuss the potential role of Treg cells in the progression of TB disease and the relevance of this knowledge for future efforts to prevent, modulate, and treat TB.

The CII-specific autoimmune T-cell response develops in the presence of FTY720 but is regulated by enhanced Treg cells that inhibit the development of autoimmune arthritis

Background

Fingolimod (FTY720) is an immunomodulating drug that inhibits sphingosine-1-phosphate binding and blocks T-cell egress from lymph nodes. We analyzed the effect of FTY720 on the autoimmune T- and B-cell response in autoimmune arthritis and studied the mechanisms by which it alters the function of T cells.

Methods

Human leukocyte antigen (HLA)-DR1 humanized mice were immunized with type II collagen (CII) and treated with FTY720 three times per week for 3 weeks. Arthritis was evaluated and autoimmune T- and B-cell responses were measured using proliferation assays, enzyme-linked immunosorbent assays, HLA-DR tetramers, and flow cytometry. The functional capacity of regulatory T (Treg) cells from FTY720-treated mice was measured using an in vitro suppression assay, and the role of Treg cells in inhibiting arthritis in FTY720-treated mice was evaluated using mice treated with anti-CD25 to deplete Treg cells.

Results

Treatment with FTY720 delayed the onset of arthritis and significantly reduced disease incidence. FTY720 did not prevent the generation of a CII-specific autoimmune T-cell response in vivo. However, as the treatment continued, these T cells became unresponsive to restimulation with antigen in vitro, and this anergic state was reversed by addition of interleukin 2. Measurements of CD4⁺CD25⁺Foxp3⁺ cells in the lymph nodes revealed that the ratio of Treg to helper T (Th) cells increased twofold in the FTY720-treated mice, and in vitro assays indicated that the regulatory function of these cells was enhanced. That FTY720 stimulation of Treg cells played a major role in arthritis inhibition was demonstrated by a loss of disease inhibition and restitution of the T-cell proliferative function after in vivo depletion of the Treg cells.

Conclusions

While FTY720 affects the recirculation of lymphocytes, its ability to inhibit the development of autoimmune arthritis involves several mechanisms, including the enhancement of Treg cell function by increasing the Treg/Th ratio and increased regulatory function on a per-cell basis. FTY720 did not inhibit the development of the autoimmune T-cell response, but disease inhibition appeared to be mediated by Treg cell–mediated suppression of the CII-specific T cells. These data suggest that specific targeting of Treg cells with FTY720 may be a novel therapy for autoimmunity.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-015-0909-6) contains supplementary material, which is available to authorized users.

Exome sequencing identifies a novel FOXP3 mutation in a 2-generation family with inflammatory bowel disease

OBJECTIVES

Inflammatory bowel disease (IBD) is heritable, but a total of 163 variants commonly implicated in IBD pathogenesis account for only 25% of the heritability. Rare, highly penetrant genetic variants may also explain mendelian forms of IBD and some of the missing heritability. To test the hypothesis that rare loss-of-function mutations can be causative, we performed whole exome sequencing (WES) on 5 members of a 2-generation family of European ancestry presenting with an early-onset and atypical form of IBD.

METHODS

WES was performed for all of the 5 family members; the mother and 3 male offspring were affected, whereas the father was unaffected. Mapping, annotation, and filtering criteria were used to reduce candidate variants. For functional testing we performed forkhead box P3 (FOXP3) staining and a T-cell suppression assay.

RESULTS

We identified a novel missense variant in exon 6 of the X-linked FOXP3 gene. The c.694A>C substitution in FOXP3 results in a cysteine-to-glycine change at the protein position 232 that is completely conserved among all vertebrates. This variant (heterozygous in the mother and hemizygous in all 3 affected sons) did not impair FOXP3 protein expression, but significantly reduced the ability of the host's T regulatory cells to suppress an inappropriate autoimmune response. The variant results in a milder immune dysregulation, polyendocrinopathy, enteropathy, and X-linked phenotype with early-onset IBD.

CONCLUSIONS

Our study illustrates the successful application of WES for making a definitive molecular diagnosis in a case of multiply affected families, with atypical IBD-like phenotype. Our results also have important implications for disease biology and disease-directed therapeutic development.

Engineered regulatory T cells coexpressing MHC class II:peptide complexes are efficient inhibitors of autoimmune T cell function and prevent the development of autoimmune arthritis

Regulatory T cells (Tregs) are critical homeostatic components in preventing the development of autoimmunity, and are a major focus for their therapeutic potential for autoimmune diseases. To enhance the efficacy of Tregs in adoptive therapy, we developed a strategy for generating engineered Tregs that have the capacity to target autoimmune T cells in an Ag-specific manner. Using a retroviral expression system encoding Foxp3 and HLA-DR1 covalently linked to the immunodominant peptide of the autoantigen type II collagen (DR1-CII), naive T cells were engineered to become Tregs that express DR1-CII complexes on their surface. When these cells were tested for their ability to prevent the development of collagen induced arthritis, both the engineered DR1-CII-Foxp3 and Foxp3 only Tregs significantly reduced the severity and incidence of disease. However, the mechanism by which these two populations of Tregs inhibited disease differed significantly. Disease inhibition by the DR1-CII-Foxp3 Tregs was accompanied by significantly lower numbers of autoimmune CII-specific T cells in vivo and lower levels of autoantibodies in comparison with engineered Tregs expressing Foxp3 alone. In addition, the numbers of IFN-γ- and IL-17-expressing T cells in mice treated with DR1-CII-Foxp3 Tregs were also significantly reduced in comparison with mice treated with Foxp3 engineered Tregs or vector control cells. These data indicate that the coexpression of class II autoantigen-peptide complexes on Tregs provides these cells with a distinct capacity to regulate autoimmune T cell responses that differs from that used by conventional Tregs.

The genetic basis of severe combined immunodeficiency and its variants

Severe combined immunodeficiency (SCID) syndromes are characterized by a block in T lymphocyte differentiation that is variably associated with abnormal development of other lymphocyte lineages (B and/or natural killer [NK] cells), leading to death early in life unless treated urgently by hematopoietic stem cell transplant. SCID comprises genotypically and phenotypically heterogeneous conditions, of which the genetic basis for approximately 85% of the underlying immunologic defects have been recently elucidated. A major obstacle in deciphering the pathogenesis of SCID syndromes is that different mutations in a single gene may give rise to distinct clinical conditions and that a similar clinical phenotype can result from mutations in different genes. Mutation analysis is now an important component of the complete evaluation of a patient with SCID since it has a dramatic impact on many aspects of this potentially life-threatening disease such as genetic counseling, prenatal diagnosis, modalities of treatment, and, eventually, prognosis. Dr Robert Good, one of the founders of modern immunology, described the SCID syndrome as “experiments of nature.” By understanding the cellular and genetic basis of these immunodeficiency diseases and, eventually, normal immunity, we optimize the “bedside to research laboratory and back again” approach to medicine.

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.

FOXP3 expression following bone marrow transplantation for IPEX syndrome after reduced-intensity conditioning

The objective of this study is to determine if immune reconstitution of FOXP3+ T regulatory cells correlates with clinical improvement of IPEX syndrome following allogeneic hematopoietic stem cell transplant. An 8-months-old male infant with a mutation in the polyadenylation site of FOXP3 gene, absence of FOXP3 protein expression and clinical manifestations of IPEX syndrome, including eczema, colitis, failure to thrive, TPN requirement, and elevated serum IgE, underwent matched unrelated hematopoietic stem cell transplant. After reduced-intensity conditioning with alemtuzumab followed by fludarabine and melphalan the patient's neutrophils engrafted day +15 and platelets day +29. Patient was a full donor chimera day +28 and +60. Intracellular FOXP3 protein expression in CD4+ T cells was absent pre-HSCT. After transplantation, percentage CD4+ T cells expressing FOXP3+CD25 bright phenotype quickly increased from 4.5 (day +29) to 23% (day +90) and continued in this trend. Foxp3 mRNA expression confirmed flow cytometry data. Serum IgE levels decreased from 5,000 IU/ml pre-transplant to 6 IU/ml on day +90, eczema resolved, and secretory diarrhea and feeding intolerance improved. T regulatory cell reconstitution is evident soon after HSCT following reduced-intensity conditioning correlating with development of full donor chimerism. Increased FOXP3 expression correlates with correction of clinical and laboratory manifestations of IPEX syndrome providing direct evidence that HSCT is a curative procedure for this disorder.

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.

FOXP3 and the regulation of Treg/Th17 differentiation

CD4 T cell lineages are marked by the signature transcription factor each lineage expresses. For example, regulatory T cells (Tregs) are characterized by expression of FOXP3, which is either induced during thymic development for natural Tregs (nTregs), or in the periphery in the presence of TGF-beta and retinoic acid for induced Tregs (iTreg). Interestingly, recent work has shown that the signature transcription factor for Th17 cells, RORgammat, is also induced by TGF-beta, thus linking the differentiation of the Treg and Th17 lineages. In the absence of a second signal from a proinflammatory cytokine, FOXP3 can inhibit RORgammat function and drive Treg differentiation. However, when the cell also receives a signal from a proinflammation cytokine (e.g., IL-6), FOXP3 function is inhibited and the Th17 differentiation pathway is induced. Therefore, it is the balance between FOXP3 and RORgammat function that determines CD4 T cell fate and the type of immune response that will be generated.

Functional analysis of FOXP3

Tolerance to self antigens is established in two ways: first in the thymus through the deletion of thymocytes expressing self-reactive T cell receptors; and second, in the periphery through multiple mechanisms involving deletion, anergy, and suppression. Dominant tolerance to self antigens in the periphery is primarily the function of the CD4(+)CD25(+)FOXP3(+) subset of T cells, which have the capability of suppressing autoreactive T cells that have escaped deletion during thymic selection. The essential role of the transcription factor FOXP3 in the development and function of these cells has been well documented. However, the underlying mechanisms by which FOXP3 controls this process are less well understood. This review will focus on the role of FOXP3 in regulating CD4 T cell function in both humans and mice, with an emphasis on recent work in human systems.

Approach to the patient with recurrent infections

Children with a history of recurrent or unusual infections present a diagnostic challenge. Differentiation between frequent infections caused by common risk factors, versus primary immune dysfunction should be based on a detailed history and physical examination and, if indicated, followed by appropriate laboratory studies. A high index of suspicion could lead to an early diagnosis and treatment of an underlying immune deficiency disease. This article presents to physicians an approach to the evaluation of children with recurrent infections. Important details from the history and physical examination, and an appropriate choice of screening laboratory test to be ordered in a given situation are discussed.

Isoform-specific inhibition of ROR alpha-mediated transcriptional activation by human FOXP3

FOXP3 is a forkhead family transcriptional repressor important for the development and function of CD4(+)CD25(+) regulatory T cells. In humans, FOXP3 is expressed as two isoforms, a full-length form and a smaller form lacking exon 2. These two isoforms are expressed in approximately equal amounts in circulating regulatory T cells, and are induced equally in freshly activated CD4(+)CD25(-) T cells. Herein, we show that FOXP3 interacts with retinoic acid receptor-related orphan receptor (ROR)alpha, and that this interaction inhibits transcriptional activation mediated by RORalpha. Full-length FOXP3, but not the isoform lacking exon 2, interacts with RORalpha, and the region of FOXP3 involved in the interaction is encoded by exon 2. Mutation of the LxxLL motif in FOXP3, located in exon 2, abolished interaction and repression by FOXP3. Additionally, the inhibition of RORalpha by FOXP3 does not require an intact forkhead domain, demonstrating a mode of FOXP3 function that is independent of DNA binding. Interestingly, expression of RORalpha in T cells leads to the expression of genes that define Th17 cells, and the expression of each of these gene was inhibited by coexpression of full-length, but not DeltaEx2, FOXP3. These data expand the possible targets of FOXP3-mediated repression and demonstrate functional differences between FOXP3 isoforms.

IPEX syndrome: Case report

INTRODUCTION IPEX syndrome, namely, a hereditary (X-linked) immunodysregulation with autoimmune polyendocrinopathy and enteropathy, as the basic manifestations, presents a rare and exceptionally severe disease. It develops due to gene mutation responsible for the synthesis of a specific protein (FOXP3), which, by differentiation and activation of regular T-lymphocytic CD4+CD25+, has the key role in the induction and maintenance of the peripheral tolerance of one's own tissue. CASE OUTLINE We present a male infant with classic clinical features of IPEX syndrome, which manifested by the end of the first month after birth, first with type 1 diabetes mellitus and chronic diarrhoea followed by dehydration and disordered development, and then with facial eczema and laboratory signs of thyroiditis without thyroid dysfunction (antithyreoglobulin antibodies 1:5500, antimicrosomal antibodies 1:40). In addition, plasma IgE level was high (517 IU/l), while antibodies to tissue transglutaminase were mildly increased (IgA 7.5 U/ml), and anti-smooth muscle and anti-DNA antibodies were absent. Based on the typical clinical features, as well as the laboratory findings, IPEX syndrome was diagnosed, which was further confirmed by proved IVS7+5G>A mutations in the FOXP3 gene. Therapy with insulin and Pronison, combined with parenteral and semielementary nutrition resulted in the patient's clinical improvement. At the age of 9 months, despite Pronison and hypoallergenic nutrition, the child had a relapse of severe and persistent diarrhoeal disorder followed by dehydration, weight loss and deterioration of general condition. Beside the complete parenteral nutrition, as well as other measures, azathioprine was introduced into the treatment, but without the desired effect. At the age of 12.5 months, due to bacteraemia and disseminated intravascular coagulation as complications, the patient ended lethally. CONCLUSION IPEX syndrome should be kept in mind in all the cases of associated type 1 diabetes mellitus and chronic diarrhoea in male neonates or infants. Although treatment results have still been modest, it is quite certain they will be far better in the near future.

Chronic granulomatous disease associated with atypical Kawasaki disease

Chronic granulomatous disease (CGD) is an infrequent inherited disorder characterized by recurrent infections and abnormal granuloma formation. Patients with CGD have an exuberant inflammatory response and an increased risk of developing autoimmunity. We present the case of a 1-year-old boy with CGD who developed several of the characteristic clinical features of Kawasaki Disease. His illness responded to intravenous immunoglobulin, aspirin, and corticosteroids.

The NLR network and the immunological disease continuum of adaptive and innate immune-mediated inflammation against self

The nucleotide-binding domain, leucine-rich repeat containing family (NLR) network has provided pivotal genetic and molecular insights into diseases that were hitherto regarded as autoimmune. The NLR-related disorders include rare monogenic autoinflammatory diseases collectively termed cryopyrin-associated periodic syndromes, Crohn's disease which is a common polygenic disease and also an association at the mechanistic level with gout and pseudogout. Unlike the classical autoimmune diseases where disease immunopathogenesis is played out primarily in the primary and secondary lymphoid organs, the immunopathogenesis of the NLR-related disorders is played out in the tissues where inflammation arises. As the genetic mutations or molecular cascades associated with the NLR-related disorders have a widespread cellular distribution, it has been somewhat enigmatic why these disorders attack certain territories, but not others. This implies that tissue-specific factors in the target organs themselves contribute to disease expression. Such examples include the high abundance of NOD2 expressing cells in the part of the gut most typically afflicted by Crohn's disease and the preferential deposition of crystals in the joints to where inflammation localises in gout and pseudogout. The NLR network is associated principally with increases in TNF or IL-1 production, both of which are key players in innate immunity. Therefore, the NLR network identifies at the genetic and molecular level a robust paradigm for innate immune activation against self. This tissue-specific-factor-associated inflammation is the diametric opposite of classical autoimmunity. Of note, the MHC class-I-associated diseases including psoriasis (HLA-Cw6) and ankylosing spondylitis (HLA-B27) show striking clinical overlaps with Crohn's disease and also some rare monogenic diseases. Thus, the NLR innate immune pathway allows the full spectrum of inflammation against self to be viewed along an immunological disease continuum with autoantibody-associated disease at one end, innate immune diseases at the other and MHC class-1-related disorders as an intermediate.

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

FoxP3 determines the development of CD4+CD25+ regulatory T (Treg) cells and represses interleukin-2 (IL-2) expression in Treg cells. However, human immunodeficiency virus type 1 (HIV-1) infects and replicates efficiently in FoxP3+ Treg cells. We report that, while inhibiting IL-2 gene expression, FoxP3 enhances gene expression from HIV-1 long terminal repeat (LTR). This FoxP3 activity requires both the N- and C-terminal domains and is inactivated by human IPEX (immunodysregulation, polyendocrinopathy, enteropathy, X-linked syndrome) mutations. FoxP3 enhances HIV-1 LTR via its specific NFkappaB binding sequences in an NFkappaB-dependent fashion in T cells but not in HEK293 cells. FoxP3 decreases level of histone acetylation at the interleukin-2 locus but not at the HIV-1 LTR. Although NFkappaB nuclear translocation is not altered, FoxP3 enhances NFkappaB-p65 binding to HIV-1 LTR. These data suggest that FoxP3 modulates gene expression in a promoter sequence-dependent fashion by modulating chromatin structure and NFkappaB activity. HIV-1 LTR has evolved to both highjack the T-cell activation pathway for expression and to resist FoxP3-mediated suppression of T-cell activation.

FOXP3 modifies the phenotypic and functional properties of regulatory T cells

In the periphery, tolerance to self antigens is mainly mediated by the CD4(+)CD25(+)FOXP3(+) subset of regulatory T cells, which can suppress the activity of autoreactive T cells that have escaped deletion in the thymus. The essential role of the transcription factor FOXP3 (forkhead box P3) in the development and function of these regulatory T cells has been well documented. It is also clear that regulatory T cells and effector T cells respond differently to T-cell receptor stimulation. In this Opinion article, we propose that these differences in responses are mediated by FOXP3, and are manifested by alterations in biochemical signalling pathways, patterns of gene expression and the appearance of cell-surface homing receptors.

Analysis of FOXP3 reveals multiple domains required for its function as a transcriptional repressor

Foxp3 has been shown to be both necessary and sufficient for the development and function of naturally arising CD4+ CD25+ regulatory T cells in mice. Mutation of Foxp3 in Scurfy mice and FOXP3 in humans with IPEX results in fatal, early onset autoimmune disease and demonstrates the critical role of FOXP3 in maintaining immune homeostasis. The FOXP3 protein encodes several functional domains, including a C2H2 zinc finger, a leucine zipper, and a winged-helix/forkhead (FKH) domain. We have shown previously that FOXP3 functions as a transcriptional repressor and inhibits activation-induced IL-2 gene transcription. To characterize the role of each predicted functional domain on the in vivo activity of FOXP3, we have evaluated the location of point mutations identified in a large cohort of patients with the immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome (IPEX) and found them to cluster primarily within the FKH domain and the leucine zipper, but also present within the poorly defined N-terminal portion of the protein. The molecular functions of each of the IPEX-targeted domains were investigated. We show that FOXP3 is constitutively localized to the nucleus and this localization requires sequences at both the amino and C-terminal ends of its FKH domain. Moreover, FOXP3 was found to homodimerize through its leucine zipper. We also identify a novel functional domain within the N-terminal half of FOXP3, which is required for FOXP3-mediated repression of transcription from both a constitutively active and a NF-AT-inducible promoter. Furthermore, we demonstrate that IPEX mutations in these domains correlate with deficiencies in FOXP3 repressor function, corroborating their in vivo relevance.

FOXP3: of mice and men

The immune system has evolved mechanisms to recognize and eliminate threats, as well as to protect against self-destruction. Tolerance to self-antigens is generated through two fundamental mechanisms: (a) elimination of self-reactive cells in the thymus during selection and (b) generation of a variety of peripheral regulatory cells to control self-reactive cells that escape the thymus. It is becoming increasing apparent that a population of thymically derived CD4+ regulatory T cells, exemplified by the expression of the IL-2Ralpha chain, is essential for the maintenance of peripheral tolerance. Recent work has shown that the forkhead family transcription factor Foxp3 is critically important for the development and function of the regulatory T cells. Lack of Foxp3 leads to development of fatal autoimmune lymphoproliferative disease; furthermore, ectopic Foxp3 expression can phenotypically convert effector T cells to regulatory T cells. This review focuses on Foxp3 expression and function and highlights differences between humans and mice regarding Foxp3 regulation.

Delayed functional maturation of natural regulatory T cells in the medulla of postnatal thymus: role of TSLP

BACKGROUND

Generation of functional (CD4+)(CD8-)CD25+ regulatory T cells (Treg) in the murine thymus depends on FoxP3. Removal of the thymus from neonatal mice has been shown to result in a multiple organ autoimmune disease phenotype that can be prevented by introducing the FoxP3+ Treg population to the animal. It has therefore, been proposed that functional FoxP3+ Treg cells are not made in the neonatal thymus; however, it remains unclear when and where functional (FoxP3+)(CD4+)(CD8-)CD25+ thymocytes are generated in postnatal thymus.

RESULTS

We report that neither FoxP3 mRNA nor protein is expressed in (CD4+)(CD8-)CD25+, or (CD4+)(CD8-)CD25- thymocytes until 3-4 days post birth, despite the presence of mature (CD4+)(CD8-)CD25+/- thymocytes in the thymus by 1-2 days after birth. (FoxP3-)(CD4+)(CD8-)CD25+ thymocytes from day 2 newborn mice show no Treg activity. Interestingly, we are able to detect low numbers of FoxP3+ thymocytes dispersed throughout the medullary region of the thymus as early as 3-4 days post birth. Expression of FoxP3 is induced in embryonic day 17 fetal thymus organ culture (FTOC) after 4-6 days of in vitro culture. Treatment of FTOCs with thymic stromal derived lymphopoietin (TSLP) enhanced expression of FoxP3, and blocking the TSLP receptor reduces FoxP3 expression in FTOC. Furthermore, TSLP stimulates FoxP3 expression in purified (CD4+)CD8- thymocytes, but not in (CD4+)CD8+, (CD4-)CD8+ and (CD4-)CD8- thymocytes.

CONCLUSION

Expression of FoxP3 or Treg maturation is ontogenically distinct and kinetically delayed from the generation of (CD4+)(CD8-)CD25+ or (CD4+)(CD8-)CD25- thymocytes in the postnatal thymus. TSLP produced from medullary thymic epithelia cells (mTEC) contributes to the expression of FoxP3 and the maturation of natural regulatory T cells. Overall, these results suggest that the development of Treg cells requires paracrine signaling during late stages of thymocyte maturation that is distinct from signaling during positive or negative selection.

DNA vaccination against tumors

DNA vaccines have been used to generate protective immunity against tumors in a variety of experimental models. The favorite target antigens have been those that are frequently expressed by human tumors, such as carcinoembryonic antigen (CEA), ErbB2/neu, and melanoma-associated antigens. DNA vaccines have the advantage of being simple to construct, produce and deliver. They can activate all arms of the immune system, and allow substantial flexibility in modifying the type of immune response generated through codelivery of cytokine genes. DNA vaccines can be applied by intramuscular, dermal/epidermal, oral, respiratory and other routes, and pose relatively few safety concerns. Compared to other nucleic acid vectors, they are usually devoid of viral or bacterial antigens and can be designed to deliver only the target tumor antigen(s). This is likely to be important when priming a response against weak tumor antigens. DNA vaccines have been more effective in rodents than in larger mammals or humans. However, a large number of methods that might be applied clinically have been shown to ameliorate these vaccines. This includes in vivo electroporation, and/or inclusion of various immunostimulatory molecules, xenoantigens (or their epitopes), antigen-cytokine fusion genes, agents that improve antigen uptake or presentation, and molecules that activate innate immunity mechanisms. In addition, CpG motifs carried by plasmids can overcome the negative effects of regulatory T cells. There have been few studies in humans, but recent clinical trials suggest that plasmid/virus, or plasmid/antigen-adjuvant, prime-boost strategies generate strong immune responses, and confirm the usefulness of plasmid-based vaccination.