Chromosomal clustering of genes controlled by the aire transcription factor

Investigating Thymic Epithelial Cell Diversity Using Systems Biology

The thymus is an intricate organ consisting of a diverse population of thymic epithelial cells (TECs). Cortical and medullary TECs and their subpopulations have distinct roles in coordinating the development and selection of functionally competent and self-tolerant T cells. Recent advances made in technologies such as single-cell RNA sequencing have made it possible to investigate and resolve the heterogeneity in TECs. These findings have provided further understanding of the molecular mechanisms regulating TEC function and expression of tissue-restricted Ags. In this brief review, we focus on the newly characterized subsets of TECs and their diversity in relation to their functions in supporting T cell development. We also discuss recent discoveries in expression of self-antigens in the context of TEC development as well as the cellular and molecular changes occurring during embryonic development to thymic involution.

A novel variant in AIRE causing a rare, non‑classical autoimmune polyendocrine syndrome type 1

Autoimmune polyendocrine syndrome type 1 (APS‑1) is a rare inherited autoimmune disease, characterized by a classic triad, including chronic mucocutaneous candidiasis, primary adrenocortical insufficiency and hypoparathyroidism. The present study investigated phenotypes and pathogenic variants in a Chinese woman with non‑classical APS‑1. Disease‑associated variants in a patient with APS‑1 were identified via targeted next generation sequencing and the variant was confirmed via Sanger sequencing. Serum levels of calcium, phosphorus, parathyroid hormone (PTH), follicle‑stimulating hormone (FSH), luteinizing hormone (LH), estradiol and urinary levels of calcium were measured. Blood count assays and bone marrow morphology were investigated. The patient was a 32‑year‑old woman who had suffered from typical carpopedal spasms since she was 7 years old. She developed syncope, primary amenorrhea, intermittent diarrhea and general fatigue in subsequent years. Hypocalcemia, hyperphosphatemia, low levels of PTH and estradiol, elevated levels of FSH and LH, and absence of erythroblasts were observed, which indicated hypoparathyroidism, primary ovarian insufficiency and pure red cell aplasia. A novel heterozygous missense variant (NM_000383.2: c.623G>T, NP_000374.1: p.Gly208Val) in exon 5 of autoimmune regulator and a reported variant (NM_000383.2: c.371C>T, NP_000374.1: p.Pro124Leu) in exon 3 were detected, of which the c.623G>T variant may be a pathogenic variation that induces APS‑1. Under a regular follow‑up and therapeutic adjustment of calcium, calcitriol, hormone replacement therapy and methylprednisolone, the endocrine function and clinical symptoms of the patient were notably improved. The results of the present study expand the known genetic and phenotypical spectra of APS‑1.

Meta-Analysis of Autoimmune Regulator-Regulated Genes in Human and Murine Models: A Novel Human Model Provides Insights on the Role of Autoimmune Regulator in Regulating STAT1 and STAT1-Regulated Genes

Autoimmune regulator (AIRE) regulates promiscuous expression of tissue-restricted antigens in medullary epithelial cells (mTEC) of the thymus. To understand the diverse effects of AIRE, it is crucial to elucidate the molecular mechanisms underlying the process of AIRE-regulated gene expression. In this study, we generated a recombinant AIRE expression variant of the TEC 1A3 human cell line, TEC 1A3 AIRE^hi, to determine genes targeted by AIRE, and using microarray analysis, we identified 482 genes showing significant differential expression (P < 0.05; false discovery rate <5%), with 353 upregulated and 129 downregulated by AIRE expression. Microarray data were validated by quantitative PCR, confirming the differential expression of 12 known AIRE-regulated genes. Comparison of AIRE-dependent differential expression in our cell line model with murine datasets identified 447 conserved genes with a number of transcription regulatory interactions, forming several key nodes, including STAT1, which had over 30 interactions with other AIRE-regulated genes. As STAT1 mutations cause dominant chronic mucocutaneous candidiasis and decreased STAT1 levels in monocytes of autoimmune polyglandular syndrome 1 (APS-1) patients, it was important to further characterize AIRE–STAT1 interactions. TEC 1A3AIRE^hi were treated with the STAT1 phosphorylation inhibitors fludarabine and LLL3 showed that phosphorylated STAT1 (p-STAT1) was not responsible for any of the observed differential expression. Moreover, treatment of TEC 1A3 AIRE^hi with STAT1 shRNA did not induce any significant variation in the expression of unphosphorylated STAT1 (U-STAT1) downstream genes, suggesting that these genes were directly regulated by AIRE but not via U-STAT1. The novel model system we have developed provides potential opportunities for further analysis of the pathogenesis of (APS-1) and the wider roles of the AIRE gene.

Neutralizing Anti-Cytokine Autoantibodies Against Interferon-α in Immunodysregulation Polyendocrinopathy Enteropathy X-Linked

Anti-cytokine autoantibodies (ACAAs) have been described in a growing number of primary immunodeficiencies with autoimmune features, including autoimmune polyendocrine syndrome type I (APS-1), a prototypical disease of defective T cell-mediated central tolerance. Whether defects in peripheral tolerance lead to similar ACAAs is unknown. Immunodysregulation polyendocrinopathy enteropathy X-linked (IPEX) is caused by mutations in FOXP3, a master regulator of T regulatory cells (T_reg), and consequently results in defective T cell-mediated peripheral tolerance. Unique autoantibodies have previously been described in IPEX. To test the hypothesis that ACAAs are present in IPEX, we designed and fabricated antigen microarrays. We discovered elevated levels of IgG ACAAs against interferon-α (IFN-α) in a cohort of IPEX patients. Serum from IPEX patients blocked IFN-α signaling in vitro and blocking activity was tightly correlated with ACAA titer. To show that blocking activity was mediated by IgG and not other serum factors, we purified IgG and showed that blocking activity was contained entirely in the immunoglobulin fraction. We also screened for ACAAs against IFN-α in a second geographically distinct cohort. In these samples, ACAAs against IFN-α were elevated in a post hoc analysis. In summary, we report the discovery of ACAAs against IFN-α in IPEX, an experiment of nature demonstrating the important role of peripheral T cell tolerance.

Twenty Years of AIRE

About two decades ago, cloning of the autoimmune regulator (AIRE) gene materialized one of the most important actors on the scene of self-tolerance. Thymic transcription of genes encoding tissue-specific antigens (ts-ags) is activated by AIRE protein and embodies the essence of thymic self-representation. Pathogenic AIRE variants cause the autoimmune polyglandular syndrome type 1, which is a rare and complex disease that is gaining attention in research on autoimmunity. The animal models of disease, although not identically reproducing the human picture, supply fundamental information on mechanisms and extent of AIRE action: thanks to its multidomain structure, AIRE localizes to chromatin enclosing the target genes, binds to histones, and offers an anchorage to multimolecular complexes involved in initiation and post-initiation events of gene transcription. In addition, AIRE enhances mRNA diversity by favoring alternative mRNA splicing. Once synthesized, ts-ags are presented to, and cause deletion of the self-reactive thymocyte clones. However, AIRE function is not restricted to the activation of gene transcription. AIRE would control presentation and transfer of self-antigens for thymic cellular interplay: such mechanism is aimed at increasing the likelihood of engagement of the thymocytes that carry the corresponding T-cell receptors. Another fundamental role of AIRE in promoting self-tolerance is related to the development of thymocyte anergy, as thymic self-representation shapes at the same time the repertoire of regulatory T cells. Finally, AIRE seems to replicate its action in the secondary lymphoid organs, albeit the cell lineage detaining such property has not been fully characterized. Delineation of AIRE functions adds interesting data to the knowledge of the mechanisms of self-tolerance and introduces exciting perspectives of therapeutic interventions against the related diseases.

Single-cell RNA-sequencing resolves self-antigen expression during mTEC development

The crucial capability of T cells for discrimination between self and non-self peptides is based on negative selection of developing thymocytes by medullary thymic epithelial cells (mTECs). The mTECs purge autoreactive T cells by expression of cell-type specific genes referred to as tissue-restricted antigens (TRAs). Although the autoimmune regulator (AIRE) protein is known to promote the expression of a subset of TRAs, its mechanism of action is still not fully understood. The expression of TRAs that are not under the control of AIRE also needs further characterization. Furthermore, expression patterns of TRA genes have been suggested to change over the course of mTEC development. Herein we have used single-cell RNA-sequencing to resolve patterns of TRA expression during mTEC development. Our data indicated that mTEC development consists of three distinct stages, correlating with previously described jTEC, mTEChi and mTEClo phenotypes. For each subpopulation, we have identified marker genes useful in future studies. Aire-induced TRAs were switched on during jTEC-mTEC transition and were expressed in genomic clusters, while otherwise the subsets expressed largely overlapping sets of TRAs. Moreover, population-level analysis of TRA expression frequencies suggested that such differences might not be necessary to achieve efficient thymocyte selection.

DNA breaks and chromatin structural changes enhance the transcription of autoimmune regulator target genes

The autoimmune regulator (AIRE) protein is the key factor in thymic negative selection of autoreactive T cells by promoting the ectopic expression of tissue-specific genes in the thymic medullary epithelium. Mutations in AIRE cause a monogenic autoimmune disease called autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy. AIRE has been shown to promote DNA breaks via its interaction with topoisomerase 2 (TOP2). In this study, we investigated topoisomerase-induced DNA breaks and chromatin structural alterations in conjunction with AIRE-dependent gene expression. Using RNA sequencing, we found that inhibition of TOP2 religation activity by etoposide in AIRE-expressing cells had a synergistic effect on genes with low expression levels. AIRE-mediated transcription was not only enhanced by TOP2 inhibition but also by the TOP1 inhibitor camptothecin. The transcriptional activation was associated with structural rearrangements in chromatin, notably the accumulation of γH2AX and the exchange of histone H1 with HMGB1 at AIRE target gene promoters. In addition, we found the transcriptional up-regulation to co-occur with the chromatin structural changes within the genomic cluster of carcinoembryonic antigen-like cellular adhesion molecule genes. Overall, our results suggest that the presence of AIRE can trigger molecular events leading to an altered chromatin landscape and the enhanced transcription of low-expressed genes.

CD28- and CD28lowCD8+ Regulatory T Cells: Of Mice and Men

Since the rebirth of regulatory (formerly known as suppressor) T cells in the early 1990s, research in the field of immune-regulation by various T cell populations has quickly gained momentum. While T cells expressing the transcription factor Foxp3 are currently in the spotlight, several other T cell populations endowed with potent immunomodulatory capacities have been identified in both the CD8⁺ and CD4⁺ compartment. The fundamental difference between CD4⁺ and CD8⁺ T cells in terms of antigen recognition suggests non-redundant, and perhaps complementary, functions of regulatory CD4⁺ and CD8⁺ T cells in immunoregulation. This emphasizes the importance and necessity of continuous research on both subpopulations of regulatory T cells (Tregs) so as to decipher their complex physiological relevance and possible synergy. Two distinct CD8-expressing Treg populations can be distinguished based on expression of the co-stimulatory receptor CD28. Here, we review the literature on these (at least in part) thymus-derived CD28^low and peripherally induced CD28^-CD8⁺ Tregs.

Aire Inhibits the Generation of a Perinatal Population of Interleukin-17A-Producing γδ T Cells to Promote Immunologic Tolerance

Aire's primary mechanism of action is to regulate transcription of a battery of genes in medullary thymic epithelial cells (mTECs) and, consequently, negative selection of effector T cells and positive selection of regulatory T cells. We found that Aire-deficient mice had expanded thymic and peripheral populations of perinatally generated IL-17A+Vγ6+Vδ1+ T cells, considered to be "early responders" to tissue stress and drivers of inflammatory reactions. Aire-dependent control of Il7 expression in mTECs regulated the size of thymic IL-17A+Vγ6+Vδ1+ compartments. In mice lacking Aire and γδ T cells, certain tissues typically targeted in the "Aire-less" disease, notably the retina, were only minimally infiltrated. IL-17A+Vγ6+Vδ1+ cells were present in the retina of wild-type mice and expanded very early in Aire-deficient mice. A putatively parallel population of IL-17A+Vγ9+Vδ2+ T cells was increased in humans lacking Aire. Thus, Aire exerts multi-faceted autoimmune control that extends to a population of innate-like T cells.

Autoimmune Regulator (AIRE) Is Expressed in Spermatogenic Cells, and It Altered the Expression of Several Nucleic-Acid-Binding and Cytoskeletal Proteins in Germ Cell 1 Spermatogonial (GC1-spg) Cells

Autoimmune regulator (AIRE) is a gene associated with autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED). AIRE is expressed heavily in the thymic epithelial cells and is involved in maintaining self-tolerance through regulating the expression of tissue-specific antigens. The testes are the most predominant extrathymic location where a heavy expression of AIRE is reported. Homozygous Aire-deficient male mice were infertile, possibly due to impaired spermatogenesis, deregulated germ cell apoptosis, or autoimmunity. We report that AIRE is expressed in the testes of neonatal, adolescent, and adult mice. AIRE expression was detected in glial cell derived neurotrophic factor receptor alpha (GFRα)(+) (spermatogonia), GFRα(-)/synaptonemal complex protein (SCP3)(+) (meiotic), and GFRα(-)/Phosphoglycerate kinase 2 (PGK2)(+) (postmeiotic) germ cells in mouse testes. GC1-spg, a germ-cell-derived cell line, did not express AIRE. Retinoic acid induced AIRE expression in GC1-spg cells. Ectopic expression of AIRE in GC1-spg cells using label-free LC-MS/MS identified a total of 371 proteins that were differentially expressed. 100 proteins were up-regulated, and 271 proteins were down-regulated. Data are available via ProteomeXchange with identifier PXD002511. Functional analysis of the differentially expressed proteins showed increased levels of various nucleic-acid-binding proteins and transcription factors and a decreased level of various cytoskeletal and structural proteins in the AIRE overexpressing cells as compared with the empty vector-transfected controls. The transcripts of a select set of the up-regulated proteins were also elevated. However, there was no corresponding decrease in the mRNA levels of the down-regulated set of proteins. Molecular function network analysis indicated that AIRE influenced gene expression in GC1-spg cells by acting at multiple levels, including transcription, translation, RNA processing, protein transport, protein localization, and protein degradation, thus setting the foundation in understanding the functional role of AIRE in germ cell biology.

AIRE: From promiscuous molecular partnerships to promiscuous gene expression

Autoimmune regulator (AIRE) is a unique transcriptional regulator that induces promiscuous expression of thousands of tissue-restricted antigens (TRAs) in medullary thymic epithelial cells (mTECs), a step critical for the induction of immunological self-tolerance. The past 15 years have seen dramatic progress in our understanding of how AIRE induces immunological self-tolerance on a molecular level. This major advancement can be greatly attributed to the identification of a large variety of proteins that physically associate with AIRE, supporting and regulating its transcription-transactivation capacity. These diverse molecular partnerships have been shown to play roles in shuttling AIRE to the nucleus, securing AIRE's interaction with nuclear matrix and chromatin, releasing RNA polymerase-II from its stalled state and potentiating AIRE-mediated gene expression, among others. In this review we discuss the relationship of AIRE with its vast and rather diverse repertoire of partners and highlight how such "promiscuous partnerships" contribute to the phenomenon of "promiscuous gene expression" in the thymus.

Single-cell transcriptome analysis reveals coordinated ectopic gene-expression patterns in medullary thymic epithelial cells

Expression of tissue-restricted self antigens (TRAs) in medullary thymic epithelial cells (mTECs) is essential for the induction of self-tolerance and prevents autoimmunity, with each TRA being expressed in only a few mTECs. How this process is regulated in single mTECs and is coordinated at the population level, such that the varied single-cell patterns add up to faithfully represent TRAs, is poorly understood. Here we used single-cell RNA sequencing and obtained evidence of numerous recurring TRA-co-expression patterns, each present in only a subset of mTECs. Co-expressed genes clustered in the genome and showed enhanced chromatin accessibility. Our findings characterize TRA expression in mTECs as a coordinated process that might involve local remodeling of chromatin and thus ensures a comprehensive representation of the immunological self.

Aire controls gene expression in the thymic epithelium with ordered stochasticity

The transcription factor Aire controls immunological tolerance by inducing the ectopic thymic expression of many tissue-specific genes, acting broadly by removing stops on the transcriptional machinery. To better understand Aire's specificity, we performed single-cell RNA-seq and DNA-methylation analysis of Aire-sufficient and Aire-deficient medullary epithelial cells (mTECs). Each of Aire's target genes was induced in only a minority of mTECs, independently of DNA-methylation patterns, as small inter-chromosomal gene clusters activated in concert in a proportion of mTECs. These microclusters differed between individual mice. Thus, our results suggest an organization of the DNA or of the epigenome that results from stochastic determinism but is 'bookmarked' and stable through mTEC divisions, which ensures more effective presentation of self antigens and favors diversity of self-tolerance between individuals.

Lymphopenia-induced proliferation in the absence of functional Autoimmune regulator (Aire) induces colitis in mice

Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) is caused by mutations in Autoimmune regulator (Aire), a transcriptional regulator of negative selection in thymus. However, Aire is also expressed in periphery, but the full range of Aire's peripheral function is unknown. Here, we transferred lymphocytes from wildtype donors into lymphopenic recipients with or without functional Aire. Following cell proliferation thus took place in Aire-sufficient or deficient environment. The wildtype lymphocytes hyperproliferated and induced disease in lymphopenic Aire(-/-) but not in Aire(+/+) recipients. The disease was characterized by diarrhea, inflammation, and colitis, and in some recipients pancreatitis, gastritis, and hepatitis was also found. Our results identify Aire as an important regulator of peripheral T cell homeostasis in gastrointestinal tissues. Given a suitable trigger the absence of peripheral Aire leads to dysregulated T cell proliferation and disease.

Brd4 bridges the transcriptional regulators, Aire and P-TEFb, to promote elongation of peripheral-tissue antigen transcripts in thymic stromal cells

Aire controls immunologic tolerance by inducing a battery of thymic transcripts encoding proteins characteristic of peripheral tissues. Its unusually broad effect is achieved by releasing RNA polymerase II paused just downstream of transcriptional start sites. We explored Aire's collaboration with the bromodomain-containing protein, Brd4, uncovering an astonishing correspondence between those genes induced by Aire and those inhibited by a small-molecule bromodomain blocker. Aire:Brd4 binding depended on an orchestrated series of posttranslational modifications within Aire's caspase activation and recruitment domain. This interaction attracted P-TEFb, thereby mobilizing downstream transcriptional elongation and splicing machineries. Aire:Brd4 association was critical for tolerance induction, and its disruption could account for certain point mutations that provoke human autoimmune disease. Our findings evoke the possibility of unanticipated immunologic mechanisms subtending the potent antitumor effects of bromodomain blockers.

Ovarian autoimmune disease: clinical concepts and animal models

The ovary is not an immunologically privileged organ, but a breakdown in tolerogenic mechanisms for ovary-specific antigens has disastrous consequences on fertility in women, and this is replicated in murine models of autoimmune disease. Isolated ovarian autoimmune disease is rare in women, likely due to the severity of the disease and the inability to transmit genetic information conferring the ovarian disease across generations. Nonetheless, autoimmune oophoritis is often observed in association with other autoimmune diseases, particularly autoimmune adrenal disease, and takes a toll on both society and individual health. Studies in mice have revealed at least two mechanisms that protect the ovary from autoimmune attack. These mechanisms include control of autoreactive T cells by thymus-derived regulatory T cells, as well as a role for the autoimmune regulator (AIRE), a transcriptional regulator that induces expression of tissue-restricted antigens in medullary thymic epithelial cells during development of T cells. Although the latter mechanism is incompletely defined, it is well established that failure of either results in autoimmune-mediated targeting and depletion of ovarian follicles. In this review, we will address the clinical features and consequences of autoimmune-mediated ovarian infertility in women, as well as the possible mechanisms of disease as revealed by animal models.

A novel mutation in autoimmune regulator gene causes autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy

BACKGROUND

Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy syndrome (APECED) is a rare autosomal recessive disease due to mutations in the autoimmune regulator (AIRE) gene, which encodes a transcription factor that induces the expression of peripheral tissue-specific antigens in medullary thymic epithelial cells.

AIM

The purpose of this study was to identify the underlying genetic cause in a Chinese family diagnosed with APECED.

METHOD

Peripheral blood samples were collected from family members. All exons of the AIRE gene and adjacent exon-intron sequences were amplified by PCR and subsequently sequenced. The functional consequence of the mutations was analyzed by cell transfection and in vitro assays.

RESULTS

A novel c.483_484insC mutation in exon 4 was identified, which resulted in a frame shift predicted to generate a truncated protein containing the first 163 AIRE amino acids followed by 52 aberrant amino acids. Confocal immunofluorescence microscopy of COS-7 cells transfected with wild-type and mutant AIRE constructs showed that wild-type AIRE protein was localized mainly in the nucleus, while mutant AIRE was localized mainly in the cytoplasm. A luciferase reporter assay showed that the identified mutation dramatically inhibited the transactivation activity of AIRE in vitro.

CONCLUSION

We identified a novel AIRE mutation which alters the intracellular location and transcription activity of AIRE, and has implications in the pathogenesis of APECED.

The biophysical and biochemical properties of the autoimmune regulator (AIRE) protein

AIRE (for autoimmune regulator) is a multidomain protein that performs a fundamental function in the thymus and possibly in the secondary lymphoid organs: the regulation, especially in the sense of activation, of the process of gene transcription in cell lines deputed to the presentation of self-antigens to the maturing T lymphocytes. The apoptosis of the elements bearing T-cell receptors with critical affinity for the exhibited self-antigens prevents the escape of autoreactive clones and represents a simple and efficient mechanism of deletional self-tolerance. However, AIRE action relies on an articulated complex of biophysical and biochemical properties, in most cases attributable to single subspecialized domains. Here a thorough review of the matter is presented, with a privileged look at the pathogenic changes of AIRE that interfere with such properties and lead to the impairment in its chief function.

Regulation of T-cell Tolerance by Lymphatic Endothelial Cells

Lymphatic endothelial cells are most often thought of as structural cells that form the lymphatic vasculature, which transports fluid out of peripheral tissues and transports antigens and antigen presenting cells to lymph nodes. Recently, it has been shown that lymphatic endothelial cells also dynamically respond to and influence the immune response in several ways. Here, we describe how lymphatic endothelial cells induce peripheral T-cell tolerance and how this relates to tolerance induced by other types of antigen presenting cells. Furthermore, the ability of lymphatic endothelial cells to alter immune responses under steady-state or inflammatory conditions is explored, and the therapeutic potential of bypassing lymphatic endothelial cell-induced tolerance to enhance cancer immunotherapy is discussed.

APECED: A Paradigm of Complex Interactions between Genetic Background and Susceptibility Factors

Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) is a rare autosomal recessive disease, caused by mutations of a single gene named Autoimmune regulator gene (AIRE) which results in a failure of T-cell tolerance. Central tolerance takes place within the thymus and represents the mechanism by which potentially auto-reactive T-cells are eliminated through the negative selection process. The expression of tissue-specific antigens (TSAs) by medullary thymic epithelial cells (mTECs) in the thymus is a key process in the central tolerance and is driven by the protein encoded by AIRE gene, the transcription factor autoimmune regulator (AIRE). A failure in this process caused by AIRE mutations is thought to be responsible of the systemic autoimmune reactions of APECED. APECED is characterized by several autoimmune endocrine and non-endocrine manifestations and the phenotype is often complex. Although APECED is the paradigm of a monogenic autoimmune disorder, it is characterized by a wide variability of the clinical expression even between siblings with the same genotype, thus implying that additional mechanisms, other than the failure of Aire function, are involved in the pathogenesis of the disease. Unraveling open issues of the molecular basis of APECED, will help improve diagnosis, management, and therapeutical strategies of this complex disease.

Expression profile of a clonal insulin-expressing epithelial cell in the thymus

BACKGROUND

Type 1 diabetes is an autoimmune disease resulting from the destruction of pancreatic beta-cells. One of the main antigens targeted in this auto reactive response is insulin. It has been shown that insulin is expressed in small amounts in the thymus, and more specifically in the medullary thymic epithelial cells (mTECs), which also express a variety of other tissue-specific antigens. This thymic expression enables the maintenance of self-tolerance, and is essential in preventing auto-immune disease. Our laboratory has created a mouse mTEC clonal cell line specifically expressing insulin in order to better understand the regulatory mechanisms of this ectopic expression of insulin. In this study, we compared the insulin expressing cell line to an insulin non-expressing mTEC line by genome-wide expression profiling.

RESULTS

The most important difference was overexpression of CD34 in the insulin expressing clone, confirmed by Real-time Rt-PCR and flow cytometry. Cells in the thymus expressing higher levels of CD34 were found to contain higher levels of insulin and, to a lesser extent, Aire, a master regulator of self-antigen expression in the thymus. The cells expressing CD34 were not enriched in CD80, a known mTEC maturity marker.

CONCLUSION

CD34 may be a specific marker for functionality, with some specificity for insulin.

Overlapping gene coexpression patterns in human medullary thymic epithelial cells generate self-antigen diversity

Promiscuous expression of numerous tissue-restricted self-antigens (TRAs) in medullary thymic epithelial cells (mTECs) is essential to safeguard self-tolerance. A distinct feature of promiscuous gene expression is its mosaic pattern (i.e., at a given time, each self-antigen is expressed only in 1-3% of mTECs). How this mosaic pattern is generated at the single-cell level is currently not understood. Here, we show that subsets of human mTECs expressing a particular TRA coexpress distinct sets of genes. We identified three coexpression groups comprising overlapping and complementary gene sets, which preferentially mapped to certain chromosomes and intrachromosomal gene clusters. Coexpressed gene loci tended to colocalize to the same nuclear subdomain. The TRA subsets aligned along progressive differentiation stages within the mature mTEC subset and, in vitro, interconverted along this sequence. Our data suggest that single mTECs shift through distinct gene pools, thus scanning a sizeable fraction of the overall repertoire of promiscuously expressed self-antigens. These findings have implications for the temporal and spatial (re)presentation of self-antigens in the medulla in the context of tolerance induction.

Regulations of gene expression in medullary thymic epithelial cells required for preventing the onset of autoimmune diseases

Elimination of potential self-reactive T cells in the thymus is crucial for preventing the onset of autoimmune diseases. Epithelial cell subsets localized in thymic medulla [medullary thymic epithelial cells (mTECs)] contribute to this process by supplying a wide range of self-antigens that are otherwise expressed in a tissue-specific manner (TSAs). Expression of some TSAs in mTECs is controlled by the autoimmune regulator (AIRE) protein, of which dysfunctional mutations are the causative factor of autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED). In addition to the elimination of self-reactive T cells, recent studies indicated roles of mTECs in the development of Foxp3-positive regulatory T cells, which suppress autoimmunity and excess immune reactions in peripheral tissues. The TNF family cytokines, RANK ligand, CD40 ligand, and lymphotoxin were found to promote the differentiation of AIRE- and TSA-expressing mTECs. Furthermore, activation of NF-κB is essential for mTEC differentiation. In this mini-review, we focus on molecular mechanisms that regulate induction of AIRE and TSA expression and discuss possible contributions of these mechanisms to prevent the onset of autoimmune diseases.

AIRE expressing marginal zone dendritic cells balances adaptive immunity and T-follicular helper cell recruitment

Autoimmune polyendocrine syndrome Type I (APS I) results in multiple endocrine organ destruction and is caused by mutations in the Autoimmune regulator gene (AIRE). In the thymic stroma, cells expressing the AIRE gene dictate T cell education and central tolerance. Although this function is the most studied, AIRE is also expressed in the periphery in DCs and stromal cells. Still, how AIRE regulated transcription modifies cell behaviour in the periphery is largely unknown. Here we show that AIRE is specifically expressed by 33D1(+) DCs and dictates the fate of antibody secreting cell movement within the spleen. We also found that AIRE expressing 33D1(+) DCs expresses self-antigens as exemplified by the hallmark gene insulin. Also, as evidence for a regulatory function, absence of Aire in 33D1(+) DCs led to reduced levels of the chemokine CXCL12 and increased co-stimulatory properties. This resulted in altered activation and recruitment of T-follicular helper cells and germinal centre B cells. The altered balance leads to a change of the early response to a T cell-dependent antigen in Aire(-/-) mice. These findings add to the understanding of how specific DC subtypes regulate the early responses during T cell-dependent antibody responses within the spleen and further define the role of AIRE in the periphery as regulator of self-antigen expression and lymphocyte migration.

A functional alternative splicing mutation in AIRE gene causes autoimmune polyendocrine syndrome type 1

Autoimmune polyendocrine syndrome type 1 (APS-1) is a rare autosomal recessive disease defined by the presence of two of the three conditions: mucocutaneous candidiasis, hypoparathyroidism, and Addison's disease. Loss-of-function mutations of the autoimmune regulator (AIRE) gene have been linked to APS-1. Here we report mutational analysis and functional characterization of an AIRE mutation in a consanguineous Chinese family with APS-1. All exons of the AIRE gene and adjacent exon-intron sequences were amplified by PCR and subsequently sequenced. We identified a homozygous missense AIRE mutation c.463G>A (p.Gly155Ser) in two siblings with different clinical features of APS-1. In silico splice-site prediction and minigene analysis were carried out to study the potential pathological consequence. Minigene splicing analysis and subsequent cDNA sequencing revealed that the AIRE mutation potentially compromised the recognition of the splice donor of intron 3, causing alternative pre-mRNA splicing by intron 3 retention. Furthermore, the aberrant AIRE transcript was identified in a heterozygous carrier of the c.463G>A mutation. The aberrant intron 3-retaining transcript generated a truncated protein (p.G155fsX203) containing the first 154 AIRE amino acids and followed by 48 aberrant amino acids. Therefore, our study represents the first functional characterization of the alternatively spliced AIRE mutation that may explain the pathogenetic role in APS-1.

Alterations of the autoimmune regulator transcription factor and failure of central tolerance: APECED as a model

Self-nonself discrimination plays a key role in inducing a productive immunity and in preventing autoimmune reactions. Central tolerance within the thymus and peripheral tolerance in peripheral lymphoid organs lead to immunologic nonresponsiveness against self-components. The central tolerance represents the mechanism by which T cells binding with high avidity to self-antigens are eliminated through the so-called negative selection. Thymic medullary epithelial cells and medullary dendritic cells play a key role in this process, through the expression of a large number of tissue-specific self-antigens involving the transcription factor autoimmune regulator (AIRE). Mutations of AIRE result in autoimmune polyendocrinopathy candidiasis ectodermal dystrophy, a rare autosomal recessive disease (OMIM 240300), which is the paradigm of a genetically determined failure of central tolerance and autoimmunity. This review focuses on recent advances in the molecular mechanisms of central tolerance, their alterations and clinical implication.

DNA methylation profile of Aire-deficient mouse medullary thymic epithelial cells

BACKGROUND

Medullary thymic epithelial cells (mTECs) are characterized by ectopic expression of self-antigens during the establishment of central tolerance. The autoimmune regulator (Aire), which is specifically expressed in mTECs, is responsible for the expression of a large repertoire of tissue-restricted antigens (TRAs) and plays a role in the development of mTECs. However, Aire-deficient mTECs still express TRAs. Moreover, a subset of mTECs, which are considered to be at a stage of terminal differentiation, exists in the Aire-deficient thymus. The phenotype of a specific cell type in a multicellular organism is governed by the epigenetic regulation system. DNA methylation modification is an important component of this system. Every cell or tissue type displays a DNA methylation profile, consisting of tissue-dependent and differentially methylated regions (T-DMRs), and this profile is involved in cell-type-specific genome usage. The aim of this study was to examine the DNA methylation profile of mTECs by using Aire-deficient mTECs as a model.

RESULTS

We identified the T-DMRs of mTECs (mTEC-T-DMRs) via genome-wide DNA methylation analysis of Aire(-/-) mTECs by comparison with the liver, brain, thymus, and embryonic stem cells. The hypomethylated mTEC-T-DMRs in Aire(-/-) mTECs were associated with mTEC-specific genes, including Aire, CD80, and Trp63, as well as other genes involved in the RANK signaling pathway. While these mTEC-T-DMRs were also hypomethylated in Aire(+/+) mTECs, they were hypermethylated in control thymic stromal cells. We compared the pattern of DNA methylation levels at a total of 55 mTEC-T-DMRs and adjacent regions and found that the DNA methylation status was similar for Aire(+/+) and Aire(-/-) mTECs but distinct from that of athymic cells and tissues.

CONCLUSIONS

These results indicate a unique DNA methylation profile that is independent of Aire in mTECs. This profile is distinct from other cell types in the thymic microenvironment and is indicated to be involved in the differentiation of the mTEC lineage.

Genetic basis of altered central tolerance and autoimmune diseases: a lesson from AIRE mutations

The thymus is a specialized organ that provides an inductive environment for the development of T cells from multipotent hematopoietic progenitors. Self-nonself discrimination plays a key role in inducing a productive immunity and in preventing autoimmune reactions. Tolerance represents a state of immunologic nonresponsiveness in the presence of a particular antigen. The immune system becomes tolerant to self-antigens through the two main processes, central and peripheral tolerance. Central tolerance takes place within the thymus and represents the mechanism by which T cells binding with high avidity self-antigens, which are potentially autoreactive, are eliminated through so-called negative selection. This process is mostly mediated by medullary thymic epithelia cells (mTECs) and medullary dendritic cells (DCs). A remarkable event in the process is the expression of tissue-specific antigens (TSA) by mTECs driven by the transcription factor autoimmune regulator (AIRE). Mutations in this gene result in autoimmune polyendocrinopathy candidiasis ectodermal dystrophy (APECED), a rare autosomal recessive disease (OMIM 240300). Thus far, this syndrome is the paradigm of a genetically determined failure of central tolerance and autoimmunty. Patients with APECED have a variable pattern of autoimmune reactions, involving different endocrine and nonendocrine organs. However, although APECED is a monogenic disorder, it is characterized by a wide variability of the clinical expression, thus implying a further role for disease-modifying genes and environmental factors in the pathogenesis. Studies on this polyreactive autoimmune syndrome contributed enormously to unraveling several issues of the molecular basis of autoimmunity. This review focuses on the developmental, functional, and molecular events governing central tolerance and on the clinical implication of its failure.

Autoimmune regulator is acetylated by transcription coactivator CBP/p300

The Autoimmune Regulator (AIRE) is a regulator of transcription in the thymic medulla, where it controls the expression of a large set of peripheral-tissue specific genes. AIRE interacts with the transcriptional coactivator and acetyltransferase CBP and synergistically cooperates with it in transcriptional activation. Here, we aimed to study a possible role of AIRE acetylation in the modulation of its activity. We found that AIRE is acetylated in tissue culture cells and this acetylation is enhanced by overexpression of CBP and the CBP paralog p300. The acetylated lysines were located within nuclear localization signal and SAND domain. AIRE with mutations that mimicked acetylated K243 and K253 in the SAND domain had reduced transactivation activity and accumulated into fewer and larger nuclear bodies, whereas mutations that mimicked the unacetylated lysines were functionally similar to wild-type AIRE. Analogously to CBP, p300 localized to AIRE-containing nuclear bodies, however, the overexpression of p300 did not enhance the transcriptional activation of AIRE-regulated genes. Further studies showed that overexpression of p300 stabilized the AIRE protein. Interestingly, gene expression profiling revealed that AIRE, with mutations mimicking K243/K253 acetylation in SAND, was able to activate gene expression, although the affected genes were different and the activation level was lower from those regulated by wild-type AIRE. Our results suggest that the AIRE acetylation can influence the selection of AIRE activated genes.

Unmodified histone H3K4 and DNA-dependent protein kinase recruit autoimmune regulator to target genes

Autoimmune regulator (AIRE) directs the expression of otherwise tissue-restricted antigens (TRAs) in medullary thymic epithelial cells, allowing their presentation to developing T cells, which leads to central tolerance. We addressed the conundrum of how AIRE is recruited to these otherwise silent genes in cells. Our studies confirmed that interactions between AIRE and the unmodified histone H3K4 (H3K4me0) are important for targeting AIRE to the mouse insulin promoter in chromatin. By replacing its H3K4me0-binding module with one that binds to the methylated H3K4me3, we redirected the mutant AIRE.ING protein to an actively transcribed gene. Nevertheless, the mutant AIRE D297A protein, which could not bind to H3K4me0, still activated the human insulin promoter on an episomal plasmid target. This targeting was due to DNA-dependent protein kinase (DNA-PK). Thus, in cells that lacked the catalytic subunit of DNA-PK (DNA-PKcs), the assembly and activity of AIRE on DNA, whether in chromatin or on episomal plasmids, was abrogated. However, by the heterologous tethering of AIRE to DNA, we could restore its activity on a plasmid target in DNA-PKcs-negative cells. Importantly, mutations in the putative DNA-binding residues in its SAND domain had no effect on the transcriptional effects of AIRE. Thus, AIRE is recruited to TRA genes in chromatin via cooperative interactions with H3K4me0 and DNA-PK.

Monogenic autoimmunity

Monogenic autoimmune syndromes provide a rare yet powerful glimpse into the fundamental mechanisms of immunologic tolerance. Such syndromes reveal not only the contribution of an individual breakpoint in tolerance but also patterns in the pathogenesis of autoimmunity. Disturbances in innate immunity, a system built for ubiquitous sensing of danger signals, tend to generate systemic autoimmunity. For example, defects in the clearance of self-antigens and chronic stimulation of type 1 interferons lead to the systemic autoimmunity seen in C1q deficiency, SPENCDI, and AGS. In contrast, disturbances of adaptive immunity, which is built for antigen specificity, tend to produce organ-specific autoimmunity. Thus, the loss of lymphocyte homeostasis, whether through defects in apoptosis, suppression, or negative selection, leads to organ-specific autoimmunity in ALPS, IPEX, and APS1. We discuss the unique mechanisms of disease in these prominent syndromes as well as how they contribute to the spectrum of organ-specific or systemic autoimmunity. The continued study of rare variants in autoimmune disease will inform future investigations and treatments directed at rare and common autoimmune diseases alike.

Beyond the hormone: insulin as an autoimmune target in type 1 diabetes

Insulin is not only the hormone produced by pancreatic β-cells but also a key target antigen of the autoimmune islet destruction leading to type 1 diabetes. Despite cultural biases between the fields of endocrinology and immunology, these two facets should not be regarded separately, but rather harmonized in a unifying picture of diabetes pathogenesis. There is increasing evidence suggesting that metabolic factors (β-cell dysfunction, insulin resistance) and immunological components (inflammation and β-cell-directed adaptive immune responses) may synergize toward islet destruction, with insulin standing at the crossroad of these pathways. This concept further calls for a revision of the classical dichotomy between type 1 and type 2 diabetes because metabolic and immune mechanisms may both contribute to different extents to the development of different forms of diabetes. After providing a background on the mechanisms of β-cell autoimmunity, we will explain the role of insulin and its precursors as target antigens expressed not only by β-cells but also in the thymus. Available knowledge on the autoimmune antibody and T-cell responses against insulin will be summarized. A unifying scheme will be proposed to show how different aspects of insulin biology may lead to β-cell destruction and may be therapeutically exploited. We will argue about possible reasons why insulin remains the mainstay of metabolic control in type 1 diabetes but has so far failed to prevent or halt β-cell autoimmunity as an immune modulatory reagent.

Mouse thymic epithelial cell lines expressing "Aire" and peripheral tissue-specific antigens reproduce in vitro negative selection of T cells

In the human thymus, AIRE (autoimmune regulator) gene is expressed in a very limited type of medullary thymic epithelial cells (mTECs) and no cognate cell lines are available, hence the molecular analysis of AIRE gene function has been difficult. To improve this situation, we attempted to isolate Aire-expressing cells and established three cell lines (Aire⁺TEC1, Aire⁺TEC2, Aire⁺DC) from the abnormally enlarged thymus, which was developed in the transgenic mice expressing SV40 T-antigen driven by the mouse Aire gene promoter. When these Aire⁺ cell lines were co-cultured with fresh thymocytes, they adhered to the majority of thymocytes and induced apoptosis as if negative selection of T-cells in the thymus is occurring in vitro. Further analysis revealed that these Aire⁺ cell lines are derived from mTECs and exhibit characteristic natures of "antigen presenting cells" including several distinct abilities: to express a variety of peripheral tissue-specific antigens, to produce immunoproteasome and immunological synapse, and to express some of TNFSFs (tumor necrosis factor super families). Thus, the newly established Aire⁺ cell lines will be invaluable for the further detailed analysis of AIRE gene function in the central tolerance of immunity and autoimmune disease.

Overexpressing autoimmune regulator regulates the expression of toll-like receptors by interacting with their promoters in RAW264.7 cells

Autoimmune regulator (Aire) is a transcriptional activator that regulates the ectopic expression of many tissue-restricted antigens in medullary thymic epithelial cells, and that has an important role in the negative selection of autoreactive T cells. However, the roles of Aire expression in peripheral lymphoid tissues and hematopoietic cells, especially monocytes/macrophages, remain poorly understood. In this study, we found that the mRNA and protein expression levels of toll-like receptor (TLR)1, TLR3, and TLR8 were notably up-regulated in a mouse macrophage-like cell line (RAW264.7) stably expressing Aire, while the expression of TLR2, TLR4, TLR5, TLR6, TLR7, and TLR9 were not significantly changed. In addition, the mRNA expression of TLR3 and TLR8 were significantly increased in primary peritoneal macrophages transiently transfected with Aire. Using chromatin immunoprecipitation and a luciferase activity assay, we also found that Aire interacted with the TLR1, TLR3, and TLR8 promoters and increased the luciferase transcriptional activity of these promoters in RAW264.7 cells. Moreover, after stimulation by Pam(3)CSK(4), a TLR1 ligand, and poly(I:C), a TLR3 ligand, we found that the mRNA expression levels of IL-1α, TNFα, iNOS, and IFNα were increased in RAW264.7 cells stably expressing Aire. Together, these data suggest that Aire has a crucial role in the recognition of pathogenic microorganisms and peripheral immune tolerance in antigen-presenting cells (APCs) by regulating the expression of TLRs.

Control of central and peripheral tolerance by Aire

The negative selection of self-reactive thymocytes depends on the expression of tissue-specific antigens by medullary thymic epithelial cells. The autoimmune regulator (Aire) protein plays an important role in turning on these antigens, and the absence of even one Aire-induced tissue-specific antigen in the thymus can lead to autoimmunity in the antigen-expressing target organ. Recently, Aire protein has been detected in peripheral lymphoid organs, suggesting that peripheral Aire plays a complementary role here. In these peripheral sites, Aire was found to regulate the expression of a group of tissue-specific antigens that is distinct from those expressed in the thymus. Furthermore, transgenic antigen expression in extrathymic Aire-expressing cells (eTACs) can mediate deletional tolerance, but the immunological relevance of Aire-dependent, endogenous tissue-specific antigens remains to be determined.

Overexpression of the IGF2-mRNA binding protein p62 in transgenic mice induces a steatotic phenotype

BACKGROUND & AIMS

The insulin-like growth-factor 2 (IGF2) mRNA binding protein p62 is highly expressed in hepatocellular carcinoma tissue. Still, its potential role in liver disease is largely unknown. In this study, we investigated pathophysiological implications of p62 overexpression in mice.

METHODS

We generated mice overexpressing p62 under a LAP-promotor. mRNA expression levels and stability were examined by real-time RT-PCR. Allele-specific expression of Igf2 and H19 was assessed after crossing mice with SD7 animals. The Igf2 downstream mediators pAKT and PTEN were determined by Western blot.

RESULTS

Hepatic p62 overexpression neither induced inflammatory processes nor liver damage. However, 2.5week old transgenic animals displayed a steatotic phenotype and improved glucose tolerance. p62 overexpression induced the expression of the imprinted genes Igf2 and H19 and their transcriptional regulator Aire (autoimmune regulator). Neither monoallelic expression nor mRNA stability of Igf2 and H19 was affected. Investigating Igf2 downstream signalling pathways showed increased AKT activation and attenuated PTEN expression.

CONCLUSIONS

The induction of a steatotic phenotype implies that p62 plays a role in hepatic pathophysiology.

Aire and T cell development

In the thymus, developing T cells that react against self-antigens with high affinity are deleted in the process of negative selection. An essential component of this process is the display of self-antigens, including those whose expression are usually restricted to specific tissues, to developing T cells within the thymus. The Autoimmune Regulator (Aire) gene plays a crucial role in the expression of tissue specific self-antigens within the thymus, and disruption of Aire function results in spontaneous autoimmunity in both humans and mice. Recent advances have been made in our understanding of how Aire influences the expression of thousands of tissue-specific antigens in the thymus. Additional roles of Aire, including roles in chemokine and cytokine expression, have also been revealed. Factors important in the differentiation of Aire-expressing medullary thymic epithelial cells have been defined. Finally, the identity of antigen presenting cells in negative selection, including the role of medullary thymic epithelial cells in displaying tissue specific antigens to T cells, has also been clarified.

Expression and function of the autoimmune regulator (Aire) gene in non-thymic tissue

Educational immune tolerance to self-antigens is induced primarily in the thymus where tissue-restricted antigens (TRAs) are presented to T lymphocytes by cells of the thymic stroma - a process known as central tolerance. The expression of these TRAs is controlled in part by a transcription factor encoded by the autoimmune regulatory (Aire) gene. Patients with a mutation of this gene develop a condition known as autoimmune-polyendocrinopathy-candidiasis-ectodermal-dystrophy (APECED), characterized by autoimmune destruction of endocrine organs, fungal infection and dental abnormalities. There is now evidence for TRA expression and for mechanisms of functional tolerance outside the thymus. This has led to a number of studies examining Aire expression and function at these extra-thymic sites. These investigations have been conducted across different animal models using different techniques and have often shown discrepant results. Here we review the studies of extra thymic Aire and discuss the evidence for its expression and function in both human and murine systems.

The role of AIRE in human autoimmune disease

The autoimmune regulator (AIRE) gene encodes a transcription factor involved in the presentation of tissue-restricted antigens during T-cell development in the thymus. Mutations of this gene lead to type 1 autoimmune polyglandular syndrome (APS-1), also termed autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) syndrome, which is characterized by the clinical presentation of at least two of a triad of underlying disorders: Addison disease, hypoparathyroidism and chronic mucocutaneous candidiasis. This Review describes the process of positive and negative selection of developing T cells in the thymus and the role of AIRE as a regulator of peripheral antigen presentation. Furthermore, it addresses how mutations of this gene lead to the failure to eliminate autoreactive T cells, which can lead to clinical autoimmune syndromes.

Recent insights into the role and molecular mechanisms of the autoimmune regulator (AIRE) gene in autoimmunity

Since many years immunologists have being tried to answer the tantalizing enigma of immunological tolerance. Complex mechanisms in both thymus (central tolerance) and peripheral lymphoid organs (peripheral tolerance) underly lymphocyte tolerance and its maintenance. The genesis of autoimmunity involves environmental and genetic mechanisms, both contributing to the disruption and deregulation of central and peripheral tolerance, allowing autoreactive pathogenetic T and B-cell clones arising. Among genetic factors the autoimmune regulator (AIRE) gene is one of the best candidates to understand the complex scenario of autoimmunity. Autoimmune polyendocrinopathy syndrome type 1 is a rare autosomal recessive disease caused by mutations in the AIRE gene. Therefore, the disorder has certainly been a powerful model to address the question concerning how a tolerant state is achieved or maintained and to explore how it has gone lost in the context of autoimmunity. AIRE has been proposed to function as a 'non classical' transcription factor, strongly implicated in the regulation of organ-specific antigen expression in thymic epithelial cells and in the imposition of T cell tolerance, thus regulating the negative selection of autoreactive T cell clones. A plethora of proposal have been suggested for AIRE's potential mechanism of action, thus regulating the negative selection of autoreactive T cells. In this review recent discoveries are presented into the role and molecular mechanisms of the AIRE protein in APECED and other autoimmune diseases.

The role of Aire in clonal selection

In his clonal selection theory, Frank Macfarlane Burnet predicted that autoreactive lymphocytes are deleted to prevent autoimmunity. This and other principles of lymphocyte behavior outlined by Burnet guided many studies that lead to our current understanding of thymic selection. Thus, when the genetic mutation responsible for autoimmune polyglandular syndrome type 1 was mapped to the autoimmune regulator (AIRE) gene, and Aire was found to be highly expressed in thymic epithelium, studying the role of Aire in negative selection made sense in the context of modern models of thymic selection. We now know Aire is a transcription factor required for the expression of many tissue-specific antigens (TSAs) in the thymus. In the absence of functional Aire, human patients and mice develop multi-organ autoimmune disease because of a defect in thymic negative selection. In addition to its role in the thymus, recent work in our lab suggests that extrathymic Aire-expressing cells have an important role in the clonal deletion of autoreactive CD8+ T cells. In this review, we summarize the latest studies on thymic and peripheral Aire-expressing cells, as well as other TSA-expressing stromal cell populations in peripheral lymphoid organs. We also discuss theoretical differences in thymic and peripheral Aire function that warrant further studies.

Transplantation of autoimmune regulator-encoding bone marrow cells delays the onset of experimental autoimmune encephalomyelitis

The autoimmune regulator (AIRE) promotes "promiscuous" expression of tissue-restricted antigens (TRA) in thymic medullary epithelial cells to facilitate thymic deletion of autoreactive T-cells. Here, we show that AIRE-deficient mice showed an earlier development of myelin oligonucleotide glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis (EAE). To determine the outcome of ectopic Aire expression, we used a retroviral transduction system to over-express Aire in vitro, in cell lines and in bone marrow (BM). In the cell lines that included those of thymic medullary and dendritic cell origin, ectopically expressed Aire variably promoted expression of TRA including Mog and Ins2 (proII) autoantigens associated, respectively, with the autoimmune diseases multiple sclerosis and type 1 diabetes. BM chimeras generated from BM transduced with a retrovirus encoding Aire displayed elevated levels of Mog and Ins2 expression in thymus and spleen. Following induction of EAE with MOG(35-55), transplanted mice displayed significant delay in the onset of EAE compared with control mice. To our knowledge, this is the first example showing that in vivo ectopic expression of AIRE can modulate TRA expression and alter autoimmune disease development.

Clues to immune tolerance: the monogenic autoimmune syndromes

Autoimmune disease affects a significant proportion of the population. The etiology of most autoimmune diseases is largely unknown, but it is thought to be multifactorial with both environmental and genetic influences. Rare monogenic autoimmune diseases, however, offer an invaluable window into potential disease mechanisms. In this review, we will discuss the autoimmune polyglandular syndrome (APS1), the immunedysregulation, polyendocrinopathy, enteropathy, X-linked syndrome (IPEX), and autoimmune lymphoproliferative syndrome (ALPS). Significantly, the information gained from the study of these diseases has provided new insights into more common autoimmune disease and have yielded new diagnostics and therapeutic opportunities.

Epigenetic regulation of promiscuous gene expression in thymic medullary epithelial cells

Thymic central tolerance comprehensively imprints the T-cell receptor repertoire before T cells seed the periphery. Medullary thymic epithelial cells (mTECs) play a pivotal role in this process by virtue of promiscuous expression of tissue-restricted autoantigens. The molecular regulation of this unusual gene expression, in particular the involvement of epigenetic mechanisms is only poorly understood. By studying promiscuous expression of the mouse casein locus, we report that transcription of this locus proceeds from a delimited region ("entry site") to increasingly complex patterns along with mTEC maturation. Transcription of this region is preceded by promoter demethylation in immature mTECs followed upon mTEC maturation by acquisition of active histone marks and local locus decontraction. Moreover, analysis of two additional gene loci showed that promiscuous expression is transient in single mTECs. Transient gene expression could conceivably add to the local diversity of self-antigen display thus enhancing the efficacy of central tolerance.

Insulin transactivator MafA regulates intrathymic expression of insulin and affects susceptibility to type 1 diabetes

OBJECTIVE

Tissue-specific self-antigens are ectopically expressed within the thymus and play an important role in the induction of central tolerance. Insulin is expressed in both pancreatic islets and the thymus and is considered to be the primary antigen for type 1 diabetes. Here, we report the role of the insulin transactivator MafA in the expression of insulin in the thymus and susceptibility to type 1 diabetes.

RESEARCH DESIGN AND METHODS

The expression profiles of transcriptional factors (Pdx1, NeuroD, Mafa, and Aire) in pancreatic islets and the thymus were examined in nonobese diabetic (NOD) and control mice. Thymic Ins2 expression and serum autoantibodies were examined in Mafa knockout mice. Luciferase reporter assay was performed for newly identified polymorphisms of mouse Mafa and human MAFA. A case-control study was applied for human MAFA polymorphisms.

RESULTS

Mafa, Ins2, and Aire expression was detected in the thymus. Mafa expression was lower in NOD thymus than in the control and was correlated with Ins2 expression. Targeted disruption of MafA reduced thymic Ins2 expression and induced autoantibodies against pancreatic islets. Functional polymorphisms of MafA were newly identified in NOD mice and humans, and polymorphisms of human MAFA were associated with susceptibility to type 1 diabetes but not to autoimmune thyroid disease.

CONCLUSIONS

These data indicate that functional polymorphisms of MafA are associated with reduced expression of insulin in the thymus and susceptibility to type 1 diabetes in the NOD mouse as well as human type 1 diabetes.

The role of AIRE polymorphisms in melanoma

Polymorphisms of AIRE, a transcription factor that up-regulates intrathymic expression of tissue-specific antigens including melanoma-associated antigens (MAAs), may variably affect the selection of MAAs-specific thymocytes, generating T-cell repertoires protecting or predisposing individuals to melanoma. We found that AIRE single nucleotide polymorphisms (SNPs) rs1055311, rs1800520 and rs1800522 were significantly more frequent in healthy subjects than in melanoma patients, independently from sex, age and stages of melanoma. The presence of these SNPs was associated with increased frequency of two T-cell clonotypes specific for MAGE-1 linking their protective effect to selection/expansion of MAA-specific T cells. Interestingly, mRNA transcribed on the rs1800520 SNP showed increased free energy than the wild type suggesting that its reduced stability may be responsible for the different activity of the polymorphic AIRE molecule. This finding may contribute at identifying subjects with increased risk of developing melanoma or patients with melanoma that may take benefit from immunotherapy.

Upregulation of phagocytic clearance of apoptotic cells by autoimmune regulator

To investigate the effect of autoimmune regulator (AIRE) on phagocytic clearance of apoptotic cells, a recombinant expression vector containing full-length human AIRE cDNA was transfected into 16HBE cells. After incubation with transfected 16HBE cells, engulfment of apoptotic HL-60 cells induced by camptothecin was detected by myeloperoxidase (MPO) staining. The change in the expression of Rac 1 in transfected 16HBE cells was determined by RT-PCR and Western blotting. The results showed that the phagocytosis percentage of the experimental group, the mock transfection group and the negative control group (non-apoptotic cells) was (25.50 + or - 3.67)%, (6.25 + or - 1.58)% and (1.0 + or - 0.67)%, respectively. Moreover, the expressions of Rac 1 mRNA and protein were up-regulated in AIRE-transfected 16HBE cells, suggesting that AIRE may function as a regulator in the phagocytic clearance of apoptotic cells by promoting the expression of Rac 1.

Spatial mapping of thymic stromal microenvironments reveals unique features influencing T lymphoid differentiation

Interaction of hematopoietic progenitors with the thymic microenvironment induces them to proliferate, adopt the T lineage fate, and asymmetrically diverge into multiple functional lineages. Progenitors at various developmental stages are stratified within the thymus, implying that the corresponding microenvironments provide distinct sets of signals to progenitors migrating between them. These differences remain largely undefined. Here we used physical and computational approaches to generate a comprehensive spatial map of stromal gene expression in the thymus. Although most stromal regions were characterized by a unique gene expression signature, the central cortex lacked distinctive features. Instead, a key function of this region appears to be the sequestration of unique microenvironments found at the cortical extremities, thus modulating the relative proximity of progenitors moving between them. Our findings compel reexamination of how cell migration, lineage specification, and proliferation are controlled by thymic architecture and provide an in-depth resource for global characterization of this control.