Expression Of Tissue-restricted Self-antigens Research Articles

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 self-tolerance induction and prevents autoimmunity, with each TRA being expressed in only a few mTECs. How this process is regulated in single mTECs and 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 provide evidence for 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, which might involve local re-modeling of chromatin and thus ensures a comprehensive representation of the immunological self.

Direct and indirect roles of the LTβR pathway in central tolerance induction

Medullary thymic epithelial cells (mTECs) play a critical role in thymic negative selection of autoreactive thymocytes, especially for thymocytes specific for peripheral tissue-restricted self-antigens (TRA). Deficiency in lymphotoxin b receptor (LTbetaR) is associated with peripheral tissue inflammation, but whether this is caused by defective negative selection has been unclear; the significance of the LTbetaR pathway for negative selection is evident in some models but not others. Here, we revisit the data and clarify the role of LTbetaR in mTEC development and function and thymic TRA expression. These processes are discussed as potential mechanisms for LTbetaR-mediated control of negative selection.

Highly variable expression of tissue‐restricted self‐antigens in human thymus: Implications for self‐tolerance and autoimmunity

Induction of T cell tolerance in the thymus (central tolerance) is essential for preventing organ-specific autoimmunity. This apparent paradox is in part explained by promiscuous expression of numerous tissue-restricted self-antigens (TRA) in medullary thymic epithelial cells (mTEC), which is highly conserved between mice and man. In animal models, the threshold of central tolerance towards such TRA is surprisingly sensitive towards minor shifts in antigen expression levels and this might also be the case in humans. To precisely assess the inter-individual variability of TRA expression in man, we determined the level of transcription of several auto-antigens in purified human mTEC and subsets thereof by quantitative RT-PCR. We detected two expression patterns: first, high variability (>20-fold) correlated with autoimmune regulator (Aire) expression and mTEC differentiation, and secondly, non-correlated low variability. Importantly, our approach revealed a significantly higher Aire-correlated insulin transcription in mTEC of carriers of the protective insulin-dependent diabetes mellitus locus 2 haplotype compared to the non-protective haplotype. The considerable, yet selective variability in thymic expression levels of target auto-antigen expression might constitute a hitherto underestimated risk factor for the susceptibility of autoimmune diseases in man.

Fremd und Selbst – Die Herausforderung für das Immunsystem

This process of thymic selection produces a repertoire of T-cells able to recognize foreign antigens but to remain nonreactive to the body's own antigens, designated "self" (central tolerance). The interaction between lymphoid and stromal cells within the thymic microenvironment is critical in order to shape the correct antigen specificity of the T-cell repertoire. The thymus has been regarded as the main site of tolerance induction to ubiquitous self-antigens, i.e. abundant blood-borne self-antigens or proteins that are expressed by thymic cells. In contrast, tolerance to ubiquitous, i.e. tissue-restricted, self-antigens has been attributed to extrathymic, peripheral mechanisms. Here I review scientific evidence which assigns a much larger role for the thymus in tissue-specific tolerance. The reason for this paradigmatic shift is, first, the discovery of ectopic expression of tissue-restricted self-antigens by thymic medullary epithelial cells (TECs) which thus present to developing T-cells a "molecular mirror of peripheral self". As a consequence, central tolerance to many peripheral self-antigens can be generated. Second, the thymus is capable to produce regulatory T-cells. Such CD4+CD25+Foxp3+ T-cells are selected by antigens presented by medullary TECs and can control in the periphery the activation of tissue-specific self-reactive T-cells by use of active inhibitory mechanisms. These new insights have implications for our understanding of self-tolerance and its breakdown in autoimmune diseases.

A CENTRAL ROLE FOR CENTRAL TOLERANCE

Recent elucidation of the role of central tolerance in preventing organ-specific autoimmunity has changed our concepts of self/nonself discrimination. This paradigmatic shift is largely attributable to the discovery of promiscuous expression of tissue-restricted self-antigens (TRAs) by medullary thymic epithelial cells (mTECs). TRA expression in mTECs mirrors virtually all tissues of the body, irrespective of developmental or spatio-temporal expression patterns. This review summarizes current knowledge on the cellular and molecular regulation of TRA expression in mTECs, outlines relevant mechanisms of antigen presentation and modes of tolerance induction, and discusses implications for the pathogenesis of autoimmune diseases and other biological processes such as fertility, pregnancy, puberty, and tumor defense.

Contrasting models of promiscuous gene expression by thymic epithelium

Medullary thymic epithelial cells (mTECs) express a broad spectrum of tissue- restricted self-antigens (TRAs), which are required for the development of central tolerance. A new study suggests that TRA expression is a specialized property of terminally differentiated mTECs. However, as discussed here, an alternative model—whereby TRA expression is regulated by conserved developmental programs active in developing mTECs—may be equally plausible.

More mature and more promiscuous

Epithelial cells in the thymus teach T cells what to ignore by presenting them with a comprehensive array of tissue-restricted self-antigens (TRAs) during development. According to Derbinski and colleagues on page 33, this promiscuous expression of TRAs—one of the mechanisms behind central T cell tolerance—increases as the epithelial cells mature. Teaching T cells how to distinguish self-tissues from foreign invaders is essential for the prevention of autoimmune disease. This education process occurs in the thymus and depends on the presentation of a diverse array of self-antigens to developing thymocytes—a function of medullary thymic epithelial cells (mTECs). T cells that recognize these self-antigens are either deleted or become regulatory T cells. Figure Self-antigens normally expressed only in individual tissues (labelled) are made in mTECs as they mature. Recent studies have shown that the transcriptional regulator Aire (autoimmune regulator), which is highly expressed in mTECs, drives the expression of many of these TRAs. But not all TRAs depend on Aire, and Aire expression is not limited to mTECs. Thus, the rules that govern the thymic expression of these tightly regulated genes remain largely mysterious. Derbinski et al. now show that the expression of a majority of TRAs increases as the mTECs differentiate in the thymus, suggesting an intricate link between mTEC maturity and TRA expression. The level of Aire expression mirrored the increase in TRA expression, consistent with the established role of Aire in this process. The authors confirm, however, that Aire does not act alone, as many TRAs are up-regulated in mature mTECs from mice lacking the transcriptional regulator. How do differentiating mTECs turn on these genes that are normally expressed only in peripheral tissue? The answer is not completely clear, but Derbinski and colleagues show that regulation occurs at many levels. Some TRAs were expressed by transcriptional read-through of genes that are clustered together in a contiguous chromosomal region. Others depended on de-repression of genes normally silenced by genetic imprinting. But exactly how mTEC differentiation triggers these changes in gene expression remains to be determined.

Ber-ACT8: new monoclonal antibody to the mucosa lymphocyte antigen.

Using a newly established HTLV-1 positive T cell line as an immunogen, a new monoclonal antibody, Ber-ACT8, was produced. It reacts with in vitro activated T cells and a small subset of normal resting T cells, but not with resting B cells or any of the 29 established human permanent cell lines tested. Immunohistological analysis of a wide spectrum of human tissues showed that Ber-ACT8 reactivity is restricted to a few T cells in the peripheral blood, the extrafollicular areas of lymph nodes and tonsils, and splenic red pulp. In the gut Ber-ACT8 labelled most intraepithelial T cells and up to 50% of lamina propria T cells. The antibody also immunostained T cells present in the oral and bronchial mucosa. Double labelling on splenic cells, fresh blood lymphocytes, and in vitro activated T cells showed that most Ber-ACT8 positive cells coexpressed CD8. Ber-ACT8 did not react with any of the 14 Hodgkin's lymphomas nor any of the 172 non-Hodgkin's lymphomas tested, with the exception of 10 cases of T cell lymphomas, five of which were located in the jejunum and associated with coeliac disease, and one B cell lymphoma, and most cases of hairy cell leukaemia tested. Parallel immunostainings with Ber-ACT8, anti-TCR-beta (beta F1), and anti-TCR-delta showed that most Ber-ACT8 positive T cells carry the TCR of alpha beta type. Comparison of Ber-ACT8 with HML-1, B-ly7, and LF61 showed essentially the same reactivity and an identical molecular target. The molecular structure recognised seems to be a trimeric molecule with components of 150, 125 and 105 kilodaltons, with the Ber-ACT8 epitope localised on the 150 kilodalton chain. The 150 kilodalton molecule contains an 0-linked carbohydrate moiety of about 10 kilodaltons. Because of its very selective distribution, the trimeric antigen is a powerful reagent for the diagnosis of gut T cell-derived T cell lymphomas and other extranodal T cell lymphomas, as well as hairy cell leukaemia.