Peripheral antigen-expressing cells and autoimmunity

Antigen-presenting cells expressing glutamate decarboxylase 67 were identified as epithelial cells in glutamate decarboxylase 67-GFP knock-in mouse thymus

Glutamate decarboxylase (GAD), which has two isoforms, GAD65, and GAD67, is responsible for synthesis of the major inhibitory neurotransmitter, gamma-aminobutyric acid. GAD is expressed predominantly in the central nervous system; recent reports suggest that GAD is also expressed in non-neuronal organs including the pancreas. In the pancreatic islets, GAD serves as one of the autoantigens in type I diabetes mellitus. Recent flow cytometric analyses have shown that a variety of self-antigens, including GAD, are ectopically transcribed and expressed in particular cell populations of the thymus, although consensus concerning the cellular phenotype has not been obtained. The aim of this study was to clarify the localization and cellular phenotype of GAD67-expressing cells in the thymus at a cellular level with a novel approach using GAD67-green fluorescent protein (GFP) knock-in mice, in which GFP is expressed specifically in GAD67-positive cells. GFP-positive cells were detected in the thymic medulla and were identified as epithelial cells by immunohistochemistry. Almost all GFP-positive cells were positive for major histocompatibility complex (MHC) class II antigen staining and were positive for both cytokeratin and Ulex Europaeus Agglutinin I, markers of medullary thymic epithelial cells, but were negative for CD11c, Gr-1, and CD45, markers of dendritic cells, macrophages, and B-lymphocytes, respectively.

Thymic transcription of neurohypophysial and insulin-related genes: impact upon T-cell differentiation and self-tolerance

The thymus is the unique lymphoid organ responsible for the generation of a diverse repertoire of T lymphocytes that are competent against non self-antigens while being tolerant to self-antigens. A vast repertoire of neuroendocrine-related genes is transcribed in the nonlymphoid cellular compartment of the thymus (thymic epithelial cells, dendritic cells and macrophages). The precursors encoded by these genes engage two types of interactions with developing T cells (thymocytes). First, they are not processed in a classical neuroendocrine way but as the source of self-antigens that are presented to pre-T cells by the major histocompatibility complex proteins of the thymus. This presentation could be responsible for the establishment of central T-cell self-tolerance to neuroendocrine functions. Second, they also deliver signal ligands that are able to bind to neuroendocrine-type receptors expressed by thymocytes. This interaction activates several types of intracellular signalling pathways implicated in the developmental process of T lymphocytes. Several experimental arguments support a role for thymic dysfunction as a crucial factor in the development of organ-specific autoimmune endocrinopathies, such as 'idiopathic' central diabetes insipidus and type 1 diabetes mellitus. The rational use of tolerogenic neuroendocrine self-antigens for the prevention/treatment of autoimmune endocrinopathies is currently under investigation.

Expression of tissue-specific autoantigens in the hematopoietic cells leads to activation-induced cell death of autoreactive T cells in the secondary lymphoid organs

Many tissue-specific antigens are expressed in specialized cells called peripheral antigen-expressing cells (PAE) in the thymus and can induce central tolerance. While thymic medullary epithelial cells are the prototypic PAE that express peripheral antigens via an aire-dependent mechanism, some studies also describe bone marrow (BM)-derived dendritic cells (DC) and macrophages as PAE in both the thymus and secondary lymphoid organs. However, the role of these cells in development of tolerance to tissue-specific antigens has not been elucidated. Here we use BM radiation chimeric mice to study the existence of hematopoietic PAE and their contribution to tolerance to tissue-specific antigens. Our results reveal that BM-derived PAE exist in both central and secondary lymphoid organs and that the expression of peripheral antigens in the BM-derived cells does not correlate with aire expression. Using double-transgenic mice expressing TCR specific for a model antigen expressed under the control of a prostate-specific promoter, we show that expression of self antigen in PAE of non-hematopoietic origin is both necessary and sufficient to induce clonal deletion. Surprisingly, while BM-derived PAE fail to induce clonal deletion, they do cause the activation-induced cell death of autoreactive cells in the secondary lymphoid organs. Thus, BM-derived PAE have a distinct function in the maintenance of tolerance to tissue-specific antigens.

Peripheral antigen-expressing cells in type 1 diabetes

Studies in both humans and rodent models provide new insight into key mechanisms regulating tolerance to self-molecules. There is evidence that tissue-specific molecules are expressed in the thymus and peripheral lymphoid tissues (PLTs) by specialized antigen-presenting cells (APCs), and that such expression is critical for self-tolerance. Insulin, a key hormone exclusively produced by pancreatic beta cells and a critical autoantigen in type 1 diabetes, provides an excellent example of a molecule with tissue-restricted expression that is ectopically expressed by APCs in both thymus and PLTs. APCs may play a role in insulin presentation in both the central and peripheral immune system. Functional data from several transgenic and knockout mouse models, some specific for the expression of insulin, help dissect the significance of self-molecule presentation by APCs and its role in autoimmune diabetes.

Central and peripheral autoantigen presentation in immune tolerance

Recent studies in both humans and experimental rodent models provide new insight into key mechanisms regulating tolerance to self-molecules. These recent advances are bringing about a paradigm shift in our views about tolerance to self-molecules with tissue-restricted expression. There is, indeed, mounting evidence that selected antigen-presenting cells (APCs) have the ability to synthesize and express self-molecules, and that such expression is critical for self-tolerance. Insulin is a key hormone produced exclusively by pancreatic beta-cells and a critical autoantigen in type 1 diabetes. It provides an excellent example of a molecule with tissue-restricted expression that is expressed ectopically by APCs. The fact that APCs expressing insulin have been demonstrated in both thymus and peripheral lymphoid tissues suggests that they may play a role in insulin presentation in both the central and peripheral immune system. Experimental mice, in which insulin expression was altered, provide functional data that help to dissect the role of insulin presentation by APCs of the immune system. This review addresses recent literature and emerging concepts about the expression of self-molecules in the thymus and peripheral lymphoid tissues and its relation to self-tolerance.

Reduced thymic expression of islet antigen contributes to loss of self-tolerance

Type 1 diabetes (T1DM) results from a failure of central and peripheral tolerance to islet cell antigens. ICA69 belongs to a group of molecules expressed predominantly in neuroendocrine tissues (including pancreatic islets), which are targets of autoimmune responses in T1DM. These molecules are also expressed in the thymus and peripheral lymphoid organs by dendritic cells. The aim of the present study was to evaluate possible variation in thymic ICA69 expression, comparing diabetes-resistant controls to T1DM-prone NOD mice. Thymic tissue was retrieved from 3- to 6-week-old female B6, NOD-H2(b), and NOD mice. Paraffin-embedded sections were stained with an ICA69-specific antibody in an immunoperoxidase assay. ICA69 staining of thymic sections from B6 and NOD.H2(b) showed strong and continual staining, yet the sections from the NOD mice showed significantly reduced staining for ICA69. Corroboration of the reduced level of ICA69 in the thymus of NOD mice has been obtained via analysis for the expression of ICA69 versus other candidate autoantigens (glutamic acid decarboxylase 65, glutamic acid decarboxylase 67, and insulin 2) in the thymus. Real-time PCR analysis, using cDNA generated from the thymus, displayed that the expression of GAD65, GAD67, and INS2 were equivalent when comparing NOD at any age to B6, BALB/cJ, and ALR/LtJ. In marked contrast, the level of ICA69 in the thymus of the NOD mice examined was significantly reduced when compared to the controls. In fact, the real-time PCR analysis strongly suggested that ICA69 was not expressed in the thymus of NOD mice. These findings support the hypothesis that the level of thymic ICA69 expression may be of importance in regulating self-tolerance in T1DM.

Low expression of insulin in the thymus of non-obese diabetic mice

Insulin is a predominant autoantigen in IDDM in man and the NOD mouse. Failure of negative selection of diabetogenic T cells in thymus may be an important pre-disposing cause of the disease. To obtain insight into negative selection against such T-cell clones the thymic expression of insulin was studied in NOD and Balb/c mice by quantitative competitive RT-PCR. We detected RNA for insulin in the thymus of 3-week-old Balb/c mice as well as in NOD mice. However, the NOD mice expressed only half as many insulin transcripts as the Balb/c mice. Also, insulin protein was detected in the thymic medulla of both Balb/c and NOD mice. Furthermore, thymic RNA preparations were investigated for the presence of insulin transcription factors. None of the known pancreatic transcription factors for insulin; Pdx-1, Pax6 or Nkx6.1 were detectable in the thymus of Balb/c mice. These results support the idea that low insulin expression in the thymus may be a predisposing cause for development of diabetes in NOD mice analogous with what has been found in humans with the disease-disposing IDDM2 allele. Furthermore, our results suggest that insulin expression in the thymus may be regulated by different principles from those in the pancreas.

Clonal deletion of simian virus 40 large T antigen-specific T cells in the transgenic adenocarcinoma of mouse prostate mice: an important role for clonal deletion in shaping the repertoire of T cells specific for antigens overexpressed in solid tumors

In addition to their overexpression in cancer cells, most of the tumor-associated Ags are expressed at low but detectable levels in normal tissues. It is not clear whether the repertoire of T cells specific for unmutated tumor Ags is shaped by negative selection during T cell development. The transgenic adenocarcinoma of mouse prostate (TRAMP) model is transgenic for the SV40 large T Ag (Tag) under the control of the rat probasin regulatory elements. Although it has been established that T lymphocytes from TRAMP mice are tolerant to SV40 Tag, the mechanism of the tolerance is largely unknown. To examine whether the T cell clonal deletion is responsible for the tolerance, we crossed the TRAMP mice with mice transgenic for a rearranged TCR specific for SV40 Tag presented by the H-2K(k). Double transgenic TRAMP/TCR mice showed profound thymic deletion of SV40 Tag-reactive T cells, including a 6- to 10-fold reduction in the total thymocyte numbers and a >50-fold reduction in phenotypically mature T cells. Consistent with this finding, we observed that the SV40 Tag and endogenous mouse probasin genes are expressed at low levels in the thymus. These results demonstrate that clonal deletion is a major mechanism for tolerance to Ags previously regarded as prostate-specific, and provide direct evidence that the T cell repertoire specific for an unmutated tumor Ag can be shaped by clonal deletion in the thymus.