Elsevier

Immunology Letters

Volume 106, Issue 2, 15 August 2006, Pages 172-179
Immunology Letters

Expression profiling of autoimmune regulator AIRE mRNA in a comprehensive set of human normal and neoplastic tissues

https://doi.org/10.1016/j.imlet.2006.06.006Get rights and content

Abstract

Defects in the autoimmune regulator (AIRE) gene cause the monogenic autoimmune disease autoimmune polyendocrinopathy syndrome type 1 (APS-1), which is characterized by a loss of self-tolerance to multiple organs. In concordance with its role in immune tolerance, AIRE is strongly expressed in medullary thymic epithelial cells (mTECs). Data on mechanisms controlling AIRE activation and the expression of this gene in other tissues are fragmentary and controversial. We report here AIRE mRNA expression profiling of a large set of normal human tissues and cells, tumor specimen and methylation deficient cell lines. On this broad data basis we found that AIRE mRNA expression is confined to mTECs in thymus and to lymph node tissue and that DNA hypomethylation contributes to transcriptional control of this gene.

Introduction

Induction and maintenance of central T-cell tolerance to peripheral organs essentially depends on intrathymic expression of otherwise tissue restricted peripheral self-antigens, termed promiscuous gene expression, which is an inherent property of medullary thymus epithelial cells (mTECs) [1]. Negative selection of potentially autoreactive T-cells recognizing these antigens prevents organ-specific autoimmunity [2]. Protocols to obtain pure populations of mTECs have boosted further dissection of the characteristics of these cells [3], [4]. One of the specific features of mTECs, in particular of mature CD80hi mTECs, is the expression of the autoimmune regulator gene AIRE, which is not or only barely detectable in other cell types within the thymus [3], [5]. The pivotal role of AIRE in the induction of central tolerance is unequivocal, as patients with loss-of-function mutations develop a rare multiorgan autoimmune disease known as autoimmune polyendocrinopathy syndrome type 1 (APS-1; OMIM 240300) or autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) [6], [7], [8]. There is strong evidence, that AIRE operates as a transcriptional regulator and mediates the induction of not all but a subset of genes promiscuously expressed in mTECs [5], [9], [10]. In line with this, targeted disruption of murine AIRE leads to reduced expression of numerous self-antigens in mTECs and development of spontaneous autoimmunity [9], [11]. As recently published, AIRE is involved additionally in the enhancement of the antigen presentation capability of mTECs [12], [13].

It is still unclear, whether AIRE plays also a role in other cell types. Extended expression profiling of human AIRE may assist in further dissecting its physiological role, in particular extrathymically. However, previous studies by Northern Blot, in situ hybridisation (ISH) and immunohistochemistry (IHC) analysis to investigate tissue distribution of human AIRE mRNA and protein have been performed in small selections rather than comprehensive collections of specimens. Furthermore, data on the expression of AIRE at the protein and mRNA level in other lymphoid organs as well as in peripheral tissues are still controversial as is the nature of the cell types expressing AIRE within a specific tissue type [6], [7], [14], [15], [16], [17].

The pool of promiscuously expressed genes in mTECs represents up to 10% of the whole genome [1], which implies a complex regulation. Until now, it is not clearly understood, how the AIRE independent portion of tissue-specific genes in mTECs is activated. Co-localization of AIRE-controlled as well as AIRE-independent promiscuously expressed genes in clusters suggests epigenetic mechanisms (e.g. DNA methylation) to contribute to this unorthodox gene expression pattern [5].

It has been reported that several members of the cancer/germline class are among the promiscuously expressed genes in mTECs [4]. Cancer/germline (CG) genes are normally strictly restricted to gametogenic cells and underlie stringent transcriptional repression in healthy somatic tissues [18]. In human cancer, however, CG genes escape silencing and are robustly expressed in varying frequencies in a broad range of different tumor types, including neoplasms of the breast, stomach, prostate, lung, liver, etc. [18]. The fact that mTECs and human cancer cells share the feature of activation of otherwise tightly controlled germ cell genes raises the question whether similar mechanisms of transcriptional regulation are operant in both cell types. So far, human cancer cells have not been analysed for AIRE expression and data on how AIRE itself is regulated is scarce [10], [19].

In an effort to extensively map the distribution of AIRE mRNA in human tissues and to understand the regulation of this gene, we performed expression profiling of AIRE mRNA in a comprehensive set of healthy organs, cancer specimen and human cancer cell lines lacking DNA methyltransferase activity.

Section snippets

Tissues, cells and cell lines

This study was approved by the local ethical review board (Ethikkomission der Ärztekammer des Landes Rheinland-Pfalz). Tumor tissues were obtained during routine diagnostic or therapeutic procedures and were stored at −80 °C until use. Healthy tissue samples were also obtained during routine surgery, e.g. as part of the safety margin required in case of excision of malignant or inflammatory lesions. Expert histopathological examination of haematoxylin/eosin stained sections of these tissues

Expression of AIRE mRNA in normal human tissues and cells

To map the tissue distribution pattern of human AIRE mRNA, we analysed a comprehensive panel of 36 different normal tissue and cell types by conventional endpoint RT-PCR and quantitative real-time RT-PCR. To address interindividual expression variances, samples from at least three different donors were analysed for the majority of tissue types.

mTECs as the main cellular source for AIRE expression represent only a small subset of cells within the thymus. An analogously small cell compartment in

Discussion

Using mRNA expression as a metric for probing function is a well-established concept and emphasizes the value of diligent mapping of tissue distribution.

As reviewed in Table 3, data in literature with regard to the expression pattern of the autoimmune regulator gene AIRE in human tissues and cells are controversial and far from being conclusive. Depending on the study one reads, AIRE mRNA levels in tissues as e.g. spleen, pancreas and adrenal cortex are described as negative, moderate or strong

Acknowledgements

We thank Sonja Schöttler for critical editing of our manuscript. The authors appreciate the efforts made by the members of the different ASZ surgical departments for their cooperation allowing rapid analysis and storage of tissue samples. This study was supported by the Combined Project Grant SFB432 (projects D1 and A14) and the Heisenberg scholarship (TU 115/2-1) of the Deutsche Forschungsgemeinschaft.

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