Target-organ specificity of autoimmunity is modified by thymic stroma and bone marrow-derived cells

Autoimmune Addison's Disease as Part of the Autoimmune Polyglandular Syndrome Type 1: Historical Overview and Current Evidence

The autoimmune polyglandular syndrome type 1 (APS1) is caused by pathogenic variants of the autoimmune regulator (AIRE) gene, located in the chromosomal region 21q22.3. The related protein, AIRE, enhances thymic self-representation and immune self-tolerance by localization to chromatin and anchorage to multimolecular complexes involved in the initiation and post-initiation events of tissue-specific antigen-encoding gene transcription. Once synthesized, the self-antigens are presented to, and cause deletion of, the self-reactive thymocyte clones. The clinical diagnosis of APS1 is based on the classic triad idiopathic hypoparathyroidism (HPT)-chronic mucocutaneous candidiasis-autoimmune Addison's disease (AAD), though new criteria based on early non-endocrine manifestations have been proposed. HPT is in most cases the first endocrine component of the syndrome; however, APS1-associated AAD has received the most accurate biochemical, clinical, and immunological characterization. Here is a comprehensive review of the studies on APS1-associated AAD from initial case reports to the most recent scientific findings.

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.

Evidence for de novo expression of thymic insulin by peripheral bone marrow-derived cells

Thymic expression of insulin has been suggested to play a major role in negative selection of autoreactive T cells and tolerance induction against pancreatic beta cells. Furthermore, the expression of insulin in peripheral antigen-presenting cells (APC) has been clearly demonstrated but whether thymic negative selection and tolerance induction also depends on peripheral influx of self-antigens (Ag) remains to be conclusively demonstrated. In this study, we wanted to test whether peripheral influx of insulin expressing cells might contribute to negative selection. In order to address this question, we used mice deficient in the Ins1 and Ins2 genes. Embryonic thymi either deficient in both insulin genes or expressing Ins2 were dissected and transplanted under the kidney capsule of athymic nude mice recipients. After indicated time points, grafted thymi were removed and analysed for insulin re-expression and for the emergence of autoreactive T cells. The analysis revealed a re-expression of Ins2 in grafted insulin deficient thymi suggesting that self-Ag expression in the thymus is not only intrinsically regulated but peripheral influx of APC capable of expressing insulin might contribute to thymic selection and tolerance induction.