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Promiscuous expression of a plethora of tissue-restricted antigens (TRAs) in medullary thymic epithelial cells (mTECs) is essential for central tolerance. This promiscuous gene expression (pGE) is characterized by inclusion of a broad range of TRAs and by its mosaic expression patterns, i.e. each antigen is only expressed in 1–3% of mTECs. It is currently unclear to which extent random and/or deterministic mechanisms are involved in the regulation of pGE. In order to address this issue, we deconstructed the transcriptional heterogeneity in mTEC to minor subsets expressing a particular TRA. We identified six delineable co-expression groups in mouse mTECs. These co-expression groups displayed a variable degree of mutual overlap and mapped to different stages of mTEC development. Co-expressed genes showed chromosomal preference and clustered within delimited genomic regions. Moreover, co-expression groups in mice and humans selected by a pair of orthologous genes preferentially co-expressed sets of orthologous genes attesting to the species conservation of pGE between mouse and human. Furthermore, co-expressed genes were enriched for specific transcription factor binding motifs concomitant with up-regulation of the corresponding transcription factors, implicating additional factors in the regulation of pGE besides the Autoimmune Regulator (Aire). Thus promiscuous transcription of self-antigens in mTECs entails a highly coordinated process, which is evolutionary strictly conserved between species. [ABSTRACT FROM AUTHOR]

Promiscuous expression of a plethora of tissue-restricted Ags (TRAs) by medullary thymic epithelial cells (mTECs) plays an essential role in T cell tolerance. Although the cellular mechanisms by which promiscuous gene expression (pGE) imposes T cell tolerance have been well characterized, the underlying molecular mechanisms remain poorly understood. The autoimmune regulator (AIRE) is to date the only validated molecule known to regulate pGE. AIRE is part of higher-order multiprotein complexes, which promote transcription, elongation, and splicing of a wide range of target genes. How AIRE and its partners mediate these various effects at the molecular level is still largely unclear. Using a yeast two-hvbrid screen, we searched for novel AIRE-interacting proteins and identified the homeodomain-interacting protein kinase 2 (HIPK2) as a novel partner. HIPK2 partially colocalized with AIRE in nuclear bodies upon cotransfection and in human mTECs in situ. Moreover, HIPK2 phosphorylated AIRE in vitro and suppressed the coactivator activity of AIRE in a kinase-dependent manner. To evaluate the role of Hipk2 in modulating the function of AIRE in vivo, we compared whole-genome gene signatures of purified mTEC subsets from TEC-specific Hipk2 knockout mice with control mice and identified a small set of differentially expressed genes. Unexpectedly, most differentially expressed genes were confined to the CD80lo mTEC subset and preferentially included AIRE-independent TRAs. Thus, although it modulates gene expression in mTECs and in addition affects the size of the medullary compartment, TEC-specific HIPK2 deletion only mildly affects AIRE-directed pGE in vivo. [ABSTRACT FROM AUTHOR]

Autoimmune diseases (AIDs) are the consequence of a breach in immune tolerance, leading to the inability to sufficiently differentiate between self and non-self. Immune reactions that are targeted towards self-antigens can ultimately lead to the destruction of the host's cells and the development of autoimmune diseases. Although autoimmune disorders are comparatively rare, the worldwide incidence and prevalence is increasing, and they have major adverse implications for mortality and morbidity. Genetic and environmental factors are thought to be the major factors contributing to the development of autoimmunity. Viral infections are one of the environmental triggers that can lead to autoimmunity. Current research suggests that several mechanisms, such as molecular mimicry, epitope spreading, and bystander activation, can cause viral-induced autoimmunity. Here we describe the latest insights into the pathomechanisms of viral-induced autoimmune diseases and discuss recent findings on COVID-19 infections and the development of AIDs. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]