ABSTRACT
Aire is a transcriptional regulator that induces the promiscuous expression of thousands of tissue-restricted antigens (TRAs) in medullary thymic epithelial cells (mTECs), a step critical for the induction of immunological self-tolerance. Studies have offered molecular insights into how Aire operates, but more comprehensive understanding of this process still remains elusive. Here we found abundant expression of the protein deacetylase Sirtuin-1 (Sirt1) in mature Aire(+) mTECs, wherein it was required for the expression of Aire-dependent TRA-encoding genes and the subsequent induction of immunological self-tolerance. Our study elucidates a previously unknown molecular mechanism for Aire-mediated transcriptional regulation and identifies a unique function for Sirt1 in preventing organ-specific autoimmunity.
Subject(s)
Central Tolerance/immunology , Sirtuin 1/immunology , Transcription Factors/immunology , Transcriptional Activation/immunology , Acetylation , Animals , Antigens/immunology , Central Tolerance/genetics , Epithelial Cells/immunology , Epithelial Cells/metabolism , Flow Cytometry , HEK293 Cells , Humans , Immunoblotting , Interleukin Receptor Common gamma Subunit/deficiency , Interleukin Receptor Common gamma Subunit/genetics , Mice, Inbred C57BL , Mice, Inbred NOD , Mice, Knockout , Mice, SCID , Mice, Transgenic , Oligonucleotide Array Sequence Analysis , Organ Specificity/immunology , Protein Binding/immunology , Reverse Transcriptase Polymerase Chain Reaction , Sirtuin 1/genetics , Sirtuin 1/metabolism , Thymus Gland/cytology , Thymus Gland/immunology , Thymus Gland/metabolism , Transcription Factors/genetics , Transcription Factors/metabolism , Transcriptome/immunology , AIRE ProteinABSTRACT
The ability of the immune system to avoid autoimmune disease relies on tolerization of thymocytes to self-antigens whose expression and presentation by thymic medullary epithelial cells (mTECs) is controlled predominantly by Aire at the transcriptional level and possibly regulated at other unrecognized levels. Aire-sensitive gene expression is influenced by several molecular factors, some of which belong to the 3'end processing complex, suggesting they might impact transcript stability and levels through an effect on 3'UTR shortening. We discovered that Aire-sensitive genes display a pronounced preference for short-3'UTR transcript isoforms in mTECs, a feature preceding Aire's expression and correlated with the preferential selection of proximal polyA sites by the 3'end processing complex. Through an RNAi screen and generation of a lentigenic mouse, we found that one factor, Clp1, promotes 3'UTR shortening associated with higher transcript stability and expression of Aire-sensitive genes, revealing a post-transcriptional level of control of Aire-activated expression in mTECs.
Subject(s)
3' Untranslated Regions/genetics , Cell Differentiation/immunology , Thymocytes/metabolism , Thymus Gland/metabolism , Animals , Cell Differentiation/genetics , Epithelial Cells/metabolism , Gene Expression Regulation/genetics , MiceABSTRACT
BACKGROUND: In order to become functionally competent but harmless mediators of the immune system, T cells undergo a strict educational program in the thymus, where they learn to discriminate between self and non-self. This educational program is, to a large extent, mediated by medullary thymic epithelial cells that have a unique capacity to express, and subsequently present, a large fraction of body antigens. While the scope of promiscuously expressed genes by medullary thymic epithelial cells is well-established, relatively little is known about the expression of variants that are generated by co-transcriptional and post-transcriptional processes. RESULTS: Our study reveals that in comparison to other cell types, medullary thymic epithelial cells display significantly higher levels of alternative splicing, as well as A-to-I and C-to-U RNA editing, which thereby further expand the diversity of their self-antigen repertoire. Interestingly, Aire, the key mediator of promiscuous gene expression in these cells, plays a limited role in the regulation of these transcriptional processes. CONCLUSIONS: Our results highlight RNA processing as another layer by which the immune system assures a comprehensive self-representation in the thymus which is required for the establishment of self-tolerance and prevention of autoimmunity.