The deacetylase Sirt1 is an essential regulator of Aire-mediated induction of central immunological tolerance (original) (raw)

Central tolerance is shaped in the thymus, a primary lymphoid organ, where immature T lymphocytes are 'educated' to become mature cells able to recognize foreign antigens while tolerating the body's own components. This educational process involves two central steps that occur in distinct anatomical compartments of the thymus: the cortex and the medulla 1. Both parts are characterized by the presence of specialized types of epithelial cells that provide the required microenvironment for T cell development 1. Although cortical thymic epithelial cells (cTECs) are critical for positive selection of developing thymocytes 2 , medullary thymic epithelial cells (mTECs) have a primary role in the negative selection of self-reactive T cells and/or generation of thymic regulatory T cells (T reg cells) 3-5. The critical role of mTECs is emphasized by their unique ability to promiscuously express and subsequently present essentially all of the body's self antigens, including those that normally have high tissue restriction 5,6. The expression of transcripts encoding many of these tissue-restricted antigens (TRAs) is regulated by a single transcriptional regulator: the autoimmune regulator Aire. Indeed, mice with a dysfunctional gene encoding Aire express only a fraction of the TRA repertoire 7 and as a result develop autoantibodies and immune infiltrates directed at multiple peripheral tissues 7,8 , which resembles the multi-organ autoimmune disorder APS-1 ('autoimmune polyendocrine syndrome type 1') observed in humans with AIRE mutations 9. The molecular mechanisms that underlie the unique ability of Aire to promote the ectopic expression of thousands of tissue-restricted genes has been a topic of considerable interest in the past decade 10,11. Aire is not a classic transcription factor that binds to consensus sequences in the promoters of its target genes but instead is a unique transcriptional regulator that recognizes specific marks of inactive chromatin, such as histone 3 not methylated at Lys4 (refs. 12-14). Several lines of evidence have shown that Aire affects mainly the elongation steps of transcription, in particular the release of stalled RNA polymerase II (refs. 15,16). Correspondingly, Aire has been shown to interact with subunits of the key regulator of the release of RNA polymerase II, the pTEFb-CDK9 complex, as well as the DNA-PK-TOP2-FACT complex 17-19 , which presumably enhances transcription efficacy by removing nucleosomes in front of the elongating RNA polymerase. Aire also interacts with the MBD1-ATF7ip complex, which is required for targeting Aire to the specific TRA-encoding loci 20. Although the studies noted above have provided very important insights into how Aire regulates gene expression, most were based on analyses of protein-protein interactions under nonphysiological conditions. Consequently, they may have missed some of the key physiological partners of Aire. We first sought to identify transcription factors and/or regulators that, like Aire, have high expression in mature mTECs and then assess their involvement in Aire-induced promiscuous gene expression. We found that the protein deacetylase Sirtuin-1 (Sirt1), known to control various physiological processes, including metabolism, development, fertility and aging 21 , was also critical for the establishment of Aire-dependent immunological self-tolerance. Specifically, we found abundant expression of Sirt1 in mature Aire + mTECs, where it was