Abstract
Aire induces ectopic expression of peripheral tissue antigens (PTAs) in thymic medullary epithelial cells, which promotes immunological tolerance. Beginning with a broad screen of histone peptides, we demonstrate that the mechanism by which this single factor controls the transcription of thousands of genes involves recognition of the amino-terminal tail of histone H3, but not of other histones, by one of Aire's plant homeodomain (PHD) fingers. Certain posttranslational modifications of H3 tails, notably dimethylation or trimethylation at H3K4, abrogated binding by Aire, whereas others were tolerated. Similar PHD finger-H3 tail-binding properties were recently reported for BRAF-histone deacetylase complex 80 and DNA methyltransferase 3L; sequence alignment, molecular modeling, and biochemical analyses showed these factors and Aire to have structure-function relationships in common. In addition, certain PHD1 mutations underlying the polyendocrine disorder autoimmune polyendocrinopathy-candidiases-ectodermaldystrophy compromised Aire recognition of H3. In vitro binding assays demonstrated direct physical interaction between Aire and nucleosomes, which was in part buttressed by its affinity to DNA. In vivo Aire interactions with chromosomal regions depleted of H3K4me3 were dependent on its H3 tail-binding activity, and this binding was necessary but not sufficient for the up-regulation of genes encoding PTAs. Thus, Aire's activity as a histone-binding module mediates the thymic display of PTAs that promotes self-tolerance and prevents organ-specific autoimmunity.