Autoimmune regulator (AIRE) is a protein that helps establish central immune tolerance, the process that limits immune responses against the body’s own tissues during lymphocyte development. Encoded by the human AIRE gene, it acts principally in specialized epithelial cells of the thymus, where it promotes the expression of proteins normally associated with other tissues. Exposure to these self-proteins helps shape the developing T-cell population. Loss of AIRE function can cause severe, multiorgan autoimmunity. (ncbi.nlm.nih.gov)
Gene and molecular properties
The human AIRE gene is located at 21q22.3, on the long arm of chromosome 21. Its product is a nuclear transcriptional regulator that forms discrete nuclear bodies and interacts with proteins involved in controlling gene expression. The italicized symbol AIRE refers to the human gene; AIRE denotes its protein product. (ncbi.nlm.nih.gov)
AIRE contains several functional regions, including an amino-terminal caspase recruitment domain (CARD), a SAND domain, and two plant homeodomain fingers, designated PHD1 and PHD2. These regions contribute to its molecular interactions and transcriptional activity. Despite their name, plant homeodomain fingers also occur in animal proteins. Mutations in different AIRE domains can disrupt its activity through distinct mechanisms, including altered subcellular localization and impaired association with chromatin. (pmc.ncbi.nlm.nih.gov)
PHD1 recognizes the amino-terminal tail of histone H3, with a preference for an unmethylated lysine at position 4, abbreviated H3K4me0. This interaction links AIRE activity to the chemical state of chromatin rather than simply to recognition of a particular DNA sequence. Experiments have shown that some disease-associated PHD1 mutations impair histone binding and the activation of target genes. (pubmed.ncbi.nlm.nih.gov)
Role in thymic self-tolerance
AIRE’s best-established function occurs in medullary thymic epithelial cells (mTECs). These cells express a broad range of tissue-restricted proteins—proteins whose expression elsewhere is normally concentrated in particular organs. Such expression in the thymus is often called promiscuous gene expression: “promiscuous” here means that the expressed genes span many tissue-specific programs. AIRE controls an important subset of this expression, allowing the thymus to display a representation of proteins encountered throughout the body. (pubmed.ncbi.nlm.nih.gov)
The relationship between AIRE and T-cell selection involves several steps:
- Self-antigen production. AIRE promotes transcription of genes encoding otherwise tissue-restricted antigens.
- Antigen presentation. Thymic epithelial cells display self-derived peptides in association with major histocompatibility complex molecules. Dendritic cells can also acquire epithelial-cell-derived antigens or peptide–MHC complexes, extending the network of self-antigen presentation.
- Selection of developing T cells. Recognition of these self-antigens can result in the removal of autoreactive cells or contribute to the development of immunoregulatory cells. (pubmed.ncbi.nlm.nih.gov)
Removal of autoreactive cells is known as clonal deletion or negative selection. AIRE does not itself directly kill T cells; it helps create the antigenic environment in which selection occurs. In Aire-deficient mice, reduced expression of peripheral tissue antigens in thymic epithelial cells is associated with organ-specific autoimmune disease. (pubmed.ncbi.nlm.nih.gov)
AIRE also contributes to the development of particular regulatory T-cell populations. These cells suppress immune responses rather than functioning primarily as conventional effector T cells. Mouse studies have demonstrated an AIRE-dependent population of FOXP3-expressing regulatory T cells generated early in life that contributes to durable self-tolerance. This finding establishes that AIRE-mediated tolerance is not limited to deleting potentially harmful cells. (pubmed.ncbi.nlm.nih.gov)
Transcriptional mechanism
AIRE’s effects involve both chromatin recognition and the machinery responsible for transcription. Its PHD1 interaction with histone H3 provides one means of associating with chromatin, while interactions with transcriptional cofactors help activate otherwise poorly expressed genes. Histone recognition is therefore one component of its mechanism, not a complete explanation of target-gene selection. (pmc.ncbi.nlm.nih.gov)
An experimentally established mechanism is recruitment of positive transcription elongation factor b (P-TEFb). This complex assists RNA polymerase II in producing extended transcripts. Experiments in cultured cells and mouse thymic epithelial cells showed that AIRE can promote transcriptional elongation, helping convert initiated transcription into productive gene expression. (pubmed.ncbi.nlm.nih.gov)
AIRE is not the sole regulator of thymic self-antigen expression. For example, the transcription factor FEZF2 controls a distinct program of tissue-restricted antigen expression independently of Aire in mice. Consequently, AIRE deficiency disrupts an important component of self-tolerance without eliminating all thymic self-antigen expression or all mechanisms of tolerance. (sciencedirect.com)
Genetic deficiency and human disease
Pathogenic loss-of-function mutations in AIRE cause autoimmune polyendocrinopathy–candidiasis–ectodermal dystrophy (APECED), also called autoimmune polyendocrine syndrome type 1 (APS-1). The classical disorder follows autosomal-recessive inheritance, with pathogenic variants affecting both gene copies. Impaired AIRE activity compromises self-tolerance and permits immune-mediated injury to multiple organs. (ncbi.nlm.nih.gov)
Characteristic manifestations include chronic mucocutaneous candidiasis, hypoparathyroidism, and adrenal insufficiency, although the combination of affected organs varies. Autoimmune damage to endocrine glands accounts for many manifestations. Susceptibility to candidiasis has been linked to autoantibodies against signaling proteins in the IL-17 pathway, which contributes to mucosal defense against Candida. (medlineplus.gov)
Some individual AIRE variants have also been reported to act in a dominant-negative manner: the altered protein inhibits activity of the normal protein produced from the other gene copy. Family studies have associated certain such variants with variable autoimmune manifestations, sometimes with later onset and incomplete penetrance. These findings are variant-specific and do not mean that every person carrying one altered AIRE copy develops autoimmune disease. (pubmed.ncbi.nlm.nih.gov)
Extrathymic expression
Aire-expressing cells have also been identified outside the thymus. Experiments in mice demonstrated that extrathymic Aire-expressing cells in secondary lymphoid organs can interact with and delete autoreactive T cells. These observations indicate a possible contribution to peripheral immune tolerance, which operates after lymphocytes leave their primary developmental organs. The experimental finding is distinct from establishing the full physiological importance of these cells in humans. (pmc.ncbi.nlm.nih.gov)
Discovery and experimental significance
The AIRE gene was identified in 1997 through positional cloning studies of APECED. Independent reports described a previously unknown gene encoding a protein with structural features suggestive of transcriptional regulation and identified mutations in affected individuals. These discoveries connected a monogenic human autoimmune disorder to a specific nuclear regulator. (archive-ouverte.unige.ch)
A 2002 study of Aire-deficient mice established a functional connection between Aire, thymic expression of peripheral tissue antigens, and protection against organ-specific autoimmunity. Subsequent biochemical studies identified histone recognition and transcriptional elongation as components of AIRE activity. Together, these experimental systems made AIRE a model for investigating how gene regulation in thymic epithelial cells shapes immune self-recognition. (pubmed.ncbi.nlm.nih.gov)
References
- AIRE autoimmune regulator [Homo sapiens (human)] - Gene - NCBIncbi.nlm.nih.gov
- AIRE gene: MedlinePlus Geneticsmedlineplus.gov
- Projection of an immunological self shadow within the thymus by the aire proteinpubmed.ncbi.nlm.nih.gov
- Mechanistic dissection of dominant AIRE mutations in mouse models reveals AIRE autoregulationpmc.ncbi.nlm.nih.gov
- The autoimmune regulator PHD finger binds to non-methylated histone H3K4 to activate gene expressionpubmed.ncbi.nlm.nih.gov
- Aire employs a histone-binding module to mediate immunological tolerance, linking chromatin regulation with organ-specific autoimmunitypmc.ncbi.nlm.nih.gov
- Transfer of Cell-Surface Antigens by Scavenger Receptor CD36 Promotes Thymic Regulatory T Cell Receptor Repertoire Development and Allo-tolerancepubmed.ncbi.nlm.nih.gov
- Thymic Dendritic Cell Subsets Display Distinct Efficiencies and Mechanisms of Intercellular MHC Transferpubmed.ncbi.nlm.nih.gov
- Immune tolerance. Regulatory T cells generated early in life play a distinct role in maintaining self-tolerancepubmed.ncbi.nlm.nih.gov
- AIRE recruits P-TEFb for transcriptional elongation of target genes in medullary thymic epithelial cellspubmed.ncbi.nlm.nih.gov
- Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy: MedlinePlus Geneticsmedlineplus.gov