Twenty Years of AIRE

Abstract

About two decades ago, cloning of the autoimmune regulator (AIRE) gene materialized one of the most important actors on the scene of self-tolerance. Thymic transcription of genes encoding tissue-specific antigens (ts-ags) is activated by AIRE protein and embodies the essence of thymic self-representation. Pathogenic AIRE variants cause the autoimmune polyglandular syndrome type 1, which is a rare and complex disease that is gaining attention in research on autoimmunity. The animal models of disease, although not identically reproducing the human picture, supply fundamental information on mechanisms and extent of AIRE action: thanks to its multidomain structure, AIRE localizes to chromatin enclosing the target genes, binds to histones, and offers an anchorage to multimolecular complexes involved in initiation and post-initiation events of gene transcription. In addition, AIRE enhances mRNA diversity by favoring alternative mRNA splicing. Once synthesized, ts-ags are presented to, and cause deletion of the self-reactive thymocyte clones. However, AIRE function is not restricted to the activation of gene transcription. AIRE would control presentation and transfer of self-antigens for thymic cellular interplay: such mechanism is aimed at increasing the likelihood of engagement of the thymocytes that carry the corresponding T-cell receptors. Another fundamental role of AIRE in promoting self-tolerance is related to the development of thymocyte anergy, as thymic self-representation shapes at the same time the repertoire of regulatory T cells. Finally, AIRE seems to replicate its action in the secondary lymphoid organs, albeit the cell lineage detaining such property has not been fully characterized. Delineation of AIRE functions adds interesting data to the knowledge of the mechanisms of self-tolerance and introduces exciting perspectives of therapeutic interventions against the related diseases.

Keywords: animal disease models; autoimmune polyendocrinopathies; immune tolerance; thymus gland; transcription factors; type-1 diabetes mellitus.

Figure 1

Schematic representation of thymic epithelial cell (TEC) differentiation. Thymic epithelial progenitor cell (TEPC) is tagged by mouse thymic stroma antibodies 20/24 (Mts 20/24), synthesizes intracellular keratins (Ks) 5 and 8 (K5 and K8, respectively), and exhibits surface markers associated with mature cortical TEC (cTEC), such as the cluster of differentiation CD205 and the thymoproteasome subunit β5t. Commitment to medullary TEC (mTEC) sublineage is restricted to claudine (Cld)-exposing elements, which, through intermediate stages of mTEC pro-precursor and precursor (pro-pmTEC and pmTEC, respectively), generate the immature mTEC (mTEC^lo). mTEC^lo differentiation into mature mTEC (mTEC^hi) is accompanied by enhancement of Ulex europaeus agglutinin 1 (UEA1) labeling and further upgrading of class-II major histocompatibility complex (MHCII) antigens and CD80. Lymphostromal interaction (thymic “crosstalk”) drives the emergence of pro-pmTECs by induction of molecules of the tumor necrosis factor-receptor super-family (TnfR-Sf), such as the lymphotoxin-β receptor (LtβR) and the receptor activator of nuclear factor Nf-κB (Rank). The transition from pro-pmTECs to pmTECs is characterized by loss of the stage-specific embryonic antigen 1 (Ssea) and results in a Rank^hi condition. Loss of Aire expression and acquisition of keratinocyte markers typify a subset of post-Aire mTECs that emerge in the postnatal thymus.

Figure 2

Schematic representation of human autoimmune regulator (AIRE). At the N-terminus, the caspase-activation and recruitment domain (CARD) and nuclear localization signal (NLS) are flanked by the SAND domain. Moving to the C-terminus, two plant-homeodomains (PHD1 and PHD2, respectively) fingers are separated by a proline-rich region (PRR). Four LxxLL (L stays for leucine) motifs are enclosed in the amino-acid chain. Preeminent domain-related properties are reported.

Figure 3

Schematic representation of autoimmune regulator (AIRE)-containing multimolecular complexes involved in initiation and post-initiation events of gene transcription. Abbreviations: Ac, acetylation; CBP, CREB-binding protein; DNA-PK, DNA-activated protein kinase; DNA-TOP, DNA-topoisomerase; PARP1, poly-(ADP-ribose) polymerase 1; BRD4, bromodomain-containing domain 4; P-TEFb, positive transcription elongation factor b; RNA-PolII, RNA-polymerase II; ts-ag, tissue-specific antigen. Reprinted (with changes) with the permission from Macmillan Publishers Ltd.: Peterson (171). Copyright 2015.

Figure 4

In the experiment by Anderson et al. (202), lethally irradiated (thus retaining only the radio-resistant stromal cells) wild-type and Aire^−/− mice were transplanted with bone marrow from mirror donors. Only Aire^−/− recipients, independently from the donor condition, exhibited organ damage, underlining that the property of preventing the autoimmune process is inherent to wild-type (Aire-sufficient) thymic stroma.

Figure 5

Two research group found that the organ damage of nude mice co-engrafted with 2′-deoxyguanosine-resistant thymic stroma from wild-type and Aire^−/− donors matched that of the animals engrafted with a single Aire^−/− stroma (221, 233). This led the authors to speculate that impairment of regulatory T cells (Treg cells) has no role in the autoimmune process caused by Aire deficiency. If it had, Treg cells generated in the wild-type stroma would prevent the onset of the disease. However, mice co-engrafted with an excess (4:1) of wild-type stroma had no organ damage, raising reasonable doubts on the earlier conclusions. The experiment was replicated with splenocytes co-transferred into recombinase-activating gene-1-deficient (Rag1^−/−) mice, and yielded similar results.

Figure 6

Schematic representation of thymocyte maturation and the processes of positive and negative selection. Thymocytes originate from bone marrow-derived pluripotent precursors, which in the most immature form lack the clusters of differentiation CD4 and CD8 and are referred to as double-negative (DN) cells. There are four stages (DN1–DN4) of DN condition, during which thymocytes move from the cortico-medullary junction to the subcapsular zone of the gland. The irreversible commitment to T-cell lineage intervenes in the passage from DN1 to DN2 condition, when expression of the recombinase-activating genes starts: only cells that succeed in rearrangement of the gene encoding the T-cell receptor (TCR) β-chain are selected for further maturation. After a brief DN4 stage, survived thymocytes become double-positive (DP) by acquisition of CD4 and CD8 and are allowed to rearrange the gene encoding the TCR α-chain. Now they deal with the processes of positive and negative selection and their fate is dictated by the interaction with the thymic stroma. Once positively selected to generate single-positive (SP) cells, the thymocytes reverse their direction and enter the medulla. While a large percentage of thymocytes do not run into an antigen fitting their TCR and die by neglect, the fate of the remainders depends on the degree of affinity with the cognate antigen. Generally, a strong affinity induces apoptosis and ensuing clonal deletion. An intermediate affinity may induce diversion to an anergic state, such as that of the regulatory T cells. In the theory of avidity, the amount of antigen determines the outcome of the interaction. There are three or four stages of maturation of the SP thymocytes, which finally reach the perivascular space as pre-recent thymic emigrants (pre-RTEs). Abbreviations: NK, natural killer; cTECs, mTECs, thymic epithelial cells (cortical, medullary); DC, dendritic cell.