Post-Aire Maturation of Thymic Medullary Epithelial Cells Involves Selective Expression of Keratinocyte-Specific Autoantigens

Abstract

The autoimmune regulator (Aire)-directed ectopic expression of tissue-specific antigens (TSAs) by mature medullary thymic epithelial cells (mTECs) has been viewed as an essential mechanism in the induction of central tolerance. Recent data suggest that the survival of mTECs extends beyond the Aire+ cell population to form the post-Aire mTEC population and Hassall’s corpuscles (HCs). The nature and function of these post-Aire epithelial cells and structures, however, have remained unidentified. In this study, we characterized in detail the end-stage development of mTECs and HCs in both Aire-sufficient and Aire-deficient mice. In addition, using a transgenic mouse model in which the LacZ reporter gene is under the control of the endogenous Aire promoter, we purified and analyzed the post-Aire mTECs to characterize their function. We showed that the end-stage maturation of mTECs closely resembles that of keratinocytes and that the lack of Aire results in a marked block of mTEC differentiation, which is partially overcome by ligands for RANK and CD40. We also provide evidence that, during mTEC development, Aire is expressed only once and during a limited 1–2 day period. The following loss of Aire expression is accompanied by a quick downregulation of MHC class II and CD80, and of most of the Aire-dependent and Aire-independent TSAs, with the exception of keratinocyte-specific genes. In the final stage of maturation, the mTECs lose their nuclei to become HCs and specifically express desmogleins (DGs) 1 and 3, which, via cross-presentation by APCs, may contribute to tolerance against these pemphigus vulgaris-related TSAs.

Aire Expression Is Inherent to Most Medullary Thymic Epithelial Cells during Their Differentiation Program

“…Because the spectrum of TRAs expressed by mTECs depends on their developmental status (22,24,25), any developmental shift toward a particular stage(s) due to a lack of Aire would result in a defect in the promiscuous gene expression that normally relies on the heterogeneity of mTECs in a developmental state. Similarly, if Aire has a role in adjusting the functional state to one that is optimal for TRA gene expression (26,27), it is conceivable that loss of Aire in all mTECs within a certain developmental window for Aire expression would also result in a defect in promiscuous gene expression.…”

A highly conserved NF‐κB‐responsive enhancer is critical for thymic expression of Aire in mice

“…Aire is rapidly upregulated for a few days [14,15] [17,24,[26][27][28]. RANK ligation activates both the classical and alternative pathways of NF-κB signaling [29].…”

Induction de la tolérance centrale dans le thymus par le facteur de transcription Aire

“…Ces cellules se caractérisent par une expression modérée des molécules de CMH II et CD80 ainsi que par la perte totale d'expression de Aire. Elles présentent aussi la particularité d'exprimer plusieurs antigènes spé-cifiques des kératinocytes [15]. La perte de Aire s'accompagne d'une…”

“…Enfin, les mTEC post-Aire perdent leurs noyaux et fusionnent pour former des corpuscules de Hassall. Ces structures sont identifiées à l'aide des marqueurs spécifiques de cellules différenciées de l'épiderme tels que les cytokératines 6 et 10, l'involucrine, le marqueur LEKTI (lympho-epithelial Kazal type related inhibitor), ainsi que les desmogléines 1 et 3 ( Figure 1) [15,16]. Chez l'homme, les corpuscules de Hassall ont le potentiel d'induire le développement de cellules nTreg [17].…”