Lineage Tracing and Cell Ablation Identify a Post-Aire-Expressing Thymic Epithelial Cell Population

Abstract

Thymic epithelial cells in the medulla (mTECs) play a critical role in enforcing central tolerance through expression and presentation of tissue-specific antigens (TSAs) and deletion of autoreactive thymocytes. TSA expression requires autoimmune regulator (Aire), a transcriptional activator present in a subset of mTECs characterized by high CD80 and MHC II expression and a lack of potential for differentiation or proliferation. Here, using an Aire-DTR transgenic line, we show that short-term ablation specifically targets Aire+ mTECs, which quickly undergo RANK-dependent recovery. Repeated ablation also affects Aire− mTECs, and using an inducible Aire-Cre fate-mapping system, we find that this results from the loss of a subset of mTECs that showed prior expression of Aire, maintains intermediate TSA expression, and preferentially migrates towards the center of the medulla. These results clearly identify a distinct stage of mTEC development and underscore the diversity of mTECs that play a key role in maintaining tolerance.

Differential Features of AIRE-Induced and AIRE-Independent Promiscuous Gene Expression in Thymic Epithelial Cells

“…AIRE is not expressed in the mTEC lo subset, which contains most immature "pre-AIRE" cells and a minority of "post-AIRE" cells (34). Accordingly, we found only minimal differences between the transcriptome of WT and KO mTEC lo (Fig.…”

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

“…5C). PostAire mTECs that reside in the mTEC high-intermediate population (19,22) had been eliminated from the thymus during long-term ablation of Aire + mTECs, and the remaining small population of CD80 high GFP 2 mTECs was considered to be mature mTECs just prior to the onset of Aire expression (pre-Aire mTECs). A similarly dramatic decrease in mature mTECs after long-term ablation of Aire + mTECs was noticed in another Aire/DTR-KI mouse generated through bacterial artificial chromosome transgenesis, although the reason for this phenomenon has not been pursued in depth (22).…”

“…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.…”

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

“…Cette sous-population cellulaire était considérée jusqu'à présent comme correspondant au stade final de maturation des mTEC. Cependant, des études de traçabilité du destin cellulaire (fate mapping) 1 , qui ont permis de suivre au cours du temps les cellules ayant exprimé Aire antérieurement, ont montré l'existence d'une population de mTEC dite « post-Aire » (Figure 1) [13,14]. 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.…”

“…À l'extrémité aminoterminale, Aire possède une région HSR (homogeneously staining region) qui contient un domaine CARD (caspase recruitment domain) permettant son oligomérisation. expression réduite de TRA, pas d'une abolition totale [14]. Il est donc plausible que les mTEC post-Aire continuent à contribuer à l'induction de la tolérance centrale bien que ce rôle reste à établir.…”