Temporal Lineage Tracing of Aire-Expressing Cells Reveals a Requirement for Aire in Their Maturation Program

Abstract

Understanding the cellular dynamics of Aire-expressing lineage(s) among medullary thymic epithelial cells (AEL-mTECs) is essential for gaining insight into the roles of Aire in establishment of self-tolerance. In this study, we monitored the maturation program of AEL-mTECs by temporal lineage tracing, in which bacterial artificial chromosome transgenic mice expressing tamoxifen-inducible Cre recombinase under control of the Aire regulatory element were crossed with reporter strains. We estimated that the half-life of AEL-mTECs subsequent to Aire expression was ∼7–8 d, which was much longer than that reported previously, owing to the existence of a post-Aire stage. We found that loss of Aire did not alter the overall lifespan of AEL-mTECs, inconsistent with the previous notion that Aire expression in medullary thymic epithelial cells (mTECs) might result in their apoptosis for efficient cross-presentation of self-antigens expressed by AEL-mTECs. In contrast, Aire was required for the full maturation program of AEL-mTECs, as exemplified by the lack of physiological downregulation of CD80 during the post-Aire stage in Aire-deficient mice, thus accounting for the abnormally increased CD80high mTECs seen in such mice. Of interest, increased CD80high mTECs in Aire-deficient mice were not mTEC autonomous and were dependent on cross-talk with thymocytes. These results further support the roles of Aire in the differentiation program of AEL-mTECs.

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

“…3). Because Airedeficient mice show an increase in mTEC high (19), we suspect that the increase in mTEC high in heterozygous Aire/hAGF-KI mice was also a reflection of the gene-dosage effect of Aire due to hypomorphic function of hAGF. Taken together, these results suggest that hAGF is a suitable surrogate marker of endogenous Aire, at least for monitoring of its expression.…”

“…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).…”

Autoimmune Polyendocrinopathy Candidiasis Ectodermal Dystrophy

“…The maturation defect in the AIREdeficient thymus can lead to insufficient presentation of TSAs, delayed migration of thymocytes and the aberrant death of mTECs that may produce activation signals to the immune system [60][61][62]. However, the molecular mechanism behind the promotion of mTEC maturation by AIRE needs to be elucidated.…”

AIRE expands: new roles in immune tolerance and beyond

“…One potential explanation for this is that Aire in MEC hi cells induces apoptosis, and this pro-apoptotic function may enhance self-tolerance by facilitating phagocytosis and cross-presentation of TSAs by thymic antigen-presenting cells 62 . In conflict with this interpretation is evidence using an inducible Cre mouse model that suggests that Aire does not affect mTEC lifespan 63 . Instead, Aire may be involved with physiologic downregulation of CD80 in mature mTECs 63 .…”