Relb acts downstream of medullary thymic epithelial stem cells and is essential for the emergence of RANK⁺ medullary epithelial progenitors

Abstract

Thymic epithelial cells (TECs) provide essential signals for αβT‐cell development, and medullary TECs (mTECs) control T‐cell tolerance through both negative selection and Foxp3+ regulatory T (Treg) cell development. Although heterogeneity within the mTEC compartment is well studied, the molecular regulators of specific stages of mTEC development are still poorly understood. Given the importance of the RANK‐RANKL axis in thymus medulla formation, we have used RANK Venus reporter mice to analyze the ontogeny of RANK+ TECs during development and correlated RANK expression with mTEC stem cells defined by SSEA‐1. In addition, we have investigated how requirements for the key regulators Foxn1 and Relb map to specific stages of mTEC development. Here, we show SSEA‐1+ mTEC stem cells emerge prior to RANK expression and are present in both nude and Relb −/− mice, providing direct evidence that mTEC lineage specification occurs independently of Foxn1 and Relb. In contrast, we show that Relb is necessary for the effective production of downstream RANK+ mTEC progenitors. Collectively, our work defines stage‐specific requirements for critical TEC regulators during medulla development, including the timing of Relb dependency, and provides new information on mechanisms controlling mTEC specification.

The non-canonical NF-κB pathway in immunity and inflammation

“…Mice deficient in non- canonical NF-κB signalling components, such as NIK, IKKα or RELB, or the canonical NF-κB signalling component TRAF6, have severe defects in mTEC development 87–91 . Recent studies suggest that the non-canonical NF-κB pathway is dispensable for initial mTEC lineage specification but is required for the further differentiation of mTEC progenitors into mature mTECs 92,93 . In addition to mediating mTEC development, non-canonical NF-κB signalling regulates the expression of autoimmune regulator (AIRE), an mTEC-specific transcription factor that is required for the expression of tissue-specific antigens and the induction of central tolerance 85 .…”

Thymus medulla under construction: Time and space oddities

“…In this issue, reports by Baik et al. and Mayer et al. reveal novel spatial–temporal peculiarities in the generation of mTEC lineages that sprout in the fetal and postnatal life, respectively.…”

“…established reaggregate thymic organ cultures (RTOCs) in which MHCII‐mismatched E15 Cld3,4 hi RANK + TECs were mixed with E15 WT thymus and their progeny was traced on the basis of distinct MHCII. The authors show that in chimeric RTOCs, E15 Cld3,4 hi RANK + cells preferentially generate MHCII hi Ly51 − CD80 −/+ TECs, indicating that RANK + TECs contain mTEC unipotent progenitors . Although future experiments are required to map the expression of additional cTEC and mTEC markers in RANK + TECs, these results uncover a novel degree of heterogeneity in early steps of the mTEC differentiation program.…”

“…provides genetic evidence for the role of chief regulators of TEC specification, such as Foxn1 and Relb , in the differentiation of RANK + mTEPs. The authors show that Foxn1 and Relb are differentially required for the development of Cld3,4 hi SSEA1 + and Cld3,4 hi RANK + TEC subtypes . While unaltered in Relb‐deficient mice, the abundance of Cld3,4 hi SSEA1 + cells was reduced in Nude mice.…”

“…While Cld3,4 hi SSEA1 + TECs exist in the E13 thymus, RANK + TECs emerge one day later within the Cld3,4 hi SSEA1 − subset and become prominently detected in the E15 thymus. Interestingly, RANK + TECs express higher levels of MHC class II (MHCII) and lower levels of CD205 than Cld3,4 hi SSEA1 + mTEPs . To address the lineage potential of RANK + TECs, Baik et al.…”