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

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.