Central immune tolerance depends on crosstalk between the classical and alternative NF-κB pathways in medullary thymic epithelial cells

•Classical NF-κB signaling via RelA and c-Rel is essential for mTEC development.•RelA and c-Rel regulate mTEC development by direct transcriptional control of Relb.•Inactivation of RelB in thymic epithelium leads to loss of mTECs and autoimmunity.•Relb transgene expression rescues defective development of Traf6-deficient mTECs.•NF-κB activity and RelB expression is present predominantly in mature mTECs.

Medullary thymic epithelial cells (mTECs) contribute to self-tolerance by expressing and presenting peripheral tissue antigens for negative selection of autoreactive T cells and differentiation of natural regulatory T cells. The molecular control of mTEC development remains incompletely understood. We here demonstrate by TEC-specific gene manipulation in mice that the NF-κB transcription factor subunit RelB, which is activated by the alternative NF-κB pathway, regulates development of mature mTECs in a dose-dependent manner. Mice with conditional deletion of Relb lacked mature mTECs and developed spontaneous autoimmunity. In addition, the NF-κB subunits RelA and c-Rel, which are both activated by classical NF-κB signaling, were jointly required for mTEC differentiation by directly regulating the transcription of Relb. Our data reveal a crosstalk mechanism between classical and alternative NF-κB pathways that tightly controls the development of mature mTECs to ensure self-tolerance.