Protection against autoimmunity is driven by thymic epithelial cell-mediated regulation of T reg development

Medullary thymic epithelial cells (mTECs) are key antigen-presenting cells mediating T cell tolerance to prevent harmful autoimmunity. mTECs both negatively select self-reactive T cells and promote the development of thymic regulatory T cells (tTregs) that mediate peripheral tolerance. The relative importance of these two mechanisms of thymic education to prevent autoimmunity is unclear. We generated a mouse model to specifically target the development and function of mTECs by conditional ablation of the NF-B–inducing kinase (NIK) in the TEC com-partment. In contrast to germline-deficient NIK−/− mice, Foxn1CreNIKfl/fl mice rapidly developed fatal T cell–dependent multiorgan autoimmunity shortly after birth. Thymic transplantation and adoptive transfer experiments demon-strated that autoimmunity arises specifically from the emergence of dysfunctional tTregs. Thus, Treg function, rather than negative selection, enforces the protection of peripheral tissues from autoimmune attack.