Thymic epithelial cells (TECs) produce glucocorticoids, which antagonize negative selection
of autoreactive thymocytes and promote a competent T cell antigen–specific repertoire. To
characterize their source, we generated a knock-in reporter mouse in which endogenous
Cyp11b1, the final enzyme in de novo production of active glucocorticoids, was fluorescently
tagged with mScarlet. Here, we find that Cyp11b1 is expressed in medullary TECs (mTECs)
but not cortical TECs or other cells in the thymus. A distinct characteristic of mTECs is the …

Thymocytes progress toward mature T cells through a series of antigen-specific selection
steps facilitated by thymic epithelial cells (TECs) in the cortex, the site of positive selection,
and medulla, the site of negative selection. TECs produce glucocorticoids (GCs), which
enhance positive selection and increase the fitness of the T cell antigen-specific repertoire,
but which TEC subpopultion does this, and how it is regulated, is unknown. We generated a
reporter mouse that expresses a fusion protein containing Cyp11b1, the enzyme that …