Dissertation Defense: Kaitlynn Donahue
Candidate: Kaitlynn Donahue
Major: Biochemistry and Molecular Biology
Advisor: Jonathan Ashwell, M.D.
Regulation of Steroid Hormone Biosynthesis by Thymic Epithelial Cells
Selective processes early in T cell development ensure that most thymocytes expressing αβ T cell antigen receptors (TCRs) are eliminated, and only self-restricted and self-tolerant cells join the peripheral T cell repertoire. Glucocorticoids produced by thymic epithelial cells (TECs) promote the survival of such thymocytes to increase the breadth and efficacy of the repertoire. There are two TEC subsets, cortical (cTECs) and medullary (mTECs), each contributing to selection at different developmental stages. The exact source and regulation of thymic-derived glucocorticoid production are not fully understood, and their precise identification will help determine whether they enhance positive selection or antagonize negative selection. It was hypothesized that glucocorticoids are synthesized by cTECs, with the potential to influence both stages of selection. To test this, a transgenic reporter mouse was utilized in which endogenous Cyp11b1, the final and essential enzyme in de novo glucocorticoid biosynthesis, was fused with fluorescent mScarlet. Cyp11b1mScarlet was detected in a lineage of mTECs that express the transcription factor Aire, which is known to drive promiscuous expression of tissue-restricted antigens (TRAs) involved in negative selection and the establishment of peripheral immune tolerance. In Aire-knockout mice, detection of Cyp11b1mScarlet, transcripts encoding enzymes required for the de novo pathway, and ex-vivo glucocorticoid synthesis were significantly reduced, supporting Aire’s role in regulating glucocorticoid biosynthesis. This presents a novel yet paradoxical role for Aire in promoting both positive and negative thymocyte selection, the combined effect serving to enlarge the pool of self-restricted yet self-tolerant T cells. This also supports a new function for Aire in coordinating the expression of genes involved in an entire biosynthetic pathway whose secondary products have known paracrine functions. To this end, Aire was found to drive de novo sex steroid biosynthesis, supporting a broader role for Aire in shaping the thymic microenvironment and paving the way for future investigations.