Paracrine rather than systemic glucocorticoids are biologically active in the thymus

Abstract

Abstract Thymocyte positive and negative selection are critical for generation of a competent and self-tolerant T cell repertoire. Glucocorticoids (GCs) protect thymocytes from T cell receptor (TCR)-induced death, and thymocyte-specific GC receptor (GR) deletion amplifies negative selection, weakening the TCR repertoire. Circulating GCs are secreted by the adrenals, but levels fluctuate widely with time of day and in response to stressors. To avoid such variation, thymus GCs might be regulated independently of the adrenals, as thymic epithelial cells (TECs) and possibly thymocytes express GC-synthetic enzymes. Whether local GC production is sufficient to affect thymocyte development in the presence of adrenal GCs, however, is unknown. Here, we have found that corticosterone, the major mouse GC, was locally elevated in the thymus compared to the blood, and that cultured thymus produced corticosterone from endogenous substrates via GC-synthetic enzyme activity. To test the source and importance of local GC synthesis in vivo, we generated mice with targeted deletion of the GC-synthetic enzyme Cyp11b1 in TECs (Cyp11b1foxn1-Cre) or thymocytes (Cyp11b1lck-Cre). As a measure of GC signaling we quantified thymocyte expression of the GC-responsive gene Gilz. Gilz mRNA was normal in Cyp11b1lck-Cre but reduced in Cyp11b1foxn1-Cre thymocytes, with a reduction equivalent to that in GR-deficient thymocytes. Basal GR signaling is thus driven overwhelmingly by TEC-rather than adrenal-derived corticosterone. These findings demonstrate the importance of paracrine GC function in vivo, and are consistent with a role for paracrine GCs in thymocyte selection.