Glucocorticoid production in the murine thymus

Glucocorticoid hormones are known to act as important modulatory factors in the develop- ment of autoimmune diseases, and to play an important role in thymic T-cell selection. There seems to be a finely balanced equilibrium between the apoptosis-inducing effects of gluco- corticoid and T cell receptor ligand binding. Here we are investigating whether glucocorticoid-induced T cell apoptosis is mainly dependent on circulating glucocorticoid levels or if the thymus itself is able to produce glucocorticoids. To this end, we attempted to demonstrate enzyme activities of the whole set of steroidogenic enzymes for the synthesis of glucocorticoids in murine thymic tissue. We isolated steroidogenic organelles from thymic tissue, incubated these with radioactive (precursor) steroids in vitro, and visualized the resulting products by thin-layer chromatography. Our results show that the thymus pos- sesses all enzymes and cofactors required for glucocorticoid production. However, an intact thymic architecture is necessary for glucocorticoid production, since 11 -hydroxylase was not detected in irradiated thymi or in a thymic epithelial cell line. The results of these experi- ments show that the whole glucocorticoid metabolism takes place within the thymus. This finding provides the biochemical basis for the in situ effects of glucocorticoid hormones on thymocyte development and selection.