Single-Cell Resolution and Quantitation of Targeted Glucocorticoid Delivery in the Thymus

Abstract

Glucocorticoids are lipid-soluble hormones that signal via the glucocorticoid receptor (GR), a ligand-dependent transcription factor. Circulating glucocorticoids derive from the adrenals, but it is now apparent that paracrine glucocorticoid signaling occurs in multiple tissues. Effective local glucocorticoid concentrations and whether glucocorticoid delivery can be targeted to specific cell subsets are unknown. We use fluorescence detection of chromatin-associated GRs as biosensors of ligand binding and observe signals corresponding to steroid concentrations over physiological ranges in vitro and in vivo. In the thymus, where thymic epithelial cell (TEC)-synthesized glucocorticoids antagonize negative selection, we find that CD4 + CD8 + TCR hi cells, a small subset responding to self-antigens and undergoing selection, are specific targets of TEC-derived glucocorticoids and are exposed to 3-fold higher levels than other cells. These results demonstrate and quantitate targeted delivery of paracrine glucocorticoids. This approach may be used to assess in situ nuclear receptor signaling in a variety of physiological and pathological contexts. Glucocorticoids signal via the GR, a ligand-dependent transcription factor, and paracrine glucocorticoid signaling occurs in the thymus. Taves et al. use chromatin-associated GRs as biosensors to estimate glucocorticoid concentrations in vitro and in vivo. In the thymus, antigen-signaled CD4 + 8 + TCR hi cells are targeted by epithelial cell-synthesized glucocorticoids to promote positive selection.