Cell-targeted steroid signaling in the thymus determines T cell repertoire strength

Abstract

Abstract Thymocyte selection is critical for generation of a competent and tolerant T cell repertoire, and glucocorticoids (GCs) antagonize T cell receptor (TCR) signaling and negative selection to strengthen the TCR repertoire. While circulating GCs from the adrenals coordinate systemic responses, the thymus contains all the enzymes needed to independently synthesize GCs. However, the biological relevance, source, and targets of locally-synthesized GCs is unclear. 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 thymic epithelial cells (Cyp11b1foxn1-Cre) or thymocytes (Cyp11b1lck-Cre). We found that expression of GC-responsive genes was reduced in Cyp11b1foxn1-Cre but not Cyp11b1lck-Cre thymocytes, confirming that epithelial cells are a major source of GCs in the mouse thymus. Cyp11b1foxn1-Cre T cells had reduced alloreactivity and antiviral responses, demonstrating a weakened TCR repertoire. To identify targets of epithelial GCs we used chromatin-associated GC receptors as a cell-specific biosensor of endogenous GC exposure. With this approach we detected and quantified targeted GC delivery to activated CD4+8+TCRhi thymocytes undergoing antigen-specific selection. Negative selection, as measured by PD-1 and Bim expression, was specifically increased in CD4+8+TCRhi thymocytes of Cyp11b1foxn1-Cre mice. These findings 1) demonstrate the biological importance of local GC synthesis, 2) show cell-specific targeting of paracrine GCs, and 3) quantitate intra-organ heterogeneity in GC levels within the thymus. GCs may similarly function as paracrine immunoregulatory signals across a range of tissues and contexts.