1 Developmental kinetics, turnover and stimulatory capacity of thymic epithelial cells.

Despite the importance of thymic stromal cells to T-cell development, relatively little is known about their biology. Here we employ single-cell analysis of stromal cells to analyse extensive changes in the number and composition of thymic stroma throughout life, revealing a surprisingly dynamic population. Phenotypic progression of thymic epithelial subsets was assessed at high resolution in young mice to provide a developmental framework. The cellular and molecular requirements of adult epithelium were studied using various mutant mice to demonstrate new crosstalk checkpoints dependent on RelB in the cortex and CD40 in the medulla. Using Ki67 and BrdU labelling, the turnover of thymic epithelium was found to be rapid, but then diminished upon thymic involution. The various defects in stromal turnover and composition that accompanied involution were rapidly reversed following sex steroid ablation. Unexpectedly, mature cortical and medullary epithelium showed a potent capacity to stimulate naïve T-cells, comparable to that of thymic dendritic cells. Overall, these studies show that the thymic stroma is a surprisingly dynamic population and may have a more direct role in negative selection than previously thought.