Thymic epithelial cells: the multi-tasking framework of the T cell "cradle".

The thymus provides the anatomical "cradle" that fosters developing thymocytes. Thymic epithelial cells (TECs) are specialized cellular components that may be viewed as a multifunctional "frame" to nurture distinct stages of thymopoiesis. A symbiotic relationship between TECs and thymocytes exists because reciprocal interactions are required to achieve complete maturation of both cell types. Here, we propose that crucial instructive signals delivered by developing thymocytes negatively regulate functional attributes of immature TECs (including the expression of Delta-like 4 (DLL4) and interleukin-7 (IL-7)) that are required during early stages of thymopoiesis, while promoting the diversification of more mature TEC subsets. Thus, the division of labour among TECs may be coordinated directly by local cellular feedback mechanisms operating within distinct thymic niches.