Thymic regulatory T cell niche size is dictated by limiting interleukin 2 from antigen-bearing dendritic cells and feedback competition

Thymic regulatory T (T(reg)) cell production requires interleukin 2 (IL-2) and agonist TCR ligands, and is controlled by competition for a limited developmental niche, but the thymic sources of IL-2 and the factors that limit access to the niche are poorly understood. Here we show that IL-2 produced by antigen-bearing dendritic cells plays a key role in T(reg) cell development, and that existing T(reg) cells limit new T(reg) cell development by competing for IL-2. . Our data suggest that antigen-presenting cells that can provide both IL-2 and a TCR ligand comprise the thymic niche, and that competition by existing T(reg) cells for a limited supply of IL-2 provides negative feedback for new T(reg) cell production.