Synchronized development of thymic eosinophils and thymocytes.

The thymus is an organ required for T cell development and is also an eosinophil-rich organ; however, the nature and function of thymic eosinophils remain unclear. Here, we characterized the gene expression and differentiation mechanism of thymic eosinophils in mice. Thymic eosinophils showed a distinct gene expression profile compared with other organ-resident eosinophils. The number of thymic eosinophils was controlled by medullary thymic epithelial cells. In Rag-deficient mice, the unique gene expression signature of thymic eosinophils was lost but restored by pre-T cell receptor signaling, which induces CD4+ CD8+ thymocyte differentiation, indicating that T cell differentiation beyond the CD4- CD8- stage is necessary and sufficient for the induction of thymic eosinophils. These results demonstrate that thymic eosinophils are quantitatively and qualitatively regulated by medullary thymic epithelial cells and developing thymocytes, respectively, suggesting that thymic eosinophils are a distinct, thymus-specific cell subset, induced by interactions with thymic cells.