Role of thymic stromal cells in thymocyte education: a comparitive analysis of different models.

The thymus is the privileged lymphoid organ for T-cell differentiation, including clonal selection. Precursor thymocytes are selected on the basis of their ability to recognize self-major histocompatibility complex molecules (MHC) that are expressed on thymic stromal cells, including epithelial cells, macrophages, and interdigitating cells/dendritic cells (IDCs/DCs). Epithelial cells have been associated with positive selection, and dendritic cells with negative selection, but the exact role played by these stromal elements is still unresolved. This review focuses on the different in vitro and in vivo approaches that have been used to elucidate the role of thymic stromal cell types in thymic function, and on a critical evaluation of these approaches.