Thymic epithelial cells provide unique signals for positive selection of cd4⁺ cd8⁺ thymocytes in vitro

Using a novel system that supports positive selection in vitro, we have investigated the cellular requirements for this process by testing the ability of individual thymic and nonthymic stromal cell types to support the maturation of CD4⁺ CD8⁺ thymocytes into CD4⁺ or CD8⁺ T cells. We show that thymic cortical epithelial ceils are unique in their ability to mediate this maturation, and suggest that in addition to TCR ligation, these cells supply specific signals for positive selection. Moreover, by demonstrating positive sdection on ECDI (1-ethyl-3-[Ydimethyl-aminopropyl]- carbodiimide)-fixed epithelial ceils in this system, we provide direct evidence that the provision of these signals involves interactions with epithelial ceil surface molecules rather than the release of soluble factors.