Characteristics of an in vitro system of thymocyte positive selection

We investigated the phenotypic changes accompanying maturation of CD4⁺CD8⁺ thymocytes in the presence of thymic stromal cells in vitro. Many of the features that are characteristic of positive selection in vivo, such as an ordered down-regulation of CD4 or CD8 together with up-regulation of the TCR-αβ complex, also occurred in vitro. Transient expression of CD69 was also observed, together with activation of IL-2 and IL-4 cytokine genes, providing further evidence that positive selection in vitro involves a pattern of gene regulation comparable to that seen in vivo.