Studies on T cell maturation on defined thymic stromal cell populations in vitro

We describe an in vitro system in which positive sdection of developing T cells takes place on defined stromal cell preparations, which include major histocompatibility complex class II⁺ epithelial cells but exclude cells of bone marrow origin. In this system, maturation of doublepositive T cell receptor negative (TCR⁻), CD4⁺8⁺ thymocytes into single-positive TCR⁺, CD4⁺ and CD8⁺ ceils takes place together with the development of functional competence. As in vivo, this maturation is associated with the upregulation of TCR levels as cells progress from double-positive to single-positive status. We also show that dass II⁺ epithelial cells in these cultures are less efficient than dendritic cells in mediating the ddetion (negative selection) of Vβ8⁺ cells by the superantigen staphylococcal enterotoxin B. For the first time, this approach provides a model in which the cellular interactions involved in both positive and negative selection can be studied under controlled in vitro conditions.