Induction of apoptosis and p53 expression in immature thymocytes by direct interaction with thymic epithelial cells

Apoptosis of normal thymocytes was shown to be triggered by several mechanisms (e.g. glucocorticoids, γ-irradiation). In the present study the authors report on thymocyte apoptosis that is induced by thymic epithelial cells. The thymocytes undergo a massive apoptotic death within 24 h of cocultivation with thymic epithelial cell monolayers derived from primary cultures (PTEC) or from a thymic epithelial cell line (TEC). Non-thymic monolayers were inactive. Apoptosis induction in this experimental model requires direct contact between the thymocytes and the thymic epithelial monolayer and can be blocked by anti-CD2 and anti-LFA-1 antibodies. The immature CD3(-/+dull) CD4⁺CD8⁺ thymocytes were the cells which undergo apoptosis. The fact that the authors are dealing with a massive apoptotic process of immature cells in the absence of exogenous antigen suggests that it involves the nonselected thymocytes. The apoptotic pathway selected by thymocytes following their culturing on TEC involves p53 expression. Indeed it was found that TEC-induced apoptosis, led to the accumulation of p53 protein that preceded the step of DNA fragmentation in freshly isolated thymocytes as well as in a glucocorticoid resistant thymoma cell line. Since glucocorticoid-induced thymocyte apoptosis is p53-independent, glucocorticoids are conceivably not involved in TEC-induced thymocyte death. The in vitro experimental model presented here may reflect the physiological sequence of events leading to thymocyte death in the thymus.