The expression, regulation and function of proapoptotic molecules produced by thymic epithelial cells

Abstract

Immature T cells, known as thymocytes develop in the thymus. During this process thymocytes rearrange and express a T cell receptor (TCR). TCR expression is a partly random process. Although this contributes to generating diversity, it also generates TCRs that are non-functional or that could illicit autoimmune disease. Therefore a stringent thymocyte developmental process ensures that only those thymocytes bearing a potentially useful TCR mature. During this developmental process 95-99% of thymocytes die. How these thymocytes are induced to die is not understood but is fundamentally important in elucidating thymocyte development. The thymic micro environment facilitates thymocyte death. This microenvironment is created by the different cells that are present. The thymus is cellularly heterogenous with thymic epithelial cells (TECs) being the predominant population. To further characterize how TECs contribute to the thymic micro environment this laboratory previously established the rat and mouse functional TEC lines TEA3A1 and BT1B, respectively. Using these cell lines this thesis examines the expression, regulation and role of factors that have been hypothesized as having a role in thymocyte death. Initially the regulation of prostaglandin E2(PGE2) was examined. It was found that PGE2 production is stimulated by ATP and synergistically stimulated by TGF-alpha and ATP. As the nervous system is a potential source of ATP this data provides a mechanism for neuronal control of thymocyte development. Secondly the expression of proapoptotic Fas ligand was characterized. Moreover evidence is provided that the role of Fas ligand in negative selection is related to its subcellular localization. Thirdly we provide evidence that one of the functions of glucocorticoids in the thymus is to maintain the viability of TECs. Furthermore preliminary evidence is provided that these cells can produce corticosterone similar to TECs in vivo and therefore may provide a useful model for characterizing the regulation of glucocorticoid synthesis by TECs. Therefore this thesis provides a synthesis of how three factors hypothesized as being involved in thymocyte development may contribute to the thymic microenvironment. Firstly by examining the expression of PGE2. Secondly by characterizing the expression of Fas ligand and thirdly by showing that glucocorticoids are important in maintaining epithelial cell viability.