Transgenic Expression of a Novel Thymic Epithelial Cell Antigen Stimulates Abberant Development of Thymocytes

We previously reported a novel thymic stromal cell Ag, HS9, as a potent molecule participating in intrathymic T cell development. HS9 Ag is expressed on thymic stromal cells especially in the cortex but not on thymocytes. In the present study, we isolated and characterized a novel cDNA, N14, encoding HS9 Ag. Sequencing analysis of N14 cDNA has revealed it to be a novel one without any significant homology to previously reported functional molecules. COS7 cells transfected with expression vectors harboring N14 cDNA became reactive with HS9-specific mAb. Northern blot analysis and in situ hybridization revealed that several tissues that are positive for HS9 mAb expressed N14 mRNA. To examine the role of this molecule in T cell development, transgenic mice were generated. In situ hybridization and immunohistochemical study showed that the transgene was significantly overexpressed on both cortical and medullar thymic stromal cells but not on thymocytes. Flow cytometric analyses showed that the percentages of mature CD4^-CD8⁺ or CD4⁺CD8^- thymocytes in transgenic mice were approximately twice and triple, respectively, those in control littermates. Moreover, substantial CD4⁺CD8⁺ thymocytes appeared to have high levels of TCR compared with peripheral T cells. Histologic examination revealed that transgenic mice had thin cortex and relatively developed medulla. These data indicate the critical role of the N14 gene in T cell development.