DISTINCT REQUIREMENTS OF POSITIVE AND NEGATIVE SELECTION FOR SELECTING CELL-TYPE AND CD8 INTERACTION

We investigated the requirements of positive and negative selection in the thymus for CD8 interaction and the selecting cell type. Thymic ep ithelial cells are known to mediate positive selection, whereas thymoc ytes fail to do so. The reason for this failure could be either the lo w amount of MHC class I molecules on thymocytes or the lack of other p roperties required for positive selection. To address this question a CD2.K(b) transgenic mouse was prepared in which the expression of the K(b) gene is under control of the CD2 promoter. In these mice the thym ocytes exhibited very high levels of K(b). The mice were crossed with two TCR transgenic mice expressing either the Des.TCR (anti-K(b)), whi ch is positively selected on K(k) and negatively on K(b), and the 2C.T CR (anti-L(d)) with positive selection on K(b) and negative on L(d). D espite the high K(b) expression on thymocytes in CD2.K(b) X 2C.TCR mic e no positive selection was observed whereas efficient negative select ion was found in CD2.K(b) X Des.TCR F1 mice. Thus, although thymocytes can negatively select, even a strong increase of MHC class I expressi on cannot convert them into positively selecting cells. This is consis tent with the notion that thymic epithelium is specialized for positiv e selection, but the respective difference between thymocytes and thym ic epithelium is not clear. The influence of CD8 was investigated usin g transgenic mice expressing a K(k)/A2 or a K(b)/A2 hybrid gene in whi ch the promoter, the alpha1, alpha2 domains were of mouse origin and t he alpha3 domain of human origin. Because the murine CD8 molecule does not efficiently bind to human HLA class I, in these mice the CD8 inte raction was impaired. In K(b)/A2 X 2C.TCR and K(k)/A2 X Des.TCR mice n o positive selection of the respective TCR was found, whereas in K(b)/ A2 X Des.TCR mice negative selection was still functional. Altogether, the results indicate that positive selection depends more strictly on CD8 interaction and cell type than negative selection.