Schönrich G, Strauss G, Müller K P, Dustin L, Loh D Y, Auphan N, Schmitt-Verhulst A M, Arnold B, Hämmerling G J
Department of Somatic Genetics, Deutsches Krebsforschungszentrum DKFZ, Heidelberg, Germany.
J Immunol. 1993 Oct 15;151(8):4098-105.
We investigated the requirements of positive and negative selection in the thymus for CD8 interaction and the selecting cell type. Thymic epithelial cells are known to mediate positive selection, whereas thymocytes fail to do so. The reason for this failure could be either the low amount of MHC class I molecules on thymocytes or the lack of other properties required for positive selection. To address this question a CD2.Kb transgenic mouse was prepared in which the expression of the Kb gene is under control of the CD2 promoter. In these mice the thymocytes exhibited very high levels of Kb. The mice were crossed with two TCR transgenic mice expressing either the Des.TCR (anti-Kb), which is positively selected on Kk and negatively on Kb, and the 2C.TCR (anti-Ld) with positive selection on Kb and negative on Ld. Despite the high Kb expression on thymocytes in CD2.Kb x 2C.TCR mice no positive selection was observed, whereas efficient negative selection was found in CD2.Kb x Des.TCR F1 mice. Thus, although thymocytes can negatively select, even a strong increase of MHC class I expression cannot convert them into positively selecting cells. This is consistent with the notion that thymic epithelium is specialized for positive selection, but the respective difference between thymocytes and thymic epithelium is not clear. The influence of CD8 was investigated using transgenic mice expressing a Kk/A2 or a Kb/A2 hybrid gene in which the promoter, the alpha 1, alpha 2 domains were of mouse origin and the alpha 3 domain of human origin. Because the murine CD8 molecule does not efficiently bind to human HLA class I, in these mice the CD8 interaction was impaired. In Kb/A2 x 2C.TCR and Kk/A2 x Des.TCR mice no positive selection of the respective TCR was found, whereas in Kb/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.
我们研究了胸腺中阳性和阴性选择对CD8相互作用及选择细胞类型的要求。已知胸腺上皮细胞介导阳性选择，而胸腺细胞则不能。胸腺细胞不能介导阳性选择的原因可能是其表面MHC I类分子数量少，或者缺乏阳性选择所需的其他特性。为了解决这个问题，制备了一种CD2.Kb转基因小鼠，其中Kb基因的表达受CD2启动子控制。在这些小鼠中，胸腺细胞表现出非常高的Kb水平。将这些小鼠与两种TCR转基因小鼠杂交，一种表达Des.TCR（抗Kb），它在Kk上进行阳性选择而在Kb上进行阴性选择；另一种表达2C.TCR（抗Ld），它在Kb上进行阳性选择而在Ld上进行阴性选择。尽管在CD2.Kb×2C.TCR小鼠的胸腺细胞上Kb表达很高，但未观察到阳性选择，而在CD2.Kb×Des.TCR F1小鼠中发现了有效的阴性选择。因此，尽管胸腺细胞可以进行阴性选择，但即使MHC I类分子表达大幅增加也不能将它们转化为进行阳性选择的细胞。这与胸腺上皮细胞专门负责阳性选择的观点一致，但胸腺细胞和胸腺上皮细胞之间的具体差异尚不清楚。利用表达Kk/A2或Kb/A2杂交基因的转基因小鼠研究了CD8的影响，其中启动子、α1和α2结构域来自小鼠，α3结构域来自人类。由于鼠源CD8分子不能有效地与人HLA I类分子结合，在这些小鼠中CD8相互作用受损。在Kb/A2×2C.TCR和Kk/A2×Des.TCR小鼠中未发现相应TCR的阳性选择，而在Kb/A2×Des.TCR小鼠中阴性选择仍然有效。总之，结果表明阳性选择比阴性选择更严格地依赖于CD8相互作用和细胞类型。
