Inefficient Positive Selection of CD4+ T Cells in Epithelial Cell Tumors of the Human Thymus
Human thymomas are epithelial tumors that are associated with a large number of non-neoplastic T cells of mainly immature phenotype, suggesting that epithelial cells of thymomas retain the function of thymic cortical epithelium. But it has not been reported that lymphocytes within thymoma are positively selected. We studied whether the single-positive cells in thymomas had been positively selected using three-color flowcytometly. A large proportion (16–85%, mean±SD 58±22%, n=17) of CD4+ single-positive T cells within the thymomas lack cell surface expression of CD3 and also of CD69, an early activation antigen that is expressed in positively selected thymocytes. Normal thymus had this CD4+CD8−CD3− population at much lower frequencies (12±7%, n=11). It has been reported that in the human, the CD4+CD8−CD3− cells are the predominant population that is intermediate between the CD4−CD8− double-negative and CD4+CD8+ double-positive stages. In contrast to CD4+ single-positive cells, most of CD8+ single-positive cells in the thymoma as well as in the normal thymus expressed CD69 and a high level of CD3. These observations could be explained partly if the epithelial cells of thymoma were unable to positively select all the immature thymocytes generated in the thymoma. This relative inefficiently of positive selection could not be attributed solely to the paucity of MHC class II expression in the thymoma.
KeywordsEpithelial Cell Tumor Normal Thymus Immature Thymocyte Human Thymoma Early Activate Antigen
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