Accumulation of Immature CD3-CD4+CD8- Single-Positive Cells That Lack CD69 in Epithelial Cell Tumors of the Human Thymus

Human thymomas are epithelial tumors that are associated with a large number of nonneoplastic T cells of mainly immature phenotype, suggesting that epithelial cells of thymomas retain the function of thymic cortical epithelium. We report here that a large proportion (16-85%, mean ± SD 58 ± 22%, n = 17) of CD4+ single-positive T cells within the thymoma lack cell surface expression of CD3 and also of CD69, an early activation antigen that is expressed by positively selected thymocytes. Normal thymus had this CD4+CD8-CD3- population at much lower levels (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 the CD8+ single-positive cells in the thymoma as well as in the normal thymus expressed CD69 and a high level of CD3. A partial explanation of these observations is that the epithelial cells of thymoma were unable to positively select all the immature thymocytes generated in the thymoma. This relative inefficiency of positive selection could not be attributed solely to the paucity of MHC class II expression in the thymoma.