Immaturity of lymphocytes in the metastatic lesions of thymoma

Thymoma is a thymic epithelial tumor which often contains a large number of immature T cells. Although the metastatic lesions are also associated with abundant lymphocytes, their characteristics have not been assessed in detail. In this study, the phenotype was analyzed and compared with those in their primary lesions. Nine metastatic thymomas were obtained from seven patients. In the metastatic lesions, CD1a⁺ cells and CD4⁺CD8⁺ cells accounted for 77.7 ± 10.6 and 52.3 ± 15.8% of all the lymphocytes, respectively. In five primary lesions and their metastatic lesions, CD3^-CD4⁺CD8^- cells accounted for 23.9 ± 16.9 and 45.2 ± 15.5% of the CD4⁺CD8^- cells, respectively. CD69 was expressed on 70.9 ± 9.5 and 53.1 ± 11.8% of the CD4⁺CD8^- cells, respectively. These results indicate that the metastatic lesions of thymoma are associated with abundant immature T cells which are phenotypically less mature than those in their primary lesions.