Three-color flow cytometric study on lymphocytes derived from thymic diseases

Background. Anterior mediastinal masses derive from a variety of diseases. Thymomas have been shown to commonly hold CD4⁺CD8⁺ double-positive (DP) lymphocytes, and identification of this subset by two-color flow cytometric study was suggested to help diagnosis of thymoma. Several other thymic diseases, however, possibly hold CD4⁺CD8⁺ DP lymphocytes. In this study, we utilized the three-color flow cytometric method for further examination of the phenotypes of lymphocytes in the thymic diseases. Materials and methods. One hundred eight specimens (77 primary and 10 metastatic thymomas, 10 thymic carcinomas, 2 thymic carcinoids, 4 malignant lymphomas, 2 seminomas, an inflammatory pseudotumor, and 2 nonneoplastic thymic hyperplasias) were subjected to the study. The expressions of CD3, CD4, and CD8 on tumor-associated lymphocytes were evaluated by three-color flow cytometric study. Results. The proportion of the CD4⁺CD8⁺ DP subset was more than 30% in all 78 lymphocyte-rich thymomas, in 2 malignant lymphomas, and in both thymic hyperplasias. CD3 expression of the CD4⁺CD8⁺ DP subset ranged from a negative to a high level in thymomas and thymic hyperplasias, while it was restricted to a particular level in CD4⁺CD8⁺ DP-type malignant lymphomas. The proportion of CD3⁺ cells in the CD4⁺CD8^- single-positive subset was consistently less than 90% in the lymphocyte-rich thymomas, while it was more than 90% in the thymic hyperplasias. Conclusion. Although identification of the CD4⁺CD8⁺ DP subset in the tumor-associated lymphocytes does not necessarily indicate thymoma, a further characterization of thymic neoplasms possessing the CD4⁺CD8⁺ DP subset was enabled by three-color flow cytometric study, suggesting the utility of this method as an anciliary tool for differential diagnosis of these diseases.