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 ancillary tool for differential diagnosis of these diseases.