An immunohistochemical study of thymic epithelial tumors. II. Lymphoid component.

The cellular composition and characteristics of lymphocytes and histiocytes in nine cases of thymoma and one case of squamous cell carcinoma of the thymus were studied immunohistochemically, using a panel of monoclonal and polyclonal antibodies. Nine cases of thymoma were subclassified into seven cases of mixed type and one case each of predominantly lymphocytic type and predominantly epithelial type. Two metastatic tumors and one recurrent tumor were included. Almost all the lymphocytes in the mixed type and lymphocytic type categories, either in primary site or metastasis, were T lymphocytes which showed the immature thymic lymphocyte-phenotype of the normal thymic cortex. On the other hand, the one epithelial thymoma contained many mature T lymphocytes along with immature T cells, as in the normal thymic medulla. In thymic squamous cell carcinoma, all infiltrating T lymphocytes were mature T lymphocytes. These findings suggest that epithelial cells of thymomas have an intimate relationship with the coexisting lymphocytes and retain functional properties of the normal thymic epithelial counterpart. On the other hand, the specific function of thymic epithelial cells is no longer maintained in thymic squamous cell carcinoma.