An immunohistochemical study of thymic epithelial tumors. I. Epithelial component.

Twenty-four cases of thymic epithelial tumors, including 18 cases of thymoma, five cases of squamous cell carcinoma, and one case of undifferentiated carcinoma, were studied immunohistochemically using monoclonal antibody Leu-7 (HNK-1) and antikeratin antibody. Seven cases of non-neoplastic thymic tissues were also studied. Leu-7 antibody stained epithelial cells in the outer cortex of the normal thymus, and antikeratin antibody stained thymic epithelial cells in both cortex and medulla of the normal thymus. Seventeen thymomas and one undifferentiated carcinoma were focally or diffusely stained with Leu-7, some showing cortical and medullary differentiation as seen in the normal thymus. On the other hand, none of the five squamous cell carcinomas were stained with Leu-7. All thymomas stained for keratin in varying degrees, and all squamous cell carcinomas were diffusely and strongly stained with antikeratin antibody. It is concluded that normal thymic epithelial cells showed zonal differentiation, and neoplastic cells were considered to retain these characteristics to some extent; i.e., thymomas had the same phenotype of epithelial cells of the cortex, especially the outer cortex of the thymus in some instances (Leu-7-positive, keratin-positive) and of both cortex and medulla (mixture of Leu-7-positive and -negative cells) with organoid arrangement in other instances, and thymic squamous cell carcinoma had the same phenotype as epithelial cells in the thymic medulla (Leu-7-negative, keratin-positive).