Immunohistological Analysis of Thymic Tumors with PE-35 Monoclonal Antibody Reactive with Medullary Thymic Epithelium¹

PE-35 mouse monoclonal antibody (MoAb) (IgG1) detecting an epithelial antigen with a molecular weight of 35,000 was characterized serologically. Immunoperoxidase staining and double immunoenzymatic staining showed that PE-35 antigen is predominantly on nonlymphoid cells in the medulla of thymus. By immunoelectron microscopy, thymic epithelial cells in the medulla were positive with PE-35 MoAb, but macrophages, interdigitating reticulum cells, and thymocytes were negative with this MoAb, which demonstrated that PE-35 is a valuable marker for medullary epithelium.

Using PE-35 and other MoAbs detecting thymic epithelial antigens (TE-3A, RFD-4, TE-4, and HLA-DR), 25 thymomas were studied, together with 6 other tumors of thymic origin. Among 25 thymomas, all 6 cases of epithelial type and 8 of 14 mixed lymphoepithelial type were positive with PE-35 MoAb, but only one of 5 lymphocytic type was positive. PE-35 antigen has a tendency to be expressed in the cases retaining medullary type thymocytes, with the phenotype of cluster of differentiation (CD) 1^-/CD3⁺/CD6⁺, and also in the area of medullary differentiation. TE-3A, RFD-4, and TE-4 MoAbs reacted with most thymoma cases regardless of the types. HLA-DR was, however, expressed on a part of thymomas and the phenotype combined with that of PE-35 was as follows: PE-35⁺/HLA-DR⁺, 8 cases; PE-35⁺/HLA-DR^-, 8 cases; PE-35^-/HLA-DR⁺, 8 cases; PE-35^-/HLA-DR^-, one case. The results suggested that thymoma may originate from different subsets and/or different stages of thymic epithelium.