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.
This work was supported in part by a Grant-in-Aid for the Comprehensive Ten-Year Strategy for Cancer Control from the Ministry of Health and Welfare; Grant-in-Aids for Cancer Research from the Ministries of Education, Science, and Culture and of Health and Welfare in Japan; and by a grant from the Cancer Research Institute, Inc., New York, NY. Preliminary results of this work were presented at the Third Workshop on Human Leukocyte Differentiation Antigens, Oxford, United Kingdom, in 1986.