PE-35-related antigen expression and CD1a-positive lymphocytes in thymoma subtypes based on Muller-Hermelink classification - An immunohistochemical study using catalyzed signal amplification

PE-35 monoclonal antibody, detecting a cell-surface antigen of various type s of carcinoma and normal epithelium, reacts exclusively with the medullary epithelium in the thymus; therefore, the antigen has been considered as a marker of medullary differentiation in thymomas. Using the catalyzed signal amplification method, which made it possible to apply PE-35 to routinely p rocessed, archival tissues, we examined expression of this antigen, togethe r with CD1a reactivity of lymphocytes, in 40 thymic epithelial tumors subcl assified using the Muller-Hermelink system. Medullary thymomas infiltrated with a small number of CD1a-negative lymphocytes were PE-35 positive, altho ugh many of the long spindle tumor cells were PE-35 negative. Mixed thymoma s and predominantly cortical thymomas, both with prominent CD1a-positive ly mphocytes, were also PE-35 positive, although some areas of the latter type were PE-35 negative. Cortical thymomas with decreased numbers of CD1a-posi tive lymphocytes were largely PE-35 negative. In well-differentiated thymic carcinomas with a few CD1a-positive lymphocytes, two cases were negative, but four cases were at least focally positive with PE-35. All high-grade th ymic carcinomas infiltrated with some CD1a-negative lymphocytes were PE-35 positive. These results suggested that medullary thymoma generally possesse s the medullary nature, although the latter tends to be lost in the long sp indle tumor cells. Mixed and predominantly cortical thymomas may have mixed medullary phenotype and cortical function. Cortical thymoma and many well- differentiated thymic carcinomas may possess the cortical nature, while the large polygonal tumor cells tend to lose immature T-lymphocyte-retaining f unction.