Demonstration of phenotypic abnormalities of thymic epithelium in thymoma including two cases with abundant Langerhans cells.

A panel of monoclonal antibodies that phenotypically define stages of normal human thymic epithelial (TE) cell maturation was used to compare thymic epithelium of nine thymomas with hyperplastic thymic epithelium in myasthenia gravis (MG) and thymic epithelium of normal thymuses. It has been shown previously that normal thymic epithelial cells express antigens of early TE cell maturation (A2B5, TE-4) throughout thymic ontogeny and acquire antigens 12/1-2, TE8, and TE-15 at 14 to 16 weeks of fetal gestation. Hyperplastic MG thymic epithelial cells expressed TE antigens in phenotypic patterns similar to that seen in normal postnatal thymus, ie, TE in subcapsular cortex and medulla was TE4+, A2B5+, and 12/1 - 2+ and Hassall's bodies were reactive with antibodies TE8 and TE15. In contrast, thymic epithelium in primary mediastinal thymomas was TE4+, A2B5+, TE8-, and greater than 75% of thymoma epithelium was 12/1 - 2-, a thymic epithelial phenotype similar to that seen on normal fetal thymic epithelium at 14 to 16 weeks fetal gestation. In one subject with a mature epithelial histologic pattern, thymoma epithelium was found to be strongly TE8+, a phenotype suggestive of a later stage of TE maturation. Lymphocytes in five of seven thymomas with immature thymic epithelial cells predominantly expressed immature thymocyte phenotype while two thymomas with immature epithelial phenotype showed a predominance of Langerhans cells and surrounding lymphocytes expressing a mature phenotype. Lymphocytes in the thymoma with differentiated epithelial cells expressed a mature thymocyte phenotype. Thus, in thymomas of varying histologic types, phenotypic abnormalities of thymic epithelium are present; these phenotypic abnormalities may reflect abnormal thymic epithelial maturation.