Expression of thymoproteasome subunit β5t in type AB thymoma.

Type AB thymoma is a thymic epithelial tumour composed of lymphocyte-poor type A and lymphocyte-rich type B components. Although it is categorised as a single entity in the classification of WHO, it shows a broad range of morphology. To investigate whether the functional characteristic of neoplastic cells in type AB thymoma relates to morphological diversity, we performed immunohistochemical analysis using anti-β5t antibody in 20 cases of type AB thymoma. β5t is a recently discovered proteasomal β subunit expressed exclusively in cortical thymic epithelial cells and tumour epithelial cells of thymomas with cortical differentiation. Consistent with our previous observation, β5t was predominantly expressed in the type B component. When the type B component was divided into three groups morphologically, β5t was expressed more frequently in cases with round to polygonal than spindle to oval tumour cells. Furthermore, the ratio of terminal deoxynucleotidyl transferase (TdT)-positive lymphocytes was increased in components with higher expression of β5t. These results indicate that the histological diversity of type AB thymoma correlates with expression of a functional marker β5t and abundance of TdT-positive lymphocytes.