Expression of Proteasome Subunit β5t in Thymic Epithelial Tumors

Recently, a proteasome beta subunit expressed exclusively in thymic cortical epithelial cells was discovered in mice and humans. This subunit, designated beta 5t, is a component of the thymoproteasome, a specialized type of proteasome implicated in thymic positive selection. To investigate whether beta 5t could serve as a marker for the differential diagnosis of thymic epithelial tumors, we performed immunohistochemical analysis using anti-beta 5t antibody in 54 cases of thymic epithelial tumors comprising 41 cases of thymomas and 13 cases of thymic carcinomas. beta 5t was detected in the neoplastic epithelial cells of thymomas. Among the subtypes of thymoma, expression of beta 5t was observed in most cases of type B thymoma (20 of 21) but not in type A thymomas (0 of 3). In type AB thymomas, beta 5t expression was variable (6 of 17). Type B3 thymomas (4 cases) were positive for beta 5t but negative for CD5, c-kit, and glucose transporter 1 (GLUT-1), which are known as diagnostic markers for thymic carcinomas. In contrast, thymic carcinomas were negative for beta 5t (0 of 13) but expressed at least one and usually all of CD5, c-kit, and GLUT-1. Thus, beta 5t and CD5/c-kit/GLUT-1 were differentially expressed in type B3 thymoma and thymic carcinoma. We tested beta 5t expression in 39 cases of tumors arising from other organs, which showed the specific expression of beta 5t in thymic epithelial tumors. This study demonstrates that beta 5t is expressed in most type B and in some type AB thymomas and is a marker useful in differentiating type B3 thymomas from thymic carcinomas when used in combination with other diagnostic markers.