Thymic epithelial cells express major histocompatibility complex (MHC) clas
s II and are involved in T-cell ontogeny. In these cells, MHC class II-asso
ciated invariant chain (CD74) is involved in antigen presentation during T-
cell selection. We studied a range of thymic epithelial neoplasms for CD74
expression by neoplastic epithelial cells to determine whether such express
ion correlates with MHC class IT expression and tumor type. Sixty-four thym
ic epithelial neoplasms (27 cases of benign thymoma, 20 cases of invasive t
hymoma, and 17 cases of true thymic carcinoma) were studied for neoplastic
epithelial cell expression of CD74 and MHC class II molecules by immunohist
ochemical staining of paraffin-embedded tissue. Neoplastic epithelial cells
in 88% of thymic carcinomas (15/17), 70% of invasive thymomas (14/20), but
only 33% of benign thymomas (9/27) were immunoreactive for CD74. A subset
of CD74-positive neoplasms was positive for MHC class II as well, with high
er relative rates of dual positivity in more aggressive neoplasms. In addit
ion, specific histologic subtypes of thymic epithelial neoplasms displayed
differing patterns of CD74 positivity. Based on these findings, CD74 and MH
C class II are useful markers for the classification of thymic epithelial n
eoplasms.