Immunohistochemical evidence for the expression of the carcinoembryonic antigen by human thymic epithelial cells in vitro and in neoplastic conditions.

Thymic epithelial cells (TECs), which are known to influence T-cell differentiation, may undergo phenotypic changes and lose some differentiation antigens (for example, the HLA-DR complex) in neoplastic conditions and when they are grown in culture. Using an indirect immunofluorescence assay, the authors investigated the expression of the carcinoembryonic antigen (CEA) by normal cultured or pathologic human TECs. This antigen, which can be regarded as a marker of undifferentiation, disappears during the normal development of epithelial tissues and reappears in neoplastic conditions. In normal as well as hyperplastic (myasthenia gravis-associated) thymuses, the epithelial network (revealed in double-labeling experiments by an anti-keratin monoclonal antibody) is virtually CEA-negative, except for the specific labeling observed on some cells of Hassall's corpuscles. In thymomatous epithelial cells, however, a strong and specific fluorescent labeling was consistently detected in all thymomas studied. Thymic epithelial cells grown in cultures from fragments of normal thymuses also expressed CEA on their cell membranes. Interestingly, the relative number of CEA-positive cells increased as a function of the age of the primary culture and reached virtually 100% when monolayers became confluent (Days 12-14). Moreover, using an ELISA assay, the authors demonstrated the presence of CEA in supernatants from TEC cultures. Interestingly, the amount of CEA in these supernatants decreased as a function of the age of the culture. In addition, a marked inhibition of TEC proliferation was observed after treating the cultures with an anti-CEA serum. Our results demonstrate that CEA is expressed not only in situ by differentiated neoplastic TECs but also by normal TECs cultured in vitro. In addition, the inhibitory action of the anti-CEA serum on TEC proliferation suggests that CEA may act physiologically as a growth factor for proliferating epithelial cells. In this respect, cultures of human TECs represent a good model for further studies.