Apoptosis, bcl-2 protein, and Fas antigen in thymic epithelial tumors.

We examined 35 thymic epithelial tumors by immunohistochemical techniques to investigate the extent of apoptosis detected by in situ end-labeling of fragmented DNA and the expression of bcl-2 and Fas. There were 22 thymomas (4 medullary, 9 mixed, 3 predominantly cortical, 6 cortical), 3 well-differentiated thymic carcinomas (WDTCs), and 10 high-grade thymic carcinomas (HGTCs), according to the Müller-Hermelink histologic classification. The HGTC showed the highest apoptotic index of the tumor cells. The apoptotic indices of the thymomas and WDTCs were significantly lower than those of the HGTCs (P < .001), but there was no significant difference among each type of thymoma and the WDTCs. bcl-2 protein was expressed in the tumor cells of the medullary-type thymomas and of the HGTCs but not in the other types of thymoma or WDTC. Fas antigen was negative in the HGTCs but was predominantly expressed in thymomas and WDTCs with advanced clinical stages (P < .05). From the immunostaining pattern, WDTC might be more likely to belong to the class of thymoma than to the category of HGTC.