Apoptosis in thymic epithelial tumors.

We studied Fas/FasL, Bcl-2, and M30 CytoDeath in five cases of normal thymus and 41 cases of thymic epithelial tumors (TETs). In normal thymus, Fas was expressed in all epithelial cells, but not in thymic lymphocytes; FasL was weakly expressed only in medullary epithelium and Hassal's corpuscles. In TETs, Fas was expressed in all epithelial cells and lymphocytes, while FasL was differently expressed in epithelial cells of different subtypes of TETs, i.e., type A (2/2), AB (12/12), B1 (0/9), B1/B2 (0/3), B2 (0/1), B2/B3 (1/3), B3 (6/10) and C (1/1), but not in lymphocytes. Bcl2 protein was strongly expressed in medullary-derived lymphocytes of normal thymus and TETs, and weakly expressed only in medullary (spindle) epithelium of TETs. On M30 CytoDeath immunostaining, apoptotic indices were very low in all TETs (0-1.2). In conclusion, since FasL was expressed on the epithelial cells of lymphocyte-depleted (LD) areas of type B2/B3 and/or LD subtypes (type A, AB, B3, C), FasL expression could be relevant for the intracytoplasmic processes of T-cell selection and the regulation of the lymphoid cells inside the tumor, at least partly. However, tumorigenesis of TETs is not necessarily induced by abrogation of apoptosis.