MHC non-restricted, CD95-independent apoptosis of immature thymocytes induced by thymic epithelial cells

The interaction of thymocytes with thymic epithelial cells in the absence of an exogenous antigen was studied in vitro. Thymic, but not splenic epithelial cells induced apoptosis of thymocytes. A thymic epithelial cell line (TEC) induced apoptosis of thymocytes but not of splenic T-cells. The target population for TEC-induced death were immature CD4⁺8⁺ (double positive), but not mature single positive thymocytes. TEC also induced DNA fragmentation in day 18 foetal thymocytes, most of which are CD4⁺8⁺ cells. Radiation leukemia virus (RadLV)-transformed thymic lymphoma clones expressing various phenotypes reflected this sensitivity, in that a CD4⁺8⁺3⁺ clone apoptosed by thymic epithelial cells or TEC. Other, single positive or double negative clones were resistant. Thymocytes from C3H (H-2(k)), C57BL/6 (H-2^b) and Balb/C (H-2(d)) mice apoptosed equally in response to either C57BL/6 thymic epithelial cells or TEC (H-2^b x H-2(d)). Likewise, thymocytes from MRLlpr((-/-)) and B6lpr((-/-)) mice, which do not express CD95 were also apoptosed by TEC. The data suggest that thymic epithelial cells induce MHC non-restricted. Fas-independent apoptosis of immature thymocytes. This response may reflect a mechanism through which thymocytes expressing TcR with no affinity to self MHC/peptide complexes are eliminated.