In vitro evidence for participation of DEC-205 expressed by thymic cortical epithelial cells in clearance of apoptotic thymocytes

Binding of apoptotic cells was compared after incubation of thymocytes with two clones of murine thymic stromal cells to which CD4⁺/CD8⁺ thymocytes attach. With the BA/10, but not the BA/2, clone, thymocytes with apoptotic morphology were bound irreversibly. These tightly bound thymocytes were further identified as apoptotic in terms of active caspase-3 and DNA fragmentation assayed in situ. FACS analysis indicated that the apoptotic thymocytes are at an early double-positive stage and results with mice mutant for the Fas gene showed that the Fas-Fas ligand system is not involved. Comparison of BA/10 and BA/2 cells showed that the former, but not the latter, can be induced to express CDR-1 antigen which is characteristic of cortical epithelial thymic stroma and constitutively express DEC-205, a surface protein common to cortical thymic epithelium and dendritic cells. Antibody NLDC-145 that is specific for the DEC-205 protein strongly reduced the number of stromal cells with bound apoptotic thymocytes. Preincubation of thymocytes in dexamethasone dramatically increased the number of bound apoptotic cells, indicating that the thymic cortical epithelial cells can participate in clearance of apoptotic thymocytes through involvement of DEC-205.