Identification of a transitional preapoptotic population of thymocytes

Apoptosis, a major form of cell death in the immune system, is characterized by cell shrinkage, chromatin condensation, and cleavage into nucleosomal fragments. Apoptosis may be the mechanism for the elimination of autoreactive and unselected CD4+ CD8+ thymocytes in the thymus. A large number of diverse agents are capable of inducing apoptosis in immature thymocytes. Rat thymocytes were treated with etoposide, a DNA topoisomerase II reactive agent, or dexamethasone, a glucocorticoid, and separated on discontinuous Percoll gradients. We have identified and isolated a transitional preapoptotic population of thymocytes that exhibited early morphologic and biochemical changes associated with apoptosis. These preapoptotic cells were intermediate in size and density between normal and apoptotic thymocytes and exhibited a decreased surface expression of both CD4 and CD8 molecules compared to control thymocytes. On ultrastructural examination, they were shown to possess sharply defined clumps of condensed chromatin abutting onto the nuclear membrane. These morphologic changes, the first detectable signs of apoptosis, occurred prior to the internucleosomal cleavage of DNA, often regarded as the biochemical hallmark of apoptosis. Nucleosomal fragments of 180 to 200 base pairs or multiples thereof were, however, detected following subsequent dramatic changes in the nuclear structure of these preapoptotic cells that resulted in morphology typical of apoptosis. These results suggest that early critical events in apoptosis precede internucleosomal cleavage of DNA.