IL-2 addiction: withdrawal of growth factor activates a suicide program in dependent T cells.

IL-2-dependent T effector cells usually die when deprived of growth factor. Cell death (as measured by plasma membrane breakdown) requires protein synthesis because it is inhibited by cycloheximide or emetine. When the DNA in several IL-2-dependent cell lines was examined following removal of IL-2, it was found that extensive chromatin cleavage precedes plasma membrane breakdown by several hours. The DNA fragments observed were not randomly generated but consisted of oligonucleosomes. This suggests that IL-2 deprivation led to activation of an endonuclease with specificity for linker DNA. DNA fragmentation, like cell death, did not occur in the presence of protein synthesis inhibitors. The protein(s) synthesized in response to IL-2 deprivation may, therefore, be the endonuclease or its activator; none of the IL-2-dependent T cells examined contain detectable endogenous endonuclease prior to IL-2 removal. DNA fragments were also found in vivo in lymph node cells draining a site of antigen administration. These results suggest that one aspect of the termination of immune responses involves activation of a cell suicide program in the expanded effector T cell clones. In this program an endonuclease is activated and chromatin is cleaved; as a result macromolecular synthesis begins to wind down so that repair synthesis stops; within a few hours, the cell lyses.