Susceptibility to cytotoxic T lymphocyte-induced apoptosis is a function of the proliferative status of the target

When fibroblasts are cultured in decreasing concentrations of serum, there is a concomitant decrease in the percentage of cells in log phase growth (8) . Culture of L929 fibroblasts in 2, 5, 10, or 20% FBS led to greater cell proliferation with increasing FBS concentration. After 2 d of growth under these conditions, [~H]thymidine uptake was 12, 29, 45, and 87 x 103 cpm, respectively, after a 1-h pulse. When assayed as CTL targets in their respective culture conditions, there was no significant difference in the membrane lysis of these cells as assessed by release of the cytoplasmic radiolabel, 5tCr ( Fig. 1 A) . However, the fragmentation of the target DNA, as assessed by liberation of a 125I-IUDR. label, increased dramatically in cells grown in increasing serum concentrations ( Fig. 1 B) . These results suggested that the degree of DNA fragmentation was related to the proliferative status of the target cell. More importantly, however, this experiment established, without any drug-induced modulation of the target cell, that the process of target cell membrane lysis induced by the CTL lethal hit is distinct from the process of CTLinduced DNA fragmentation.