The effect of caspase-inhibitors on radiation induced apoptosis in human peripheral blood lymphocytes: an electron microscopic approach

Resting lymphocytes are sensitive to radiation damage and die by apoptosis. We investigated the effect of caspase-inhibitors on radiation induced apop tosis in human peripheral blood lymphocytes. Lymphocytes were irradiated in vitro with 5 Gy Co-60-gamma-rays and cultured for 24 hours in the presence or absence of the caspase-inhibitors zVAD-fmk and zDEVD-fmk. Cell death wa s evaluated by electron microscopy. Irradiation in the absence of the inhib itors resulted in about 30% dead cells, almost all showing typical apoptoti c morphologies. Addition of either one of the inhibitors could not rescue c ells from death. Part of the dead lymphocytes (about 65%) still showed typi cal nuclear characteristics of apoptotic cells: sharply marginated, condens ed chromatin, clumped into one sphere or into a crescent shaped mass. The r emaining part of the dead cells had ultrastructural characteristics, aberra nt from apoptic cells: clumping of the chromatin was less pronounced and le ss sharply marginated. Irregular clumps were formed. Data indicate that par t of the lymphocytes go in apoptosis in a caspase-independent way. The othe r part shows caspase-dependent apoptosis with respect to the nuclear events .