The mechanism of T-cell mediated cytotoxicity. VIII. Zeiosis corresponds to irreversible phase (programming for lysis) in steps leading to lysis.

The combined use of an improved technique for inactivating cytotoxic T cells during the lytic reaction, with time lapse cinematography and isotope release assay, have shown that the initiation of the morphological phase of zeiosis corresponds to the time when the target cell is irreversibly programmed to lyse. It is suggested that rubidium release occurs during the phase of zeiosis. The rate of release of chromium is the result of two phases of variable length, The reversible phase (before programming for lysis) and the irreversible phase from the initiation of zeiosis to the final lytic event. The time required for programming for lysis to occur depends on the number of T cells reacting with the target cell. Thus at high ratios in tubes, where multiple interactions are possible, most target cells are programmed to lyse within 10 min. However, under conditions when T-cell:target-cell conjugates are kept in suspension to prevent multiple interactions, programming for lysis can take several hours. This provides an explanation for the apparent difference in timing of zeiosis and programming for lysis in previous publications. It is also shown that further T-cell interactions with the target cell after programming for lysis (i.e. during the irreversible phase), markedly influence the rate of chromium release. This provides an explanation for the fact that chromium release takes at least 3 h to reach plateau levels after inactivation of the T cells, whereas at high effector cell ratios, maximum levels of chromium release can occur within 1 h.