Death of cells by apoptosis following attachment of specifically allergized lymphocytes in vitro

A low rate of spontaneous cell death by apoptosis was found in a DBA/2 mouse mastocytoma growing in culture. It was not significantly altered by addition of splenic lymphocytes from normal C57BL mice, but was massively enhanced by lymphocytes from mice previously immunized against the tumor. Cells showing ultrastructural changes of early apoptosis in the latter experiments had lymphocytes firmly attached to their surfaces, suggesting that cellular immune attack induces apoptosis directly: the cell budding to produce membrane-bounded fragments with well preserved organelles characteristic of the later stages of the process was associated with lymphocyte detachment. These cell fragments or apoptotic bodies were not phagocytosed by viable cells as they are when apoptosis takes place in tissues, and underwent secondary degeneration with ballooning of organelles and rupture of membranes while still suspended in the medium. The major wave of lymphocyte-induced apoptosis occurred within the first 3 h of incubation. The number of well preserved apoptic bodies reached a peak at 1 1/2 hr and thereafter declined, whereas cell debris increased progressively. The maximum rate of release of absorbed, Cr from tumour cells coincided in time with the wave of apoptosis, but difficulty in assessing the relative rates of formation and secondary degeneration of apoptotic bodies made it impossible to determine whether the release was particularly associated with one or the other of these processes. That cellular immune attack should induce a type of cell death known to occur in normal tissue has implications for the hypothesis that the immune system continuously monitors cells and eliminates those that undergo significant antigenic change