The ability of mouse lymphoid cells to show non-specific cytotoxicity to 51Cr-labelled mastocytoma cells in vitro was assessed by the release of 51Cr.
Little cytotoxicity was shown by normal lymphoid cells. However, incubation for several days with mitogens for T cells, such as PHA, Con A, P. graveolens and lentil mitogen, activated spleen cells for cytotoxic killing providing PHA was present during the killing reaction. Pokeweed mitogen, which is an inferior mitogen for T cells, was less active. Three agglutinins, which did not transform T cells, were ineffective. Con A and PWM also activated cortisone-resistant thymus cells.
Activated T cells only killed target cells when a plant agent was present during the killing reaction. The T cell mitogens, which are also strong agglutinins, were effective. PWM was less effective and agents which were not T cell mitogens, including leucoagglutinins, had a variable and usually small effect.
Cell division was not essential for the activation of cytotoxic cells as colcemid diminished but did not abolish activation. DNA synthesis was not required during the killing reaction as judged by the failure of hydroxyurea, an inhibitor of DNA synthesis, to block cytotoxicity when added 2 hr before the killing reaction.
The relation between the number of lymphocytes added and the number of target cells killed was compatible with the `one hit' hypothesis that an effective interaction between a single attacking cell and a target cell led to target cell death.
Non-specific cytotoxicity does not require a strong antigenic difference between attacking and target cells as syngeneic lymphocytes will kill mastocytoma cells. Killing of human cell lines also occurs. The data suggest that the present type of non-specific cytotoxicity is due to activated T cells (presumably blasts) and that plant mitogens are required both for the conversion of the resting T cells to activated cells and to approximate the activated T cells to the target cells and/or initiate a special terminal cytotoxic activation.