Characterization of natural cytotoxicity in vitro in a spontaneous rat tumor model

Spontaneous rat tumor cells were tested in a 12-hr Cr-release assay for susceptibility or resistance to cytolysis by lymphoid effector cells from untreated rats. Most target cells were intermediate to highly susceptible (up to 85% lysis at an effector-to-target ratio of 50:1), but we observed nonsusceptible target cells, too. Cold target inhibition experiments confirmed the susceptibility classifications obtained in the direct assay. The functional properties of effector cells paralleled closely the known properties of natural killer cells. The NK responsiveness of various rat strains did not coincide with the incidence of spontaneous tumors.