Differential susceptibility of avian sarcoma cells derived from different periods of tumor growth to natural killer cell activity.

Tumors which are induced in chickens by avian sarcoma virus usually grow actively for 2 to 3 weeks and then regress. We have cultured tumor cells from each of the active and regression periods of neoplastic growth and compared them in terms of sensitivity to natural killer (NK) activity and to specific cell-mediated cytotoxicity. The results indicate that those cells which derive from regressing sarcomas are sensitive to NK activity but resistant to specific cell-mediated cytolysis. In contrast, tumor cells derived from the period of active neoplastic growth can be lysed effectively in either type of assay, although they do not appear to be as susceptible as do "regressor" cells to the action of NK effectors. The addition of autologous virus to these various reaction mixtures is inhibitory to specific cell-mediated cytotoxicity but not to NK-mediated lysis. These data suggest that NK activity may play an important role in the elimination of tumor cells of the "regressor" phenotype, which otherwise appear to be resistant to immune cytolysis.