Characterization of the effector cells mediating an "innocent bystander" cytotoxicity reaction induced by a syngeneic mammary adenocarcinoma in mice.

Spleen cells from BALB/c mice bearing a syngeneic mammary adenocarcinoma nonspecifically destroy xenogeneic targets following in vitro induction with mammary tumor-associated antigens. Studies were undertaken to characterize the effector cell population(s) mediating this "innocent bystander" cytotoxicity reaction. Fractionation experiments using phagocyte-depleted spleen cells revealed that the effector population was adherent to nylon wool columns. Flow cytometric analysis of the nylon-adherent cells revealed the presence of a minor population of Thy 1.2+ cells. Following treatment of the nylon-adherent cells with anti-Thy 1.2 and complement, the cytotoxic activity was abolished. Furthermore, when those cells expressing the Thy 1.2 antigen were positively selected by cell sorting, they were able to mediate the cytotoxic reaction. In contrast, nylon-adherent Thy 1.2-negative cells were unable to mediate the reaction following selection by cell sorting. Depletion studies with anti-Lyt 1 or anti-Lyt 2 and complement also abolished this cytotoxic response. Additional studies demonstrated that nylon-adherent spleen cells from mammary tumor-bearing mice were not able to lyse natural killer cell-sensitive targets. These data suggest that the cells which effect tumor antigen-induced "innocent bystander" cytotoxicity, or their activated precursors, are nylon-adherent Thy 1.2+, Lyt 1+2+ T-cells.