Induction of “Innocent Bystander” Cytotoxicity in Nonimmune Mice by Adoptive Transfer of L3T4⁺Lyt-1⁺2^-Mammary Tumor Immune T-Cells

Spleen cells from BALB/c mice, bearing a syngeneic mammary adenocarcinoma, nonspecifically lyse xenogeneic target cells following in vitro reexposure to mammary tumor-associated antigens. The effectors of this reaction have been shown to be Thy-1⁺ Lyt-1⁺2⁺ nylon-adherent cells. Tumor-immune spleen cells are also able to induce nonimmune spleen cells to express nonspecific cytotoxicity. Studies were undertaken to determine whether this inducer activity is mediated by the effector population or by a functionally distinct subset. Cell separation studies demonstrated that the inducers and effectors of innocent bystander cytotoxicity can be separated based upon the property of adherence to nylon wool. Further examination of the nylon-nonadherent inducer population indicated that the phenotype is L3T4 + Lyt-1⁺2^-. By flow cytometry the inducer subset was shown to express a higher density of Thy 1 antigen than that expressed by the cytotoxic effectors. When adult thymectomized mice were implanted with mammary tumors, nonspecific effectors were not generated. In contrast, spleen cells from the same experimental animals were able to induce nonspecific cytotoxicity in normal mice following adoptive transfer of their spleen cells. Thus, these data support the conclusion that during the course of mammary tumor growth, at least two functionally and phenotypically distinct cell populations interact for the expression of 'innocent bystander' cytotoxicity.