Effector cell analysis of tumor cell rejection in vivo in two syngeneic tumor systems exhibiting distinct in vitro cytotoxic mechanisms.

The nature of the in vivo anti-tumor effector cells was investigated in two different tumor system, MH134 hepatoma and X5563 plasmacytoma, in which tumor-specific antibodies and cytotoxic T lymphocytes (CTL), respectively, mediate in vitro tumor cell lyses. Winn assays utilizing MH134- and X5563-immune spleen cells revealed that in both tumor systems, tumor neutralization was produced exclusively by a tumor-specific immune Lyt-1 T cell subpopulation which was depleted of antibody-producing B cells or T cell subset(s) capable of generating CTL responses. These Lyt-1 T cells could exert their in vivo tumor-protective function under conditions in which MH134 tumor-specific antibody was not detected or in T cell-depleted recipient mice (B cell mice) in which CTL precursors were not recruited, indicating that their activities do not depend on the induction of antibody or CTL response. These results are discussed in the context of the relationships (1) between effector systems detected in in vitro cytotoxicity tests and effector mechanisms responsible for in vivo tumor protection, and (2) between epitopes or molecules required for triggering in vitro and in vivo effectors against the tumor.