Destruction of experimental malignant melanoma by mediators of cellular immunity.

Listeria monocytogenes (LM) in admixture with B-16 melanoma suppresses local tumor development in syngeneic C57BL/6 mice. In vitro, LM-immune peritoneal and splenic cells are cytotoxic to B-16. Induction of cell-mediated immunity to LM antigens are required for the killing effect, since effector cells from LM-"immune" athymic nude mice are unable to kill tumor cells in vitro. Further, elimination of macrophages by a specific antiserum plus complement abrogates the cytotoxic effect of peritoneal cells. Peritoneal or splenic adherent or nonadherent cells are not cytotoxic, whereas combination of these two cell populations in the presence of the specific antigen can kill the B-16 target cells. A factor, probably lymphotoxin, released by the intact effector cells in the culture fluid mediates tumor cell destruction in vitro. Production of this factor requires cooperation of macrophages with specifically sensitized thymus-derived cells.