The serum of BCG-infected mice treated with endotoxin contains a substance (tumor necrosis factor, TNF) which mimics the tumor-necrotizing action of endotoxin itself. TNF is not residual endotoxin, but a factor released from host cells, probably macrophages. TNF induced in the same way in rats and rabbits also causes necrosis of transplanted murine tumors. Unlike endotoxin, TNF is toxic in vitro for neoplastic murine and human cell lines but not for mouse embryo culture. TNF has striking effects on immunologic reactions in vitro, some like those of endotoxin and others unlike those of endotoxin. TNF is a glycoprotein; its molecular weight is less than 70,000. Highly purified preparations do not contain lysosomal or nonlysosomal serum enzymes, interferon or prostaglandin E1.
