Sinusoidal rat liver cells spontaneously kill tumor cells in vitro. They have the same preferences as do spleen cells for certain types of tumor cells, YAC-1, P815, BSP73Asml, BSP73As, EB, EsB, and L5222. Metastasizing tumor cells are less sensitive than their nonmetastasizing counterparts.

Not all effector cells are Kupffer cells. These noumacrophage killer cells share some features with classical natural killers: (a) fast reactions (4 h); (b) high toxicity against YAC-1 cells; (c) sensitivity to anti-asialo Gm1 globulin; (d) similar age dependency; (e) short biological halflife (∼1 day) (deduced from radiation experiments); (b) silica particle insensitivity; and (g) nonadherence. The natural killing potency of the liver is higher than that of the spleen.

The reduction of tumoricidal capacity of the liver in germ-free animals suggests environmental influences. Tumoricidal capacity (organ capacity) is increased in rats chronically fed thioacetamide, carbon tetrachloride (CCl4), dimethylaminoazobenzene, and N-nitrosomorpholin.

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