Inhibitory effect of endotoxin on the growth of plasma cell tumor.

Small amounts (0.1 ng to 5.0 mug) of Escherichia coli endotoxin protect normal female BALB/c mice against challenge with low doses of syngeneic mineral oil-induced 315 plasma cell tumor. Significant protection was most evident when mice were treated with endotoxin 11 days and 5 days before inoculation with 50 to 100 tumor cells i.p., and endotoxin treatment continued twice a week for the entire experiment. Tumors induced by 10,000 cells s.c. were similarly affected by this treatment. The antitumor action of endotoxin was obliterated when higher challenges of tumor cells or solid tumor pieces were used. Omission of endotoxin pretreatment resulted in a loss of the effect against i.p.-induced tumors but not against s.c.-induced tumor.