Bacterial lipoplysaccharide (LPS) induces the release of factors into the serum which enable mice to reject experimental tumors. One such factor is tumor necrosis factor which causes acute necrosis of syngeneic sarcoma transplants in mice. Effective therapeutic use of tumor necrosis factor is limited, however, by its toxicity. We show here that the efficacy of tumor necrosis factor can be substantially increased by combining its application with low doses of LPS. Our data suggest that LPS exerts its antitumor effects by engaging more than one defense mechanism.

Characteristic for the activation of a biological system is a concomitant induction of negative feedback mechanisms which antagonize the initial stimulus. Interference with the negative feedback response may substantially increase biological reactions. We show here that the blocking of two negative feedback responses occurring as a consequence of treatment with LPS, namely the production of prostaglandin E and the generation of suppressor T-lymphocytes, increases dramatically the ability of mice to reject tumor transplants.

Thus, through appropriate combination of different factors one may reduce the dose of each below toxic levels and through interference with negative feedback responses increase the efficacy of antitumor reagents. We consider our findings in the context of formulating an effective immunotherapy of malignancies and as a promising step toward it.

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