Previous experimental data from various laboratories indicate that endotoxin of gram-negative oral microorganisms might be one of the most important bacterial products involved in bone resorption during periodontitis. Immunologically nonspecific tolerance to endotoxins in rats was induced by repeated application of Serratia marcescens trichloroacetic acid-extracted endotoxin. Silk ligature was placed on the second maxillary molar of the endotoxin-tolerant rats as well as of control rats in which tolerance to endotoxin had not been induced. The animals were sacrificed 8 days later. The rats showed no specific immune response to the tolerance-inducing endotoxin as measured by passive hemagglutination and by the lymphoblast assays, but we found that bone resorption was significantly reduced in the endotoxin-tolerant rats as compared with ligature-treated animals in which tolerance to endotoxin had not been induced.
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