Endotoxin inhibition of macrophage-mediated bone resorption

The mechanisms by which bacterial endotoxins (ETX) elicit bone loss in septic osteolytic lesions and in organ cultures of bone rudiments have never been clearly established. The possible mechanisms for ETX action include: (a) the stimulation of osteoclast proliferation; (b) the stimulation of synthesis of secondary agents known to elicit bone resorption, e.g., prostaglandins; (c) the stimulation of the resorptive activity of osteoclasts. In the absence of extant methods for isolating and culturing osteoclasts, we have explored the last possibility by evaluating the action of ETX in a bone resorption system consisting of a putative osteoclast precursor, the macrophage, cocultured with isotopically labeled devitalized bone. We have observed the following: 1. ETX from several species of bacteria (Escherichia coli, Shigella flexneri, and S. minnesota) suppress bone resorption (i.e.,⁴⁵Ca release) mediated by thioglycollate-elicited peritoneal macrophages. This inhibition occurs at ETX concentrations as low as 0.5 μg/ml and is evident within the initial 24 h of incubation. In marked contrast, ETX does not alter the resorptive activity of resident peritoneal macrophages. 2. The suppression of bone resorption by ETX does not depend on the presence of serum complement nor is it a manifestation of reduced cell viability or cell bone-particle binding. Moreover, prolonged pretreatment of elicited cells with ETX does not reduce their subsequent resorptive activity. 3. The suppressive action of ETX is partially reversed by polymyxin B, an observation which implicates the lipid A component of ETX in the inhibitory process. 4. PGE₁, PGE₂, and indomethacin at concentrations as high as 10^-5M do not alter macrophage-mediated resorption; neither does indomethacin modify the action of ETX when the two agents are used concurrently. However, PGE₁ and PGE₂ can mitigate the suppressive action of ETX. The latter result indicates but does not define a role for prostaglandin in the ETX phenomenon. We suggest the ETX elicits bone loss in vivo by stimulating osteoclast proliferation or prostaglandin synthesis, and not by directly evoking enhanced bone resorption by osteoclasts or other osteolytic cells.