Reversal of defective IL-6 production in lipopolysaccharide-tolerant mice by phorbol myristate acetate

The development of LPS tolerance has been suggested to be mediated by an inhibition of cytokine synthesis. Here we have studied serum IL-6 and TNF levels in mice after LPS administration. Repeated administration of LPS (35 μg daily for 4 days) to mice induced a refractoriness (tolerance) to subsequent administrations of LPS in terms of induction of circulating IL-6 and TNF. To investigate the mechanism by which LPS down-regulates its own induction of cytokine synthesis and the relationship between IL-6 and TNF production, we attempted to revert the inhibition of IL-6 and TNF production using agents like PMA or IFN-γ, previously reported to activate macrophage production of cytokines. Pretreatment with PMA (4 μg, 10 min before LPS) partially restored IL-6 production in LPS-tolerant mice given 2 μg LPS. On the other hand, PMA did not restore TNF induction in LPS-tolerant mice, even when administered with high doses of LPS (up to 200 μg). A similar reversal of LPS resistance to IL-6, but not TNF, induction by PMA was observed in genetically LPS-resistant C3H/HeJ mice. IFN-γ also restored, although to a lesser extent than PMA, IL-6 production. However, unlike PMA, IFN-γ could also partially restore TNF production in LPS-tolerant mice, although only when LPS was administered at high doses. By contrast with PMA, IFN-γ was clearly more active in restoring TNF synthesis than that of IL-6. Similar results were obtained in genetically LPS-unresponsive C3H/HeJ mice. These data Jggest that different mechanisms are implicated in the inhibition of IL-6 and TNF synthesis in LPS-tolerant mice and that part of this inhibition can be overcome by PMA or IFN-γ.