Independent Signal Transduction Pathways for IL-1 and TNF in LPS-Tolerant Macrophages

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Macrophages rendered "tolerant" by pretreatment with low-dose endotoxin (LPS p) release less TNF and more IL-1 in response to a second activating endotoxin exposure (LPS a). We hypothesized that LPS p pretreatment alters signal transduction pathways for TNF and IL-1 independently. The effect of pretreatment with LPS p alone was compared to pretreatment with LPS p plus defined second-messenger pathway agonists or antagonists. Murine peritoneal macrophages were pretreated in vitro for 4 hr with LPS p or PMA or LPS p plus protein kinase C inhibitor (PKCi) or 8-bromo-cAMP. Cells were then washed and cultured with medium alone for 20 hr. Macrophages were also pretreated with LPS p plus indomethacin for the total 24-hr pretreatment interval. Cells were then stimulated for 24 hr with LPS a, after which supernatant TNF and IL-1 were measured by bioassay. In the absence of LPS p, mediators were increased by LPS a in a dose-dependent manner. LPS p pretreatment inhibited TNF and augmented IL-1 in response to LPS a. Pretreatment with PMA partially reproduced LPS p pretreatment. Pretreatment with PKCi alone increased both TNF and IL-1 release by LPS a. The combination of LPS p plus PKCi pretreatment further enhanced IL-1 release without affecting TNF inhibition. The addition of indomethacin had a similar effect. The combination of LPS p plus 8-bromo-cAMP blocked the augmentation of IL-1 without changing TNF inhibition. Macrophage endotoxin tolerance following LPS p pretreatment alters LPS a-triggered TNF and IL-1 release by independent signal transduction pathways.