Independent signal transduction pathways for IL-1 and TNF in LPS-tolerant macrophages

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