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Cellular signaling mechanisms associated with the inhibition of endotoxin tolerance by IFNgamma and GM-CSF(Adib-Conquy & Cavaillon, J. Biol. Chem. 2002, 277; 27927).
Endotoxin tolerance was initially described as a failure of animals to develop fever in response to an injection of bacterial endotoxin after they had been pretreated with this molecule. The observation was extended to survival a lethal dose of lipopolysaccharide after a previous injection of a sub-lethal dose of LPS. In vitro, endotoxin tolerance is characterized by a decreased production of pro-inflammatory cytokines by cultured leukocytes in response to LPS following a first exposure to the same stimulus. Gamma interferon (IFNgamma) and granulocyte monocyte-colony stimulating factor (GM-CSF) are immunostimulatory cytokines that prime monocytes and prevent endotoxin tolerance. We have shown that the deactivating effects of LPS, as well as the priming effects of IFNgamma and GM-CSF or their capacity to restore tumor necrosis factor (TNF) production by LPS-tolerized human monocytes, are independent of the modulation of TLR4 or MD-2. In monocytes pretreated with IFNgamma or GM-CSF, IRAK expression was up-regulated. After LPS stimulation, an increased IRAK kinase activity, a higher MyD88/IRAK association and a stronger NF-kappaB activation were observed. In contrast, in LPS-tolerized monocytes, IRAK-1 expression and kinase activity, IRAK/MyD88 association and NF-kappaB activation were inhibited. Furthermore, we showed that the prevention of tolerance by IFNgamma and GM-CSF was independent of IRAK-1 kinase activity. Our results suggest that these cytokines prevent endotoxin tolerance, induced by low but not by high doses of LPS, by inhibiting IRAK-1 degradation and by promoting its association with MyD88 after a second LPS stimulation, which in turn leads to NF-kappaB activation and TNF production
Compartmentalization of endotoxin tolerance(Fitting et al. J. Infect. Dis. 2004, 189, 1295).
Endotoxin tolerance has been defined and analyzed either entirely in vivo or entirely in vitro mostly with conventional LPS, extracted according to Westphal’s method, and purified according to conventional methods. In contrast, often referred to an endotoxin tolerance phenomenon, the hyporeactivity of circulating leukocytes reported in patients with sepsis is an ex vivo observation. Therefore, our objective was to set up an ex vivo model of endotoxin tolerance. Mice were injected intravenously with LPS and their leukocytes derived from different compartments were challenged in vitro with LPS or heat-killedStaphylococcus aureus (HKSA) and TNF production was measured. A rapid (1-3 h) and dramatic decrease in the production of TNF in response to LPS was observed with circulating leukocytes, splenocytes, peritoneal cells and bone-marrow cells after LPS injection. In contrast, LPS-induced TNF production by bronchoalveolar cells was reduced far less and only very briefly. The kinetics of acquisition of tolerance and recovery were different for the various compartments. “Cross-tolerance” with HKSA did not parallel the phenomenon of endotoxin tolerance as observed with LPS, in agreement with data suggesting that cross-tolerance between TLR2 and TLR4 agonists could not be generalized. These data show that endotoxin tolerance, as monitored by ex vivo analysis, is compartmentalized and that bronchoalveolar cells are less likely than peritoneal, splenic or marrow cells to develop endotoxin tolerance.