MAP KINASE AND NF-kB HYPERACTIVATION UPON TLR LIGAND STIMULATION IN IL-6 TRANSGENIC MICE

Background: IL-6 is a pleotropic cytokine that has been shown to play an important role in the pathogenesis of several inflammatory diseases. Beside its direct effects in the early phases of inflammation, IL-6 may amplify inflammatory responses therefore contributing to maintaining chronic inflammation and enhancing tissue damage. We have recently observed increased serum levels of inflammatory cytokines and increased lethality in IL-6 transgenic mice injected with TLR ligands (Carvello et al, unpublished), suggesting that exposure to IL-6 in vivo modulates inflammatory responses. Objectives: To evaluate molecular mechanisms underlying the increased sensitivity to TLR ligands of IL-6 transgenic mice. Methods: Cytokine production (IL-6, IL-1, TNF-alpha) by splenic mononuclear cells of IL-6 transgenic and wild-type mice was measured by ELISA following in vitro stimulation with TLR ligands (LPS, 1mcg/ml; LTA, 10mcg/ml). The activation status of the MAP kinases (MAPK), p38 and ERK, was evaluated using phosphospecific antibodies in Western Blot, cytometric analysis and confocal microscopy. Nuclear translocation of NF-kB was visualized by confocal microscopy with anti-p65 antibody. Results: Similarly to the in vivo condition, splenic mononuclear cells from IL-6 transgenic mice stimulated with LPS or LTA in an ex vivo setting produced increased levels (greater than 2 folds) of TNF-alpha and IL-6. We then analyzed MAPK and NF-kB, which are main pathways induced upon TLR stimulation. We found an hyperactivation of p38 MAPK both at basal level and upon LPS or LTA stimulation in peritoneal macrophages from IL-6 transgenic mice. We also found an hyperactivation of ERK MAPK. Moreover, we found an increased NF-kB nuclear translocation, both at basal level and upon LPS or LTA stimulation in cells from IL-6 transgenic mice. Studies with specific pharmacological inhibitors are ongoing in order to demonstrate a causal role of these signalling pathways activation in the increased cytokine production. Conclusion: Our data show that cells from IL-6 transgenic mice present increased inflammatory cytokine production upon LPS or LTA stimulation, which correlates with hyperactivated MAPK and NF-kB signalling pathways, providing insights on the molecular mechanisms underlying the increased sensitivity to TLR ligands of IL-6 transgenic mice. Since macrophage activation syndrome is often triggered by infections, the ability of IL-6 to favour activation of signalling pathways of TLR responses may have relevance in the mechanisms leading to macrophage activation syndrome. This effect of IL-6 may also be relevant in the context of chronic inflammation where enhanced responses to TLR ligands induced by IL-6 may contribute to the maintenance and exacerbation of chronic inflammation.