Bacterial LPS triggers monocytes and macrophages to produce several inflammatory cytokines and mediators. However, once exposed to LPS, they become hyporesponsive to a subsequent endotoxin challenge. This phenomenon is defined as LPS desensitization or tolerance. Previous studies have identified some components of the biochemical pathways involved in negative modulation of LPS responses. In particular, it has been shown that the IL-1R-related protein ST2 could be implicated in LPS tolerance. The natural ligand of ST2 was recently identified as IL-33, a new member of the IL-1 family. In this study, we investigated whether IL-33 triggering of ST2 was able to induce LPS desensitization of mouse macrophages. We found that IL-33 actually enhances the LPS response of macrophages and does not induce LPS desensitization. We demonstrate that this IL-33 enhancing effect of LPS response is mediated by the ST2 receptor because it is not found in ST2 knockout mice. The biochemical consequences of IL-33 pretreatment of mouse macrophages were investigated. Our results show that IL-33 increases the expression of the LPS receptor components MD2 (myeloid differentiation protein 2) and TLR-4, the soluble form of CD14 and the MyD88 adaptor molecule. In addition, IL-33 pretreatment of macrophages enhances the cytokine response to TLR-2 but not to TLR-3 ligands. Thus, IL-33 treatment preferentially affects the MyD88-dependent pathway activated by the TLR.

Septic shock from bacterial endotoxin, triggered by the release of lipopolysaccharide (LPS) molecules from the outer wall of Gram-negative bacteria, is a major cause of human death for which there is no effective treatment once the complex inflammatory pathways stimulated by these small amphipathic molecules are activated. Here we report that plasma gelsolin, a highly conserved human protein, binds LPS from various bacteria with high affinity. Solid-phase binding assays, fluorescence measurements, and functional assays of actin depolymerizing effects show that gelsolin binds more tightly to LPS than it does to its other known lipid ligands, phosphatidylinositol 4,5-bisphosphate and lysophosphatidic acid. Gelsolin also competes with LPS-binding protein (LBP), a high-affinity carrier for LPS. One result of gelsolin-LPS binding is inhibition of the actin binding activity of gelsolin as well as the actin depolymerizing activity of blood serum. Simultaneously, effects of LPS on cellular functions, including cytoskeletal actin remodeling, and collagen-induced platelet activation by pathways independent of toll-like receptors (TLRs) are neutralized by gelsolin and by a peptide based on gelsolin residues 160-169 (GSN160-169) which comprise part of gelsolin's phosphoinositide binding site. Additionally, TLR-dependent NF-kappaB translocation in astrocytes appears to be blocked by gelsolin. These results show a strong effect of LPS on plasma gelsolin function and suggest that some effects of endotoxin in vivo may be mediated or inhibited by plasma gelsolin.