C1 Inhibitor Prevents Endotoxin Shock Via a Direct Interaction with Lipopolysaccharide

Abstract

C1 inhibitor (C1INH) is beneficial in animal models of endotoxemia and sepsis. However, the mechanism(s) of C1INH protection remain(s) ill-defined. In this study, we demonstrated that both active C1INH and reactive center-cleaved, inactive C1INH protected mice from lethal Gram-negative endotoxemia. Both forms of C1INH blocked the LPS-binding protein-dependent binding of Salmonella typhimurium LPS to the murine macrophage cell line, RAW 264.7, and suppressed LPS-induced TNF-α mRNA expression. Inhibition of LPS binding to RAW 264.7 cells was reversed with anti-C1INH Ab and was more efficient when C1INH was incubated first with LPS rather than with the cells. C1INH also suppressed LPS-induced up-regulation of TNF-α mRNA in whole human blood. The interaction of C1INH with LPS was directly demonstrated both by ELISA and by nondenaturing PAGE, but deletion of the amino-terminal 97-aa residues abrogated this binding. Therefore, C1INH, in addition to its function as a serine protease inhibitor, has a novel anti-inflammatory function mediated via its heavily glycosylated amino-terminal non-serpin domain.

C1 inhibitor: Biologic activities that are independent of protease inhibition

“…Other data suggest that contact system activation does not play an important role (Feletou et al, 1996). Our data indicated that not only did intact, active C1 inhibitor protect from endotoxin shock, but that treatment with inactive, reactive center cleaved C1 inhibitor also was effective (Liu et al, 2003). Both active and inactive C1 inhibitor also prevented both local and systemic vascular permeability changes in response to endotoxin (Liu et al, 2005b).…”

“…At least one other protein, MD-2, is associated with CD14 and TLR4 on the cell surface; together, these make up the LPS receptor (da Silva Correia and Ulevitch, 2002;Juan et al, 1995;Stelter et al, 1997;Stelter et al, 1999;Viriyakosol and Kirkland, 1995;Viriyakosol et al, 2000). The binding of C1 inhibitor to LPS (or lipid A) was inhibited by fetal bovine serum, which contains LBP, and by an LBP peptide that includes the binding site for LBP (Liu et al, 2003).…”

“…In a variety of different animal models of Gram negative endotoxin shock and sepsis, survival is significantly improved, together with several physiologic measurements, by treatment with C1 inhibitor (Fischer et al, 1997;Giebler et al, 1999;Guerrero et al, 1993;Jansen et al, 1998;Liu et al, 2003;Scherer et al, 1996;Schmidt et al, 1999a;Schmidt et al, 1999b). C5a very likely plays a role in the pathogenesis of septic shock (Barton and Warren, 1993;Czermak et al, 1999;Strachan et al, 2000).…”

“…C1 inhibitor prevented the binding of Salmonella typhimurium LPS to the murine macrophage cell line, RAW 264.7, and to human blood cells. Direct binding assays showed that C1 inhibitor interacted with LPS, thereby blocking LPS-induced TNF-α expression (Liu et al, 2003). LPS, a component of the outer membrane of Gram negative bacteria, consists of an inner lipid A component, which is responsible for the pathophysiology of endotoxin shock, and an outer polysaccharide component.…”

“…Initial data suggest that, in some instances, these reactions may be significant contributors to the anti-inflammatory activity of this important protease inhibitor. These include at least four different binding reactions (Table 2): (1) An interaction with a variety of extracellular matrix components (Patston and Schapira, 1997); (2) inhibition of alternative complement pathway activation via binding of C1 inhibitor to C3b which prevents its interaction with factor B, in a manner analogous to that of factor H (Jiang et al, 2001); (3) an interaction with both E and P selectins on endothelial cells that is mediated by one or more Lewis x tetrasaccharides expressed on C1 inhibitor, and which results in suppression of leukocyte rolling and transmigration Cai et al, 2005); (4) binding to endotoxin from several different gram negative bacteria that results in suppression of endotoxin shock by interference with the interaction of endotoxin with its receptor complex on macrophages (Liu et al, 2003;Liu et al, 2004;Liu et al, 2005a;Liu et al, 2005b); (5) direct binding to gram negative bacteria that results in suppression of the development of sepsis (unpublished data). This review will briefly summarize the limited information available on the first two reactions and will describe in more detail the data relating to the interactions with selectins and with endotoxin lipopolysaccharide (LPS).…”