Oxidized phospholipid inhibition of toll-like receptor (TLR) signaling is restricted to TLR2 and TLR4: roles for CD14, LPS-binding protein, and MD2 as targets for specificity of inhibition

The generation of reactive oxygen species is a central feature of inflammation that results in the oxidation of host phospholipids. Oxidized phospholipids, such as 1-palmitoyl-2-arachidonyl-sn-glycero-3-phosphorylcholine (OxPAPC), have been shown to inhibit signaling induced by bacterial lipopeptide or lipopolysaccharide (LPS), yet the mechanisms responsible for the inhibition of Toll-like receptor (TLR) signaling by OxPAPC remain incompletely understood. Here, we examined the mechanisms by which OxPAPC inhibits TLR signaling induced by diverse ligands in macrophages, smooth muscle cells, and epithelial cells. OxPAPC inhibited tumor necrosis factor-alpha production, IkappaBalpha degradation, p38 MAPK phosphorylation, and NF-kappaB-dependent reporter activation induced by stimulants of TLR2 and TLR4 (Pam3CSK4 and LPS) but not by stimulants of other TLRs (poly(I.C), flagellin, loxoribine, single-stranded RNA, or CpG DNA) in macrophages and HEK-293 cells transfected with respective TLRs and significantly reduced inflammatory responses in mice injected subcutaneously or intraperitoneally with Pam3CSK4. Serum proteins, including CD14 and LPS-binding protein, were identified as key targets for the specificity of TLR inhibition as supplementation with excess serum or recombinant CD14 or LBP reversed TLR2 inhibition by OxPAPC, whereas serum accessory proteins or expression of membrane CD14 potentiated signaling via TLR2 and TLR4 but not other TLRs. Binding experiments and functional assays identified MD2 as a novel additional target of OxPAPC inhibition of LPS signaling. Synthetic phospholipid oxidation products 1-palmitoyl-2-(5-oxovaleryl)-sn-glycero-3-phosphocholine and 1-palmitoyl-2-glutaryl-sn-glycero-3-phosphocholine inhibited TLR2 signaling from approximately 30 microm. Taken together, these results suggest that oxidized phospholipid-mediated inhibition of TLR signaling occurs mainly by competitive interaction with accessory proteins that interact directly with bacterial lipids to promote signaling via TLR2 or TLR4.     

Absence of thrombin-activatable fibrinolysis inhibitor protects against sepsis-induced liver injury in mice

Thrombin-activatable fibrinolysis inhibitor (TAFI), also known as carboxypeptidase R, has been implicated as an important negative regulator of the fibrinolytic system. In addition, TAFI is able to inactivate inflammatory peptides such as complement factors C3a and C5a. To determine the role of TAFI in the hemostatic and innate immune response to abdominal sepsis, TAFI gene-deficient (TAFI-/-) and normal wild-type mice received an i.p. injection with Escherichia coli. Liver TAFI mRNA and TAFI protein concentrations increased during sepsis. In contrast to the presumptive role of TAFI as a natural inhibitor of fibrinolysis, TAFI-/- mice did not show any difference in E. coli-induced activation of coagulation or fibrinolysis, as measured by plasma levels of thrombin-anti-thrombin complexes and D-dimer and the extent of fibrin depositions in lung and liver tissues. However, TAFI-/- mice were protected from liver necrosis as indicated by histopathology and clinical chemistry. Furthermore, TAFI-/- mice displayed an altered immune response to sepsis, as indicated by an increased neutrophil recruitment to the peritoneal cavity and a transiently increased bacterial outgrowth together with higher plasma TNF-alpha and IL-6 levels. These data argue against an important part for TAFI in the regulation of the procoagulant-fibrinolytic balance in sepsis and reveals a thus far unknown role of TAFI in the occurrence of hepatic necrosis.     

Mrp8 and Mrp14 are endogenous activators of Toll-like receptor 4, promoting lethal, endotoxin-induced shock

To identify new components that regulate the inflammatory cascade during sepsis, we characterized the functions of myeloid-related protein-8 (Mrp8, S100A8) and myeloid-related protein-14 (Mrp14, S100A9), two abundant cytoplasmic proteins of phagocytes. We now demonstrate that mice lacking Mrp8-Mrp14 complexes are protected from endotoxin-induced lethal shock and Escherichia coli-induced abdominal sepsis. Both proteins are released during activation of phagocytes, and Mrp8-Mrp14 complexes amplify the endotoxin-triggered inflammatory responses of phagocytes. Mrp8 is the active component that induces intracellular translocation of myeloid differentiation primary response protein 88 and activation of interleukin-1 receptor-associated kinase-1 and nuclear factor-kappaB, resulting in elevated expression of tumor necrosis factor-alpha (TNF-alpha). Using phagocytes expressing a nonfunctional Toll-like receptor 4 (TLR4), HEK293 cells transfected with TLR4, CD14 and MD2, and by surface plasmon resonance studies in vitro, we demonstrate that Mrp8 specifically interacts with the TLR4-MD2 complex, thus representing an endogenous ligand of TLR4. Therefore Mrp8-Mrp14 complexes are new inflammatory components that amplify phagocyte activation during sepsis upstream of TNFalpha-dependent effects.     

Class B scavenger receptor types I and II and CD36 targeting improves sepsis survival and acute outcomes in mice

Class B scavenger receptors (SR-Bs), such as SR-BI/II or CD36, bind lipoproteins but also mediate bacterial recognition and phagocytosis. In evaluating whether blocking receptors can prevent intracellular bacterial proliferation, phagocyte cytotoxicity, and proinflammatory signaling in bacterial infection/sepsis, we found that SR-BI/II- or CD36-deficient phagocytes are characterized by a reduced intracellular bacterial survival and a lower cytokine response and were protected from bacterial cytotoxicity in the presence of antibiotics. Mice deficient in either SR-BI/II or CD36 are protected from antibiotic-treated cecal ligation and puncture (CLP)-induced sepsis, with greatly increased peritoneal granulocytic phagocyte survival (8-fold), a drastic diminution in peritoneal bacteria counts, and a 50-70% reduction in systemic inflammation (serum levels of IL-6, TNF-α, and IL-10) and organ damage relative to CLP in wild-type mice. The survival rate of CD36-deficient mice after CLP was 58% compared with 17% in control mice. When compensated for mineralocorticoid and glucocorticoid deficiency, SR-BI/II-deficient mice had nearly a 50% survival rate versus 5% in mineralo-/glucocorticoid-treated controls. Targeting SR-B receptors with L-37pA, a peptide that functions as an antagonist of SR-BI/II and CD36 receptors, also increased peritoneal granulocyte counts, as well as reduced peritoneal bacteria and bacterium-induced cytokine secretion. In the CLP mouse sepsis model, L-37pA improved survival from 6 to 27%, reduced multiple organ damage, and improved kidney function. These results demonstrate that the reduction of both SR-BI/II- and CD36-dependent bacterial invasion and inflammatory response in the presence of antibiotic treatment results in granulocyte survival and local bacterial containment, as well as reduces systemic inflammation and organ damage and improves animal survival during severe infections.     

Oxidized phospholipid-induced inflammation is mediated by Toll-like receptor 2

Oxidative tissue damage is a hallmark of many chronic inflammatory diseases. However, the precise mechanisms linking oxidative changes to inflammatory reactions remain unclear. Herein we show that Toll-like receptor 2 (TLR2) translates oxidative tissue damage into inflammatory responses by mediating the effects of oxidized phospholipids. Intraperitoneal injection of oxidized 1-palmitoyl-2-arachidonyl-sn-3-glycerophosphorylcholine (OxPAPC) resulted in upregulation of inflammatory genes in wild-type, but not in TLR2(-/-) mice. In vitro, OxPAPC induced TLR2 (but not TLR4)-dependent inflammatory gene expression and JNK and p38 signaling in macrophages. Induction of TLR2-dependent gene expression required reducible functional groups on sn-2 acyl chains of oxidized phospholipids, as well as serum cofactors. Finally, TLR2(-/-) mice were protected against carbon tetrachloride-induced oxidative tissue damage and inflammation, which was accompanied by accumulation of oxidized phospholipids in livers. Together, our findings demonstrate that TLR2 mediates cellular responses to oxidative tissue damage and they provide new insights into how oxidative stress is linked to acute and chronic inflammation.     

Oxidized phospholipids mediate occludin expression and phosphorylation in vascular endothelial cells

Oxidized l-alpha-1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (OxPAPC), a component of minimally modified LDL, induces production of proinflammatory cytokines and development of atherosclerotic lesions. We tested the hypothesis that OxPAPC alters expression, phosphorylation, and localization of tight junction (TJ) proteins, particularly occludin, a transmembrane TJ protein. OxPAPC reduced total occludin protein and increased occludin phosphorylation dose dependently (10-50 microg/ml) and time dependently in bovine aortic endothelial cells. OxPAPC decreased occludin mRNA and reduced the immunoreactivity of zonula occludens-1 at the cell-cell contacts. Furthermore, OxPAPC increased the diffusive flux of 10-kDa dextran in a dose-dependent manner. O2-* production by bovine aortic endothelial cells increased nearly twofold after exposure to OxPAPC. Also, enzymatic generation of O2-* by xanthine oxidase-lumazine and H2O2 by glucose oxidase-glucose increased occludin phosphorylation, implicating reactive oxygen species as modulators of the OxPAPC effects on occludin phosphorylation. Superoxide dismutase and/or catalase blocked the effects of OxPAPC on occludin protein content and phosphorylation, occludin mRNA, zonula occludens-1 immunoreactivity, and diffusive flux of 10-kDa dextran. These findings suggest that changes in TJ proteins are potential mechanisms by which OxPAPC compromises the barrier properties of the vascular endothelium. OxPAPC-induced disruption of TJs, which likely facilitates transmigration of LDL and inflammatory cells into the subendothelial layers, may be mediated by reactive oxygen species.     

Reduced adrenal response and increased mortality after systemic Klebsiella pneumoniae infection in interleukin-6-deficient mice.

During bacterial infections, both the immune system and the hypothalamus-pituitary-adrenal (HPA) axis are activated. The role of IL-6 in the activation of the HPA axis during bacterial sepsis is not fully understood. The aim of the present study was to investigate the role of endogenous IL-6 in a potentially lethal infection with Klebsiella pneumoniae and the concomitant activation of the HPA axis. We examined the mortality of IL-6-/- and IL-6+/+ mice after intravenous (i.v.) infection with K. pneumoniae as well as the bacterial outgrowth in several organs. Subsequently, the influence of endogenous IL-6 on the effect of i.v. administration of K. pneumoniae on the plasma levels of corticosterone and the pro-inflammatory cytokines TNF-alpha and IL-1alpha was investigated in these mice. The present study demonstrates that IL-6-/- mice are more susceptible than IL-6+/+ mice to a systemic Gram-negative infection with K. pneumoniae, leading to increased outgrowth of microorganisms in the organs of the mice. Moreover, this infection is associated with a reduced adrenal response in IL-6-/- mice. We conclude that IL-6-/- mice are more susceptible to Gram-negative bacterial infections, which is mainly due to an impaired recruitment of granulocytes to the site of infection in the absence of IL-6. Furthermore, the reduced adrenal response may be an explanation for the strong inflammatory response with higher TNF-alpha plasma levels in IL-6-/- mice.     

IL-10-deficient mice demonstrate multiple organ failure and increased mortality during Escherichia coli peritonitis despite an accelerated bacterial clearance

To determine the role of endogenous IL-10 in local antibacterial host defense and in the development of a systemic inflammatory response syndrome during abdominal sepsis, IL-10 gene-deficient (IL-10(-/-)) and wild-type (IL-10(+/+)) mice received an i.p. injection with Escherichia coli. Peritonitis was associated with a bacterial dose-dependent increase in IL-10 concentrations in peritoneal fluid and plasma. The recovery of E. coli from the peritoneal fluid, blood, and lungs was diminished in IL-10(-/-) mice, indicating that endogenous IL-10 impaired bacterial clearance. Despite a lower bacterial load, IL-10(-/-) mice had higher concentrations of TNF, macrophage inflammatory protein-2 and keratinocyte in peritoneal fluid and plasma, and demonstrated more severe multiple organ damage as indicated by clinical chemistry and histopathology. Furthermore, IL-10(-/-) mice showed an increased neutrophil recruitment to the peritoneal cavity. To examine the role of elevated TNF levels in the altered host response in IL-10(-/-) mice, the effect of a neutralizing anti-TNF mAb was determined. Anti-TNF did not influence the clearance of E. coli in either IL-10(+/+) or IL-10(-/-) mice. Furthermore, anti-TNF did not affect leukocyte influx in the peritoneal fluid, multiple organ damage, or survival in IL-10(+/+) mice. In IL-10(-/-) mice, anti-TNF partially attenuated neutrophil recruitment and multiple organ damage, and prevented the increased lethality. These data suggest that although endogenous IL-10 facilitates the outgrowth and dissemination of bacteria during E. coli peritonitis, it protects mice from lethality by attenuating the development of a systemic inflammatory response syndrome by a mechanism that involves inhibition of TNF release.     

Ecto-5'-nucleotidase (CD73) decreases mortality and organ injury in sepsis

The extracellular concentrations of adenosine are increased during sepsis, and adenosine receptors regulate the host's response to sepsis. In this study, we investigated the role of the adenosine-generating ectoenzyme, ecto-5'-nucleotidase (CD73), in regulating immune and organ function during sepsis. Polymicrobial sepsis was induced by subjecting CD73 knockout (KO) and wild type (WT) mice to cecal ligation and puncture. CD73 KO mice showed increased mortality in comparison with WT mice, which was associated with increased bacterial counts and elevated inflammatory cytokine and chemokine concentrations in the blood and peritoneum. CD73 deficiency promoted lung injury, as indicated by increased myeloperoxidase activity and neutrophil infiltration, and elevated pulmonary cytokine levels. CD73 KO mice had increased apoptosis in the thymus, as evidenced by increased cleavage of caspase-3 and poly(ADP-ribose) polymerase and increased activation of NF-κB. Septic CD73 KO mice had higher blood urea nitrogen levels and increased cytokine levels in the kidney, indicating increased renal dysfunction. The increased kidney injury of CD73 KO mice was associated with augmented activation of p38 MAPK and decreased phosphorylation of Akt. Pharmacological inactivation of CD73 in WT mice using α, β-methylene ADP augmented cytokine levels in the blood and peritoneal lavage fluid. These findings suggest that CD73-derived adenosine may be beneficial in sepsis.     

Bacterial peptidoglycan-associated lipoprotein is released into the bloodstream in gram-negative sepsis and causes inflammation and death in mice

Gram-negative bacterial sepsis commonly causes organ dysfunction and death in humans. Although circulating bacterial toxins trigger inflammation in sepsis, little is known about the composition of bacterial products released into the blood during sepsis or the contribution of various bacterial components to the pathogenesis of sepsis. We have shown that diverse Gram-negative bacteria release bacterial peptidoglycan-associated lipoprotein (PAL) into serum. The present studies explored release of PAL into the blood during sepsis and tested the hypothesis that PAL contributes to bacterial virulence and inflammation in Gram-negative sepsis. Released PAL was detected in the blood of 94% of mice following cecal ligation and puncture. Picomolar to nanomolar levels of PAL stimulated macrophages and splenocytes from lipopolysaccharide-hyporesponsive (C3H/HeJ) mice. Injection of PAL into C3H/HeJ mice stimulated production of serum cytokines and increased pulmonary and myocardial expression of inflammatory markers. PAL caused death in sensitized C3H/HeJ mice. Mutant Escherichia coli bacteria with reduced levels of PAL or truncated PAL were less virulent than wild-type bacteria, as indicated by higher survival rates and lower circulating levels of interleukin 6 and bacteria in a model of peritonitis in lipopolysaccharide-responsive mice. The studies suggest that PAL may be an important bacterial mediator of Gram-negative sepsis.     

Regulation of Ca(2+)/calmodulin-dependent protein kinase phosphatase (CaMKP) by oxidation/reduction at Cys-359

Oxidized phospholipids are known to be key signaling molecules in the onset of several diseases involving inflammation. The aim of this study was to evaluate the effect of oxidized phosphatidylserines (oxPS) in modulating the immune system, through cytokine production. Flow cytometry analysis was used to evaluate the oxPS capacity to induce the expression of different cytokines by monocytes, myeloid dendritic cells (mDCs) and DCs CD14(-/low)CD16(+). oxPS were formed during oxidation induced by the hydroxyl radical. Among the four families of oxPS studied, only oxPS modified in the polar head with formation of a terminal hydroperoxyacetaldehyde upregulated the production of cytokines IL-8 and TNF-α by monocytes and DCs subsets (mDCs and CD14(-/low)CD16(+) DCs). This family of oxPS showed the capacity to upregulate the production of IL-1β, IL-6, and MIP-1β from the same type of cells. A significant raise in the percentage of monocytes and dendritic cells producing the studied cytokines was observed, when compared with basal control. Oxidation products modified in the fatty acyl chain did not upregulate TNF and IL-8. oxPS with terminal hydroperoxyacetaldehyde has pro-inflammatory properties. This outcome may help to understand the biological role of phosphatidylserine oxidation products in inflammatory processes and in dysfunctions of immune system.     

Amelioration of sepsis by inhibiting sialidase-mediated disruption of the CD24-SiglecG interaction

Suppression of inflammation is critical for effective therapy of many infectious diseases. However, the high rates of mortality caused by sepsis attest to the need to better understand the basis of the inflammatory sequelae of sepsis and to develop new options for its treatment. In mice, inflammatory responses to host danger-associated molecular patterns (DAMPs), but not to microbial pathogen-associated molecular patterns (PAMPs), are repressed by the interaction [corrected] of CD24 and SiglecG (SIGLEC10 in human). Here we use an intestinal perforation model of sepsis to show that microbial sialidases target the sialic acid-based recognition of CD24 by SiglecG/10 to exacerbate inflammation. Sialidase inhibitors protect mice against sepsis by a mechanism involving both CD24 and Siglecg, whereas mutation of either gene exacerbates sepsis. Analysis of sialidase-deficient bacterial mutants confirms the key contribution of disrupting sialic acid-based pattern recognition to microbial virulence and supports the clinical potential of sialidase inhibition for dampening inflammation caused by infection.     

Distinct different contributions of the alternative and classical complement activation pathway for the innate host response during sepsis

Complement activation represents a crucial innate defense mechanism to invading microorganisms, but there is an eminent lack of understanding of the separate contribution of the different complement activation pathways to the host response during sepsis. We therefore investigated different innate host immune responses during cecal ligation and puncture (CLP)-induced sepsis in mice lacking either the alternative (fD(-/-)) or classical (C1q(-/-)) complement activation pathway. Both knockout mice strains showed a significantly reduced survival and increased organ dysfunction when compared with control mice. Surprisingly, fD(-/-) mice demonstrated a compensated bacterial clearance capacity as control mice at 6 h post CLP, whereas C1q(-/-) mice were already overwhelmed by bacterial growth at this time point. Interestingly, at 24 h after CLP, fD(-/-) mice failed to clear bacteria in a way comparable to control mice. However, both knockout mice strains showed compromised C3 cleavage during sepsis. Investigating potential causes for this discrepancy, we were able to demonstrate that despite normal bacterial clearance capacity early during the onset of sepsis, fD(-/-) mice displayed increased inflammatory cytokine generation and neutrophil recruitment into lungs and blood when compared with both control- and C1q(-/-) mice, indicating a potential loss of control over these immune responses. Further in vitro experiments revealed a strongly increased Nf-κB activation capacity in isolated neutrophils from fD(-/-) mice, supporting this hypothesis. Our results provide evidence for the new concept that the alternative complement activation pathway exerts a distinctly different contribution to the innate host response during sepsis when compared with the classical pathway.     

MyD88 is pivotal for the early inflammatory response and subsequent bacterial clearance and survival in a mouse model of Chlamydia pneumoniae pneumonia

Chlamydia pneumoniae is the causative agent of respiratory tract infections and a number of chronic diseases. Here we investigated the involvement of the common TLR adaptor molecule MyD88 in host responses to C. pneumoniae-induced pneumonia in mice. MyD88-deficient mice were severely impaired in their ability to mount an acute early inflammatory response toward C. pneumoniae. Although the bacterial burden in the lungs was comparable 5 days after infection, MyD88-deficient mice exhibited only minor signs of pneumonia and reduced expression of inflammatory mediators. MyD88-deficient mice were unable to up-regulate proinflammatory cytokines and chemokines, demonstrated delayed recruitment of CD8+ and CD4+ T cells to the lungs, and were unable to clear the pathogen from their lungs at day 14. At day 14 the MyD88-deficent mice developed a severe, chronic lung inflammation with elevated IL-1beta and IFN-gamma leading to increased mortality, whereas wild-type mice as well as TLR2- or TLR4-deficient mice recovered from acute pneumonia and did not show delayed bacterial clearance. Thus, MyD88 is essential to recognize C. pneumoniae infection and initiate a prompt and effective immune host response against this organism leading to clearance of bacteria from infected lungs.     

Divergent response to LPS and bacteria in CD14-deficient murine macrophages

Gram-negative bacteria and the LPS constituent of their outer membranes stimulate the release of inflammatory mediators believed to be responsible for the clinical manifestations of septic shock. The GPI-linked membrane protein, CD14, initiates the signaling cascade responsible for the induction of this inflammatory response by LPS. In this paper, we report the generation and characterization of CD14-null mice in which the entire coding region of CD14 was deleted. As expected, LPS failed to elicit TNF-alpha and IL-6 production in macrophages taken from these animals, and this loss in responsiveness is associated with impaired activation of both the NF-kappaB and the c-Jun N-terminal mitogen-activated protein kinase pathways. The binding and uptake of heat-killed Escherichia coli, measured by FACS analysis, did not differ between CD14-null and wild-type macrophages. However, in contrast to the findings with LPS, whole E. coli stimulated similar levels of TNF-alpha release from CD14-null and wild-type macrophages at a dose of 10 bioparticles per cell. This effect was dose dependent, and at lower bacterial concentrations CD14-deficient macrophages produced significantly less TNF-alpha than wild type. Approximately half of this CD14-independent response appeared to be mediated by CD11b/CD18, as demonstrated by receptor blockade using neutrophil inhibitory factor. An inhibitor of phagocytosis, cytochalasin B, abrogated the induction of TNF-alpha in CD14-deficient macrophages by E. coli. These data indicate that CD14 is essential for macrophage responses to free LPS, whereas other receptors, including CD11b/CD18, can compensate for the loss of CD14 in response to whole bacteria.     

CD16 promotes Escherichia coli sepsis through an FcR gamma inhibitory pathway that prevents phagocytosis and facilitates inflammation

Sepsis, a leading cause of death worldwide, involves proinflammatory responses and inefficient bacterial clearance. Phagocytic cells play a crucial part in the prevention of sepsis by clearing bacteria through host innate receptors. Here we show that the FcRgamma adaptor, an immunoreceptor tyrosine-based activation motif (ITAM)-bearing signal transduction subunit of the Fc receptor family, has a deleterious effect on sepsis. FcRgamma(-/-) mice show increased survival during peritonitis, owing to markedly increased E. coli phagocytosis and killing and to lower production of the proinflammatory cytokine tumor necrosis factor (TNF)-alpha. The FcRgamma-associated receptor that inhibits E. coli phagocytosis is FcgammaRIII (also called CD16), and its absence protects mice from sepsis. FcgammaRIII binds E. coli, and this interaction induces FcRgamma phosphorylation, recruitment of the tyrosine phosphatase SHP-1 and phosphatidylinositide-3 kinase (PI3K) dephosphorylation. Decreased PI3K activity inhibits E. coli phagocytosis and increases TNF-alpha production through Toll-like receptor 4. We identified the phagocytic receptor negatively regulated by FcRgamma on macrophages as the class A scavenger receptor MARCO. E. coli-FcgammaRIII interaction induces the recruitment of SHP-1 to MARCO, thereby inhibiting E. coli phagocytosis. Thus, by binding FcgammaRIII, E. coli triggers an inhibitory FcRgamma pathway that both impairs MARCO-mediated bacterial clearance and activates TNF-alpha secretion.     

C1 inhibitor-mediated protection from sepsis

C1 inhibitor (C1INH) protects mice from lethal Gram-negative bacterial LPS-induced endotoxin shock and blocks the binding of LPS to the murine macrophage cell line, RAW 264.7, via an interaction with lipid A. Using the cecal ligation and puncture (CLP) model for sepsis in mice, treatment with C1INH improved survival in comparison with untreated controls. The effect was not solely the result of inhibition of complement and contact system activation because reactive center-cleaved, inactive C1INH (iC1INH) also was effective. In vivo, C1INH and iC1INH both reduced the number of viable bacteria in the blood and peritoneal fluid and accelerated killing of bacteria by blood neutrophils and peritoneal macrophages. In vitro, C1INH bound to bacteria cultured from blood or peritoneal fluid of mice with CLP-induced sepsis, but had no direct effect on bacterial growth. However, both C1INH and iC1INH enhanced the bactericidal activity of blood neutrophils and peritoneal exudate leukocytes. C1INH-deficient mice (C1INH-/- mice) subjected to CLP had a higher mortality than did wild-type littermate mice. Survival of C1INH-/- mice was significantly increased with two doses of C1INH, one given immediately following CLP, and the second at 6 h post-CLP. C1INH may be important in protection from sepsis through enhancement of bacterial uptake by, and/or bactericidal capacity of, phagocytes. Treatment with C1INH may provide a useful additional therapeutic approach in some patients with peritonitis and/or sepsis.