Human C1-Inhibitor Suppresses Malaria Parasite Invasion and Cytoadhesion via Binding to Parasite Glycosylphosphatidylinositol and Host Cell Receptors.

Plasmodium falciparum-induced severe malaria remains a continuing problem in areas of endemicity, with elevated morbidity and mortality. Drugs targeting mechanisms involved in severe malaria pathology, including cytoadhesion of infected red blood cells (RBCs) to host receptors and production of proinflammatory cytokines, are still necessary. Human C1-inhibitor (C1INH) is a multifunctional protease inhibitor that regulates coagulation, vascular permeability, and inflammation, with beneficial effects in inflammatory disease models, including septic shock. We found that human C1INH, at therapeutically relevant doses, blocks severe malaria pathogenic processes by 2 distinct mechanisms. First, C1INH bound to glycan moieties within P. falciparum glycosylphosphatidylinositol (PfGPI) molecules on the parasite surface, inhibiting parasite RBC invasion and proinflammatory cytokine production by parasite-stimulated monocytes in vitro and reducing parasitemia in a rodent model of experimental cerebral malaria (ECM) in vivo. Second, C1INH bound to host CD36 and chondroitin sulfate A molecules, interfering with cytoadhesion of infected RBCs by competitive binding to these receptors in vitro and reducing sequestration in specific tissues and protecting against ECM in vivo. This study reveals that C1INH is a potential therapeutic antimalarial molecule able to interfere with severe-disease etiology at multiple levels through specific interactions with both parasite PfGPIs and host cell receptors.

The role of the complement and contact systems in the dextran sulfate sodium-induced colitis model: the effect of C1 inhibitor in inflammatory bowel disease.

The complement and contact systems may be involved in the pathophysiological process of inflammatory bowel disease (IBD). C1 inhibitor (C1INH) is the most important inhibitor of both the complement and contact systems. We evaluated the role of these systems and the effect of both active and inactive forms of C1INH (iC1INH) in dextran sulfate sodium (DSS)-induced colitis mouse model. Three percent DSS was used in drinking water to induce colitis in complement C3-deficient (C3(-/-)) mice, bradykinin type 2 receptor deficient (Bk(2)R(-/-)) mice, and C57BL/6 mice. After ten days DSS exposure, C3(-/-) mice exhibited markedly less weight loss than wild-type (WT) mice (12 +/- 3.3% vs. 30 +/- 1.2%, P < 0.05) and developed a milder disease-activity index (DAI), histological score, colon shortening, and myeloperoxidase (MPO) elevation (P < 0.05, respectively). The Bk(2)R(-/-) mice were not protected from the disease. Seven-day treatment with either native C1INH or iC1INH reduced the severity of the disease in WT mice, as indicated by decreased weight loss (15 +/- 1.8%, 14 +/- 2.1% vs. 30 +/- 1.2%, P < 0.05, respectively), DAI, intestinal tissue damage, and MPO elevation compared with untreated WT DSS control mice (P < 0.05, respectively). These findings suggest that complement plays a role in the development of DSS-induced colitis and that blockade of the complement system might be useful for the acute phase of IBD treatment. C1INH, however, leads to an amelioration of DSS-induced colitis via a mechanism that does not involve the inhibition of complement or contact system activation but does result in significant suppression of leukocyte infiltration.

Biological activities of C1 inhibitor.

Broadly speaking, C1 inhibitor plays important roles in the regulation of vascular permeability and in the suppression of inflammation. Vascular permeability control is exerted largely through inhibition of two of the proteases involved in the generation of bradykinin, factor XIIa and plasma kallikrein (the plasma kallikrein-kinin system). Anti-inflammatory functions, however, are exerted via several activities including inhibition of complement system proteases (C1r, C1s, MASP2) and the plasma kallikrein-kinin system proteases, in addition to interactions with a number of different proteins, cells and infectious agents. These more recently described, as yet incompletely characterized, activities serve several potential functions, including concentration of C1 inhibitor at sites of inflammation, inhibition of alternative complement pathway activation, inhibition of the biologic activities of gram negative endotoxin, enhancement of bacterial phagocytosis and killing, and suppression of the influx of leukocytes into a site of inflammation. C1 inhibitor has been shown to be therapeutically useful in a variety of animal models of inflammatory diseases, including gram negative bacterial sepsis and endotoxin shock, suppression of hyperacute transplant rejection, and treatment of a variety of ischemia-reperfusion injuries (heart, intestine, skeletal muscle, liver, brain). In humans, early data appear particularly promising in myocardial reperfusion injury. The mechanism (or mechanisms) of the effect of C1 inhibitor in these conditions is (are) not completely clear, but involve inhibition of complement and contact system activation, in addition to variable contributions from other C1 inhibitor activities that do not involve protease inhibition.

New treatments addressing the pathophysiology of hereditary angioedema.

Hereditary angioedema is a serious medical condition caused by a deficiency of C1-inhibitor. The condition is the result of a defect in the gene controlling the synthesis of C1-inhibitor, which regulates the activity of a number of plasma cascade systems. Although the prevalence of hereditary angioedema is low - between 1:10,000 to 1:50,000 - the condition can result in considerable pain, debilitation, reduced quality of life, and even death in those afflicted. Hereditary angioedema presents clinically as cutaneous swelling of the extremities, face, genitals, and trunk, or painful swelling of the gastrointestinal mucosa. Angioedema of the upper airways is extremely serious and has resulted in death by asphyxiation.Subnormal levels of C1-inhibitor are associated with the inappropriate activation of a number of pathways - including, in particular, the complement and contact systems, and to some extent, the fibrinolysis and coagulation systems.Current findings indicate bradykinin, a product of contact system activation, as the primary mediator of angioedema in patients with C1-inhibitor deficiency. However, other systems may play a role in bradykinin's rapid and excessive generation by depleting available levels of C1-inhibitor.There are currently no effective therapies in the United States to treat acute attacks of hereditary angioedema, and currently available agents used to treat hereditary angioedema prophylactically are suboptimal. Five new agents are, however, in Phase III development. Three of these agents replace C1-inhibitor, directly addressing the underlying cause of hereditary angioedema and re-establishing regulatory control of all pathways and proteases involved in its pathogenesis. These agents include a nano-filtered C1-inhibitor replacement therapy, a pasteurized C1-inhibitor, and a recombinant C1-inhibitor isolated from the milk of transgenic rabbits. All C1-inhibitors are being investigated for acute angioedema attacks; the nano-filtered C1-inhibitor is also being investigated for prophylaxis of attacks. The other two agents, a kallikrein inhibitor and a bradykinin receptor-2 antagonist, target contact system components that are mediators of vascular permeability. These mediators are formed by contact system activation as a result of C1-inhibitor consumption.

Increased vascular permeability in C1 inhibitor-deficient mice mediated by the bradykinin type 2 receptor.

Heterozygosity for C1 inhibitor (C1INH) deficiency results in hereditary angioedema. Disruption of the C1INH gene by gene trapping enabled the generation of homozygous- and heterozygous-deficient mice. Mating of heterozygous-deficient mice resulted in the expected 1:2:1 ratio of wild-type, heterozygous, and homozygous-deficient offspring. C1INH-deficient mice showed no obvious phenotypic abnormality. However, following injection with Evans blue dye, both homozygous and heterozygous C1INH-deficient mice revealed increased vascular permeability in comparison with wild-type littermates. This increased vascular permeability was reversed by treatment with intravenous human C1INH, with a Kunitz domain plasma kallikrein inhibitor (DX88), and with a bradykinin type 2 receptor (Bk2R) antagonist (Hoe140). In addition, treatment of the C1INH-deficient mice with an angiotensin-converting enzyme inhibitor (captopril) increased the vascular permeability. Mice with deficiency of both C1INH and Bk2R demonstrated diminished vascular permeability in comparison with C1INH-deficient, Bk2R-sufficient mice. These data support the hypothesis that angioedema is mediated by bradykinin via Bk2R.

C1 inhibitor: biologic activities that are independent of protease inhibition.

C1 inhibitor therapy improves outcome in several animal models of inflammatory disease. These include sepsis and Gram negative endotoxin shock, vascular leak syndromes, hyperacute transplant rejection, and ischemia-reperfusion injury. Furthermore, some data suggest a beneficial effect in human inflammatory disease. In many inflammatory conditions, complement system activation plays a role in pathogenesis. The contact system also very likely is involved in mediation of damage in inflammatory disease. Therefore, the beneficial effect of C1 inhibitor has been assumed to result from inhibition of one or both of these systems. Over the past several years, several other potential anti-inflammatory effects of C1 inhibitor have been described. These effects do not appear to require protease inhibition and depend on non-covalent interactions with other proteins, cell surfaces or lipids. In the first, C1 inhibitor binds to a variety of extracellular matrix components including type IV collagen, laminin, entactin and fibrinogen. The biologic role of these reactions is unclear, but they may serve to concentrate C1 inhibitor at extravascular inflammatory sites. The second is a non-covalent interaction with C3b that results in inhibition of formation of the alternative pathway C3 convertase, a function analogous to that of factor H. The third is an interaction with E and P selectins on endothelial cells that is mediated by the Lewis(x) tetrasaccharides that are expressed on C1 inhibitor. These interactions result in suppression of leukocyte rolling and transmigration. The fourth interaction is the binding of C1 inhibitor to Gram negative bacterial endotoxin that results in suppression of endotoxin shock by interference with the interaction of endotoxin with its receptor complex on macrophages. Lastly, C1 inhibitor binds directly to Gram negative bacteria, which leads to suppression of the development of sepsis, as demonstrated in the cecal ligation and puncture model. These observations suggest that C1 inhibitor is a multi-faceted anti-inflammatory protein that exerts its effects through a variety of mechanisms including both protease inhibition and several different non-covalent interactions that are unrelated to protease inhibition.

Mechanism of angioedema in first complement component inhibitor deficiency.

Since shortly after the discovery that hereditary angioedema resulted from deficiency of first complement component (C1) inhibitor, the characterization of the mediator of angioedema has been a major goal. However, because C1 inhibitor regulates activation of both the contract and complement systems, identification of the mediator was not immediately accomplished. For a number of years, some studies appeared to indicate involvement of one system, whereas other studies suggested involvement of the other. However, the vast majority of the evidence accumulated over the past years indicates quite clearly that the major mediator is bradykinin. Therefore, unregulated contact system activation is the defect that leads directly to the development of angioedema.

Deficiencies of C1 inhibitor.

Hereditary and acquired deficiencies of the C1 inhibitor result in a single prominent symptom, namely angioedema. Angioedema may involve the skin, the gastrointestinal tract or the upper airway. Genetically determined defects in C1INH cause hereditary angioedema. The defect may be acquired as the result of an auto-antibody to C1INH or be due to the generation of anti-idiotypic antibody to monoclonal immunoglobulins as occurs in various B cell lymphoproliferative diseases. Androgens provide prophylaxis against attacks of angioedema. There is no widely approved treatment for acute attacks of angioedema although several promising drugs are now in the final stages of clinical trials.

Approaches toward reversal of increased vascular permeability in C1 inhibitor deficient mice.

C1 inhibitor (C1INH) deficient mice have increased vascular permeability that can be demonstrated by the extravasation of Evans Blue dye. This vascular leak is reversed with protease inhibitors, such as C1INH itself, DX88 (a recombinant variant Kunitz domain plasma kallikrein inhibitor), and the bradykinin receptor type 2 antagonist, Hoe140. The studies described here were undertaken for the following reasons: (1) To provide a more quantitative analysis of the effects of these interventions; (2) to provide data to further test the hypothesis that increased vascular permeability results from contact system activation with kallikrein-mediated release of bradykinin; (3) to test the hypothesis that the amino terminal non-serpin domain of C1INH modulates access to complex proteases, such as kallikrein complexed with high molecular weight kininogen (HK); and (4) to determine whether attenuated androgens or estrogens exert a direct effect on C1INH synthesis. To characterize the differences in these reagents, the dose-response and the rate of reappearance of increased vascular permeability in C1INH(-/-) mice were determined for the following agents: human plasma-derived C1INH, a recombinant Kunitz domain plasma kallikrein inhibitor (DX88), a bradykinin receptor antagonist (Hoe140), and a recombinant C1INH with an amino terminal truncation at amino acid 98 and substitution of the P2 Ala with a Val (Cserp98,A443V). C1INH and Cserp98,A443V were equivalent in activity, which provides further support for the hypothesis that the vascular leak is mediated by bradykinin and suggests that the amino terminal domain neither enhances nor interferes with access to kallikrein within the kallikrein-HK complex. DX88 was effective at very low doses, as was Hoe140. The duration of action of Hoe140 was quite prolonged. The data indicate that, in the mouse, neither danazol nor estrogens have a significant effect on C1INH synthesis.

Biological effects of C1 inhibitor.

C1 inhibitor is a serine proteinase inhibitor (serpin) that regulates activation of both the complement and contact systems. Regulation of complement system activation takes place through inactivation of the classical pathway proteases, C1r and C1s, the lectin pathway protease, MASP2, and perhaps via inhibition of alternative pathway activation by reversible binding to C3b. Regulation of contact system activation takes place through inactivation of plasma kallikrein and coagulation factor XIIa. Deficiency of C1 inhibitor results in hereditary angioedema, which is characterized by recurrent episodes of localized angioedema of the skin, gastrointestinal mucosa or upper respiratory mucosa. A variety of clinical, in vitro and animal experiments indicate that the mediator of increased vascular permeability in hereditary angioedema is bradykinin. Animal models suggest that in addition to its utility in therapy of hereditary angioedema, C1 inhibitor may prove useful in a variety of other diseases including septic shock, reperfusion injury, hyperacute transplant rejection, traumatic and hemorrhagic shock, and the increased vascular permeability associated with thermal injury, interleukin-2 therapy and cardiopulmonary bypass. The therapeutic effect in these disease models very likely results from a combination of complement system activation, contact system activation and perhaps from other activities of C1 inhibitor. These other activities include a direct interaction with endotoxin, which may help to prevent endotoxic shock and an interaction with selectin molecules on endothelial cells, which may serve both to concentrate C1 inhibitor at sites of inflammation and to inhibit the transmigration of leukocytes across the endothelium.

The pathogenesis of hereditary angioedema.

Hereditary angioedema (HAE), which is characterized by episodic localized angioedema of the skin or mucosa, results from heterozygous deficiency of the plasma protease inhibitor, C1 inhibitor (C1INH). The most obvious biologic role of C1INH, therefore, is prevention of excessive vascular permeability. A variety of data indicate that this role is primarily a product of regulation of the contact system proteases, factor XIIa and plasma kallikrein. The C1INH deficient mouse, although it does not have episodes of cutaneous angioedema, does have increased vascular permeability which is reversed by treatment with C1INH, with the plasma kallikrein inhibitor, DX88, and with the bradykinin 2 receptor (Bk2R) antagonist, Hoe140. In addition, mice deficient in both C1INH and the Bk2R do not have increased vascular permeability. These analyses strengthen the argument that angioedema is mediated by bradykinin. This mouse also provides a system to test new potential therapeutic approaches. In addition to its role in the regulation of vascular permeability, C1INH also is an important modulator of inflammatory responses via regulation of activation of both the contact and the complement systems, and very likely via activities unrelated to protease inhibition.

Estrogen-dependent inherited angioedema.

Classic forms of hereditary angioedema are characterized clinically by recurrent episodes of angioedema, biochemically by reduced C1 inhibitor level and/or function, and genetically by a heterogeneous group of mutations in the C1 inhibitor gene that have an autosomal dominant mode of transmission. Androgens and estrogens have significant clinical effects in patients with hereditary angioedema, and tend to have antagonist effects of the levels of C1 inhibitor protein. Androgens increase the levels of C1 inhibitor protein, reduce attacks of angioedema, and thus are an important therapy for patients. The mechanisms by which the sex steroid hormones achieve these effects are not understood. The recent recognition of a novel estrogen-dependent form of angioedema may offer important insights into the mechanisms by which the sex hormones exert their effects, and the pathogenesis and treatment of both estrogen-dependent and classic forms of hereditary angioedema.

Normal transcription of the C1 inhibitor gene is dependent upon a polypurine-polypyrimidine region within the promoter.

Analysis of the transcriptional activity of C1 inhibitor (CIINH) promoter reporter constructs with mutations in the R-Y region indicate that triplex formation by this region is not a predictor of transcriptional activity and that normal promoter function depends on the interaction of trans acting factors with specific elements within this region. Electrophoretic mobility shift assay (EMSA) of Hep3B nuclear extracts using the wild type promoter probe (nucleotides -98 to -9) yielded four major bands. Incubation of the same extracts with probes lacking the HNF-1 site resulted in the disappearance of one band. Supershift assays indicate that HNF-1alpha is the only previously identified protein that is present in the EMSA bands. Southwestern blot analysis detected four bands (M(r)s -130, 75, 65 and 20 kDa). These data suggest that the -98 to -9 region of the C1INH promoter interacts with at least four proteins, one of which is HNF-1alpha.

The effect of C1 inhibitor on myocardial ischemia and reperfusion injury.

BACKGROUND: Activation of the complement system has been demonstrated to be an important mechanism in the mediation of myocardial ischemia and reperfusion (MIR) injury. C1 inhibitor (C1INH) has been shown to be beneficial in experimental MIR models. The underlying mechanism of this effect has been assumed to result primarily from inhibition of complement system activation. We recently demonstrated that C1INH plays a direct role in suppression of leukocyte transmigration in the mouse intestinal ischemia and reperfusion model. The purpose of this study was to investigate the mechanism of the beneficial effect of C1INH in mouse MIR model. METHODS: C57BL/6, C1INH-deficient (C1INH(-/-)), and C3-deficient mice (C3(-/-)) were subjected to 30-min (C57BL/6 and C1INH(-/-)) or 60-min (C3(-/-)) occlusion of the left anterior descending branch of the coronary artery followed by 4-h reperfusion. C1INH or reactive center cleaved inactive C1INH (iC1INH) was injected intravenously 5 min before reperfusion. RESULTS: Myocardial infarct size relative to the area at risk or relative to left ventricular area was significantly reduced in C1INH-treated wild-type, C1INH(-/-), and C3(-/-) mice compared with vehicle-treated mice. MIR induced an increase in myocardial polymorphonuclear neutrophil accumulation and plasma cardiac specific troponin I levels in vehicle-treated MIR mice, while C1INH treatment significantly attenuated these effects. iC1INH had a similar protective effect. CONCLUSIONS: These results suggested that C1INH prevented MIR injury in mice and that this cardioprotective effect may not solely result from complement inhibition, but might be also contributed by inhibiting leukocyte recruitment into ischemic tissue, an effect that is not mediated via protease inhibition.

C1 inhibitor suppresses the endotoxic activity of a wide range of lipopolysaccharides and interacts with live gram-negative bacteria.

Human C1 inhibitor (C1INH) prevents endotoxin shock via a direct interaction with Gram-negative bacterial lipopolysaccharide (LPS) and improves survival in animal models of sepsis. In this report, we further characterize the interaction of C1INH with LPS and whole live bacteria. We investigate C1INH interactions with LPS from five different strains of Gram-negative enteric bacteria known to participate in the pathogenesis of human sepsis. Treatment with C1INH improved survival in mice with endotoxin shock induced by LPS from Salmonella enterica serovar typhimurium as previously shown, as well as LPS from Escherichia coli O55:B5 and Pseudomonas aeruginosa, and a trend to improved survival was observed when Klebsiella pneumoniae and Serratia marcescens LPS were used. Enzyme-linked immunosorbent assay and native polyacrylamide gel electrophoresis shift experiments demonstrated a direct interaction of C1INH with LPS from all the strains studied. The binding of both native and reactive center-cleaved, inactive C1INH results in inhibition of LPS-induced proinflammatory cytokine production. Furthermore, we demonstrate the ability of C1INH to bind at the surface of only a restricted number of whole live Gram-negative bacteria as well as mutant bacteria expressing a truncated LPS lacking the O-antigen. These data reveal the interaction of C1INH with a wide range of enteric bacterial LPS and strongly suggest that the interaction between C1INH and the surface of Gram-negative microorganisms is determined by the length of the polysaccharide chain of the endotoxin molecule.

C1 inhibitor, a multi-functional serine protease inhibitor.

C1 inhibitor (C1INH) is a serpin that regulates both complement and contact (kallikrein-kinin) system activation. It consists of a serpin domain that is highly homologous to other serpins and an amino terminal non-serpin mucin-like domain. Deficiency of C1INH results in hereditary angioedema, a disease characterised by episodes of angioedema of the skin or the mucosa of the gastrointestinal tract or the oropharynx. Although early data suggested that angioedema was mediated via complement system activation, the preponderance of the data indicate that bradykinin is the mediator. In the past few years, it has become apparent that C1INH has additional anti-inflammatory functions independent of protease inhibition. These include interactions with leukocytes that may result in enhanced phagocytosis, with endothelial cells via E- and P-selectins that interfere with leukocyte rolling and in turn results in suppression of transmigration of leukocytes across the endothelium, and interactions with extracellular matrix components that may serve to concentrate C1INH at sites of inflammation. In addition, C1INH suppresses gram negative sepsis and endotoxin shock, partly via direct interaction with endotoxin that interferes with its interaction with macrophages, thereby suppressing tumour necrosis factor-a and other inflammatory mediators. C1INH treatment improves outcome in a number of disease models, including sepsis and other bacterial infections, possibly malaria, ischaemia-reperfusion injury (intestinal, hepatic, muscle, cardiac, brain), hyper-acute transplant rejection, and other inflammatory disease models. Recent data suggest that this effectiveness is the result of mechanisms that do not require protease inhibition, in addition to both complement and contact system activation.

Hereditary angioedema: a current state-of-the-art review, III: mechanisms of hereditary angioedema.

OBJECTIVE: To review the available evidence on the pathophysiologic mechanism of episodes of edema in hereditary angioedema (HAE). DATA SOURCES: MEDLINE and PubMed were searched using the following keywords: hereditary angioedema, C1 inhibitor, complement system, contact system, and bradykinin. STUDY SELECTION: Studies were selected based on their relevance to the pathophysiologic features of HAE. RESULTS: Early studies from the 1970s and 1980s disagreed as to whether the symptoms in HAE were mediated via complement or contact system activation. Studies have demonstrated that, in vitro, in C1 inhibitor (C1-INH)-deficient plasma, only contact system activation results in generation of a vascular permeability enhancing factor. Furthermore, individuals who express a variant C1-INH that is a normal inhibitor of contact system proteases but is deficient in the ability to inactivate complement system proteases do not develop angioedema. The blood of patients with HAE, during attacks, contains elevated levels of cleaved high-molecular-weight kininogen and bradykinin. Last, C1-INH-deficient mice develop increased vascular permeability that is mediated via contact system activation. CONCLUSIONS: Hereditary angioedema attacks are mediated by bradykinin generated via contact system activation. The specific factors that trigger attacks remain unclear.

The effect of C1 inhibitor on intestinal ischemia and reperfusion injury.

Complement activation and neutrophil stimulation are two major components in events leading to ischemia and reperfusion (IR) injury. C1 inhibitor (C1INH) inhibits activation of each of the three pathways of complement activation and of the contact system. It is also endowed with anti-inflammatory properties that are independent of protease inhibition. The goal of these studies was to investigate the role and mechanism of C1INH in alleviating IR-induced intestinal injury. C57BL/6, C1INH-deficient (C1INH(-/-)), bradykinin type 2 receptor-deficient (Bk2R(-/-)), and C3-deficient mice (C3(-/-)) were randomized into three groups: sham operated control, IR, and IR + C1INH-treated groups. Ischemia was generated by occlusion of the superior mesenteric artery followed by reperfusion. C1INH or reactive center-cleaved inactive C1INH (iC1INH) was injected intravenously before reperfusion. IR resulted in intestinal injury in C57BL/6, C1INH(-/-), Bk2R(-/-), and C3(-/-) mice with significantly increased neutrophil infiltration into intestinal tissue. In each mouse strain, C1INH treatment reduced intestinal tissue injury and attenuated leukocyte infiltration compared with the untreated IR group. C1INH inhibited leukocyte rolling in the mesenteric veins of both C57BL/6 and C3-deficient mice subjected to IR. C1INH treatment also improved the survival rate of C57BL/6 and C1INH(-/-) mice following IR. Similar findings were observed in the IR animals treated with iC1INH. These studies emphasize the therapeutic benefit of C1INH in preventing intestinal injury caused by IR. In addition to the protective activities mediated via inhibition of the complement system, these studies indicate that C1INH also plays a direct role in suppression of leukocyte transmigration into reperfused tissue.

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.