Interplay between C1-inhibitor and group IIA secreted phospholipase A2 impairs their respective function.

High levels of human group IIA secreted phospholipase A2 (hGIIA) have been associated with various inflammatory disease conditions. We have recently shown that hGIIA activity and concentration are increased in the plasma of patients with hereditary angioedema due to C1-inhibitor deficiency (C1-INH-HAE) and negatively correlate with C1-INH plasma activity. In this study, we analyzed whether the presence of both hGIIA and C1-INH impairs their respective function on immune cells. hGIIA, but not recombinant and plasma-derived C1-INH, stimulates the production of IL-6, CXCL8, and TNF-α from peripheral blood mononuclear cells (PBMCs). PBMC activation mediated by hGIIA is blocked by RO032107A, a specific hGIIA inhibitor. Interestingly, C1-INH inhibits the hGIIA-induced production of IL-6, TNF-α, and CXCL8, while it does not affect hGIIA enzymatic activity. On the other hand, hGIIA reduces the capacity of C1-INH at inhibiting C1-esterase activity. Spectroscopic and molecular docking studies suggest a possible interaction between hGIIA and C1-INH but further experiments are needed to confirm this hypothesis. Together, these results provide evidence for a new interplay between hGIIA and C1-INH, which may be important in the pathophysiology of hereditary angioedema.

Icatibant use in Brazilian patients with hereditary angioedema (HAE) type 1 or 2 and HAE with normal C1-INH levels: findings from the Icatibant Outcome Survey Registry Study.

BACKGROUND: Hereditary angioedema can be caused by C1-Inhibitor (C1-INH) deficiency and/or dysfunction (HAE-1/2) or can occur in patients with normal C1-INH (HAE nC1-INH). METHODS: The Icatibant Outcome Survey (IOS; NCT01034969) registry monitors the safety and effectiveness of icatibant for treating acute angioedema. OBJECTIVE: Present findings from Brazilian patients with HAE-1/2 and HAE nC1-INH participating in IOS. RESULTS: 42 patients were enrolled (HAE-1/2, n = 26; HAE nC1-INH, n = 16). Median age at symptom onset was significantly lower with HAE-1/2 vs. HAE nC1-INH (10.0 vs. 16.5y, respectively; p = 0.0105), whereas median age at diagnosis (31.1 vs. 40.9y; p = 0.1276) and the median time between symptom onset and diagnosis (15.0 vs. 23.8y; p = 0.6680) were numerically lower vs. HAE nC1-INH, respectively. One icatibant dose was used for > 95% of HAE attacks. Median (range) time-to-event outcomes were shorter for patients with HAE nC1-INH vs. HAE-1/2, including time to first administration (0.5 [0-96.0] vs. 1.0 [0-94.0]h, respectively), time from first administration to complete resolution (1.0 [0-88.0] vs. 5.5 [0-96.0]h, respectively), and total attack duration (7.0 [0.3-99.0] vs. 18.5 [0.1-100.0]h, respectively). Mean (SD) time from attack onset to resolution was significantly shorter for patients with HAE nC1-INH vs. HAE-1/2 (9.8 [18.7] vs. 19.6 [24.0]h, respectively; p = 0.0174). 83 adverse events (AEs) in 42 patients were reported; most were mild (66.3%) or moderate (13.3%) and non-serious (75.9%). The most common icatibant-related AE was injection site erythema (HAE-1/2, 34.6%; HAE nC1-INH, 18.8%). STUDY LIMITATIONS: This was an observational study without a treatment comparator and that relied on patient recall. CONCLUSIONS: Findings demonstrate effectiveness and tolerability of icatibant in Brazilian HAE patients.

Molecular Basis for Bordetella pertussis Interference with Complement, Coagulation, Fibrinolytic, and Contact Activation Systems: the Cryo-EM Structure of the Vag8-C1 Inhibitor Complex.

Complement, contact activation, coagulation, and fibrinolysis are serum protein cascades that need strict regulation to maintain human health. Serum glycoprotein, a C1 inhibitor (C1-INH), is a key regulator (inhibitor) of serine proteases of all the above-mentioned pathways. Recently, an autotransporter protein, virulence-associated gene 8 (Vag8), produced by the whooping cough pathogen, Bordetella pertussis, was shown to bind to C1-INH and interfere with its function. Here, we present the structure of the Vag8-C1-INH complex determined using cryo-electron microscopy at a 3.6-Å resolution. The structure shows a unique mechanism of C1-INH inhibition not employed by other pathogens, where Vag8 sequesters the reactive center loop of C1-INH, preventing its interaction with the target proteases.IMPORTANCE The structure of a 10-kDa protein complex is one of the smallest to be determined using cryo-electron microscopy at high resolution. The structure reveals that C1-INH is sequestered in an inactivated state by burial of the reactive center loop in Vag8. By so doing, the bacterium is able to simultaneously perturb the many pathways regulated by C1-INH. Virulence mechanisms such as the one described here assume more importance given the emerging evidence about dysregulation of contact activation, coagulation, and fibrinolysis leading to COVID-19 pneumonia.

A Novel C1-Esterase Inhibitor Oxygenator Coating Prevents FXII Activation in Human Blood.

The limited hemocompatibility of currently used oxygenator membranes prevents long-term use of artificial lungs in patients with lung failure. To improve hemocompatibility, we developed a novel covalent C1-esterase inhibitor (C1-INH) coating. Besides complement inhibition, C1-INH also prevents FXII activation, a very early event of contact phase activation at the crossroads of coagulation and inflammation. Covalently coated heparin, as the current anticoagulation gold standard, served as control. Additionally, a combination of both coatings (C1-INH/heparin) was established. The coatings were tested for their hemocompatibility by dynamic incubation with freshly drawn human whole blood. The analysis of various blood and plasma parameters revealed that C1-INH-containing coatings were able to markedly reduce FXIIa activity compared to heparin coating. Combined C1-INH/heparin coatings yielded similarly low levels of thrombin-antithrombin III complex formation as heparin coating. In particular, adhesion of monocytes and platelets as well as the diminished formation of fibrin networks were observed for combined coatings. We could show for the first time that a covalent coating with complement inhibitor C1-INH was able to ameliorate hemocompatibility. Thus, the early inhibition of the coagulation cascade is likely to have far-reaching consequences for the other cross-reacting plasma protein pathways.

SERPING1 mutation update: Mutation spectrum and C1 Inhibitor phenotypes.

C1 inhibitor (C1Inh) deficiency is responsible for hereditary angioedema (C1-INH-HAE) and caused by variants of the SERPING1/C1INH/C1NH gene. C1Inh is the major control of kallikrein-kinin system. C1Inh deficiency leads to its uncontrolled activation, with subsequent generation of the vasoactive peptide bradykinin. This update documents 748 different SERPING1 variants, including published variants and additional 120 unpublished ones. They were identified as heterozygous variants (n = 729), as homozygous variants in 10 probands and as compound heterozygous variants (nine combinations). Six probands with heterozygous variants exhibited gonadal mosaicism. Probands with heterozygous (n = 72) and homozygous (n = 1) variants were identified as de novo cases. Overall, 58 variants were found at positions showing high residue conservation among serpins, and have been referred to as a mousetrap function of C1Inh: reactive center loop, gate, shutter, breach, and hinge. C1Inh phenotype analysis identified dysfunctional serpin variants with failed serpin-protease association and a residual 105-kDa species after incubation with target protease. Regarding this characteristic, in conditions with low antigenic C1Inh, 74 C1-INH-HAE probands presented with an additional so-called intermediate C1-INH-HAE phenotype. The present update addresses a comprehensive SERPING1 variant spectrum that facilitates genotype-phenotype correlations, highlighting residues of strategic importance for serpin function and for identification of C1Inh deficiency as serpinopathy.

Low Levels of IgM and IgA Recognizing Acetylated C1-Inhibitor Peptides Are Associated with Systemic Lupus Erythematosus in Taiwanese Women.

The objective of this study was to identify novel acetylation (Ac) modifications of the C1-inhibitor (C1-INH) and explain the association of the levels of autoantibodies against acetylated C1-INH peptides with the risk of developing systemic lupus erythematosus (SLE). Ac modifications of the C1-INH were identified and validated through in-gel digestion, nano-liquid chromatography-tandem mass spectrometry, immunoprecipitation, and Western blotting by using serum protein samples obtained from patients with SLE and age-matched healthy controls (HCs). In addition, the levels of serum C1-INH, Ac-protein adducts, and autoantibodies against unmodified and acetylated C1-INH peptides were measured. C1-INH levels in patients with SLE were significantly lower than those in HCs by 1.53-fold (p = 0.0008); however, Ac-protein adduct concentrations in patients with SLE were significantly higher than those in HCs by 1.35-fold (p = 0.0009). Moreover, immunoglobulin M (IgM) anti-C1-INH367-385 Ac and IgA anti-C1-INH367-385 Ac levels in patients with SLE were significantly lower than those in HCs. The low levels of IgM anti-C1-INH367-385 (odds ratio [OR] = 4.725, p < 0.001), IgM anti-C1-INH367-385 Ac (OR = 4.089, p = 0.001), and IgA anti-C1-INH367-385 Ac (OR = 5.566, p < 0.001) indicated increased risks for the development of SLE compared with HCs.

A mechanism for hereditary angioedema with normal C1 inhibitor: an inhibitory regulatory role for the factor XII heavy chain.

The plasma proteins factor XII (FXII) and prekallikrein (PK) undergo reciprocal activation to the proteases FXIIa and kallikrein by a process that is enhanced by surfaces (contact activation) and regulated by the serpin C1 inhibitor. Kallikrein cleaves high-molecular-weight kininogen (HK), releasing the vasoactive peptide bradykinin. Patients with hereditary angioedema (HAE) experience episodes of soft tissue swelling as a consequence of unregulated kallikrein activity or increased prekallikrein activation. Although most HAE cases are caused by reduced plasma C1-inhibitor activity, HAE has been linked to lysine/arginine substitutions for Thr309 in FXII (FXII-Lys/Arg309). Here, we show that FXII-Lys/Arg309 is susceptible to cleavage after residue 309 by coagulation proteases (thrombin and FXIa), resulting in generation of a truncated form of FXII (δFXII). The catalytic efficiency of δFXII activation by kallikrein is 15-fold greater than for full-length FXII. The enhanced rate of reciprocal activation of PK and δFXII in human plasma and in mice appears to overwhelm the normal inhibitory function of C1 inhibitor, leading to increased HK cleavage. In mice given human FXII-Lys/Arg309, induction of thrombin generation by infusion of tissue factor results in enhanced HK cleavage as a consequence of δFXII formation. The effects of δFXII in vitro and in vivo are reproduced when wild-type FXII is bound by an antibody to the FXII heavy chain (HC; 15H8). The results contribute to our understanding of the predisposition of patients carrying FXII-Lys/Arg309 to angioedema after trauma, and reveal a regulatory function for the FXII HC that normally limits PK activation in plasma.

Transcriptional profiling, molecular cloning, and functional analysis of C1 inhibitor, the main regulator of the complement system in black rockfish, Sebastes schlegelii.

C1-inhibitor (C1inh) plays a crucial role in assuring homeostasis and is the central regulator of the complement activation involved in immunity and inflammation. A C1-inhibitor gene from Sebastes schlegelii was identified and designated as SsC1inh. The identified genomic DNA and cDNA sequences were 6837 bp and 2161 bp, respectively. The genomic DNA possessed 11 exons, interrupted by 10 introns. The amino acid sequence possessed two immunoglobulin-like domains and a serpin domain. Multiple sequence alignment revealed that the serpin domain of SsC1inh was highly conserved among analyzed species where the two immunoglobulin-like domains showed divergence. The distinctiveness of teleost C1inh from other homologs was indicated by the phylogenetic analysis, genomic DNA organization, and their extended N-terminal amino acid sequences. Under normal physiological conditions, SsC1inh mRNA was most expressed in the liver, followed by the gills. The involvement of SsC1inh in homeostasis was demonstrated by modulated transcription profiles in the liver and spleen upon pathogenic stress by different immune stimulants. The protease inhibitory potential of recombinant SsC1inh (rSsC1inh) and the potentiation effect of heparin on rSsC1inh was demonstrated against C1esterase and thrombin. For the first time, the anti-protease activity of the teleost C1inh against its natural substrates C1r and C1s was proved in this study. The protease assay conducted with recombinant black rockfish C1r and C1s proteins in the presence or absence of rSsC1inh showed that the activities of both proteases were significantly diminished by rSsC1inh. Taken together, results from the present study indicate that SsC1inh actively plays a significant role in maintaining homeostasis in the immune system of black rock fish.

Regulation of Complement and Contact System Activation via C1 Inhibitor Potentiation and Factor XIIa Activity Modulation by Sulfated Glycans - Structure-Activity Relationships.

The serpin C1 inhibitor (C1-INH) is the only regulator of classical complement activation as well as the major regulator of the contact system. Its importance is demonstrated by hereditary angioedema (HAE), a severe disease with potentially life-threatening attacks due to deficiency or dysfunction of C1-INH. C1-INH replacement is the therapy of choice in HAE. In addition, C1-INH showed to have beneficial effects in other diseases characterized by inappropriate complement and contact system activation. Due to some limitations of its clinical application, there is a need for improving the efficacy of therapeutically applied C1-INH or to enhance the activity of endogenous C1-INH. Given the known potentiating effect of heparin on C1-INH, sulfated glycans (SG) may be such candidates. The aim of this study was to characterize suitable SG by evaluating structure-activity relationships. For this, more than 40 structurally distinct SG were examined for their effects on C1-INH, C1s and FXIIa. The SG turned out to potentiate the C1s inhibition by C1-INH without any direct influence on C1s. Their potentiating activity proved to depend on their degree of sulfation, molecular mass as well as glycan structure. In contrast, the SG had no effect on the FXIIa inhibition by C1-INH, but structure-dependently modulated the activity of FXIIa. Among the tested SG, ß-1,3-glucan sulfates with a Mr ≤ 10 000 were identified as most promising lead candidates for the development of a glycan-based C1-INH amplifier. In conclusion, the obtained information on structural characteristics of SG favoring C1-INH potentiation represent an useful elementary basis for the development of compounds improving the potency of C1-INH in diseases and clinical situations characterized by inappropriate activation of complement and contact system.

Genetics of Hereditary Angioedema Revisited.

Contemporary genetic research has provided evidences that angioedema represents a diverse family of disorders related to kinin metabolism, with a much greater genetic complexity than was initially considered. Convincing data have also recently been published indicating that the clinical heterogeneity of hereditary angioedema due to C1 inhibitor deficiency (classified as C1-INH-HAE) could be attributed at least in part, either to the type of SERPING1 mutations or to mutations in genes encoding for enzymes involved in the metabolism and function of bradykinin. Alterations detected in at least one more gene (F12) are nowadays considered responsible for 25 % of cases of hereditary angioedema with normal C1-INH (type III hereditary angioedema (HAE), nlC1-INH-HAE). Interesting data derived from genetic approaches of non-hereditary angioedemas indicate that other immune pathways might be implicated in the pathogenesis of HAE. More than 125 years after the recognition of the hereditary nature of HAE by Osler, the heterogeneity of clinical expressions, the genetics of this disorder, and the genotype-phenotype relationships, still presents a challenge that will be discussed in this review. Large scale, in-depth genetic studies are expected not only to answer these emerging questions but also to further elucidate many of the unmet aspects of angioedema pathogenesis. Uncovering genetic biomarkers affecting the severity of the disease and/or the effectiveness of the various treatment modalities might lead to the prevention of attacks and the optimization of C1-INH-HAE management that is expected to provide a valuable benefit to the sufferers of angioedema.

The Alzheimer's disease peptide ß-amyloid promotes thrombin generation through activation of coagulation factor XII.

UNLABELLED: Essentials How the Alzheimer's disease (AD) peptide ß-amyloid (Aß) disrupts neuronal function in the disease is unclear. Factor (F) XII initiates blood clotting via FXI, and thrombosis has been implicated in AD. Aß triggers FXII-dependent FXI and thrombin activation, evidence of which is seen in AD plasma. Aß-triggered clotting could contribute to neuronal dysfunction in AD and be a novel therapeutic target. SUMMARY: Background ß-Amyloid (Aß) is a key pathologic element in Alzheimer's disease (AD), but the mechanisms by which it disrupts neuronal function in vivo are not completely understood. AD is characterized by a prothrombotic state, which could contribute to neuronal dysfunction by affecting cerebral blood flow and inducing inflammation. The plasma protein factor XII triggers clot formation via the intrinsic coagulation cascade, and has been implicated in thrombosis. Objectives To investigate the potential for Aß to contribute to a prothrombotic state. Methods and results We show that Aß activates FXII, resulting in FXI activation and thrombin generation in human plasma, thereby establishing Aß as a possible driver of prothrombotic states. We provide evidence for this process in AD by demonstrating decreased levels of FXI and its inhibitor C1 esterase inhibitor in AD patient plasma, suggesting chronic activation, inhibition and clearance of FXI in AD. Activation of the intrinsic coagulation pathway in AD is further supported by elevated fibrin levels in AD patient plasma. Conclusions The ability of Aß to promote coagulation via the FXII-driven contact system identifies new mechanisms by which it could contribute to neuronal dysfunction and suggests potential new therapeutic targets in AD.

Effects of a plasma-derived C1 esterase inhibitor on hemostatic activation, clot formation, and thrombin generation.

Hereditary angioedema (HAE) is a rare, autosomal dominant disease in which C1 esterase inhibitor (C1 INH) is deficient or dysfunctional. Package inserts for nanofiltered C1 esterase inhibitor (C1 INH-nf) products contain warnings about thrombotic events. The objective of this study was to evaluate the effect of C1 INH-nf on hemostatic activation, clot formation, and thrombin generation. Ten healthy volunteers provided blood samples for thromboelastometry using the ROTEM system. Platelet-poor samples were prepared for thrombin generation studies. C1 INH-nf was added to samples at final concentrations of 0.14, 0.7, 1.4, 2.8, and 7.0 U/ml. Recombinant factor VIIa and prothrombin complex concentrate were used as procoagulant controls, and antithrombin and desirudin were used as anticoagulant controls. C1 INH-nf had no procoagulant effect on hemostasis based on thromboelastometry, regardless of the final concentration or activating reagent used (P > 0.05 for all comparisons of C1 INH-nf versus negative control). C1 INH-nf 2.8 and 7.0 U/ml concentrations had a statistically significant anticoagulant effect on maximum clot firmness (P < 0.05 for all comparisons of C1 INH-nf versus negative control), with a magnitude similar to that observed with desirudin. C1 INH-nf had no effect on thrombin generation lag time, peak thrombin generation, or thrombin generation rate, regardless of the final concentration or activating reagent used (P < 0.05 for all comparisons of C1 INH-nf versus negative controls). We found no evidence of a procoagulant effect of C1 INH-nf when studied ex vivo at concentrations up to 10-fold higher than those achieved with clinical dosing in patients with HAE.

Biochemical comparison of four commercially available C1 esterase inhibitor concentrates for treatment of hereditary angioedema.

BACKGROUND: For safe and efficacious treatment of hereditary angioedema, C1 esterase inhibitor (C1-INH) concentrates should have high purity and high amounts of functional protein. As no pharmacopoeia requirements exist for C1-INH concentrate lot release, biochemical characteristics as declared by the manufacturers may not be compared directly. This study compared the characteristics and purity profiles of four commercially available C1-INH concentrates. STUDY DESIGN AND METHODS: The analysis included one transgenic (Ruconest) and three plasma-derived (Berinert, Cetor, Cinryze) C1-INH concentrates. C1-INH antigen concentration was determined by nephelometry, total protein (specific activity) with a Bradford assay, purity by size-exclusion chromatography and gel electrophoresis, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry was performed. RESULTS: Functionality (inversely proportional to antigen-to-activity ratio) was lowest for Ruconest (1.67), followed by Cetor (1.42), Berinert (1.24), and Cinryze (1.22). Specific activity (U/mg) and purity (%) were highest in Ruconest (12.13; 98.6) and Berinert (11.57; 97.0), followed by Cinryze (10.41; 89.5) and Cetor (9.01; 88.6). Main protein bands were found for all plasma-derived products at approximately 105 kDa, and for Ruconest, at approximately 98 kDa. Additional bands in the plasma-derived products were α1-antichymotrypsin, ceruloplasmin, Factor C3 (Cinryze/Cetor), and immunoglobulin heavy constant mu (Berinert). CONCLUSION: Ruconest has a very high purity profile but is not identical to the human C1-INH protein. Of the plasma-derived products, Berinert has the highest purity profile. The impact of the nontherapeutic proteins identified has not yet been evaluated. For harmonization of the analysis for drug release, we recommend the establishment of regulatory requirements for purity determination and the implementation of threshold levels in C1-INH concentrates.

Pasteurized and nanofiltered, plasma-derived C1 esterase inhibitor concentrate for the treatment of hereditary angioedema.

Hereditary angioedema (HAE) is a relatively rare autosomal dominant disorder that is typically characterized by recurrent episodes of edema in various body locations. It is most commonly caused by an inherited deficiency of functionally active C1 esterase inhibitor (C1-INH). Replacement therapy with a human plasma-derived C1-INH concentrate is recommended for the treatment and prophylaxis of acute attacks of HAE due to C1-INH deficiency (HAE-C1-INH). This article will discuss the current therapies available for the treatment of HAE-C1-INH, latest treatment guidelines, results of several studies demonstrating the efficacy and safety of the plasma-derived, pasteurized and nanofiltered C1-INH concentrate Berinert(®) (CSL Behring GmBH, Marburg, Germany), and future perspectives for the treatment and management of HAE-C1-INH.

Oversulfated chondroitin sulfate inhibits the complement classical pathway by potentiating C1 inhibitor.

Oversulfated chondroitin sulfate (OSCS) has become the subject of multidisciplinary investigation as a non-traditional contaminant in the heparin therapeutic preparations that were linked to severe adverse events. In this study, it was found that OSCS inhibited complement fixation on bacteria and bacterial lysis mediated by the complement classical pathway. The inhibition of complement by OSCS is not due to interference with antibody/antigen interaction or due to consumption of C3 associated with FXII-dependent contact system activation. However, OSCS complement inhibition is dependent on C1 inhibitor (C1inh) since the depletion of C1inh from either normal or FXII-deficient complement plasma prevents OSCS inhibition of complement activity. Surface plasmon resonance measurements revealed that immobilized C1inhibitor bound greater than 5-fold more C1s in the presence of OSCS than in presence of heparin. Although heparin can also inhibit complement, OSCS and OSCS contaminated heparin are more potent inhibitors of complement. Furthermore, polysulfated glycosaminoglycan (PSGAG), an anti-inflammatory veterinary medicine with a similar structure to OSCS, also inhibited complement in the plasma of dogs and farm animals. This study provides a new insight that in addition to the FXII-dependent activation of contact system, oversulfated and polysulfated chondroitin-sulfate can inhibit complement activity by potentiating the classical complement pathway regulator C1inh. This effect on C1inh may play a role in inhibiting inflammation as well as impacting bacterial clearance.

Model-based evaluation of similarity in pharmacokinetics of two formulations of the blood-derived plasma product c1 esterase inhibitor.

A novel formulation of C1 esterase inhibitor concentrate, a plasma product used in the treatment of hereditary angioedema (HAE), was studied in a clinical trial for similarity in pharmacokinetics (PK) compared with the reference product. Direct trial data were limited given the availability of patients, and therefore a modeling approach was used to study similarity. Type I error of the study was evaluated using simulations based on retrospective data. A population PK modeling analysis was performed on data from the trial. Analysis of variance was carried out on results of a noncompartmental PK analysis (NCA) of the clinical data. Simulations showed that type I error was inflated to 62% (P < .05) when bioequivalence criteria (confidence intervals within 80%-125%) were adhered to strictly. In the clinical trial, 13 HAE patients were evaluable. The population PK analysis showed no significant differences in PK parameters, whereas confidence intervals for all parameters were within 80% to 125%. The relative differences in area under the curve, incremental recovery, and mean residence time estimated using NCA were all close to 1. The novel formulation showed similar PK characteristics to the original formulation. The model-based approach showed that strict criteria for PK comparison could not be applied in this analysis.

C1-inhibitor: more than a serine protease inhibitor.

C1-inhibitor (C1-inh) is a crucial regulator of the activation of plasmatic cascade systems involved in inflammation contributing to the homeostasis in the generation of proinflammatory mediators. The importance of C1-inh is illustrated by patients with hereditary angioedema where decreased levels of C1-inh lead to an uncontrolled generation of vasoactive peptides resulting in potential life-threatening subcutaneous edema. Recent publications, however, suggest that the anti-inflammatory properties of C1-inh do not strictly depend on its capacity to regulate the complement and contact phase system. This review summarizes the biochemical characteristics of C1-inh and its role in the regulation of plasmatic cascade systems as well as the role of the nonserpin domain.

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.

Human complement regulators C4b-binding protein and C1 esterase inhibitor interact with a novel outer surface protein of Borrelia recurrentis.

The spirochete Borrelia recurrentis is the causal agent of louse-borne relapsing fever and is transmitted to humans by the infected body louse Pediculus humanus. We have recently demonstrated that the B. recurrentis surface receptor, HcpA, specifically binds factor H, the regulator of the alternative pathway of complement activation, thereby inhibiting complement mediated bacteriolysis. Here, we show that B. recurrentis spirochetes express another potential outer membrane lipoprotein, termed CihC, and acquire C4b-binding protein (C4bp) and human C1 esterase inhibitor (C1-Inh), the major inhibitors of the classical and lectin pathway of complement activation. A highly homologous receptor for C4bp was also found in the African tick-borne relapsing fever spirochete B. duttonii. Upon its binding to B. recurrentis or recombinant CihC, C4bp retains its functional potential, i.e. facilitating the factor I-mediated degradation of C4b. The additional finding that ectopic expression of CihC in serum sensitive B. burgdorferi significantly increased spirochetal resistance against human complement suggests this receptor to substantially contribute, together with other known strategies, to immune evasion of B. recurrentis.