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The role of complement in cryoglobulin-induced immune complex glomerulonephritis.

Many forms of glomerulonephritis are triggered by Ab localization in the glomerulus, but the mechanisms by which this induces glomerular inflammation are not fully understood. In this study we investigated the role of complement in a mouse model of cryoglobulin-induced immune complex glomerulonephritis. Several complement-deficient mice on a C57BL/6 and BALB/c genetic background were used and compared with strain-matched, wild-type controls. Cryoglobulinemia was induced by i.p. injection of 6-19 hybridoma cells producing an IgG3 cryoglobulin with rheumatoid factor activity against IgG2a of allotype a present in BALB/c, but not C57BL/6, mice. Thus, the cryoprecipitate in C57BL/6 mice consisted of the IgG3 cryoglobulin only (type I cryoglobulinemia) compared with IgG3-IgG2a complexes in BALB/c (type II cryoglobulinemia). The survival of mice was not affected by complement deficiency. Glomerular influx of neutrophils was significantly less in C3-, factor B-, and C5-deficient mice compared with wild-type and C1q-deficient mice. It did not correlate with C3 deposition, but did correlate with the amount of C6 deposited. Deficiency of CD59a, the membrane inhibitor of the membrane attack complex, did not induce an increase in neutrophil infiltration, suggesting that the generation of C5a accounts for the effects observed. There was no apparent difference between cryoglobulinemia types I and II regarding the role of complement. Our results suggest that in this model of cryoglobulin-induced glomerulonephritis the neutrophil influx was mediated by C5 activation with the alternative pathway playing a prominent role in its cleavage. Thus, blocking C5 is a potential therapeutic strategy for preventing renal injury in cryoglobulinemia.

Animals↗

Complement in lung disease.

Complement proteins play an integral role in both innate and adaptive immune responses of the host. Complement activation leads to the formation of bioactive molecules including the anaphylatoxins, C3a and C5a, and the lytic membrane attack complex (C5b-9). These molecules trigger a series of events that culminate in the recruitment of phagocytic cells, release of cytokines/chemokines and reactive oxygen species, enhanced expression of adhesion molecules and apoptosis at the site of inflammation. Several animal models provide evidence that this series of events forms the basis for the pathophysiology found in many lung diseases, such as asthma and acute respiratory distress syndrome. Clinical data further confirm these findings. This review briefly discusses recent data from such studies.

Animals↗

Improvement of the anti-C3 activity of compstatin using rational and combinatorial approaches.

Compstatin is a 13-residue cyclic peptide that has the ability to inhibit the cleavage of C3 to C3a and C3b. The effects of targeting C3 cleavage are threefold, and result in hindrance of: (i) the generation of the pro-inflammatory peptide C3a, (ii) the generation of opsonin C3b (or its fragment C3d), and (iii) further complement activation of the common pathway (beyond C3) with the end result of the generation of the membrane attack complex. We will report on our progress on: (i) rational design of more active compstatin analogues based on the three-dimensional structure of compstatin, (ii) experimental combinatorial design based on the generation of a phage-displayed peptide library partially randomized with the implementation of structure-induced restraints, and (iii) theoretical combinatorial design based on a novel computational optimization method, structure-induced restraints and flexible structural templates. All three approaches have resulted in analogues with improved activities. Currently, the lead analogue has the sequence acetyl-I[CVYQDWGAHRC]T-NH(2) (where the brackets denote cyclization), and is 16-fold more active than the parent peptide. We will also report on our progress towards understanding the dynamic character of compstatin using molecular dynamics simulations. The identification of an ensemble of interconverting conformers of compstatin with variable populations is a first step towards the incorporation of dynamic elements in the design of new analogues using dynamics-activity relationships in addition to structure-activity relationships.

Animals↗

The control of homologous lysis.

Complement activation unleashes powerful effector mechanisms against which host cells are protected by homologous restriction factors. These factors are glycolipid-anchored membrane proteins that either induce C3 convertase dissociation (for example decay-accelerating factor) or prevent the full development of the membrane attack complex (for example homologous restriction factor and CD59). In this article Peter Lachmann explores the biology and biochemistry of these important and intriguing molecules.

Animals↗

Retrometabolic syntheses of astaxanthin (3,3'-dihydroxy-beta,beta-carotene-4,4'-dione) conjugates: a novel approach to oral and parenteral cardio-protection.

Disodium disuccinate astaxanthin has potent cardioprotective effects in animals, with demonstrated preclinical efficacy in the rat, rabbit, and canine models of experimental infarction. It has been effective in subchronic and acute dosing regimens after parenteral administration, and recently published data in rats demonstrate that oral cardioprotection is also readily achieved. Myocardial salvage in the canine can reach 100% with a 4-day subchronic dosing regimen; single-dose I.V. cardioprotection, when given 2 hours before experimental coronary occlusion, is on average two-thirds of that achieved with the subchronic regimen in dogs. In conscious animals, no effects on hemodynamic parameters have been observed. Recently, the beneficial properties of this prototypical astaxanthin conjugate have been extended to include second- and third-generation compounds with improved pharmacokinetic and/or potency profiles. The primary mechanism of cardioprotection appears to be antioxidant activity: potent direct scavenging of the lynchpin radical in ischemia-reperfusion injury, superoxide anion, has been documented in appropriate model systems. In addition, modulation of serum complement activity, reduction of the levels of deposition of C-reactive protein (CRP) and the membrane attack complex (MAC) in infarcted tissue, and reduction in oxidative stress markers from the arachidonic acid and linoleic acid pathways also suggest a significant anti-inflammatory component to the mechanism of cardioprotection. Favorable plasma protein binding has been demonstrated in vitro for several astaxanthin conjugates; this binding capacity overcomes the supramolecular assembly of the compounds that occurs in aqueous solution, which in itself improves the stability and shelf-life of aqueous formulations. Astaxanthin readily populates cardiac tissue after metabolic hydrolysis of both oral and parenteral administration of the astaxanthin ester derivates, providing a reservoir of cardioprotective agent with a significant half-life due to favorable ADME in mammals. Due to the well-documented safety profile of astaxanthin in humans, disodium disuccinate astaxanthin may well find clinical utility in cardiovascular applications in humans following successful completion of preclinical and clinical pharmacology and toxicology studies in animals and humans, respectively.

Animals↗

Intact pig pancreatic islet function in the presence of human xenoreactive natural antibody binding and complement activation.

BACKGROUND: The expression of xenogeneic epitopes and the activation of human complement by adult pig islets after prolonged culture have hitherto not been described. MATERIALS AND METHODS: Freshly isolated and cultured islets were analyzed by fluorescence-activated cell sorter analysis, fluorescence microscopy, and immunohistology for expression of Gal(alpha1,3)Gal epitopes, binding of human xenoreactive natural antibodies (XNA), and complement deposition. RESULTS: Freshly isolated and cultured islets showed detectable Gal(alpha1,3)Gal expression and human XNA binding limited to intraislet capillary endothelial cells. No significant modification in Gal(alpha1,3)Gal expression and human XNA binding levels was detected in adult pig islets cultured for up to 4 days compared with freshly isolated islets. Incubation of pig islets with human serum demonstrated the deposition of C3, C4, and membrane attack complex, but not factor B with a similar pattern to XNA. However C3 and C4 showed a more widespread deposition. Despite complement activation, no cytotoxic effect on islets was detected after 4 hr of incubation with human serum capable of killing porcine endothelial cells. Even after 4 days of culture in 50% intact human serum, pig islets retained both their normal morphology and a normal insulin response to glucose stimulation. CONCLUSIONS: Neither islet cell lysis nor, more importantly, any alteration in beta cell function occurred, which suggests that adult pig islets may not be directly damaged by serum after xenotransplantation in humans. Nevertheless, complement activation in vivo could trigger rapid cellular rejection mechanisms through islet cell opsonization and release of bioactive fragments.

Animals↗

Effect of the sugar chain of soluble recombinant CD59 on complement inhibitory activity.

A soluble recombinant CD59#77 (rCD59#77), consisting of 77 amino acids starting from the N terminus of membrane-bound CD59, was prepared using a gene expression system in CHO cells. The rCD59#77 preparation was composed of glycosylated and non-glycosylated forms (G and NG forms). Unexpectedly, NG form was 7 times more potent than G form in complement inhibitory activity. Postulating that sialic acids on G-form molecules make it difficult for rCD59#77 to access nascent membrane attack complexes on the cell surface, the sialic acids were removed by neuraminidase treatment. However, the inhibitory activity was not changed. Next, one of two putative N-glycosylation sites was mutated by substituting Gln18 for Asn18. The mutant, designated rCD59#77(N/Q), had no sugar moiety and was as active as the NG form of rCD59#77. These results suggest that the bulky sugar moiety at Asn18 is not necessary for the complement-inhibitory activity of rCD59 and actually hampers that function.

Animals↗

Resistance of Actinobacillus pleuropneumoniae to bactericidal antibody and complement is mediated by capsular polysaccharide and blocking antibody specific for lipopolysaccharide.

Actinobacillus pleuropneumoniae is resistant to complement-mediated killing, even in the presence of specific Ab. Our studies focused on identifying the mechanism(s) responsible for this resistance. Encapsulated A. pleuropneumoniae was susceptible to killing in precolostral calf serum (PCS) but not in normal serum as a complement source in the presence of anti-capsular polysaccharide (CP) IgG. In contrast, two capsule-deficient mutants were sensitive to killing in normal serum and one was sensitive to killing in PCS alone. Electron microscopy demonstrated that A. pleuropneumoniae serotype 5a synthesized a thick, adherent CP that bound anti-CP Ab distant from the outer membrane. The CP of A. pleuropneumoniae did not prevent complement activation or the attachment of C3 to the cell surface. However, the CP did limit the amount of C9, a component of the membrane attack complex, that bound to A. pleuropneumoniae in PCS. A second mechanism of serum resistance was a result of an LPS-specific Ab present in the IgG fractions of normal swine serum, swine anti-K17 serum, and guinea pig anti-K17 LPS that blocked anti-CP IgG complement-mediated killing of A. pleuropneumoniae. Incubation of swine anti-K17 IgG with purified K17 LPS depleted Abs specific for K17 LPS but not for K17 proteins and removed all blocking activity. Immune swine serum containing this blocking Ab reduced the deposition of C9 on A. pleuropneumoniae in the presence of anti-CP IgG and also directed the deposition of C9 to sites on the bacteria in which the bound C9 was easily eluted. Thus, CP and anti-LPS Ab may act synergistically or at different stages of infection to limit the ability of complement to eliminate A. pleuropneumoniae.

Actinobacillus pleuropneumoniae↗

Complement receptors in HIV infection.

The complement system plays an important role in the antimicrobial defense of the organism. Its components recognize a large variety of pathogens and target them for destruction, either directly by formation of a membrane attack complex or indirectly by recruiting phagocytic cells. In addition, it has several functions in cell activation, clearance of immune complexes, control of inflammatory reactions, chemotaxis and autoimmunity. For mediation of all these tasks of the complement system, complement receptor molecules on the cell surface play a key role. Current knowledge on structure, function, signal transduction and associated molecules is briefly summarized here. The role of complement receptors for human immunodeficiency virus (HIV)-associated pathogenesis is ambiguous and varies depending on cell type. On the one hand, complement receptors support the infected host to manage HIV infection and to defend itself, at least partially, against viral spreading throughout the organism. Such complement receptor-mediated supporting mechanisms are activation of immune cells and lysis of viral particles and infected host cells. On the other hand, HIV employs complement receptors to intrude more easily into various cell types, to become localized into lymph follicles and to activate viral replication in latently infected cells. This review summarizes the complex interaction of virus and complement receptors in HIV infection for different cell types.

Animals↗

Interaction of complement proteins C5b-6 and C5b-7 with phospholipid vesicles: effects of phospholipid structural features.

Complement components C5b-6 and C7 assemble to form C5b-7, which then interacts with membranes and commits the membrane attack complex to a target site. This protein-membrane association event was investigated to determine possible structural features that could contribute to a selective membrane attack. This system may also suggest general properties of protein-membrane insertion events. Initial binding of C5b-6 to membranes could potentially determine the site of assembly. However, binding of C5b-6 to membranes required phosphatidylglycerol or phosphatidic acid produced from egg phosphatidylcholine while binding of C5b-6 to phosphatidylcholine, phosphatidylserine, or phosphatidylinositol was undetectable. Binding to phosphatidic acid was irreversible, and the bound C5b-6 could no longer interact with C7. In contrast, C5b-7 interacted with all phospholipids tested. The rate-limiting process was the interaction of C5b-6 and C7, which displayed bimolecular properties and an activation energy of 37 kcal/mol. The C5b-7 complex showed 20-fold selectivity for small unilamellar phospholipid vesicles over large unilamellar vesicles. Vesicles carrying high negative charge densities were selected over neutral vesicles by a factor of about 5. Vesicles formed from phospholipids with short, saturated hydrocarbon side chains (dimyristoylphosphatidylcholine and dipalmitoylphosphatidylcholine) were about 5-fold less effective than those formed from phospholipids with natural fatty acid distributions. The gel vs. fluid state had little influence on C5b-7 insertion.(ABSTRACT TRUNCATED AT 250 WORDS)

1,2-Dipalmitoylphosphatidylcholine↗

Roles of the complement system in human neurodegenerative disorders: pro-inflammatory and tissue remodeling activities.

Complement is an important component of the innate immune response with the capacity to recognize and clear infectious challenges that invade the CNS through a damaged blood brain barrier. For instance, the membrane attack complex is involved in cytotoxic and cytolytic activities while other smaller fragments lead to cell activation (chemotaxis) and phagocytosis of the intruders. It is noteworthy that there is a growing body of evidence that uncontrolled complement biosynthesis and activation in the CNS can contribute to exacerbate the neuronal loss in several neurodegenerative disorders. We provide here an insightful review of the double-edged sword activities of the local innate complement system in the CNS and discuss further the potential therapeutic avenues of delivering complement inhibitors to control brain inflammation.

Animals↗

Role of complement regulatory membrane proteins in ischaemia-reperfusion injury of rat gastric mucosa.

BACKGROUND: The role of complement in ischaemia-reperfusion injury has not been well investigated. 5I2 is a monoclonal antibody (mAb) directed against a rat membrane inhibitor of the C3 convertase step, which is the rat counterpart of mouse Crry/p65. 6D1 is a mAb against rat CD59 which inhibits the formation of membrane attack complexes. METHODS: We visualized the tissue distribution of these membrane inhibitors in rat gastrointestinal tract by immunohistochemical staining with the appropriate mAb. Then, we tested the hypothesis that complement regulatory proteins protect rat gastric mucosa against ischaemia-reperfusion stress by using these mAbs. Gastric mucosal integrity was continuously monitored by measuring the blood-to-lumen clearance of [51Cr]-labelled ethylenediaminetetraacetic acid (EDTA) under control conditions, during ischaemia and after reperfusion. RESULTS: Rat 6D1 and 5I2 antigens were both widely distributed and predominantly expressed on smooth muscle and endothelial cells in gastrointestinal tracts. Blockade of complement regulatory proteins with 5I2 and 6D1 mAbs resulted in a significant increase in [51Cr]-EDTA clearance after reperfusion. CONCLUSIONS: These findings support the hypothesis that endogenous complement regulatory proteins may act as important protective factors against ischaemia-reperfusion stress in rat gastric mucosa.

Animals↗

Effect of soluble complement receptor type 1 on reperfusion edema and neutrophil migration after lung allotransplantation in swine.

OBJECTIVE: Soluble complement receptor type 1 inhibits complement activation by blocking C3 and C5 convertases of the classical and alternative pathways. We evaluated the effect of soluble complement receptor type 1 on lung allograft reperfusion injury. METHODS: Left lung transplantation was performed in 13 weight-matched pigs (25 to 31 kg) after prolonged preservation (20 hours at 1 degree C). One hour after reperfusion the recipient contralateral right lung was excluded to assess graft function only. Complement activity and C3a levels were measured after reperfusion and at the end of the assessment. Extravascular lung water index, intrathoracic blood volume, and cardiac output were assessed during a 5-hour observation period. Gas exchange and hemodynamics were monitored. At the end of the 5-hour assessment period, myeloperoxidase assay and bronchoalveolar lavage were performed to assess neutrophil migration, and C5b-9 (membrane attack complex) deposits in the allograft were detected by immunohistochemistry. Two groups were studied. In group II (n = 6) recipient animals were treated with soluble complement receptor type 1 (15 mg/kg) 15 minutes before reperfusion. Group I (n = 7) served as the control group. RESULTS: Serum complement activity was completely inhibited in group II. In contrast to group I, C5b-9 complexes were not detected in group II allograft tissue samples. C3a was reduced to normal levels in group II (p = 0.00005). Extravascular lung water index was higher in group I animals throughout the assessment period (p = 0.035). No significant difference in allograft myeloperoxidase activity (p = 0.10) and polymorphonuclear leukocyte count of the bronchoalveolar lavage fluid (p = 0.057) was detected. CONCLUSION: Inhibition of the complement system by soluble complement receptor type 1 blocks local complement activation in the allograft and reduces posttransplantation reperfusion edema but does not improve hemodynamic parameters.

Animals↗

Survival of Helicobacter pylori From complement lysis by binding of GPI-anchored protectin (CD59).

BACKGROUND & AIMS: Although Helicobacter pylori is sensitive to complement lysis in vitro, chronic infection persists for years. We tested whether H. pylori acquires complement resistance by binding glycolipid-tailed inhibitors from the host. METHODS: Gastric biopsy specimens from H. pylori-infected patients (n = 10) and noninfected controls (n = 6) were analyzed for complement deposition and expression of the complement regulators protectin (CD59) and DAF. Protectin binding and complement sensitivity analyses were performed with the NCTC strain 11637 (CagA(+)) and 2 clinical isolates 9:0 (CagA(+)) and 67:20 (CagA(-)). RESULTS: In the noninfected mucosa, protectin was strongly expressed on the membranes of epithelial cells, but in the infected epithelia the expression was granular and more focused to the mucus. H. pylori bacteria in the gastric pits were often positive for protectin but negative for C5b-9. An opposite pattern was seen on the surface mucosa. In vitro analyses using (125)I-CD59 and bacteriolysis assays showed that protectin bound to H. pylori and protected CagA(+) strains against complement killing. In an enzyme-linked immunosorbent assay, the binding of CD59 correlated inversely with the appearance of the C5b-9 neoantigen. CONCLUSIONS: Binding of protectin inhibits membrane attack complex assembly on H. pylori and may thereby contribute to their survival on the gastric mucosa.

Adult↗

[Expression and role of complement regulatory proteins on human gametes and pre-implantation embryos].

Human gametes and pre-implantation embryos express selectively several complement regulatory proteins. Membrane cofactor protein (MCP, CD46) and decay accelerating factor (DAF, CD55) are regulators for C3 convertases and protectin (CD59) is an inhibitor of the membrane attack complex. These three proteins were identified on human sperm and found to be functional. CD55 and CD59 were both expressed by the plasmic membrane of unfertilized oocytes and pre-implantation embryos. CD46 was not present on unfertilized oocytes but appeared at the 6/8 cell-stage embryo when human gene expression first occurs. Complement receptor 1 (CR1, CD35) and MHC class I antigens were not found on oocytes neither on embryos. Such a selective expression of complement regulatory proteins associated with the lack of MHC class I antigens may represent an immune protective mechanism by which human gametes and pre-implantation embryos escape from complement-mediated damage during their travel through the female genital tract. Indeed uterine, tubal and follicular fluids contain all the components of the complement cascade, including classical and alternative pathways. Nevertheless participation of CD46 and CD59 in cell to cell interaction during fertilization and/or implantation cannot be excluded. CD59 is an adhesive molecule involved in the rosette phenomena and CD46 has been described as the human receptor for measles virus, which binds through a fusion protein. Monoclonal antibodies raised against these two proteins (CD46 and CD59) are able to inhibit heterospecific fertilization between zona-free hamster oocytes and human spermatozoa suggesting the role of these proteins during fertilization.

Animals↗

Mutations in the putative lipid-interaction domain of complement C9 result in defective secretion of the functional protein.

Complement protein C9 assembles with C5, C6, C7, C8 on the surface of target cells to form the lytic membrane attack complex (MAC). During MAC assembly and insertion into the target membrane, the hydrophilic, globular C9 partially unfolds to expose a hydrophobic lipid interaction domain. Several copies of amphiphilic C9 subsequently polymerize to form the characteristic ring-like MAC. Using a combined photoaffinity label and computer modeling approach, two amphipathic helices in a segment encompassing the amino acids 293-334 have been predicted to interact with membrane lipids. To elucidate the mechanism of C9 lipid binding and insertion, site-directed mutagenesis was used to change the amphipathic character of the helices. While some conservative amino acid replacements such as Thr307 by a Leu were tolerated and yielded fully active C9 when expressed in COS cells, successive changes of Leu305 into Val, Ala, and Glu on the hydrophobic site of the first helix gave rise to only partly or not secreted C9. All non-conservative amino acid replacements introduced on either side of the helices resulted in non-secreted C9 that was subsequently degraded intracellularly, indicating the importance of the correct folding of the presumptive transmembrane domain during biosynthesis. A natural secretion-incompetent mutant was found in which Val293, located in the proposed lipid-binding region, was lacking. Taken together, these findings suggest that the high incidence of homozygous C9 deficiencies may be due to a blockage in intracellular transport and secretion due to point mutations in this 'hot spot' region of the molecule.

Affinity Labels↗

Disodium disuccinate astaxanthin (Cardax): antioxidant and antiinflammatory cardioprotection.

Disodium disuccinate astaxanthin (Cardax), DDA) has cardioprotective effects in the rat, rabbit, and canine models of experimental infarction. It is highly effective by parenteral administration in subchronic and acute dosing regimens. Unpublished data in rats suggest that oral cardioprotection is also readily achievable. DDA-induced myocardial salvage in the canine can reach 100% with a 4-day subchronic dosing regimen. At a single i.v. dose DDA is cardioprotective, when given 2 h before experimental coronary occlusion, but the protection is on the average two-thirds of that achieved with the subchronic regimen in dogs. In conscious animals DDA has no effects on hemodynamic parameters. The primary mechanism of cardioprotection appears to be antioxidant activity involving direct scavenging of superoxide anion, the lynchpin radical in ischemia-reperfusion injury. In addition, modulation of serum complement activity, as well as the reduction in the levels of C-reactive protein (CRP) and the membrane attack complex (MAC) in infarcted tissue suggest a significant antiinflammatory component in the mechanism of cardioprotective action of DDA. Stoichiometric binding of the meso-form of the compound to human serum albumin (HSA) has been demonstrated in vitro. This binding capacity overcomes the supramolecular assembly of the compound in aqueous solution, which by itself improves the stability and shelf life of aqueous formulations. Non-esterified astaxanthin readily enters cardiac tissue after either oral or parenteral administration, providing a reservoir of a cardioprotective agent with a significant half-life due to favorable ADME in mammals. Due to the well-documented safety profile of non-esterified astaxanthin in humans, disodium disuccinate astaxanthin may well find clinical utility in cardiovascular indications in humans following successful completion of preclinical and clinical pharmacology and toxicology studies.

Animals↗

Detection of refolding conformers of complement protein C9 during insertion into membranes.

Human complement protein C9 is a hydrophilic serum glycoprotein responsible for efficient expression of the cytotoxic and cytolytic functions of complement. It assembles on the surface of a target cell together with C5, C6, C7 and C8 to form the membrane attack complex (MAC) and therefore has to change structure to become an integral membrane protein. As the protein assumes a stable structure in an aqueous environment, the question arises as to how it can enter the hydrophobic interior of a membrane. During MAC assembly C9 polymerizes into a circular structure, termed poly(C9) (ref. 8), which is responsible for the cylindrical electron microscopic appearance of the MAC. The suggestion has been made that C9 must at least partly unfold in order to enter a membrane and also that polymerization of the molecule is intimately linked to insertion and cytotoxicity. The extent of unfolding and the mechanism of polymerization are not understood, nor is it known precisely which parts of the molecule participate in the proposed structural changes. We have been able to capture refolding C9 conformers during membrane insertion with the help of sequence-specific anti-peptide antibodies. Some of these antibodies inhibit C9-mediated haemolysis but not C9 polymerization, while others have the opposite effect. This suggests that the two processes are independent.

Complement C9↗

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