Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Complement C5b”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 703 records · Page 39Linked to original sources
PubMed1996

Specific complement inhibition with heparin-coated extracorporeal circuits.

BACKGROUND: Although it is well established that heparin-coated extracorporeal circuits reduce complement activation during cardiac operations, little in vivo information is available on the reduction in alternative and classic pathway activation. METHODS: In a prospective, randomized study involving patients undergoing coronary artery bypass grafting with standard full heparinization, we compared heparin-coated circuits (Duraflo II) (10 patients) with uncoated circuits (10 patients) and assessed the extent of initiation of complement activation by detecting iC3 (C3b-like C3) concentrations, classic pathway activation by C4b/c (C4b, iC4b, C4c) concentrations, terminal pathway activation by soluble C5b-9 concentrations, and C3 activation by C3a (C3a desArg) and C3b/c (C3b, iC3b, C3c) concentrations. RESULTS: Heparin-coated extracorporeal circuits significantly reduced circulating complement activation product C3b/c and soluble C5b-9 concentrations at the end of cardiopulmonary bypass and after protamine sulfate administration compared with the uncoated circuits, but not iC3, C4b/c, or C3a concentrations. CONCLUSIONS: Heparin-coated extracorporeal circuits reduce complement activation through the alternative complement pathway, probably at the C3 convertase level, and, consequently, the terminal pathway. C3b/c seems to be a more sensitive marker than C3a to assess complement activation during cardiac operations.

Aged↗
PubMed1987

Structure and specificity of complement receptors.

Fifteen to 16 cell surface proteins which interact with soluble components of the complement system have now been identified. Most of these--CR1, CR2, CR3, "CR4", DAF, HSV-1 c glycoprotein, Gp 45-70, p150,95, cell-surface Factor H, and a 90 kD protein--interact with C3 or C4 and their degradation products. Other receptors for C1q, Factor H, C5a, and the C5b-9 complex have been identified. Receptors for additional complement proteins such as Factor B or its fragments are likely to exist. Complement receptors have a wide tissue distribution and have major roles in controlling the turnover of the complement system, and regulating behaviour and growth of leukocytes.

Animals↗
PubMed1993

Interaction between apolipoproteins A-I and A-II and the membrane attack complex of complement. Affinity of the apoproteins for polymeric C9.

We have previously observed enhanced binding of HDL and apolipoproteins A-I and A-II to human endothelial cells exposed to activated complement. Induction of these binding sites required complement activation through C9, suggesting a specific role for the C9 component of the C5b-9 complex. We now report that specific and saturable binding sites for apoA-I and -A-II are expressed by C9 polymers (polyC9), whereas little binding was observed to native monomeric C9. These data suggested an interaction of the apoproteins with a site(s) which is exposed only upon C9 polymerization, and also suggested that binding of the apoproteins to this new site might interfere with assembly of C9 into the polyC9 tubule and insertion into the cell membrane. ApoA-I was found to inhibit zinc-catalyzed polymerization of C9 in a concentration-dependent fashion. Formation of SDS-resistant C9 polymers was completely inhibited at apoA-I or -A-II concentrations > or = 5 microM. ApoA-I also produced a concentration-dependent inhibition of C9 incorporation into C5b-9 complexes on endothelial cells, which was accompanied by a corresponding decrease in SDS-resistant C9 polymers associated with the cell membrane. In summary, the ability of the HDL apoproteins A-I and A-II to interact with an activation-dependent conformer(s) of the C9 component of the C5b-9 complex appears to explain the expression of HDL binding sites on endothelial cells exposed to complement. These apoproteins are also inhibitors of C9 polymerization, which may underlie the protective effect of HDL for blood cells exposed to activated complement.

Amino Acid Sequence↗
PubMed1994

Hereditary C6 deficiency in a strain of PVG/c rats.

A chance observation has led to the discovery of a strain of PVG rats (PVG/c-) which are deficient in complement (C) component C6. Analysis of total haemolytic activity (CH50) of PVG/c- serum revealed an absent CH50 activity compared with serum of other rat strains and of a PVG/c rat (PVG/c+) that showed normal C activity. Thus, the PVG/c- rat was unable to activate the C5b-9 membrane attack complex. To gain insight into the complement abnormalities, analysis of individual C components was performed. Testing the PVG/c- serum in a C6 haemolytic assay and using deficient human sera showed a deficiency of C6 in the PVG/c- rat. Highly purified human C6 and human sera deficient in other components were able to reconstitute the CH50 activity of the PVG/c- rat. The possibility that an inactivator of C was present in PVG/c- serum was excluded. The deficiency was found to be inheritable and under the control of an autosomal recessive gene. Furthermore, tissue antigens and immunity of the PVG/c- rat were found to be identical to those determined in the PVG/c+ rat. With regard to their health status, the PVG/c- animals seem to have no disadvantages compared with PVG/c+ rats when held under the same conditions within the protected environment of animal facilities. Taken together, both rat strains provide an unique animal model for studying the biological role of C, particularly the C5b-9 membrane attack complex in experimental medicine.

Animals↗
PubMed2000

Predominant role for C5b-9 in renal ischemia/reperfusion injury.

Previous work has indicated that complement is a mediator of ischemia/reperfusion (I/R) injury. To investigate the components of complement responsible for this effect, we examined a model of renal I/R injury in C3-, C4-, C5-, and C6-deficient mice. We occluded the renal arteries and veins (40-58 minutes) and, after reperfusion (0-72 hours), assessed renal structural and functional injury. C3-, C5-, and C6-deficient mice were protected from renal I/R injury, whereas C4-deficient mice were not protected. C6-deficient mice treated with antibody to block C5a generation showed no additional protection from I/R injury. Reconstitution with C6 alone restored the I/R injury in C6-deficient mice. Tubular epithelial cells were the main structures damaged by complement-mediated attack, and, in contrast, the renal vessels were spared. Neutrophil infiltration and myeloperoxidase activity were reduced in C-deficient mouse kidney, but by a similar extent in C3-deficient and C6-deficient mice. We conclude that the membrane attack complex of complement (in which C5 and C6 participate) may account for the effect of complement on mouse renal I/R injury. Neither C5a-mediated neutrophil infiltration nor the classic pathway, in which C4 participates, appears to contribute to I/R injury in this model. By contrast with other organs, such as the heart, the primary effect of complement in the ischemic area is on the parenchymal cell rather than the vascular endothelial cell. The membrane attack complex of complement is a potential target for prevention of I/R injury in this model.

Animals↗
PubMed1982

Immunological basis of serum resistance of Neisseria gonorrhoeae.

The immunological basis for resistance of certain strains of Neisseria gonorrhoeae to the bactericidal action of normal human serum was studied by investigating the potential role of factors which are known to interfere with each of the sequential steps that result in immune lysis of Gram-negative bacteria. Strains of N. gonorrhoeae were characterized as serum-sensitive (sers) or serum-resistant (serr) on the basis of their sensitivity to lysis by the sera of six normal individuals. Neither intrinsic resistance to the lytic action of activated human complement nor inaccessibility of the cell membrane to C5b accounted for serr. Outer membrane lipopolysaccharide (LPS) was the target antigen for lytic antibody in normal human sera. The gross chemical composition and molecular size of the LPS of the strains were heterogeneous and no consistent patterns of differences between those extracted from serr and from sers strains were found. Neither IgA nor IgG 'blocking' antibody in normal human serum was responsible for serr. We conclude that serr results from the absence from the LPS of the strains of antigenic loci for the lytic antibody in most normal human sera, or, expressed as a function of the host, the absence from the sera of most normal humans of lytic antibody directed against LPS antigenic loci for immune lysis.

Antibodies, Bacterial↗
PubMed2005

Prevention of acute vascular rejection by a functionally blocking anti-C5 monoclonal antibody combined with cyclosporine.

BACKGROUND: Inhibition of the complement cascade at C5 prevents formation of pro-inflammatory molecules C5a and C5b-9, which play a key role in allograft rejection. The present study was undertaken to determine whether blocking terminal complement with anti-C5 monoclonal antibody (mAb) alone or combined with cyclosporine (CsA) would prevent acute vascular rejection (AVR) in a mouse cardiac allograft model. METHODS: C3H mouse hearts were transplanted into BALB/c mice and randomized into five groups with the following treatments: (1) no treatment; (2) CsA alone; (3) control mAb; (4) anti-C5 mAb alone; and (5) anti-C5 mAb and CsA. RESULTS: Allografts in untreated or control mAb-treated recipients were rapidly rejected at 8.0+/-0.6 and 8.2+/-0.8 days, respectively. These grafts exhibited typical AVR, characterized by vasculitis, hemorrhage, and thrombosis. A high level of complement activity was also demonstrated in these animals. High-dose CsA was not able to inhibit complement activation or AVR, and grafts were rejected in 15.5+/-1.1 days. Anti-C5 monotherapy completely inhibited complement activation and attenuated AVR, but grafts were rejected in 8.3+/-0.5 days by acute cellular rejection. In contrast, a combination of anti-C5 mAb and CsA successfully achieved indefinite graft survival (>100 days). This combined therapy completely inhibited terminal complement activation and prevented both humoral- and cellular-mediated rejection. CONCLUSIONS: Combination therapy of anti-C5 mAb and CsA achieves indefinite graft survival in a mouse cardiac allograft model. These data suggest that inhibition of terminal complement using anti-C5 mAb may be an effective therapeutic adjunct to prevent AVR in clinical transplantation.

Acute Disease↗
PubMed1992

Complement-mediated killing of susceptible gram-negative bacteria: an elusive mechanism.

Exposure of gram-negative bacteria to a source of antibody and complement frequently results in efficient cell killing; this effect follows the deposition of C5b-9 membrane attack complexes onto the bacterial surface. This review examines the molecular and physiological events culminating in C5b-9 deposition and cell death and advances an hypothesis that may account for cytoplasmic-membrane damage following complement-mediated perturbation of the outer membrane.

Animals↗
PubMed1993

Structure of complement poly-C9 determined in projection by cryo-electron microscopy and single particle analysis.

The ring-like complement 'lesions' found on membranes of complement lysed cells comprise a complex of components C5b through C9 that coalesce to form hollow cylinders which penetrate the membrane bilayer and create lytic pores. Walls of these C5b-9 membrane attack complex cylinders may consist primarily of the C9 component, since samples of purified, isolated C9 can polymerize into cylindrical structures which appear identical with the fully assembled C5b-9 complex. The structure of these poly-C9 molecules has been investigated using the techniques of cryo-electron microscopy and single particle analysis. Sets of single poly-C9 particles viewed as rings were selected from cryo-EM images, then particles were aligned and treated by correspondence analysis to identify the principle interparticle similarities and variations. The highest ranking variation found was the presence or absence of a dense inner ring of protein density. Other important variations were interpreted as different types of particle tilt. These results were used in selecting a subgroup of untilted particles for averaging and symmetry analysis. The rotational power spectrum of the initial average suggested 13-fold symmetry. The 13-fold symmetry was used to select and group particles for further analysis. Individual particles were 13-fold rotational averaged and those with enhanced peripheral features were placed into either a right-handed subgroup or into a left-handed subgroup based on orientation of the peripheral features. Particles within each group were aligned and averaged, and a poly-C9 structure was produced which shows important structural details and from which the C9 monomer structure can be deduced. The poly-C9 structure contains a dense inner ring of diameter between 113-181 A and which is modulated into 13 discrete peaks with peak-to-peak separation of approx. 35 A. The dense inner ring is surrounded by a less dense, concentric outer rim extending to 254 A diameter. The outer rim contains projections that are contiguous with the inner peaks but are skewed relative to the ring radius to produce the appearance of a pin-wheel. These projections correspond with the peripheral features picked up in the rotationally averaged individual particles; the left- or right-handed orientation of projections may result from the up/down orientation of individual particles in ice. The C9 monomer structure within the cylinder is suggested by the density distribution. The monomer would be a rod with diameter of 35 A, oriented parallel to the cylinder axis and would be roughly perpendicular to a membrane.(ABSTRACT TRUNCATED AT 400 WORDS)

Complement C9↗
PubMed1993

Complement component C5 modulates the systemic tumor necrosis factor response in murine endotoxic shock.

Patients with disseminated Neisseria meningitidis infections (meningococcemia) suffer from a fulminant shock syndrome that is accompanied by extraordinarily high concentrations in serum of tumor necrosis factor (TNF). People with homozygous deficiencies of late complement components (C5, C6, C7, and C8) experience a high incidence of disseminated neisserial infections yet suffer from an attenuated form of the disease. The mechanisms that account for this disparity in host response are unclear, but they may in part be related to differences in the systemic TNF response that are modulated by terminal complement components (C5 to C9). The role of C5 in the modulation of the systemic endotoxin-induced TNF response was studied with matched strains of C5-deficient (B10 D2/Osn) and complement-sufficient (B10 D2/Nsn) mice. Following lipopolysaccharide (LPS) administration, complement-sufficient mice exhibited more rapid increases in pulmonary and hepatic vascular permeabilities than did C5-deficient controls. Complement-sufficient mice developed acute passive hepatic congestion, they appeared more ill than C5-deficient mice, and they exhibited a twofold greater rise in serum TNF activity compared with that by C5-deficient mice. C5-deficient mice reconstituted with normal serum before an LPS injection exhibited pulmonary and hepatic vascular permeability increases and serum TNF levels approaching those observed in complement-sufficient mice. Alveolar and peritoneal macrophages isolated from complement-sufficient and C5-deficient mice and incubated in heat-inactivated serum did not exhibit differences in TNF mRNA expression or secreted TNF activity following stimulation with LPS. However, incubation of macrophages in complement-sufficient mouse serum (before LPS stimulation) resulted in increased TNF mRNA expression and TNF activity compared with those in cells incubated in C5-deficient serum. In vitro studies employing human complement components and peripheral blood monocytes revealed that recombinant C5a, in the presence or absence of LPS, can induce increased concentrations of TNF and that C5b to C9 had no additional modulatory effect on the TNF response. These data suggest that C5 modulates the endotoxin-triggered TNF response. The role of complement components distal to C5 (i.e., C5b to C9) in the endotoxin-triggered TNF response remains unclear.

Animals↗
PubMed1988

Properdin, the terminal complement components, thrombospondin and the circumsporozoite protein of malaria parasites contain similar sequence motifs.

Properdin is a plasma glycoprotein which stabilizes the C3bnBb enzyme complex of the alternative pathway of the complement system. Unlike the classical pathway, which is initiated by interaction of C1q with the Fc regions of IgG or IgM antibodies in immune complexes, the alternative pathway can be directly activated via binding of C3b to surfaces of foreign organisms. The stabilized C3bnBbP complex activates components C3 and C5 resulting in opsonization of foreign material (via C3b) and assembly of the membrane attack complex (via C5b) on target cells. Therefore properdin greatly enhances complement-mediated clearance and inactivation mechanisms in both natural and acquired resistance to infection. This paper shows that the primary amino acid sequence of properdin is composed mainly of six repeating motifs, each of approximately 60 amino acids, and that similar sequences are found in thrombospondin, the circumsporozoite protein of malaria parasites and regions of the membrane-attack components of complement. These similarities may provide insight into the mechanisms by which parasites avoid host defences mediated by complement.

Amino Acid Sequence↗
PubMed2003

The actin cytoskeleton facilitates complement-mediated activation of cytosolic phospholipase A2.

Cytosolic PLA(2)-alpha (cPLA(2)) and metabolites of arachidonic acid (AA) are key mediators of complement-dependent glomerular epithelial cell (GEC) injury. Assembly of C5b-9 increases cytosolic Ca(2+) concentration and results in transactivation of receptor tyrosine kinases and activation of PLC-gamma 1 and the 1,2-diacylglycerol (DAG)-PKC pathway. Ca(2+) and PKC are essential for membrane association and increased catalytic activity of cPLA(2). This study addresses the role of the actin cytoskeleton in cPLA(2) activation. Depolymerization of F-actin by cytochalasin D or latrunculin B reduced complement-dependent [(3)H]AA release, as well as the complement-induced increase in cPLA(2) activity. These effects were due to inhibition of [(3)H]DAG production and PKC activation, implying interference with PLC. Complement-dependent [(3)H]AA release was also reduced by jasplakinolide, a compound that stabilizes F-actin and organizes actin filaments at the cell periphery, and calyculin A, which induces condensation of actin filaments at the plasma membrane. The latter drugs did not affect [(3)H]DAG production, suggesting their inhibitory actions were downstream of PKC. Neither cytochalasin D, latrunculin B, nor calyculin A affected association of cPLA(2) with microsomal membranes, and cytochalasin D and latrunculin B did not alter the localization of the endoplasmic reticulum. Stable transfection of constitutively active RhoA induced formation of stress fibers, stabilized F-actin, and attenuated the complement-induced increase in [(3)H]AA. Thus in GEC, cPLA(2) activation is dependent, in part, on actin remodeling. By regulating complement-mediated activation of cPLA(2), the actin cytoskeleton may contribute to the pathophysiology of GEC injury.

Actin Cytoskeleton↗
PubMed1988

Molecular mechanisms of lymphocyte-mediated killing.

Lymphocytes kill tumor cells and other target cells by a contact-dependent mechanism. Killer lymphocytes acquire cytoplasmic granules upon lymphokine stimulation and these granules in turn contain potent cytotoxic mediators, some of which have been characterized recently. These include (i) a pore-forming protein (perforin) of 70 kDa that polymerizes into transmembrane tubules in the presence of calcium and that is structurally related to the terminal components (C5b-6, C7, C8 and C9) of the complement cascade, (ii) a cytokine related to tumor necrosis factor and lymphotoxin that causes DNA fragmentation in target cells, and (iii) a family of serine esterases presently without any known function. The relevance of these mediators in lymphocyte-mediated killing is currently being investigated. The identification and characterization of additional putative mediators of cytotoxicity in the future will undoubtedly provide us with a better understanding of the molecular basis of cell-mediated killing.

Antibody-Dependent Cell Cytotoxicity↗
PubMed1993

Glomerular C3c localization indicates ongoing immune deposit formation and complement activation in experimental glomerulonephritis.

In antibody-mediated glomerular disease, deposits of C3 (C3b) are common and are degraded by factor I to C3c and C3d. However, the kinetics of C3b degradation in glomerulonephritis have not been defined. To do this, we studied three models of complement-dependent glomerulonephritis with established C3 deposits (passive Heymann nephritis, cationized immunoglobulin G membranous nephropathy, and concanavalin A-anticoncanavalin A glomerulonephritis). C3b deposition was halted by administration of cobra venom factor, and the disappearance of C3c and C3d from glomeruli was measured with specific antibodies and quantitative fluorescence densitometry. Results showed that C3c deposits were reduced by over 85% within 24 hours in all three models. C3c clearance was unaffected by site or mechanism of deposit formation. C3d deposits persisted despite lack of ongoing complement activation. In passive Heymann nephritis when disease activity was monitored by urinary C5b-9 excretion, C3c was cleared in parallel with return of urine C5b-9 excretion to normal values. We conclude that glomerular deposits of C3c are cleared within 24 hours of cessation of complement activation. Positive staining for C3 utilizing antibody specific for the C3c portion documents recent complement activation usually reflecting new immune deposit formation.

Animals↗
PubMedDate not supplied

Pro-inflammatory complement activation by the A beta peptide of Alzheimer's disease is biologically significant and can be blocked by vaccinia virus complement control protein.

The amyloid plaque is the hallmark of Alzheimer's disease (AD). The transmembrane domain and a portion of the C-terminus (A beta) of the amyloid precursor protein, are known to form the nucleus of the amyloid plaque. It has been demonstrated recently, using in vitro assays, that the A beta peptide can activate both the classical (antibody-independent) and alternate pathways of complement activation. The proposed complement activation is due to the binding of A beta to the complement components C1q and C3, respectively, which initiate formation of the proinflammatory C5a and C5b-9 membrane attack complex. In this report, we have investigated the in vitro findings for the likely complement-dependent proinflammatory properties of the Alzheimer's disease A beta peptide. We have performed experiments using congenic C5-deficient and C5-sufficient mice injected with synthetic A beta and recombinant polypeptide (C-100) containing A beta. Injection of C-100 into C5-sufficient mice induced a clear increase in the number of polymorphonuclear cells (neutrophils) at the site of injection due to complement activation and the subsequent release of proinflammatory chemtoactic factors. In sharp contrast, the C5-deficient mice did not show any increase in cellular influx. The vaccinia virus complement control protein, an inhibitor of both the classical and alternate pathway can down-regulate the biologically significant activation of complement by A beta, as demonstrated by an in vitro immunassay. The therapeutic down-regulation of A beta-caused complement activation could greatly alleviate the progression of some of the chronic neurodegeneration characteristic of Alzheimer's disease.

Alzheimer Disease↗
PubMed2001

Formation of high-affinity C5 convertases of the alternative pathway of complement.

Cleavage of C5 by C5 convertase is the last enzymatic step in the complement activation cascade leading to the formation of the cytolytic proteolytically activated form of C5 (C5b)-9 complex. In the present study, we examined the effect of the density of C3b (the proteolytically activated form of C3) on the function of the noncatalytic subunit of natural surface-bound forms of the enzyme. A comparison of the kinetic parameters of C5 convertases assembled on three surfaces (zymosan, rabbit erythrocytes, and sheep erythrocytes) were similar and revealed that the average K:(m) decreased approximately 28-fold (5.2-0.18 microM) when the density of C3b was increased from approximately 18,000 to 400,000 C3b/cell. Very-high-affinity C5 convertases were generated when preformed C3 convertases were allowed to self amplify by giving them excess C3. These convertases exhibited K(m) from 0.016 to 0.074 microM, well below the normal plasma concentration of C5 in blood (0.37 microM). The results suggest that in serum convertases formed with monomeric C3b will be relatively inefficient in capturing C5 but will continue to cleave C3 opsonizing the cell surface for phagocytosis, whereas convertases formed with C3b-C3b complexes in areas of high C3b density will primarily cleave C5. The catalytic rate of these convertases approaches maximum velocity, thereby switching the enzyme from cleavage of C3 to cleavage of C5, and production of the cytolytic C5b-9 complex.

Animals↗
PubMed2000

Local activation of the complement system in endoneurial microvessels of diabetic neuropathy.

Quantitative immunocytochemical analysis of complement proteins (CP) was performed on sural nerve biopsies from 15 patients with diabetic neuropathy (DN) and 18 nondiabetic patients with other forms of chronic neuropathy (ON). The mean age of the patients and the pathological severity of the neuropathy were similar in both groups. The percentage of patients that expressed strongly immunoreactive CP in the walls of endoneurial microvessels was significantly greater in DN than in ON for all proteins tested. C3d neoantigen was expressed in 100% of DN cases compared with 17% of ON; and membrane attack complex (MAC), C5b-9 neoantigen, in 93% of DN and 17% of ON. In the cases with DN, 81% of endoneurial microvessels, as identified by the endothelial marker, Ulex europaeus, contained C5b-9 neoantigen deposits, compared with 22% in those of ON, and the staining in DN was significantly more intense. Expression of the neoantigens of C3d and C5b-9 in nerve implies local activation of the complement system. In DN, activation of the complement pathway and formation of the MAC could injure blood vessels and adversely affect the circulation in the endoneurium.

Adult↗
PubMed1985

Functional domains of the alpha subunit of the eighth component of human complement: identification and characterization of a distinct binding site for the gamma chain.

The purified gamma subunit of the eighth component of human complement (C8) was used to characterize its site of interaction within C8 and to probe the ultrastructure of membrane-bound C5b-8 and C5b-9 complexes. Purification of gamma was accomplished by separating the disulfide-linked alpha-gamma subunit from the noncovalently associated beta chain and subjecting the former to limited reduction, alkylation, and ion-exchange chromatography. Upon mixing, purified alpha and gamma exhibited a high affinity for each other, as evidenced by their ability to form a noncovalent, equimolar complex at dilute concentrations and in the presence of excess serum albumin. Purified gamma also exhibited an affinity for C8', a previously described derivative that is functionally similar to C8 although it is composed of only alpha and beta. These results indicate that alpha possesses a specific site for interaction with gamma and that this site is preserved in the isolated subunit. Furthermore, this site remains accessible when alpha is associated with beta. In related experiments, gamma was found to specifically associate with membrane-bound C5b-8' and C5b-(8')9 complexes. These results indicate that the site for gamma interaction remains accessible on alpha in C5b-8' and is not shielded by C9 within C5b-(8')9. It is concluded that the gamma subunit of C8 is located on the surface of membrane-bound C5b-8 and C5b-9.

Binding Sites↗