Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Complement Membrane Attack Complex”

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 127 records · Page 7Linked to original sources
PubMed1989

Immunoelectron microscopic localization of membrane attack complex and hepatitis B e antigen in membranous nephropathy.

Immunoelectron microscopy was used to localize membrane attack complex (MAC) and hepatitis B e (HBe) antigen in renal tissue specimens from a total of 9 patients with membranous nephropathy (MN); 6 with MN associated with a hepatitis B virus (HBV) infection, 2 with idiopathic MN, and 1 with lupus nephritis. All the patients were proteinuric, and 2 patients were classified as stage I-II, 6 as stage II, and 1 as stage IV. MAC, along with IgG and C3, was distributed within the subepithelial electron dense deposits in all the stages. MAC was also stained in the striated membranous structures within the glomerular basement membrane and mesangial matrix of some patients. In HBV-associated MN, HBe antigen was localized in the subepithelial electron dense deposits of 5 patients, while it was absent from the subepithelial deposits in a patient that was sero-positive for hepatitis B s antigen but negative for HBe antigen. This patient also lacked MAC deposition in these loci. These results suggest that MAC is associated with the formation of subepithelial deposits and proteinuria in MN. In HBV-associated MN, HBe antigen-antibody immune complex makes up the subepithelial deposits and is likely to activate the terminal components of complement in situ.

Complement Membrane Attack Complex↗
PubMed1989

Complement-mediated endothelial cell damage in immune complex vasculitis of the skin: ultrastructural localization of the membrane attack complex.

Activation of the complement system is an important element in our concept of the pathomechanism of immune complex (IC) vasculitis. Both deposition of IC and attraction of polymorphonuclear leukocytes (PMN) are effected by products of complement activation. Actual tissue damage, however, is believed to be caused by PMN penetrating the vessel wall. Our former finding that deposits of membrane attack complex of complement (MAC) are found predominantly in skin lesions of patients with IC vasculitis and not in perilesional skin, has raised the question whether the complement system itself (by way of the MAC) contributes to tissue damage. Our present study shows the ultrastructural localization of MAC in lesional and clinically uninvolved skin in two patients with a cutaneous IC vasculitis. Lesional skin deposits of MAC were found on endothelial cells (EC) of upper dermal vessels and on infiltrating PMN. Uninvolved skin deposits of MAC were found on some EC, but clearly to a lesser extent than on EC of the lesional skin. In the skin of two healthy controls MAC was only found sporadically on EC. Deposits of MAC on EC in the lesional skin were often associated with a typical form of local cell swelling. This local form of endothelial cell swelling was incidentally seen in vessels of clinically uninvolved skin, but not in the skin of the two controls. The association of the endothelial cell swelling with deposits of MAC suggests that the complement system can have a direct damaging effect on EC in IC vasculitis by the assembly of MAC on the endothelial cell membrane.

Aged↗
PubMed1985

Inside-out vesicles prepared from complement lysed sheep red blood cells.

Antibody sensitized sheep red blood cells were lysed to 68% by diluted human serum. The ghosts were shown to undergo vesiculation induced by low ionic strength buffer and gentle shearing forces like osmotically lysed ghosts. About 45% of the vesicles derived from complement lysed ghosts (as referred to vesicle protein) remained floating on top of a linear 3-18% dextran T110 gradient during ultracentrifugation. Using antibodies as probes for sideness about 90% of the floating vesicles displayed an inside-out orientation. Floating inside-out vesicles bear a slightly reduced but significant number of C9 molecules on their inverted extracellular side of the membrane as compared to the (leaky or collapsed) vesicles banding in the gradient. This finding suggests that the floating is not due to a lack of C5b-9 (under the assumption that all C9 was bound to C5b-9). It is concluded that lysis with diluted human serum may result partly in ineffective C5b-9 complexes and/or in lesions with limited permeability which do not impair vesicle floating on top of polymer gradients.

Animals↗
PubMed1984

Measurement of the ratio of the eighth and ninth components of human complement on complement-lysed membranes.

The mole ratio of the eighth (C8) and ninth (C9) components of human complement on membranes carrying the cytolytic C5b-9 complex was measured by direct binding assays. Erythrocytes from two different species were used as the membrane system. Antibody-treated sheep erythrocytes carrying a relatively small number of precursive membrane-bound C5b-7 complexes were prepared by exposure to human C8-depleted serum. These complexes were subsequently converted to C5b-8 by addition of saturating amounts of C8. Parallel binding assays using 125I-C8 were used to determine the exact amount bound and thus the number of C5b-8 complexes per cell. These cells were subsequently incubated with excess 125I-C9 and the amount bound relative to C8 on the membrane was measured. Results indicated the C8:C9 ratio remained constant at approximately 1:4 as the number of complexes varied from 40 to 310 per cell. Similar results were obtained regardless of whether C8 and C9 were added sequentially or simultaneously to cells bearing C5b-7. For comparison, experiments were also performed using membranes that contained a high number of complexes. Here, rabbit erythrocytes which carried approximately 25 000 C5b-7 per cell were incubated with limited amounts of C8 to form C5b-8 complexes on the membrane surface, the exact number of which was measured by 125I-C8 binding assays. When erythrocytes prepared in this manner were incubated with excess 125I-C9, the ratio of C8:C9 on the membrane was found to be essentially constant at approximately 1:3 as the number of these complexes varied from 50 to 4000 per cell.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1988

Complement induces a transient increase in membrane permeability in unlysed erythrocytes.

The effects of low concentrations of human serum on antibody-sensitized sheep erythrocytes (EA) were studied. We report that exposure to low concentrations of serum induced a large but transient increase in the membrane permeability of those EA that do not lyse. This change in the permeability of the erythrocyte membrane resulted in net uptake of Na+ and decrease in cell K+, without affecting the total internal cation content. Although exposure to serum also allowed for net uptake of larger molecules like L-glucose, it did not lead to cell swelling. Experiments with sera genetically deficient in one of the terminal complement components showed that C8, but not C9, was required to produce the observed change in membrane permeability. Therefore, we propose that the C5b-8 complex can mediate the transient increase in permeability observed in unlysed erythrocytes during complement activation by whole serum.

Animals↗
PubMed1991

C6 depletion reduces proteinuria in experimental nephropathy induced by a nonglomerular antigen.

The role of the complement membrane attack complex, C5b-9, in mediating glomerular injury has been well defined in models of membranous nephropathy induced by antibody to endogenous glomerular epithelial cell membrane antigens. The effect of selective C6 depletion (to prevent C5b-9 formation) on morphologic characteristics and proteinuria in a model of in situ subepithelial immune complex nephritis induced by an exogenous cationized antigen (human immunoglobulin G (IgG)) followed by rabbit antibody to human IgG was studied. Selective C6 depletion was achieved by repeated administration of a goat antibody to rat C6. Other groups were treated with cobra venom factor to induce generalized complement depletion and with sublethal irradiation to deplete circulating leukocytes. In C6-depleted rats, C6 levels were reduced to less than 3% of baseline throughout the 2 days of the study compared with over 100% in controls. At 4 h after disease induction, glomerular deposition of antigen and antibody were similar in C6D and control groups by immunofluorescence and by direct measurement of glomerular deposition of radiolabeled antigen and antibody (cationized 131I human IgG, 9.1 +/- 0.1 micrograms/38,000 glomeruli in C6D versus 9.8 +/- 0.9 in controls; P = was not significant; rabbit 125I-labeled anti-human IgG, 104 +/- 10 ng in C6D versus 80 +/- 9 ng in controls; P = was not significant). Circulating C3 levels and glomerular C3 deposition were also similar in C6D and control groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1989

Regulatory control of complement on blood platelets. Modulation of platelet procoagulant responses by a membrane inhibitor of the C5b-9 complex.

Antibody against a membrane inhibitor of the C5b-9 complex has been used to investigate regulatory control of the terminal complement proteins on blood platelets. Monospecific rabbit antibody (alpha-P18) was raised against the purified 18-kDa erythrocyte membrane inhibitor of C5b-9 (Sugita, Y., Nakano, Y., and Tomita, M. (1988) J. Biochem. (Tokyo) 104, 633-637). In addition to its interaction with erythrocytes, this antibody (and its Fab) bound specifically to platelet membranes. In immunoblots of cell membrane proteins prepared under non-reducing conditions, alpha-P18 bound specifically to an 18-kDa erythrocyte membrane protein and to a 37-kDa platelet membrane protein. Absorption of this antibody by platelet membranes competed its binding to the purified 18-kDa erythrocyte protein, suggesting that epitopes expressed by the erythrocyte 18-kDa C5b-9 inhibitor are common to the platelet. When bound to the platelet surface, the Fab of alpha-P18 increased C9 activation by membrane C5b-8, monitored by exposure of a complex-dependent C9 neo-epitope. Although alpha-P18 caused little increase in the cytolysis of platelets treated with C5b-9 (total release of lactate dehydrogenase less than 5%), it markedly increased the cell stimulatory responses induced by these complement proteins, including, secretion from platelet alpha- and dense granules, conformational activation of cell surface GP IIb-IIIa, release of membrane microparticles from the platelet surface, and exposure of new membrane binding sites for components of the prothrombinase enzyme complex. Prior incubation of C5b67 platelets with 100 micrograms/ml alpha-P18 (Fab) lowered by approximately 10-fold the half-maximal concentration of C8 required to elicit each of these responses (in the presence of excess C9). Incubation with alpha-P18 (Fab) alone did not activate platelets, nor did incubation with this antibody potentiate the stimulatory responses of platelets exposed to other agonists. These data indicate that a membrane inhibitor of the C5b-9 complex normally serves to attenuate the procoagulant responses of blood platelets exposed to activated complement proteins, and suggest the mechanism by which a deletion or inactivation of this cell surface component would increase the risk of vascular thrombosis.

Blood Platelets↗
PubMed1988

Deposition of terminal C5b-9 complement complexes on erythrocytes and leukocytes during cardiopulmonary bypass.

Hemolysis, leukopenia, a hemostatic deficit, and nonspecific systemic reactions collectively known as the postperfusion syndrome develop in patients who undergo cardiopulmonary bypass. We now report that terminal C5b-9 complement complexes are deposited on erythrocytes and polymorphonuclear neutrophilic leukocytes during cardiopulmonary bypass. Plasma samples taken from 48 unselected patients during and at the end of cardiopulmonary bypass contained raised levels of fluid-phase SC5b-9 complement complexes, indicating that the complement sequence had been activated to completion. Various degrees of overt intravascular hemolysis were observed in all the patients, and lysed erythrocyte membranes were recovered from the blood samples. Immunoassays performed with use of antibodies to C5b-9 neoantigens demonstrated the presence of C5b-9 on red-cell ghosts but not on intact erythrocytes. The appearance of ghosts carrying C5b-9 always coincided with hemolysis. Furthermore, granulocytes isolated from 20 patients during bypass were all found to carry C5b-9 complexes, whereas cells isolated before or 24 hours after surgery carried no C5b-9. The neoantigen-positive material present in detergent extracts of granulocytes sedimented in a broad peak (25 to 40 sedimentation coefficient [S]) in sucrose-density gradients, exactly as did pore-forming C5b-9 complexes. Deposition of C5b-9 on blood cells during cardiopulmonary bypass may be partly responsible for the hemolysis and may augment granulocyte activation by the stimulation of arachidonate metabolism in those cells.

Autoantibodies↗
PubMed1987

Haemolytic 'efficiency' of C5b-9 complexes in drug-induced immune haemolysis: role of cellular C5b-9 distribution.

We report on quantitative analyses of C5b-9 binding to target red blood cells (RBC) lysed through the action of drug (nomifensine)-dependent antibodies (ddab) and anti-RBC antibodies (warm plus cold agglutinins). Immunoradiometric assays showed that, at any given degree of haemolysis, more C5b-9 was bound to cells sensitized with anti-RBC antibodies compared to cells lysed via ddab. Thus, C5b-9 complexes deposited through the action of ddab appeared to be haemolytically more 'efficient' than those deposited via anti-RBC antibodies. However, data obtained from flow-cytometric assays and immunoelectron microscopy demonstrated a major role for the distribution (total number) of C5b-9 lesions among the cells within a given cell population. Enhanced haemolysis with a given total amount of C5b-9 complexes appeared to be derived from a more even distribution of these complexes among the cells rather than from higher lytic efficiency of individual complexes. Since ddab bind with low affinity to cells, we suggest that these antibodies diffuse from cell to cell and cause extensive haemolysis through the deposition of relatively few C5b-9 complexes at each site. In contrast, high affinity antibodies remain largely bound to relatively restricted sites causing deposition of larger amounts of C5b-9 on, and hence lysis of, numerically fewer cells. With respect to the net intravascular haemolytic effect, our findings may explain why complement-activating antibodies of low affinity are often more detrimental than high affinity antibodies.

Antibody Affinity↗
PubMed1986

Repolarization of the membrane potential of blood platelets after complement damage: evidence for a Ca++ -dependent exocytotic elimination of C5b-9 pores.

Gel-filtered blood platelets exposed to complement proteins C5b-9 have previously been shown to undergo a reversible depolarization of membrane potential (Em) in the absence of lytic plasma membrane rupture. In this paper, we examine the mechanism by which C5b-9 damaged platelets restore their basal electrochemical state, despite increased ion conductance due to membrane insertion of these cytolytic serum proteins. Repolarization of Em after formation of the C5b-9 membrane pore is shown to be accompanied by a Ca++-dependent vesiculation of the platelet surface, which results in the release of these proteins from the plasma membrane and a restoration of the membrane's functional integrity. This exocytotic elimination of C5b-9 complexes from the plasma membrane is accompanied by a ouabain-inhibitable repolarization of Em, which presumably reflects restoration of transmembrane cation gradients by the plasma membrane Na/K ATPase. The role of external Ca++ in the platelet's response to membrane-insertion of the C5b-9 proteins is discussed both in the context of the known cellular effects of this ion and in the context of recent observations suggesting sublytic changes in platelet function after complement-mediated plasma membrane damage.

Adult↗
PubMed1987

Immunohistochemical detection of the membrane and fluid-phase terminal complement complexes C5b-9(m) and SC5b-9. Consequences for interpretation and terminology.

Activation of the terminal pathway of complement on a membrane results in the generation of the membrane-damaging terminal C5b-9(m) complement complex, whereas the non-lytic water-soluble SC5b-9 complex is formed when complement is activated in the fluid phase. Both forms of the terminal complement complex (TCC) can be immunohistochemically detected, but not distinguished, by antibodies recognizing neoantigens in the complexes. By means of monoclonal antibodies against C9 neoantigens and against the S-protein, it was demonstrated that deposits of the TCC in tissue sections may be either in the form of C5b-9(m) or SC5b-9. The consequences of this for the interpretation of the histochemical data and the terminology of the two complexes are discussed.

Adult↗
PubMed1981

Permeability characteristics of complement-damaged membranes: evaluation of the membrane leak generated by the complement proteins C5b-9.

Permeability characteristics of the membrane lesion generated by the terminal complement proteins are considered in light of recent observations that the measured diffusion of solute across complement-damaged membranes does not conform to the "doughnut hole" model of a discrete transmembrane pore formed by the inserted C5b-9 complex. By using the measured kinetics of steady-state tracer isotope diffusion of nonelectrolytes across resealed erythrocyte ghost membranes treated with C5b-9, a new transport model is developed. This model considers the apparent membrane lesion strictly in terms of the operational criteria of a functional conducting pathway for the observed diffusing solute, independent of a priori assumptions about the geometry or molecular properties of the membrane lesion. With this definition of the unit membrane lesion and the assumption that the exclusion size of the conducting pathway varies directly with the multiplicity of bound C5b-9 (as suggested by previous measurements under conditions of varying input of C5b-9), numerical estimates of te apparent permeability of the complement-damaged membrane to four diffusing nonelectrolytes are derived. These results suggest that the pathway for a particle diffusing across the complement lesion cannot be a pore and is functionally equivalent to an aqueous leak pathway, free of pore constraints. Implications of these results are discussed in terms of current molecular models for the mechanism of membrane damage by the complement proteins.

Biological Transport↗
PubMed1983

Size distribution and stability of the trans-membrane channels formed by complement complex C5b-9.

We have performed double marker sieving experiments with molecules ranging from ca. 0.5-3 nm dia. in order to evaluate the size distribution of the channels formed by complement in resealed sheep erythrocyte ghosts. Evidence is presented that marker release through the channels reached equilibrium between the ghosts and the extracellular fluid in a period of 3 hr and that the channels are stable at 37 degrees C for this period of time. Under these experimental conditions we have observed a differential in the endpoint release of inositol and sucrose, which indicates that some of the ghosts carried channels measuring between 0.7 and 0.9 nm dia. No differential was observed between release of sucrose and raffinose (0.9 and 1.1 nm mol. dia., respectively). Comparisons between sucrose and inulin (0.9 and 3 nm mol. dia, respectively) showed a difference in marker release. Also, there was substantial release of inulin, indicating the presence of channels above 3 nm in dia. Hence, the present data indicate formation of channels in three size ranges, namely, 0.7-0.9 nm, 0.9-3 nm and greater than 3 nm.

Animals↗
PubMed1982

Cutaneous localization of the membrane attack complex in discoid and systemic lupus erythematosus.

Biopsy specimens of skin lesions from three patients with discoid lupus erythematosus and six patients with systemic lupus erythematosus contained the membrane attack complex, which comprises C5b through C9, as well as immune complexes at the dermal-epidermal junction. The basilar epithelium in these areas was vacuolated and edematous, and the dermis contained an inflammatory infiltrate. In contrast, 19 of 29 specimens of normal-appearing skin from patients with discoid or systemic lupus erythematosus showed only immune complexes at the dermal-epidermal junction, without the membrane attack complex. The other 10 specimens, all from patients without cutaneous involvement, showed neither immune complexes nor membrane attack complexes. These data suggest that immune complexes within skin lesions selectively generate the assembly of the membrane attack complex, which mediates membrane injury. A synergistic interaction of immune complexes and cofactors may be required to activate complement in areas of skin that are predisposed to tissue injury.

Adult↗
PubMed1985

Formation of soluble immune complexes by complement in sera of patients with various hypocomplementemic states. Difference between inhibition of immune precipitation and solubilization.

To examine whether the ability of complement to form soluble immune complexes plays a role in preventing immune complex-mediated diseases, we analyzed the capacity of complement to inhibit immune precipitation (IIP) and to solubilize preformed immune aggregates (SOL) in 23 sera of patients with various hypocomplementemic states, and we correlated the results of these studies with the clinical syndromes found in the various patients. In sera with deficiency or depletion of early classical pathway components, IIP was profoundly altered, whereas SOL was delayed but in the normal range. In contrast, in sera with C3 depletion but intact classical pathway and in properdin-deficient serum, IIP was initially preserved, whereas SOL was abolished. Since the incidence of immune complex diseases in various hypocomplementemic states correlates with the severity of IIP defects, but not with reduced SOL, it is suggested that IIP is an essential biological function of complement that prevents the rapid formation of insoluble immune complexes in vivo.

Antigen-Antibody Complex↗
PubMed1987

Paroxysmal nocturnal hemoglobinuria type III. Lack of an erythrocyte membrane protein restricting the lysis by C5b-9.

The complement-mediated lysis is inefficient when complement and target cells are homologous with regard to the species. In erythrocytes from patients suffering from paroxysmal nocturnal hemoglobinuria (PNH), the species restriction is lost: PNH-erythrocytes (PNH-E) are susceptible to lysis by human complement. In human erythrocytes (huE) the species restriction is ascribed to an integral membrane protein, designated C8-binding protein (C8bp). In the present study, we tested membranes of PNH-E type III for the presence of C8bp. A protein with C8-binding capacity could not be detected. C8bp, which was isolated from the membrane of huE, inhibited the lysis of PNH-E by C5b-9 as well as the C9 polymerization. Thus, addition of C8bp restored the species restriction in PNH-E. In conclusion, we propose that lack of C8bp might represent the defect in PNH-E type III membranes, which is responsible for their enhanced lytic susceptibility towards lysis by the late complement components.

Complement C8↗
PubMed1988

Paroxysmal nocturnal hemoglobinuria. Enhanced stimulation of platelets by the terminal complement components is related to the lack of C8bp in the membrane.

Recently, a protein isolated from the membrane of human E, the so-called C8 binding protein (C8bp), has been described. C8bp is characterized as a 65-kDa protein that binds to C8 and inhibits the C5b-9-mediated lysis in a homologous system. In the present study, membranes of peripheral blood cells were tested for the presence of C8bp by SDS-PAGE and immunoblotting. In all cells a protein band reacting with anti-C8bp was seen, the Mr, however, was only about 50 kDa. To further analyze the 50-kDa protein, we isolated the protein by phenol-water extraction and isoelectric focusing from papain-treated platelets. The isolated protein behaved similar to the E-derived C8bp: it inhibited the lysis of model target cells by C5b-9. To examine the function of C8bp in platelets, we tested platelets from patients suffering from paroxysmal nocturnal hemoglobinuria (PNH). These platelets were deficient in C8bp, being in accordance with their higher lytic susceptibility in vitro. In response to sublytic C5b-9 doses, the PNH platelets released considerably more serotonin and thromboxane B2 than normal platelets. By addition of purified C8bp, the thromboxane B2 release was suppressed, indicating that C8bp not only restricts the lytic complement attack, but also regulates the C5b-9-mediated stimulation of target cells. Thus, lack of C8bp might not only result in enhanced hemolysis, but also in enhanced stimulation of platelets, which in turn might contribute to the thrombotic complications seen in some PNH-type III patients.

Blood Platelets↗