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PubMed1975

On the mechanism of cytolysis by complement: evidence on insertion of C5b and C7 subunits of the C5b,6,7 complex into phospholipid bilayers of erythrocyte membranes.

The doughnut hypothesis of cytolysis by complement [Mayer, M. M. (1972) Proc. Nat. Acad. Sci. USA 69, 2954-2958] describes an annular structure made up of C5b-9 (complement factors C5b, C6, C7, C8, and C9) which becomes inserted in the lipid bilayer of the cell membrane, thus creating a hole. We now present initial explorations of this hypothesis. EAC1-6 and EAC1-7 (sheep erythrocytes carrying rabbit antibody and complement factors C1 through C6 or C1 through C7, respectively), prepared with either 125I-C3 or 125I-C5 were incubated with trypsin and the release of bound 125I was measured. In the case of 125I-C3, all of the radioactivity was released by trypsin from both intermediates. With 125I-C5, trypsin released all of the 125I from EAC1-6, but only 40-55% from EAC1-7. Possible reasons for resistance of the C5b subunit in EAC1-7 to tryptic digestion are discussed; in terms of the doughnut hypothesis it would be due to shielding by lipid molecules as a consequence of insertion into the lipid bilayer. In accord with this interpretation we have also found that C5b in EAC1-7, but not in EAC1-6, resists elution by 0.3 M NaC1. Similarly, we have found that 125I-C7 in EAC1-7 resists stripping by trypsin. Hence, we now propose the hypothesis that hydrophobic polypeptide chains from the C5b and the C7 subunits of C5b,6,7 complex become inserted in the phospholipid bilayer and that subsequent reactions with C8 and C9 open a channel across the membrane.

Animals↗
PubMed1975

Cyanate as an inactivator of complement proteins.

Sodium cyanate added to normal human serum or serum from patients with sickle-cell disease resulted in the functional inactivation of C3, C5, C6, C7, and the C3b inactivator, but not C8 and C9. Final concentrations as low as 0.5 mM in serum caused inactivation of 12 to 64% of the C3 after 8 hr at 37 degrees C. The activity of the inactivated C3, C5, and C3b inactivator was not restored by dialysis. Most of the functional activity of C3 in cyanate-treated sera was destroyed by very small quantities of 14C-labeled cyanate that was bound to the protein. C3 inactivation by cyanate occurred in heated sera (50 degrees C, 30 min) and sera treated with EDTA, probably indicating that one mechanism for inactivation was by a direct carbamylation reaction. Both C3 and C5 showed two anodal-migrating forms in two dimensional antigen-antibody crossed electrophoresis in some sera treated with low concentrations of cyanate. Measurements of circular dichroism of highly purified carbamylated C3 showed no detectable changes in structure even though most of the functional activity was destroyed. Purified, inactive C3 that was carbamylated with 14C-labeled cyanate was capable of binding to EAC142, but the resulting EAC1423 was weakly positive for immune adherence and negative for agglutination with anti-C3 antiserum. Unlabeled, cell-bound C3b on EAC142 was not susceptible to cyanate action as shown by no loss in immune adherence and positive agglutination with anti-C3 antiserum. The C3b inactivator was more susceptible to cyanate than C3 in a short time period, whereas both were inactivated after 8 hr. Since cyanate is currently being evaluated as a treatment for sickle-cell disease, the inactivation of C3 by the drug is an important consideration for such patients who are already deficient in C3 dependent heat-labile opsonins that aid in host defense.

Anemia, Sickle Cell↗
PubMed1980

The critical role of complement in experimental pneumococcal sepsis.

Using a guinea pig model fo bacteremia due to Streptococcus pneumoniae serotype 7, opsonization by the classical and alternative pathways of complement activation was studied in immune and nonimmune animals. Depletion of the alternative complement pathway and complement components C3-C9 resulted in a significant, lethal defect of intravascular clearance in both normal and immune animals. Preopsonization corrected the initial clearance defect in complement-depleted animals. Maximal rates of clearance of bacteremia occurred in immune, normal animals, Immune, C4-deficient animals had clearance curves similar to normal, nonimmune animals. Thus, optimal clearance of pneumococcal bacteremia requires an intact alternative an classical pathway of complement activation. In the nonimmune animal, the alternative pathway provides the primary host defense against infection, whereas after immunization, optimal clearance of bacteremia requires an intact classical pathway of complement activation. However, immunization does not alter the lethal clearance defect in complement-depleted animals.

Animals↗
PubMed2005

Thrombus formation induced by antibodies to beta2-glycoprotein I is complement dependent and requires a priming factor.

We monitored the number of intravascular platelet-leukocyte aggregates (PLAs) and thrombotic occlusions (TOs) by intravascular microscopy in the mesentery of rats receiving antiphospholipid (aPL) immunoglobulin G (IgG) purified from the sera of patients with antiphospholipid syndrome. aPL IgG had no procoagulant effect, but it caused rapid endothelial deposition of fibrinogen, followed by PLA and TO in rats receiving an intraperitoneal injection of bacterial lipopolysaccharide 3 hours before IgG infusion. Anti-beta2-glycoprotein I-depleted aPL IgG failed to induce PLAs and TOs. C3 and C9 colocalized with aPL IgG on the mesenteric vessels. The number of PLAs and TOs was markedly reduced in C6-deficient rats and in animals treated with anti-C5 miniantibody, suggesting the contribution of the terminal complement (C) complex to the aPL antibody-mediated intravascular thrombosis. In conclusion, our data indicate that antibodies to beta2-glycoprotein I trigger coagulation subsequent to a priming proinflammatory factor and that the terminal C complex is the main mediator of the coagulation process.

Animals↗
PubMed1990

Regulatory control of the terminal complement proteins at the surface of human endothelial cells: neutralization of a C5b-9 inhibitor by antibody to CD59.

Functionally inhibitory antibody to the plasma membrane complement inhibitor CD59 has been used to investigate control of the terminal complement proteins at the endothelial cell surface. Antibodies against purified human erythrocyte CD59 (polyclonal anti-CD59 and monoclonal antibodies [MoAbs] 1F1 and 1F5) were found to bind specifically to monolayers of cultured human umbilical vein endothelial cells, and by Western blotting to recognize an 18- to 21-Kd endothelial protein. When bound to the endothelial monolayer, anti-CD59 (immunoglobulin G or Fab fragment) potentiated membrane pore formation induced upon C9 binding to C5b-8, and augmented the C5b-9-induced cellular responses, including stimulated secretion of von Willebrand factor and expression of catalytic surface for the prothrombinase enzyme complex. Although potentiating endothelial responses to the terminal complement proteins, anti-CD59 had no effect on the response of these cells to stimulation by histamine. Taken together, these data suggest that human endothelial cells express the CD59 cell surface inhibitor of the terminal complement proteins, which serves to protect these cells from pore-forming and cell-stimulatory effects of the C5b-9 complex. These data also suggest that the inactivation or deletion of this cell surface regulatory molecule would increase the likelihood for procoagulant changes in endothelium exposed to complement activation in plasma.

Antibodies, Monoclonal↗
PubMed1994

Defective regulation of complement by the sickle erythrocyte: evidence for a defect in control of membrane attack complex formation.

A prominent clinical manifestation of sickle cell disease (SCD) is hemolytic anemia. Although complement activation can lead to intravascular hemolysis, its role in the hemolysis of SCD is not known. Because normal red blood cells induced to vesiculate by treatment with calcium and ionophore become sensitive to damage by activated complement and because sickle cells release microvesicles as they circulate, we postulated that sickle cells might also be unusually sensitive to complement-dependent hemolysis. Complement activation is tightly regulated on the membrane of the normal erythrocyte; therefore, defective complement regulation by the sickle cell would be necessary for complement-dependent hemolysis to occur. These studies show a defect in the regulation of membrane attack complex (C5b-9) formation in sickle erythrocytes, particularly in the most dense cells. The defect is characterized by increased binding of C5b-7 and of C9 to denser sickle cells and results in increased susceptibility of sickle cells to C5b-9-mediated (reactive) lysis initiated by either C5b6 or activated cobra venom factor. Among the densest sickle cells, irreversibly sickled cells are especially sensitive to reactive lysis. The similarity of this defect to that previously described in a patient with paroxysmal nocturnal hemoglobinuria suggests that complement-mediated hemolysis could play a role in the anemia of SCD.

Adenosine Triphosphate↗
PubMed2002

CD59 blocks not only the insertion of C9 into MAC but inhibits ion channel formation by homologous C5b-8 as well as C5b-9.

Activation of the complement system on the cell surface results in the insertion of pore forming membrane attack complexes (MAC, C5b-9). In order to protect themselves from the complement attack, the cells express several regulatory molecules, including the terminal complex regulator CD59 that inhibits assembly of the large MACs by inhibiting the insertion of additional C9 molecules into the C5b-9 complex. Using the whole cell patch clamp method, we were able to measure accumulation of homologous MACs in the membrane of CD59(-) human B-cells, which formed non-selective ion channels with a total conductance of 360 +/- 24 pS as measured at the beginning of the steady-state phase of the inward currents. C5b-8 and small-size MAC (MAC containing only a single C9) can also form ion channels. Nevertheless, in CD59(+) human B-cells in spite of small-size MAC formation, an ion current could not be detected. In addition, restoring CD59 to the membrane of the CD59(-) cells inhibited the serum-evoked inward current. The ion channels formed by the small-size MAC were therefore sealed, indicating that CD59 directly interfered with the pore formation of C5b-8 as well as that of small-size C5b-9. These results offer an explanation as to why CD59-expressing cells are not leaky in spite of a buildup of homologous C5b-8 and small-size MAC. Our experiments also confirmed that ion channel inhibition by CD59 is subject to homologous restriction and that CD59 cannot block the conductivity of MAC when generated by xenogenic (rabbit) serum.

Animals↗
PubMed1974

Failure of cobra venom factor to prevent the generalized Shwartzman reaction and loss of renal cortical fibrinolytic activity.

Previous studies have shown that renal cortical fibrinolytic activity is absent in rabbits given one or two injections of endotoxin. Using a fibrin slide technic, we have studied the effect of endotoxin on cortical fibrinolytic activity in rabbits depleted of terminal complement components (C3-C9) by cobra venom factor. The loss of cortical fibrinolytic activity induced by endotoxin is not prevented by pretreatment with cobra venom factor. Additionally, this complement depletion did not prevent development of glomerular fibrin deposition, cortical necrosis or thrombocytopenia in the generalized Shwartzman reaction.

Animals↗
PubMed1976

Complement component profiles in urticaria, dermatitis herpetiformis, and alopecia areata.

Insignificant variation was noted in the mean levels of the complement components CIQ, C4, C2, C3, C5 and C3PA between various groups of sera from patients with urticaria, angio-oedema, dermatitis herpetiformis, alopecia areata, or hay fever. No deficiencies were found. Elevated C9 levels in chronic urticaria and angio-oedema reflected its nature as an acute phase protein. The assessment of C components in single or sequential serum samples revealed only marginal variations. Similar results were recorded for plasminogen the immunoglobulins, G, A and M. Some evidence is provided for the primary involvement of the kallikrein-kinin system in urticaria.

Alopecia Areata↗
PubMed1999

Complement resistance in Borrelia burgdorferi strain 297: outer membrane proteins prevent MAC formation at lysis susceptible sites.

Two variants of Borrelia burgdorferi strain 297, complement-resistant wild-type (WT297) and complement-sensitive mutant (MUT297), were used as a model to study the mechanism of resistance to the alternative complement pathway in this organism. No difference in the quantity of membrane attack complex (MAC) deposition on WT297 and MUT297 was observed after 2 h incubation with normal human serum (NHS), at which time 4% of WT297 and 95% of MUT297 were killed. The polymerization of C9 bound to WT297 and MUT297 was demonstrated by immunoblotting using an anti-C9 polyclonal antibody. Immunofluorescence and thin-section immunoelectron microscopy showed MAC to be diffusely distributed on the outer membrane of both variants. Furthermore, MAC appeared to be tightly bound to the surface of both variants as demonstrated by elution studies. Protease treatment rendered WT297 susceptible to killing by NHS, suggesting that outer membrane proteins may be associated with complement resistance of WT297. One- and two-dimensional gel electrophoreses showed that proteins of 20 and 30 kDa, and 66 kDa were present in WT297 but were absent or sparse in trypsin-treated WT297 and MUT297. Interestingly, immunoblotting using a polyclonal antibody against C3 showed that C3 fragments appeared to bind different acceptors on WT297 than on trypsin-treated WT297, or MUT297. Therefore, the binding of C3 fragments to acceptors on WT297, in contrast to MUT297, may not direct the formation of the MAC to lysis-susceptible sites on the surface of the bacterium, resulting in the complement resistance of WT297.

Antibodies, Monoclonal↗
PubMed1996

Early complement components in Alzheimer's disease brains.

Activation products of the early complement components C1, C4 and C3 can be found colocalized with diffuse and fibrillar beta-amyloid (beta/A4) deposits in Alzheimer's disease (AD) brains. Immunohistochemically, C1-esterase inhibitor (C1-Inh) and the C1 subcomponents C1s and C1r can not, or only occasionally, be detected in plaques or in astrocytes. The present finding that C1q, C1s and C1-Inh mRNA are present in both AD and control brains suggests that the variable immunohistochemical staining results for C1r, C1s and C1-Inh are due to a rapid consumption, and that the inability to detect C1s, C1r or C1-Inh is probably due to the dissociation of C1s-C1-Inh and C1r-C1-Inh complexes from the activator-bound C1q into the fluid phase. Employing monoclonal antibodies specific for different forms of C1-Inh, no complexed C1-Inh could be found, whereas inactivated C1-Inh seems to be present in astrocytes surrounding beta/A4 plaques in AD brains. These findings, together with our finding (using reverse transcriptase-polymerase chain reaction) that C1-Inh is locally produced in the brain, suggest that in the brain complement activation at the C1 level is regulated by C1-Inh. Immunohistochemically, no evidence for the presence of the late complement components C5, C7 and C9, or of the membrane attack complex (MAC), was found in beta/A4 plaques. In contrast to the mRNA encoding the early components, that of the late complement components appears to be hardly detectable (C7) or absent (C9). Thus, without blood-brain-barrier impairment, the late complement components are probably present at too low a concentration to allow the formation of the MAC, which is generally believed to be responsible for at least some of the neurodegenerative effects observed in AD. Therefore, the present findings support the idea that in AD, complement does not function as an inflammatory mediator through MAC formation, but through the action of early component activation products.

Aged↗
PubMed1998

Identity of the residues responsible for the species-restricted complement inhibitory function of human CD59.

The membrane-anchored glycoprotein CD59 inhibits assembly of the C5b-9 membrane attack complex (MAC) of human complement. This inhibitory function of CD59 is markedly selective for MAC assembled from human complement components C8 and C9, and CD59 shows little inhibitory function toward MAC assembled from rabbit and many other non-primate species. We have used this species selectivity of CD59 to identify the residues regulating its complement inhibitory function: cDNA of rabbit CD59 was cloned and used to express human/rabbit CD59 chimeras in murine SV-T2 cells. Plasma membrane expression of each CD59 chimera was quantified by use of a 5'-TAG peptide epitope, and each construct was tested for its ability to inhibit assembly of functional MAC from human versus rabbit C8 and C9. These experiments revealed that the species selectivity of CD59 is entirely determined by sequence contained between residues 42 and 58 of the human CD59 polypeptide, whereas chimeric substitution outside this peptide segment has little effect on the MAC inhibitory function of CD59. Substitution of human CD59 residues 42-58 into rabbit CD59 resulted in a molecule that was functionally indistinguishable from native human CD59, whereas the complementary construct (corresponding residues of rabbit CD59 substituted into human CD59) was functionally indistinguishable from rabbit CD59. Based on the solved solution structure of CD59, these data suggest that selectivity for human C8 and C9 resides in a cluster of closely spaced side chains on the surface of CD59 contributed by His44, Asn48, Asp49, Thr51, Thr52, Arg55, and Glu58 of the polypeptide.

Amino Acid Sequence↗
PubMed1983

Inhibition of the lytic action of cell-bound terminal complement components by human high density lipoproteins and apoproteins.

Human serum lipoproteins are known to participate in or modify several immunologically relevant responses, including the inhibition of target cell lysis initiated by fluid-phase C5b-7 (reactive lysis). We now report that human high density lipoproteins (HDL) can inhibit the complement (C) lytic mechanism after C5b-7, C5b-8, and even C5b-9 have been bound to the target membrane. This inhibitory activity of serum or plasma copurifies in hydrophobic chromatography with antigenically detected apolipoprotein A-I (apoA-I), the major HDL apoprotein, and with HDL in CsCl density gradient ultracentrifugation. Although HDL is more active than its apoproteins in fluid-phase inhibition of C5b-7-initiated reactive lysis, the HDL apoproteins are more effective after C5b-7, C5b-8, or C5b-9 have become bound to human or sheep erythrocytes (E). Highly purified HDL apoproteins, apoA-I and apoA-II, both have greater inhibitory activity than whole HDL on a protein weight basis, and some evidence has been obtained that apoA-I dissociating spontaneously from HDL may be the principal inhibitory moiety in physiological situations. HDL lipids themselves are inactive. The HDL-related inhibitors are ineffective when incubated with EC5b-7 and removed before C8 and C9 are added, and only minimally effective on cell-bound C5b-8 sites before C9 is added. They exert their most prominent inhibitory activity after C9 has been bound to EC5b-8 at low temperature, but before the final temperature-dependent, Zn(++)-inhibitable membrane damage steps have occurred. Therefore, HDL or its apoproteins do not act to repair already established transmembrane channels, but might interfere either with insertion of C9 into the lipid bilayer or with polymerization of C9 at C5b-8 sites. This heat-stable inhibitory activity can be demonstrated to modify lysis of erythrocytes in whole serum, i.e., it does not depend upon artificial interruption of the complement membrane attack sequence at any of the above-mentioned stages. Contributions of the target membrane itself to the mechanism of inhibition are suggested by the observations that, in contrast to sheep or normal human E, lysis of guinea pig E or human E from patients with paroxysmal nocturnal hemoglobinuria is inhibited poorly. This is the first description of a naturally occurring plasma inhibitor acting on the terminal, membrane-associated events in complement lysis. Although further study is required to assess the physiologic or immunopathologic significance of this new function of HDL, the HDL apoproteins or their relevant fragments should be useful experimentally as molecular probes of the lytic mechanism.

Animals↗
PubMed1993

Structure of the human C6 gene.

The terminal components of the complement system (C6-C9) are related proteins, differing in size and complexity. They seem to be typical mosaic proteins, composed of modules which are homologous with parts of other proteins. Individual elements in a mosaic protein are often bounded by introns in the gene, and where they are duplicated within a polypeptide, partial gene duplication within the gene is responsible. It is often found in such genes that the intron/exon boundaries are of the class 1 type. We have examined the boundaries of 17 of the 18 exons of C6 and five of C7. When considered with published data for C9, only one of the protein elements appears to follow the conventional pattern. These data suggest a more complex evolutionary history for the genes of the terminal complement components than had been anticipated and challenge the notions both that discovery of a recognized protein module is of predictive value in relation to gene structure and that these genes evolved from the simple to the complex.

Amino Acid Sequence↗
PubMed1998

Imaging bioluminescent indicators shows Ca2+ and ATP permeability thresholds in live cells attacked by complement.

A series of permeability thresholds to Ca2+ metabolites and macromolecules, occurring at different times when cells are attacked by complement, has been established by imaging HeLa cells transiently expressing a recombinant cytosolic fusion protein of firefly luciferase and aequorin (luciferase-aequorin) to measure changes in ATP and cytosolic free Ca2+. Nuclear fluorescence of propidium was used as a measure of permeability to small molecules, and luciferase activity imaged to assess lysis. The rise in cytosolic free Ca2+ observed after C9 attack preceded by at least 60 s both the increase in propidium fluorescence, measured in single cells, and the decrease in ATP monitored by luciferase light emission. These effects were dependent on the concentration of C9. At concentrations of C9 up to 4 micrograms/ml no loss of luciferase-aequorin protein was detected at the end of the experiment. Thus the membrane integrity of the cells remained intact, even though the cells were permeable to propidium. These results confirmed our earlier observations that propidium permeability in cells attacked by complement was not a reliable measure of cell death. They also show that it is vital to take account of cellular heterogeneity if the mechanisms by which cells respond to membrane pore former attack are to be correctly interpreted.

Adenosine Triphosphate↗
PubMed1976

Human platelet-initiated formation and uptake of the C5-9 complex of human complement.

We have studied the interaction of radiolabeled complement components with normal human platelets, platelets from a patient with paroxysmal nocturnal hemoglobinuria, and rabbit platelets in the absence of known complement activators or in the presence of cobra venom factor (CVF). When unwashed platelets in platelet-rich plasma, or washed platelets suspended in serum or autologous plasma, were incubated for 30 min, C3 and terminal components (C5, C8, and C9) were found to bind to them. The terminal components were shown to be bound as the C5-9 complex, rather than as individual proteins, by eluting them from the platelet membrane and examining their behavior on ultracentrifugation. They cosedimented at a rate characteristic of the stable C5-9 complex (22S). As many as 370-1,380 C5-9 complexes/platelet were calculated to have been bound during the incubation period. The complex so formed did not differ by ultracentrifugational criteria from that binding to rabbit platelets after CVF activation of complement. Though C3 was not included in the complex, it did not appear to be bound by nonspecific absorption. It could not be removed by washing but rather was eluted by the freeze-thaw technique used to elute the C5-9 complex. Incubation of radiolabeled components in platelet-free plasma did not result in C5-9 complex formation, indicating an initiating role for platelets in this reaction. In contrast to platelets, erythrocytes incubated in analogous plasma did not induce detectable C5-9 formation. Neither EDTA, phenylmethylsulfonylfluoride, nor epsilon-amino-N-caproic acid prevented platelet-initiated formation of C5-9, suggesting that the reaction may involve mechanisms of complement activation not previously described.

Aminocaproates↗
PubMed1978

Haemolytic complement in peripheral lymph of normal men.

The haemolytic activity of nine individual components of complement and the concentrations of C1q, C1s, C4, C3, C3PA and C9 proteins were measured in the leg lymph and serum of four normal men. The mean lymph/serum ratio for total haemolytic complement was 0.257, for C1H50 it was 0.138, for C4H50 0.105, for C2H50 0.279, for C3H50 0.063, for C5H50 0.266, for C6H50 0.145, for C7H50 0.25, for C8H50 0.244 and for C9H50 0.253. The mean lymph/serum ratio for complement proteins was: for C1q 0.048, for C1s 0.06, for C4 0.199, for C3 0.225, for C9 0.244 and for C3PA 0.263. The low haemolytic activity of total complement and of all components in lymph seems to be dependent on the low complement protein concentration. The remarkably low lymph C1q protein concentration may play a physiological role in controlling total complement activity in lymph and interstitial fluid. The total complement haemolytic activity in lymph exceeded the haemolytic activity of C1 and C3, which may indicate the possibility of independent activation of C5-7 and C8-9. The considerably low C3H50 level, with high C3 protein concentration, at present lacks a proper explanation.

Adolescent↗
PubMed1994

C5b-9 increases albumin permeability of isolated glomeruli in vitro.

Deposition of antibody and activation of the complement cascade are important in both naturally occurring glomerulonephritis and in experimental models including passive Heymann nephritis. We studied the effect of antibody and complement on albumin permeability of isolated glomeruli to determine the role of the terminal complement components (C5-C9) in mediating the proteinuria in nephritis. Isolated glomeruli were treated with anti-Fx1a (Heymann antibody) and then incubated them with pooled human serum, serum in which complement had been inactivated by heat, or serum deficient in C6 or C7. The albumin reflection coefficient (sigma albumin) was calculated from the volumetric response of glomeruli to transcapillary oncotic gradients produced by albumin or high molecular weight neutral dextran (252 kD). Convectional permeability to albumin (Palbumin) was calculated as 1-sigma albumin. Albumin permeability of control glomeruli was not different from 0. Albumin permeability was not altered by antibody alone but was increased to 0.65 +/- 0.04 when antibody treated glomeruli were incubated for 10 minutes with pooled serum as a source of complement. Heat treatment of serum to inactivate complement prevented the increase in permeability. Incubation for 10 minutes with serum without antibody pretreatment caused a lesser increase in permeability of isolated glomeruli (0.18 +/- 0.06). Serum deficient in either C6 or C7 did not cause an increase in albumin permeability of antibody pre-treated glomeruli, but incubation with a combination of these sera (now containing the complete cascade) increased permeability to the same extent as did pooled normal serum (0.58 +/- 0.04).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗