Assembly of the functional membrane attack complex of human complement: formation of disulfide-linked C9 dimers

Glutathione inhibits antibody and complement-mediated immunologic cell injury via multiple mechanisms

Antioxidant glutathione (GSH) plays an important role in the regulation of immunity. However, little is known about its effects on humoral immunity, especially its action on effector molecules like antibody and complement. Given that these molecules contain abundant disulfide bonds, we speculated that GSH might influence the action of these proteins via its thiol function. Using a model of a glomerular mesangial cell (MC) lysis induced by antibodies plus complement, we addressed this hypothesis. Exposure of rat MCs to anti-Thy-1 antibody plus complement or anti-MC rabbit serum caused a complement-dependent cell lysis, which was completely blocked by GSH. Moreover, GSH potently prevented the antibody-mediated agglutination of red blood cells and aggregation of antibody-sensitized microspheres. Further analysis revealed that GSH inhibited antibody binding to antigens and promoted the conversion of the antibodies to its reduced forms. GSH also potently inhibited the formation and deposition of C5b-9 in MCs and suppressed both the classic and alternative complement activation pathway. Lastly, GSH attenuated P38 activation, an oxidative sensitive kinase that partially mediated the antibody- and complement-dependent MC lysis. Depletion of GSH via inhibiting gamma-glutamylcysteine synthetase or xCT transporter augmented P38 activation and sensitized MCs to the cell lysis. Collectively, our results indicate that GSH protects cells from immunological cell damage via mechanisms involving inhibition of antibody binding to the antigens, suppression of complement activation and augmentation of cellular defense mechanism. Our study provides novel mechanistic insights into the actions of GSH in the regulation of immune responses and suggests that GSH might be used to treat certain immune disorders.

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Little information is available regarding the role of GSH on humoral immunity.

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GSH inhibited antibody-triggered and complement-mediated immune responses.

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GSH interfered with antibody binding to cell surface antigens via its thiol function.

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GSH inhibited both the classic and alternative complement activation pathways.

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GSH increased cell resistance to immunological injury via inhibition of P38.

C5 complement deficiency in a Saudi family, molecular characterization of mutation and literature review

INTRODUCTION

Complement deficiencies are rare primary immunodeficiency disorders, the diagnosis of which is often underestimated. Only a small number of molecular studies have been carried out for the characterization of the underlying genetic defects in these cases.

PURPOSE

Reporting the first family from the Arabian Gulf region with multiple members affected by meningococcemia and abscent serum complement 5 (C5). We tried to correlate clinical, biochemical and molecular genetics features of this family.

METHODS

Determination of the serum level of all complement proteins including the terminal cascade (C5-9), followed by mutation analysis on DNA extracted from fresh blood samples of each alive family member.

RESULTS

Molecular studies showed a homozygous nonsense mutation in exon 1, with the change of cytosine to thymine at position 55 (55C > T) leading to change of the glutamine amino acid at position 19 to a stop codon (Q19X), and serologically absence of C5 in the serum. A similar but compound heterozygous mutation has been reported in one African-American family. previously.

CONCLUSION

Characterization of the underlying mutations in C5 deficient families is important, to understand this uncommon complement deficiency, and try to elucidate structure-function relationships in the C5 gene. This report also highlights the importance of complement screening in cases of sporadic meningococcal Infections, especially in communities with high prevalence of consanguineous marriages, which will ensure timely and adequate clinical interventions.

Topology of the membrane-bound form of complement protein C9 probed by glycosylation mapping, anti-peptide antibody binding, and disulfide modification

The two N-linked oligosaccharides in native human C9 were deleted by site-specific mutagenesis. This aglycosyl-C9 did not differ from its native form in hemolytic and bactericidal activity. A new N-glycosylation site (K311N/E313T) was introduced into the turn of a helix-turn-helix [HTH] fold that had been postulated to form a transmembrane hairpin in membrane-bound C9. This glycosylated form of human C9 was as active as the native protein suggesting that the glycan chain remains on the external side of the membrane and that translocation of this hairpin is not required for membrane anchoring. Furthermore, flow cytometry provided evidence for the recognition of membrane-bound C9 on complement-lysed ghosts by an antibody specific for the HTH fold. A new N-glycosylation site (P26N) was also introduced close to the N-terminus of C9 to test whether this region was involved in C9 polymerization, which is thought to be required for cytolytic activity of C9. Again, this glycosylated C9 was as active as native C9 and could be induced to polymerize by heating or incubation with metal ions. The two C-terminal cystines within the MACPF domain could be eliminated partially or completely without affecting the hemolytic activity. Free sulfhydryl groups of unpaired cysteines in such C9 mutants are blocked since they could not be modified with SH-specific reagents. These results are discussed with respect to a recently proposed model that, on the basis of the MACPF structure in C8alpha, envisions membrane insertion of C9 to resemble the mechanism by which cholesterol-dependent cytolysins enter a membrane.

Molecular characterization of three new mutations causing C5 deficiency in two non-related families

Deficiencies in complement components are rare diseases whose diagnosis is often underestimated. In addition, in only a few cases molecular studies have been carried out for the characterization of the underlying genetic defects. To date, studies involving C5-deficient patients are scarce. The aim of the present report is to characterize the biochemical and molecular complement deficiency in two non-related families with one or more members showing no detectable hemolytic complement activity (CH50<50 U/ml) and reporting a history of several episodes of meningitis. Protein deficiency was assessed by means of hemolytic assays, bi-dimensional double immunodiffusion, ELISA and Western blot of patients' sera. Molecular studies were carried out by PCR and RT-PCR of DNA and RNA, respectively, both extracted from fresh blood samples of each family member. In Family A, only the propositus had complete C5 deficiency. Molecular studies showed that he was heterozygous for two changes in the C5 gene. One of the mutations was also carried by the father (c.1883_1884AG<CTCT) and the second (c.2536T>C, Y846H) was a de novo mutation. In Family B, the two C5-deficient members share the homozygous nonsense mutation c.892C>T (Q298X) in exon 9. The characterization of these new mutations is interesting in order to elucidate structure-function relationships in the C5 gene and it also helps to understand the molecular basis of this uncommon deficiency. Moreover, this report highlights the importance of complement screening in cases of repeated meningococcal infections in order to establish its involvement and to consider adequate clinical recommendations such as prophylactic antibiotics or meningococcal vaccines.

Early complement activation and decreased levels of glycosylphosphatidylinositol-anchored complement inhibitors in human and experimental diabetic retinopathy

Diabetic retinal microangiopathy is characterized by increased permeability, leukostasis, microthrombosis, and apoptosis of capillary cells, all of which could be caused or compounded by activation of complement. In this study, we observed deposition of C5b-9, the terminal product of complement activation, in the wall of retinal vessels of human eye donors with 9 +/- 3 years of type 2 diabetes, but not in the vessels of age-matched nondiabetic donors. C5b-9 often colocalized with von Willebrand factor in luminal endothelium. C1q and C4, the complement components unique to the classical pathway, were not detected in the diabetic retinas, suggesting that C5b-9 was generated via the alternative pathway, the spontaneous activation of which is regulated by complement inhibitors. The diabetic donors showed a prominent reduction in the retinal levels of CD55 and CD59, the two complement inhibitors linked to the plasma membrane by glycosylphosphatidylinositol anchors, but not in the levels of transmembrane CD46. Similar complement activation in retinal vessels and selective reduction in the levels of retinal CD55 and CD59 were observed in rats with a 10-week duration of streptozotocin-induced diabetes. Thus, diabetes causes defective regulation of complement inhibitors and complement activation that precede most other manifestations of diabetic retinal microangiopathy. These are novel clues for probing how diabetes affects and damages vascular cells.

Isolation and characterization of a complement-activating lipid extracted from human atherosclerotic lesions

The major characteristics of human atherosclerotic lesions are similar to those of a chronic inflammatory reaction, namely fibrosis, mesenchymal cell proliferation, the presence of resident macrophages, and cell necrosis. Atherosclerosis exhibits in addition the feature of lipid (mainly cholesterol) accumulation. The results of the present report demonstrate that a specific cholesterol-containing lipid particle present in human atherosclerotic lesions activates the complement system to completion. Thus, lipid could represent a stimulatory factor for the inflammatory reaction, whose underlying mechanistic basis may be, at least in part, complement activation. The complement-activating lipid was purified from saline extracts of aortic atherosclerotic lesions by sucrose density gradient centrifugation followed by molecular sieve chromatography on Sepharose 2B. It contained little protein other than albumin, was 100-500 nm in size, exhibited an unesterified to total cholesterol ratio of 0.58 and an unesterified cholesterol to phospholipid ratio of 1.2. The lipid, termed lesion lipid complement (LCA), activated the alternative pathway of complement in a dose-dependent manner. Lesion-extracted low density lipoprotein (LDL) obtained during the purification procedure failed to activate complement. Specific generation of C3a desArg and C5b-9 by LCA indicated C3/C5 convertase formation with activation proceeding to completion. Biochemical and electron microscopic evaluations revealed that much of the C5b-9 present in atherosclerotic lesions is membraneous, rather than fluid phase SC5b-9. The observations reported herein establish a link between lipid insudation and inflammation in atherosclerotic lesions via the mechanism of complement activation.

SC5b-7, SC5b-8 and SC5b-9 complexes of complement: ultrastructure and localization of the S-protein (vitronectin) within the macromolecules

Purified terminal components of the complement system were used together with purified S-protein, the inhibitor of the membrane attack complex, to generate the soluble complexes SC5b-7, SC5b-8 and SC5b-9. These complexes were purified by ultracentrifugation in sucrose density gradients with 50-70% yield, exhibiting sedimentation coefficients of 20 S, 21 S and 23 S, respectively. In Ouchterlony double-diffusion analysis, the purified complexes gave a line of identity against all antisera of the precursor components indicating that complex formation had occurred. The identity of the complexes was also revealed by the appearance of all subunit components after polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Since the inhibitor function of S-protein in the terminal complement cascade should also be manifested in the morphology of the macromolecules generated, the ultrastructures of the three complexes were analyzed by electron microscopy. In contrast to aggregated (C5b-7)n and (C5b-8)n, negatively stained SC5b-7 and SC5b-8 imaged mostly as monomeric irregularly shaped cylindrical structures, whereas SC5b-9 less than 27 S) appeared as wedge-shaped structure lacking the tubular polymerized C9. (All three complexes were also generated in the presence of biotinyl-S-protein and labeled with avidin-gold conjugates as electron-dense marker). Analysis of the modified complexes in electron micrographs demonstrated that the complexes were marked exclusively at one site of their ultrastructures, suggesting this region to be the location of S-protein and the critical site for membrane binding of C5b-7 or C5b-8 and for initiation of C9 polymerization. These results support recent findings in which the function of S-protein as complement inhibitor was dependent on conformational changes of the protein molecule with concomitant exposure of the heparin-binding domain.

Ninth component of complement: self-aggregation and interaction with lipids

We have investigated environmental conditions that might be of importance for the polymerization of the ninth component (C9) of human complement. In disagreement with earlier reports summarized by Tschopp et al. [Tschopp, J., Müller-Eberhard, H. J., & Podack, E. R. (1982) Nature (London) 298, 534-538] we find no evidence for significant aggregation or loss of hemolytic activity of C9 when incubated at 37 degrees C even after 12 days of incubation. Higher temperatures cause denaturation of the protein and formation of stringlike aggregates. In contrast, short-term proteolysis with 1% (w/w) trypsin at room temperature causes rapid polymerization of part of the C9 into tubular structures (poly-C9), and the remainder of the monomeric C9 is digested. This polymerization reaction is inhibitable by trypsin inhibitor; alpha-thrombin and proteinase K are ineffective in creating polymers. A second discrepancy to the earlier reports is our finding that monomeric C9 immediately interacts with small unilamellar lipid vesicles (SUV) without a required heating step. As a result of this interaction about half of the C9 aggregates to form strings and tubules, and these aggregates cause agglutination of vesicles. The other half of the C9 associates with a second population of SUV without causing a change in Stokes' radius of these vesicles, and no proteinaceous structures are detectable on the vesicle surface by electron microscopy. When these two vesicle populations are tested for their membrane integrity, no release of an encapsulated fluorescent marker can be detected, nor is there leakage of potassium ions across the bilayer membrane since a membrane diffusion potential can be developed.(ABSTRACT TRUNCATED AT 250 WORDS)

Proteolytic modification of human complement protein C9: loss of poly(C9) and circular lesion formation without impairment of function

We have compared the ability of thrombin-cleaved C9 (C9n) with that of native C9 to produce tubular or ring-like poly(C9) and to express the classical complement lesion on target membranes. Three procedures were used to produce poly(C9): (i) limited proteolysis with trypsin, (ii) interaction with small unilamellar lipid vesicles, and (iii) incubation with a 2- to 4-fold molar excess of ZnCl2. In contrast to C9, which could be converted to tubular poly(C9), C9n was converted to smaller peptides by the first procedure and was aggregated into string-like poly(C9) by the other two methods. C9-depleted human serum (R-9 serum) was reconstituted with either C9 or C9n and these sera were then used to lyse sensitized sheep erythrocytes. Numerous classical complement lesions could be detected on ghost membranes obtained from cells lysed by C9-reconstituted R-9 serum but only a few on ghost membranes produced by C9n-reconstituted R-9 serum. C9n was shown to be hemolytically as active as C9 even when tested under "single-hit" conditions and it was about twice as efficient when compared with C9 in releasing sucrose and inulin from resealed ghosts. These results are interpreted to indicate that formation of the classical complement lesion is only incidental to lysis and not an obligatory event and that enlargement of the "functional pore size" of the complement lesion is not linked to formation of a circular membrane attack complex.

Complement protein C9 labeled with fluorescein isothiocyanate can be used to monitor C9 polymerization and formation of the cytolytic membrane lesion

Human complement protein C9 was covalently labeled with the fluorescent chromophore fluorescein isothiocyanate (FITC) with only a small reduction in the cytolytic activity of the protein. Polymerization of the labeled protein--either by incubating with lipid vesicles treated with complement proteins C5b-8 (activating the C5b-9 membrane lesion) or by heating the protein [Tschopp, J., Muller-Eberhard, H.J., & Podack, E.R. (1982) Nature (London) 298, 534]--resulted in a 40-60% decrease in the fluorescence emission from FITC. The decrease in total fluorescence was accompanied by an increase in the steady-state anisotropy following activation and polymerization of FITC-C9 by C5b-8 membranes, while heat-induced aggregation of the protein resulted in a dramatic depolarization of fluorescence. Only small changes in either the absorbance spectrum or fluorescence lifetime of the chromophore were detected upon FITC-C9 polymerization. Evidence is presented that the measured changes in FITC fluorescence upon C9 activation are due to self energy transfer between closely apposed fluorescein chromophores which occur in the polymerized form of the protein. The significance of these observations to the molecular structure of the assembled C5b-9 complex is discussed, as are the potential applications of this fluorescent derivative of C9.

The influence of electrochemical gradients of Na+ and K+ upon the membrane binding and pore forming activity of the terminal complement proteins

The hemolytic activity of the terminal complement proteins (C5b-9) towards erythrocytes containing high potassium concentration has been reported to be dramatically increased when extracellular Na+ is substituted isotonically by K+ (Dalmasso, A.P., et al., 1975, J. Immunol. 115:63-68). This phenomenon was now further investigated using resealed human erythrocyte ghosts (ghosts), which can be maintained at a nonlytic osmotic steady state subsequent to C5b-9 binding: (1) The functional state of C5b-9-treated ghosts was studied from their ability to retain trapped [14C]-sucrose or [3H]-inulin when suspended either in the presence of Na+ or K+. A dramatic increase in the permeability of the ghost membrane to both nonelectrolytes - in the absence of significant hemoglobin release - was observed for C5b-9 assembly in the presence of external K+. (2) The physical binding of the individual 125I-labeled terminal complement proteins to ghost membranes was directly measured as a function of intra- and extracellular K+ and Na+. The uptake of 125I-C7, 125I-C8, and 125I-C9 into membrane C5b-9 was unaltered by substitution of Na+ by K+. (3) The binding of the terminal complement proteins to ghosts subjected to a transient membrane potential generated by the K+-ionophore valinomycin (in the presence of K+ concentration gradients) was measured. No significant change in membrane binding of any of the C5b-9 proteins was detected under the influence of both depolarizing and hyperpolarizing membrane potentials.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation of membrane attack complex of complement from myelin membranes treated with serum complement

The interaction between complement and myelin membranes and its possible role in myelin damage and in the disposal of damaged myelin in vivo is of interest because activation of complement generates both opsonin(s) and membrane attack complex of complement. In our studies on the role of complement in demyelination, we have shown that isolated myelin activates serum complement in the absence of myelin-specific antibody and that membrane attack complex of complement is the required factor in antibody-mediated demyelination of mouse cerebellar explant cultures. In the present study, we examined whether activation of serum complement by myelin is associated with the formation of membrane attack complex of complement in myelin membranes. Extracts of myelin-associated proteins following incubation of myelin with fresh serum were studied by ultracentrifugation on a sucrose density gradient for detection of C5b-9 neoantigen. The subunit structure of C5b-9 was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, electroblotting, and immunostaining. Results indicate that the macromolecular complex consisting of late-acting complement components, C5-C9, was assembled in the target myelin membranes.

Molecular composition of the terminal membrane and fluid-phase C5b-9 complexes of rabbit complement. Absence of disulphide-bonded C9 dimers in the membrane complex

The terminal membrane C5b-9(m) and fluid-phase SC5b-9 complexes of rabbit complement were isolated from target sheep erythrocyte membranes and from inulin-activated rabbit serum respectively. In the electron microscope, rabbit C5b-9(m) was observed as a hollow protein cylinder, a structure identical with that of human C5b-9(m). Monodispersed rabbit C5b-9(m) exhibited an apparent sedimentation coefficient of 29 S in deoxycholate-containing sucrose density gradients, corresponding to a composite protein-detergent molecular-weight of approx. 1.4 X 10(6). Protein subunits corresponding to human C5b-C9 were found on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. By densitometry, there were consistently six molecules of monomeric C9 present for each monomeric C5b-8 complex. Fluid-phase rabbit SC5b-9 was a hydrophilic 23 S ma macromolecule that differed in subunit composition from its membrane counterpart in that it contained S-protein and only two to three molecules of C9 per monomer complex. The data are in accord with the previous report on human C5b-9 that C5b-9(m) contains more C9 molecules than SC5b-9 [Ware & Kolb (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 6426-6430]. They corroborate the previous molecular-weight estimate of approx. 10(6) for C5b-9(m) and thus support the concept that the fully assembled, unit lesion of complement is a C5b-9 monomer [Bhakdi & Tranum-Jensen (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 1818-1822]. They also show that C9 dimer formation is not required for assembly of the rabbit C5b-9(m) protein cylinder, or for expression of its membrane-damaging function.

Terminal membrane C5b-9 complex of human complement: transition from an amphiphilic to a hydrophilic state through binding of the S protein from serum

The membrane-damaging C5b-9(m) complex of complement is a cylindrically structured, amphiphilic molecule that is generated on a target membrane during complement attack. Isolated C5b-9(m) complexes are shown here to possess the capacity of binding a protein, termed "S"-protein, that is present in human plasma. Binding of this protein apparently shields the apolar surfaces of C5b-9(m), since the resulting "SC5b-9(m)" complex is hydrophilic and no longer aggregates in detergentfree solution. Dispersed SC5b-9(m) complexes exhibit an apparent sedimentation coefficient of 29S in sucrose density gradients, corresponding to a molecular weight of approximately 1.4 million. SDS PAGE analyses indicate binding of 3-4 molecules of S-protein per C5b-9(m) complex. These data are consistent with a monomer nature and molecular weight of 1-1.1 million of the C5b-9(m) complex. Ultrastructural analysis of SC5b-9(m) shows preservation of the hollow cylindrical C5b-9(m) structure. Additional material, probably representing the S-protein itself, can be visualized attached to the originally membrane-embedded portion of the macromolecule. The topography of apolar surfaces on a molecule thus appears directly probed and visualized through the binding of a serum protein.