Synthesis of Complement Components C5, C6, C7, C8 and C9 in Vitro by Human Monocytes and Assembly of the Terminal Complement Complex

Monocytes cultured under serum-free conditions secreted protein which bound covalently and non-covalently to agarose beads, an activator of the alternative pathway of complement. There was a significantly binding of monoclonal anti-C3c antibodies, polyclonal anti-C5, anti-C6, anti-C7, anti-C8, and anti-C9 antibodies, and of a monoclonal antibody against a neoantigen of polymerized C9 to agarose beads incubated with the monocytes for 24, 48, 72 or 96 h. From these results, we conclude that monocytes produce C5, C6, C7, C8 and C9 that assemble as the terminal complement complex on the surface of the agarose beads. Activation by agarose of the alternative pathway with generation of particle bound C3 and C5 convertases is a prerequisite for the subsequent formation of the terminal complement complex. Whether SC5b-9 or the membrane attack of complement (C5b-9) is formed on the beads will be examined.     

Synthesis of Complement by Alveolar Macrophages from Patients with Sarcoidosis

Sarcoidosis is a granulomatous disorder of unknown aetiology. Alveolar macrophages (AM) in sarcoidosis release a variety of mediators important to the pathogenesis of the disease. Complement is essential for the inflammatory response and we investigated whether there were any major defects in the potential for sarcoidosis AM to synthesize complement in vitro. AM from 11 patients with active sarcoidosis and three healthy controls were cultured under serum-free conditions. There was a significant binding of polyclonal (anti-C5, -C6, -C7, -C8) and monoclonal anti-complement antibodies (anti-C3c and anti-C9 neoepitope (aE11] to agarose beads incubated with unstimulated AM for 24, 48, or 72 h. A significant and inhibitable production of soluble C3c, C5, C9, and S-protein was found in the harvested medium as detected by enzyme immunoassays. Activated C3 and C9 were also detected based on neoepitope expression. Presence of co-cultured agarose beads reduced the amount of soluble S-protein due to deposition on the agarose. We argue that the C9 neoepitope is an integral part of the terminal complement complex (TCC), both in the fluid and solid phase when bound to the agarose. In the fluid phase, SC5b-9 was generated, whereas the agarose-bound S-protein is assumed not to be associated with TCC on the beads. The results demonstrate for the first time that AM from sarcoidosis patients synthesize the functional alternative and terminal pathway of complement.     

Human Alveolar Macrophages Synthesize Active Complement Components C6, C7, and C8 in Vitro

We investigated whether serum-free human alveolar macrophage cultures synthesize active C6, C7, and C8. There was a significant binding of polyclonal anti-human C6 antibodies to agarose beads incubated with unstimulated macrophages for 24 or 48 h. Endotoxin stimulation of the macrophages was necessary for significant binding of polyclonal anti-C7 and anti-C8 antibodies to agarose beads co-cultured for 48 or 96 h. Two monoclonal antibodies (poly C9-MA and MCaE11) specific for a neoantigen of polymerized C9 in the terminal complement complex (TCC), bound to beads mainly incubated with endotoxin stimulated macrophages. The MCaE11 was more sensitive than the poly C9-MA in detecting the C9 neoantigen on beads incubated with the macrophages or human serum diluted 1:16. We thus conclude that human alveolar macrophages synthesize active C6, C7, and C9 that together with C5 and C9, assemble as the TCC on co-cultured agarose beads. Activation of the alternative pathway on the agarose with generation of fixed C3 and C5 convertases is a prerequisite for the subsequent generation of the TCC.     

Human Umbilical Vein Endothelial Cells Synthesize Functional C3, C5, C6, C8 and C9 In Vitro

Human endothelial cells (EC), cultured serum-free, synthesize de novo protein which increasingly bind to agarose beads (an alternative pathway activator), until a plateau phase is reached after 24-48 h. EC synthesize functional C3, C5, C6, C8 and C9, which were detected on co-cultured agarose beads, using relevant polyclonal anti-complement antibodies. Two monoclonal anti-C9 neoepitope antibodies (aE11, poly C9-MA) bound to the co-cultured beads, showing that the terminal complement complex (TCC) (C5b-9) was assembled on the beads. This also suggests that C7 is synthesized. There seems to be a positive correlation between the amount of agarose-bound labelled protein and agarose-bound complement. The results indicate that EC produce and secrete the components for the functional alternative and terminal pathways of complement.     

Synthesis of C3, C5, C6, C7, C8, and C9 by Human Fibroblasts

We investigated the ability of human fibroblasts to produce the components of the final common pathway (C3-C9) of complement in vitro by co-culturing an alternative complement activator (agarose beads) with the cells. The test system involved incubation of beads with anti-complement antibodies followed by radioactive-labelled anti-Ig detection antibodies. Subsequently, the beads were examined in a radioimmunoassay. Our results indicate that human fibroblasts produce C3, C5, C6, C7, C8, and C9. A neoepitope selectively expressed on activated C9 was detected, indicating assembly of the terminal complement complex and thus formation of a functional terminal complement pathway by the fibroblasts.     

Human peritoneal macrophages. Production in vitro of the active terminal complement components C5 to C9 and a functional alternative pathway of complement. Brief report

Endotoxin-stimulated human peritoneal macrophages were cultured in serum-free medium with agarose beads. Monospecific antibodies to human C3c, C3g, C5, C6, C7, C8, C9 and to C9-neoantigen bound to the beads. This shows that activated C3 and the terminal complement complex (TCC), made from complement components C5 to C9, were generated on the beads. De novo synthesis was confirmed by agarose binding of tritium-labelled protein. Moreover, C3-derivatives and C9-neoantigen were detected on normal serum-treated agarose beads but not on beads kept in factor B-depleted or heat-inactivated sera, implying that an intact alternative complement pathway was required for our findings. The macrophages thus synthesize the active complement components of the alternative and terminal pathways in vitro.     

C3-independent immune haemolysis: haemolysis of EAC14oxy2 cells by C5-C9 without participation of C3.

C3-independent immune haemolysis was studied using EAC14oxy2 cells and purified C5, C6, C7, C8 and C9. We have found that EAC14oxy2 cells were lysed by C5-C9 and that haemolysis occurs, even after pretreatment of the cells and the C5-C9 preparation with anti-C3. This indicates that EAC14oxy2 can be lysed by C5-C9 without any participation of C3. In contrast, EAC1 and EAC14 cells are not lysed by C5-C9, suggesting that our C5-C9 preparation lacks activated complement components, such as C3bBb, C5b6 or C(56)a. Based on our study of the haemolysis of EAC14oxy2, we have determined that: (i) EAC14oxy2 cells are lysed by a preparation of C5, C6, C7, C8 and C9, but no lysis occurs when any one of these complement is absent, (ii) for significant haemolysis of EAC14oxy2, a higher concentration of C5 is necessary as compared to the C5 requirement when haemolysis occurs in the presence of C3, (iii) the degree of haemolysis is linearly related to the concentration of C5 and does not reach a plateau, despite the addition of as much as 3,200 U of C5, and (iv) the degree of haemolysis is linearly related to the concentration of cell bound C42. These observations suggest that, in the absence of C3, the C3 convertase C42 can activate C5 directly, resulting in the formation of the membrane attack complex, C5b-9.     

Formation of the Membrane Attack Complex of Complement (MAC) on Erythrocytes from Monocyte-Produced Terminal Complement Components

By using antibodies against C5, C6, C7, C8, and C9, we found that terminal complement components were deposited on IgM-coated sheep erythrocytes (EIgM) kept in serum-free endotoxin-stimulated monocyte cultures for 24 or 48 h. Monoclonal antibodies revealed C9 neoantigens on the EIgM. There was no specific binding of an anti-S protein antibody, which reacts with the SC5b-9 complex, to the EIgM. Controls were native sheep erythrocytes (E) treated similarly which, in contrast to EIgM, do not activate the classical pathway of complement. Cycloheximide (1.0 microgram/ml) in the cell cultures resulted in no specific binding of the anti-C9 antibodies to EIgM. A fraction of the EIgM was lysed during incubation with the monocytes. We conclude that the monocytes secrete C5, C6, C7, C8, and C9, which form the membrane attack complex of complement (C5b-9) on the EIgM.     

C56 formation in the reaction mixture of isolated complement components through the classical complement pathway

The mechanism of hemolysis of unsensitized erythrocytes by a mixture of 9 isolated, human-derived complement components, C1s, C4, C2, C3, C5, C6, C7, C8 and C9 (C1s-C9) was studied. Of the tested erythrocytes, guinea pig erythrocytes (Egp) were the most susceptible to lysis by C1s-C9, followed by human and sheep erythrocytes. Contamination of the isolated complement components by C56 was ruled out. It was determined that a factor was generated in the reaction mixture of C1s, C4, C2, C3, C5 and C6 (C1s-C6), which had lytic activity against Egp when C7, C8 and C9 were added. We found that the lytic factor was similar to C56 in the following properties: (1) the activity of the lytic factor decreased when incubated with isolated C7 prior to its reaction with Egp; (2) the lytic factor did not bind to Egp by itself but it did bind in the presence of C7; (3) EDTA did not have any inhibitory effect on the lytic factor; (4) the activity of the lytic factor decreased by treatment with anti-C5 and anti-C6 but not by treatment with anti-C3 and anti-C4, and (5) gel filtration of the reaction mixture (C1s-C6) indicated that the elution volumes of the lytic factor and of isolated C56 were similar. Thus, it is likely that C56 is generated in the reaction mixture of C1s-C6 and the lytic factor binds to unsensitized erythrocytes together with C7, to form an intermediate EC567 which is susceptible to lysis by the action of C8 and C9.     

Combined inhibition of complement and CD14 abolish E. coli-induced cytokine-, chemokine- and growth factor-synthesis in human whole blood

The relative role of complement and CD14 in E. coli-induced cytokine synthesis in an in vitro human whole blood model of sepsis was examined. Fresh lepirudin-anticoagulated whole blood was incubated with E. coli for 2h. Monoclonal antibodies or a C5a receptor antagonist were used to block complement. Inflammatory mediators (n=27) were measured by multiplex technology, selected cytokine mRNA by real time PCR, and CD11b, oxidative burst and phagocytosis by flow cytometry. E. coli significantly increased 18 of the 27 inflammatory mediators, including proinflammatory cytokines (TNF-alpha, IL-6, INF-gamma and IL-1beta), chemokines (IL-8, MCP-1, MIP-1alpha, MIP-1beta, eotaxin and IP-10), growth factors (VEGF, FGF-basic, G-CSF and GM-CSF) and other interleukins (IL-9, IL-15 and IL-17). Notably, the increases in all mediators were abolished by a combined inhibition of CD14 and complement using anti-C2 and anti-factor D in combination, whereas the relative effect of the inhibition of complement and CD14 varied. In comparison, a C5a receptor antagonist and anti-CD14 in combination reduced cytokine synthesis less efficiently. Real time PCR analysis confirmed that the cytokine synthesis was blocked at the mRNA level. Similarly, E. coli-induced CD11b up-regulation, oxidative burst and phagocytosis was totally inhibited by CD14, anti-C2 and anti-factor D in combination after 2h incubation. In conclusion, the combined inhibition of complement using anti-C2, anti-factor D and CD14 almost completely inhibits the E. coli-induced inflammatory response. The combined approach may therefore be a new treatment regimen in Gram-negative sepsis.     

Human Alveolar Macrophages Synthesize the Functional Alternative Pathway of Complement and Active C5 and C9 in Vitro

Attachment of protein to agarose beads cultured with macrophages in protein-free medium containing 3H-leucine, shows that de novo synthesis of protein with affinity to the beads takes place. We also found that monoclonal antibodies against human C3c, C3g, and a C9-neoantigen as well as polyclonal antibodies against human C5 and C9, bound to agarose beads that had been kept with the macrophage cultures. Demonstration of C3 derivatives on the agarose beads shows that the essential complement factors of the alternative pathway are synthesized and have been activated by the beads. Deposition of C5 and the detection of a neoantigen of C9 on the beads, indicates that the whole terminal complement pathway has been formed and activated. We conclude that human alveolar macrophages form in vitro the functional alternative pathway of complement, C5 and C9, and we have indirect evidence for synthesis of C6, C7, and C8.     

Receptors on guinea-pig erythrocytes specific for cell-bound fourth component of human complement (C4).

Guinea-pig erythrocytes have receptors for heterologous (human and rabbit) complement activated by the classical pathway on cell surfaces. This was shown in the present study by rosette-forming reactions of guinea-pig erythrocytes and human lymphocytes or sheep erythrocytes pre-treated with antibody and human R3 complement. The binding is temperature-dependent and is enhanced by treating the guinea-pig erythrocytes with neuraminidase. The receptors were shown to be specific for C4 by inhibition tests employing a range of anti-human complement antibodies (including anti-Clq, -Cl inhibitor, -C4, -C2, -C3 and -C3b inactivator). Of these reagents, only anti-C4 inhibited the receptor activity, indicating that the guinea-pig erythrocyte C4-receptors differ from those on lymphocytes, monocytes, polymorphonuclear leucocytes and human erythrocytes which are reported to react with both C3b and C4b. In contrast to the strong affinity observed for heterologous C4, guinea-pig erythrocytes appear to react very weakly, if at all, with homologous C4.     

Cercarial glycocalyx of Schistosoma mansoni activates human complement.

Human complement activation by cercariae and schistosomula of the human parasite Schistosoma mansoni was studied in vitro. Cercariae are composed of tails which are shed after infection of the host and bodies which transform into the larvae or schistosomula after infection. After incubation in fresh normal human serum (NHS), cercarial tails bound more anti-C3 antibodies than did cercarial bodies (CB), and the tails were rapidly lysed, while the attached CB remained intact. Complement activation by cercariae was dependent on the alternative pathway but was independent of antibody, as shown by C3 deposition by hypogammaglobulinemic human sera. By transmission microscopy, the fibrillar glycocalyx on both CB and tails was stained by NHS but not by heat-inactivated serum (HI-NHS). The glycocalyx was labeled with periodate and tritiated borohydride, and parasites were incubated in NHS and HI-NHS. After solubilization, the labeled glycocalyx on organisms incubated in NHS but not HI-NHS bound anti-C3 antibodies. Of the CB incubated with eserine sulfate to prevent transformation, 78% +/- 10% were dead after culture for 24 h in NHS. In contrast, 21% +/- 12% of the CB were dead after culture in HI-NHS. Schistosomula incubated in NHS bound 37% of the amount of anti-C3 antibodies bound by cercariae but were not killed by NHS. In conclusion, the cercarial glycocalyx activated human complement, and schistosomula were less susceptible to killing than cercariae because they had less glycocalyx and activated less complement.     

Differential expression of the C5a receptor and complement receptors 1 and 3 after LPS stimulation of neutrophils and monocytes

Animal experiments recently suggested that administration of anti-C5a, anti-C5a receptor or soluble complement receptor type-1 may be of value in the treatment of septic shock. Because results regarding C5a receptor expression (C5a-R, CD-88) have been found to differ between septic animals and patients, the aim of this study was to investigate the neutrophil and monocyte receptor expression of CD-88 and complement receptor-1 (CR-1, CD-35) after stimulation with lipopolysaccharide (LPS) ex vivo. Whole blood or isolated neutrophils and monocytes from healthy people were incubated with LPS in a dose range of 0.1-1000 ng/ml. The expressions of CD-88 and CD-35 were analysed by means of flow cytometry. For comparison, the expressions of complement receptor-3 (CR-3, CD-11b/CD-18), Fc-gamma receptor type-I (CD-64) and CEACAM-8 (CD-66b) were also investigated. In whole blood, CD-88 expression on neutrophils was reduced (P < 0.05). The expressions of CD-35 and CD-11b were increased both on neutrophils (P < 0.001; P < 0.05) and on monocytes (P < 0.001; P < 0.001). No effect was observed on isolated cells. In agreement with the findings in septic patients, LPS reduced the neutrophil C5a-R expression, whereas the expressions of CR-1 and CR-3 were increased. The effects of LPS were indirect and were mediated via factors in the blood. The clinical significance of this is not known, but may be associated with decreased chemotaxis.     

CD59 efficiently protects human NT2-N neurons against complement-mediated damage

The complement regulatory protein CD59 controls cell survival by the inhibition of C5b-9 formation on the cell membrane. Loss of CD59 increases the susceptibility of cells to complement-mediated damage and lysis. Deposition of IgM can induce complement activation with subsequent cell death. We have previously demonstrated the presence of CD59 on human NT2-N neurons. In this study, we investigated the functional role of CD59 for NT2-N cell survival after IgM-mediated complement activation. Complement activation was induced on NT2-N neurons with human serum following incubation with the IgM monoclonal antibody A2B5 reacting with a neuronal cell membrane epitope. Deposition of C1q and C5b-9 was detected on the cell membrane and sC5b-9 in the culture supernatant. Specific inhibition of complement was obtained by the C3 inhibitor compstatin, and by anti-C5/C5a MoAb. CD59 was blocked by the MoAb BRIC 229. Membrane damage of propidium iodide-stained NT2-N cells was confirmed by immunofluorescence microscopy and degeneration of neuronal processes was shown with crystal violet staining. A2B5, but not the irrelevant control IgM antibody, induced complement activation on NT2-N neurons after incubation with a human serum, as detected by the deposition of C1q. A marked membrane deposition of C5b-9 on NT2-N neurons with accompanying cell death and axonal degeneration was found after the blocking of CD59 with MoAb BRIC 229 but not with an isotype-matched control antibody. Compstatin and anti-C5 monoclonal antibodies which blocked C5 activation efficiently inhibited complement activation. In conclusion, CD59 is essential for protecting human NT2-N neurons against complement-mediated damage, which is known to occur in a number of clinical conditions including stroke.     

Report of studies on monoclonal antibodies (MABS) to complement components

Twenty-five monoclonal antibodies (MABS) to complement components were evaluated according to the ISBT/ICSH protocol by twelve laboratories. Seven detected some form of C3c, but one of them, 174, did not react with EiC3b, although it was positive with 'EC3b' (Fruitstone). 174 may detect some form of enzyme sensitive C3b antigen, but C3a was not evaluated (present on 'EC3b' Fruitstone). Twelve of the antibodies were anti-C3d, one was an anti-C3g and five were anti-C4c.     

Flow cytometric analysis of immunophagocytosis using sensitized fluorescent microspheres bearing C3b

We analyzed the phagocytic activity of purified human monocytes using fluorescent latex beads sensitized with IgG or IgG.C3 by flow cytometry. To prepare IgG-sensitized latex beads (BA), BSA-coated latex beads (B) were incubated with diluted rabbit IgG anti-BSA. To bind complement components, BA were incubated with whole serum pretreated with K-76 monocarboxylic acid (K-76COOH). K-76COOH inhibits the activity of factor I and C5, resulting in deposition of C1, C4b, C2a, C3b on BA (BAC). Phagocytic activity was assessed by percent phagocytosis and phagocytic index (PI). To eliminate the effects of non-phagocytosed latex beads, subtraction of the data at 4 degrees C from 37 degrees C was performed. Percent phagocytosis for 60 min. was B 5.0%, BA 18.3%, and BAC 57.5%, and PI (ingested latex beads/100 cells) was B 7.9, BA 36.8, and BAC 152.7, respectively. In addition, K-76COOH caused dose dependent inhibition on IgG.C3 mediated phagocytosis. Comparison of inhibition pattern on BAC and BA indicated that K-76COOH directly inhibited C3.C3-receptor binding.     

补体C5b-9复合物促进树突状细胞成熟

探讨补体C5b-9复合物对树突状细胞成熟及免疫学功能的影响。rhGM-CSF+rhIL-4体外诱导单核细胞分化为不成熟树突状细胞,体外于不成熟树突状细胞表面用补体蛋白纯品组装CSb-9,37℃,5%CO_2温育96 h,流式分析细胞表型及抗原捕获能力;与同种异体CD4~+和CD8~+T细胞共培养检测其免疫刺激功能;ELISA检测细胞因子分泌。结果显示,亚溶解型C5b-9处理DC表面标志CD83、HLA、CD80、CD86、B7-H1、B7-H3、B7-H4以及BTLA等表达上调;DC分泌IL-12及TNF-α上调,抗原捕获能力降低;C5b-9处理DC刺激CD4~+T细胞活化及分泌IFN-γ、IL-2能力增强。结论:补体C5b-9复合物可以促进树突状细胞成熟,衔接天然免疫和特异性免疫。     

C3- and T-cell-dependent adjuvant activity of in vivo formed immune complexes.

The effects of polyclonal antibodies to mouse serum components on the primary humoral immune response of mice in vivo were studied. It was observed that rabbit IgG to complement component C3 and albumin and mouse IgG to C5, but also heat-aggregated non-immune rabbit IgG, enhanced the agglutinating antibody response to sheep erythrocytes (SRBC). Since the increase in response was only observed when antigen and antibodies were administered via the same route (i.p.), immunological adjuvant activity was implicated. Ineffectiveness of anti-C5 IgG in C5-deficient mice indicated that the antibody-induced adjuvant activity is mediated by in vivo formed immune complexes (IC). The adjuvant activity of IC was reduced by selective C3-depletion of animals, pointing to a requirement of C3. The effect of variations in other parameters was studied with anti-C3 and anti-C5 IgG as immunoadjuvant. The immunostimulatory effect was most pronounced when the antibodies were administered simultaneously with or shortly before antigen. Treatment of animals with antibodies one or two days before antigen, however, resulted in a suppression of the response. The response to thymus-independent antigens was not enhanced by anti-C3 nor by anti-C5 IgG. Optimal adjuvant activity of anti-C3 IgG was observed at low antigen doses. Nude mice were insensitive to the immunopotentiating effect of anti-C3 and so was the F1 progeny of BALB/c male and CBA/N female mice expressing a B-cell maturation defect. C5 deficiency and lipopolysaccharide (LPS) non-responsiveness did not affect the adjuvant activity of in vivo formed C3-anti-C3 IC.     

Flow cytometric evaluation of material-induced platelet and complement activation

Flow cytometry is used to characterize the activation state of platelets and leukocytes within whole blood after contact for 4 h at 37 degrees C with various materials under conditions of low shear. The contact involved adding heparinized whole blood to small diameter tubes that were connected to two arms extending from a rocking platform. For all surfaces (polyethylene, polypropylene, Silastic, PVA hydrogel) tested there was strong evidence of platelet activation in the bulk blood: platelet-derived microparticles. P-selectin expression and platelet-leukocyte aggregates. Only contact with PVA hydrogel surfaces led to dramatic increases in CD11b up-regulation on monocytes and neutrophils that was inhibited by complement inhibition (sCRI). Flow cytometry was also used to evaluate the effectiveness of various agents to inhibit material-induced complement activation. The assay involved incubating 10 microm polystyrene beads for 1 h with serum at 37 degrees C before isolating the beads so as to label them with a monoclonal antibody against a neoantigen on SC5b-9. The beads were then identified by flow cytometry and the fluorescence associated with their SC5b-9 level recorded. The ability of C1-INH, pentamidine and benzamidine to moderately inhibit SC5b-9 levels suggests a role for classical complement activation in material-induced complement activation.