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PubMed1996

Comparison of complement activation in vitro by different Echinococcus granulosus extracts.

In the present study we have investigated and compared in vitro the specific complement (C) activating activity of three metacestode preparations of Echinococcus granulosus. Extracts from hydatid cyst fluid (HCF-ext), protoscoleces (PSC-ext) and hydatid cyst membrane (HCM-ext) activated human C producing C3 conversion and generating the C5b6 complex and the terminal C complex (TCC). HCM-ext showed much lower C activating activity than PSC-ext and HCF-ext. Moreover, its ability to generate C5b6 and TCC was lower than its ability to convert C3. On the other hand, PSC-ext and HCF-ext proved to be good C activators when their specific C activating activities were compared with that of inulin. However, PSC-ext produced lower levels of TCC than those produced by HCF-ext, in spite of the fact that both produced practically the same levels of C3d and C5b6. These results may be consistent with the existence of several mechanisms of C modulation involved in the defence of the parasite against host C damage.

Animals↗

PubMed1997

Complement-activating ability of leucocytes from patients with complement factor I deficiency.

Previous studies from this laboratory have shown that normal peripheral blood B cells are capable of activating complement via the alternative pathway (AP), that the activation is associated with complement receptor type 2 (CR2) expression, and that erythrocytes at normal blood levels partially inhibit the activation. The purpose of the present study was to investigate whether factor I (FI) deficiency, which leads to continued formation of the AP convertase (C3bBb) resulting in the consumption of factor B and C3 and large scale generation of C3b fragments, affects the phenotype and/or function of the patients' B cells. Using flow cytometry, peripheral blood leucocytes (PBL) from two FI-deficient patients were investigated for expression of complement receptors and complement regulatory proteins, in vivo-deposited C3 fragments and in vitro complement-activating ability. CR1 levels on B cells were significantly lower in FI-deficient patients than in normal individuals, whereas CR2 levels were found to be reduced, although not to a significant extent. CR1 levels on monocytes and polymorphonuclear leucocytes (PMN) were found to be normal or slightly raised. All leucocyte subpopulations were found to be covered in vivo with C3b fragments. AP activation on B cells from FI-deficient patients in homologous serum was significantly reduced compared with that for normal individuals, whereas no in vitro activation was seen in autologous serum. In addition, the in vivo-bound C3b fragments were degraded to C3d,g when the patients' PBL were incubated in homologous serum containing EDTA. Finally, the patients, erythrocytes failed to exert any inhibition on AP activation in homologous serum.

Antigens, CD↗

PubMed1980

Analysis of the different types of leukocyte membrane complement receptors and their interaction with the complement system.

The specificity, distribution, and structure of 8 different types of leukocytes membrane complement (C) receptors (CR1, CR2, CR3, and receptors for C1q, beta 1H, C3e, C3a, and C5a) are discussed. Recent data are reviewed on the synthesis of C components by macrophages and B lymphocytes, and how these components may function in the activation of these two cell type by the C system. Commonly used C receptor assay procedures are evaluated in terms of both specificity and sensitivity. Specific assay procedures are recommended for measuring CR1 (C4b-C3b receptor), CR2 (C3d receptor), CR3 (C3bi receptor), and the beta 1H receptor. Assays include both rosette and fluorescence procedures for detection of C receptors on either mouse or human leukocytes. Primary systems have been selected for optimal sensitivity and specificity, and where possible, acceptable alternative systems that are less sensitive or specific are suggested for laboratories lacking facilities for C purification.

Animals↗

PubMed1984

Factor I deficiency and C3 nephritic factor: immunochemical findings and association with Neisseria meningitidis infection in two patients.

The complement system was examined in two patients with systemic Neisseria meningitidis infections, both of whom had reduced or nondetectable CH50 as analysed by both pathways. C3 measured by conventional technique revealed 19% anti-C3c-reactive protein in the plasma of patient 1 and 3% in patient 2. Patient 1 had circulating C3b but no detectable C3c, C3d, or C4d, whereas patient 2 had normal levels of C3c and C4d and strongly elevated levels of C3d. Factor B analysis revealed no demonstrable native factor B and small amounts of Bb in patient 1 and normal concentration of native factor B plus trace amounts of Bb in patient 2. The depletion of C3 in both patients was due to uncontrolled activation caused by complete factor I deficiency (patient 1) and circulating C3 nephritic factor (patient 2). Both parents of patient 1 had factor I concentrations below (mean-2 SD) that seen in normal healthy individuals (n = 20). Circulating immune complexes (IC) were demonstrated in patient 1 only, whereas serum from both patients had strongly reduced capacity to solubilize preformed IC.

Adolescent↗

PubMed1996

Inherited complement C3 deficiency: reduced C3 mRNA and protein levels in a Laotian kindred.

To determine the molecular basis of complement C3 deficiency in a Laotian kindred, the homozygous C3-deficient male propositus was studied. By ELISA, this individual's serum was determined to contain approximately 4 microg/ml C3 (0.3% of normal). In accord with this result, anti-C3 immunoprecipitation of [35S]-methionine-labeled fibroblasts from this C3D individual revealed pro-C3 of normal size (180,000 Mr), but in significantly reduced amounts (approximately 1% of normal fibroblasts), that was processed and secreted with normal-size alpha- and beta-chains. In addition, C3-specific mRNA of normal size (5.2 kb) but in reduced quantity (approximately 1% of normal) was detected in this individual's fibroblasts by Northern analysis. The nucleotide sequence of the transcriptional initiation site, the promoter, and the IL-1beta/IL-6 cis-regulatory elements of the C3-deficient gene are normal in this C3-deficient individual, indicating that the low C3 mRNA and protein levels are not caused by reduced C3 transcription that is the result of a cis-mutation. Moreover, cDNA sequencing studies revealed no defect in the C3-deficient mRNA, including the areas mutated in four previously characterized C3-deficient patients. These data indicate that (1) C3 protein deficiency in this Laotian patient results from reduced levels of C3-specific mRNA, (2) the small amount of expressed C3 protein is processed and secreted normally from the deficient cells, and (3) the molecular genetic defect(s), although not yet delineated, is different from those described in other C3-deficient individuals, thereby providing additional evidence for numerous mutations that cause inherited C3 deficiency in humans.

Cells, Cultured↗

PubMed1992

Calcium-dependent and calcium-independent signals in the conglutinin-binding assay (KgBa) for immune complexes. Influence of anti-collagen-antibodies.

A solid phase ELISA conglutinin-binding assay (KgBa) was evaluated for the detection of circulating immune complexes. ELISA wells were coated with purified bovine conglutinin and incubated with test sera. Bound IgG was detected with enzyme labelled anti-immunoglobulin. Heat aggregated IgG which had been "solubilized" (i.e., complement treated by incubation with serum) was employed as a reference. The binding of the complement-reacted IgG to solid phase conglutinin was found to be calcium-dependent and inhibitable with N-acetyl-D-glucosamine (GlcNAc). Prolonged incubation (4 days) of aggregated IgG with serum at 37 degrees C abolished the binding to conglutinin, a finding consistent with the complete degradation of deposited C3b to C3c and C3d. The solubilized IgG that bound to solid phase conglutinin was found by gel chromatography to be of high molecular weight (greater than 600 kDa). Binding of IgG to solid phase bovine conglutinin was also observed to a variable degree in normal and pathological sera. However, in this situation the IgG binding was largely calcium-independent, was not inhibited by GlcNAc and did not decrease after prolonged incubation of the serum at 37 degrees C. The reactive IgG eluted on gel chromatography at the position of monomeric IgG suggesting binding via the antigen binding sites. Binding of this IgG was inhibited by both collagen type II and purified conglutinin. These observations suggest that the assay detects cross-reacting autoantibodies against collagen epitopes, or, alternatively, antibodies against the dietary antigen, bovine conglutinin.

Acetylgalactosamine↗

PubMed1986

Ligand binding by the p150,95 antigen of U937 monocytic cells: properties in common with complement receptor type 3 (CR3).

U937 cells (a monocytic cell line) grown in the presence of phorbol myristate acetate were surface labeled with 125I and the iC3b-binding proteins isolated by affinity chromatography on iC3b-Sepharose in the presence of divalent cations. Three polypeptides of 170, 150 and 95 kDa were found to bind specifically to iC3b-Sepharose. The polypeptides of 170 and 95 kDa were identified as the alpha and the beta subunits of CR3 by immunoprecipitation with OKM1 monoclonal antibody. The 150-kDa polypeptide was not immunoprecipitated by antibodies to the alpha subunit of CR3 or LFA-1. However, the 150-kDa polypeptide, together with the 95-kDa polypeptide, was immunoprecipitated with an anti-beta subunit-specific antibody IB4, which immunoprecipitates LFA-1, CR3 and p150,95. These results indicated that the 150-kDa polypeptide is the alpha subunit of the p150,95 antigen. The binding of p150,95 and CR3 to iC3b-Sepharose is specific as neither binds to C3u-Sepharose. A monoclonal antibody, 3.9, which immunoprecipitated the 150 and a 95-kDa polypeptide from U937 cells was characterized as being directed against the alpha-subunit of the p150,95 antigen. Phorbol myristate acetate-stimulated U937 cells from rosettes with EAC3b and EAiC3b but not with EAC3d cells. Monoclonal antibody 3.9 does not inhibit either type of rosetting, but we were unable to exclude a role for p150,95 in adherence of iC3b-coated particles. Since there is no rosetting with C3d-bearing particles it is unlikely that p150,95 is a receptor for C3d, a role for p150,95 which has been suggested by others (Wright, S.D., Licht, M.R. and Silverstein, S.C., Fed. Proc. 1984. 43: 413).

Antibodies, Monoclonal↗

PubMed2000

Each of the three binding sites on complement factor H interacts with a distinct site on C3b.

Factor H (fH) restricts activation of the alternative pathway of complement at the level of C3, both in the fluid phase and on self-structures, but allows the activation to proceed on foreign structures. To study the interactions between fH and C3b we used surface plasmon resonance analysis (Biacore(R)) and eight recombinantly expressed fH constructs containing fragments of the 20 short consensus repeat domains (SCRs) of fH. We analyzed the binding of these constructs to C3b and its cleavage products C3c and C3d. Three binding sites for C3b were found on fH. Site 1 was localized to the five amino-terminal SCRs (SCR1-5), and its reciprocal binding site on C3b was found to be lost upon the cleavage of C3b to C3c and C3d. Site 2 on fH was localized by exclusion probably within or near SCRs 12-14 (fragment SCR8-20 bound to C3b, C3c, and C3d; SCR8-11 did not bind to C3b at all; and SCR15-20 bound only to the C3d part of C3b). Site 3 on fH for C3b was localized to the carboxyl-terminal SCRs 19-20, and its reciprocal binding site was mapped to the C3d part of C3b. In conclusion, we confirmed and mapped three binding sites on fH for C3b and demonstrated that the three binding sites on fH interact with distinct sites on C3b. Multiple reciprocal interactions between C3b and fH can provide a basis for the different reactivity of the alternative pathway with different target structures.

Binding Sites↗

PubMed2002

X-ray crystal structure of the C4d fragment of human complement component C4.

C4 fulfills a vital role in the propagation of the classical and lectin pathways of the complement system. Although there are no reports to date of a C4 functional activity that is mediated solely by the C4d region, evidence clearly points to it having a vital role in a number of the properties of native C4 and its major activation fragment, C4b. Contained within the C4d region are the thioester-forming residues, the four isotype-specific residues controlling the C4A/C4B transacylation preferences, a binding site for nascent C3b important in assembling the classical pathway C5 convertase and determinants for the Chido/Rodgers (Ch/Rg) blood group antigens. In view of its functional importance, we undertook to determine the three-dimensional structure of C4d by X-ray crystallography. Here we report the 2.3A resolution structure of C4Ad, the C4d fragment derived from the human C4A isotype. Although the approximately 30% sequence identity between C4Ad and the corresponding fragment of C3 might be expected to establish a general fold similarity between the two molecules, C4Ad in fact displays a fold that is essentially superimposable on the structure of C3d. By contrast, the electrostatic characteristics of the various faces of the C4Ad molecule show marked differences from the corresponding faces of C3d, likely reflecting the differences in function between C3 and C4. Residues previously predicted to form the major Ch/Rg epitopes were proximately located and accessible on the concave surface of C4Ad. In addition to providing further insights on the current models for the covalent binding reaction, the C4Ad structure allows one to rationalize why C4d is not a ligand for complement receptor 2. Finally the structure allows for the visualization of the face of the molecule containing the binding site for C3b utilized in the assembly of classical pathway C5 convertase.

Amino Acid Sequence↗

PubMed1980

Covalent binding and hemolytic activity of complement proteins.

We report the inactivation of the third component of complement (C3) by hydroxylamine. C3 hemolytic and covalent binding activities decline with identical kinetics, demonstrating a direct correlation between the two activities. We conclude that covalent, surface-bound C3b is hemolytically active. The inactivation of C3 is first order with respect to hydroxylamine. We also studied C3 inactivation with [14C]methylamine. The inactivation corresponds quantitatively with the labeling of C3 in the C3d domain. The data obtained support the following hypothesis: there is an internal thioester within C3 which becomes highly reactive on activation to C3b, and C3b binds to receptive surfaces by transfer of the acyl function of the thioester to a hydroxyl group on the receptive surface. This proposed model for the reaction of C3 with receptive surfaces also applies to C4, which binds to membrane surfaces covalently and is able to be inactivated by hydroxylamine and methylamine. C5, on the other hand, is not inactivated by treatment with the amines.

Binding Sites↗

PubMed2002

Engagement of complement receptor 2 on the surface of B cells from patients with systemic lupus erythematosus contributes to the increased responsiveness to antigen stimulation.

B cells from patients with systemic lupus erythematosus (SLE) display increased responses following cross-linking of the surface antigen receptor. We explored the possibility that the increased responses are at least partially due to simultaneous cross-linking of the complement receptor 2 (CR2). To this end, we stimulated fresh B cells from SLE patients with an anti-IgD antibody conjugated to the Epstein-Barr virus gp350 protein, which binds to CR2, and recorded the free intracytoplasmic calcium response during the first 10 min. Despite the fact that SLE B cells were found to express half as many surface CR2 as normal B cells, both peak responses and the percentage of responding cells were significantly increased in the former. These observations suggest that regulatory molecules such as CR2 are involved in the increased B cell responses in SLE patients. We propose that certain immune complexes that circulate in the sera of SLE patients that have anti-surface immunoglobulin specificities and are decorated with natural ligands of CR2, such as C3d, elicit and promote B cell overactivity.

Adult↗

PubMed1974

Receptor for soluble C3 and C3b on human lymphoblastoid (RAJI) cells. Properties and biologocal significance.

This study describes the presence of a receptor for fluid phase human C3 and C3b on Raji cell membranes. The binding of C3 and C3b was demonstrated indirectly by a fluoresceinated anti-C3 serum and directly by using radioiodinated proteins. No other complement proteins or serum factors were needed to mediate binding of C3 and C3b to the receptor. The possibility of enzymatic cleavage of C3 before or after its attachment on the cell membrane was ruled out by the demonstration of antigenically intact C3 on Raji cells. Inhibition and dissociation of Raji cell-EAC1423 rosettes by C3 and C3b indicated that both of these proteins bind to the same receptor site or closely associated receptor sites on Raji cells. C3b-bearing Raji cells were immune adherence negative, indicating that C3b binding to the receptor is brought about through the immune adherence region of the molecule and not the C3d portion. The C3 receptor on Raji cell membranes is uniformly distributed and can move on the membrane plane. Approximately 4 x 10(5) molecules of C3 or C3b bind per Raji cell. The receptor had a higher affinity for C3 than C3b, as was shown by uptake experiments and inhibition of Raji cell-EAC1423 rosette formation. Apart from the described receptor for C3 and C3b another specific receptor for C3b inactivator-cleaved C3b (C3d) bound to red cells was shown to be present on Raji cells. Raji cells cultured in medium containing fresh normal human serum and cobra venom factor were lysed. Similar results were obtained when C3b-bearing Raji cells were cultured in medium with fresh normal human serum. The lytic effect could be abolished by inactivating serum C3 proactivator (C3PA) and required C6. It was concluded that C3b bound to the Raji cell membrane activates the complement system through the alternate pathway and results in membrane damage and cytolysis. It is postulated that cell destruction by this mechanism may play an important role in vivo in controlling cell growth.

Animals↗

PubMed1982

Cell-surface-marker phenotyping in patients with leukemic non-Hodgkin lymphomas (NHL) of low and intermediate malignancy.

Certain markers for B lymphocytes (SIg, EAIgG, EAIgMC3b, EAIgMC3d, M-R) and T lymphocytes (E-R, EAIgG, EAIgM) were applied in order to characterize circulating neoplastic cells in non-Hodgkin lymphomas (NHL) of low and intermediate malignancy. In all cases, the diagnoses were based on the histological examination of lymph nodes, bone marrow, or skin biopsies. According to the Kiel classification, the diagnoses were as follows: chronic lymphocytic leukemia (CLL) n = 145. Immunocytoma (Ic) n = 39. Centrocytic (Cc) and centroblastic/centrocytic lymphoma (Cb/Cc) n = 17. Hairy-cell leukemia (HCL) n = 10. Prolymphocytic leukemia (PLL) n = 6 and Sézary's syndrome n = 3. In only 8 of the 220 cases did the leukemia cells show T characteristics. In leukemic B-cell lymphoma, a uniform phenotype was observed for B-CLL, characterized by a weak SIgM staining with or without SIgD, the presence of C3d-receptors and a high percentage of M-R. This phenotype was also detected in one third of the cases of immunocytoma. The cells of B-PLL and leukemic CC and CB/CC were characterized by a stronger SIg staining, a variable formation of complement receptors and, in most cases, absence of M-R. In all cases of B-cell lymphoma, the monoclonality of the cell proliferation could be confirmed by the restriction to a single L-chain type. This also applies to the 10 cases of HCL, although in the majority of these cases, several heavy-chain classes could be detected at the malignant cells.

Antigens, Surface↗

PubMed1977

Kinetic studies on the fragmentation of the third component of complement (C3) by trypsin.

The kinetics of cleavage of C3 by trypsin was analyzed by electrophoresis in agarose and in polyacrylamide gels containing sodium dodecyl sulfate and the data obtained were used to construct an anatomical model for C3 showing the sites of tryptic attack, the fragments generated, and their composition. Trypsin was shown to cleave C3 in a stepwise fashion. The attack was initially directed at the alpha-polypeptide chain and resulted in the generation of C3a and C3b. Further cleavage of the alpha-chain of C3b, converted it into C3b1 and then into C3d and C3c. Cleavage of the beta-chain by trypsin occurred only at the C3c stage with the release of a small peptide (m.w. 12,000) from C3c and the formation of C3c'. On immunoelectrophoresis, C3c' had a less anodal mobility compared to the beta1A mobility of C3c. C3a, once formed could be further cleaved to give residual fragments with decreasing net positive charge. Exposure of C3 to acid conditions, pH 5.0 or below, rendered the molecule exceedingly susceptible to tryptic degradation.

Citrates↗

PubMed1985

Deposition of C3b and iC3b onto particulate activators of the human complement system. Quantitation with monoclonal antibodies to human C3.

Monoclonal antibodies were used to determine the number and molecular form of C3 bound to particulate activators of the complement (C) system by human serum. Sheep erythrocytes (E) coated with IgM (EIgM) and IgG (EIgG) were used to study activation of the classical pathway (CP). Yeast (Y), rabbit erythrocytes (ER), and five species of bacteria (Escherichia coli, Staphylococcus aureus, Streptococcus pneumoniae type 3, Streptococcus pyogenes, and Hemophilus influenzae type b) were used to study activation of the alternative pathway (AP). The deposition of C3b onto EIgM and EIgG incubated in C7-deficient human serum was dependent on the serum concentration. At all serum concentrations tested, there was complete conversion of C3b to iC3b. Kinetic analysis of C3b deposition and conversion to iC3b indicated that these events occurred almost simultaneously; the reaction was completed by 15 min. The deposition of C3 onto the AP activators ER and Y was also dependent on serum concentration, and ER, but not Y, required the presence of Mg-EGTA and thus the activation of only the AP. C3b deposition and conversion to iC3b on Y was complete in 15 min, with 82% of bound C3 converted to iC3b. For ER, maximum C3 deposition required 30 min in both the presence and absence of Mg-EGTA. However, after 1 h of incubation, 74% of bound C2 was iC3b in the absence of Mg-EGTA, compared with only 52% in the presence of Mg-EGTA. Thus, even on AP activators, a large portion of C3b may be converted to iC3b, and this conversion is probably controlled by elements on the particle's surface. Studies with the five species of bacteria yielded similar results. Approximately 3-5 X 10(4) molecules of C3 were bound per microorganism, with opsonization being completed in 30 min. Remarkably, only 16-28% of bound C3 was in the form of iC3b, even after 2 h of incubation. The presence or absence of Mg-EGTA, or the addition of purified CR1 to the reaction mixture, did not significantly effect the ratio of C3b to iC3b. Finally, SDS-PAGE and autoradiography of particle-bound 125I-C3 fragments confirmed that there was no conversion of iC3b to C3d,g or C3d. The data obtained about the opsonization of bacteria suggest that the predominant form of C3 that is encountered by inflammatory phagocytes may be C3b.

Animals↗

PubMed1977

The role of membrane receptors for C3b and C3d in phagocytosis.

In this paper we re-examine the roles of particle-bound IgG and C3 in phagocytosis of sheep erythrocytes (E) by monolayers of purified human monocytes and polymorphonuclear leukocytes (PMN). We conclude that two fragments of the C3 molecule, that is, C3b and C3d, can function as opsonins if the phagocyte has the appropriate membrane receptors. Monocytes, that bind both C3b and C3d, respond to both as opsonins. PMN, which do not bind C3d, respond only to particles opsonized with C3b. C3 and IgG have separate roles in phagocytosis. IgG, through its Fc fragment, directly stimulates particle ingestion, but is relatively inefficient at inducing particle binding. On the other hand, C3 primarily mediates the binding of the particle via complement receptors. A marked synergy exists between C3 and IgG in inducing phagocytosis. Thus, opsonization of the particle with C3 can be a necessary condition for particle ingestion, although by itself C3 does not trigger phagocytosis. The opsonic effect of C3 can be mimicked by a variety of nonimmunologic agents which enhance binding of the particle to the phagocyte without directly stimulating ingestion. The contact-inducing agents used include centrifugation of particle and phagocyte, high molecular weight dextran, protamine, and treatment of E with neuraminidase. These results suggest that the role of C3 in opsonization is mainly or exclusively one of establishing contact between particle and phagocyte.

Agglutination↗

PubMed1988

On the mechanisms of haemodialysis-induced neutropenia: a study with five new and re-used membranes.

A prospective study was undertaken in 12 haemodialysed patients successively treated on five new as well as re-used dialyser membranes, that is cuprophane, cellulose acetate, polysulphone, polycarbonate, and polyacrylonitrile. A significant reduction of neutrophils occurred with every membrane during their first use, which improved only with cuprophane upon re-use. Thrombocytopenia was noted only when neutropenia reached very low values. Monocyte reduction occurred on cuprophane, cellulose acetate and polycarbonate, but did not improve during second use. C3d accumulation paralleled the time course of neutropenia only with cuprophane and cellulose acetate. Plasma collected at the extreme of neutropenia induced aggregation of control and predialysis cells, but did not aggregate autologous dialysed neutrophils collected at 5 min. Our data indicate that the mechanism linking complement activation to neutropenia is probably triggered by more than one factor.

Aged↗

PubMed1978

Evidence that bovine conglutinin reacts with an early product of C3b degradation, and an improved conglutination assay.

When EAC43b were treated with heated serum in EDTA, reactivity with bovine conglutinin appeared rapidly, even at 0 degrees C, and almost simultaneously with the loss of C3b rosetting capacity. At the time conglutinability first appeared, there was no detectable decrease in I-A or hemolytic C3 activity, and no detectable C3 antigen release from the cells. With prolonged exposure to heated serum in EDTA, I-A (immune adherence) and hemolytic C3 activity were lost. If this exposure was at 37 degrees C, C3 antigen became strongly detectable in the supernatant fluid, and eventually conglutinability was markedly reduced or lost, whereas C3d rosettes were unaffected. We suggest that bovine conglutinin reacts with some early product of C3b degradation, rather than with C3d, and propose that this intermediate be designated C3k. We have developed a semi-quantitative assay for bovine conglutinin, utilizing a Coulter Counter to register the decrease in total particles due to red cell aggregation. By using this method, we have detected conglutination with mouse complement (C) as well as with that from man and the guinea pig.

Animals↗