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Alteration in H chain V region affects complement activation by chimeric antibodies.

Chimeric antibodies to the synthetic polypeptide (Tyr, Glu)-Ala-Lys ((T,G)-A-L) were used to examine C activation by human IgG1. Two IgG1 antibodies, which contained mouse L chains and H chains with mouse V domains and human C domains, differed only in their VH domain. Ag binding and C activation by these antibodies were analyzed by ELISA. When limiting amounts of Ag were used in the assays, the antibodies required different quantities of Ag for optimal binding, suggesting that the antibodies bind to different epitopes on the (T,G)-A-L molecule. However, when competitive inhibition assays were performed with an optimal concentration of Ag, there were no differences in relative binding affinities for (T,G)-A-L or dissociation characteristics of the antibodies. C activation was examined at optimal Ag concentration to ensure equivalent binding of two IgG1 antibodies to Ag. After combination with immobilized Ag, these two antibodies bearing different V regions exhibited marked differences in the binding of C components C1q and C3d. When present in equal amounts in the assay, antibody 10B activated C and bound more C1q and C3d than antibody B11. These results indicate that V region differences can affect C activation by IgG.

Amino Acid Sequence↗

Identification of the C3b receptor-binding domain in third component of complement.

We report here that complement receptor type one (CR1) binds to a region of C3b that is contained within the NH2 terminus of the alpha' chain. In an enzyme-linked immunosorbent assay, CR1 bound to C3b, iC3b, and C3c but not to C3d, and this binding was inhibited by soluble C3b and C3c. Further attempts to generate a small C3 fragment capable of binding CR1 were unsuccessful. However, elastase degradation of C3 generated four species of C3c (C3c I-IV), two of which bound CR1. NH2-terminal sequence analysis and sodium dodecyl sulfate-gel electrophoresis of the C3cs indicated that the beta chains and the 40,000-dalton COOH-terminal alpha' chain fragments were identical; the NH2-terminal alpha' chain fragments of C3c I-IV varied from 21,000 to 27,000 daltons and accounted for the differential binding to CR1. C3c-I and II, which do not bind CR1, were missing 8 and 9 residues from the NH2 terminus of the alpha' chain when compared with the intact alpha' chain of C3b. C3c-III and IV, which bind CR1, had NH2 termini identical to the intact NH2-terminal alpha' chain of C3b. Using iodinated concanavalin A and endoglycosidase H, we showed that the NH2-terminal alpha' chains of C3c-I and III were glycosylated, while C3c-II and IV were not. Therefore, these data indicated that the amino terminus of the NH2-terminal alpha' chain fragment of C3c was responsible for binding CR1 while the COOH terminus of this fragment was not involved since the presence or absence of this region in C3c did not affect CR1 binding to C3c. Subsequently, two peptides were synthesized from the NH2-terminal alpha' chain fragment of C3c: X42, 42 residues in length from the NH2 terminus and C30, 30 residues in length from the COOH terminus. X42 inhibited binding of CR1 to C3b, and this effect was also observed with antipeptide antibodies against the X42 peptide. The C30 and other C3-derived peptides and antipeptide antibodies had no effect on the binding of CR1 to C3b.

Amino Acid Sequence↗

Human complement (C3b) receptors defined by a mouse monoclonal antibody.

The aim of the present study was to prepare a monoclonal antibody against human C3 receptors. The monoclonal antibody termed C3RTo5, which is characterized in this study, inhibited the ligand binding of C3b receptors of human erythrocytes, neutrophils and lymphocytes, but did not block the ligand binding of C3bi and C3d receptors. C3RTo5 reacted only with cells that express C3b receptors. It did not react with cells that do not express C3 receptors or with cells that express C3 receptors other than C3b receptors. The reactivity of C3RT05 against C3b receptors of human erythrocytes, neutrophils, glomerular cells, tonsil cells, or spleen cells could be removed by absorption with human erythrocytes or tonsil cells, whereas absorption with human peripheral T cells or sheep erythrocytes had no effect. In immunoprecipitation studies, a glycoprotein with a mol. wt of 205,000 could be isolated with C3RTo5 from non-ionic detergent lysate of tritiated tonsil cells. A rabbit antiserum prepared against this glycoprotein was able to stain C3b receptor-positive cells, inhibit C3b receptor ligand-binding activity and, furthermore, to precipitate a 205,000 mol. wt component. The results of this study indicate that C3RTo5 is a monoclonal antibody with selective reactivity to C3b receptors and presumably to the binding sites within the receptor molecule. Using C3RTo5 further strong evidence was obtained that membrane-bound C3b receptors have a mol. wt of 205,000.

Animals↗

Complement activation by cross-linked B cell-membrane IgM.

The B cell membrane IgM (mIgM) occurs in a monomeric form incapable of activating C. However, when cross-linked by a polyvalent ligand, mIgM may activate C by assuming a polymeric structure like secreted IgM. This possibility was tested with CR2- lymphoma cells, which did not activate C spontaneously. When CR2-deficient mIgM(lambda)+ Ramos cells were treated with F(ab')2 goat anti-lambda, then exposed to human serum, a marked C3 deposition took place, as examined by the flow cytometry. Similarly, C3 deposition on mIgM(kappa)+, mIgD(kappa)+ P32 cells was induced by F(ab')2 of either anti-kappa or anti-mu. Anti-delta was without effect, but the C3 deposition resulting from anti-kappa was markedly enhanced after mIgD was modulated by anti-delta. The mIgM-cross-linked cells bound C1q, and C3 deposition on these cells was abrogated by depletion of C1q, but not Factor B nor D, from serum. The C1-binding step of the mIgM-mediated C activation was inhibited by monomeric Fab' of polyclonal anti-mu containing a blocking Ab to the hemolytic activity of human IgM Forssman Ab. A large proportion of C3 deposits on mIgM-cross-linked cells was found to be associated with mIgM in the form of C3dg or C3d. These results demonstrate that cross-linked mIgM indeed triggers the classical pathway of C.

B-Lymphocytes↗

Characterization of C3 receptors on cultured rat glomerular endothelial cells.

In this study we characterized C3 receptors on cultured rat glomerular endothelial cells (GEnC), using immunochemical and molecular techniques. GEnC membrane proteins were immunoprecipitated with a polyclonal antibody directed towards mouse complement receptor 2 (CR2). This anti-MCR2 immunoprecipitated GEnC proteins of 120 and 150 kDa. By immunohistochemistry, anti-MCR2 stained GEnC in rat glomeruli in vivo. Given the presence of CR2-like proteins on GEnC, subsequent studies were done to determine whether GEnC had C3-binding proteins. GEnC proteins of 80, 200, and 300 kDa specifically bound to columns of rat C3d-Sepharose and C3b-Sepharose, illustrating that these proteins were binding to the C3d portion of C3. The 80, 200, and 300 kDa C3d-binding proteins were distinct from the 120 and 150 kDa anti-MCR2 reactive proteins, as shown by immunoabsorption studies. Next, a specific cDNA probe for rat CR2 was generated by RT-PCR. Oligonucleotides were chosen from highly conserved regions in mouse and human CR2 spanning 224 bases, with the rationale that these would also be conserved in the rat. A 224 bp PCR product was generated from both rat GEnC and rat kidney cDNA, illustrating the presence of CR2 mRNA in these tissues. By Northern analysis, the CR2 PCR product hybridized to mRNA of 2 and 5 kb from GEnC. The 5 kb transcript was also identified in rat kidney mRNA. Therefore, proteins immunologically related to mouse CR2 are present in GEnC in vitro and in vivo. C3d-binding proteins of 80, 200, and 300 kDa are also present on rat GEnC, yet these appear to be immunologically distinct from the proteins identified by anti-MCR2. Whether the GEnC CR2 mRNA transcripts of 2 and 5 kb are translated into the 80 and 200 kDa C3d-binding proteins or the 120 and 150 kDa mouse CR2-like proteins remains to be defined.

Animals↗

Monovalent ligands of complement receptor 2 inhibit whereas polyvalent ligands enhance anti-Ig-induced human B cell intracytoplasmic free calcium concentration.

We have performed experiments to investigate the role of ligands for complement receptor 2 (CR2) in human B cell activation. Flow microfluorimetry was used to assess changes in free intracytoplasmic calcium concentration [Ca2+] in indo-loaded B cells, immediately after exposure to anti-mu antibody and to monovalent or polyvalent CR2 ligands. As monovalent ligands we used the C3d fragment and synthetic C3 peptides (peptides P14, residues 1201-1214, and P28, residues 1187-1214). As polyvalent ligands we used i) an intact monoclonal mouse anti-CR2 antibody (HB5) and its F(ab')2 fragment, ii) tetravalent P13 [residues 1202-1214) 4-template), and iii) P28 conjugated to BSA (molar ratio 5/1). Anti-CR2 antibody HB5, tetravalent P13, and P28 conjugated to BSA, enhanced the ability of F(ab')2 fragments of the IgG fraction of goat anti-human mu antibody to increase human B cell [Ca2+]i. In contrast, the monomeric CR2 ligands C3d and P28 inhibited the anti-mu-induced increase in human B cell [Ca2+]i. Multivalent P13, P28, and the HB5, by themselves, did not affect B cell [Ca2+]i. These experiments suggest that the valence of the CR2 ligands is crucial for the nature (synergistic vs antagonistic) of the message transmitted through the CR2.

Antigens, Differentiation, B-Lymphocyte↗

Formation of C3-IgG complexes in serum by aggregated IgG and by non-immunoglobulin activators of complement.

We studied the generation of C3-IgG complexes during the activation of C3 in serum by aggregated human IgG (AHG), zymosan or cobra venom factor (CVF). C3-IgG complexes were detected by specific radioimmunoassays: samples to be tested were incubated with anti-IgG Sepharose, and complexes that had bound to the Sepharose were detected by incubation with either 125I-anti-C3c or 125I-anti-C3d, g. Incubation of serum with as little as 6 micrograms AHG per ml, for 30 min at 37 degrees, resulted in the generation of C3-IgG complexes. When serum was incubated with zymosan or CVF, C3-IgG complexes were also generated. AHG appeared to be more effective in the generation of C3-IgG complexes than CVF. We calculated that AHG (2 mg/ml) caused about 36% of the C3 to be fixed to IgG, CVF (400 micrograms/ml) about 14%. Finally, the presence of C3 fixed to IgG in serum incubated with CVF was demonstrated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE), followed by immunoblotting. This study indicates that the formation of C3-IgG complexes in serum is not only induced by immune complexes but also by non-immunoglobulin activators. Therefore, C3-IgG complexes might be considered as complement activation products, and their detection in patients' samples should not be considered as conclusive evidence for the presence of immune complexes.

Antigen-Antibody Complex↗

An inherited deficiency of the third component of complement, C3, in guinea pigs.

Hereditary deficiency of the third component of complement, C3, is found very seldom in the human. C3 deficiency is associated with severe bacterial infections revealing the central role of C3 in complement activation via the classical or alternative pathway. We describe a new hereditary C3 deficiency in strain 2 guinea pigs. Serum from these animals had a markedly reduced lytic activity in a standard assay for complement-dependent, antibody-mediated cytotoxicity. In functional assays of individual components, the hemolytic activity of the components C4, C2, C5 and of factors B, D and H was in the normal range. The functional C3 titer, and similarly C3 antigenic activity in the serum of these C3-deficient animals (C3D) was on average only 5.7% of normal activity. Typing the animals with alloantisera or monoclonal antibodies to guinea pig Ia-antigens revealed that the C3D animals had the major histocompatibility complex-haplotype of inbred strain 2 guinea pigs (B.1, Ia.2,4). The C3 defect is not linked to the major histocompatibility complex and, in addition, is not linked to a C3a receptor deficiency. Macrophages and hepatocytes of the C3D animals have an unimpaired capacity for synthesis and secretion of C3 as measured by enzyme-linked immunosorbent assay. There was no indication for hypercatabolism of normal C3 by the animals as shown by plasma clearance of 125I-radiolabeled C3. Thrombocytes of the C3D animals responded normally to stimulation with purified C3a in an ATP-release assay without an indication for a desensitization in vivo. Possibly the fault resides in an enhanced susceptibility of their own C3 to proteolysis. However, C3 partially purified from the plasma of the C3D animals or secreted by hepatocytes exhibited no obvious structural differences to purified normal C3 in sodium dodecyl sulfate-polyacrylamide gel electrophoresis or in immunoblotting. The C3D serum had a reduced bactericidal activity compared to normal or to C4-deficient serum. Nevertheless, the animals are apparently healthy without an indication for increased frequency of bacterial infections. These guinea pigs provide an unique model for analysis of the biological functions of C3 in vivo and in vitro without the need for artificial C3-depletion procedures with all their known and unknown side-effects.

Animals↗

Purification and characterization of IgG immunoconglutinins from patients with systemic lupus erythematosus: implications for a regulatory function.

The levels of IgG immunoconglutinins in plasma from patients with rheumatoid arthritis, systemic lupus erythematosus and primary biliary cirrhosis were monitored by ELISA. High levels of IgG immunoconglutinins were found mainly in plasma from patients with systemic lupus erythematosus. These immunoconglutinins bound to microtitre plate-fixed C3, C3b and C3c but poorly to C3d. This binding was inhibited by particle-bound C3b and iC3b but not by the corresponding soluble fragments. Furthermore, Western blot analysis revealed no immunoconglutinin-binding to reduced C3 peptides and no binding was shown to soluble C3 alpha and beta chain by ELISA. IgG immunoconglutinins were purified from three plasma specimens by affinity chromatography on activated thiol sepharose ATS/C3 fragments. Two immunoconglutinin preparations that preferentially recognize ATS-C3b, inhibited C5-convertase function by 50-100% while one immunoconglutinin that recognized ATS-C3d,g had no effect. The two former immunoconglutinins also inhibited all three factor I cleavages in C3 alpha chain but the latter inhibited only the third cleavage. None of the immunoconglutinins affected the binding of complement-coated anti-BSA/BSA complexes to CR1 (CD35) on human erythrocytes, but the two immunoconglutinins that inhibited all factor I cleavages also inhibited the factor I-induced release of anti-BSA/BSA complexes from CR1. The results show that immunoconglutinins recognize specific epitopes on bound C3 fragments and that they are able to modulate C3-mediated functions.

Antibody Specificity↗

Erythrocyte complement C3b receptor (CR1) levels and immune complex-induced manifestations in patients with primary Sjögren's syndrome.

To assess the importance of low C3b receptor (CR1) numbers on erythrocytes for the development of clinical features in patients with primary Sjögren's syndrome (SS), two groups of patients were selected for investigation--one without (n = 13), and one with (n = 21) extraglandular, possibly immune complex-induced, clinical manifestations during the disease course. A preponderance (p less than 0.01) of low CR1 levels was found in patients with primary SS (mean 57%) as compared with normal controls (mean 70%), but the CR1 levels did not differ between the two groups of patients. The CR1 values did not correlate with the levels of circulating immune complexes and C3d. Although the preponderance of low CR1 levels is pathogenetically intriguing, it appears to be of little help in the clinical assessment of patients with primary SS.

Antigen-Antibody Complex↗

C3b and c3d receptor sites on human fibroblasts derived from several human tissues.

The C3b and C3d receptor sites on one cell line of human diploid fibroblasts (WI-38) were reported in previous papers [3, 4]. In this paper we describe that C3b and C3d receptor sites can also be detected in fibroblast cell lines derived from other human tissues. We consider that C3b and C3d receptors are normally found on the cell surfaces of all human fibroblasts.

Binding Sites↗

A comparative study of normal B cells and the EBV-positive Burkitt's lymphoma cell line, Raji, as activators of the complement system.

Contradictory reports regarding the ability of complement receptor type 2 (CR2,CD21) on normal B cells to activate complement (C') via the alternative pathway (AP), prompted us to compare the performance of human peripheral blood B cells and the Epstein-Barr virus-positive Burkitt's lymphoma cell line, Raji (a well characterized AP activator) by using flow cytometry. Measured in terms of the membrane deposition of C3 fragments per cell, Raji cells were significantly (6- to 26-fold) more effective as complement activators than were normal B cells. Raji cells were also found to express approximately four to five times as many CR2 as normal B cells. In addition, they distinguished themselves by displaying a greater Ca(2+)-dependent activation, with pooled normal human sera (NHS) as the complement source, and by degrading unprotected C3b fragments from iC3b to C3dg/C3d at a significantly lower rate than the B cells. The Ca2+ dependency of Raji cell activation was found to be partially a result of classical pathway (CP) triggering by specific antibodies in the NHS, although other triggering mechanisms may also be involved. If the influence of these variations between Raji cells and normal B cells was excluded, by relating deposition of anti-C3d-reactive fragments, during AP activation, to the number of CR2 expressed, the difference in performance between the two cell types was found to be insignificant.

Animals↗

Red blood cell-bound C3d in normal subjects and in random hospital patients.

Red blood cells (RBC) drawn into EDTA from 103 normal adults, 26 newborn infants, and 203 randomly selected hospital patients, and into citrate-phosphate-dextrose (CPD) from 87 Red Cross donors, were evaluated for RBC-bound C3d by agglutination with anti-C3d and by a radiolabeled anti-antiglobulin technique. Positive agglutination reactions were observed only with RBC from 26 hospital patients; however, by the radiolabeled antiglobulin method, C3d was demonstrable on all RBC, but its amount varied widely among different subjects. A normal range for RBC-bound C3d was established. RBC-bound C3d levels were not related to the age (18-88 years) or sex of the subjects and remained unchanged over 9 months. Red blood cells taken into CPD had more RBC-bound C3d than cells taken into EDTA; cord RBC had less than adult RBC. In 39 percent of the hospital patients (excluding those with autoimmune hemolytic anemia) RBC-bound C3d was above the normal range and a good correlation was found between values obtained by the radiolabeled anti-antiglobulin method and by agglutination titration scores. There was a threshold level of RBC-bound C3d below which agglutination reactions were negative. Evaluation of RBC-bound C3d in health and disease is important for determining optimal characteristics of anticomplement antiglobulin reagents.

Adolescent↗

Interaction of target cell-bound C3bi and C3d with human lymphocyte receptors. Enhancement of antibody-mediated cellular cytotoxicity.

The occurrence and distribution of distinct receptors for three C3 fragments on purified human blood lymphocytes were studied by rosette formation. Indicator cells were bovine, chicken, or sheep erythrocytes (E) bearing up to 100,000 molecules of human C3b (EC3b) without antibody. EC3b was converted to C3bi-bearing-E (EC3bi) with purified C3b inactivator (factor I) and beta1H (factor H), and to C3d-bearing E (EC3d) by treatment of EC3bi with trypsin. Using bovine E (Eb) as indicators, approximately 11% of the lymphocytes bound EbC3b, 6% bound EbC3bi and 2% bound EbC3d. Fractionation of the lymphocytes by adsorption to monolayers of C3-fragment-bearing Eb or by rosetting indicated that most of the cells with receptors for C3b were distinct from those having receptors for C3bi and/or C3d. Cells from two lymphoblastoid cell lines (Raji and Daudi) formed strong rosettes with EC3b, which were weak. 51Cr-labeled E was used as a target in antibody, C3-fragment-bearing E was not lysed by the lymphocytes. However, at suboptimal concentrations of IgG enhancing capacity of the fragments occurred in the order of C3bi greater than C3d greater than C3b. In addition, C3-fragment-bearing cells inhibited the lysis of antibody-coated cells not concluded that target cell bound C3 fragments enhance ADCC by improving contact between target cells and those effector cells which have C3 receptors. Cell-bound C3 effector cells. It is proposed that certain lymphocytes are capable of interacting with C3bi in addition to C3b and C3d and that C3bi and C3d have a greater regulatory effect on their cytolytic function than C3b.

Animals↗

Ligands of CR2 do not interfere with C3 fragment fixation or enhanced NK sensitivity of Raji cells treated with human serum.

Raji cells activate the alternative complement pathway (ACP) and fix C3 fragments when incubated in human serum (HS). Earlier experiments have shown that CR2 molecules are involved in this phenomenon and the opsonized cells have elevated sensitivity to the lytic effect of CR3-bearing NK cells. We show here that Raji cells treated with CR2 site-specific ligands, (C3d, OKB-7 and HB-5 mAbs, and a synthetic peptide which binds to CR2) generated and bound C3 fragments after exposure to HS. The elevated lytic sensitivity of HS-treated cells was not altered by the presence of the various CR2 ligands. Thus, the membrane-bound C3 fragments are not fixed at the C3dg receptor binding site.

Antibodies, Monoclonal↗

Deposition of the terminal C5b-9 complement complex in infarcted areas of human myocardium.

Poly- and monoclonal antibodies to neoantigens of the human C5b-9 complement complex, as well as polyclonal antibodies to C5, C8, and C9, were used to detect and identify C5b-9 deposits in human myocardial tissue. Immunocytochemical studies were performed on fresh-frozen autopsy material derived from patients with myocardial infarctions; in addition, in 17 of these patients, paraffin sections of formalin-fixed tissue were investigated. Sixteen autopsies from patients with noncardiac diseases were analyzed as controls. Without exception, C5b-9 positivity was registered selectively and exclusively on and in myocardial cells located within the zones of infarction. The selectivity of staining was confirmed by control reactions for succinic dehydrogenase activity performed in adjacent, respective double-stained sections. Most intensive staining with anti-neoantigen antibodies was observed in the peripheral areas of the infarctions. Weak staining for C3d, rather strong staining for C5 and C9, and intermediate staining with anti-C8 antibodies were observed in the same localizations. Stainings for C4 and IgA were negative, whereas immunocytochemical reactions for IgG and IgM revealed an irregular and very weak staining. Only very weak staining was also observed with a monoclonal antibody to complement S-protein, indicating that the terminal complement components were deposited mainly in the form of membrane-damaging C5b-9 complexes. Immunocytochemical staining for C5b-9 was found to represent a most sensitive tool for detection of ischemic myocardial lesions, permitting easy detection even of single cell necroses. As a working hypothesis, we suggest that initial ischemia may cause loss of the ability of the heart muscle cells to regulate complement turnover at the membrane level. The resulting deposition of C5b-9 on the cell membranes may contribute to functional disturbance and irreversible damage of myocardial cells during the infarction process.

Adult↗

Differential coating of human red blood cells with C4 or C3 in a low ionic strength medium.

An EDTA-containing, low ionic strength medium, pH 5.1, can be used for incubation of blood to obtain coating of red blood cells with C4 or C3. The coating achieved depends on incubation temperature (4 or 37 degrees C) and on whether or not CaCl2 or MgCl2 is added to the medium. The optimal concentration of EDTA appears to be 2 mM under the present conditions. This is equimolar to the final concentration of the added CaCl/MgCl2. The choice between CaCl and MgCl2 depends on whether blood without anticoagulant or ACD blood is employed. Coated cells keep satisfactorily for 5 weeks at 4 degrees C in Alsever's solution as well as after freezing and thawing. C3-coated cells can be converted to C3d-coated ones by incubation in normal, compatible, EDTA-containing serum at 37 degrees C.

Calcium Chloride↗

Activation of the terminal complement cascade in renal infarction.

Ischemic injury is an important cause of functional derangement in the kidney. The complement (C) system has previously been shown to be an important mediator of ischemic tissue injury in myocardial infarction. In the present study we therefore investigated the possible role of C in renal ischemic lesions. The deposition and distribution of various C components (C1q, C3c, C3d, C4, C5, C6, C9) and regulators [vitronectin, clusterin and protectin (CD59)] in human renal infarction lesions were studied by indirect immunofluorescence microscopy. Deposition of components of the terminal C complex (TCC), as well as vitronectin and clusterin, were observed throughout the infarcted areas. The strongest deposits were seen on the membranes of tubular epithelial cells and in the tubular lumina of the infarction areas, especially in the border zone between normal and infarcted tissue. Using markers for different segments of tubuli (Tamm-Horsfall glycoprotein and brush border antigens) it was possible to localize deposits of TCC predominantly to the proximal tubuli. In the glomeruli of the infarcted areas deposits of TCC were seen as a crescent-like pattern at and immediately beneath the Bowman's capsule. The expression of cell membrane-associated protectin was diminished in tubular epithelial cells of the infarction lesions. A clue for the possible mechanism of C activation in renal infarction was obtained from in vitro experiments, in which the contact of normal human serum with urine was observed to lead to the generation of TCC.(ABSTRACT TRUNCATED AT 250 WORDS)

Complement Activation↗