Location of the inter-chain disulfide bonds of the third component of human complement

Targeted disruption of the murine gene coding for the third complement component (C3)

Complement is a system of more than 30 proteins found both in plasma and on cell membranes. The complement system has several important functions in the immune response including initiation of inflammation, neutralization and elimination of pathogens, regulation of antibody responses, clearance of immune complexes and disruption of cell membranes. Under certain conditions complement may, however, act as a mediator of deleterious inflammatory reactions and complement activation has been implicated in the pathogenesis of autoimmune disorders, atherosclerosis, neurodegenerative diseases, bioincompatibility reactions and decompression sickness. Using gene targeting, we have generated mice deficient for the third complement component (C3). These mice represent an animal model in which complement activation by any pathway is prevented at an early stage. The C3-deficient mice should be valuable for the study of the roles of the complement system in vivo in a variety of physiological and pathological situations.

Conformational epitopes of C3 reflecting its mode of binding to an artificial polymer surface

The aim of the study was to investigate the incompletely understood mechanisms of complement (C) activation and binding on artificial biomaterials. Polystyrene in the form of microtitre plates was used as target for C binding, detectable by ELISA using monoclonal anti-C3 antibodies specific for conformational epitopes expressed by bound C3 and C3 fragments. C3 binding in whole blood/plasma/serum is maximal at low dilutions and occurs predominantly by C activation. At higher dilutions, C3 binding occurs at approximately 1/3 of maximal levels and is solely an effect of adsorption. C3 adsorption in the lower serum dilution range, occurs at low but clearly detectable levels. Comparative epitope analysis between C3 fragments, actively bound to polystyrene in the presence of serum, and of iC3b bound to sheep erythrocytes, clearly indicates that C3 binding/activation on polystyrene takes place as a C3 convertase-mediated reaction, which in serum/plasma is followed by a secondary factor I-dependent degradation of the bound C3b into iC3b. The neo-epitope analysis of serum-contacting polystyrene revealed that the adsorbed C3, throughout the entire serum dilution range tested, deposits in a state closely similar to that observed for purified C3 at a high packing density. Polystyrene surfaces with adsorbed purified C3 expressing this epitope profile were found to mediate APW dependent deposition of C3b in pig serum, presumably by forming a hybrid convertase with porcine Bb. These data therefore suggest that adsorbed C3 on serum-contacting polystyrene surfaces may initiate complement activation via the APW.

Disulfide bridges in human complement component C3b

The disulfide bridges of human complement component C3b, derived from C3 by removal of the 77-residue C3a, have been determined. The 10 bridges are Cys537-Cys794, Cys605-Cys640, Cys851-Cys1491, Cys1079-Cys1136, Cys1336-Cys1467, Cys1367-Cys1436, Cys1484-Cys1489, Cys1496-Cys1568, Cys1515-Cys1639, and Cys1615-Cys1624. Including the 3 bridges in C3a (Cys670-Cys698, Cys672-Cys705, and Cys685-Cys706) previously determined by high-resolution X-ray crystallography [Hoppe-Seyler's Z. Physiol. Chem. 361 (1980) 1389-1399] all disulfide bridges of C3 are localized. C3 and the strongly related C4 and C5 are members of the alpha 2-macroglobulin superfamily. The predicted bridge patterns of C4 and C5 are discussed and compared with that of alpha 2-macroglobulin.

Conformational differences between surface-bound and fluid-phase complement-component-C3 fragments. Epitope mapping by cDNA expression

In previous studies a subset of complement-component-C3 (C3) epitopes, C3(D), expressed in denatured and surface-bound C3 and C3 fragments, has been described. These epitopes were detected by antibodies raised against denatured C3. In the present study we used a cDNA expression strategy to localize epitopes recognized by monoclonal and polyclonal anti-C3(D) antibodies. First, DNAse I digestion of C3 cDNA was used to generate 200-300 bp fragments. These cDNA fragments were expressed as beta-galactosidase-C3 fusion proteins using the lambda gt11 vector. The fusion proteins were tested by Western-blot analysis for reactivity with monoclonal and polyclonal anti-C3 antibodies, and the location of the epitopes were determined by sequencing the cDNA fragments. Affinity-purified polyclonal anti-C3(D) antibodies specific for denatured C3 reacted strongly with the C3 fusion fragments corresponding to segments of the 40 kDa subunit of C3c (residues 1477-1510) and the C3d fragment (residues 1117-1155 and 1234-1294) of C3. Adsorption of the polyclonal antibodies with a mixture of EAC3b and EAC3bi (degradation fragments of C3 bound to sheep erythrocytes) abolished binding to fusion proteins spanning the C3d region, but not the 40 kDa fragment of C3c. No effect was seen with the corresponding soluble C3 fragments. The monoclonal anti-C3(D) antibodies (mAbs) 7D326.1 and 7D331.1, specific for EAC3b and EAC3bi, bound to a fusion protein corresponding to amino acid residues 1312-1404, whereas mAb 7D9.2, specific for EAC3d, reacted with a fusion protein spanning amino acid residues 1082-1118. mAbs 4SD11.1 and 4SD18.1, which did not bind to any physiological C3 fragment, detected a fusion protein covering residues 1477-1510. In summary, the segments of C3 represented by amino acid residues 1082-1118, 1117-1155, 1234-1294 and 1312-1404 accommodate C3(D) epitopes that are expressed by erythrocyte-bound C3 fragments, but not by the corresponding fluid-phase fragment, whereas the segments spanning residues 973-1026 and 1477-1510 contain C3(D) epitopes that are exposed exclusively in denatured C3 and therefore hidden in physiological fragments of the protein.

Generation of iC3 at the interface between blood and gas

Earlier studies have shown that C3 can be denatured when blood comes in contact with a polystyrene surface. This study was undertaken to see if similar denaturation of C3 occurs at the gas-plasma interface which is found in all kinds of oxygenator used during cardio-pulmonary operations. An in vitro system consisting of gas bubbling through human blood, serum or plasma was used. The generation of C3a, as an indicator of complement activation, and iC3 and iC3 fragments were monitored. Both C3a and iC3/iC3 fragments levels were increased during bubbling. In contrast to the C3a level, no reduction in iC3/iC3 fragments formation was seen in the presence of EDTA, indicating that it was independent of complement activation. The rate of iC3/iC3 fragments generation was unaffected by the composition of the gas (pure oxygen, pure nitrogen or air), suggesting that the denaturation of C3 indeed occurred at the serum-gas interface. C3 and iC3/iC3 fragments were isolated from bubbled EDTA-chelated serum by PEG precipitation and chromatography on FPLC, using a Mono S column and detected by two ELISAs, specific for native C3 and iC3/iC3 fragments. After 240 min approximately 20% of the total amount of C3 consisted of intact iC3 and it was confirmed that this population bound to human erythrocytes.

Neoantigens in complement component C3 as detected by monoclonal antibodies. Mapping of the recognized epitopes by synthetic peptides

The different fragments of the third complement component, C3, generated upon complement activation/inactivation have the ability to bind to several other complement components and receptors as well as to proteins of foreign origin. These multiple reactivities of C3 fragments are associated with a series of conformational changes occurring in the C3 molecule during its degradation. The conformations acquired by the different C3 fragments are also associated with the exposure of neoantigenic epitopes that are specific for (a) particular fragment(s). In order to study these epitopes and thus the conformational changes occurring in C3, monoclonal antibodies (mAbs) recognizing such epitopes were produced in Balb/c mice after immunization with denatured human C3. Two of the three antibodies (7D84.1 and 7D264.6) presented in this study recognized predominantly surface-bound iC3b, and one mAb (7D323.1) recognized both surface-bound and fluid-phase iC3b. Although none of the mAbs recognized any other fluid-phase C3 fragment, all three antibodies detected micro-titre-plate-fixed C3b and iC3b, but not C3c or C3d. In addition to the reaction with human C3, mAb 7D323.1 also bound to micro-titre-plate-fixed rabbit C3. The epitopes recognized by the three mAbs were further localized by using synthetic peptides that were designed on the basis of the differential binding of the mAbs to the C3 fragments. All three antibodies reacted with C3-(924-965)-peptide, which represents the region of C3 between the kallikrein-cleavage site (923-924) and the elastase-cleavage site (965-966). On the basis of the binding of the mAbs to five different overlapping peptides spanning the region between residues 924 and 965 of the human C3 sequence, and the sequence similarity between human C3 and rabbit C3 within this area, the epitopes recognized by these antibodies are mapped. The contribution of the individual amino acid residues in the formation of the epitopes is discussed.

Purification of chicken C3 and a structural and functional characterization

A major plasma protein from chicken, analogous to mammalian complement component C3, was purified by the removal of plasminogen, precipitation with polyethyleneglycol, and ion-exchange chromatography. Purification was guided by a rabbit antiserum specific to chicken C3. The yield of native C3 was 27%, and purity and functional activity was assessed by SDS-PAGE, immunoprecipitation techniques, and the ability of the purified C3 to restore the haemolytic activity of C3-depleted chicken serum. Monoclonal antibodies were raised against purified chicken C3. These antibodies were characterized and used to prepare an immunosorbent column to deplete chicken plasma specifically of C3. Chicken C3 has a mol.wt of 185,000-195,000 and a two-chain structure with an alpha chain (118,000) and beta chain (68,000). Complement activation leads to changes in the electrophoretic mobility of chicken C3 and to a decrease in mol.wt to 144,000 corresponding to the release of a 15,000 C3a and a 34,000 C3d/C3dg fragment. Chicken C3 exists in multiple molecular forms with pI values of 6.4-6.6. A genetic polymorphism of chicken C3 based on electrophoretic mobility has not yet been detected after analysis of more than 500 individuals. The function of chicken C3 is dependent on a reactive thioester because treatment of purified chicken C3 with methylamine causes functional inactivation of C3.

Activation of the classical and alternative pathways of complement by Treponema pallidum subsp. pallidum and Treponema vincentii

Both in vivo and in vitro studies have indicated that complement plays an important role in the syphilitic immune responses. Few quantitative data are available concerning activation of the classical pathway by Treponema pallidum subsp. pallidum, and no information is available on treponemal activation of the alternative pathway. Activation of both pathways was compared by using T. pallidum subsp. pallidum and the nonpathogen T. vincentii. With rabbit and human sources of complement, both organisms rapidly activated the classical pathway, as shown by hemolysis of sensitized sheep erythrocytes and by the generation of soluble C4a. With human sources of complement, both organisms also activated the alternative pathway, as shown by hemolysis of rabbit erythrocytes and by the generation of soluble C3a in the presence of magnesium ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). During incubation, organisms remained actively mobile and did not lyse, indicating that activation was a function of complement reactivity with the intact outer treponemal surface. In addition, freshly harvested T. pallidum subsp. pallidum immediately activated both pathways of complement; preincubation of organisms did not enhance complement reactivity. T. vincentii was a more potent activator of this pathway. T. pallidum subsp. pallidum contained almost four times as much surface sialic acid as T. vincentii did. When sialic acid was enzymatically removed from T. pallidum subsp. pallidum, enhanced activation of the alternative pathway was detected. It is proposed that T. pallidum subsp. pallidum retards complement-mediated damage by the alternative pathway through surface-associated sialic acid. This may be an important virulence determinant that enables these organisms to readily disseminate through the bloodstream to infect other tissues.

Domain structure of human alpha 2-macroglobulin. Characterization of a receptor-binding domain obtained by digestion with papain

Digestion of methylamine-treated alpha 2-macroglobulin (alpha 2M X MA) with catalytic amounts of papain at pH 4.5 has been investigated. Cleavage of Lys(1313)-Glu resulted in two major products, which could be separated by gel chromatography: a large disulfide bridged fragment set nearly the size of intact alpha 2M X MA, and an 18 kDa fragment, constituting the carboxy-terminal domain of alpha 2M. This domain contained the receptor recognition site, exposed as a result of cleavage of the internal beta-cysteinyl-gamma-glutamyl thiol esters in alpha 2M. Compared with alpha 2M-trypsin complex the apparent affinity for binding to rat hepatocyte receptors was 0.1 and 2% at 4 and 37 degrees C, respectively. The receptor-binding domain presumably forms a compact globular beta-barrel-type structure, stable at pH 2.5-9.0. Chemical modification experiments suggest that receptor binding is contributed by a determinant formed by the precise folding of the polypeptide chain.

Detection of disulphide bonds and localization of interchain linkages in the third (C3) and the fourth (C4) components of human complement

Disulphide bonds contribute significantly to the maintenance of structural/functional integrity of many proteins. Therefore it was of interest to study the distribution and the effect of disulphides on conformation of complement components C3 and C4. These proteins are precursors of several fragments with various binding sites and distinct physiological functions. The constituents of C3c (beta, alpha 27, alpha 43) and those of C4c (beta, alpha 27, alpha 16, gamma) were investigated, since other fragments of C3 or C4 do not participate in interchain linkages. Inter-and intra-chain disulphide bonds in C3c and C4c were localized by using a modification of conventional SDS (sodium dodecyl sulphate)/polyacrylamide-gel electrophoresis such that the change in mobility of disulphide-bond-containing proteins can be detected throughout the transition from a non-reduced to a fully reduced state. Several forms of the alpha 43 fragment from C3, and of the gamma-chain of C4, with different mobilities can exist, depending on the number of intra-chain disulphide bonds reduced. The intermediates (heterodimers) generated by a partial reduction of C3c or C4c were characterized by two-dimensional SDS/polyacrylamide-gel electrophoresis performed in the absence, then in the presence, of beta-mercaptoethanol. The inter-chain linkages in C3c were determined to be beta-alpha 27 and alpha 27- alpha 43, thus indicating the presence of only one interchain bond in C3. The two interchain bonds in C4c are beta-alpha 27 and alpha 16-gamma. The third interchain bond in C4 (alpha 27-gamma, tentative) remains to be determined.