Antigens of complement factor C3 involved in the interactions with factors I and H

A Robust Method to Store Complement C3 With Superior Ability to Maintain the Native Structure and Function of the Protein

Complement components have a reputation to be very labile. One of the reasons for this is the spontaneous hydrolysis of the internal thioester that is found in both C3 and C4 (but not in C5). Despite the fact that ≈20,000 papers have been published on human C3 there is still no reliable method to store the protein without generating C3(H2O), a fact that may have affected studies of the conformation and function of C3, including recent studies on intracellular C3(H2O). The aim of this work was to define the conditions for storage of native C3 and to introduce a robust method that makes C3 almost resistant to the generation of C3(H2O). Here, we precipitated native C3 at the isoelectric point in low ionic strength buffer before freezing the protein at -80°C. The formation of C3(H2O) was determined using cation exchange chromatography and the hemolytic activity of the different C3 preparations was determined using a hemolytic assay for the classical pathway. We show that freezing native C3 in the precipitated form is the best method to avoid loss of function and generation of C3(H2O). By contrast, the most efficient way to consistently generate C3(H2O) was to incubate native C3 in a buffer at pH 11.0. We conclude that we have defined the optimal storage conditions for storing and maintaining the function of native C3 without generating C3(H2O) and also the conditions for consistently generating C3(H2O).

C3 adsorbed to a polymer surface can form an initiating alternative pathway convertase

Contact between blood and a biomaterial surface induces an immediate complement-mediated inflammatory response. Under these conditions, the alternative pathway of complement is often initiated and amplified on the biomaterial surface. Adsorption of a protein such as C3 to a polymer surface induces conformational changes in the protein. Based on the expression on adsorbed C3 of conformational neoepitopes specific for bound C3 fragments, we have hypothesized that adsorbed C3 is able to bind factor B and form a functional C3,Bb convertase. Using a quartz crystal microbalance to monitor binding of proteins to a polymer surface, we have demonstrated that a functional C3-containing alternative pathway convertase can be formed, in particular, in the presence of properdin. These data indicate that adsorption of C3 induces conformational changes that turn C3 into a C3b-like molecule that is able to participate in the functioning of the alternative convertase, and they suggest a new mechanism for complement activation on a biomaterial surface.

Evidence for iC3 generation during cardiopulmonary bypass as the result of blood-gas interaction

Earlier we have shown that iC3 is generated at the blood-gas interface in vitro and that the generation of this molecule is independent of complement activation and the composition of the gas. In order to investigate whether iC3 is also generated during cardiopulmonary bypass where blood comes into contact with oxygen bubbles, two bubble oxygenators were incubated at 37 degrees C with human heparinized blood. A continuous increase in the level of iC3 was shown in the oxygen-perfused bubble oxygenator (up to 100 nmol/l after 180 min) in contrast to the unbubbled control. Similarly, in plasma drawn from patients undergoing cardiopulmonary bypass using either bubble or membrane oxygenators, the levels of iC3 were shown to increase continuously during the operation. Furthermore, this form of C3 was found to be susceptible to cleavage by factor I. The formation of iC3 at the blood-gas interface in vivo could be a mechanism by which gas bubbles induce clinical manifestations associated with complement activation, e.g. during cardiopulmonary bypass, adult respiratory distress syndrome and decompression sickness.

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.

X-ray scattering study of hagfish protease inhibitor, a protein structurally related to complement and alpha 2-macroglobulin

A protease inhibitor from hagfish blood plasma, homologous to human alpha 2-macroglobulin, has been studied in solution using small-angle X-ray scattering; the radius of gyration, R, was found to be 7.0 nm, the molecular weight 340,000 +/- 20,000, and the largest distance within the molecule, Dmax, 22 nm. When the inhibitor reacts with chymotrypsin, its 1:1 chymotrypsin complex is found to be more compact than the native molecule, R = 6.1 nm. A very similar conformational change is observed after the protein is reacted with methylamine. The data are consistent with models consisting of two equal elliptic cylinders with the same size as the one used as a model for the complement proteins C3 and C4 [cf. Osterberg et al. (1989) Eur. J. Biochem. 183, 507-511]. In the model for the native protein, these cylinders are arranged in an extended form, and in the one for the methylamine derivative (or chymotrypsin complex), they are closer together so that the projection of their elliptic surfaces forms an angle of about 70 degrees. These models for the hagfish protease inhibitor were expanded to models for the twice as large human alpha 2-macroglobulin using symmetry operations, and the resulting alpha 2-macroglobulin models were found to agree with those emerged from earlier studies involving electron microscopy and X-ray scattering methods.

Factor H

While the mouse and human H proteins are structurally and functionally similar, they differ in their genetics. Whereas there is no evidence in humans for more than one gene; in mice the H locus is complex. Based on cDNA sequence and hybridization analysis of genomic cosmid clones, there are at least three distinct genes, all highly related to one another. The consensus repeating unit that comprises this molecule has obviously been duplicated numerous times, since it is present in many other molecules. Thus, it is not surprising to discover that there are several genes related to H in the mouse. A similar case has been described for two other members of this family. In humans, CR1 cDNA hybridizes to two distinct genomic clusters in the CR1 locus (Wong et al. 1989), and in mice, mCRY hybridizes to two regions in the genome, one on chromosome 1 and another on chromosome 8 (Aegerter-Shaw et al. 1987). It will be of interest to see if any other members of this family display as complex a genetic locus as murine H.

Large intramolecular movement in human complement protein C3 induced by methylamine. A small-angle scattering study using monoclonal antibodies as markers

The reaction of methylamine with complement protein C3, which involves cleavage of a labile thiol ester bond, yields a large intramolecular rearrangement. This is shown by small-angle neutron and X-ray scattering using a Fab antibody as a marker. For the C3(Fab) 1:1 complex, the methylamine reaction yields an increase in the radius of gyration, R, from 4.6 nm to 6.0 nm. In the absence of Fab the corresponding R values increase from 4.4 nm to 5.1 nm. It is estimated that the methylamine-induced increase in R may correspond to a movement of the epitope to a position 5 nm away from the centre of gravity of the C3 molecule. In agreement with this finding, the maximum distance within the C3(Fab) complex increases from 16 nm to 22 nm as a result of the methylamine reaction. In order to explain this conformational change, it is tentatively suggested that the methylamine-induced cleavage of the C3 thiol ester bond leads to a domain rotation within the C3 molecule. In agreement with this idea, the data is consistent with a model which enables a globular domain within the molecule to rotate without redistributing the molecular mass more than that corresponding to the radii of gyration observed.

Distinctive expression of neoantigenic C3(D) epitopes on bound C3 following activation and binding to different target surfaces in normal and pathological human sera

Binding of C3 to sheep erythrocytes in a serum-free milieu (EAC14oxy2, EAC142) has previously been shown to mimic the antigenic change that occurs upon denaturation of C3 in sodium dodecyl sulphate (SDS), whereby neoantigenic C3(D) epitopes are exposed. The present paper deals with C3 bound to various target surfaces which are known to modulate the functional properties of C3 in different ways. Bound C3 fragments on serum-treated human aggregated gammaglobulin, zymosan, rabbit and sheep erythrocytes, and on circulating immune complexes isolated from sera of patients with rheumatoid arthritis and systemic lupus erythematosus, were shown to be mainly in the iC3b form. By RIAs, employing polyclonal antibodies, the range of C3(D) antigenic epitopes of 125I-labelled SDS denatured C3 expressed by the particle-bound iC3b was monitored. The physiologically bound iC3b on all tested particles expressed wide range of C3(D) epitopes and each type of particle-bound C3 exposed its individual range. By competition ELISA specific C3(D) alpha epitopes were monitored, employing monoclonal antibodies. A distinct difference in the expression of these epitopes was observed in iC3b bound to various test particles in the presence of normal serum and in iC3b present on circulating immune complexes from pathological sera. Considering that the neoantigenic C3(D) epitopes have been shown to be associated with different functions of C3, the distinctive antigenic expression of each type of serum-treated particle might reflect different functional forms of the protein.

Production of mouse monoclonal antibodies that detect distinct neoantigenic epitopes on bound C3b and iC3b but not on the corresponding soluble fragments

Polyclonal antibodies raised in rabbits against sodium dodecyl sulphate (SDS)-denatured and reduced human complement factor C3 have in recent studies been shown to lack any reactivity towards native C3 but to react with antigens distinctly expressed by SDS-denatured C3 (C3(D) antigens). These antigens are also neoantigens specific for physiologically bound C3 and appear to be involved in the interaction of C3 with other complement components. The present investigation deals with production of mouse monoclonal antibodies against C3(D) antigens. To accomplish this two different immunization and screening procedures employing C3 preparations of known C3(D) expression were tested. From each group 14 clones were randomly selected and the reactivity of these and of a control group of 14 additional monoclonal anti-human C3 antibody preparations raised against native soluble C3 and C3b, was investigated in ELISA and immunoblotting. The procedure which employed denatured reduced C3 as both immunogen as well as screening antigen was shown to be superior for obtaining anti-C3(D) antibodies. Altogether 16 clones producing antibodies against C3(D) antigens were found. All of them bound to the C3 alpha-chain, 14 to C3c and one to C3d, and eight monoclonal antibodies specific for neoantigens of C3(D) type on bound C3b and/or iC3b were obtained. The majority of these detected neoantigenic epitopes in the 25,000 N-terminal fragment of the C3 alpha-chain specifically exposed by bound iC3b, but one monoclonal antibody was specific for the 36,000 C-terminal alpha-chain fragment and for both bound C3b and iC3b.