Identification and alignment of a thiol ester site in the third component of guinea pig complement

Structure at 1.44 A resolution of an N-terminally truncated form of the rat serum complement C3d fragment

Complement component C3 plays a key role in the complement-mediated immune defence, and occupies a central position within the complement cascade system. One of its degradation products, C3dg, was purified from rat serum and crystallised in two different crystal forms as N-terminally truncated fragment. Despite the truncation and the lack of a significant portion of the N-terminus as compared to C3d, the structure of the fragment is highly similar to that of recombinant human C3d (Nagar et al., Science 280 (1998) 1277-1281). Structural details of the reactive site have been obtained, suggesting a possible mode of thioester bond formation between Cys-1010 and Gln-1013 and thioester bond cleavage in the transacylation reaction involving His-1126. The truncation at the N-terminus of C3d leads to the exposure of a surface of the molecule that favours dimerisation, so that in both crystal forms, the fragment is present as a dimer, with monomers related by a two-fold axis.

Rabbit complement lyses tumor cells without massive C3 deposition

These experiments were performed to determine why rabbit complement lyses tumor cells very efficiently, while not having particularly strong activity in hemolytic assays or in any other complement assay. The target cells used were human tumor cells coated with three different mouse IgG(2a) monoclonal antibodies, and complement from 5 mammalian species were tested. In antibody titration experiments, rabbit complement was found to lyse target cells at a relatively low antibody concentration, insufficient to allow lysis by complement of other species. Since this result was still observed after absorption of rabbit serum with target cells, the potency of rabbit complement cannot be attributed to the presence of natural antibodies. We then assayed C3 deposition on target cells, using two types of (125)I-labeled anti-C3 Abs to measure C3 deposition: goat antibodies specific for C3 of the human, guinea pig, rabbit, rat or mouse, and chicken antibodies to human C3 which cross-react with C3 of other mammals. Unexpectedly, complement of the human, rat, guinea pig, and BUB mouse deposited large amounts of C3 on the surface of target cells, while rabbit complement deposited 100-1,000 fold less. We discuss the possible reasons that C3 deposition does not correlate with cytotoxicity, and may indeed be inversely related. These data indicate that there is a fundamental difference in the complement cascade between rabbits and the other species tested. The potent lytic activity of rabbit complement is likely to be related to this difference, although the mechanism is not yet understood.

The reaction mechanism of the internal thioester in the human complement component C4

A key step in the elimination of pathogens from the body is the covalent binding of complement proteins C3 and C4 to their surfaces. Proteolytic activation of these proteins results in a conformational change, and an internal thioester is exposed which reacts with amino or hydroxyl groups on the target surface to form amide or ester bonds, or is hydrolysed. We report here that the binding of the human C4A isotype involves a direct reaction between amino-nucleophiles and the thioester. A two-step mechanism is used by the C4B isotype. The histidine at position 1,106(aspartic acid in C4A) first attacks the thioester to form an acyl-imidazole intermediate. The released thiol then acts as a base to catalyse the transfer of the acyl group to amino- and hydroxyl-nucleophiles, including water.

Purification and characterization of porcine C3. Studies of the biologically active protein and its split products

Separation techniques for obtaining pure and biologically active swine C3 have been improved in this study. Using these procedures and through the further characterization of porcine C3, the possibilities for developing more specific techniques for the analysis of the complement system in swine have been improved. Plasma was initially treated with protease inhibitors, polyethylene glycol (PEG)-fractionation, plasminogen-depletion and a rapid chromatographic desalting step. The essential fractionation was carried out by DEAE-Sephacel chromatography. Contaminants were removed by size-exclusion (Sepharose CL-6B)- and hydroxylapatite-chromatography. The final recovery reached 56% with 73% retaining specific hemolytic activity. The amino acid composition (98.33%), the functional compatibility and the secondary structure of fragments and intact protein indicate a high degree of homology with human C3. In contrast with the findings of earlier studies was the considerable immunologic cross-reactivity observed with human C3, and the size difference between the human and the swine C3-beta subunit, which was found to be 10 kDa lighter than the human analogue. The finding that the swine C3b/iC3b/C3c fragments do not separate from C3 by agarose electrophoresis, unlike the human analogues, demonstrated that this commonly used simple parameter for the detection of complement activation cannot be used in the porcine model.

A rapid FPLC method for purification of the third component of human and guinea pig complement

A method is described for the purification of human and guinea pig C3 from small amounts of serum. This procedure requires only two steps--polyethylene glycol (PEG) precipitation and fast protein liquid chromatography (FPLC) Mono Q HR 10/10 ion exchange chromatography. The protocol takes less than two hours to complete and yields 4-6 mg of purified C3. Similar results, in terms of antigenic and functional recovery, were obtained for both human and guinea pig components. About 67% of C3 antigen was recovered from eluted fractions with fully preserved specific activity. Isolated C3 was over 95% pure as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography; this level of purity was confirmed by the absence of any observable contamination as assessed by immunoelectrophoresis using high titer anti-whole human serum. This method allows rapid and reproducible purification of fully active human or guinea pig C3 on a daily basis.

Regulatory system of guinea-pig complement C3b: tests for compatibility of guinea-pig factors H and I with human factors

Two proteins that are involved in cleavage of methylamine-treated C3 of guinea-pig origin (C3(MA)gp) have been isolated from guinea-pig serum. One of them functioned as a cofactor of human factor I (Ihu) for cleavage of C3(MA)gp and its molecular size was 150 kDa. The other was functionally pure and able to cleave C3(MA)gp together with human factor H (Hhu). They appear to be analogous to human factors H and I in the guinea-pig and will be referred to as Hgp and Igp. Methylamine-treated human C3 [C3(MA)hu] was not a compatible substrate for Hgp or Igp: little cleavage of C3(MA)hu was observed if human factor H (Hhu) or I was substituted with the guinea-pig counterpart. C3(MA)gp, on the other hand, served as a substrate, though less efficiently, for Hhu and Ihu. Human C4b-binding protein (C4bp) and membrane cofactor protein (MCP) as well as Hhu could participate in cleavage of C3(MA)gp by Igp or Ihu. In these assays, C3(MA)gp was degraded again less efficiently than C3(MA)hu. Interestingly, human C3b/C4b receptor (CR1) mediated factor I-dependent cleavage of C3(MA)hu and C3(MA)gp to a similar extent regardless the sources of factor I. These results suggest that factor I-dependent C3b regulatory system is species-specific except in the case of CR1, which may function as a cofactor irrespective of species.

Regulatory system of guinea-pig complement C3b: two factor I-cofactor proteins on guinea-pig peritoneal granulocytes

Complement factor I is a plasma protease serving for proteolytic inactivation of C3b together with its cofactor. We have identified two factor I-cofactor activities in solubilized extracts of guinea-pig peritoneal granulocytes using guinea-pig factor I (Igp) and fluorescent-labeled methylamine-treated guinea-pig C3 (f-C3(MA)gp). One of these eluted from a chromatofocusing column between pH 7.6-7.1, and the other at about pH 5.7. These two cofactor fractions both interacted with Igp and, to a lesser degree, with human factor I (Ihu) on C3(MA)gp cleaving it into an inactive C3bi analogue, but did not cleave methylamine-treated human C3 (C3(MA)hu) together with Igp or Ihu. These factors are therefore species specific. The neutral and acidic fractions with cofactor activity contained C3(MA)gp-binding proteins with a doublet of 55 kDa and 42 kDa, and a singlet of 160 kDa, respectively, on SDS-PAGE. These proteins may be membrane cofactor protein (MCP) and C3b/C4b receptor (CR1) of guinea-pigs.

Isolation and characterization of the third complement component of axolotl (Ambystoma mexicanum)

1. Using a monoclonal anti-human C3 antibody and a polyclonal anti-cobra venom factor antibody as probes, a protein homologous to the mammalian third complement component (C3) was purified from axolotl plasma and found to be axolotl C3. 2. Axolotl C3 consists of two polypeptide chains (Mr = 110,000 and 73,000) linked by disulfide bonds. An internal thiolester bond in the alpha chain was identified by the incorporation of [14C]methylamine and NH2-terminal sequence from the C3d fragment of C3. 3. Digestion of C3 by trypsin resulted in the cleavage of both the alpha and beta chains, generating fragments with a cleavage pattern similar to that of human C3. 4. The amino acid composition of axolotl C3 and the amino acid sequences of the thiolester site (and the surrounding amino acids), the cleavage site for the C3-convertase, and one of the factor I cleavage sites are similar to C3 from other vertebrates. 5. In contrast to human C3, which has concanavalin A binding carbohydrates on both the alpha and beta chains, only the beta chain of axolotl C3 contains such carbohydrates.

Lipopolysaccharides as complement inhibitors by complex formation with the purified third complement component (C3)

Lipopolysaccharides (LPS) from different bacteria in smooth or rough form (Y. enterocolitica, Y. pseudotuberculosis, E. coli, S. typhimurium, S. marcescens) strongly inhibited hemolytic C3 in incubation mixtures with purified C3. LPS from a core deficient mutant was still reactive, whereas lipid A no longer affected C3 activity. The physical state of LPS was critical for its effect on C3. Strand-like LPS structures formed by Ca++-induced aggregation of solubilized LPS, as shown by electron microscopy, demonstrated the highest reactivity with C3. Inhibition of hemolytic C3 was found to be due to complex formation between LPS and C3 by a hydrophobic reaction. The binding capacity of 1 microgram LPS-R and LPS-S was as high as 125 ng C3 and 56 ng C3, respectively. The C3b fragment required different reaction conditions for maximal binding. The strong binding capacity of LPS for the complement component C3 raises the possibility that LPS act as inhibitors of complement by interruption of the reaction cascade at local infectious sites with gram-negative bacteria.

Substances that can trigger activation of the alternative pathway of complement have anti-melanoma activity in mice

Intraperitoneal (i.p.) injection of substances that can ignite the alternative pathway of complement, namely isolated human C3b or C3(H2O), guinea-pig C3(H2O) or cobra venom factor, or conventionally prepared zymosan, will reproducibly and very significantly increase the mean survival time of C57BL mice previously inoculated i.p. with melanoma cells. The effect is greater at higher doses and earlier post-inoculation (p.i.) administration, but the substances are active at low doses (30-100 micrograms/mouse) if given early enough. It is likely that C3b or C3(H2O) was the previously unidentified anti-tumour factor activated in serum by S. aureus treatment or serum fractionation and described elsewhere. Activation of the alternative pathway of complement appears to have potential interest for cancer therapy.