Isolation and characterization of the third and fifth components of rabbit complement

The complement system of the goat: haemolytic assays and isolation of major proteins

BACKGROUND

The aim of the present study was to develop a haemolytic assay for the study of the complement system in dairy goats (Capra aegagrus hircus) and to characterize the major goat complement system proteins.

RESULTS

The commonly used sheep erythrocyte sensitized with rabbit antibodies were not sensitive to lysis by goat serum, but the combination of human red blood cells (RBC) plus rabbit antibodies was the best option found for goat complement assay. A buffer based on HEPES instead of the classical veronal (barbitone) was developed. Three proteins were isolated: factor H, C1q and C3 and these were compared with the corresponding human proteins. A novel affinity chromatography technique was developed for isolation of factor H.

CONCLUSIONS

Human RBC plus rabbit antibodies were a suitable option for haemolytic assays. The isolated proteins are similar to the human counterparts.

Classical pathway evaluation

This unit describes several assay methods that can be used to determine the functional status of the classical pathway of complement and to quantitate its component proteins. The classical pathway includes C1qrs, C2, C4, C3, C5, C6, C7, C8, and C9, listed in the order in which they interact. Two CH(50) assay procedures are presented that test total classical pathway function; one is carried out in test tubes, whereas the alternate protocol describes a variation that is carried out in 96-well microtiter plates. Three support protocols describe preparing serum and erythrocyte-antibody complexes (EA) for use in CH(50) assays,and titrating of hemolysin for use in EA preparation. As an alternative to functional assays, radial immunodiffusion (RID) methods are also presented. These can be used to measure the concentrations of most circulating complement proteins and to evaluate the functional status of C1-esterase inhibitor. A fourth support protocol provides a method to determine antibody concentrations for RID assays.

Monoclonal antibodies to rabbit alpha-2-macroglobulin and their use in a sensitive ELISA assay

alpha-2-macroglobulin (alpha-2-M), a plasma proteinase inhibitor, plays an important role in the pathogenesis of lung inflammation. The purpose of this study was to develop a sensitive ELISA assay for rabbit alpha-2-M to allow us to define the role of this protein in a rabbit model of lung inflammation/injury. Therefore, we developed hybridomas which secrete monoclonal antibodies (mAbs) against rabbit alpha-2-M. From the antibodies produced, two (5B6 and 3C5) were selected. Both of them were of the IgG1 subclass. 5B6 reacted with native alpha-2-M as well as with the "fast" form of alpha-2-M (native alpha-2-M or the "slow" form is converted to the "fast" form by reaction with a proteinase). On the other hand, 3C5 reacted only with "fast" form of alpha-2-M. With these antibodies, we developed two ELISA assays which were used to determine the concentration of alpha-2-M in the lung fluids from rabbits with lung injury.

Activation of complement by tissue plasminogen activator, but not acute cerebral ischemia, in a rabbit model of thromboembolic stroke

Although complement activation is associated with tissue injury during inflammatory and ischemic states, complement activation in states of acute cerebral ischemia before and after administration of tissue plasminogen activator (TPA) has not yet been examined and is the focus of this investigation. Twenty-four New Zealand White rabbits weighing 3 to 3.5 kg were used for this study. Of these, 20 were subjected to intracranial autologous clot embolization via the internal carotid artery. Three hours postembolization, rabbits received an intravenous infusion of TPA (6.3 mg/kg, 20% bolus with the remainder infused over a 2-hour interval; 12 animals) or vehicle (eight animals). All animals were observed for a total of 7 or 8 hours postembolization. These two groups were compared to a cohort undergoing sham operation with subsequent TPA infusion (four animals). Plasma samples to quantify complement component C5 hemolytic activity (C5H5O) were obtained at the following time points: 30 minutes before and after clot embolization; 1 hour before and 1 hour after the initiation of therapy with TPA or vehicle and at the completion of the protocol; 7 to 8 hours after clot embolization. The C5 activation was not detected as the result of acute cerebral ischemia. However, animals receiving TPA with or without concomitant clot embolization exhibited C5 activation as assessed by a reduction in C5 hemolytic function, both 1 hour after initiation of TPA infusion (78.7 +/- 10.3% and 77.5 +/- 9.9% of baseline value, respectively; mean +/- standard error of the mean [SEM]) and at the end of the protocol, 2 hours after the completion of the TPA infusion (72.5 +/- 8.8% and 53.3 +/- 8.1%, respectively; mean +/- SEM, p < 0.05, each group). This study supports the conclusion that TPA, but not acute cerebral ischemia, may activate the complement cascade in this rabbit model of thromboembolic stroke.

Purification and characterization of equine complement factor C3

A rapid method for purifying equine C3 which yields milligram quantities of pure C3 is described. Protein from equine plasma was selectively precipitated with polyethylene glycol, and the C3 was purified by anionic and cationic exchange HPLC. The yield from this procedure was 12%. The purified C3 was composed of an alpha chain (118 kD) and a beta chain (68 kD) linked by at least one disulfide bond, and it had an isoelectric point of 4.7. Amino acid analysis indicated a strong conservation of amino acid usage between equine and human C3. The N-terminal sequences of the alpha and beta chains were homologous to human, mouse, and rat C3, and activation of C3 produced breakdown products similar in molecular weight to C3b and iC3b of other species. Equine C3 appeared to be functionally dependent upon a reactive thiolester as treatment of fresh equine serum with methylamine abrogated its hemolytic activity.

Isolation of the fifth component of the bovine complement system

Bovine C5 has been isolated from fresh bovine serum by a five-step procedure: polyethylene glycol precipitation, sequential ion-exchange chromatography on DEAE-Sephacel and CM-Sephadex, hydroxylapatite chromatography, and affinity chromatography. The purified C5 was a protein of apparent molecular weight 202,000 +/- 9,000 composed of two chains: an alpha-chain of molecular weight 127,000 +/- 5,000 and a beta-chain of molecular weight 74,000 +/- 2,000. The alpha-chain was cleaved by Sepharose-CVF.Bb (a cobra venom factor (CVF)-induced C3/C5 alternative pathway convertase) in the absence of any C3 or C3b. The monocarboxylic acid form of K-76, a sesquiterpene compound isolated from the culture filtrates of Stachybotris complementi, inhibited the alternative pathway of bovine serum, and the inhibited hemolytic activity was restored, in a dose dependent manner, by bovine C5. This provided the basis for a C5 functional assay throughout the purification procedure. The purified C5 showed species specificity and was functionally distinct from bovine C3.

Measurement of complement activation in rabbit plasma or serum using monoclonal antibodies against C5a

Eight murine monoclonal antibodies (MoAb) raised against the major zymosan-induced chemotactic factor in rabbit serum were found to neutralize the chemotactic activity induced by lipopolysaccharides (LPS) and antigen-antibody complexes. A 15 kDa antigen was identified in plasma incubated with LPS by immunoblot analysis with MoAb. This is similar to the molecular weight of the major zymosan-induced chemotactic factor. Both the generation of this 15 kDa antigen and chemotactic activity were abrogated in a heat-inactivated plasma. A cross-reaction to human C5a was demonstrated for three MoAb (5H8B9, 4B1C11, and 2A5E3) in an indirect enzyme-linked immunosorbent assay (ELISA) of partially purified C5a and by the isolation of zymosan-induced chemotactic activity by affinity chromatography. MoAb 5H8B9 and 4B1C11 were able to neutralize the chemotactic activity in human zymosan-activated serum. MoAb 2A5E3 was able to bind 125I-labelled human C5a des Arg. We conclude that these MoAb are directed against rabbit C5a. MoAb 5B2C5 and 2B1A2, which are directed to different antigenic binding sites on C5a, may be applied in a sandwich ELISA for the detection and quantification of C5a des Arg in rabbit serum or plasma. The sandwich ELISA can be performed directly on serum or plasma samples without having to precipitate native C5. Complement activation is demonstrated by measuring the increased generation of C5a des Arg in rabbit plasma or serum activated with LPS, zymosan, antigen-antibody complexes, or cobra venom factor.

Preparation and characterization of monoclonal antibodies against the fifth component of rabbit complement (C5)

By immunizing mice genetically deficient in C5 we were able to obtain a group of monoclonal antibodies to rabbit C5 that cross-react with C5 from a wide variety of mammalian sera, including mouse. The specificity of the monoclonal antibodies was against native C5 and C5b but not C5a. The antibodies strongly inhibit the expression of C5 hemolytic activity. We suggest that these monoclonal antibodies will be useful for studying C5 as well as providing a way to selectively deplete C5 from plasma in vitro or in vivo.

Isolation of decay-accelerating factor (DAF) from rabbit erythrocyte membranes

A regulatory protein purified from rabbit erythrocyte stroma had decay-accelerating activity to C5 convertases on hemolytic intermediate cells (EAC1-3b, P). Its molecular weight was estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under non-reducing conditions to be 66,000, slightly smaller than that of human decay-accelerating factor (DAF). The amino acid composition of the protein resembled that of human DAF. The protein was incorporated into sheep erythrocytes. The incorporation and the decay-accelerating activity were inhibited by low density lipoprotein. It did not bind to rabbit C3-Sepharose in a column under conditions in which human CR1 bound, but its interaction with rabbit C3 was evident from the retardation of its elution from the column. These results indicate that the protein purified from rabbit erythrocyte stroma was DAF.

Differences and similarities between human and rabbit neutrophil granulocyte responses in vitro: the effects of zymosan-activated plasma, phorbol myristate acetate and n-formyl-methionyl-leucyl-phenylalanine

The amount of reactive oxygen intermediates (ROI) generated by activated polymorphonuclear neutrophils (PMN), as well as the closeness of contact between PMN and vessel wall, may determine whether PMN activators will induce the adult respiratory distress syndrome. We examined the ROI-generating and aggregating effects of zymosan activated plasma (ZAP), phorbol myristate acetate (PMA) and n-formyl-methionyl-leucyl-phenylalanine (FMLP), on isolated human and rabbit PMN. PMA, after a short lag phase, induced a large and long-lasting increase in ROI generation. The initial peak response was higher and more rapid in human than in rabbit cells. The reaction to FMLP occurred almost instantaneously, but was much weaker than that to PMA, and ROI generation returned to near baseline in less than 10 min. No species difference was seen. ZAP caused an FMLP-like ROI response in human cells, whereas no response was observed in rabbit PMN. PMN aggregation was induced by all three activators, most markedly by PMA. No species difference was detected for PMA; FMLP gave a stronger aggregation of rabbit than of human PMN, however, while the opposite was true for ZAP. In conclusion, ZAP was a potent stimulus for PMN aggregation, but had modest (or no) effects on the production of ROI. Marked differences between human and rabbit PMN responses were observed.

A simple hemolytic assay for bovine complement component C3

A simple, one-step, alternative pathway (AP) hemolytic assay for bovine C3 has been developed. Methylamine was used to prepare a bovine serum reagent, R3, functionally depleted of C3. The addition of purified bovine C3 to the R3 reconstituted, in a dose-dependent manner, the hemolytic activity for unsensitized heterologous erythrocytes. The assay was used to determine relative levels of C3 in different bovine serum samples. Human C3 and bovine C3 were interchangeable in the assay. Reconstitution of bovine and human R3 reagents with homologous or heterologous C3, in the presence of different species of erythrocytes, provided evidence that cell surface regulation of the homologous hemolytic AP may not be limited to the assembly and activity of the C3 convertase. The AP assay was more sensitive and less complex to perform than a standard classical pathway assay for bovine C3.

Nucleotide sequence of cDNA and derived amino acid sequence of rabbit complement component C3 alpha-chain

The nucleotide sequence coding for 726 amino acid residues of the alpha-chain of rabbit C3 was determined from a cDNA clone. Subfragments of the cDNA produced by restriction endonucleases were inserted into the bacteriophage M13 and sequenced using the dideoxynucleotide technique. The derived amino acid sequence was compared with those of human and mouse C3, which have been previously reported [by De Brujn, M.H.L. and Fey, G.H. (1985) Proc. Natl. Acad. sci. USA 82, 708, and Westel, R.A. et al. (1984) J. Biol. chem. 259, 13857, respectively]. There was 79% or 78% homology in amino acid sequence between rabbit and human or mouse C3, respectively. All of the cysteinyl residues were conserved among the three molecules, and the sequence around the thioester site was also highly conserved. Several regions having low homology were found: one of them was the small fragment released by factor I cleavage.

A simplified method for the preparation of EAC14 intermediate cells using human serum treated with methylprednisolone

The inhibitory effect of methylprednisolone 21-succinate ester (MPS) on the activities of complement components in human serum was studied by incubating human serum with various concentrations of MPS at 37 degrees C for 30 min and then measuring the residual activity of each component in human serum. The formation of EAC1 and EAC14 by C1 and C4 respectively, were only weakly inhibited by MPS at a final concentration of 10 mg/ml. In contrast, the same concentration of MPS completely inhibited the capacity of C2, C3, C5 and C6-9 to induce respective succeeding intermediates. On the basis of these findings a simplified method was devised for the preparation of EAC14 intermediates using human serum pretreated with MPS.

Isolation and characterization of the third component of bovine complement

C3 was obtained from bovine serum by polyethylene glycol precipitation and chromatography on DEAE-Sephadex A-50, CM-Sephadex A-50 and Sephacryl S-200. The protein has a molecular weight of 183,000 (alpha-chain 114,000 and beta-chain 69,000). A CVF-induced bovine C3 convertase (Sepharose-CVF.Bb) cleaved C3 into C3a (11,000) and C3b (172,000) as shown by SDS-polyacrylamide gel electrophoresis. Isoelectricfocusing of C3 demonstrated at least three electrophoretic variants with pI 6.55-6.85. The isolated protein promoted the formation and action of a C3 convertase in the presence of purified bovine factors B and D. A monospecific antiserum prepared in rabbits failed to cross react with human C3 or CVF. C3c was identified as a contaminant during the isolation of C3.

Inflammation and mediators of lung injury

This article covers the major pathways involved in acute inflammation in mammals with a particular emphasis on their relevance to the bovine species. It focuses on the potential and proven contributions of these systems to pulmonary defense mechanisms and lung pathology. The article also points out what is known and where gaps in our information exist as well as promising areas for research in the coming years.

Additive effect of intravascular complement activation and brief episodes of hypoxia in producing increased permeability in the rabbit lung

Systemic complement activation with intravascularly administered cobra venom factor (CVF) or infusion of either zymosan-activated rabbit plasma or a fifth component of complement fragment with anaphylatoxin activity in the rabbit have not caused significant increases in bronchoalveolar lavage albumin in rabbits (Webster, R. O., G. L. Larsen, B. C. Mitchell, A. J. Goins, and P. M. Henson. 1982. Am. Rev. Respir. Dis. 125:335-340). To assess if another stimulus (hypoxia) acting in concert with complement activation can produce significant lung injury, rabbits were challenged with CVF alone, 10 min of 12% oxygen alone, or CVF followed by a 10-min exposure to 12% oxygen. Either stimulus alone caused no significant changes in arterial oxygen, pulmonary resistance, or dynamic compliance during the 240 min of observation after either stimulus, and neither stimulus alone caused increased albumin accumulation in bronchoalveolar lavage over a 30-min period at the end of the experiment. However, the combination of insults significantly altered arterial oxygen, pulmonary resistance, and dynamic compliance while also increasing albumin and neutrophils recovered by lavage. The increase in lavage albumin did not appear to be due to hemodynamic events in that the pulmonary artery pressure increased acutely after CVF infusion and again during the hypoxic exposure, but was normal when albumin accumulation in the lung was measured. Neutrophil depletion with nitrogen mustard abolished all of these changes induced by CVF plus hypoxia. In addition, meclofenamate pretreatment and infusion during the 4-h study abolished the increases in lavage albumin and neutrophils as well as the increase in pulmonary artery pressure after CVF. Meclofenamate pretreatment did not, however, block accumulation of albumin in the lung (interstitium). We conclude that complement activation, as an isolated event, will not cause a significant increase in lavage albumin in this model. However, combining complement activation with an episode of hypoxia will lead to an increase in lavage albumin that is dependent on the presence of neutrophils for its expression. Meclofenamate treatment will prevent increases in lavage albumin and neutrophils while not preventing albumin accumulation in the lung (interstitium), suggesting a product of the cyclooxygenase pathway of arachidonic acid metabolism is needed to produce movement of albumin and/or neutrophils across the alveolar epithelium in this model.

Identification of C3b as the major serum protein that stimulates prostaglandin and thromboxane synthesis by macrophages

We have previously reported that heterologous, homologous and autologous sera, all stimulated rabbit alveolar macrophages to synthesize prostaglandins (PG). Gel permeation chromatography of serum showed multiple fractions possessing this stimulatory activity, with the major one at 150-160K daltons. In the present study, we have shown that: (a) Fresh rabbit serum stimulated PG release by macrophages. (b) Serum depleted of C3 and C5 lost its stimulatory activity. (c) Trypsinized serum, sera activated by aggregated IgG and zymosan, partially purified C3, C5 and the C3, C5 preparation or purified C3 activated by zymosan, all stimulated PG release by macrophages with the following order of potency: activated C3, C5 = activated C3 = zymosan-activated serum greater than trypsinized serum = aggregated IgG-activated serum greater than partially purified C3, C5 = serum. PGE2 was the predominant PG synthesized by stimulated macrophages. However, thromboxane (TX) production seemed to be more selectively enhanced i.e., increase in TX production was more pronounced than the increase in PGE release. To further identify the active complement component, we blocked the C3b receptor (C3 b R) by preincubating macrophages with anti-C3bR, and showed that subsequent treatment with activated C3 and C5 failed to elicit any PG release. This pretreatment with anti-C3bR had no inhibitory effect on subsequent zymosan stimulation of PG release. Thus we concluded that C3b was the major serum protein that stimulates PG synthesis by macrophages.

Characterization of the third component of pig complement and its utilization in a C3b receptor study

The third component of the pig complement system (C3) was isolated in hemolytically active form and characterized. The C3 component is a beta-globulin with the molecular weight of 191,000 and is composed of 2 non-identical polypeptide chains of Mr 112,000 and 74,000. The isolated C3 can be used for the detection of the C3b receptor on the membranes of heterologous peritoneal macrophages.

Acute local inflammation alters synthesis, distribution, and catabolism of third component of complement (C3) in rabbits

In order to evaluate the basis for changes in plasma concentrations of the third component of complement (C3) during inflammation, we injected purified radiolabeled C3 into normal New Zealand White rabbits and into rabbits with turpentine-induced pleurisy. In the normal animals, C3 was distributed between the intravascular compartment (75%) and the extravascular space (25%), with an exchange rate of 1.8 +/- 0.1% of the plasma pool per hour. The fractional catabolic rate (FCR) was 2.7 +/- 0.3% of the C3 plasma pool per hour, the synthesis rate was 1.0 +/- 0.2 mg C3/kg per h, and the plasma concentration was 1.23 +/- 0.3 mg C3/ml. Rabbits with turpentine-induced inflammation showed a shift of the volume of C3 distribution in favor of the extravascular compartment. In addition, the rate by which 125I-C3 was cleared from the circulation increased by 29% and was related to the appearance of 20% of the C3-bound circulating radioactivity in the affected pleural cavity at the zenith of inflammation. The FCR, calculated by measuring urinary excretion of radiolabel, increased by only 9% and was probably related to the C3 degradation that was observed in the pleural fluid during the early stages of inflammation. The plasma C3 concentration reached a peak at 230% of the baseline concentration, owing to an increase in the rate of synthesis by as much as 480%. The latter increase could be blocked by cycloheximide, an inhibitor of protein synthesis. We conclude that the increase of plasma C3 in the acute phase is due to stimulated synthesis, which is partially offset by a rise in FCR and by a shift of protein to the site of inflammation.

In vivo clearance and tissue distribution of C5a and C5a des arginine complement fragments in rabbits

We have previously shown a marked difference in the inflammatory response to human C5a or C5a des arginine (Arg) instilled in rabbit lungs. These studies raised the question of where C5a and C5a des Arg are processes in vivo and what role neutrophils may play in the tissue distribution of these two mediators. After intravenous injection of purified, biologically active 125I-C5a or 125I-C5a des Arg, adult rabbits were serially bled and then killed at various time intervals. Although greater than 50% of the injected radioactivity was cleared from the circulation within 2 min for both mediators, C5a des Arg persisted in the circulation longer than C5a. C5a instillation caused an acute neutropenia, whereas C5a des Arg caused a less severe and more prolonged neutropenia, preceding a neutrophilic response observed with both mediators. Clearance of the mediators was primarily seen in the highly vascularized organs: the lung, spleen, liver, and kidney. A time-dependent accumulation was seen initially in the lung, followed by the spleen, liver, and kidney. Histologic examination showed a marked increase in the number of neutrophils within the lung and spleen. Depletion of circulating neutrophils by nitrogen mustard pretreatment of rabbits showed no change in the amount of labeled mediator bound in the lung, whereas splenic accumulation was dependent on the presence of neutrophils. These results indicate that C5a and C5a des Arg are rapidly removed from the circulation by specific accumulation in vascularized tissues. Clearance by the lung was not affected by neutrophil depletion, whereas clearance by the spleen was dependent on neutrophils. These experiments further suggest there are neutrophil-dependent and neutrophil-independent mechanisms involved in the removal of C5a and C5a des Arg from the circulation and that binding of C5 fragments in the pulmonary vasculature may precede and then induce neutrophil sequestration.

Chemotactic activity generated from the fifth component of complement by plasma kallikrein of the rabbit

Rabbit plasma kallikrein incubated with rabbit C5 resulted in the generation of chemotactic and secretagogue activity for rabbit neutrophils. This effect on C5 appeared to be due to kallikrein itself and not to a contaminating enzyme, because it could be inhibited by anti-kallikrein IgG or by soybean trypsin inhibitor to the same extent the kinin generation by the same kallikrein preparation was inhibited by these agents. The chemotactic response was consistent with the generation of a C5a-like peptide from C5 because the effect could be partially inhibited by carboxypeptidase N and was related to the generation of a small (approximately 14,000 mol wt) fragment of C5. No direct chemotactic response was detectable for kallikrein, activated Hageman factor, high-molecular weight kininogen, or intact C5. Incubation of Kallikrein, high-molecular weight kininogen, and Hageman factor together, so that activation of all three proteins occurred, did not results in the generation of detectable chemotactic activity.