Isolation and characterization of the third component of bovine 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.

Isolation and characterization of factor I of the bovine complement system

OBJECTIVE

To isolate and characterize factor I of the bovine complement system. Sample Population-Serum obtained from the blood of beef cattle.

PROCEDURES

Serum samples were fractionated to yield factor I by means of sequential precipitation, ion-exchange, and gel-filtration chromatography. The protein was identified throughout the procedure on the basis of its ability to degrade the alpha'-chain of bovine C3b in the presence of bovine factor H. Electrophoresis in polyacrylamide gels was used to assess the degradation of C3b and determine the molecular weights of factor I and its component polypeptide chains.

RESULTS

Bovine factor I had an apparent molecular weight of 94 kd and consisted of 2 disulfide-bonded polypeptides that had apparent molecular weights of 51 and 42 kd (under reducing conditions). Factor H was required for the factor I cleavage of the alpha'-chain of bovine C3b into iC3b. A similar cofactor effect was provided by trypsinized bovine erythrocytes or erythrocyte ghosts. Bovine properdin was prepared and shown to be a single polypeptide chain of 58 kd in the reduced form.

CONCLUSIONS AND CLINICAL RELEVANCE

Bovine factor I can be purified from serum by a simple 4-step procedure. It is structurally and functionally comparable to factor I of other species, and its purification completes the isolation and characterization of all the soluble components of the bovine alternative complement pathway.

The spotted wolffish (Anarhichas minor Olafsen) complement component C3: isolation, characterisation and tissue distribution

The complement component C3 was isolated from spotted wolffish (Anarhichas minor Olafsen) serum by polyethylene glycol precipitation, anion exchange chromatography and gel filtration. Silver staining in SDS-PAGE and rabbit anti-wolffish C3 antiserum used in Western blotting revealed that spotted wolffish C3 contains two polypeptide chains, M(r)65 and 115kDa, respectively. The high molecular weight alpha-chain of the C3 incorporated 14C-methylamine suggesting that it contained a reactive thioester group. The deduced amino acid sequence, after screening a liver cDNA expression library, showed that the wolffish C3 contained key amino acids for binding C3 convertase, factor H, I and properdin. Also, high degree of homology to other vertebrate C3 was found in the beta-alpha junction site. Phylogenetic tree analysis indicated that the Japanese flounder and spotted wolffish that belong to order pleuronectiformes and perciformes, respectively, are phylogenetically close species. Immunohistochemical experiments showed that liver hepatocytes and blood contained C3, and in situ hybridisation experiments revealed that liver hepatocytes expressed C3.

Degradation of bovine complement C3 by trichomonad extracellular proteinase

Bovine trichomoniasis is a local infection of the reproductive tract making interaction with mucosal host defenses crucial. Since the parasite is susceptible to killing by bovine complement, we investigated the role of the third component of complement (C3) in host parasite interactions. Bovine C3 was purified by anionic and cationic exchange chromatography. The purified protein was characterized by immunoreactivity, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and peptide sequencing of the amino terminus of the beta chain. When purified bovine C3 was incubated for varying time periods with trichomonad extracellular proteinases, SDS-PAGE gels revealed digestion of the alpha chain to small fragments. Such degradation in vivo would prevent formation of C3b and completion of the complement cascade, resulting in evasion of killing. To evaluate the relevance of this data, we determined whether C3 was present in bovine genital secretions. With a quantitative ELISA assay, C3 could be demonstrated in both uterine and vaginal washes. To our knowledge, this is the first demonstration of bovine C3 in genital secretions. The C3 concentration increased significantly in vaginal secretions by 8 and 10 weeks in heifers infected with Tritrichomonas foetus. An increase was also seen in uterine secretions of infected heifers, but sample numbers were insufficient for statistical analysis. Transcription of the major extracellular cysteine proteinase (TFCP8) was demonstrated in T. foetus cells from uterine secretions of infected heifers by RT-PCR and Southern blotting. The results indicate that C3 may be important in genital defense and that trichomonad extracellular proteinases may play a role in evasion of complement-mediated killing.

Alternative pathway of complement in ruminants: role in infection

Besides being initiated by antigen-antibody complexes, the classical pathway also can be activated by two innate mechanisms: (1) A family of proteins called collectins, which resemble C1q, bypass the activation of C1q. (2) Acute phase proteins, belonging to the family of pentraxins, activate the classical pathway by binding to C1q. The term 'alternative' complement pathway is a misnomer. This system is a primary primitive immune mechanism. It is phylogenetically older than the classical pathway. Contrary to the classical pathway, it does not require development of a specific immune response before getting into action. It acts within minutes after the microorganism has entered the body. The alternative pathway is continually activated at a low controlled rate but amplified by the surface of intruding microorganisms. It has the capacity to distinguish between self and non-self. Many nonpathogenic microorganisms are killed by the alternative pathway of complement. Pathogens have developed evasion mechanisms to escape the killing effect of this pathway. The kinetics of the activation of the alternative pathway of ruminants differs from that of mouse and man. The difference might be mediated by conglutinin.

Deposition of complement components on Streptococcus agalactiae in bovine milk in the absence of inflammation

Bovine milk is generally considered to be almost devoid of complement, on the basis of undetectable hemolytic activity, unless inflammation recruits plasma components. This study examines the deposition of complement components on a mastitis-causing isolate of Streptococcus agalactiae by enzyme-linked immunosorbent assay (ELISA). Neat milk from mid-lactating, uninflamed mammary glands (normal milk) effected marked C3 deposition on bacteria. Kinetic studies showed a protracted lag period (30 to 45 min) preceding C3 deposition, which required about 2 h to reach a maximum. Experiments with diluted serum suggested that this slow C3 deposition resulted mainly from the low concentration of certain components of complement in milk. Bacteria incubated in neat milk readily bound bovine conglutinin, indicating the presence of iC3b. Elution of covalently bound C3 fragments with hydroxylamine confirmed the deposition of C3b and iC3b on bacteria. Deposition of C4 on bacteria was not detected in neat milk, suggesting that C3 deposition did not result from the activation of the classical pathway. This was not the result of a lack of antibodies. However, C4 deposition could be obtained by adding purified bovine C1q to normal milk, and C3 deposition was accelerated, suggesting the participation of the classical pathway. The deposition of C1q on antibody-sensitized bacteria was impeded by milk compared with that of C1q diluted in phosphate-buffered saline. Concentrations of C1q in normal milk were very low, ranging from 150 to 250 ng/ml. Overall, these findings indicate that C1q was a limiting factor of the classical pathway in normal milk. The capacity of milk to deposit C3 fragments on mastitis-causing S. agalactiae prompts further studies to investigate its role in opsonophagocytosis.

Determination of optimal in vitro conditions for caprine alternative complement pathway assay

Optimal in vitro testing conditions for caprine alternative complement pathway assay were determined. Effects of the following variables were tested: heterologous erythrocytes; pH, ionic strength and Mg2+ ion concentration of the complement diluent; incubation time and temperature. Rabbit erythrocytes were the optimal target cells. The optimal buffer conditions were: pH 8.0, ionic strength 0.06 mmol NaCl and 5mmol Mg2+ ion. Optimal incubation time and temperature were 75 min and 30 degrees C, respectively.

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.

Degradation of bovine C3 by serine proteases from parasites Hypoderma lineatum (Diptera, Oestridae)

Purified enzymes of Hypoderma lineatum (Insecta, Oestridae), were assayed for their proteolytic activity on bovine C3 in normal cattle sera. The products of cleavage by these serine proteases (hypodermins A, B, and C), were analysed by electrophoresis in SDS polyacrylamide gels followed by immunoblotting. The enzymatic attack was initially directed at the alpha polypeptide chain by hypodermin A at a concentration of 1 micrograms/ml of serum and by hypodermin B at 5 micrograms/ml. The generated peptides differed in their molecular size from those produced during natural degradation of C3 in a control serum by physiologically relevant enzymes. Hypodermin A, at a concentration of 1 microgram/ml, also caused a cleavage of the beta chain. At 5 micrograms/ml, hypodermin A induced total degradation of the C3 molecule. Hypodermin B (5 micrograms/ml) starts splitting C3 near cleavage sites of factor I. Bovine C3 appears to be highly sensitive to hypodermins A and B in normal sera. Apparent molecular sizes and alignment of the bovine C3 cleavage products are presented schematically. Hypodermin C, a collagenolytic enzyme, had no effect on C3 in normal sera. The biological consequences for the immunopathological reactions associated with hypodermosis are discussed.

A simple isolation procedure for functionally pure components of the bovine alternative complement pathway (ACP) C3 convertase and bovine conglutinin (K)

A simple multicomponent isolation procedure for bovine C3, factor B, factor D and conglutinin (K) from a single serum sample is described. The components of the alternative pathway C3 convertase were isolated in milligram quantities from 800 ml bovine serum and were found to be functionally pure with respect to each other and to factors H and I.

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.

Isolation of bovine complement factor H

A bovine serum protein, initially recognized by its inhibitory effect on the hemolytic activity of the bovine alternative pathway was isolated from fresh bovine serum by polyethylene glycol precipitation and chromatography on DEAE-Sephacel, CM-Sephadex A-50 and Sephadex G-200. The protein, a single chain polypeptide with an apparent molecular weight of 158,000, was identified as factor H, a regulatory protein of the alternative complement pathway. Functional characterization of this protein as factor H was based on the following properties: binding to C3b, inhibition of factor B binding to C3b, cofactor activity in the cleavage of C3b by factor I, inhibition of fluid phase alternative pathway C3 convertase (C3b.Bb) formation and activity, and species-specific inhibition of the alternative pathway mediated hemolysis of heterologous erythrocytes. A monospecific rabbit antiserum against bovine factor H failed to react with human serum factor H.