Factor D of the alternative pathway of bovine complement: Isolation and characterization

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.

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.

Gallysin-1, an antibacterial protein isolated from hemolymph of Galleria mellonella

An inducible hemolysin with antibacterial properties was isolated from the hemolymph of immune Galleria mellonella larvae. The Galleria-derived lysin, named Gallysin-1, was shown to have an apparent molecular weight of 75,000 and to be relatively heat stable at 56 degrees C. Although Gallysin alone was not bactericidal it caused sufficient damage of the outer cell membranes of Pseudomonas aeruginosa RP4 and Escherichia coli K176 to release beta-lactamase from the periplasm. In the presence of either purified Galleria lysozyme or egg white lysozyme Gallysin-1 had potent antibacterial activity against gram-negative bacteria. Gallysin-1 killed osmotically shocked P. aeruginosa and E. coli that suggests that it can also attack exposed inner cell membranes of gram-negative bacteria. The identification of Gallysin-1 recognizes another distinct member of the bactericidins involved in insect immunity.

Immune lysis of Trypanosoma congolense: generation of a soluble covalent complex of variant surface glycoprotein and bovine complement component C3b

Organisms of Trypanosoma congolense variant antigenic type TC13 (VAT TC13) were incubated, at 37 degrees C for 60 min, with fresh bovine serum in the presence of antibody specific for the variant surface glycoprotein (VSG). Upon immune lysis, soluble VSG (54 kDa) and a larger complex (about 225 kDa), containing VSG, was detected in the supernatant fluid of the mixture. Neither soluble VSG nor the VSG complex were detected when fresh bovine serum was incubated with organisms of T. congolense in the absence of specific antibody. Within a narrow range of low antibody concentration, the release of soluble VSG and the formation of the VSG complex were correlated with the amount of specific antibody added to the mixture. The VSG complex could be precipitated with rabbit antibodies specific for VSG of VAT TC13 or antibodies specific for bovine complement C3. The VSG complex was detected by Western blot with rabbit anti-VSG of VAT TC13 as well as rabbit antibovine C3. The complex was found to consist of VSG covalently bound to bovine complement component C3b. Potential pathophysiological implications are discussed.

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.

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.

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.