A comparison of methods for the molecular quantitation of the fourth component of human complement

Complement: A nostalgic journey

The scientific career and research contributions of Hans J. Müller-Eberhard to the field of complement research are presented in historical context, and interpreted with regard to the state of the field and the research technologies available when the contributions were made.

On the site of C4 deposition upon complement activation via the mannan-binding lectin pathway or the classical pathway

The mannan-binding lectin (MBL) pathway and the classical pathway of complement activation are initiated by the binding of the recognition structure of the initiator complexes, MBL and C1q, respectively, to their ligands, i.e. carbohydrate structures or immune complexes. Proenzymes associated with MBL or C1q are then activated and generate C3 convertase through the activation of C4 and C2. The cleavage product of C4, C4b, attaches covalently to nearby hydroxyl or amino groups. The current picture is that C2 must then attach to C4b before being cleaved by the same associated proteases into the enzymatically active fragment, C2b. This suggests a stringent requirement for the deposition of C4b very close to the initiator complex, or indeed onto the initiator complex. We examined the possibility of C4b being bound to the initiator complex by a solid-phase assay, allowing for the selective elution of the initiator complexes, followed by quantification of the C4b being eluted and the C4b remaining on the solid phase. Also, we estimated the generation of complexes between the released initiator complex and C4b. More than 99% of deposited C4b was bound directly to the solid phase rather than to the initiator complex. Our approach cannot answer the question of the whereabouts of the C2 when it is cleaved.

Two clusters of acidic amino acids near the NH2 terminus of complement component C4 alpha'-chain are important for C2 binding

Previous work has indicated a role for the NH2-terminal segment of the C3 alpha'-chain in the binding interactions of C3b with a number of its protein ligands. In particular, we have identified two clusters of acidic residues, namely, E736 and E737 and to a lesser extent D730 and E731, as being important in the binding of C3b to factor B and complement receptor 1 and the binding of iC3b to complement receptor 3. Whereas human C3 and C4 have an overall sequence identity of 29%, over a segment near the NH2 termini of their respective alpha'-chains the sequence identity is 56% (70% chemical similarity). Given the functional similarity between the C4b-C2 and C3b-B interactions in the respective formation of the classical and alternative pathway C3 convertases, as well as the sequence conservation of two acidic clusters, we hypothesized that residues 744EED and 749DEDD within the NH2-terminal segment of the C4 alpha'-chain would mediate in part the binding of C2 to C4b. We tested this hypothesis using three independent approaches. Site-directed mutagenesis experiments revealed that replacing subsets of the charged residues by their isosteric amides within either acidic cluster resulted in molecules having reduced C2 binding activity. Moreover, a synthetic peptide (C4 residues 740-756) encompassing the two acidic clusters was a specific inhibitor of the binding of C2 to red cell-associated C4b. Finally, Ab raised against the above peptide was able to block the interaction between red cell-associated C4b and fluid phase C2. Taken together, these results strongly suggest that the NH2-terminal acidic residue-rich segment of C4 alpha'-chain contributes importantly to the interaction of C4b with C2.

Identification of residues within the 727-767 segment of human complement component C3 important for its interaction with factor H and with complement receptor 1 (CR1, CD35)

Mapping approaches employing blocking antibodies and synthetic peptides have implicated the 727-767 segment at the NH2 terminus of C3b alpha'-chain as contributing to the interactions with factor B, factor H, and CR1. Our previous mutagenesis study on the NH2-terminal acidic cluster of this segment identified residues Glu-736 and Glu-737 as contributing to the binding of C3b to factor B and CR1 but not factor H. We have now extended the charged residue mutagenic scan to cover the remainder of the segment (738-767) and have assessed the ability of the C3b-like C3(H2O) form of the mutant molecules to interact with factor H, CR1, and membrane cofactor protein (MCP) using a cofactor-dependent factor I cleavage assay as a surrogate binding assay. We have found that the negatively charged side chains of Glu-744 and Glu-747 are important for the interaction between C3(H2O) and factor H, a result in general agreement with an earlier synthetic peptide study (Fishelson, Z. (1991) Mol. Immunol. 28, 545-552) which implicated residues within the 744-754 segment in H binding. The interactions of the mutants with soluble CR1 (sCR1) revealed two classes of residues. The first are residues required for sCR1 to be an I cofactor for the first two cleavages of alpha-chain. These are all acidic residues and include the Glu-736/Glu-737 pair, Glu-747, and the Glu-754/Asp-755 pairing. The second class affects only the ability of sCR1 to be a cofactor for the third factor I cleavage and include Glu-744 and the Lys-757/Glu-758 pairing. The dominance of acidic residues in the loss-of-function mutants is striking and suggests that H and CR1 contribute basic residues to the interface. Additionally, although there is partial overlap, the contacts required for CR1 binding appear to extend over a wider portion of the 727-767 segment than is the case for factor H. Finally, none of the mutations had any effect on the interaction between soluble MCP and C3(H2O), indicating that despite its functional homology to H and CR1, MCP differs in its mode of binding to C3b/C3(H2O).

Cold-dependent activation of complement: recognition, assessment, and mechanism

Cold-dependent activation of complement (CDAC) is a phenomenon characterized by low hemolytic complement activity in chilled serum. Complement component levels are normal when measured immunologically, and there is normal hemolytic activity in EDTA plasma or serum maintained at 37 degrees C. Little attention has been paid to CDAC except in Japan, and current unfamiliarity with it, even by clinical immunologists, can lead to confusion and unnecessary laboratory tests. A 66-year-old patient with a complex medical history is described whose complement tests showed abnormalities characteristic of CDAC. Evidence for classical complement pathway activation in the cold was obtained by CH50 measurements, by hemolytic C4 determinations, by C4a, C3a, and C4d generation, and by quantitating C1s-C1r-(C1 inhibitor)2 complexes. A good correlation was observed among these parameters. Cryoprecipitates were absent. CDAC activity has persisted for over 5 years and is greater at 13 than at 4 degrees C. Activation is ablated by heating at 56 degrees C and restored by the addition of C1 to the heated serum. Adsorption by streptococcal protein G-Sepharose and precipitation by 2.5% polyethylene glycol support the hypothesis that CDAC is caused by aggregated IgG. The CDAC factor(s) also induces complement activation in normal serum but has not interfered with Raji cell or C1q binding tests or with FACS analysis. More limited studies of a second individual experiencing CDAC yielded similar results.

Functional properties of heterogeneous human asialo-C4 and its isotypes C4A and C4B

The fourth component of human complement (C4) is encoded at two separate but closely linked loci within the MHC on the short arm of chromosome 6. Thus, there are two types of C4 protein in most individual and pooled normal human sera (NHS): C4A and C4B. Incubation of individual sera, pooled NHS, or purified heterogeneous C4 (C4A/C4B) with bacterial sialidase at 37 degrees C increased C-mediated hemolysis of antibody-sensitized sheep erythrocytes 1.54- to 1.93-fold. Comparative studies of Tmax of human C2, using asialo-C4 or buffer-treated C4 on EAC1gp and extrapolation to time 0 indicated a z value 4-fold higher with asialo-C4. This indicated that more hemolytically active C42 complexes are available with sialidase-treated C4 compared to untreated C4. There was no appreciable difference in the % 125I-C4 bound to EAC1gp (sialidase- or buffer-treated). Sera from two different blood donors with C4A3 phenotype (C4BQ0), two different donors with C4B1 phenotype (C4AQ0), and serum from an individual heterozygous deficient at both C4A3 and C4B1 regions (A3, AQ0; B1, BQ0) were investigated. The C4 allotypes, purified from these sera, were treated with sialidase; the C4A3 was enhanced in hemolytic assays by sialidase-treatment (1.52- to 2.3-fold), whereas the C4B1 allotype was not enhanced. Fluorometric determinations revealed that approximately the same percentage of sialic acid was released from sialidase-treated C4A3 and C4B1. Therefore, the increase in hemolytic titer observed after treatment of NHS or purified heterogeneous C4 with sialidase is a property of C4A3 but not a property of C4B1.

Trichinella spiralis: activation of complement by infective larvae, adults, and newborn larvae

The ability of Trichinella spiralis to activate complement (C) has been addressed by several investigators. However, these investigators employed methods in which either detection of C fragments on the parasite surface or the adherence of leukocytes to the parasite was considered an indication of C activation. The present studies were undertaken to examine: (a) whether activation of C occurs via the classical and/or alternative pathway, (b) at which stage(s) of the parasite C activating capacity is acquired, and (c) what molecular entities of the epicuticle and/or cuticle are responsible for initiating C activation. Our studies indicate that T. spiralis activates C primarily via the alternative pathway (and weakly via the classical pathway) since incubation of parasites obtained from infected mice with either normal human serum (NHS) or Mg.EGTA-NHS, followed by incubation (1 hr, 37 degrees C) with antibody-sensitized sheep erythrocytes or rabbit erythrocytes, respectively, showed a time-and parasite number-dependent depletion of C. Although the three stages of T. spiralis, i.e., infective larvae, adults and newborn larvae, are capable of activating C, the newborn appears to be the most potent activator, especially when parasite number and size are taken into consideration. Further evidence of C activation is obtained from SDS-PAGE and Western blot analysis in which homogenates of parasites preincubated with NHS showed the presence of C3, C9, and C1q, whereas controls without serum were negative. Since isolated C1q was also capable of directly binding to the surface of adults and infective larvae, it is postulated that their cuticle and/or epicuticle may possess surface structures which serve as binding sites for C1q.(ABSTRACT TRUNCATED AT 250 WORDS)

Substitution of a single amino acid (aspartic acid for histidine) converts the functional activity of human complement C4B to C4A

The C4B isotype of the fourth component of human complement (C4) displays 3- to 4-fold greater hemolytic activity than does its other isotype C4A. This correlates with differences in their covalent binding efficiencies to erythrocytes coated with antibody and complement C1. C4A binds to a greater extent when C1 is on IgG immune aggregates. The differences in covalent binding properties correlate only with amino acid changes between residues 1101 and 1106 (pro-C4 numbering)--namely, Pro-1101, Cys-1102, Leu-1105, and Asp-1106 in C4A and Leu-1101, Ser-1102, Ile-1105, and His-1106 in C4B, which are located in the C4d region of the alpha chain. To more precisely identify the residues that are important for the functional differences, C4A-C4B hybrid proteins were constructed by using recombinant DNA techniques. Comparison of these by hemolytic assay and binding to IgG aggregates showed that the single substitution of aspartic acid for histidine at position 1106 largely accounted for the change in functional activity and nature of the chemical bond formed (ester vs. amide). Surprisingly, substitution of a neutral residue, alanine, for histidine at position 1106 resulted in an increase in binding to immune aggregates without subsequent reduction in the hemolytic activity. This result strongly suggests that position 1106 is not "catalytic" as previously proposed but interacts sterically/electrostatically with potential acceptor sites and serves to "select" binding sites on potential acceptor molecules.

Murine complement component C4 and sex-limited protein: identification of amino acid residues essential for C4 function

Murine sex-limited protein (Slp) is an isotype of murine complement component C4 that shares 95% sequence identity with C4 as well as the intramolecular thioester necessary for C4 function but has no complement activity. Slp is nonfunctional at least in part because it is not cleaved by the activated form of complement protease C1s (C1s), which proteolytically activates C4 in the classical complement pathway. Slp is also distinct from C4 in that its expression in some mouse strains is under testosterone control. In the present studies, we used site-directed mutagenesis of C4 and expression of the mutant proteins in cultured cells to identify the amino acid substitutions in Slp that are responsible for resistance to C1s cleavage. We focused on sequence changes immediately downstream of the cleavage site in C4 because the arginine at that site is conserved in Slp, but the downstream sequences diverge substantially, with six differences in the first 7 residues followed by a 3-residue deletion in Slp. We found that a C4 mutant carrying only the 3-residue deletion is not cleaved by C1s and has essentially no hemolytic activity, whereas a mutant carrying only the six replacement changes is cleaved by C1s and has normal hemolytic activity. Both mutants have intact thioesters. A third mutant in which two acidic residues in the segment deleted in Slp were replaced by aliphatic residues is also cleaved by C1s, has an intact thioester group, and has normal hemolytic activity. These results indicate that the downstream mutations are responsible for the resistance of Slp to C1s cleavage and suggest that the length rather than the specific sequence of this segment is critical in determining susceptibility to the protease.

Role of the fourth complement component (C4) in the regulation of contact sensitivity. II. Qualitative differences between C4 molecules from high- and low-C4 mouse strains

Lymph node cells collected from CBA/J mice 4 days after painting with picryl chloride induce contact sensitivity in naive recipient mice by virtue of hapten IgM immuno complexes. The immunizing capacity of these cells ("4-day" cells) is abolished after incubation of the cells with a C4-deficient guinea pig serum reconstituted with plasma or purified C4 from mice with high C4 levels (C4h), but not with plasma or purified C4 from mice with low C4 levels (C41). The inhibition of the immunizing capacity of 4-day cells is due to the activation of the early components of the classical complement pathway which is likely to result in the solubilization of membrane-bound immunocomplexes. However, the same amounts of CBA/J and BALB/c C4 have a different effect in inhibiting the induction of contact sensitivity by 4-day cells. In fact, by dose-response experiments, we have found that the amount of C41 able to inhibit the induction of contact sensitivity is about threefold higher than that of C4h. Analysis of the covalent binding ability of C4h and C41 reveals that C4h is able to bind to the surface of 4-day cells more efficiently than C41 and this probably accounts for the difference of the two C4 molecules in inhibiting the immunizing capacity of 4-day cells. Results are discussed in terms of different reactivities of C4h and C41 with the surface of 4-day cells.

Studies on the mechanism of C3b inhibition of immune complex induced C1 activation

We had previously demonstrated that in normal human serum (NHS) nascent C3b inhibited C1 activation by immune complexes (IC). We have now investigated the mechanism of this feedback inhibition. For these studies, EA-IgG were added to solutions containing physiological concns of purified C1, C1-In, C2, C3 and C4. Mixtures were then incubated at 37 degrees C for 30 min. Western blot and autoradiographic analyses revealed that almost half of the IgG molecules had become covalently linked to C3b in a 1:1 complex with the C3 alpha' chain of C3b being bound to the heavy chain of IgG. IgG-C3b and free IgG were separated by ion exchange chromatography and immune complexes were formed with each. The consumption of complement in NHS by EA-IgG and EA-(IgG-C3b) were then compared. The results indicate that binding of C3b to IgG did not significantly inhibit the C1 activating potential of the IgG. Thus feedback inhibition is not due to the binding of C3b to IgG. An alternative mechanism was next explored. After incubation of EA-IgG with C1 through C3, EA were separated from supernatant fluid by centrifugation. It was determined that one-third to one-half of the IgG had been released from the erythrocytes. Release appears not to have been due to C3b binding to IgG, since the released IgG-C3b readily bind to fresh sheep erythrocyte (E), and since IgG that was free of C3b was also released from EA by complement, it is more likely that C3b binding to the E caused the dissociation of antibody. These results indicate that under physiological conditions, the C1 activating potential of an immune complex is greatly reduced as the result of the binding of nascent C3b to the antigen moiety of the IC, thereby causing the displacement of complement activating antibody. In addition to IgG, IgG-C3b is also released from the IC.

Quantitation of (C1INH)2 C1r-C1s complexes in glomerulonephritis as an indicator of C1 activation

C1 activation was assessed in several forms of glomerulonephritis by radioimmunoassay quantitation of circulating (C1INH)2 C1r-C1s complexes (INC). Eight patients with active systemic lupus erythematosus (SLE) and nephritis had elevated serum INC (mean = 15.3 vs control = 5.8, P less than 0.01). Their INC levels were normal during remission. Serum INC had a weak inverse correlation with serum C1q greater than 3 mg/dl (r = 0.42, P = 0.02). In longitudinal studies, serum INC also had a weak inverse correlation with serum C3 and C4. Only 1 of 10 patients with type I and 1 of 15 with type III membrano-proliferative glomerulonephritis (MPGN) had elevated serum INC. No patient with type II MPGN had elevated levels. Two of 10 patients with poststreptococcal glomerulonephritis (P-SGN) had elevated serum INC, but all normalized with convalescence. Patients with IgA nephropathy had normal serum INC. The data demonstrate the importance of C1 activation in SLE and P-SGN. The mechanism of complement activation in types I and III MPGN remains unclear; the data suggest, but do not prove, that C1-independent complement activation may occur in these patients.

Functional and morphological characterization of cultures of Kupffer cells and liver endothelial cells prepared by means of density separation in Percoll, and selective substrate adherence

This paper presents a study on the structure and function of Kupffer cells (KC) and liver endothelial cells (LEC) isolated by a simple and rapid technique involving 1) perfusion of the liver with collagenase; 2) cell separation by means of density centrifugation in Percoll; and 3) cell culture, taking advantage of the fact that KC and LEC differ in their preferences for growth substrate. The KC, which attach and spread under serum-free conditions on surfaces of glass or plastic during the first 15 min in culture exhibit a typical macrophage-like morphology including membrane ruffling and a heterogenous content of vacuoles. Moreover, these cells express (a) Fc receptors (FcR) for binding and phagocytosis of erythrocytes covered with immune globulin G (E-IgG), and (b) complement receptors (CR) for binding and serum dependent phagocytosis of erythrocytes covered with either human C3b or mouse inactivated C3b (iC3b). The cells also bind fluid phase fluoresceinated C3b. Approximately 30% of the KC express immune response-associated (Ia)-antigens. The LEC attach and spread on fibronectin coated surfaces, but not on glass or plastic surfaces, during the first two hours in culture with or without serum, and are morphologically distinct from KC. Cultured LEC are well spread out with no membrane ruffling and with numerous large vesicles surrounding the regularly shaped nucleus. These cells bind, but do not ingest E-IgG via the FcR, but no binding of fluid phase C3b or particle fixed C3b or iC3b can be observed. Incubation of LEC with fluorescein amine conjugates of ovalbumin or formaldehyde treated serum albumin, but not with fluoresceinated native serum albumin, results in accumulation of fluorescence specifically localized in the large perinuclear vesicles. Neither KC nor any other cell types tested have the ability to accumulate fluorescence upon incubation with these compounds. Ia-antigens are not present on the LEC. Cytochemical demonstration of unspecific esterase, acid phosphatase, and peroxidase reveals different patterns and intensities of staining in KC as compared to LEC.

Cigarette smoke can activate the alternative pathway of complement in vitro by modifying the third component of complement

Cigarette smoking is associated with significant increases in the number of pulmonary mononuclear phagocytes and neutrophils. A potent chemoattractant for these cells is C5a, a peptide generated during complement (C) activation. We, therefore, investigated the possibility that cigarette smoke could activate the complement system in vitro. Our results show that factor(s) (mol wt less than 1,000) present in an aqueous solution of whole, unfiltered cigarette smoke can deplete the hemolytic capacity of whole human serum in a dose-dependent manner. The particle-free, filtered gas phase of cigarette smoke is inactive. The smoke factor(s) do not activate serum C1, but do deplete serum C4 activity. Treatment of purified human C3 with whole smoke solution modifies the molecule such that its subsequent addition to serum (containing Mg/EGTA to block the classical pathway) results in consumption of hemolytic complement by activation of the alternative pathway. Smoke-modified C3 shows increased anodal migration in agarose electrophoresis, but this is not due to proteolytic cleavage of the molecule as evidenced by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In contrast to methylamine-treated C3, C3 treated with smoke is only partially susceptible to the action of the complement regulatory proteins Factors H and I. In addition, smoke-modified C3 has diminished binding to Factor H as compared with methylamine-treated C3. Finally, smoke-modified C3 incorporates [14C]methylamine which suggests that the thiolester bond may be intact. These data indicate that aqueous whole cigarette smoke solution can modify C3 and activate the alternative pathway of complement, perhaps by a previously unrecognized mechanism. Should this occur in vivo, complement activation might partly account for the extensive pulmonary leukocyte recruitment observed in smokers.

Inhibition of cleavage of the third component of human complement (C3) by its small cleavage fragment, C3a: inhibition occurs with the classical-pathway, but not the alternative-pathway, C3 convertase

Activation of the third component of complement (C), C3, is central to the functioning of the C system in inflammation. Cleavage of C3 by the C3 convertases of both the classical and alternative pathways results in the formation of two split products, C3b and C3a. C3a inhibited cleavage of C3 by the classical-pathway C3 convertase. The inhibition varied in a concn-dependent relationship, with a concn of approximately 40 micrograms/ml yielding 50% inhibition. Removal of the carboxy terminal arginine from the C3a did not alter the inhibition. C3a did not inhibit cleavage of C3 by the alternative C pathway C3 convertase, or cleavage of C5 by C5 convertase. The C3-cleaving capacity of EAC142oxy that had been previously incubated with C3a could be recovered completely by washing the cells, indicating that the C3a binding to the EAC42oxy cell must have been reversed without having had an effect on the amount of C2 bound. Ribonuclease, a molecule of similar size and charge to C3a, did not affect C3 cleavage and C3a inhibition was not reduced by providing a surface for non-specific adsorption of the C3a, suggesting that the effect of C3a on C3 cleavage was not mediated by non-specific interaction with cell surfaces. C3a inhibited the C3-cleaving capacity of the fluid-phase enzyme, C42oxy, to the same degree as it inhibited the cell-bound enzyme, EAC42oxy, indicating that the C3a must interact with the C42 complex directly. Inhibition of C3 cleavage by C3a is the first demonstration of product inhibition of a complement enzyme. It may provide another control of C3 activation.

Studies of complement autoactivatability in hereditary angioedema: direct relationship to functional C-1-INA and the effect of classical pathway activators

It has been observed earlier that the hemolytic complement in diluted sera obtained from patients with hereditary angioedema (HAE) undergoes spontaneous decay when incubated at 37 degrees C. Employing individual serum from patients at different stages of this disease it was demonstrated that this spontaneous loss of hemolytic complement also occurs without dilution and is directly linked to the absence of functional C-1-INA. Incubation of HAE serum resulted in a loss of activity which appears to be dependent upon the concentration of functional C-1-INA. While C-1-INA levels less than 50 micrograms/ml lead to rapid depletion with time, reconstitution of deficient sera with highly purified C-1-INA or of undiluted NHS inhibited spontaneous activation. Furthermore, NHS was rendered susceptible to autoactivation when its C-1-INA was depleted by passage over an anti-C-1-INA Sepharose 4B affinity column in the presence of 10 mM EDTA, indicating that in the absence of functional C-1-INA, C1 undergoes an uninhibited spontaneous autoactivation which leads to the consumption of C4 and C2 but not C3. Consumption of C3 was observed, however, in HAE sera that contained a significant amount of immune complexes. Incubation of HAE sera with highly purified Hageman factor fragment (5 micrograms/ml), or aggregated IgG (2 mg/ml) was found to accelerate the rate of decay when compared to untreated samples while sera from patients under treatment with Danazol or Stanozolol failed to autoactive. These results suggest that, the absence of C-1-INA, may, by itself trigger the dissociation and autoactivation of C1 in the sera of such patients; however, the presence of other complement activators accelerates the reaction. This inherent property of HAE sera, i.e., spontaneous autoactivation at 37 degrees C, may be a useful screening test but direct determination of C-1-INA activity is required to establish the precise diagnosis.

Binding of human C4 to C-reactive protein-pneumococcal C-polysaccharide complexes during activation of the classical complement pathway

Sequential interaction of CRP-PnC aggregates, made at slight CRP excess, with purified human C1, C4 and C2oxy resulted in formation of an effective C3-convertase, indicating the binding of C1, C4 and C2 on the aggregates. Immunoprecipitation experiments demonstrated that, following cleavage of 125I-C4 by CRP-PnC-C1 complexes, approximately 3% of the 125I-C4 was bound to CRP while a lower percentage was bound to PnC, CRP-C4 complexes could also be demonstrated by substituting 125I-CRP for 125I-C4. The nature of the CRP-C4 bond was examined by electrophoretic analysis. Complexes of 125I-C4-CRP prepared as earlier were incubated at 100 degrees C for 2 min in buffer containing 2% SDS and 5% beta-mercaptoethanol and subjected to electrophoresis in SDS-containing polyacrylamide gradient slab gels. Autoradiography of the dried gels revealed the presence of high mol. wt bands containing the alpha'-chain of C4b. CRP could also be demonstrated in these high mol. wt bands which apparently represented covalent complexes between the alpha'-chain of C4b and CRP monomers. Since CRP contains no detectable carbohydrate, it seems likely that an amide bond is formed between the two proteins.

Mechanisms of activation of the classical pathway of complement by Hageman factor fragment

The mechanism by which a fragment of activated Hageman factor (HFf) activates the classical pathway of complement in serum or platelet-poor plasma has been further delineated. When serum or platelet-poor plasma was incubated with various concentrations of HFf, the total complement hemolytic activity was reduced in a dose-dependent manner. This activation appears to be due to the direct interaction of HFf with macromolecular C1, since incubation of purified C1 with HFf resulted in dissociation of the subunits with concomitant reduction of C1r antigenicity that is indicative of C1 activation. HFf-dependent activation was prevented by prior treatment of HFf with the active site-directed inhibitor, H-D-proline-phenylalanine-arginine chloromethyl ketone or with a specific inhibitor of activated HF derived from corn. Incubation of HFf with highly purified C1r also resulted in activation of C1r as assessed directly using a synthetic substrate or indirectly by activation of C1s and consumption of C2. However, incubation of HFf with highly purified C1s resulted in formation of activated C1s (C1s-) but this was less efficient than HFf activation of C1r. We therefore conclude that activation of C1 in macromolecular C1 is the result of HFf conversion of C1r to C1r; activation of C1s then occurs primarily by C-1r and to a lesser degree by the direct action of HFf.

Isolation of tryptic fragments of human C4 expressing Chido and Rodgers antigens

It has been shown previously that methylamine is incorporated into the alpha-chain of human C4, resulting in a loss of haemolytic function and the appearance of a free thiol group in the molecule. In the present study it was demonstrated that a fragment resembling C4d is liberated from C4 by trypsin. The fragment--Try-C4d--contains both the methylamine binding site and the free thiol group. When separated on DEAE-Sepharose, four types of Try-C4d, differing in charge and size, could be defined. The size difference was found to parallel the presence of Chido and Rodgers blood group antigens. Fragments of Mr 30,000 carried the Rodgers antigen and the Chido antigen was expressed on fragments of Mr 28,000.

The effect of C4 on C1 binding and activation

Cell-bound human C4 enhances the uptake of C1 by immunoglobulin-carrying cells. The effect is more pronounced with small amounts of IgG than with large amounts. Furthermore, the presence of C4 on the cell surface dramatically promotes the enzymatic effect of C1 on C2. The C1 binding is efficiently blocked, and the C1 activation is almost abolished by pretreating IgG-bearing cells with protein A from Staphylococcus aureus. In contrast, protein A has no effect on cells carrying C4 in addition to IgG. Protein A added in small amounts to IgG-coated sheep erythrocytes inhibits the haemolysis by subsequently added human serum, whereas even larger amounts of protein A has no effect when added to cells carrying C4 as well. Cells with and without C4 take up protein A equally well.

Isolation of component C4 of human complement and its polypeptide chains

Component C4 of human complement was purified from fresh frozen plasma with a yield of 25% using an initial batch separation with quaternary diethyl-(2-hydroxypropyl)aminoethyl--Sephadex followed by column chromatography on DEAE-cellulose and gel filtration in Sephadex G-200. The final product was homogenous according to polyacrylamide gel electrophoresis and immunochemical methods. Low-speed sedimentation-equilibrium analyses revealed a molecular weight of 189,000, using a value of 0.736 ml/g for the partial specific volume. The polypeptide chains of reduced and alkylated C4 were separated on DEAE-Sepharose in the presence of 8 M urea. Gel filtration in Sepharose 4B in 6 M guanidine hydrochloride revealed molecular weights of 88,000, 72,000 and 32,000 for the alpha, beta and gamma chain respectively. The amino acid compositions of component C4 and its constitutive chains were also determined.

The binding of complement component C3 to antibody-antigen aggregates after activation of the alternative pathway in human serum

Preformed immune aggregates, containing antigen and either IgG (immunoglobulin G) or F(ab')2 rabbit antibody, were incubated with normal human serum under conditions allowing activation of only the alternative pathway of complement. Both the IgG and F(ab')2 immune aggregates bound C3b, the activated form of the complement component C3, in a similar manner, 2-3% of the C3 available in the serum being bound to the aggregates as C3b, and the rest remaining in the fluid phase as inactive C3b or uncleaved C3. It was found that the C3b was probably covalently bound to the IgG in the aggregates, since C3b-IgG complexes could be demonstrated on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, after repeated washing with buffers containing high salt or boiling under denaturing conditions. Incubation of the C3b-antibody-antigen aggregates in buffers known to destroy ester linkages had little effect on the C3b-IgG complexes, which suggested that C3b and IgG might be linked by an amide bond. Two main types of C3b-IgG complexes were found that had apparent mol.wts. of 360000 and 580000, corresponding to either one to two C3b molecules respectively bound to one molecule of antibody. On reduction of the C3b-IgG complexes it was found that the beta-chain, but not the alpha'-chain, of C3b was released along with all the light chain of IgG but only about half or less of the heavy chain of IgG. These results indicate that, during activation of the alternative pathway of complement by immune aggregates containing IgG antibody, the alpha'-chain of C3b may become covalently bound at one or two sites in the Fd portion of the heavy chain of IgG.

Activation of the classical pathway of complement by Hageman factor fragment

A fragment of activated Hageman factor (HFf) has been demonstrated to activate the classical pathway of complement in a manner that is analogous to complement activation by antigen-antibody complexes or aggregated IgG. Thus C1, C4, C2, C3, and C5 were found to be depleted on addition of HFf to serum. The reduction of serum hemolytic activity was maximal upon addition of 5 micrograms HFf and an incubation time of 60 min at 37 degrees C. Consumption of the total complement activity and of the individual components proceeded in a dose-dependent fashion. No comparable activity was observed when equimolar concentrations of either the native Hageman factor (HF) or two-chain activated form of Hageman factor (HFa) were incubated with serum. Further, the ability of HFf to convert serum C3 and C4 was similar to that of aggregated IgG as assessed by immunoelectrophoresis. This function of HFf appeared to be independent of plasminogen (or plasmin) since plasminogen-free serum was indistinguishable from normal serum. Radial double immunodiffusion experiments using antiserum to C1q, C1r, and C1s on HFf-treated serum demonstrated the dissociation of the C1 trimolecular complex, with concomitant reduction of C1r antigenicity that is indicative of C1 activation. Thus, HFf appears to lead to C1 activation upon incubation with serum or when incubated with partially purified C1. This may represent a control link between activation of the intrinsic coagulation-kinin pathway and the initiation of the classical complement cascade.

Interaction between the labile binding site of human C4 and methylamine

Human complement component C4 was irreversibly inactivated by low concentrations of methylamine at slightly alkaline pH. The inactivated C4 molecules lost the ability to bind to EAC1 cells but retained th capacity to participate in the formation of classical pathway C3 convertase in the fluid phase. 14C-methylamine was incorporated into the alpha-chain at a ratio of 1 mol methylamine per mol C4.

The binding of human complement component C4 to antibody-antigen aggregates

The binding of human complement component C4 to antibody-antigen aggregates and the nature of the interaction have been investigated. When antibody-antigen aggregates with optimal C1 bound are incubated with C4, the C4 is rapidly cleaved to C4b, but only a small fraction (1-2%) is bound to the aggregates, the rest remaining in the fluid phase as inactive C4b. It has been found that C4b and th antibody form a very stable complex, due probably to the formation of a covalent bond. On reduction of the C4b-immunoglobulin G (IgG) complex, the beta and gamma chains, but not the alpha' chain, of C4b are released together with all the light chain, but only about half of the heavy chain of IgG. The reduced aggregates contain two main higher-molecular-weight complexes, one shown by the use of radioactive components to contain both IgG and C4b and probably therefore the alpha' chain of C4b and the heavy chain of IgG, and the other only C4b and probably an alpha' chain dimer. The aggregates with bound C1 and C4b show maximal C3 convertase activity, in the presence of excess C2, when the alpha'-H chain component is in relatively highest amounts. When C4 is incubated with C1s in the absence of aggregates, up to 15% of a C4b dimer is formed, which on reduction gives an alpha' chain complex, probably a dimer. The apparent covalent interaction between C4b and IgG and between C4b and other C4b molecules cannot be inhibited by iodoacetamide and hence cannot be catalysed by transglutaminase (factor XIII). The reaction is, however, inhibited by cadaverine and putrescine and 14C-labelled putrescine is incorporated into C4, again by a strong, probably covalent, bond. It is suggested that a reactive group, possibly an acyl group, is generated when C4 is activated by C1 and that this reactive group can react with IgG, with another C4 molecule, or with water.

Kinetics of the agglutination of IgG-coated latex particles by C1q: the influence of heat-labile serum components

Interaction between human C1q and IgG coated latex particles has been studied by means of a standard aggregometer equipment. A dose-dependent agglutination was observed and 100 ng of C1q were readily detected. The kinetics of the agglutination was also monitored. Serum, partially purified C1, and high molecular weight fractions from Sephadex G-200 fractionated serum produced agglutination only in the presence of EDTA. In the absence of this chelator these products disintegrated preformed C1q-IgG-latex particle agglutinates. This disagglutinating principle is heat-sensitive and tentatively macromolecular C1 dependent. The most probable basis of the activity is the competition between C1, with a high affinity for IgG particles, and C1q. The inability of C1 to induce particle agglutination might be caused by the C1 subunits C1r and C1s sterically inhibiting the subunit C1q to bridge between the particles.

The assembly of early components of complement on antibody-antigen aggregates and on antibody-coated erythrocytes

Radioimmune assays were developed to assay the binding of complement components C1q, C1s and C4 to antibody aggregates and to cell-bound antibody. The binding of the components was compared with the haemolytic activity and with the capacity to form the C3 convertase activity in the presence of excess C2. The destruction of whole complement and of C4 activity is similar per 1,000 molecules of antibody in aggregates and cell-bound antibody, as is the binding of C1g and C1s, the latter being in a 1:2 molar ratio. The binding of C4 is about 12 times greater, per 1,000 molecules of antibody, on cells than in aggregates. However, the effective C4 molecules, as judged by the formation of C3 convertase activity, are much more similar on cells and aggregates. An assembly mechanism of the early components of complement on antibody-coated cells, which is compatible with these results, is suggested.

The biochemistry of complement

Current biochemical studies of the complement system are illustrated by description of the activation of complement by the classical pathway after interaction with antibody aggregates. This is described in terms of the structures of the components involved, their assembly and the mechanism of activation.

A solid phase fluorescent immunossay for the quantitation of the C4 component of human complement

A non-competitive method for the determination of the C4 component of human complement in serum is described. This procedure involves use of a specific antibody covalently attached to derivatized polyacrylamide beads and a fluorescently labeled specific antibody. Reproducible results were achieved for C4 in serum in the range of 10 mg/dl to 170 mg/dl within 2 h. C4 levels as low as 150 ng/ml can also be measured. Fluorescent immunoassay and radial immunodiffusion were used to determine C4 levels in healthy adults. Good agreement was found between the two methods.

Assay for the two different types of lymphocyte complement receptors

This paper describes the assay system for two different types of lymphocyte complement receptors, the immune adherence receptor (C3b receptor) and the C3d receptor.

Synthesis of the second component of complement by long-term primary cultures of human monocytes

A method has been developed for preparation of confluent monolayers of human monocytes from small volumes of blood and for maintenance of these pure monocyte cultures for up to 16 wk in vitro. These cells phagocytosed 5.7 mum diameter latex beads, rosetted with erythrocytes coated with IgG or with C3, killed Listeria monocytogenes, and synthesized both lysozyme and the second component of complement. Lysozyme was secreted at a rate of approximately 50,000 mol/min per cell for at least 12 wk in cultures. The maximal rate of C2 synthesis and secretion was considerably less; i.e., approximately 30 mol/min per cell between the 2nd and 12th wk in culture. Monocytes produced little C2 during the first 6 days in culture after which a marked increase in the rate of C2 production was noted. This increase was coincident with morphologic evidence of monocyte maturation.

Enzymatic activity of the second component of complement

Isolated C2 and C2i preparations were able to hydrolyze a number of synthetic esters containing basic amino acids, among which N-alpha-acetylglycyl-L-lysine methyl ester (AcGlyLysOMe) was most susceptible. The cleaving activity was a property of the C2 molecule, since it correlated with the presence of C2 on analyses of C2 preparations by ultracentrifugation in sucrose gradients, filtration through Sephadex G-200 columns, and on electrophoresis in acrylamide gels. Furthermore, acrylamide gel electrophoretic studies showed a shift in hydrolytic activity from the position occupied by C2 to that characteristic of C2i after incubation of C2 with C1s. The action was enzymatically mediated as evidenced by a bell-shaped pH activity curve, a linear dependence on C2 concentration, and the presence of Michaelis-Menten kinetics. The Michaelis constant for cleavage of AcGlyLysOMe by C2 was 1.8 X 10(-2) mol. Cleavage of C2 by C1s increased C2 enzymatic activity, yet chemical oxidation of the molecule, although enhancing hemolytic acitivity, failed to increase C2 hydrolytic activity. The observed enzymatic activity of C2 was found to be relevant to the function of C2 in the C42 complex, since AcGlyLysOMe competitively inhibited the C42 mediated cleavage of C3 in free solution and the C42 dependent binding of C3 to cells.

Phagocytes and C4 in paraproteinaemia

Immunological studies were performed on patients with multiple myeloma. A defect in polymorphonuclear leucocyte (PMN) function as evidenced by diminished adherence of these cells to nylon fibre columns was detected in 16, and low levels of the fourth component of complement (C4) were observed in 14, of the 26 patients studied. Twelve of the patients with low C4 exhibited the defect in PMN adhesiveness whereas only four of the 12 patients with normal C4 showed the defect. The PMN defect was not caused solely by the low C4, since PMNs from seven patients with hereditary angioedema, which is associated with low levels of C4, did not show the defect. The low C4 and defect in PMN adhesiveness occurred primarily in patients with IgG myeloma; all but one of the patients with IgA myeloma, macroglobulinaemia, or light chain disease were normal in both parameters. Results of skin window studies indicated that patients with the PMN defect also had a defect in the early PMN inflammatory response. The defect in PMN adhesiveness could be completely corrected by incubating the cells in normal plasma. Binding of the C4 to paraprotein could not be demonstrated, and C1 activation was found to be caused only by one of 10 isolated paraproteins studied. These studies indicate that patients with paraproteinaemia have immunological abnormalities in addition to low immunoglobulin levels and suggest that these abnormalities may be involved in the pathogenesis of the recurrent infections commonly associated with this disease.