Activation of the first component of complement

The quaternary structure in solution of human complement subcomponent C1r2C1s2

C1r2C1s2 is a subcomponent of first component C1 of the complement cascade. Previously two distinct models for its structure have been described, in which C1r2C1s2 is either a linear rod-like assembly of the globular domains found in each of C1s and C1r, or these domains are arranged to form an asymmetric X-shaped structure. These two models were evaluated by using hydrodynamic simulations and neutron scattering. The data on C1s, C1s2 and C1r are readily represented by straight hydrodynamic cylinders, but not C1r2 or C1r2C1s2. Tests of the X-structure for C1r2 and C1r2C1s2 successfully predicted the experimental sedimentation coefficients, thus supporting this model. Neutron scattering analyses on C1s and C1r2 are consistent with a linear structure for C1s, but not for C1r2. An X-shaped structure for C1r2 was found to give a good account of the neutron data at large scattering angles. The total length of the C1s and C1r monomers was determined as 17-20 nm, which is compatible with electron microscopy. On the basis of the known sequences of C1r and C1s, this length is accounted for by a linear arrangement of a serine-proteinase domain (length 4 nm), two short consensus repeat domains (2 x 4 nm), and a globular entity containing the I, II and III domains (4-7 nm).

Comparison of the role of tyrosine residues in human IgG and rabbit IgG in binding of complement subcomponent C1q

Treatment of covalently cross-linked or heat-aggregated oligomers of human IgG with 4 mM-tetranitromethane abrogated their C1q-binding activity. In contrast, tetranitromethane modification of rabbit IgG oligomers, under identical conditions, had no effect upon their C1q-binding activity. The tetranitromethane treatment led to nitration of about ten tyrosine residues per IgG molecule in both species, and the modification was specific for tyrosine residues. Reduction of the nitrated protein with Na2S2O4 did not lead to recovery of C1q-binding activity in human IgG oligomers or to loss of activity in rabbit IgG oligomers. Tryptic peptides from the nitrated proteins were isolated and a peptide containing nitrotyrosine-319 was recovered from human IgG, as well as peptides from both species corresponding to the region around nitrotyrosine-278. These data are consistent with the inactivation of C1q-binding activity in human IgG being the result of nitration of tyrosine-319; the rabbit IgG is unaffected by nitration because position 319 is phenylalanine. The evidence supports the C1q-receptor site proposed by Burton, Boyd, Brampton, Easterbrook-Smith, Emanuel, Novotny, Rademacher, van Schravendijk, Sternberg & Dwek [(1980) Nature (London) 288, 338-344]: residues 316-338.

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.

The molecular genetics of components of the complement system

Rapid progress has been made recently on the elucidation of the structural components of the complement system by the application of recombinant DNA techniques. The derived amino acid sequences of most of the complement proteins are now available through cDNA cloning, and significant progress has been made in the discovery of the genetic organization of the corresponding genes. The linkage of some of the complement component genes has been established through the study of phenotypic genetics. Of particular interest has been the mapping of two clusters of genes which encode proteins involved in the activation of C3. C2, C4 and factor B, three of the structural components of the classical and alternative pathway C3 convertases, are encoded by genes which map to the MHC on human chromosome 6. The linkage of the genes with each other in a 100 kb segment of DNA has been established through the isolation of overlapping cosmid clones of genomic DNA, and PFGE has defined the molecular map position of these genes within the class III region of the MHC. The regulatory proteins factor H, C4BP, CR1 and DAF, which are involved in the control of C3 convertase activity, are encoded by closely-linked genes (termed the regulators of complement activation or RCA linkage group) that have been mapped to human chromosome 1. PFGE has defined the linkage of the CR1, C4BP and DAF genes, together with the CR2 gene in an 800 kb segment of DNA, and it is clear that this technique will eventually be applied to the molecular mapping of other complement genes in relation to their flanking loci. Polymorphism is a feature of many of the complement proteins, especially those encoded by genes in the MHC class III region. Of these, C4 is by far the most polymorphic, and differences in gene size and gene number, in addition to the functional and antigenic differences in the gene products, have been recognized. Null alleles at either of the C4 loci are rather common and may be important susceptibility factors in some HLA-associated diseases, particularly SLE. The molecular basis of complement deficiency states has begun to be elucidated. In many cases, the deficiency is not caused by a major gene deletion or rearrangement, and techniques which detect single point mutations in DNA (Cotton et al, 1988) will have to be applied to fully characterize the nature of the defect.

The distortive mechanism for the activation of complement component C1 supported by studies with a monoclonal antibody against the "arms" of C1q

A mouse monoclonal antibody (IgG1 isotype) against human C1q (MAb 130) is presented that activates C1 in serum through its antigen-binding sites at an optimal molar ratio of 3 MAbs:1 C1q. The antibody does not inhibit binding of C1q to IgG. Experiments with pepsin- and collagenase-digested C1q showed that MAb 130 binds to the fibril-like strands (arms) of C1q, close to the globular heads. Bivalency of MAb 130 was a requirement for C1-activation, but not for binding to C1q. Increasing the segmental flexibility of the intact antibody by reduction and alkylation destroyed its capacity to activate C1. A MAb against the globular heads of C1q completely inhibited C1-activation by aggregated IgG (AHG), but did not prevent activation by MAb 130. C1, reconstituted by adding C1q-stalks that lack the globular heads to C1q-depleted serum was not activated by AHG, whereas activation by MAb 130 was not affected. Activation of serum-C1 by AHG and MAb 130 was inhibited by addition of excess purified C1-inhibitor in a comparable and dose-dependent manner. Sucrose-gradient analysis indicated a predominance of stable complexes of a single C1q-molecule with three MAbs at the optimal activating ratio. When isolated and added to C1q-depleted serum, these complexes activated C1 efficiently. A mechanism for activation by MAb 130 is proposed that supports the "distortive" model of C1-activation.

Complement activation occurs through both classical and alternative pathways prior to onset and resolution of adult respiratory distress syndrome

We have previously reported that plasma concentrations of the terminal complement (C) complex (TCC), C5b-9, increased significantly 2 days prior to onset of adult respiratory distress syndrome (ARDS) and also 1 day preceding its resolution. To determine the pathway of complement activation that preceded development and resolution of this acute inflammatory lung injury in septic patients, we quantified the C1rC1s-C1 inhibitor complex and the C3bP complex, which are generated following activation of classical and alternative complement pathways, respectively. Two days prior to diagnosis of ARDS, the plasma C1rC1s-C1 inhibitor complex and C3bP complex levels increased 22 and 14%, respectively. Furthermore, significant correlations were identified between concentrations of the TCC and C1rC1s-C1 inhibitor complex (r = 0.73, P = 0.003) and also with the levels of the TCC and C3bP complex (r = 0.81, P = 0.002) before onset of ARDS. Equally of interest, the C1rC1s-C1 inhibitor complex and C3bP complex concentrations increased 68 and 35%, respectively, 1 day before resolution of ARDS. Similarly, significant elevations of TCC concentrations preceding resolution of ARDS correlated with C1rC1s-C1 inhibitor complex (r = 0.66, P = 0.02) and also with C3bP complex (r = 0.72, P = 0.002) levels. Our results indicate that both the classical and alternative complement pathways are activated prior to onset of ARDS and also before its resolution in septic patients.

Assignment of the complement serine protease genes C1r and C1s to chromosome 12 region 12p13

C1r and C1s are distinct, but structurally and functionally similar, serine protease zymogens responsible for the enzymatic activity of the first component of complement (C1). Recent comparisons indicate a significant degree of sequence similarity between C1r and C1s and support the hypothesis that they are related by gene duplication. Complementary DNA probes for human C1r and C1s do not cross-hybridize even at mild stringency conditions and are therefore gene-specific. Using a panel of 25 human-rodent cell hybrids, we have independently assigned the C1r and the C1s genes to chromosome 12. In situ hybridization analyses were consistent with these assignments, showing in addition that both C1r and C1s are located on the short arm of the chromosome in the region p13. These data suggest that the homologous C1r and C1s genes have remained closely linked after duplication of a common ancestor. The C1r and C1s loci also provide useful polymorphic DNA markers for the short arm of chromosome 12.

Subunit interactions in the first component of complement, C1

Interactions between C1q and other subunits of C1 were analyzed by sucrose gradient ultracentrifugation. A zone of dilute, radioiodine labelled C1q was sedimented through uniform concentrations of either C1r2C1s2, C1r2, C1r2 or C1s(2). The dissociation constants were found to be 3 x 10(-9) M and 6 x 10(-9) M for C1r2C1s2 and C1r2 binding respectively. Hill coefficients of 1 indicated no cooperativity in these bindings. Positive cooperativity was found in binding of C1s to C1q. Dissociation constants of 2 x 10(-6) M and 5 x 10(-8) M were obtained form computer modelling of a two step binding mechanism. No interaction was detected between C1q and activated C1r2. The data indicate that most of the interactions between C1q and C1r2C1s2 originates from a strong binding to the C1r2 moiety of the zymogen complex. This interaction is lost upon activation of C1r2.