Rapid genome sequencing identifies novel variants in complement factor I

Complement factor I deficiency (CFID; OMIM #610984) is a rare immunodeficiency caused by deficiencies in the serine protease complement factor I (CFI). CFID is characterized by predisposition to severe pneumococcal infection, often in infancy. We report a previously healthy adolescent male who presented with respiratory failure secondary to pneumococcal pneumonia and severe systemic inflammatory response. Rapid genome sequencing (rGS) identified compound heterozygous variants in CFI in the proband, with a novel maternally inherited likely pathogenic variant, a single nucleotide deletion resulting in premature stop (c.1646del; p.Asn549ThrfsTer25) and a paternally inherited novel likely pathogenic deletion (Chr 4:110685580-110692197del).

Heme Interferes With Complement Factor I-Dependent Regulation by Enhancing Alternative Pathway Activation

Hemolysis, as a result of disease or exposure to biomaterials, is characterized by excess amounts of cell-free heme intravascularly and consumption of the protective heme-scavenger proteins in plasma. The liberation of heme has been linked to the activation of inflammatory systems, including the complement system, through alternative pathway activation. Here, we investigated the impact of heme on the regulatory function of the complement system. Heme dose-dependently inhibited factor I-mediated degradation of soluble and surface-bound C3b, when incubated in plasma or buffer with complement regulatory proteins. Inhibition occurred with factor H and soluble complement receptor 1 as co-factors, and the mechanism was linked to the direct heme-interaction with factor I. The heme-scavenger protein hemopexin was the main contaminant in purified factor I preparations. This led us to identify that hemopexin formed a complex with factor I in normal human plasma. These complexes were significantly reduced during acute vasoocclusive pain crisis in patients with sickle cell disease, but the complexes were normalized at their baseline outpatient clinic visit. Hemopexin exposed a protective function of factor I activity in vitro, but only when it was present before the addition of heme. In conclusion, we present a mechanistic explanation of how heme promotes uncontrolled complement alternative pathway amplification by interfering with the regulatory capacity of factor I. Reduced levels of hemopexin and hemopexin-factor I complexes during an acute hemolytic crisis is a risk factor for heme-mediated factor I inhibition.

Thrombomodulin mutations in atypical hemolytic-uremic syndrome

BACKGROUND

The hemolytic-uremic syndrome consists of the triad of microangiopathic hemolytic anemia, thrombocytopenia, and renal failure. The common form of the syndrome is triggered by infection with Shiga toxin-producing bacteria and has a favorable outcome. The less common form of the syndrome, called atypical hemolytic-uremic syndrome, accounts for about 10% of cases, and patients with this form of the syndrome have a poor prognosis. Approximately half of the patients with atypical hemolytic-uremic syndrome have mutations in genes that regulate the complement system. Genetic factors in the remaining cases are unknown. We studied the role of thrombomodulin, an endothelial glycoprotein with anticoagulant, antiinflammatory, and cytoprotective properties, in atypical hemolytic-uremic syndrome.

METHODS

We sequenced the entire thrombomodulin gene (THBD) in 152 patients with atypical hemolytic-uremic syndrome and in 380 controls. Using purified proteins and cell-expression systems, we investigated whether thrombomodulin regulates the complement system, and we characterized the mechanisms. We evaluated the effects of thrombomodulin missense mutations associated with atypical hemolytic-uremic syndrome on complement activation by expressing thrombomodulin variants in cultured cells.

RESULTS

Of 152 patients with atypical hemolytic-uremic syndrome, 7 unrelated patients had six different heterozygous missense THBD mutations. In vitro, thrombomodulin binds to C3b and factor H (CFH) and negatively regulates complement by accelerating factor I-mediated inactivation of C3b in the presence of cofactors, CFH or C4b binding protein. By promoting activation of the plasma procarboxypeptidase B, thrombomodulin also accelerates the inactivation of anaphylatoxins C3a and C5a. Cultured cells expressing thrombomodulin variants associated with atypical hemolytic-uremic syndrome had diminished capacity to inactivate C3b and to activate procarboxypeptidase B and were thus less protected from activated complement.

CONCLUSIONS

Mutations that impair the function of thrombomodulin occur in about 5% of patients with atypical hemolytic-uremic syndrome.

Effects of sulphated polyanions on functions of complement factor H

Heparin and two dextran sulphate preparations with a low or high average molecular mass (M(r) 5000 and 5 x 10(5), respectively) enhanced binding of radioactively labelled complement factor H to the complement protein C3b, coupled to Sepharose 4B, maximally 2.5-4-fold within a polyanion concn range of 12.5-400 micrograms/ml. Despite this, heparin or low molecular mass dextran sulphate had no effect on the activity of H as a cofactor of complement factor I, when C3b bound to Sepharose 4B was used as a substrate, and high molecular mass dextran sulphate inhibited. Heparin or low molecular mass dextran sulphate had also no effect on the decay-accelerating activity of factor H on the alternative pathway C3 convertase, C3b,Bb, and high molecular mass dextran sulphate inhibited this activity, too, regardless of whether Sepharose 4B or sheep erythrocytes were used as carriers of C3b,Bb. These results suggest strongly that fluid phase heparin or dextran sulphate do not inhibit activation of the alternative pathway of complement by augmenting functions of H.

Expression of complement components of the alternative pathway by glioma cell lines

Glioma cell lines express proteins of the complement alternative pathway, namely C3, factor B, factor H, and factor I. Secretion of these proteins was shown by a sensitive and specific ELISA. C3 and factor H were rapidly secreted by glioma cell line CB193 and reached a concentration of 140 ng/ml/10(6) cells after 72 h of culture. Factor B and factor I were secreted at a lower rate and reached concentrations of 25 and 15 ng/ml/10(6) cells, respectively. Western blot and immunoprecipitation experiments showed that secreted proteins were identical to the corresponding plasma proteins. For factor H, besides the well known 150-kDa species, an additional polypeptide of 45 kDa with factor H immunoreactivity was observed. This species corresponded to the N-terminal truncated form found in plasma. In preliminary experiments, we observed control of these syntheses by cytokines. IL-1 beta significantly increased C3 secretion, with no effect on factor H. Secretion of factor H was enhanced by IFN-gamma. These results show that a glioma cell line could be a useful tool to study complement biosynthesis by glial cells in humans.

Cell-surface regulation of the human alternative pathway of complement. Sheep but not rabbit erythrocytes express factor I-dependent cofactor activity

When incubated in normal human serum, rabbit erythrocytes are haemolysed as a result of activation of the alternative pathway of complement (APC), but sheep erythrocytes do not spontaneously activate the human APC under physiological conditions. The mechanism for this difference has been attributed to differences in the relative affinity of membrane-bound C3b for its natural ligands, factor B and factor H, that favour the formation and stability of the APC C3 convertase on rabbit erythrocytes and inhibit convertase activity on sheep erythrocytes. Previous studies have also suggested that factor I inactivated C3b on sheep erythrocytes more effectively than on rabbit erythrocytes. Further, sheep erythrocytes have recently been shown to have a membrane protein that associates non-covalently with cell-bound C3b, but rabbit erythrocytes lack a predominant C3b binding protein. Together, these results suggested the possibility that sheep but not rabbit erythrocytes have a membrane constituent with factor I cofactor activity. To investigate this hypothesis, rabbit and sheep erythrocytes bearing radiolabelled C3b were treated either with factor I or with factor I and factor H, and conversion to iC3b was assessed by autoradiography. Factor I caused a concentration-dependent conversion of C3b to iC3b on sheep erythrocytes; however, only trace amounts of C3b on rabbit erythrocytes were degraded even when high concentrations of factor I (83 micrograms/ml) were used. While C3b on rabbit erythrocytes was converted to iC3b by the combination of factor H and factor I, much less factor H was required for the same degree of conversion of C3b on sheep erythrocytes. Treatment of sheep erythrocytes with neuraminidase had no effect on either factor I-dependent cofactor activity or the capacity of factor H to serve as a factor I cofactor. Sheep erythrocytes did not manifest decay accelerating activity, however, suggesting that the factor I cofactor constituent is a functional analogue of the human membrane cofactor protein.

In vitro biosynthesis of complement factor I by human endothelial cells

We have studied the secretion of the complement regulatory protein factor I by human umbilical vein endothelial cells (HUVEC). Northern and Western blot analysis and biosynthetic labeling experiments indicate that HUVEC secrete factor I at very low levels in basal conditions and that this secretion is significantly enhanced by interferon-gamma. Analysis of the proteolytic inactivation of C3b by HUVEC supernatants show that factor I is secreted in a functional form and can promote the specific proteolytic inactivation of C3b to iC3b. Together with previous studies establishing the secretion of complement factor H by HUVEC, this work demonstrates that the endothelial cell is able to secrete in its environment two complement regulatory proteins, factor I and factor H, which can mediate the degradation of C3b to iC3b. The secretion of factor I by HUVEC provides a useful in vitro model to analyze the modulation of this secretion and may be relevant to the local deposition of iC3b at the surface of the endothelium during the inflammatory reaction.

Preferential inactivation of the C5 convertase of the alternative complement pathway by factor I and membrane cofactor protein (MCP)

Human C3b bound to the ghost of sheep erythrocytes (E*) via activation of the alternative complement pathway (E*AC3b) consists of four major constituents on SDS-PAGE of 350, 260, 210 and 180 kDa. 350 kDa C3b is a dimeric form of C3b in which the alpha' chain of one C3b binds covalently to that of the other C3b. This complex is presumed to serve as a core for the alternative pathway C5 convertase. The other C3b populations are monomers complexed with membrane proteins or sugars. Using E*AC3b (C3b labeled) as a substrate, we have investigated functional properties of membrane cofactor protein (MCP), which is an integral membrane protein with C3b-binding and factor I-dependent cofactor activities. In conjunction with factor I, MCP was found to degrade the protein-bound C3b preferentially including the 350 kDa dimer. There was a similar but lesser tendency of this selective cleavage of C3b-dimer by CR1 but not by factor H or C4bp. In contrast to CR1 and factor H, detergent solubilization of EAC3b was required for MCP to fully express its cofactor activity for this selective degradation of C3b. We next separated the C3b dimer from the monomers and assessed their ability to assemble the alternative C5 convertase. The C3b dimer but not the monomers expressed C5 convertase activity following the addition of factors B and D, C5 and Ni2+. Kinetic analysis of the degradation of the C3b dimer by MCP and factor I suggested that only one C3b was efficiently converted to C3bi and this occurred concomitant with a decrease in C5 convertase activity. These results suggest that MCP has the ability to more efficiently interact with protein-bound C3b and that this may relate as well to its preferential ability to irreversibly inactivate the C5 convertase.

The mouse factor H allotypes with multiple amino acid replacement, H.1 and H.2 show indistinguishable co-factor activity for factor I

The authors report the functional analysis of the purified mouse factor H allotypes H.1 and H.2, which were clearly distinguished from each other by an immunodiffusion test. Both allotypes acted as a co-factor for factor I in cleaving mouse C3b and we found no significant difference between their activities. The results strongly suggest that the function of mouse factor H for the co-factor activity has been well conserved between two allotypes.

Regulatory system of guinea-pig complement C3b: tests for compatibility of guinea-pig factors H and I with human factors

Two proteins that are involved in cleavage of methylamine-treated C3 of guinea-pig origin (C3(MA)gp) have been isolated from guinea-pig serum. One of them functioned as a cofactor of human factor I (Ihu) for cleavage of C3(MA)gp and its molecular size was 150 kDa. The other was functionally pure and able to cleave C3(MA)gp together with human factor H (Hhu). They appear to be analogous to human factors H and I in the guinea-pig and will be referred to as Hgp and Igp. Methylamine-treated human C3 [C3(MA)hu] was not a compatible substrate for Hgp or Igp: little cleavage of C3(MA)hu was observed if human factor H (Hhu) or I was substituted with the guinea-pig counterpart. C3(MA)gp, on the other hand, served as a substrate, though less efficiently, for Hhu and Ihu. Human C4b-binding protein (C4bp) and membrane cofactor protein (MCP) as well as Hhu could participate in cleavage of C3(MA)gp by Igp or Ihu. In these assays, C3(MA)gp was degraded again less efficiently than C3(MA)hu. Interestingly, human C3b/C4b receptor (CR1) mediated factor I-dependent cleavage of C3(MA)hu and C3(MA)gp to a similar extent regardless the sources of factor I. These results suggest that factor I-dependent C3b regulatory system is species-specific except in the case of CR1, which may function as a cofactor irrespective of species.

A mechanism of activation of the alternative complement pathway by the classical pathway: protection of C3b from inactivation by covalent attachment to C4b

In this work we studied the role of the classical pathway complement component C4b in the activation of the alternative pathway. It was found that nascent C3b attaches with high efficiency to C4b and that C3b in C4bC3b complexes is protected from inactivation by factors H and I. Activation of C3 by factors B and D in the presence of Mg2+ ions and excess C4b led to 35% incorporation of nascent C3b into C4bC3b complexes in the fluid phase. In comparison, when human IgG was tested as an acceptor under similar conditions, only 12% of generated C3b was incorporated into IgGC3b complexes. The half-life time of dissociation of C3b from purified C4bC3b complexes was approximately 2.3 h at 37 degrees C. C4b in these complexes protected C3b from inactivation as effectively as any known alternative pathway activator. Thus, C3b bound to C4b was tenfold more stable than free C3b or C3b bound to a nonactivating surface. In comparison, the protection provided by attachment to human IgG was only 67% of that of C4b. The results provide an explanation for observations of alternative pathway recruitment following classical pathway activation and for the stability of the classical pathway C5 convertase on surfaces which do not provide protection for C3b from factors H and I.

Complement alternative pathway activation and control on membranes of human lymphoid B cell lines

Membrane regulatory molecules normally prevent complement activation by autologous cells, therefore we compared the membrane control system of human lymphoid cell lines which activate or not human complement through the alternative pathway (AP). Membrane expression of decay-accelerating factor (DAF), membrane cofactor protein (MCP), complement receptors (CR)1, CR2 and H was measured either by radioimmunoassay or enzyme-linked immunosorbent assay on cell lysates. Soluble extracts of isolated membranes were tested functionally for their ability to accelerate the decay of C3bBb C3-convertase and allow the cleavage of C3b by factor I. Both regulatory functions were detected in solubilized membranes of Ramos cells, which do not activate the AP, as well as on the potent AP activator, Raji. Raji cells were found to express CR2, DAF and MCP molecules, while MCP was the only known regulatory protein detected on Ramos cells which expressed neither CR1, nor CR2, H or DAF. The I-cofactor activity of both Raji and Ramos cells was immunoprecipitated by anti-MCP, but the decay-accelerating activity was not adsorbed by anti-DAF nor by any of the available antibodies. Two EBV genome-negative cell lines (BJAB, BL41) were tested before and after in vitro conversion by EBV. As previously shown, EBV-converted cell lines activate the AP more efficiently than EBV- cell lines. At the same time, EBV superinfection induces an increase of both AP regulatory functions of cell membranes and enhances the expression of DAF, MCP and CR2. The results of this study show that complement activation by lymphoid cell lines is not related to an impaired autologous control of these cells, but that the expression of regulatory molecules increases together with the appearance of activating structures on the cell surface. Our results also suggest the occurrence of a new factor involved in the decay-accelerating activity on BL lines.

Expression of complement alternative pathway proteins by endothelial cells. Differential regulation by interleukin 1 and glucocorticoids

We have studied the secretion of proteins of the alternative pathway of complement C3, factor B and factor H by human umbilical vein endothelial cells (HUVEC). Results showed that factor H and factor B are quantitatively secreted in abundance whereas C3 could only be detected when the cells are maintained in culture during long periods of time. Interferon-gamma stimulated factor H, factor B and, to a lesser extent, C3 secretions. Interleukin (IL) 1 had a differential effect on spontaneous C3, factor B and factor H secretions. In the presence of IL 1, there was a significant secretion of C3 occurring within a short period of culture. IL 1 also stimulated factor B secretion. There was a synergistic stimulating effect between IL 1 and interferon-gamma to bring C3 and factor B productions by HUVEC to very high levels. In contrast, factor H secretion was consistently inhibited by IL 1. Local increase in C3 and factor B secretions by endothelial cells in the presence of IL 1 may have important implications in the inflammatory reaction. In striking contrast, the glucocorticoid dexamethasone (DXM) had modulatory effects which are consistent with its anti-inflammatory properties. DXM, at therapeutic concentrations, decreased C3 and factor B secretions and increased factor H secretion. Local modulation of complement protein secretion by DXM appears to be a new mechanism by which this glucocorticoid may control inflammation.

Regulatory system of guinea-pig complement C3b: two factor I-cofactor proteins on guinea-pig peritoneal granulocytes

Complement factor I is a plasma protease serving for proteolytic inactivation of C3b together with its cofactor. We have identified two factor I-cofactor activities in solubilized extracts of guinea-pig peritoneal granulocytes using guinea-pig factor I (Igp) and fluorescent-labeled methylamine-treated guinea-pig C3 (f-C3(MA)gp). One of these eluted from a chromatofocusing column between pH 7.6-7.1, and the other at about pH 5.7. These two cofactor fractions both interacted with Igp and, to a lesser degree, with human factor I (Ihu) on C3(MA)gp cleaving it into an inactive C3bi analogue, but did not cleave methylamine-treated human C3 (C3(MA)hu) together with Igp or Ihu. These factors are therefore species specific. The neutral and acidic fractions with cofactor activity contained C3(MA)gp-binding proteins with a doublet of 55 kDa and 42 kDa, and a singlet of 160 kDa, respectively, on SDS-PAGE. These proteins may be membrane cofactor protein (MCP) and C3b/C4b receptor (CR1) of guinea-pigs.

Inhibition of factor I by diisopropylfluorophosphate. Evidence of conformational changes in factor I induced by C3b and additional studies on the specificity of factor I

The factor I-mediated cleavage of C3b, using factor H as a cofactor was completely inhibited by diisopropylfluorophosphate (DFP) when factor I and C3b were incubated with DFP before the addition of factor H. Inhibition, although to a lesser degree, was observed when factor H was present during DFP-exposure. No inhibition in factor I activity was seen when factor I and H were incubated with DFP either alone or together. It was also demonstrated that the 38-kDa subunit of factor I bound radiolabeled DFP when factor I and C3b together were exposed to DFP. These observations suggest that factor I interacts with C3b in a manner that exposes its catalytic site to DFP, an interaction that is independent of factor H. The inhibitory effect by DFP on factor I led us to further investigate the factor I cleavage products of iC3b, inasmuch as previous reports were ambiguous as to whether digestion occurs in the presence of DFP. Digestion of C3b bound to activated thiol Sepharose (ATS-C3b) in the presence of factor H at low pH and ionic strength and in serum by complement activation produced C3d,g-like fragments with apparent molecular mass of 41 and 43 kDa. These fragments were shown to have three different N-terminal and two different C-terminal ends. The major fragments had N-terminal sequences starting with Glu933, as shown by sequence determination. Traces of fragments extending beyond this point were also found, shown by Western blot analysis using a panel of mAb previously shown to bind to epitopes exposed within a region of C3 spanning residues 929 to 943, as well as a shorter fragment starting with Glu938. When digestion of C3b is carried out in the presence of DFP, the factor I level necessary for digestion is elevated and may explain how the first two cleavages producing iC3b but not the following giving C3d,g, can occur. The finding of several factor I cleavage sites in the C3d,g region of C3 demonstrates that factor I has a broad specificity, mainly for arginyl bonds. It has also been shown to digest a lysyl bond exposed in ATS-bound C3b.

Functional consequences of the genetic polymorphism of the third component of complement

The third component of complement, the central protein of the complement cascade, occurs in two principal allotypes, C3S and C3F. An excess frequency of the F allotype has been implicated in a number of disease states, including some forms of glomerulonephritis. These associations have been explained by functional differences between C3S and C3F. We examined several complement functions, using purified preparations of C3S or C3F. The C3S allotype was 1.3 times more efficient than C3F in a hemolytic assay employing sensitized sheep erythrocytes; this difference was shown to arise from a slightly more efficient deposition of C3F on the cell surface. These differences are trivial and of much less magnitude than the functional differences between C4A and C4B. There were no differences between allotypes in their ability to be converted to inactive C3b (C3bi) by complement factors H and I or by CR1 and factor I. No significant differences were seen between the allotypes and their ability to support solubilization of preformed immune complexes.

Regulation of proteolytic activity of complement factor I by pH: C3b/C4b receptor (CR1) and membrane cofactor protein (MCP) have different pH optima for factor I-mediated cleavage of C3b

C3b/C4b receptor (CR1) and membrane cofactor protein (MCP) are integral membrane glycoproteins with factor I-dependent cofactor activity. They bind to C3b, allowing factor I to cleave C3b at two sites (first and second cleavage), which results in the generation of C3bi, a hemolytically inactive form which is a ligand for complement receptor type three (CR3). C3bi is further degraded by factor I and CR1 (third cleavage) to C3dg (a ligand for complement receptor type two, CR2) and C3c. Using two different substrates, fluid-phase C3b and cell-bound C3b, the cleavage of C3b by MCP and factor I was compared to that by CR1 and factor I under various conditions. The optimal pH for the first and second cleavage of either substrate was 6.0 for MCP and 7.5 for CR1. The third cleavage was mediated only by CR1 and factor I, the optimal pH being 8.0. Low ionic conditions enhanced the C3b binding and cofactor activity of both CR1 and MCP. The efficiency of binding C3b to CR1 or MCP was maximal at pH 6.2. The isoelectric point (pI) of MCP was acidic (approximately 4.0), while that of CR1 was 6.8. Therefore, compared to CR1, MCP possesses distinct functional profiles relative to C3b-binding and factor I-cofactor activity.

Metabolism of C3 and factor B in patients with congenital factor I deficiency

The metabolism of complement factor B and C3 was analyzed in three previously described patients with congenital deficiency of factor I (C3b/C4b inactivator). Samples taken at steady state contained elevated levels of Ba and Bb, whereas native factor B was not detectable. Following plasma infusion in two of the patients Ba was rapidly cleared (within 8-12 hr) from the circulation and native factor B increased transiently, reaching normal levels within 22-24 hr. In parallel with the increase in Ba, factor B decreased during the following days, reaching preinfusion level after seven days. This was in contrast to a continued increase in C3 concentration, which was still within the normal range 15 days after infusion, despite the presence of only trace amounts of native B. In vitro complement activation experiments, employing purified C3Nef IgG as alternative pathway activator and aggregated IgG as classical pathway activator, were performed on selected serum samples (base-line, 12 hr and 15 days postinfusion samples) from the plasma infusion series. It was demonstrated that (a) C3Nef could not induce alternative pathway C3-conversion in factor B depleted samples and (b) C3-degradation by CP-activation could still occur in B-depleted samples, although at a much slower rate compared to normal human serum. These results may partly explain the difference in kinetics of factor B and C3 metabolism seen after plasma infusion in patients with factor I deficiency.(ABSTRACT TRUNCATED AT 250 WORDS)

Pasteurisation of a factor I (C3b inactivator) concentrate from human plasma

A method of pasteurising a concentrate of factor I (C3b inactivator) prepared from human blood plasma is described. As well as obtaining a high yield of factor I by this virus inactivation procedure it is furthermore possible, by adjustment of the stabiliser concentration, to obtain a very pronounced reduction in the concentration of factor B, a component of the complement system which is unwanted in the treatment of factor I deficiency. The product so produced is in two respects superior to plasma infusions for factor I replacement therapy: it has been submitted to virus inactivation and factor B depletion.

Factor C3f is a spasmogenic fragment released from C3b by factors I and H: the heptadeca-peptide C3f was synthesized and characterized

C3f, a heptadeca-peptide having the amino acid sequence of NH2-Ser-Ser-Lys-Ile-Thr-His-Arg-Ile-His-Trp-Glu-Ser-Ala-Ser-Leu-Leu-Arg- COOH, is liberated during the catabolic degradation of C3b in serum. The amino acid sequence of C3f is known both from the cDNA-derived structure of C3 and from protein analysis after isolation of the natural factor. C3f was synthesized by solid phase peptide synthesis. Both natural and synthetic C3f had identical retention times by RP-18 high performance liquid chromatography (HPLC) analysis and the respective amino acid compositions agreed with the expected theoretical values. C3f, but not des-Arg-C3f, was weakly spasmogenic inducing contraction of guinea pig ileum at a level of 5-10 x 10(-6) M. Since C3f and C3a were cross-tachyphylactic, it was concluded that these two spasmogens compete for the same receptors. Both C3f and des-Arg-C3f at concns of 1-4 x 10(-4) M enhanced vascular permeability in guinea pig skin. These observations further suggest that C3f functionally resembles C3a anaphylatoxin. Formation of C3f in human serum following CVF activation of C3 could be demonstrated by radioimmunoassay (RIA). Digestion of C3f with purified human serum carboxypeptidase N produced C3f-desArg. These observations suggest that when serum complement protein C3 undergoes conversion to C3b, further degradation by Factors H and I readily generates C3f. C3f is a weak spasmogen that functions like C3a anaphylatoxin and C3f-desArg is a major metabolite in serum.

A new polymorphic variant of human complement factor I

Sera from 305 individuals were typed for factor I, and a new variant, tentatively designated FI C, was found. All other samples were FI B except for seven samples from Chinese that were of the FI AB phenotype. This suggests that polymorphism of factor I may be rare in Caucasians.

Mapping of the human complement factor I gene to 4q25

A detailed genetic and physical map of human complement factor I (IF) using somatic cell hybrids, in situ hybridization, and genetic linkage is reported. The gene has been localized to band 4q25. The order GC-INP10-ADH3-EGF-IF-IL2-MNS is proposed for genes on 4q on the basis of genetic and physical mapping techniques. A BclI polymorphism found with the IF probe demonstrated a maternal origin for a de novo deletion of chromosome 4 that was used in physically mapping the gene. The genetic and physical distances around band 4q24 suggest that 1 cM is approximately 1.2 million bp of DNA. This work provides a useful addition to the map of 4q.

Epstein-Barr virus regulates activation and processing of the third component of complement

Serum incubated with purified EBV was found to contain C3 cleavage fragments characteristic of C3c. Since the cofactors necessary for such cleavage of C3b by factor I are not normally present in serum, EBV was tested for factor I cofactor activity. Purified EBV from both human and marmoset EBV-producing cell lines was found to act as a cofactor for the factor I-mediated breakdown C3b to iC3b and iC3b to C3c and C3dg. EBV also acted as a cofactor for the factor I-mediated cleavage of C4b to iC4b and iC4b to C4c and C4d. EBV from both the human and marmoset cell lines accelerated the decay of the alternative pathway C3 convertase. The classical pathway C3 convertase was unaffected. Multiple lines of evidence eliminated the possibility that marmoset or human CR1 was responsible for the functional activities of EBV preparations. The spectrum of activities was different from CR1 in that EBV and EBV-expressing cell lines failed to rosette with C3b or particles bearing C3b, the primary functional assay for CR1, and EBV did not accelerate classical pathway C3 convertase decay, another property of CR1. In addition, CR1 could not be detected immunologically on marmoset or human EBV-expressing cells and mAbs to CR1 failed to alter EBV-produced decay acceleration and factor I cofactor activities, although the antibodies blocked the same CR1-dependent functional activities. The multiple complement regulatory activities exhibited by purified EBV derived from human and marmoset cells differ from those of any of the known C3 or C4 regulatory proteins. These various activities would be anticipated to provide survival value for the virus by subverting complement- and cell-dependent host defense mechanisms.

Human complement factor I: analysis of cDNA-derived primary structure and assignment of its gene to chromosome 4

Factor I is a serine proteinase of complement which together with one of several specific cofactors cleaves activation products of the third and fourth components of complement (C3b and C4b) and modulates the activity of C3 convertase. A heterodimer glycoprotein (Mr = 88,000), factor I is synthesized as a single-chain precursor, prepro-I, which undergoes intracellular proteolytic processing. The human hepatoma line HepG2, however, secretes predominantly the single-chain precursor pro-I. In order to determine the molecular basis for this apparent processing defect, factor I cDNA clones were isolated from a HepG2 mRNA-derived library. Sequencing of the largest insert, HI1971, revealed that it contains 14 base pairs of 5' untranslated region, the complete coding sequence for the 583-residue prepro-I (NH2-signal peptide-heavy chain-linking peptide-light chain-COOH), two polyadenylation signals within the 200-base pair 3' untranslated region, and a portion of poly(A) tail. Analysis of the derived protein structure 1) reveals a mosaic multidomain structure of the heavy chain; 2) demonstrates structural similarity between intracellular conversion of pro-I and activation of other serine proteinase zymogens; and 3) indicates that the light chain of factor I resembles most closely the active subunit of tissue plasminogen activator among all serine proteinases and factor D among complement proteinases. Furthermore, this protein sequence was compared to the sequences of factor I cDNA clones isolated from normal human liver libraries and found to be identical. By exclusion, this defines as cellular the basis for the inefficient processing of pro-I by the HepG2 line. Chromosomal localization by the somatic cell hybrid method maps the factor I gene to chromosome 4.

Regulation of the alternative pathway of complement by pH

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired hemolytic anemia. The abnormal PNH erythrocytes are highly susceptible to complement-mediated lysis in vitro, especially at pH 6.4. Lysis has been shown to be due to alternative pathway activation. The purpose of this study was to determine why lysis of PNH erythrocytes is increased at acidic pH. The results presented demonstrate that at pH 6.4: binding of C5 and Factor B to C3b deposited on human erythrocytes is markedly enhanced; generation of the two C3 convertases, C3(H2O), Bb and C3b,Bb is increased; and control of C3b on human erythrocytes by CR1 and Factor I is diminished. In addition, it was found that rabbit erythrocytes, which activate the human alternative pathway, are also lysed much better at pH 6.4 than at pH 7.4. These results indicate that the optimal pH for the initiation and amplification of the alternative complement pathway, and probably also for the activation of the membrane attack complex, is 6.4.

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.

Characterization of primary amino acid sequence of human complement control protein factor I from an analysis of cDNA clones

A cDNA clone of the mRNA coding for the human complement system control protein Factor I has been isolated. The predicted amino acid sequence obtained from the DNA sequence demonstrates a protein consisting of a heavy chain (Mr 35,400) linked to a light chain (Mr 27,600), both of which contain three sites for N-linked glycosylation. The light chain has clear homology with other serine proteinases, most notably in the region of the catalytically active and structurally important amino acids and shares some of the features characteristic of the plasminogen activators. The heavy chain has a clear 'mosaic' nature typical of the plasma serine proteinases; in particular it contains class A and class B LDL (low-density lipoprotein) receptor repeats with conserved cysteine residues similar to those found in other complement proteins.

Anti-idiotypic antibodies in antisera against human C3 and factor H and their application in the enrichment of antibodies specific for H-binding domains of C3

Antisera separately raised against C3 and factor H should contain antibodies against the binding domains by which these factors interact. Because these interacting domains are likely to be sterically complementary to each other, antibodies specific for these domains should also be sterically complementary or of anti-idiotypic specificity. To test this hypothesis, rabbit anti-human C3 IgG antibodies which bound to Sepharose-coupled rabbit anti-factor H IgG (anti-H-binding anti-C3) were separated by affinity chromatography. In a control experiment, the anti-human factor H column was found to bind 10 times more anti-C3 antibodies than a Sepharose column coupled with nonimmune rabbit IgG. The binding specificity of the control eluate was indistinguishable from the original anti-C3 preparation, whereas the anti-H-binding anti-C3 was shown to be enriched in regard to antibodies directed against the 42 kd fragment of the C3 alpha-chain of C3c, as demonstrated by immunoblotting of electrophoretically separated polypeptides of C3c and C3d in polyacrylamide gels. In competition binding studies, factor H was 25 times more potent as inhibitor of the anti-H-binding anti-C3 than of the original anti-C3 preparation. This similarity in binding specificity also paralleled a functional similarity in that anti-H-binding anti-C3 in the presence of factor I effected a 20% degradation of the alpha'-chain of C3b into fragments of 65, 45, and 42 kd which are the normal degradation products in the presence of factors I and H. These results suggest that the affinity procedure described resulted in the enrichment of anti-C3 antibodies with selective specificities for factor H-binding domains of C3, from a polyclonal antibody source.

Glycoprotein C of herpes simplex virus 1 is an inhibitor of the complement cascade

Mammalian cells in culture express membrane receptors for C3b when infected with HSV-1. C3b binding is mediated by glycoprotein C (gC), a virus-specified membrane glycoprotein. In view of the inhibitory functions of other C3b-binding proteins, we studied the capacity of gC to modulate complement activation. Glycoprotein C was purified from HSV-1-infected cells by immunoaffinity chromatography. Glycoprotein C, but not a control viral glycoprotein, demonstrated dose-dependent acceleration of decay of C3bBb sites. In addition, gC produced a dose-dependent, time-independent depression of the overall hemolytic efficiency of C3bBb sites. Inhibition of C5b6-initiated reactive lysis of cells bearing C3b, but not cells bearing antibody alone, by gC suggests that the second effect represents interference with the C3b-C5/5b interaction. This hypothesis is supported by the failure of gC to inhibit reactive lysis when added after C5b67 insertion into target cells. Glycoprotein C does not accelerate C14b2a decay, nor does it impair classical pathway hemolytic efficiency when excess C5 is present. By limiting available C5/5b, some gC inhibition of C3b-C5/5b interactions can be unmasked in the classical pathway system. Glycoprotein C is devoid of factor I co-factor activity. HSV-1 gC is a modulator of complement activation, especially via the alternative pathway, and may represent a novel viral mechanism for evading host defense processes.

Mechanism of resistance to lysis by the alternative complement pathway in Trypanosoma cruzi trypomastigotes: effect of specific monoclonal antibody

Trypanosoma cruzi G strain epimastigotes were lysed by normal human serum (NHS) through activation of the alternative complement pathway (ACP), whereas metacyclic trypomastigotes were resistant to lysis. Epimastigotes and metacyclics with equivalent amounts of C3b deposited on their surface bound factor B with similar affinities. In contrast, factor H bound with higher affinity to metacyclics than to epimastigotes. Both T. cruzi forms with bound C3b were extensively (60 to 80%) lysed after formation of surface C3-convertase and the addition of a C3-C9 complement source. In the presence of factors H and I, or incubation with NHS with EDTA, the percentage of lysis of metacyclics decreased faster than that of epimastigotes with increasing incubation times. These data suggest, as a possible mechanism of resistance to lysis in metacyclic trypomastigotes, the higher binding affinity of factor H to C3b and the inactivation of the latter by serum regulatory proteins. Metacyclics were lysed by NHS, through ACP, in the presence of human immune serum to T. cruzi or anti-T. cruzi monoclonal antibody, but not with the Fab fragment of the latter, which recognizes a 90,000 m.w. antigen from T. cruzi metacyclics. Protection of parasite-bound C3b from serum control proteins was observed when parasites were incubated, before C3 deposition, with the lytic monoclonal antibody but not with its Fab fragment or a nonrelated IgG control. When C3b was deposited on metacyclics before antibody binding, C3b inactivation occurred. In the lysis of metacyclics, through ACP activation, binding of antibody apparently creates new acceptor sites which prevent the activity of serum regulatory proteins.

Purification of the B lymphocyte receptor for the C3d fragment of complement and the Epstein-Barr virus by monoclonal antibody affinity chromatography, and assessment of its functional capacities

The human C3d receptor (complement receptor type 2, CR2), that also serves as the B lymphocyte receptor for the Epstein-Barr virus, was purified from detergent lysates from the B lymphoblastoid cell lines, SB and Raji, by monoclonal antibody affinity chromatography using the anti-CR2 monoclonal antibody, HB-5. Relative to the concentration of cellular protein and receptor that was initially solubilized by detergent, the procedure provided a 37,000-fold purification with a 40-50% recovery of CR2. The purified receptor presented a single Coomassie blue-stained band when analyzed by SDS-PAGE, and it retained its function of binding to C3-Sepharose. The N-terminus of CR2 was blocked. The amino acid composition was significantly similar to that of the C3b/C4b receptor, factor H and C4 binding protein, suggesting that CR2 may be a member of this newly defined protein family. However, CR2 did not exhibit the regulatory functions of these proteins, namely, the decay dissociation of the classical or alternative pathway C3 convertases and serving as a cofactor for the cleavage of C3b.

Release of immune complexes bound to erythrocyte complement receptor (CR1), with particular reference to the role of factor I

The release of 125I-bovine serum albumin (BSA)-anti-BSA immune complexes (IC) bound to human erythrocyte complement receptors (E-CR1) was studied. IC were complement-solubilized in normal human serum (NHS), and reacted with human erythrocytes at conditions optimal for binding of the IC to E-CR1. E-CR1-bound IC could be released by the addition of NHS or purified factor I. Factor I-deficient or I-depleted serum mediated no release, and addition of purified factor I restored the release. Factor H was not required for the release of IC. The kinetics of IC release was influenced by the NHS concentration, the presence of EDTA, and the time of prior storage of the erythrocytes at 4 degrees C. NHS (1:5 to 1:10) in the presence of EDTA caused nearly maximal release within 10-20 min at 37 degrees C. In the absence of EDTA the NHS-induced IC release was markedly slower. IC released within the first 30 min showed significant rebinding to new E. The release of IC was not associated with loss of the IC binding activity of E-CR1. The NHS-mediated release of IC could be inhibited by rabbit anti-CR1 and by a mixture of protease inhibitors. Release induced by purified factor I was also inhibited by protease inhibitors. The affinity of IC binding to E-CR1 was reduced after cleavage of CR1-bound C3b-IC to iC3b-IC by factor I.

CNBr cleavage of the light chain of human complement factor I and alignment of the fragments

The light chain and heavy chain of reduced and alkylated human complement Factor I were purified by high-pressure gel-permeation chromatography. CNBr cleavage of Factor I light chain yielded four major fragments, which were purified by gel filtration. N-Terminal sequence analysis of the CNBr-cleavage fragments allowed identification of 200 of the approx. 240 amino acid residues of the light chain. An alignment is proposed, based on sequence analysis of peptides obtained after cleavage at arginine residues of the light chain and on homology of the sequence determined with that of other serine proteinases. The sequence around the active-site serine residue was established and three potential attachment sites for carbohydrate moieties were identified.

Serum profiles of the regulatory complement proteins during the progression of renal damage in human glomerulonephritis

Persistent activation of the complement system after escape from control by the regulatory proteins results in an excessive release of split products with inflammatory properties. Serum levels of the regulatory proteins of the complement system (Cl-INH, H, I and AT III) were assayed by the radial immunodiffusion technique in 521 serum samples from 124 patients with idiopathic chronic glomerulonephritis divided into two groups: 81 cases with normal renal function (NRF), and 43 cases with deteriorated renal function (DRF). Significant high mean levels of Cl-INH, H, I and AT III were found in patients with NRF, when they were compared with DRF patient groups. Positive correlations between the regulatory proteins and the corresponding complement components were found in patients with NRF, whereas they were reduced considerably in patients with DRF. Serial measurement of the control proteins showed a decay of the levels of Cl-INH, H and I in patients with progressive impairment of renal function. The presence of high levels of regulatory proteins suggest that a control mechanism may occur in patients with NRF; measurement of these proteins could, therefore, be of prognostic value, as the presence of high levels are indicative of a block of the complement system.

Factor I co-factor activity of CR1 overcomes the protective effect of IgG on covalently bound C3b residues

We have shown previously that C3b resides in a protected site when it is covalently bound to IgG (C3b-IgG). Such C3b displays a reduced affinity for factor H, with consequent enhanced survival in the presence of factors H and I and increased capacity for promoting alternative pathway consumption of C3. Because erythrocyte CR1 may be a major co-factor for factor I-mediated inactivation of immune complex-borne C3b in blood, we have examined the effect of covalently bound IgG on the C3b-CR1 interaction. Binding of monomeric C3b and C3b-IgG to human erythrocyte CR1 demonstrates identical ionic strength dependence for both species. Identical numbers of binding sites with indistinguishable affinities are detected by both ligands. Cleavage of the alpha'-chain of C3b and the alpha'-heavy chain of C3b-IgG proceeds at the same rate when erythrocyte CR1 serves as co-factor for factor I. Unlike factor H, CR1 supports a second cleavage of fluid-phase iC3b alpha'1 chain (free or bound to IgG) that generates C3c and a 33,000 m.w. fragment, which bears antigenic markers characteristic of C3g. Inactivation of C3b and C3b-IgG by CR1 and factor I also occurs at physiologic ionic strength, but proceeds very slowly relative to rates attainable with sub-physiologic inputs of factor H. CR1 does not recognize IgG-bound C3b as being in a protected site but, because of low binding affinity at physiologic ionic strength, is probably highly dependent on multivalent ligand-receptor interactions to efficiently exert its co-factor functions. Thus, inactivation of C3b-IgG heterodimers or small immune complexes bearing limited numbers of C3b residues may remain largely factor H-dependent in vivo, with resultant enhanced C3b survival.

Regulation and deregulation of the fluid-phase classical pathway C3 convertase

Three mechanisms that regulate the formation and function of the fluid-phase classical pathway C3 convertase (C4b2a) have been elucidated: a) a temperature-mediated intrinsic decay of the enzyme; b) an extrinsic accelerated decay mediated by the effect of the serum protein C4b-binding protein (C4-bp); and c) the inactivation of C4b in the C4b-C4b-p complex by the proteolytic action of C3b/C4b inactivator (I), which cleaves the alpha 1-chain of C4b yielding C4d (alpha 2-chain), and C4c (alpha 3-, alpha 4-, beta-, gamma-chains). A fourth mechanism is described based on the observation that the IgG fraction of the serum of certain patients with glomerulonephritis contains a protein that prevents the intrinsic and C4-bp-mediated decay of surface-bound C4b2a. This protein prolongs the half-life of fluid-phase C4b2a from 10 min to more than 5 hr, increasing the utilization of C3. It also inhibits the decay mediated by C4-bp by preventing the dissociation of C2a from the C4b, 2a complex. In addition, I alone or in the presence of C4-bp fails to cleave the alpha 1-chain of C4b in the stabilized C4b, 2a complex. This protective property of the stabilizing factor (NFc) requires the presence of C2a because C4b was not protected unless it was bound to C2a. Therefore, NFc provides a mechanism by which the serum regulatory proteins are bypassed.

Characterization of a soluble form of the C3b/C4b receptor (CR1) in human plasma

A radioimmunoassay with the use of soluble 125I-Fab monoclonal anti-CR1 and rabbit IgG anti-CR1 bound to Staphylococcus aureus particles was employed to detect and quantitate CR1 antigen in human plasma. Among 16 normal individuals the concentration of soluble CR1 in plasma ranged from 13 to 81 ng/ml, and a similar range of concentration was found in plasma from 15 patients having systemic lupus erythematosus (SLE). The amount of plasma CR1 in normal donors, but not in SLE patients, significantly correlated with the number of CR1 sites on erythrocytes (r = 0.90, p less than 0.001), and was 7.1% of the amount of receptor that was present on erythrocytes in blood. The concentration of soluble CR1 was not diminished by ultracentrifugation or ultrafiltration of plasma, was not affected by various modes of anti-coagulation or even by clotting of blood, and did not change during incubation of blood at 4 degrees C for up to 4 hr. On sucrose density gradient ultracentrifugation of plasma the CR1 was distributed as a broad peak that overlapped the plasma protein profile. The Mr of plasma CR1 was identical to that of erythrocyte CR1 when assessed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate and immunoblotting. In addition, the plasma form of CR1 exhibited the same structural phenotype as did receptor from erythrocytes of the same individual. CR1 antigen purified from plasma was as active as CR1 from erythrocytes in promoting the cleavage by factor I of C3b to iC3b, C3c, and C3dg. Therefore, a functionally and structurally intact form of soluble CR1 resides in plasma.

Prevention of immune precipitation by purified components of the alternative pathway

The role of the alternative pathway in complement-mediated prevention of immune precipitation has been investigated by the use of BSA-anti-BSA immune complex (IC) purified components. For immune precipitation to be prevented all six alternative pathway components (C3, factors B D, P, H and I) were required. In the absence of one or both of the control proteins H and I, excessive fluid phase turnover of C3 occurred with precipitation of IC. Kinetic studies showed that in the presence of the control proteins, an initial phase of precipitation occurred, and was followed by a phase of resolubilization of IC. When the efficiency of classical and alternative pathways in the prevention of immune precipitation was compared it was found that the classical pathway proteins were more effective than the alternative pathway components. A reaction mixture containing the components of both pathways was no better than the classical pathway protein alone. 125I-C3 was bound to IC which had been rendered soluble in the presence of classical or alternative pathway components. A molar ratio of two molecules C3b:five molecules IgG was calculated. Other complement components which were bound to IC which had been formed in the presence of serum were C1q, C4, C2, C3, C5, P and H. Factors B and I were not detected. Our findings suggest that the alternative pathway is of secondary importance to the classical pathway in the prevention of immune precipitation.

Regulatory proteins for the activated third and fourth components of complement (C3b and C4b) in mice. II. Identification and properties of complement receptor type 1 (CR1)

We identified on the membrane of mouse spleen cells a polypeptide of Mr 190,000 (S190), with binding affinity for the mouse third component of the complement system (C3). S190, purified by affinity chromatography on C3-Sepharose, has properties resembling those of the human C3 receptor type 1 (CR1). Thus, S190, like CR1, served as a cofactor for the C3b inactivator (I)-mediated cleavage of fluid-phase C3b into iC3b, and had cofactor activity comparable to that of serum factor H (H). S190 also acted as a cofactor for the cleavages of membrane-bound C3b or membrane-bound iC3b into C3c (Mr 140,000) and C3dg (Mr 40,000) by serum factor I. As is the case with CR1, the specific activity of S190 for the cleavages leading to C3c-C3dg formation was approximately 100-fold greater than that of H. We therefore conclude that S190 and CR1 are analogous proteins.

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.

Mechanisms of complement activation by crystalline cholesterol

The mechanism by which cholesterol crystals activate complement in human serum has been studied. Crystals treated with serum and washed with buffer contain a fixed C3/C5 convertase. Its generation is dependent on the presence of divalent cations (and of factor B). The cholesterol-fixed convertase is subject to decay and can be regenerated by factors B and D. C2 in combination with C1 is not essential but enhances the convertase formation. These findings indicate that it is predominantly the alternative C3/C5 convertase C3bBb(P) that assembles on cholesterol during exposure to human serum. By the use of different antisera and immunofluorescence a C3 fragment, probably C3b, was demonstrated on serum-treated crystals. Its fixation is resistant to washing with urea, and with buffers of differing pH: by hydroxylaminolysis the C3 fragment dissociates from the crystals. This indicates a covalent ester bond linking the labile binding site of activated C3 to the hydroxyl group of cholesterol. Cholesterol acetate does not fix C3 nor acquire a C3-cleaving activity upon contact with serum. In addition, cholesterol crystals bind factor I (C3b inactivator) and in this way may facilitate fixation and amplification of the alternative C3/C5 convertase.

C3b covalently bound to IgG demonstrates a reduced rate of inactivation by factors H and I

We have prepared C3b covalently linked to IgG via a hydroxylamine-sensitive bond between the C3b alpha' chain and sites predominantly, but not exclusively, located in the IgG heavy chain. This C3b species displays relative resistance to inactivation by factors H and I when compared with free C3b. This resistance appears to be due entirely to reduced affinity of C3b-IgG for factor H. Resistance to inactivation is not conferred on C3b by binding to another serum glycoprotein of similar size, ceruloplasmin, and may be a special property of IgG. C3b-IgG demonstrates an enhanced capacity to consume serum C3 relative to C3b. These alterations of the behavior of C3b when bound to IgG may in part explain the augmentation of alternative pathway activity by IgG. In addition, IgG-induced protection of C3b might influence both complement-mediated killing and phagocytosis of bacteria, as well as modify the in vivo handling of IgG-containing soluble immune complexes.

C-reactive protein inhibits pneumococcal activation of the alternative pathway by increasing the interaction between factor H and C3b

We previously studied two alternative pathway activators, Streptococcus pneumoniae and positively charged liposomes, which react with C-reactive protein (CRP). Binding of CRP to these surfaces initiates classical pathway but blocks alternative pathway activation. In this study, we investigated the mechanism of this inhibition using S. pneumoniae, R36a. R36a were pretreated with CRP (CRP-R36a) or buffer and incubated with C2-deficient human serum to which 125I-labeled C3 had been added. The amount of specific 125I-C3 binding was decreased from 8200 mol/CFU on R36a to 2200 mol/CFU on CRP-R36a. In contrast, when the same experiment was performed with purified factors B, D, P, and C3, in the absence of regulatory proteins, specific 125I-C3 uptake was slightly lower on R36a (6100 mol/CFU) than on CRP-R36a (8100 mol/CFU). The ability of the fixed C3b to inactivate factor B in the presence of factor D was equivalent on the two surfaces. The binding of the regulatory factor H to C3b fixed to R36a and CRP-R36a was compared by using purified 125I-labeled factor H. The ratio of factor H bound to C3 bound was twofold greater on CRP-R36a than on R36a. This increase was found by using C2-deficient serum or purified factors B, D, P, and C3 to fix C3b to the surfaces. The ability of CRP to inhibit C3 binding to R36a was restored by the addition of factors H and I to factors B, D, P, and C3. These results indicate that CRP inhibits alternative pathway activation by increasing regulation of bound C3.

Paroxysmal nocturnal hemoglobinuria: deficiency in factor H-like functions of the abnormal erythrocytes

Erythrocytes from patients with paroxysmal nocturnal hemoglobinuria (PNH) contained a subpopulation that lacked membrane-associated Factor H-like activity present on normal human erythrocytes. Initial deposition of C3b on the erythrocytes was effected using a fluid phase C3 convertase. The cells were then treated with fluorescein-labeled C3 and the cell-bound C3 convertase. Analysis utilizing the fluorescence-activated cell sorter revealed two distinct cell populations, one of which was highly fluorescent, indicating a large number of C3b molecules per cell. Only this population (43%) was susceptible to lysis (44%) when exposed to acidified serum before C3b deposition. The less fluorescent population resembled normal human erythrocytes and was not affected by prior treatment with acidified serum. Since C3b deposition occurred almost exclusively on the complement-sensitive cells in the PNH erythrocyte population, these cells could be examined for the Factor H-like regulatory activities without prior isolation. These functions include enhancement of inactivation of erythrocyte-bound C3b by Factor I and acceleration of the decay of erythrocyte-bound C3 convertase, C3b,Bb. It was found that C3b on PNH erythrocytes was 100-fold less susceptible to inactivation by Factor I than C3b on normal human erythrocytes. The half-life at 22 degrees C of C3b,Bb on PNH erythrocytes was threefold greater than on normal human erythrocytes and similar to that of the enzyme bound to particles that do not possess Factor H-like activity. These observations suggest that the abnormal susceptibility of PNH erythrocytes to lysis by complement is due to a functional deficiency in one or more of the Factor H-like proteins present on normal human erythrocytes.

Degradation of human complement component C4b in the presence of the C4b-binding protein-protein S complex

Vitamin K-dependent protein S and the higher-molecular-weight form of C4b-binding protein (C4bp-high) interact, forming a 1:1 complex with a KD of approx. 1 X 10(-7) M [Dahlbäck (1983) Biochem. J. 209, 847-856]. In the present study the effect of protein S on the degradation of C4b by Factor I (C3b inactivator) and C4bp was investigated both in fluid phase and on cell surfaces, with the use of highly purified components. Fluid-phase degradation of C4b was monitored on sodium dodecyl sulphate/polyacrylamide-slab-gel electrophoresis, and the effect on surface-bound C4b was estimated by haemolytic assay. No effect of protein S could be demonstrated in any of the systems used. Thus, although bound to C4bp, protein S is neither involved in, nor does it affect, the interaction between C4bp and C4b. This indicates that the binding sites on the C4bp molecule for protein S and for C4b are independent and different.