Anti-factor H antibody-positive C3 glomerulonephritis secondary to poststreptococcal acute glomerulonephritis with diabetic nephropathy

Poststreptococcal acute kidney glomerulonephritis (PSAGN) has been seen in adults in recent years, especially in patients with type 2 diabetes mellitus, and the renal prognosis has not always been good. There have been cases of PSAGN in which complete remission was not achieved and hematuria and proteinuria persisted, leading to end-stage renal disease. Previous reports showed that the patients subjected to PSAGN have an underlying defect in regulating the alternative pathway of complement, and they identified that antibodies to the C3 convertase, C3 nephritic factors (C3NeF), are involved. C3NeF stabilizes C3 convertase, sustains C3 activation, and causes C3 glomerulonephritis (C3GN). On the other hand, factor H is a glycoprotein that suppresses the overactivation of the alternative pathway by decaying the C3 convertase. Anti-factor H (aFH) antibodies interfere with factor H and cause the same activation of the alternative pathway as C3NeF. However, a limited number of reports describe the clinical course of C3GN with aFH antibodies. We encountered a 49-year-old Japanese man with type 2 diabetes mellitus. He was referred to our hospital because of his elevated serum creatinine, proteinuria, hematuria, and developed edema on both legs. He was diagnosed as PSAGN at the first kidney biopsy, and his renal function improved and edema and hematuria disappeared, but proteinuria persisted after 5 months. He was diagnosed as C3GN at the second kidney biopsy. In our case, no C3NeF was detected. However, a high titer of aFH antibodies was detected in stored serum from the initial presentation, providing a unified diagnosis of aFH antibody-positive C3GN secondary to PSAGN. He progressed to end-stage renal disease (ESRD) and hemodialysis was started. The persistence of high levels of aFH autoantibodies may have caused C3GN secondary to PSAGN due to activating the alternative complement pathway, which eventually worsened the nephropathy and led to ESRD.

A Nef Precursor or Benign Counterpart: High Prevalence of Healthy Subjects with Antibody Reactivity to the C3-Convertase

There are two anticonvertase antibody populations—Nephritic Factors which cause complement dysregulation and C3CAbs which do not. C3CAbs are highly prevalent in the healthy adult population (>95% prevalence). C3CAbs suggest a direction of study to determine the origin of Nephritic Factors.

The oxygen carrier M101 alleviates complement activation, which may be beneficial for donor organ preservation

During organ transplantation, ischemia/reperfusion injury and pre-formed anti-HLA antibodies are the main cause of delayed graft function and recovery through the activation of the complement system. By supplying oxygen during transplantation, M101 is suspected to avoid complement activation, however, a direct effect exerted by M101 on this pathway is unknown. This was tested by using functional assays (lymphocytotoxic crossmatch test, C3d Luminex-based assay, 50% complement hemolysis [CH50], and 50% alternative complement pathway [AP50/AH50]), and quantitative assays (C3, C3a, C4, C5, C5a, C6, C7, C8, C9 and sC5b-9). M101 interferes with the anti-HLA lymphocytotoxic crossmatch assay, and this effect is complement-dependent as M101 inhibits the classical complement pathway (CH50) in a dose-dependent and stable manner. Such inhibition was independent from a proteolytic effect (fractions C3 to C9) but related to a dose-dependent inhibition of the C3 convertase as demonstrated by exploring downstream the release of the anaphylatoxins (C3a and C5a), C3d, and sC5b-9. The C3 convertase inhibition in the presence of M101 was further demonstrated in the AP50/AH50 assay. In conclusion, the use of M101 avoids the activation of the complement pathway, which constitutes an additional advantage for this extracellular hemoglobin to preserve grafts from ischemia/reperfusion injury and preformed anti-HLA antibodies.

Immobilization Strategies for Functional Complement Convertase Assembly at Lipid Membrane Interfaces

The self-assembly formation of complement convertases-essential biomacromolecular complexes that amplify innate immune responses-is triggered by protein adsorption. Herein, a supported lipid bilayer platform was utilized to investigate the effects of covalent and noncovalent tethering strategies on the self-assembly of alternative pathway C3 convertase components, starting with C3b protein adsorption followed bythe addition of factors B and D. Quartz crystal microbalance-dissipation (QCM-D) experiments measured the real-time kinetics of convertase assembly onto supported lipid bilayers. The results demonstrate that the nature of C3b immobilization onto supported lipid bilayers is a key factor governing convertase assembly. The covalent attachment of C3b to maleimide-functionalized supported lipid bilayers promoted the self-assembly of functional C3 convertase in the membrane-associated state and further enabled successful evaluation of a clinically relevant complement inhibitor, compstatin. By contrast, noncovalent attachment of C3b to negatively charged supported lipid bilayers also permitted C3b protein uptake, albeit membrane-associated C3b did not support convertase assembly in this case. Taken together, the findings in this work demonstrate that the attachment scheme for immobilizing C3b protein at lipid membrane interfaces is critical for downstream C3 convertase assembly, thereby offering guidance for the design and evaluation of membrane-associated biomacromolecular complexes.

Inhibitory effects of rosmarinic acid on pterygium epithelial cells through redox imbalance and induction of extrinsic and intrinsic apoptosis

Pterygium is a common tumor-like ocular disease, which may be related to exposure to chronic ultraviolet (UV) radiation. Although the standard treatment for pterygium is surgical intervention, the recurrence rate of pterygium is high when no effective inhibitory drug is used after surgery. Rosmarinic acid (RA) is a polyphenol antioxidant with many biological activities, including anti-UV and anti-tumor properties. This study aimed to examine the inhibitory effects of RA on pterygium epithelial cells (PECs). Methylthiazolyldiphenyl-tetrazolium bromide (MTT) assay was used to examine the cell cytotoxicity of PECs after RA treatment. A fluorescent probe, DCFH-DA (2',7'-dichlorofluorescin diacetate), was stained with PECs to measure intracellular reactive oxygen species (ROS) levels. Antioxidant activity assays were used to measure the levels of superoxide dismutase (SOD) and catalase (CAT) in PECs. Western blot analysis was used to determine the protein expression of nuclear factor E2-related factor 2 (Nrf2), heme oxygenase 1 (HO-1), quinone acceptor oxidoreductase 1 (NQO1), and apoptosis-associated proteins. RA significantly reduced the cell viability of the PECs. Treatment with RA remarkably increased the Nrf2 protein expression levels in the nucleus, HO-1 and NQO1 protein expression levels, and the activities of SOD and CAT. As a result, intracellular ROS levels in PECs were decreased. Additionally, the induction of extrinsic apoptosis on PECs by RA was associated with increasing expressions levels of Fas, Fas-associated protein with death domain (FADD), tumor necrosis factor-alpha (TNF-α), and caspase 8 protein. Moreover, the induction of intrinsic apoptotic cell death in PECs was confirmed through upregulation of cytochrome c, Bax, caspase 9, and caspase 3 and downregulation of Bcl-2 and pro-caspase 3. Our study demonstrated that RA could inhibit the viability of PECs through regulation of extrinsic and intrinsic apoptosis pathways. Therefore, RA may have potential as a therapeutic medication for pterygium.

Cooperation between decay-accelerating factor and membrane cofactor protein in protecting cells from autologous complement attack

Decay-accelerating factor (DAF or CD55) and membrane cofactor protein (MCP or CD46) function intrinsically in the membranes of self cells to prevent activation of autologous complement on their surfaces. How these two regulatory proteins cooperate on self-cell surfaces to inhibit autologous complement attack is unknown. In this study, a GPI-anchored form of MCP was generated. The ability of this recombinant protein and that of naturally GPI-anchored DAF to incorporate into cell membranes then was exploited to examine the combined functions of DAF and MCP in regulating complement intermediates assembled from purified alternative pathway components on rabbit erythrocytes. Quantitative studies with complement-coated rabbit erythrocyte intermediates constituted with each protein individually or the two proteins together demonstrated that DAF and MCP synergize the actions of each other in preventing C3b deposition on the cell surface. Further analyses showed that MCP's ability to catalyze the factor I-mediated cleavage of cell-bound C3b is inhibited in the presence of factors B and D and is restored when DAF is incorporated into the cells. Thus, the activities of DAF and MCP, when present together, are greater than the sum of the two proteins individually, and DAF is required for MCP to catalyze the cleavage of cell-bound C3b in the presence of excess factors B and D. These data are relevant to xenotransplantation, pharmacological inhibition of complement in inflammatory diseases, and evasion of tumor cells from humoral immune responses.

Functions and relevance of the terminal complement sequence

The terminal complement sequence is initiated upon cleavage of C5 with liberation of C5a anaphylatoxin, and involves the assembly of macromolecular C5b-9 complexes either on cell surfaces or in plasma. Cell-bound C5b-9 complexes generate transmembrane pores that can cause cell death, or they can elicit secondary cellular reactions triggered, for example, by passive flux of calcium ions into the cells. In vivo functions of the fluid-phase SC5b-9 complex have not yet been defined, but the identity of S-protein with vitronectin (serum spreading factor) provokes the anticipation that significant biological functions of this complex do exist. The terminal complement sequence may fulfil protective functions when it is triggered on alien cells that are marked for destruction. Dysregulation in the complement sequence may, however, result in detrimental attack by C5b-9 on autologous cells. Examples include not only autoimmune disease states, but also the activation of complement on dead or dying cells, and bystander attack on blood cells during cardiopulmonary bypass. Methods for detecting and quantifying C5b-9 are outlined, and the potential usefulness of such assays in clinical research is discussed.

A modification of cellulose that facilitates the control of complement activation

Complement activation by Cuprophan hemodialysis membranes has been linked to a variety of pathological sequelae (neutropenia and various cardiopulmonary manifestations) seen in the clinical setting. The modification of reactive surface hydroxyl groups on regenerated cellulose with various dicarboxylic-acid anhydrides has been found to significantly limit the complement-activating potential of these materials. Of the anhydrides tested, maleic anhydride appears to display the most dramatic and consistent diminution of complement activation compared to unmodified cellulose (0-10% of control values for C3b deposition and C3a/C5a production). Current evidence suggests that this maleated derivative facilitates the factor-H control of C3 and C5 convertase activity and thus may help limit complement activation by normal regulatory mechanisms. In addition, this modification may help limit the production of other inflammatory mediators that may result in diminished levels of cellular activation.

Monoclonal antibodies against components of the classical pathway of complement

Activation of the classical pathway of complement involves several binding and enzymatic cleavage processes. Binding and enzymatic activation results in the appearance of new structures in the individual components. This report describes the different activation steps for C1q, C1r, C1s, C4 and C2 and summarizes monoclonal antibodies reported so far which recognize either conserved epitopes or activation-dependent epitopes with particular emphasis on neoepitopes occurring during the activation cascade.

Covalent association of C3b with C4b within C5 convertase of the classical complement pathway

The C5 convertase of the classical complement pathway is a complex enzyme consisting of three complement fragments, C4b, C2a, and C3b. Previous studies have elucidated functional roles of each subunit (4, 6, 7), but little is known about how the subunits associate with each other. In this investigation, we studied the nature of the classical C5 convertase that was assembled on sheep erythrocytes. We found that one of the nascent C3b molecules that had been generated by the C3 convertase directly bound covalently to C4b. C3b bound to the alpha' chain of C4b through an ester bond, which could be cleaved by treatment with hydroxylamine. The ester bond was rather unstable, with a half-life of 7.9 h at pH 7.4 and 37 degrees C. Formation of the C4b-C3b dimer is quite efficient; e.g., 54% of the cell-bound C3b was associated with C4b when 25,000 molecules of C4b and 12,000 molecules of C3b were present per cell. Kinetic analysis also showed the efficient formation of the C4b-C3b dimer; the rate of dimer formation was similar to or even faster than that of cell-bound monomeric C3b molecules. These results indicate that C4b is a highly reactive acceptor molecule for nascent C3b. High-affinity C5-binding sites with an association constant of 2.1 X 10(8) L/M were demonstrated on C4b-C3b dimer-bearing sheep erythrocytes, EAC43 cells. The number of high-affinity C5-binding sites coincided with the number of C4b-C3b dimers, but not with the total number of cell-bound C3b molecules. Anti-C4 antibodies caused 80% inhibition of the binding of C5 to EAC43 cells. These results suggest that only C4b-associated C3b serves as a high-affinity C5 binding site. EAC14 cells had a small amount of high-affinity C5 binding sites with an association constant of 8.1 X 10(7) L/M, 100 molecules of bound C4b being necessary for 1 binding site. In accordance with the hypothesis that C4b-associated C4b might also serve as a high-affinity C5-binding site, a small amount of C4b-C4b dimer was detected on EAC14 cells by SDS-PAGE analysis. Taken together, these observations indicate that the high-affinity binding of C5 is probably divalent, in that C5 recognizes both protomers in the dimers. The high-affinity binding may allow selective binding of C5 to the convertase in spite of surrounding monomeric C3b molecules.

In vitro inhibition of the classical pathway of human complement by a natural microbial product, colistin sulphate

Colistin sulphate was found to be an inhibitor of the classical pathway of the complement system. The main sites of inhibition were the interaction of EAC14 with C2 and EAC142 with C3. It also inhibited EAC14 formation from EA and C2-deficient serum, EAC1-7 formation from EAC1-3, C5, C6 and C7 and the interaction of EAC1-7 with C8 and C9, though less efficiently. It did not inhibit formation of C3/C5 convertase of the alternative pathway. The inhibition of the classical pathway was reversible since hemolytic activity was completely restored after dialysis.

The effect of ethylenedinitrilotetraacetic acid (EDTA) on the reaction between the guinea-pig C5 convertase and guinea-pig C5

Guinea-pig C5 was reacted with EAC1423 in the washed-cell intermediate assay in the presence of glucose gelatin veronal buffer (GGVB), Zn2+-GGVB (0.025 mM), GGVB2+ containing Ca2+ and/or Mg2+ or EDTA (0.013 M)-GGVB. The EDTA inhibited the formation of competent SAC14235, while Ca2+ and/or Mg2+ had a slight enhancing effect compared to GGVB alone and Zn2+ gave a four-fold increase. Similar results were obtained by using human C5 with guinea-pig C5 convertase and functionally pure guinea-pig C6, C7, C8 and C9. When guinea-pig C6 was incorporated into these various reaction mixtures with guinea-pig C5, its addition markedly reduced the inhibition by EDTA, while Zn2+ still showed an enhancing effect. These results demonstrate that EDTA inhibited formation of competent SAC14235 by preventing activation of C5. The association of C6 with C5 can partially overcome the inhibition of C5 conversion by EDTA and may account for C5 activity in reaction mixtures containing C-EDTA.

The complement components of the major histocompatibility locus

Polymorphism of complement components, recognized by differences in either their antigenic specificity or their electrophoretic mobility, together with studies of inherited deficiencies, has enabled many of their structural genes to be mapped. In humans, three genes (for C2, C4, and factor B) have been placed between HLA-D and HLA-B on chromosome 6 and in mice, C4 between H2-I and H2-D, chromosome 17. Structural studies show that these components have exceptional features. C2 and factor B which contain the proteolytic active site of the C3 and C5 convertases are of the classical and alternative pathway respectively and are similar in structure and function. Both are novel types of serine proteases. C4 (as C3) contains an intrachain thioester bond essential for hemolytic activity. Molecular genetic investigations are determining the relative positions of these genes, and their precise structure, and should clarify their relation to the inherited diseases which are associated with defects in this section of the human genome.

Interaction of Zn2+ with guinea-pig C5 convertase and guinea-pig C5

A comparison of two methods of C5 activation, the standard method (EAC14 + C2, C3, C5, C6, and C7) and the washed-cell intermediate method (EAC1423 + C5 and washed), demonstrated that formation of hemolytically competent SAC14235 was reduced in the washed-cell method. Addition of Zn2+ in this method increased the formation of competent SAC14235 to the approximate level of the standard method. The optimum concn range of Zn2+ was 0.006-0.025 mM. In the standard method Zn2+ had no significant enhancing effect on the formation of competent SAC14235. Zn2+ had a greater affinity for EAC1423 than for fluid-phase C5 when reacted with each separately. Maximum enhancement, however, was obtained when Zn2+ was present during the reaction of C5 with EAC1423. The effect of Zn2+ on C5 activity was not related to the stabilizing property of C6 on cell-bound C5. In the washed-cell method there was an inverse relationship between the concn of C2 or C3 and the concn of C5 required to generate one competent SAC14235/cell. Over a 10-fold range of C2 or C3 concn there was an approximate four-fold increase in the number of competent SAC14235/cell formed in the presence of Zn2+. Zn2+ enhances formation of competent SAC14235 on cells that have a limited ability to activate C5 even though the C2 and C3 on the convertase are present in excess.

Inhibition of classical C5 convertase in the complement system by factor H

This paper described the influence of factor H on the haemolytic activity of the classical C5 convertase. Factor H showed little effect on the interaction of C5 with EAC1,4b,2a,3b cells bearing low numbers of C3b sites, but displayed the inhibitory effect on the interaction of C5 with the intermediate cells bearing high numbers of C3b sites. The higher the number of C3b sites on the cells, the greater the degree of the inhibition by factor H. The inhibition by factor H was accompanied by the inhibition of consumption of C5 from the fluid phase, indicating that factor H inhibits the activity of C5 convertase, not the binding of activated C5 to the cells.

The activation of human complement component C5 by a fluid phase C5 convertase

Complement component C5 is converted to C5a and C5b by the cobra venom factor-dependent C3/C5 convertase CVF,Bb (EC 3.4.21.47). The C5 convertase produces selective proteolytic cleavage of an arginyl-leucine peptide bond at positions 74-75 in the alpha chain of C5. Circular dichroism studies in both the far and near UV regions provide evidence that a conformational change accompanies the C5 activation process. When C5 is activated by CVF,Bb in the presence of complement component C6, the C5b,6 complex is formed. However, when C6 is added after C5 has been converted to C5b, the C5b,6 complex fails to form. Therefore, the activation of C5 results in a transient binding site for C6. Hydrophobic sites are probably exposed upon C5 activation because C5b undergoes aggregation when C5 is converted to C5b in the absence of C6. Transmission electron micrographs of the C5 molecule indicate a multilobal, irregular ultrastructure with estimated dimensions of 104 X 140 X 168 A. Aggregated C5b has the appearance of globular particles with a diameter range of 350-700 A. Although C5 shares a number of features with the third component of complement, including a similar ultrastructure and partial sequence homology, C5 is devoid of the unusual thiol ester linkage found in C3. It is the labile thiol ester that permits covalent attachment between C3 and nucleophilic acceptors. In contrast, interactions between C5 and C6 or C5 and membranes remain noncovalent.

The conversion of human complement component C5 into fragment C5b by the alternative-pathway C5 convertase

The cleavage of human complement component C5 to fragment C5b by the alternative pathway C5 convertase was studied. The alternative-pathway C5 convertase on zymosan can be represented by the empirical formula zymosan--C3b2BbP. Both properdin-stabilized C3 and C5 convertase activities decay with a half life of 34 min correlating with the loss of the Bb subunit. The C5 convertase functions in a stepwise fashion: first, C5 binds to C3b and this is followed by cleavage of C5 to C5b. The capacity to bind C3b is a stable feature of component C5, as C5b also has this binding capacity. Component C5, unlike component C3, does not form covalent bonds with zymosan after activation, and C5 is not inhibited by amines. Therefore C5, although similar in structure to C3, does not appear to contain the internal thioester group reported for C3 and C4.

Purification from guinea pig erythrocyte stroma of a decay-accelerating factor for the classical c3 convertase, C4b,2a

A protein with decay-accelerating activity for the classical C3 convertase, C4b2a, has been isolated on the basis of this function from guinea pig erythrocyte stroma. The isolation procedure for decay-accelerating factor of stroma (DAF-S) utilizes butanol extraction and chromatography on DEAE-Sephacel, hydroxylapatite, and phenyl Sepharose. Purified DAF-S has a m.w. of 60,000 and 65,000 on reduced and unreduced SDS gels, respectively, and exhibits m.w. of 30,000 and 175,000 on alkaline gradient gels, suggesting multiples of a 60,000-65,000 subunit. Purified DAF-S elicited a monospecific antiserum whose IgG fraction neutralized the decay-accelerating activity for C4b,2a affixed to 10(7) sheep erythrocytes (EAC1,4,2) in a dose-response fashion. The monospecific antiserum diluted up to 1:5120 agglutinated 1 x 10(6) guinea pig erythrocytes, but not sheep or human erythrocytes, suggesting that DAF-S, an integral membrane protein, has species-specific antigens that are expressed on the surface of the guinea pig erythrocyte.

Activation of the fifth and sixth component of the complement system: similarities between C5b6 and C(56)a with respect to lytic enhancement by cell-bound C3b or A2C, and species preferences of target cell

Brief shift of purified C5 and C6 at 0 degrees C to pH 6.4, followed by immediate neutralization, results in the generation of a factor, designated C(56)a, that lyses erythrocytes together with C7, C8, and C9. We compared C(56)a and C5b6 generated by an alternative-pathway convertase, with regard to their action on different target cells. We found tht C(56)a is similar to C5b6 in the following properties: 1) Together with C7, C(56)a forms a stable intermediate on either sheep or guinea pig erythrocytes. 2) Membrane-bound C3b, or A2C incorporated in the membrane, enhances lysis by C(56)a-9, as well as lysis by C5b6-9. We also found that the lysis of EC(56)a7 or EC5b67 intermediates by C8 and C9 depends on the species of the erythrocytes and the species of C8 and C9. Thus, lysis of sheep erythrocytes is more efficient with guinea pig C8 and C9 than with human C8 and C9. In the case of guinea pig erythrocytes, this relationship is reversed, i.e., these cells lyse more efficiently when human C8 and C9 are used. Enhancement of lysis by membrane-bound C3b or A2C does not abrogate this species incompatibility pattern.

Isolation of C4-binding protein from guinea pig plasma and demonstration of its function as a control protein of the classical complement pathway C3 convertase

A decay-accelerating factor of the classical complement pathway C3 convertase, C4b,2a, has been purified to homogeneity from guinea pig plasma by a 5-step procedure that includes 5% polyethyleneglycol-4000 (PEG-4000) precipitation, Sepharose 6B gel filtration, heparin-Sepharose chromatography, DE-52 anion exchange chromatography, and Sepharose-C4gp affinity chromatography. The protein elicited a monospecific antiserum in a rabbit and was found with the Mancini technique in both normal and C4-deficient guinea pig plasma at a concentration of 60 microgram/ml. The purified protein gave a single stained band of 550,000 m.w. on SDS-PAGE under nonreducing conditions and a single band of 72,000 m.w. with reduction and alkylation. On the basis of its m.w. and subunit structure, ability to bind to a C4 affinity column, and ability to regulate the classical C system by accelerating the decay of the classical C3 convertase this protein represents the guinea pig analog of the human C4-binding protein.

Relative resistance of the F-42-stabilized classical pathway C3 convertase to inactivation by C4-binding protein

The sera of some patients with SLE contain an IgG antibody (F-42) directed against the classical pathway C3 convertase (C-42), which is capable of stabilizing C42 in a dose-dependent manner. The half-life (T 1/2) of C42 is prolonged by F-42. In order to determine whether C4-binding protein was capable of reversing stabilization of C42, stabilized and unstabilized cell-bound C42 were exposed to purified C4-bp and the convertase activity was assessed. C4-bp was capable of accelerating the decay of C42 in a dose-dependent manner; 2 microgram/ml C4-bp reduced the T 1/2 of C42 from 5 to 2.5 min at 30 degrees C. On the other hand, 16 microgram C4-bp could reverse stabilization of C42 by F-42 from T 1/2 = 78 min to a T 1/2 - 40 min; 128 microgram C4-bp reduced the T 1/2 of stabilized C42 to 4 min. Functional inactivation of C42 occurs via enhanced decay-dissociation of C2 from the convertase by C4-bp, as shown by the release of 125I-C2i from the cell-bound convertase. Stabilization of C42 by F-42 is caused by prevention of decay-dissociation of 125I-C2. F-42 was also capable of stabilizing C4oxy2 even further, as shown by prolongation of the T 1/2 of cell-bound C4 oxy2 to a T 1/2 of at least 300 min at 30 degrees C.

The hemolytic equivalence of human, guinea pig and canine complement proteins

The ability of functionally pure human, guinea pig and canine complement proteins to fulfill the hemolytic function of a substituted analagous component in an otherwise totally homogeneous complement sequence was investigated by means of conventional hemolytic assays. A striking feature was the observation that for hemolysis to occur, differing requirements for enzyme-substrate homology were exhibited by each of the three species of C3 and C5 convertases (C3 and C5 CVA). The results suggest that different strategies of molecular interaction evolved in complement systems of different species.

Activation of the classical complement pathway by nephritic factor bound to the alternative pathway C3/C5 convertase

Nephritic Factor (NF), the potent alternative pathway activator, which is occasionally found in association with certain types of nephritis has recently been identified as an IgG class autoantibody specific for the C3 convertase (C3bB) of the alternative pathway. In these studies we have examined the possibility that the cell-bound NF-stabilized C3 convertase (EC3 bBNF) binds and activates the first component of the classical pathway of complement. EC3bBNF bound C1q, and the extent of binding was dependent upon the number of NF molecules bound per cell and decreased parallel to the dissociation and release of NF from the cells. Interaction of C1 with bound NF resulted in its activation as shown by the proteolytic conversion of proenzyme C1s to its activated form C1s. As was the case with C1q binding, C1 activation was dependent on the number of NF molecules bound per cell. Thus the NF-stabilized C3 convertase binds and activates C1.

The properdin system: composition and function

This article summarized the physicohemical data on the factors which compose the properdin system in guinea pig and man. The following other topics are discussed: (1) Activation of the properdin system; (2) Formation of the initiating and amplification C3 convertases; (3) Formation of the C5 convertase, and (4) Regulatory control mechanisms of the properdin system.