Cryoproteins in Acute Serum Sickness: Correlations Between Anticxn Cryoprecipitation, Immune Complexes and Immune Elimination in Acute Experimental Serum Sickness

To further define the relationship between serum cryoprecipitates and immune complexes, experimental serum sickness was produced in rabbits using radiolabelled bovine serum albumln (BSA) aei antigen.

BSA cryoprecipitation was not detected befors immune complexes appeared in the circulation. It reached peak quantities while immune complexes were present and BSA was undergoing immune elinination. The time come of BSA cryoprecipitation suggests that the BSA which appears in the cryopreclpitates may be in the form of immune couplexea.     

Intrauterine Growth Retardation of Unknown Etiology. I. Serum Complement and Circulating Immune Complexes in Mothers and Infants

ABSTRACT: Complement (C) and circulating immune complexes (CIC) levels were measured in 22 full-term pregnant women and 15 of their small-for-gestational-age (SGA) offspring in order to seek evidence supporting an immunological etiology for placental lesions related to idiopathic intrauterine growth retardation. We used 19 normal full-term pregnant women and 18 of their infants with birthweight above the 25th centile of the ponderal curve as a control population for this study. C levels were significantly lower in mothers of SGA infants than in controls (146.6 ± 46.6 and 183.6 ± 36.6 respectively, p < 0.01). CIC were present in the sera of 5 out of 22 mothers of the SGA group and in 3 out of the 15 infants sera. No CIC were found in the sera of mothers or infants from the control group. Placental lesions were observed in 14 out of the 22 (64%) cases studied in the SGA group and in 1 of 11 (9%) of the controls. Two placentas from SGA infants showed acute atherosis, and deposits of IgM and C₃ were found in their vessel walls. These data are in favor of an immunological mechanism for intrauterine growth retardation of unknown etiology.     

Defective Solubilization of Immune Complexes and Activation of the Complement System in Patients with Pulmonary Tuberculosis

Introduction  Association between inherited deficiencies of the complement components and immune complex disease indicates the importance of the complement system in the handling of circulating immune complexes. High levels of circulating immune complexes are seen in pulmonary tuberculosis. This study is, therefore, aimed to look at the concentration of circulating immune complexes, the status of complement-mediated immune complex handling, and the extent of complement activation in untreated pulmonary tuberculosis compared to treated pulmonary tuberculosis patients and healthy controls. Results  High immune complex levels, decreased complement-mediated solubilization, and increased activation of the complement system were observed in untreated tuberculosis patients. Conclusions  The results obtained from the present study suggest that complement mediated solubilization is less in patients with tuberculosis, and this defective solubilization is likely to take part in a vicious cycle involving immune complex deposition and complement activation and, thus, may lead to disease progression depending on the nature of the defect.     

Serum and Plasma Fibronectin Binds to Complement Reacted Immune Complexes Primarily via C1q

The binding of fibronectin to human Clq, C3b, and complement-reacted immune complexes (IC) was investigated by enzyme-linked immunosorbent assays. Microplates were coated with BSA followed by incubation with rabbit-anti-BSA IgG or F(ab')2 fragments of rabbit anti-BSA. Incubation of the solid phase with serum at 37 degrees C caused attachment of Clq and C3b. Addition of EDTA to the serum inhibited the binding of C3b, but not Clq, whereas substitution of the anti-BSA IgG on the solid phase with the F(ab')2 fragments abrogated the Clq, but not the C3b binding. Fibronectin binding was observed after incubation of the solid-phase IC with serum or plasma at conditions where Clq was also bound, whereas only minor amounts of fibronectin bound to the solid phase IC via C3b. Purified fibronectin showed a dose-dependent binding to solid-phase IC pretreated with Clq or fibronectin-depleted serum, confirming that the binding of fibronectin to IC largely occurred via Clq. Significantly smaller amounts of fibronectin were bound to solid-phase IC incubated with plasma instead of serum, despite the higher fibronectin concentration in plasma. This difference was found not to be due to a fibrinogen-fibronectin interaction in plasma. Binding of fibronectin to preformed fluid phase IC incubated with serum was demonstrated by SDS-PAGE analysis of PEG-precipitated IC.     

Intrauterine Growth Retardation of Unknown Etiology: II. Serum Complement and Circulating Immune Complexes in Maternal Sera and Their Relationship With Parity and Chronic Villitis

ABSTRACT: Significantly lower CH50 levels were found in women with small for gestational age (SGA) infants. The lowest values corresponded to nulliparous with placental chronic villitis (124.0 ± 10.6). Three out of five mothers with circulating immune complexes from SGA group were nulliparous, having placental chronic villitis. An immunological derangement in women with SGA infants is proposed for the development of placental lesions, mainly in nulliparous mothers with a lower previous exposure to fetal antigens.     

Circulating Immune Complexes and Rheumatoid Factors in Patients with Farmer''s Lung

Circulating IgG immune complexes (CICs), IgM rheumatoid factors (RFs), complement level (C3 and C4), and IgG antibodies to Aspergillus fumigatus, Aspergillus umbrosus, Thermoactinomyces vulgaris, and Micropolyspora faeni were analysed in the sera of 14 patients with farmer's lung (FL), 10 in the acute and four in the subacute phase of the disease. Ten spouses of FL patients served as exposed healthy controls. C3 and C4 were measured fluoronephelometrically. C3 levels were above the normal range and C4 levels near the upper limit of the normal range in both patients and controls; no statistically significant difference between patients and controls were observed. CICs were determined by enzyme immunoassays (EIA) of conglutinin-binding (KgB) and Clq-binding (ClqB). CIC levels above the normal range were detected in 12 (KgB-EIA) and nine (ClqB-EIA) of the patients and three (KgB-EIA) and six (ClqB-EIA) of the controls. No statistically significant differences were found in the mean levels between patients and controls. In contrast, RF levels in the acute phase of FL were significantly higher in the patients (P less than 0.01) than in the controls. CICs correlated positively with most microbial IgG antibodies, but negatively with RFs. RFs also correlated negatively with microbial IgG antibodies, as well as with both C3 and C4. In FL, the increased RF level may, in the absence of increased CIC and decreased complement levels, represent an immune response (IgM anti-IgG autoantibodies) induced by local mechanisms of IgG immune complexes in the lungs.     

Oral Contraception,Circulating Immune Complexes,Antiethinylestradiol Antibodies,and Thrombosis*

ABSTRACT: Circulating immune complexes were detected in women on oral contraceptives (OC) by a simple antigen nonspecific method using precipitation of serum in 25% saturated ammonium sulfate (CIC-AS). A significant correlation was found between the presence of CIC-AS and the OC vascular risk. A radioimmunoassay with tritiated ethinylestradiol indicated that CIC-AS contained antiethinylestradiol antibodies (anti-EE Ab) in a number of OC users, but indicated also that 1) anti-EE Ab may be found in cases with no detectable CIC-AS, 2) CIC-AS containing no anti-EE Ab are found in nonusers, and 3) even in OC users the CIC-AS may contain antibodies to other ligands than EE. The study demonstrated also that, in OC users with a vascular complication, anti-EE Ab were more frequently detected (78% of cases) than CIC-AS (60%). Moreover, among OC users with CIC-AS, anti-EE Ab were found in 95% of women with a vascular complication and only 37% of current healthy users. The detection of anti-EE Ab appears to be more predictive, with regard to the vascular risk of OC, than the detection of CIC-AS.     

Circulating Immune Complexes and Complement C4 Null Alleles in Patients Operated on for Premature Atherosclerotic Peripheral Vascular Disease

Circulating immune complexes can lead to vascular inflammation and premature atherosclerosis and the fourth component of complement, C4, plays an important role in the removal of immune complexes. The objective of this study was to analyze the relation between circulating immune complexes and C4 null alleles in patients operated on for peripheral vascular disease before the age of 50. The prevalence of circulating immune complexes and null alleles of C4 (C4Q0) was determined in 62 patients with peripheral atherosclerosis requiring surgery before 50 years of age and in a matched control group. C4A and C4B null alleles (C4A*Q0, C4B*Q0) were determined by electrophoresis of plasma, followed by immunofixation. C4A and C4B concentrations were measured by ELISA. Circulating immune complexes were determined by sucrose density gradient centrifugation and gel filtration. There was no difference in the distribution of C4Q0 between patients and controls. The patients had higher prevalences and levels of circulating immune complexes. This was correlated with the presence of C4Q0, especially C4A*Q0. There was an inverse correlation of concentration of circulating immune complexes with C4A levels and with ratio of C4A/B levels. Thus, a significant proportion of patients with premature peripheral atherosclerosis had circulating immune complexes and C4A*Q0 enhanced the propensity to immune complex formation. This might represent one mechanism for vascular damage in this patient group.     

Determination of Severity of Murine IgA Nephropathy by Glomerular Complement Activation by Aberrantly Glycosylated IgA and Immune Complexes

The pathogenic roles of glomerular deposition of components of the complement cascade in IgA nephropathy (IgAN) are not completely clarified. To investigate the pathologic role of complement pathways in IgAN, two IgAN-prone mouse models were examined. Grouped ddY (gddY) mice showed significant high proteinuria, severe glomerular lesions, and extracellular matrix expansion compared with high serum IgA (HIGA) mice but with similar intensity of glomerular IgA deposition. Glomerular activation of the classical, lectin, and alternative pathways was demonstrated by significantly stronger staining for complement (C)3, C5b-9, C1q, C4, mannose-binding lectin (MBL)-A/C, MBL-associated serine protease-2, and factor B and properdin in gddY mice than in HIGA mice. Similarly, the serum levels of IgA-IgG2a/IgM and IgA–MBL-A/C immune complexes and polymeric IgA were significantly higher in gddY mice than in HIGA mice. Moreover, the serum levels of aberrantly glycosylated IgA characterized by the binding of Sambucus nigra bark lectin and Ricinus communis agglutinin I were significantly higher in gddY mice than in HIGA mice. This aberrancy in glycosylation was confirmed by monosaccharide compositional analysis of purified IgA using gas-liquid chromatography. This study is the first to demonstrate that aberrantly glycosylated IgA may influence the formation of macromolecular IgA including IgA-IgG immune complexes and subsequent complement activation, leading to full progression of IgAN.IgA nephropathy (IgAN) was first reported in 1968 by Berger and Hinglais¹ and is a common form of progressive primary glomerulonephritis. The major histologic characteristics of IgAN are mesangial cell proliferation and matrix expansion with granular deposition of IgA, predominantly polymeric IgA1 (pIgA1),² and complement (C)3 in the glomerular mesangial areas. This deposition in glomeruli may provide insights into IgAN pathogenesis. In IgAN, IgA1 has aberrant glycosylation of O-glycans in the hinge region^3,4 and is predominantly in the polymeric form (pIgA).^2,5–7 However, the pathologic interaction between the complement cascade and pIgA has not been completely clarified in IgAN.Three different pathways of complement activation have been described. The classical pathway is triggered by activation of the C1 complex (composed of one molecule of C1q, two molecules of C1r, and two molecules of C1s, thus forming C1qr2s2).This activation occurs when C1q binds to IgM- or IgG-antigen complexes or when C1q binds directly to the surface of a pathogen. The lectin pathway is homologous to the classical pathway but contains opsonin, mannose-binding lectin (MBL), and ficolins instead of C1q. This pathway is activated by binding of MBL to mannose residues on the pathogen surface, thereby leading to activation of MBL-associated serine proteases (MASP-1 and MASP-2). These proteases split C4 into C4a and C4b and C2 into C2a and C2b. Similar to that in the classical pathway, C4b and C2a then bind together to form C3 convertase. The alternative pathway is triggered by spontaneous C3 hydrolysis [C3(H₂O)] directly due to the breakdown of the thioester bond. This change in shape allows the binding of plasma protein factor B [C3(H₂O)Bb, C3 convertase]. This convertase cleaves C3 proteins into C3a and C3b, which are then capable of covalently binding to a pathogenic membrane surface. Properdin is a component of the alternative pathway. It forms complexes with C3b to stabilize the alternative C3 convertase that further cleaves more C3. In all three pathways, C3 convertase cleaves and activates component C3 by forming C3a and C3b. It also cleaves C5 into C5a and C5b. C5b initiates the membrane attack pathway by forming a membrane attack complex with other recruited complement components (C6, C7, C7, C8, and multiple C9 molecules). Membrane attack complex is the cytolytic end product of the complement cascade. It forms a transmembrane channel that causes osmotic lysis of the target cell.We used IgAN animal models to investigate IgAN pathogenesis. The ddY mouse is an animal model of spontaneous IgAN. Glomerular lesions in this mouse include mesangial proliferation and extracellular matrix expansion with paramesangial IgA deposition that closely resemble those found in human IgAN.⁸ In pooled serum and glomerular elutes of 16-, 40-, and 60-week-old ddY mice, the ratio of dimeric IgA and pIgA in total IgA increases markedly with age.⁹ However, the IgAN incidence in commercially available ddY mice is highly variable.¹⁰ The high serum IgA (HIGA) mouse is one of the IgAN models derived from the ddY mouse.¹¹ This mouse is an inbred strain derived from ddY mice by selective mating of animals with high serum IgA levels. No renal abnormalities manifest in the HIGA mouse until approximately 10 weeks of age, with serum IgA levels increasing dramatically thereafter and becoming elevated by 25 weeks of age. Although this mouse shows high serum IgA levels, the increase is not associated with the severity of glomerular injury and the incidence of IgAN. We have evaluated commercially available ddY mice using serial renal biopsies obtained at 20, 40, and 60 weeks of age. These mice were divided into three groups: early-onset (approximately 20 weeks), late-onset (approximately 40 weeks), and quiescent (even at 60 weeks) mice.¹² In ddY mice with disease onset, the levels of serum IgA-IgG2a immune complex (IC) correlated strongly with the severity of glomerular lesions,¹³ although no significant correlation was observed between serum IgA levels and the incidence of glomerular injury among the groups. We bred early-onset mice in a specific pathogen–free room and established a mouse IgAN model with an almost 100% incidence of the disorder.¹⁴ This mouse strain was called grouped ddY (gddY). Although the HIGA and gddY models are derived from ddY mice, their ratios of disease onset, phenotype, and disease severity are very different. In this study, we examined these differences from the aspect of complement activation.     

Binding and Activation of the First Complement Component by Soluble Immune Complexes: Effect of Complex Size and Composition

The interaction between soluble immune complexes and the first component of complement (C1) was studied. Complexes were prepared from purified bovine thyroglobulin (BTg) or tetanus toxoid (TT) and immunospecific IgG antibodies. Purified human precursor C1 was incubated with dilutions of the preparations, and the inhibition of C1 haemolytic activity was determined as a measure of C1-binding. The activation of C1 was assessed by measuring the amount of C4 consumed by generated C1. The molar antibody/antigen (Ab/Ag) ratio of BTg--anti-BTg mixtures strongly influenced their C1-binding and C1-activating capacities: mixtures with high Ab/Ag ratios were by far the most efficient. On the other hand, the Ab/Ag ratio had only a limited influence on the activity of TT--anti-TT complexes. The effect of complex size was investigated by ultracentrifugation of antibody-antigen mixtures on calibrated sucrose density gradients followed by C1-binding and -activation experiments with the fractions obtained. For both types of immune complex, the C1-binding and -activating capacities increased markedly with increasing complex size. Thus, both the size and the Ab/Ag ratio of soluble immune complexes influence their capacity to activate the classical complement pathway. The effect of the Ab/Ag ratio, however, may also be dependent on the antigen molecule(s) present in the complexes.     

Circulating Heme Oxygenase-1 and Complement Activation in Transplant-Associated Thrombotic Microangiopathy

Transplant-associated thrombotic microangiopathy (TA-TMA) is a severe complication in patients after hematopoietic stem cell transplantation. The pathogenesis of TA-TMA is still unclear. Previous studies showed that complement activation plays an important role in the development of TA-TMA. However, no data showed which kind of complement component triggers this process. In this study we found that heme oxygenase-1, which could induce decay-accelerating factor (DAF) and inhibit the membrane-attack complex, was significantly decreased in patients with TA-TMA. DAF levels in the TA-TMA group were in line with the levels in the myocardial infarction group but were lower than levels in the healthy, noncomplication, infection, and graft-versus-host disease groups (P < .05). Human umbilical vein endothelial cells (HUVECs) incubated with TA-TMA plasma showed lower DAF levels compared with that incubated with normal human plasma. Notably, treatment with N-acetylcysteine (NAC), a drug against oxidation, increased the level of DAF. NAC could also inhibit complement activation in HUVECs incubated with TA-TMA plasma. Taken together, we propose that NAC represents a new potential therapy for patients facing TA-TMA.     

Immunoglobulin G, A, and M Responses in Serum and Circulating Immune Complexes Elicited by the 16-Kilodalton Antigen of Mycobacterium tuberculosis

The 16-kDa cytosolic antigen of M. tuberculosis was purified to homogeneity by molecular sieving chromatography, and the diagnostic potential of the antigen was evaluated in various categories of patients by enzyme-linked immunosorbent assay (ELISA). The immunoglobulin G (IgG), IgA, and IgM antibody levels to 16-kDa antigen were estimated in the two polar groups, namely, smear- and culture-positive pulmonary tuberculosis (S⁺C⁺) patients and healthy subjects (HS). Sensitivities of 62, 52 and 11% with specificities of 100, 97, and 95% were obtained for the three isotypes, respectively. The total number of positives by a combination of the three isotypes was analyzed in the polar groups, and the sensitivity improved to 83% with a specificity of 93%. Even when a combination of IgG and IgA alone was considered, the sensitivity was 82% with a specificity of 97%. Polyethylene glycol precipitation of the circulating immune complex (CIC) in sera was carried out. The CIC-bound antibodies to 16-kDa antigen were assessed by ELISA in the S⁺C⁺, S⁻C⁺, and S⁻C⁻ categories of patients. Measuring the IgG-IgA-IgM combination positivities of the CIC-bound antibodies gave sensitivities of 97.5, 100, and 45.3%, respectively. The specificity of the assay with these combinations was maintained at 95.4%.     

A Survey for Circulating Immune Complexes in Patients with Acute Myocardial Infarction

A new assay for the detection of circulating C1q-binding immune complexes (IC) is described. The assay makes use of solid-phase C1q and iodinated soluble protein A, extracted from the cell wall of Staphylococcus aureus. In a model system the assay could detect heat-aggregated IgG down to a concentration of about 50 ng/ml. This method and three other assays, previously described, were used to survey the appearance of IC activity in sera from hospitalized patients with acute myocardial infarction. Depending on the assay system used, from 56% to 66% of the patients investigated were found to develop circulating IC. The earliest appearance of circulating IC was noted 5 days after infarction. The highest incidence of positive reactions and the strongest reactions occurred 2 to 3 weeks after hospitalization; thereafter the IC positiveness tapered off, and all patients were negative 6 weeks after infarction.     

Circular Dichroism of Immune Complexes, IgG and Faby with Unique Antigenic Determinants from Rheumatoid Serum

IgG-IgG and IgG-IgM complexes were isolated from one rheumatoid arthritis (RA) serum by affinity chromatography to immobilized F(ag')2 gamma of specific antibodies against unique determinants in the complexes. Both IgG nd IgM, when isolated from these complexes, contained the unique determinants. The circular dichroism (CD) spectrum of IgG differed from that of normal IgG at neutral pH. At pH 3 both IgG and IgM displayed normal CD spectra, and only one third of the molecules now had affinity for the immobilized ligand. The molecules with affinity at pH 3 exhibited an abnormal CD spectrum at pH 3, and a normal CD spectrum was obtained only of those components that lacked affinity. One-third of the Fab gamma isolated from the IgG and IgG complexes with the unique determinants contained the unique determinants that were lacking in the rest of the same principal the Fc gamma fragments. The CD of the two Fab gamma preparations showed the same principal differences as the CD of the molecules with and without affinity to the ligand at acidic pH.     

Polynucleotide Immune Complexes in Serum and Glomeruli of Patients with Systemic Lupus Erythematosus

Several types of antipolynucleotide antibodies were eluted by acid buffer or deoxyribonuclease treatment of glomeruli obtained from nine kidneys from patients with systemic lupus erythematosus (SLE). Anti-SDNA antibodies were found concentrated over serum levels in eight eluates, anti-NDNA in six eluates and anti-RNA Pr in four eluates; anti-DSRNA antibodies were not demonstrable in any eluate tested. Deoxyribonuclease treatment eluted a high incidence and greater quantity of anti-NDNA and anti-SDNA antibody, whereas anti-RNA Pr antibody was mainly eluted by acid buffer. Simultaneous studies of antibody and antigen in serial serum specimens and in glomeruli suggested that complexes of SDNA antibody or antigen excess were frequently deposited in SLE kidneys, in addition to complexes containing anti-NDNA and anti-RNA Pr. It was observed that studies of antibody titers alone were inadequate for predicting the types of complexes deposited in the kidney. Either antigen excess could obscure detection of humoral antibody or extremely high titers of antibody as observed for RNA Pr are not conducive to the formation of kidney localizing immune complexes in the absence of antigen. Immunofluorescence studies demonstrated the presence of SDNA antigen in most cases from which anti-SDNA antibody was eluted providing direct evidence for the presence of SDNA-anti-SDNA complexes in renal glomeruli. A study of complement components indicated that Clq was absent from cases in which little or no SDNA was deposited in renal glomeruli; although all nephritic kidneys demonstrated C3 deposits. Several hypotheses accounting for this observation are discussed, including the probable utilization of the alternate pathway by certain types of complexes and a direct reaction between C1q and circulating or tissue-bound NDNA or SDNA.     

Complement Activation is Associated with the Presence of Specific Human Immunodeficiency Virus (HIV)-Anti-HIV Immune Complexes in Patients with Acquired Immunodeficiency Syndrome-Related Complex or Lymphoadenopathy Syndrome

The complement system was examined in a group of eight patients (six with lymphoadenopathy syndrome (LAS); two with acquired immunodeficiency syndrome (AIDS)-related complex (ARC], who were found to be human immunodeficiency virus (HIV)-positive, for the presence of specific HIV-anti-HIV complexes. A significant impairment of the classical and/or alternative pathway was found associated with the presence of cleavage fragments of C3 and/or B and a significant reduction in the complement factors studied. Ultracentrifugation fractions of serum samples obtained from one of the patients were assessed for the detection of specific HIV-anti-HIV (GP41-anti-GP41) complexes and were incubated with normal human serum to determine their complement activation capacity. A clear complement activation was found with the fraction in which a clear peak of HIV-anti-HIV (GP41-anti-GP41) immune complexes was present. The results demonstrate that specific immune complexes and complement activation are sometimes concomitantly present in patients with AIDS-related disease and that specific immune complexes may be one of the causal factors of the pathogenesis of complement activation in these patients. Possible consequences for the severe immune regulation with relevance to the dramatic failure in treating the virus effectively are discussed.     

Immune Complexes Isolated from Patients with Pulmonary Tuberculosis Modulate the Activation and Function of Normal Granulocytes

Circulating immune complexes (ICs) are associated with the pathogenesis of several diseases. Very little is known about the effect of ICs on the host immune response in patients with tuberculosis (TB). The effects of ICs isolated from patients with TB in modulating the release of calcium, cytokines, and granular proteins were studied in normal granulocytes, as were their chemotactic, phagocytic, and oxidative burst processes. ICs from TB patients induced decreased production of cytokines and platelet-activating factor (PAF) from normal granulocytes. ICs from TB patients also induced enhanced chemotaxis and phagocytosis but caused diminished oxidative burst. This was accompanied by an increased release in intracellular calcium. On the other hand, ICs from TB patients induced increased release of the granular proteins human neutrophil peptides 1 to 3 (HNP1–3). Thus, ICs from patients with TB exhibit a profound effect on granulocyte function with activation of certain effector mechanisms and dampening of others.     

生物材料体外动态血清接触对CH50及补体的影响

选用硅橡胶管（ＳＲ）、涤纶纤维材料（ＰＥＴＦ）、聚氯乙烯管（ＰＶＣ）及用醋酸纤维素（ＣＡ）、聚醚砜（ＰＥＳ）、聚氨酯涂层的聚氯乙烯管，分别与血清在３７℃下进行循环接触，根据接触前（０ｍｉｎ）、接触后５、１５、６０、１８０ｍｉｎ取血清，用ＩＬ－Ｍｏｎａｒｃｈ７６１型生化分析仪测定Ｃ３、Ｃ４，用试管法测定ＣＨ５０，实验结果表明，ＣＡ，ＰＥＴ，ＳＲ材料接触后血清的Ｃ３、Ｃ４，ＣＨ５０明显低于接触前，其中     

On the Interaction of the First Complement Component C1 and its Subunit Clq with Solid-Phase IgM Immune Complexes

The interaction of C1 and C1q with solid-phase anti-dextran MOPC-104E IgM was studied. An enzyme-linked immunosorbent assay (ELISA) was used to detect bound C1q. The results revealed that immobilized IgM is converted to the functionally active 'staple' conformation by the specific polyvalent ligand dextran (B 1355/S). C1q is fixed to IgM dependent on the antigen concentration, and its binding might be explained by assuming a functional binding constant (K) of approximately 10(9) M-1. Molecules bound with a K in the range of 10(7) M-1 cannot be detected by this ELISA procedure. The fixation of C1q saturated with an excess of the C1r2S2-tetramer differs from that of free C1q. C1q incorporated in the C1 complex rapidly dissociates independently of the antigen concentration. Since the complement binding sites are located at definite positions on the IgM molecule because of its pentameric structure, it is suggested that the distinguishable association properties of C1 and C1q are brought about from the altered flexibility of the C1q molecule complexed with C1r2S2.     

Increased Amounts of C4-Containing Immune Complexes and Inefficient Activation of C3 and the Terminal Complement Pathway in a Patient with Homozygous C2 Deficiency and Systemic Lupus Erythematosus

Plasma and serum samples from a patient with homozygous C2 deficiency and severe systemic lupus erythematosus who responded with full clinical remission after plasma infusions were examined for immune complexes (IC), C3 activation products, and the terminal complement complex (TCC). Plasma contained large amounts of C4-containing IC but no C3-containing IC or complement activation products. Classical pathway activation in vitro did not lead to C3 activation or TCC formation as seen in normal serum, but a very efficient binding of C1q and C4 was found. No disturbances in alternative pathway activation were observed. The results indicate an impaired formation of C3-containing IC and an inefficient clearance of C4-containing IC, supporting the idea of a causal relationship between the dysfunctional classical pathway, pathophysiology, and clinical manifestations in this patient.