Circulating IgA-immune complexes in Henoch-Schönlein purpura. A longitudinal study of their relationship to disease activity and vascular deposition of IgA

In Henoch-Schönlein purpura immune complexes in inflamed vessel walls characteristically contain immunoglobulin A(IgA). To determine whether IgA is also the predominant immunoglobulin in circulating immune complexes, we compared the results of three immune complex assays with specificities for different classes of immunoglobulins in a longitudinal study of 37 patients (30 children and seven adults) with Henoch-Schönlein purpura. Circulating IgA-containing immune complexes were detected by their reactivity with a low avidity anti-IgA antibody in 27 of the 37 patients. IgA was simultaneously present in cutaneous vessel walls in 95 percent of the patients with circulating IgA-containing immune complexes. High levels of IgA-containing immune complexes were found only during the initial phase of the disease. Immune complexes containing bound complement breakdown products were demonstrated by binding to conglutinin. IgA was found in these immune complexes in 17 patients, IgG in 17 and IgM in nine patients. There was no apparent relation with the class of immunoglobulin in the deposits. Conglutinin-binding immune complexes were present later in the course of the disease and after remission. C1q-binding immune complexes were only found in two patients. These findings suggest that immune complexes-containing IgA may initiate the vasculitis of Henoch-Schönlein purpura, whereas complement-reacted immune complexes containing immunoglobulins of the other classes appear in the circulation in a later phase.

Isolation and characterization of rat complement factor B and its interaction with cell-bound human C3

Factor B was isolated from fresh rat plasma by sequential chromatography on QAE-A50, Biorex-70, gel filtration on Sephadex G-200 superfine and rechromatography on QAE-A50. That Brat was isolated in its native form was indicated by its migration during immunoelectrophoresis and by its capacity to react with cobra venom factor (CoVF) in the presence of human D to form a C3 convertase capable of cleaving purified rat C3 and human C3. The recovery of Brat was between 8 and 15%; the final material was homogeneous according to SDS-PAGE analysis. Reduction of Brat with DTT in the presence of urea and SDS did not produce detectable peptides of lower molecular weight. Both reduced and unreduced Brat had an apparent molecular weight of 100,000. An antiserum against Brat induced in rabbits recognized only one protein in fresh rat plasma as indicated by immunodiffusion and immunoelectrophoretic analysis. Zymosan treatment of rat serum resulted in the cleavage of Brat into two fragments with alpha and gamma mobility. Native Brat has a beta electrophoretic mobility. The plasma concentration of Brat in Wistar rats was 215 +/- 38 microgram/ml (mean +/- SD).

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.

Allergic vasculitis. A histological and immunofluorescent study of lesional and non-lesional skin in relation to circulating immune complexes

This study concerns 57 patients who fulfilled histological criteria for the diagnosis allergic vasculitis. For 37 of these patients, biopsy specimen were available from lesional and adjacent non-lesional skin. Histological signs of vasculitis were found at both sites, but in clinically normal skin the perivascular infiltrate was less dense and neutrophils and eosinophils were sparse or absent. Fibrin was found in only ten patients and occurred less frequently in non-lesional skin. Deposits of immune complexes and/or complement were detected by immunofluorescence in 49 of the 57 patients. Hardly any differences between lesional and non-lesional skin were found with immunofluorescent microscopy. Circulating immune complexes were detected in 45 of the 56 available sera. A relationship was found between the class of immunoglobulin in immune complexes in the vessel wall and in the circulation. Moreover, the class of immunoglobulin seemed to be related to the course, the extracutaneous involvement, and the presence of associated diseases. No explanation was found for the histological differences observed between lesional and non-lesional skin.

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.

Isolation and characterization of feline C3 and evidence for the immune complex pathogenesis of feline infectious peritonitis

Infections of cats with feline peritonitis (FIP) virus are usually inapparent but may lead to fatal polyserositis. We have recently advanced the hypothesis that immune complexes play an essential role in the pathogenesis of the condition. To support this hypothesis, the role of the third component of complement in FIP was investigated. In the present paper, the isolation of C3 from normal cat serum and some of its physical and immunologic properties are described. The final protein had an apparent m.w. of 185,000 and was composed of 2 polypeptide chains with m.w. of 128,000 and 71,000, respectively. When tested against whole cat serum, an antiserum raised in rabbits against purified C3 recognized only 1 protein whose identity with C3 was established. With the aid of this antiserum, depositions of C3 in renal glomeruli of FIP-affected cats were demonstrated by immunofluorescence. Their localizaton coincided with that of deposited IgG, thereby supporting the concept of an immune complex pathogenesis of FIP.

Isolation and characterization of feline C3 and evidence for the immune complex pathogenesis of feline infectious peritonitis

Infections of cats with feline peritonitis (FIP) virus are usually inapparent but may lead to fatal polyserositis. We have recently advanced the hypothesis that immune complexes play an essential role in the pathogenesis of the condition. To support this hypothesis, the role of the third component of complement in FIP was investigated. In the present paper, the isolation of C3 from normal cat serum and some of its physical and immunologic properties are described. The final protein had an apparent m.w. of 185,000 and was composed of 2 polypeptide chains with m.w. of 128,000 and 71,000, respectively. When tested against whole cat serum, an antiserum raised in rabbits against purified C3 recognized only 1 protein whose identity with C3 was established. With the aid of this antiserum, depositions of C3 in renal glomeruli of FIP-affected cats were demonstrated by immunofluorescence. Their localizaton coincided with that of deposited IgG, thereby supporting the concept of an immune complex pathogenesis of FIP.

The effect of lidocaine on the processing of soluble immune aggregates and immune complexes by peritoneal macrophages

In a search for new techniques which would specifically identify the various steps of the handling of immune complexes by macrophages the local anaesthetic, lidocaine, was investigated. Lidocaine was shown to inhibit selectively the digestion of immune complexes and immunoglobulin aggregates by macrophages whereas it had little effect on the binding or ingestion steps. Due to the reversible inhibitory effect of lidocaine, this drug may be used in future studies to assess the role of extracellular stimuli on the fate of intracellular immune complexes.

Circulating and tissue-bound immune complexes in allergic vasculitis: relationship between immunoglobulin class and clinical features

Sixty-two patients with allergic vasculitis and histological evidence of leukocytoclastic vasculitis were examined to determine whether clinical features, such as course and degree of systemic involvement, could be related to the class of the immunoglobulins in immune complexes (IC) in both the circulation and the vessel wall. Circulating IC were detected with the 125I-C1q-binding assay (C1q-BA), an anti-IgA inhibition binding assay (a-IgA-Inh BA) and the conglutinin-binding assay (Con-BA). Stabilized heat-aggregated IgG, IgA and IgM were used to determine the immunoglobulin class specificity of these assays. With the C1q-BA only aggregated IgG were detected, with the a-IgA-Inh Ba only aggregates containing IgA. With the Con-BA IgG, IgA or IgM in reactive aggregates were identified with class-specific antibodies. For patients with acute cutaneous vasculitis all assays were negative. The a-IgA-Inh BA was frequently positive in sera of patients with chronic cutaneous and acute systemic vasculitis; in the latter group conglutinin-binding IC of the IgG, IgA and IgM class were also detected. Levels in the C1q-BA were high for patients with chronic systemic vasculitis. Comparison of the results of the IC assays with the immunofluorescence studies of the cutaneous vessel walls of the same patients showed agreement between the results of the C1q-BA and deposition of IgG and the results of the a-IgA-Inh BA and IgA deposition. The class of immunoglobulin in conglutinin-binding IC did not correspond as well with the immunoglobulins in vessel walls. This study shows that certain clinical features of allergic vasculitis are related to the composition of the IC in the circulation and in the vessel wall.

Stabilization of the classical pathway C3 convertase C42, by a factor F-42, isolated from serum of patients with systemic lupus erythematosus

Sera from sixteen patients with SLE were investigated for the presence of a factor which would conserve convertase activity on preformed EAClgp 4hu2hu for 30 min at 30 degrees in EDTA. Although such a factor could not be detected readily in the sera, chromatography on DE-52 cellulose yielded fractions appearing as three peaks in one patient and as two peaks in a second patient. These peaks were capable of conserving C42 activity and were designated as F-42. Purification of F-42 from the second peak eluting between 4 and 7 mS on DE-52 was obtained by SP-C50, S-300 and QAE-A50 chromatography. F-42 exhibited charge heterogeneity upon SP-C50 chromatography. On polyacrylamide gel electrophoresis the final material migrated as one band, which coincided with the position of F-42 activity upon eluation from a parallel gel. F-42 had an apparent molecular weight of 150,000 and reacted with anti-IgG in Ouchterlony analysis. Sepharose-bound anti-IgG was capable of neutralizing F-42 activity. The purified material was shown to prolong the half-life (T 1/2) of performed cell-bound C42 in GVB-EDTA at 30 degrees from 5 to 80 min.

Circulating immune complexes and thyroid-stimulating immunoglobulins before, during, and after antithyroid drug therapy in patients with Graves' disease

Sera from patients with Graves' disease were assayed for thyroid-stimulating immunoglobulins (TSI) and circulating immune complexes (CIC) before, during, and after antithyroid drug treatment. 41 (72%) of 57 untreated patients had a positive TSI index and 17 (30%) had CIC. In untreated patients CIC were significantly more common in patients with a negative TSI index than in those with a positive one. It is suggested that because CIC prevent TSI from being detected the TSI index may be negative in Graves' disease. Early in the course of the treatment CIC developed in 85% of the patients; in all of these patients TSI disappeared. Later in the course of treatment when CIC could no longer be detected TSI did not reappear. These findings indicate that the development CIC could be the manifestation of the re-establishment of a natural tolerance to thyroid-stimulating hormone receptors in patients with Graves' disease.

Experimental Streptococcus sanguis endocarditis: immune complexes and renal involvement

An increase of agglutinins and circulating immune complexes (CIC) was observed in the sera of rabbits after the induction of bacterial endocarditis (BE) by intravenous injection of live streptococci into animals with a non-bacterial thrombotic endocarditis. Rabbits immunized for 12 days with heat-killed streptococci developed higher levels of agglutinins and CIC than did those unimmunized during BE. The levels remained elevated in the immunized rabbits after the induction of streptococcal endocarditis. In rabbits with very high levels of CIC complement activation occurred in vivo, possibly via the classical pathway. Kidneys from both unimmunized and immunized rabbits with streptococcal endocarditis contained bacteria and also showed inflammatory reactions but only the renal tissue of immunized rabbits with endocarditis showed a little mesangial deposition of immunoglobulin. The findings in this study indicate that in rabbits, renal inflammation due to BE is initiated more often by septic emboli than by immune complex deposition, at least during the first 12 days after infection.

The Role of Complement in the Binding and Degradation of Immunoglobulin Aggregates by Macrophages

The role of complement (C) in the processing of soluble immune complexes by guinea pig peritoneal macrophages was studied in a homologous system in vitro by using isolated stable IgG2 aggregates as a model for immune complexes.

Degradation of immunoglobulin aggregates by macrophages is markedly stimulated in the presence of fresh serum, an effect that was shown to be mediated via the classical pathway of C. After treatment with serum, aggregates were isolated from the serum proteins by sucrose gradient ultracentrifugation and subsequently presented to the macrophages; it appeared that the stimulating effect was mediated via the binding of C to the aggregates and is probably therefore not due to generation of free C split products in the medium.

Degradation by macrophages of serum-treated aggregates with comparable sedimentation rates increased as the concentration of the serum in which the aggregates had been incubated increased. Furthermore, a stimulated degradation of aggregates could be correlated with the ability of the aggregates to activate the C system as assessed by C4 consumption. Enhanced degradation of the aggregates was eliminated when the C receptors of the macrophages were removed by trypsin treatment before incubation with the aggregates. These results thus suggest that the effect observed is dependent on the ability of the aggregates to bind and activate C components and requires the presence of intact cellular C receptors. Experiments in which the rate of binding of the aggregates to the macrophage cell membrane was compared with the ingestion and digestion of membrane-bound aggregates showed that C increased the rate of binding but had no effect on the subsequent processing of the aggregates.

These studies thus demonstrate that C plays an important role in the clearance of soluble immunoglobulin aggregates by mononuclear phagocytes.

Aggregated human immunoglobulin G stabilized by albumin: a standard for immune complex detection

Stabilized preparations of heat-aggregated human immunoglobulin G (A-IgG) of restricted size were made by separating A-IgG by sucrose density gradient ultracentrifugation in the presence of bovine serum albumin (BSA). Periodic recentrifugation of stored fractions of the radiolabelled A-IgG indicated that the initial sedimentation characteristics were preserved. Pooled fractions of A-IgG stored for up to 16 months had the same functional activity as freshly prepared A-IgG of corresponding size when assessed by activation of the first component of complement and consumption of C4 and CH50 in normal human serum. It was also found that the reactivity of A-IgG in the C1q binding assay (C1Q-BA) and the conglutinin binding assay (Con-BA) was not altered by long-term storage of these A-IgG. Testing different batches of [125I]C1q and conglutinin with the same batch of stabilized A-IgG showed variations due to the instability of both [125I]C1q and conglutinin. The influence of these variations on the quantification of the levels of immune complexes in sera was reduced by using stable A-IgG as a reference. The assays were compared to determine the effect of the size of the aggregate. The C1Q-BA detected preferentially A-IgG of large size, while size had no influence in the Con-BA. These results suggest that the stability of A-IgG in BSA is such that this preparation may be used as a reliable standard for immune complex assays.

Isolation of the Fourth Component (C4) of Rat Complement

The fourth component of rat complement was purified to homogeneity by sequential chromatography of rat plasma in benzamidine on QAE-A50, SP-C50, hydroxyapatite, and gel filtration on Bio-Gel A 1.5. The final material was homogeneous on SDS-PAGE analysis and had a calculated m.w. of 198,000. A monospecific antibody against rat C4 was obtained from immunized rabbits. The concentration of rat C4 in the plasma of normal 4-month-old Wistar rats was 190 ± 34 µg/ml (mean ± 1 S.D.).

Isolation of the fourth component (C4) of rat complement

The fourth component of rat complement was purified to homogeneity by sequential chromatography of rat plasma in benzamidine on QAE-A50, SP-C50, hydroxyapatite, and gel filtration on Bio-Gel A 1.5. The final material was homogeneous on SDS-PAGE analysis and had a calculated m.w. of 198,000. A monospecific antibody against rat C4 was obtained from immunized rabbits. The concentration of rat C4 in the plasma of normal 4-month-old Wistar rats was 190 +/- 34 microgram/ml (mean +/- 1 S.D.).

Clearance and glomerular localization of preformed DNP anti-DNP immune complexes

The in vivo behaviour of well-defined immune complexes in rats was studied using complexes derived from DNP-conjugated bovine thyroglobulin (DNP-BTG) and purified specific goat anti-DNP IgG. Both clearance and glomerular localization were mainly dependent on the nature of the antigen. Soluble immune complexes formed with DNP17-BTG were cleared faster and showed a more marked localization in the glomerular mesangium than complexes formed with DNP3.4-BTG. A slight increase in the antibody to antigen ratio seemed to facilitate mesangial localization of soluble immune complexes. Insoluble immune complexes showed temporary localization as microemboli in the lumina of glomerular and peritubular capillaries. This study thus shows that not only the size and composition of the complexes but also the nature of the antigen within the complex can influence the clearance and organ localization of circulating immune complexes.

Further evidence for the antibody nature of C3 nephritic factor (C3NeF)

C3 nephritic factor (C3NeF), found in the sera of some patients with membranoproliferative glomerulonephritis, has been shown to be composed of two heavy and two light chains, like IgG; in addition it shares antigenic determinants with IgG. Purified C3NeF binds to the amplification convertase of complement, C3b,Bb, and thereby prevents decay of its C3-cleaving potential. The capability of C3NeF to bind to C3b,Bb was used as a means for purifying C3NeF to homogeneity. The investigation described in this report suggests that binding of C3NeF to C3b,Bb occurs via the Fab portion of the molecule. Pepsin treatment of eight C3NeF preparations resulted in an average loss of 76% of C3NeF functional activity. Papain treatment induced a loss of approximately 90%. The decrease in functional activity could be attributed to the accelerated rate of dissociation of 125I-F(ab')2 and 125I-Fab fragments from stabilized cell-bound C3b,Bb. The dissociation rate of 125I-F(ab')2 from C3b,Bb was comparable with the decay of the functional activity of C3b,Bb stabilized by F(ab')2 or Fab fragments of C3NeF. Although these results suggest that the stabilizing activity of C3NeF is mediated by the Fab portion of the molecule, it was found that the Fc portion also contributes to its functional activity.

Further Evidence for the Antibody Nature of C3 Nephritic Factor (C3NeF)

C3 nephritic factor (C3NeF), found in the sera of some patients with membranoproliferative glomerulonephritis, has been shown to be composed of two heavy and two light chains, like IgG; in addition it shares antigenic determinants with IgG. Purified C3NeF binds to the amplification convertase of complement, C3b,Bb, and thereby prevents decay of its C3-cleaving potential. The capability of C3NeF to bind to C3b,Bb was used as a means for purifying C3NeF to homogeneity. The investigation described in this report suggests that binding of C3NeF to C3b,Bb occurs via the Fab portion of the molecule. Pepsin treatment of eight C3NeF preparations resulted in an average loss of 76% of C3NeF functional activity. Papain treatment induced a loss of approximately 90%. The decrease in functional activity could be attributed to the accelerated rate of dissociation of 125I-F(ab′)2 and 125I-Fab fragments from stabilized cell-bound C3b,Bb. The dissociation rate of 125I-F(ab′)2 from C3b,Bb was comparable with the decay of the functional activity of C3b,Bb stabilized by F(ab′)2 or Fab fragments of C3NeF. Although these results suggest that the stabilizing activity of C3NeF is mediated by the Fab portion of the molecule, it was found that the Fc portion also contributes to its functional activity.